Chapter 7-3 – Cell Boundaries

The Plasma Membrane: Cell Membrane Regulates what enters and leaves the cell. Provides protection and support. Highly selective barrier!!!! Student Misconceptions and Concerns 1. Students often think of the function of cell membranes as mainly containment, like that of a plastic bag. Consider relating the functions of membranes to our human skin. For example, membranes and our skin (a) detect stimuli, (b) engage in gas exchange, and (c) serve as sites of excretion and absorption. Teaching Tips 1. The hydrophobic and hydrophilic ends of a phospholipid molecule naturally create a lipid bilayer. The hydrophobic edges of the layer will seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water (a simple cell). Further, because of these hydrophobic properties, lipid bilayers are naturally self-healing. That all of this organization naturally emerges from the properties of phospholipids is worth sharing with your students. 2. You might wish to share a very simple analogy that seems to work with some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy to find exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.)

What the plasma membrane is made out of. Outside of cell Hydrophilic head Hydrophobic tail Phospholipid Cytoplasm (inside of cell) phospholipid bilayer of membrane – semi permeable Figure 4.6 The plasma membrane structure (part 1)

The Plasma Membrane: A Fluid Mosaic of Lipids and Proteins Most membranes have specific proteins and cholesterol embedded in the phospholipid bilayer. Two main types of proteins help regulate traffic across the membrane and perform other functions. Integral and peripheral. Student Misconceptions and Concerns 1. Students often think of the function of cell membranes as mainly containment, like that of a plastic bag. Consider relating the functions of membranes to our human skin. For example, membranes and our skin (a) detect stimuli, (b) engage in gas exchange, and (c) serve as sites of excretion and absorption. Teaching Tips 1. The hydrophobic and hydrophilic ends of a phospholipid molecule naturally create a lipid bilayer. The hydrophobic edges of the layer will seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water (a simple cell). Further, because of these hydrophobic properties, lipid bilayers are naturally self-healing. That all of this organization naturally emerges from the properties of phospholipids is worth sharing with your students. 2. You might wish to share a very simple analogy that seems to work with some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy to find exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.)

The Plasma Membrane: A Fluid Mosaic of Lipids and Proteins Surface Carbohydrates function in cell recognition, cell signaling and cell adhesion. Cholesterol helps keep the phospholipids spaced apart. Student Misconceptions and Concerns 1. Students often think of the function of cell membranes as mainly containment, like that of a plastic bag. Consider relating the functions of membranes to our human skin. For example, membranes and our skin (a) detect stimuli, (b) engage in gas exchange, and (c) serve as sites of excretion and absorption. Teaching Tips 1. The hydrophobic and hydrophilic ends of a phospholipid molecule naturally create a lipid bilayer. The hydrophobic edges of the layer will seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water (a simple cell). Further, because of these hydrophobic properties, lipid bilayers are naturally self-healing. That all of this organization naturally emerges from the properties of phospholipids is worth sharing with your students. 2. You might wish to share a very simple analogy that seems to work with some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy to find exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.)

Cytoplasm (inside of cell) Figure 4.UN12 Outside of cell Phospholipid Hydrophilic Protein Hydrophobic Hydrophilic Cytoplasm (inside of cell) Figure 4.UN12 Summary of Key Concepts: The Plasma Membrane: A Fluid Mosaic of Lipids and Proteins

Fluid mosaic model of membrane Figure 4.6b Outside of cell Proteins Hydrophilic region of protein Hydrophilic head Hydrophobic tail Hydrophobic regions of protein Cytoplasm (inside of cell) Fluid mosaic model of membrane Figure 4.6 The plasma membrane structure (part 2)

Membrane Protein Functions Cells must control the flow of materials in and out of the cell. Membrane proteins perform many functions. Cell signaling = relays messages to inside of cell Cytoskeletal attachment = cell shape Transport = a tunnel that substances pass through Intercellular joining = linking adjacent cells Cell-cell recognition = sugar (carbohydrate) ID tags Enzymatic activity = assembly line of chemical pathway Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 8

Intercellular joining Cell-cell recognition Figure 5.11 Cell signaling Enzymatic activity Cytoplasm Fibers of extracellular matrix Cytoskeleton Cytoplasm Attachment to the cytoskeleton and extracellular matrix Transport Intercellular joining Cell-cell recognition Figure 5.11 Primary functions of membrane proteins

The Plasma Membrane: A Fluid Mosaic of Lipids and Proteins The plasma membrane is a fluid mosaic. Fluid because molecules can move freely past one another. A mosaic because of the diversity of proteins in the membrane.

Cell Walls – ONLY IN PLANTS!! Plant cells have rigid cell walls surrounding the membrane. Plant cell walls made of cellulose protect the cells porous enough to allow water, oxygen and carbon dioxide to pass through easily. maintain cell shape keep cells from absorbing too much water. Student Misconceptions and Concerns 1. Students often think of the function of cell membranes as mainly containment, like that of a plastic bag. Consider relating the functions of membranes to our human skin. For example, membranes and our skin (a) detect stimuli, (b) engage in gas exchange, and (c) serve as sites of excretion and absorption. Teaching Tips 1. The hydrophobic and hydrophilic ends of a phospholipid molecule naturally create a lipid bilayer. The hydrophobic edges of the layer will seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water (a simple cell). Further, because of these hydrophobic properties, lipid bilayers are naturally self-healing. That all of this organization naturally emerges from the properties of phospholipids is worth sharing with your students. 2. You might wish to share a very simple analogy that seems to work with some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy to find exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.) 12

Diffusion Through Cell Boundaries Living cells exists in a liquid environment in order to survive. The plasma membrane plays a crucial role in regulating the movement of dissolved molecules from one side of the membrane to the liquid of the other side. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 13

Diffusion Through Cell Boundaries – Measuring Concentration Cytoplasm of a cell contains a solution of many different substances in water. The plasma membrane is semi-permeable, which means the membrane regulates what can go into and out of the cell. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 14

In a solution particles are constantly moving! Diffusion In a solution particles are constantly moving! The particles collide with one another and spread out randomly. As a result of this movement, particles tend to move from areas where they are more concentrated to areas where they are less concentrated. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids.

Diffusion Movement of molecules from areas of high concentration to areas of low concentration Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids.

Diffusion – no energy required!!!! Molecules of dye Membrane Net diffusion Net diffusion Equilibrium Diffusion – no energy required!!!! Figure 5.12 Passive transport: diffusion across a membrane (part 1)

Osmosis and Water Balance The diffusion of water across a selectively permeable membrane is osmosis. Water is moving from where it is more concentrated to where it is less concentrated. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 18

Osmosis and Water Balance When we compare two solutions we say that… a hypertonic solution has a greater concentration of solutes a hypotonic solution has a lesser concentration of solutes an isotonic solution has an equal concentration of solutes Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 19

Osmosis and Water Balance Hypotonic solution Hypertonic solution Sugar molecule Selectively permeable membrane Osmosis Figure 5.13 Osmosis (step 1)

Osmosis and Water Balance Hypotonic solution Hypertonic solution Isotonic solutions Osmosis Sugar molecule Selectively permeable membrane Osmosis Figure 5.13 Osmosis (step 2)

Water Balance in Cells Osmoregulation is the control of water balance within a cell or organism. Organisms must have a way to balance the water that exits and enters their cells. The cells in our body are not in danger of swelling because our cells are bathed in fluids like blood that are isotonic. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 22

(a) Isotonic solution (b) Hypotonic solution (c) Hypertonic solution Animal cell H2O H2O H2O H2O Normal Lysing Shriveled Plant cell Plasma membrane H2O H2O H2O H2O Flaccid (wilts) Turgid (normal) Shriveled (a) Isotonic solution (b) Hypotonic solution (c) Hypertonic solution Figure 5.14 The behavior of animal and plant cells in different osmotic environments

Animal cell H2O H2O A cell in an isotonic solution in which the concentration of solutes is the same inside and outside the cell. The cell is balanced and behaves normally. Normal Plant cell H2O H2O Flaccid (wilts) Isotonic solution Figure 5.14 The behavior of animal and plant cells in different osmotic environments (part 1)

Animal cells don’t have cell walls and will eventually burst!! H2O Animal cells don’t have cell walls and will eventually burst!! A cell in a hypotonic solution such as fresh water. Because the cell has a higher concentration of solutes than the fresh water, water rushes into the cell and the cell swells. Bursting H2O The cell wall of the plant cell prevents the plant cell from bursting! Turgid (normal) Hypotonic solution Figure 5.14 The behavior of animal and plant cells in different osmotic environments (part 2)

H2O A cell in a hypertonic solution in which the concentration of solutes in the solution is higher than the concentration of solutes inside the cell, which results in the movement of water out of the cell and the cell shrinking. Shriveled Plasma membrane The plasma membrane pulls away from the cell wall in the process of plasmolysis, which usually kills the cell. H2O Shriveled Hypertonic solution Figure 5.14 The behavior of animal and plant cells in different osmotic environments (part 3)

When plant cells lose water the plasma membrane pulls away from the cell wall and the plant cell shrivels resulting in the plant wilting. Figure 5.15 Plant turgor

Diffusion across Membranes Some substances do not cross membranes without the help from channel proteins embedded in the plasma membrane. These substances cross the plasma membrane by a process called facilitated diffusion through protein channels. No energy input is needed, the molecules diffuse from areas of higher concentration to areas of lower concentration through the channel protein. high concentration low concentration Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 28

Active Transport: The Pumping of Molecules across Membranes Active transport requires that a cell expend energy to move molecules across a membrane against a concentration difference, meaning, from areas of low solute concentration to areas of high solute concentration. This is done by proteins embedded in the plasma membrane called protein pumps. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 29

Active transport Lower solute concentration Solute ATP Figure 5.16-2 Lower solute concentration Solute ATP Higher solute concentration Active transport Figure 5.16 Active transport (step 2)

Membrane Transport Summary Passive Transport (requires no energy) Active Transport (requires energy) Diffusion Facilitated diffusion Osmosis Higher solute concentration Higher solute concentration Higher water concentration (lower solute concentration) Solute Solute Solute Water Solute ATP Lower solute concentration Lower water concentration (higher solute concentration) Lower solute concentration Figure 5.UN03 Summary of Key Concepts: Passive Transport, Osmosis, and Active Transport

Exocytosis and Endocytosis: Traffic of Large Molecules That Requires Energy Endocytosis – process of taking materials into the cell by means of infoldings, or pockets of the plasma membrane that form into vesicles once inside the cell. Large molecules, clumps of food or even whole cells can be taken into the cell this way. Phagocytosis – “cell eating” example = ameobas Pinocytosis – “cell drinking” Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 32

outside of the cell plasma membrane cytoplasm vesicle Endocytosis outside of the cell plasma membrane cytoplasm vesicle Figure 5.18 Endocytosis

Exocytosis and Endocytosis: Traffic of Large Molecules That Requires Energy Exocytosis is the secretion of large molecules within transport vesicles to the outside of the cell. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 34

Exocytosis vesicle Outside of cell Plasma membrane Cytoplasm Figure 5.17 Exocytosis