Cell Membrane Structure and Function chapter 5 Cell Membrane Structure and Function 1

Chapter 5 At a Glance 5.1 How Is the Structure of the Cell Membrane Related to Its Function? 5.2 How Do Substances Move Across Membranes? 5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate? © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? All the membranes of a cell have a similar basic structure Proteins suspended in or attached to phospholipid bilayer Otherwise, membranes differ among tissue types and environments Membranes function in isolation, regulation of substances and reactions, communication, and attachments within and between cells Ten thousand membranes stacked atop one another would scarcely equal the thickness of a book’s page. © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Fluid mosaic model Molecules in a fluid flow past one another Phospholipid bilayer Various biological molecules, some embedded, some attached, form a patchwork Glycolipids, glycoproteins, recognition proteins, enzymes, transport proteins, receptor proteins, anchoring proteins, cholesterol The fluid mosaic model was proposed in 1972 by S.J. Singer and G.L. Nicolson. © 2017 Pearson Education, Ltd.

Figure 5-1 The plasma membrane (interstitial fluid, outside) extracellular matrix phospholipid bilayer carbohydrate glycolipid binding site phospholipid pore glycoprotein cholesterol Figure 5-1 The plasma membrane protein connection protein enzyme receptor protein transport protein cytoskeleton (cytosol, fluid inside cell) © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Phospholipid Polar (hydrophilic) “head” Two nonpolar (hydrophobic) fatty acid “tails” © 2017 Pearson Education, Ltd.

hydrophobic tails hydrophilic head glycerol phosphate choline Figure 5-2 hydrophobic tails hydrophilic head Figure 5-2 A phospholipid glycerol phosphate choline © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Water surrounds all cells, even in multicellular eukaryotes Weakly salty interstitial fluid surrounds animal cells In water, phospholipids spontaneously arrange into a phospholipid bilayer Hydrophilic heads form hydrogen bonds with water molecules Hydrophobic tails cluster within © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? At body temperature, phospholipids shift about rapidly Phospholipids are not bonded together Fluidity of the membranes Allows shock absorption Allows membranes to merge Allows for cell shape change © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Cholesterol and saturation of fatty acids help stabilize membranes Reduce fluidity at higher temperatures Increase fluidity at lower temperatures Cholesterol reduces permeability Gives cell greater control over substances entering and leaving Saturated fatty acids (straight) pack tightly, decreasing fluidity. Unsaturated fatty acids (kinked) pack loosely, increasing fluidity. © 2017 Pearson Education, Ltd.

more saturated fatty acids more unsaturated fatty acids Figure E5-1 more saturated fatty acids less fluidity Figure E5-1 Tail kinks in phospholipids increase membrane fluidity more unsaturated fatty acids greater fluidity © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Hydrophobic molecules can readily diffuse through the phospholipid bilayer Many molecules used by cells are hydrophilic Cannot move through nonpolar, hydrophobic fatty acid tails Movement of these molecules relies on the mosaic of membrane-associated proteins © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Five major categories of membrane proteins Enzymes Recognition proteins Transport proteins Receptor proteins Connection proteins © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Glycoproteins (glyco, “sweet”) Membrane proteins that bear carbohydrate groups from the outer member surface Enzymes Promote chemical reactions that synthesize or break apart biological molecules For example, enzymes involved in ATP synthesis are embedded in the inner mitochondrial membrane. Plasma membrane enzymes help synthesize the supportive extracellular matrix that fills spaces between animal cells. In cells lining the small intestine, plasma membrane enzymes complete the breakdown of carbohydrates and proteins as these nutrients are taken into the cells. Glycolipids, carbohydrates attached to lipids, also help establish identity. © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Recognition proteins Glycoproteins Carbohydrates attached to membrane proteins Project from outer membrane surface of the cell Serve as identification tags © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Transport proteins Span bilayer Regulate movement of hydrophilic molecules across membrane Some form pores or channels Can be opened or closed Others bind substances and conduct them through the membrane May require energy © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Receptor proteins Most cells bear many types, spanning bilayer Each features binding site specific to messenger molecule Binding activates protein Direct receptor action Indirect receptor action © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Direct receptor action Some receptor proteins act as both receptors and ion channels (transporters) Messenger molecule causes a channel to open in that same protein Indirect receptor action Messenger molecule triggers cascade of reactions elsewhere within the cell Example of direct receptor action: skeletal muscle cells (which move the body). Membrane protein binds the neurotransmitter acetylcholine, which opens an ion channel in the same protein and allows ions that stimulate the muscle cell to contract to flow. Most neurotransmitters and hormones act via indirect receptor action. © 2017 Pearson Education, Ltd.

Figure 5-3 Receptor protein activation (interstitial fluid) Na+ Na+ messenger molecule neurotransmitter (cytosol) series of reactions Figure 5-3 Receptor protein activation (a) Direct receptor action (b) Indirect receptor action © 2017 Pearson Education, Ltd.

5.1 How Is the Structure of the Cell Membrane Related to Its Function? Connection proteins Anchor cell membrane Some help maintain cell shape by linking the membrane to the cytoskeleton Some connect the cytoskeleton with extracellular matrix Some link adjacent cells © 2017 Pearson Education, Ltd.

Which of the following is nonpolar? Fatty acid tails Glucose Water Phospholipid heads Question: 5-1 Answer: a Diff: Moderate Text Ref: Section 5.1 Skill: Factual Also relates to: Chapter 2 Notes: Water and all membrane proteins are polar molecules. The phospholipid head groups are also polar owing to their inherent charge. Only the fatty acid tails, which extend deep into the lipid bilayer, are nonpolar. © 2017 Pearson Education, Ltd.

Which of the following is nonpolar? Fatty acid tails Glucose Water Phospholipid heads Question: 5-1 Answer: a Diff: Moderate Text Ref: Section 5.1 Skill: Factual Also relates to: Chapter 2 Notes: Water and all membrane proteins are polar molecules. The phospholipid head groups are also polar owing to their inherent charge. Only the fatty acid tails, which extend deep into the lipid bilayer, are nonpolar. © 2017 Pearson Education, Ltd.

An organism living at the equator has more saturated phospholipids in its cell membranes than an organism living at the South Pole. Why? In cold climates, more unsaturated fats with kinked tails are needed to maintain the fluidity of the cell membranes. In cold climates, more saturated fats with kinked tails are needed to maintain the fluidity of the cell membranes. In warm climates, more unsaturated fats with kinked tails are needed to maintain the fluidity of the cell membranes. In warm climates, more saturated fats with kinked tails are needed to maintain the fluidity of the cell membranes. Question: 5-3 Answer: a Diff: Easy Text Ref: Section 5.1 Skill: Conceptual Also relates to: Chapter 3 Notes: Unsaturated fats have double bonds in the tails, making the tails kinked (instead of straight, as in saturated fats). The kinked structure of the unsaturated phospholipids makes them more fluid. This is very important in colder environments, where the molecules tend to move slower. Organisms living in warm environments have fewer unsaturated fats in their cell membranes than organisms living in cold environments. This question will help students remember Chapter 3, where saturated and unsaturated fats were introduced, and help them see the application of these different kinds of lipids. © 2017 Pearson Education, Ltd.

An organism living at the equator has more saturated phospholipids in its cell membranes than an organism living at the South Pole. Why? In cold climates, more unsaturated fats with kinked tails are needed to maintain the fluidity of the cell membranes. In cold climates, more saturated fats with kinked tails are needed to maintain the fluidity of the cell membranes. In warm climates, more unsaturated fats with kinked tails are needed to maintain the fluidity of the cell membranes. In warm climates, more saturated fats with kinked tails are needed to maintain the fluidity of the cell membranes. Question: 5-3 Answer: a Diff: Easy Text Ref: Section 5.1 Skill: Conceptual Also relates to: Chapter 3 Notes: Unsaturated fats have double bonds in the tails, making the tails kinked (instead of straight, as in saturated fats). The kinked structure of the unsaturated phospholipids makes them more fluid. This is very important in colder environments, where the molecules tend to move slower. Organisms living in warm environments have fewer unsaturated fats in their cell membranes than organisms living in cold environments. This question will help students remember Chapter 3, where saturated and unsaturated fats were introduced, and help them see the application of these different kinds of lipids. © 2017 Pearson Education, Ltd.

The phospholipid tails of a cell membrane face one another because _____. they are hydrophilic—repelled by water they are hydrophobic—attracted to water they are hydrophilic—attracted to water they are hydrophobic—repelled by water Question: 5-4 Answer: d Diff: Easy Text Ref: Section 5.1 Skill: Factual Also relates to: Chapter 2 Notes: The concepts of hydrophilic and hydrophobic are introduced at the end of Chapter 2, but they are more appropriately explained in the context of the plasma membrane. © 2017 Pearson Education, Ltd.

The phospholipid tails of a cell membrane face one another because _____. they are hydrophilic—repelled by water they are hydrophobic—attracted to water they are hydrophilic—attracted to water they are hydrophobic—repelled by water Question: 5-4 Answer: d Diff: Easy Text Ref: Section 5.1 Skill: Factual Also relates to: Chapter 2 Notes: The concepts of hydrophilic and hydrophobic are introduced at the end of Chapter 2, but they are more appropriately explained in the context of the plasma membrane. © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Substance movement vocabulary Solute = sustance that can be dissolved (dispersed into individual atoms, molecules, or ions) in a solvent Solvent = fluid capable of dissolving a solute Concentration = amount of solute in a given volume of solvent Gradient = difference in certain property (temperature, pressure, charge, concentration) between two adjacent regions Energy must be expended to create and maintain a gradient. Gradients decrease over time unless an impenetrable barrier separates adjacent regions or energy is expended. © 2017 Pearson Education, Ltd.

Animation: Plasma Membrane Structure and Transport © 2017 Pearson Education, Ltd. 28

5.2 How Do Substances Move Across Membranes? Diffusion Net movement of solutes from regions of high concentration to regions of low concentration, or “down” their concentration gradient Produced, over time, by motion of solutes Eventually solutes will be evenly dispersed Sped by increasing concentration gradient or temperature Recall that atoms, molecules, and ions are in constant motion, bombarding each other and the structures around them. © 2017 Pearson Education, Ltd.

diffuse into the water; water molecules diffuse into the dye. Figure 5-4 Dye molecules diffuse into the water; water molecules diffuse into the dye. Both dye molecules and water molecules are evenly dispersed. A drop of dye is placed in water. dye molecules Figure 5-4 Diffusion of a dye in water water molecule © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Cells must generate and maintain concentration gradients to stay alive Plasma membranes are selectively permeable Proteins allow only specific solutes to permeate, helping maintain concentration gradients Substances are allowed to permeate in two ways Passive transport Energy-requiring transport © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Passive transport: diffusion of substances down their concentration gradient Energy-requiring transport: cell must expend energy to move substances across membranes © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Passive transport Diffusion of substances down their gradients Does not require energy Includes Simple diffusion Facilitated diffusion Osmosis © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Simple diffusion Direct diffusion through the phospholipid bilayer Very small molecules with no net charge H2O (slowly), O2, CO2 Lipid-soluble molecules Alcohol, certain vitamins, steroid hormones Rate increases with gradient, temperature, smaller molecular sizes, and greater solubility in lipids. © 2017 Pearson Education, Ltd.

Figure 5-1 The plasma membrane (interstitial fluid, outside) extracellular matrix phospholipid bilayer carbohydrate glycolipid binding site phospholipid pore glycoprotein cholesterol Figure 5-1 The plasma membrane protein connection protein enzyme receptor protein transport protein cytoskeleton (cytosol, fluid inside cell) © 2017 Pearson Education, Ltd.

(interstitial fluid) O2 phospho- lipid bilayer (cytosol) Figure 5-5a (interstitial fluid) O2 phospho- lipid bilayer Figure 5-5a Types of diffusion through the plasma membrane (cytosol) (a) Simple diffusion through the phospholipid bilayer © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Facilitated diffusion Most polar molecules Too large and insoluble in lipids Ions (K+, Na+, Cl–, Ca2+) Charges attract water molecules, forming an aggregation too large to move through the bilayer Must use specific transport proteins (carrier and channel) Preexisting gradient (no energy required) © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Carrier proteins Equipped with regions that loosely bind specific solutes (certain ions, sugars, small proteins) Binding causes protein to change shape and transfer bound molecules across the membrane © 2017 Pearson Education, Ltd.

glucose carrier protein (b) Facilitated diffusion through Figure 5-5b glucose Figure 5-5b Types of diffusion through the plasma membrane carrier protein (b) Facilitated diffusion through carrier proteins © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Channel proteins Feature pores that may be open or closed Some are more selective than others Ion channel proteins are small and highly selective Many help maintain concentration gradients Aquaporins (“water pores”) Specialized water channels Size and charges lining the pore contribute to selectivity. © 2017 Pearson Education, Ltd.

Cl channel protein (c) Facilitated diffusion through channel proteins Figure 5-5c Cl Figure 5-5c Types of diffusion through the plasma membrane channel protein (c) Facilitated diffusion through channel proteins © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Osmosis Diffusion of water across selectively permeable membranes in response to gradients of concentration, pressure, or temperature From a region of high water concentration (fewer solutes) to one of low water concentration (more solutes) © 2017 Pearson Education, Ltd.

H2O aquaporin (d) Osmosis through aquaporins Figure 5-5d H2O Figure 5-5d Types of diffusion through the plasma membrane aquaporin (d) Osmosis through aquaporins or the phospholipid bilayer © 2017 Pearson Education, Ltd.

Animation: Osmosis and Diffusion © 2017 Pearson Education, Ltd. 44

5.2 How Do Substances Move Across Membranes? Isotonic (iso = “same”) Solution with equal concentrations Water moves equally in both directions; no net movement Hypertonic (hyper = “greater than”) Greater concentration of solute Hypotonic (hypo = “below”) More dilute concentration of solute Water tends to move from hypotonic to hypertonic solutions until balanced. © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Osmosis is critical to many biological processes Water uptake by roots, absorption of dietary water by intestines, reclamation of water in kidneys In isotonic solution (like the interstitial fluid of animals), there is no net movement of water When a cell is placed in a hypertonic solution, water leaves the cell, causing it to shrivel When a cell is placed in a hypotonic solution, water enters the cell, causing it to swell and eventually burst Although the concentrations of specific solutes are rarely the same both inside and outside of cells, the total concentrations of water and solutes inside and outside are equal. © 2017 Pearson Education, Ltd.

Figure 5-6 The effects of osmosis on red blood cells (a) Red blood cells in an isotonic solution (b) Red blood cells in a hypertonic solution (c) Red blood cells in a hypotonic solution © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Most plant cells have a large central vacuole with many aquaporins The vacuole’s contents are hypertonic to the surrounding cytosol, which in turn is usually hypertonic to fluid outside cells Water flows into the cytosol, then into the vacuole Turgor pressure Inflates the cell, forcing the cytosol within its plasma membrane against cell wall Without water, plants lose turgor and wilt © 2017 Pearson Education, Ltd.

Figure 5-7 Turgor pressure in plant cells cytoplasm central vacuole When water is plentiful, it fills the central vacuole, pushes the cytoplasm against the cell wall, and helps maintain the cell’s shape. Water pressure supports the leaves of this impatiens plant. (a) Turgor pressure provides support cell wall plasma membrane Figure 5-7 Turgor pressure in plant cells When water is scarce, the central vacuole shrinks and the cell wall is unsupported. Deprived of the support from water, the plant wilts. © 2017 Pearson Education, Ltd. (b) Loss of turgor pressure causes the plant to wilt

5.2 How Do Substances Move Across Membranes? Energy-requiring transport Cell must expend energy to move substances across membranes Crucial for maintaining gradients, acquiring food, excreting wastes, cell-to-cell communication Includes Active transport Endocytosis Exocytosis Active transport occurs when transporting against a concentration gradient. Endo- or exocytosis occur when moving greater volumes in or out of the cell. © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Active transport Energy is spent to move molecules against their gradient Builds potential Active transport proteins are often referred to as pumps, moving solutes “uphill” This potential can be used to relay electrical signals (nerves) or generate ATP (by allowing an established gradient to turn a molecular turnstyle, ATP synthase). © 2017 Pearson Education, Ltd.

Figure 5-8 Active transport (interstitial fluid) The transport Energy from ATP The protein protein binds both ATP and Ca2+. changes the shape of the transport protein and moves the ion across the membrane. releases the ion and the remnants of ATP (ADP and P) and closes. Ca2+ binding site ADP ATP binding site Figure 5-8 Active transport ATP P ATP Ca2+ (cytosol) © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Endocytosis (“inside cell”) Materials are engulfed from the extracellular environment and transported within the cell via vesicles Includes Pinocytosis Receptor-mediated endocytosis Phagocytosis May lead to the formation of a food vacuole, where, after fusing with a lysosome, digestion ensues This potential can be used to relay electrical signals (nerves) or generate ATP (by allowing an established gradient to turn a molecular turnstyle, ATP synthase). © 2017 Pearson Education, Ltd.

Figure 5-9 Figure 5-9 Pinocytosis A dimple forms The plasma membrane in the plasma membrane. forms a vesicle that buds into the cytosol. A deepening pit encloses fluid from outside the cell. (interstitial fluid) vesicle containing interstitial fluid (cytosol) (a) Pinocytosis (interstitial fluid) Figure 5-9 Pinocytosis (cytosol) © 2017 Pearson Education, Ltd. (b) TEM of pinocytosis

molecules and membrane dimples inward. A coated vesicle forms. Figure 5-10 coated pit (interstitial fluid) molecule to take in receptor protein (cytosol) coating protein coated vesicle A coated pit begins to form. Receptors bind molecules and membrane dimples inward. A coated vesicle forms. Figure 5-10 Receptor-mediated endocytosis © 2017 Pearson Education, Ltd.

Figure 5-11 Figure 5-11 Phagocytosis (interstitial fluid) food particle pseudopods food vacuole (cytosol) (a) Phagocytosis Figure 5-11 Phagocytosis (b) An Amoeba engulfs a Paramecium (c) A white blood cell engulfs a disease-causing fungal cell © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Exocytosis (“outside cell”) Materials to be expelled are transported via vesicles and released, diffusing into the extracellular fluid © 2017 Pearson Education, Ltd.

(interstitial fluid) secreted material plasma membrane plasma membrane Figure 5-12 (interstitial fluid) secreted material plasma membrane plasma membrane vesicle Figure 5-12 Exocytosis (cytosol) © 2017 Pearson Education, Ltd.

Animation: Endocytosis and Exocytosis © 2017 Pearson Education, Ltd. 59

Table 5-1 Table 5-1 © 2017 Pearson Education, Ltd.

5.2 How Do Substances Move Across Membranes? Given a spherical cell, the larger its diameter, the farther its innermost contents are from the plasma membrane Volume increase more rapidly than surface area Some cells (nerve cells, muscle cells) greatly increase the surface area available to serve their considerable volumes via elongation If a cell is too big, needed molecules (O2) can’t make the journey to all parts of the cell and wastes can’t make the journey out of the cell before the cell “suffocates” or becomes life-threateningly septic; there is not enough surface area to serve the volume. © 2017 Pearson Education, Ltd.

9 4 surface area 1 27 8 1 volume 1 2 radius 3 Figure 5-13 Figure 5-13 Surface area and volume relationships If the radius of a sphere increases by a factor of 3, then the volume increases by a factor of 27, but the surface area only increases by a factor of 9. radius 3 © 2017 Pearson Education, Ltd.

Which is NOT a form of active transport? Pinocytosis Receptor-mediated transport Phagocytosis Osmosis Question: 5-6 Answer: d Diff: Easy Text Ref: Section 5.2 Skill: Factual Notes: Pinocytosis, receptor-mediated transport, and phagocytosis are all energy-requiring processes for the uptake of substances into cells. Osmosis is a passive transport process dependent only on a transmembrane water concentration gradient. © 2017 Pearson Education, Ltd.

Which is NOT a form of active transport? Pinocytosis Receptor-mediated transport Phagocytosis Osmosis Question: 5-6 Answer: d Diff: Easy Text Ref: Section 5.2 Skill: Factual Notes: Pinocytosis, receptor-mediated transport, and phagocytosis are all energy-requiring processes for the uptake of substances into cells. Osmosis is a passive transport process dependent only on a transmembrane water concentration gradient. © 2017 Pearson Education, Ltd.

Which cell has the highest surface area to volume ratio? A cell with a radius of 10 units A cell with a radius of 20 units A cell with a radius of 50 units A cell with a radius of 100 units Question: 5-7 Answer: a Diff: Easy Text Ref: Section 5.2 Skill: Application Also relates to: Chapter 2 Notes: The cell with the smallest radius will have the largest surface area to volume ratio. However, the largest cell will have the highest absolute volume. It is therefore easier for a smaller cell to transport materials into and out of its cytoplasm than it is for a larger cell with more cytoplasm through which the transported material has to move. © 2017 Pearson Education, Ltd.

Which cell has the highest surface area to volume ratio? A cell with a radius of 10 units A cell with a radius of 20 units A cell with a radius of 50 units A cell with a radius of 100 units Question: 5-7 Answer: a Diff: Easy Text Ref: Section 5.2 Skill: Application Also relates to: Chapter 2 Notes: The cell with the smallest radius will have the largest surface area to volume ratio. However, the largest cell will have the highest absolute volume. It is therefore easier for a smaller cell to transport materials into and out of its cytoplasm than it is for a larger cell with more cytoplasm through which the transported material has to move. © 2017 Pearson Education, Ltd.

Which membrane process requires molecules to bind to a membrane protein? Water movement through channels Oxygen diffusion through the membrane Receptor-mediated transport Lipid movement across membranes Question: 5-9 Answer: c Diff: Easy Text Ref: Section 5.2 Skill: Factual Notes: This question provides an example of a protein-assisted form of transport involving the binding of the transported molecule to a membrane receptor before it moves across the membrane. All the other examples of transfer across biological membranes do not require the attachment of the transported molecule to a membrane protein. © 2017 Pearson Education, Ltd.

Which membrane process requires molecules to bind to a membrane protein? Water movement through channels Oxygen diffusion through the membrane Receptor-mediated transport Lipid movement across membranes Question: 5-9 Answer: c Diff: Easy Text Ref: Section 5.2 Skill: Factual Notes: This question provides an example of a protein-assisted form of transport involving the binding of the transported molecule to a membrane receptor before it moves across the membrane. All the other examples of transfer across biological membranes do not require the attachment of the transported molecule to a membrane protein. © 2017 Pearson Education, Ltd.

The salt concentration of the fluid surrounding a cell is more concentrated than the fluid inside the cell. The cell will _____. shrink swell stay the same transport salt into the cell Question: 5-11 Answer: a Diff: Moderate Text Ref: Section 5.2 Skill: Application Notes: Students have a hard time with the concept of tonicity. They need to remember that water follows solute. If they can remember this, they can understand in what direction the water will move. The most immediate effect here is for water to move OUT of the cell, resulting in the cell’s shrinking. However, over a more extended period of time the cell may transport salt into its cytoplasm, preventing additional water loss. © 2017 Pearson Education, Ltd.

The salt concentration of the fluid surrounding a cell is more concentrated than the fluid inside the cell. The cell will _____. shrink swell stay the same transport salt into the cell Question: 5-11 Answer: a Diff: Moderate Text Ref: Section 5.2 Skill: Application Notes: Students have a hard time with the concept of tonicity. They need to remember that water follows solute. If they can remember this, they can understand in what direction the water will move. The most immediate effect here is for water to move OUT of the cell, resulting in the cell’s shrinking. However, over a more extended period of time the cell may transport salt into its cytoplasm, preventing additional water loss. © 2017 Pearson Education, Ltd.

5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate? Intercellular junctions link cells and allow cells to communicate In animals Adhesive junctions Tight junctions Gap junctions In plants Plasmodesmata © 2017 Pearson Education, Ltd.

5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate? Adhesive junctions Animals only Linking and anchoring proteins connect cytoskeletons of adjacent cells Desmosomes are one type Join cells in tissues that are repeatedly stretched (skin, intestines, heart) © 2017 Pearson Education, Ltd.

(a) Adhesive junction (desmosome) Figure 5-14a plasma membranes of adjacent cells linking proteins intermediate filaments Figure 5-14a Links between cells anchoring proteins (a) Adhesive junction (desmosome) © 2017 Pearson Education, Ltd.

5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate? Tight junctions Animals only Formed by proteins that span plasma membranes at corresponding sites on adjacent cells Leakproof (bladder, skin, stomach, brain) Tight junction proteins of adjacent cells fuse to one another, forming a stitch-like pattern. © 2017 Pearson Education, Ltd.

plasma membranes of adjacent cells tight junction proteins Figure 5-14b plasma membranes of adjacent cells tight junction proteins Figure 5-14b Links between cells (b) Tight junctions © 2017 Pearson Education, Ltd.

5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate? Gap junctions Animals only Formed of six-sided protein tubes called connexons Align pores, connecting cytosol of adjacent cells Small hydrophilic molecules may pass, but proteins and organelles cannot Allow for coordination and communication (nervous signals, muscle cell contractions) © 2017 Pearson Education, Ltd.

plasma membranes of adjacent cells connexons pore (c) Gap junctions Figure 5-14c plasma membranes of adjacent cells connexons pore Figure 5-14c Links between cells (c) Gap junctions © 2017 Pearson Education, Ltd.

5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate? Plasmodesmata Plants only Channels linking adjacent cells, allowing for movement of large molecules Lined with plasma membrane and filled with cytosol Continuous from one cell to another Plasmodesmata are similar to gap junctions in function, but there is no relation. © 2017 Pearson Education, Ltd.

plasma membranes cell walls plasmodesmata (d) Plasmodesmata Figure 5-14d plasma membranes cell walls Figure 5-14d Links between cells plasmodesmata (d) Plasmodesmata © 2017 Pearson Education, Ltd.

What type of cell junctions are needed for a tissue that must prevent fluids from leaking across its cell layer? Desmosomes Tight junctions Gap junctions Plasmodesmata Question: 5-12 Answer: b Diff: Hard Text Ref: Section 5.3 Skill: Application Notes: All organs that must retain fluids have their cells joined by tight junctions to make a strong seal. Other types of junctions provide for more loose connections or for cell-to-cell transfer of ions or small molecules. © 2017 Pearson Education, Ltd.

What type of cell junctions are needed for a tissue that must prevent fluids from leaking across its cell layer? Desmosomes Tight junctions Gap junctions Plasmodesmata Question: 5-12 Answer: b Diff: Hard Text Ref: Section 5.3 Skill: Application Notes: All organs that must retain fluids have their cells joined by tight junctions to make a strong seal. Other types of junctions provide for more loose connections or for cell-to-cell transfer of ions or small molecules. © 2017 Pearson Education, Ltd.

Animal cells communicate with one another through _____. desmosomes tight junctions gap junctions plasmodesmata Question: 5-14 Answer: c Diff: Easy Text Ref: Section 5.3 Skill: Factual Notes: This question can reinforce the difference between gap junctions in animals and plasmodesmata in plants. © 2017 Pearson Education, Ltd.

Animal cells communicate with one another through _____. desmosomes tight junctions gap junctions plasmodesmata Question: 5-14 Answer: c Diff: Easy Text Ref: Section 5.3 Skill: Factual Notes: This question can reinforce the difference between gap junctions in animals and plasmodesmata in plants. © 2017 Pearson Education, Ltd.