Biology: Life on Earth Lecture for Chapter 5 Teresa Audesirk • Gerald Audesirk • Bruce E. Byers Biology: Life on Earth Eighth Edition Lecture for Chapter 5 Membrane Structure and Function Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 5 Opener A rattlesnake prepares to strike.

Chapter 5 Outline 5.1 How Is the Structure of a Membrane Related to Its Function, p. 82 5.2 How Do Substances Move Across Membranes? p. 85 5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate? p. 95

Section 5.1 Outline 5.1 How is the Structure of a Membrane Related to Its Function? The Plasma Membrane Isolates the Cell While Allowing Communication with Its Surroundings Membranes Are “Fluid Mosaics” in Which Proteins Move Within Layers of Lipids The Phospholipid Bilayer Is the Fluid Portion of the Membrane A Mosaic of Proteins Is Embedded in the Membrane

Plasma Membrane Functions of the plasma membrane Isolates the cell’s contents from environment Regulates exchange of essential substances Communicates with other cells Creates attachments within and between other cells Regulates biochemical reactions

Membranes Are “Fluid Mosaics” Membranes are dynamic, ever-changing structures

Membranes Are “Fluid Mosaics” “Fluid mosaic” model of a membrane proposed in 1972 A lumpy, constantly shifting mosaic of “tiles” or proteins Proteins float around in a sea of phospholipids

FIGURE 5-1 The plasma membrane The plasma membrane is a bilayer of phospholipids that form a fluid matrix in which various proteins (blue) are embedded. Many proteins have carbohydrates attached to them, forming glycoproteins. Three of the five major types of membrane proteins are illustrated here: recognition, receptor, and transport proteins.

The Phospholipid Bilayer Phospholipids are the basis of membrane structure Polar, hydrophilic head Two non-polar, hydrophobic tails

FIGURE 5-2 Phospholipid

The Phospholipid Bilayer The cell exterior and interior face watery environments

The Phospholipid Bilayer Hydrophobic and hydrophilic interactions drive phospholipids into bilayers Double row of phospholipids Polar heads face outward and inward Non-polar tails mingle within the membrane Cholesterol in animal membranes keeps them flexible

FIGURE 5-3 Phospholipid bilayer of cell membrane

The Phospholipid Bilayer Phospholipid bilayer is a flexible, fluid membrane to allow for cellular shape changes

The Phospholipid Bilayer Individual phospholipid molecules are not bonded to one another Some of the phospholipids have unsaturated fatty acids, whose double bonds introduce “kinks” into their “tails” The above features make the membrane fluid

FIGURE 5-4 (part 1) Kinks in phospholipid tails increase membrane fluidity

FIGURE 5-4 (part 2) Kinks in phospholipid tails increase membrane fluidity

Membrane Proteins Form a Mosaic Proteins are embedded in the phospholipid bilayer Some proteins can float and drift Other proteins are anchored by protein filaments in the cytoplasm Many proteins have attached carbohydrates (glycoproteins)

FIGURE 5-1 The plasma membrane The plasma membrane is a bilayer of phospholipids that form a fluid matrix in which various proteins (blue) are embedded. Many proteins have carbohydrates attached to them, forming glycoproteins. Three of the five major types of membrane proteins are illustrated here: recognition, receptor, and transport proteins.

Membrane Proteins Form a Mosaic Categories of membrane proteins Receptor Proteins Recognition Proteins Enzymatic Proteins Attachment Proteins Transport Proteins

Membrane Proteins Form a Mosaic Receptor Proteins Trigger cellular responses upon binding specific molecules, e.g. hormones Recognition Proteins Serve as identification tags on the surface of a cell

FIGURE 5-5 Receptor activation

Membrane Proteins Form a Mosaic Enzymes Promote chemical reactions that synthesize or break apart biological molecules Attachment Proteins Anchor the cell membrane to inner cytoskeleton, to proteins outside the cell, and to other cells

Membrane Proteins Form a Mosaic Transport Proteins Include channel and carrier proteins Regulate import/export of hydrophilic molecules

Section 5.2 Outline 5.2 How Do Substances Move Across Membranes? Molecules in Fluids Move in Response to Gradients Movement Across Membranes Occurs by Both Passive and Active Transport Passive Transport Includes Simple Diffusion, Facilitated Diffusion, and Osmosis Osmosis Plays an Important Role in Cells

Section 5.2 Outline 5.2 How Do Substances Move Across Membranes? continued Active Transport Uses Energy to Move Molecules Against Their Concentration Gradients Cells Engulf Particles or Fluids by Endocytosis Exocytosis Moves Material Out of the Cell Exchange of Materials Across Membranes Influences Cell Size and Shape

Movement of Molecules in Fluids Definitions relevant to substance movement A fluid is a substance that can move or change shape in response to external forces A solute is a substance that can be dissolved (dispersed as ions or molecules) in a solvent A solvent is a fluid capable of dissolving a solute

Movement of Molecules in Fluids Definitions relevant to substance movement (continued) The concentration of molecules is the number of them in a given volume unit A gradient is a physical difference in temperature, pressure, charge, or concentration in two adjacent regions

Movement of Molecules in Fluids Why molecules move from one place to another Substances move in response to a concentration gradient Molecules move from high to low concentration (diffusion) until dynamic equilibrium is reached

FIGURE 5-6 (part 1) Diffusion of a dye in water

FIGURE 5-6 (part 2) Diffusion of a dye in water

FIGURE 5-6 (part 3) Diffusion of a dye in water

Movement of Molecules in Fluids The greater the concentration gradient, the faster the rate of diffusion Diffusion cannot move molecules rapidly over long distances

Movement Across Membranes Concentration gradients of ions and molecules exist across the plasma membranes of all cells There are two types of movement across the plasma membrane Passive transport Energy-requiring transport

Table 5-1 Transport Across Membranes

Movement Across Membranes Passive transport Substances move down their concentration gradients across a membrane No energy is expended Membrane proteins and phospholipids may limit which molecules can cross, but not the direction of movement

Movement Across Membranes Energy-requiring transport Substances are driven against their concentration gradients Energy is expended

Passive Transport Plasma membranes are selectively permeable Different molecules move across at different locations and rates A concentration gradient drives all three types of passive transport: simple diffusion, facilitated diffusion, and osmosis

Passive Transport Simple diffusion Lipid soluble molecules (e.g. vitamins A and E, gases) and very small molecules diffuse directly across the phospsholipid bilayer

FIGURE 5-7a Diffusion through the plasma membrane (a) Simple diffusion: gases such as oxygen and carbon dioxide and lipid-soluble molecules can diffuse directly through the phospholipids.

Passive Transport Facilitated diffusion Water soluble molecules like ions, amino acids, and sugars diffuse with the aid of channel and carrier transport proteins

FIGURE 5-7b (part 1) Diffusion through the plasma membrane (b) Facilitated diffusion through a channel protein: channels (pores) allow passage of some water-soluble molecules, principally ions, that cannot diffuse directly through the bilayer.

FIGURE 5-7b (part 2) Diffusion through the plasma membrane (b) Facilitated diffusion through a channel protein: channels (pores) allow passage of some water-soluble molecules, principally ions, that cannot diffuse directly through the bilayer.

FIGURE 5-7c (part 1) Diffusion through the plasma membrane (c) Facilitated diffusion through a carrier protein.

FIGURE 5-7c (part 2) Diffusion through the plasma membrane (c) Facilitated diffusion through a carrier protein.

FIGURE 5-7c (part 3) Diffusion through the plasma membrane (c) Facilitated diffusion through a carrier protein.

FIGURE 5-7c (part 4) Diffusion through the plasma membrane (c) Facilitated diffusion through a carrier protein.

Passive Transport Osmosis – the special case of water diffusion Water diffuses from high concentration (high purity) to low concentration (low purity) across a membrane Dissolved substances reduce the concentration of free water molecules (and hence the purity of water) in a solution

Passive Transport The flow of water across a membrane depends on the concentration of water in the internal or external solutions

FIGURE 5-8 Isotonic solution

Passive Transport Comparison terms for solutions on either side of a membrane Isotonic solutions have equal concentrations of water and equal concentrations of dissolved substances No net water movement occurs across the membrane

FIGURE 5-8 Isotonic solution

Passive Transport Comparison terms for solutions on either side of a membrane (continued) A hypertonic solution is one with lower water concentration or higher dissolved particle concentration Water moves across a membrane towards the hypertonic solution

Passive Transport Comparison terms for solutions on either side of a membrane (continued) A hypotonic solution is one with higher water concentration or lower dissolved particle concentration Water moves across a membrane away from the hypotonic solution

FIGURE 5-9 Hypotonic solution

Passive Transport The effects of osmosis are illustrated when red blood cells are placed in various solutions

FIGURE 5-10 The effects of osmosis (a) If red blood cells are immersed in an isotonic salt solution, there is no net movement of water across the plasma membrane. The red blood cells keep their characteristic dimpled disk shape. (b) A hypertonic solution, with more salt than is in the cells, causes water to leave the cells and the cells to shrivel. (c) A hypotonic solution, with less salt than is in the cells, causes water to enter; the cells swell, and may burst.

FIGURE 5-10a The effects of osmosis (a) If red blood cells are immersed in an isotonic salt solution, there is no net movement of water across the plasma membrane. The red blood cells keep their characteristic dimpled disk shape.

FIGURE 5-10b The effects of osmosis (b) A hypertonic solution, with more salt than is in the cells, causes water to leave the cells and the cells to shrivel.

FIGURE 5-10c The effects of osmosis (c) A hypotonic solution, with less salt than is in the cells, causes water to enter; the cells swell, and may burst.

Passive Transport Osmosis explains why fresh water protists have contractile vacuoles Water leaks in continuously because the cytosol is hypertonic to fresh water Salts are pumped into the vacuoles, making them hypertonic to the cytosol Water follows by osmosis and is then expelled by contraction

FIGURE 4-16 (part 1) Contractile vacuoles Many freshwater protists contain contractile vacuoles. (a) Water constantly enters the cell by osmosis. In the cell, water is taken up by collecting ducts and drains into the central reservoir of the vacuole. (b) When full, the reservoir contracts, expelling the water through a pore in the plasma membrane

Active Transport Cells need to move some substances against their concentration gradients Figure: 19-2 part a Title: Viral structure and replication part a Caption: (a) A cross section of the virus that causes AIDS. Inside, genetic material is surrounded by a protein coat and molecules of reverse transcriptase, an enzyme that catalyzes the transcription of DNA from the viral RNA template after the virus enters the host cell. This virus is among those that also have an outer envelope that is formed from the host cell's plasma membrane. Spikes made of glycoprotein (protein and carbohydrate) project from the envelope and help the virus attach to its host cell.

Active Transport Active-transport membrane proteins move molecules across using ATP Proteins span the entire membrane Often have a molecule binding site and an ATP binding site Often referred to as pumps Figure: 19-2 part a Title: Viral structure and replication part a Caption: (a) A cross section of the virus that causes AIDS. Inside, genetic material is surrounded by a protein coat and molecules of reverse transcriptase, an enzyme that catalyzes the transcription of DNA from the viral RNA template after the virus enters the host cell. This virus is among those that also have an outer envelope that is formed from the host cell's plasma membrane. Spikes made of glycoprotein (protein and carbohydrate) project from the envelope and help the virus attach to its host cell.

FIGURE 5-12 Active transport Active transport uses cellular energy to move molecules across the plasma membrane against a concentration gradient. A transport protein (blue) has an ATP binding site and a recognition site for the molecules to be transported, in this case calcium ions (Ca2+)Notice that when ATP donates its energy, it loses its third phosphate group and becomes ADP + P.

Endocytosis Cells import large particles or substances via endocytosis

Endocytosis Plasma membrane pinches off to form a vesicle in endocytosis Types of endocytosis Pinocytosis Receptor-mediated endocytosis Phagocytosis

Endocytosis Types of endocytosis Pinocytosis (“cell drinking”) brings in droplet of extracellular fluid

FIGURE 5-13a Pinocytosis The circled numbers correspond to both the diagram and the electron micrograph.

FIGURE 5-13b Pinocytosis The circled numbers correspond to both the diagram and the electron micrograph.

Endocytosis Types of endocytosis Receptor-mediated endocytosis moves specific molecules into the cell

FIGURE 5-14 Receptor-mediated endocytosis The circled numbers correspond to both the diagram and the electron micrograph.

FIGURE 5-14 (part 1) Receptor-mediated endocytosis The circled numbers correspond to both the diagram and the electron micrograph.

FIGURE 5-14 (part 2) Receptor-mediated endocytosis The circled numbers correspond to both the diagram and the electron micrograph.

Endocytosis Types of endocytosis Phagocytosis (“cell eating”) moves large particles or whole organisms into the cell

FIGURE 5-15a Phagocytosis

FIGURE 5-15b Phagocytosis

FIGURE 5-15c Phagocytosis

Exocytosis Exocytosis Vesicles join the membrane, dumping out contents in exocytosis

FIGURE 5-16 Exocytosis Exocytosis is functionally the reverse of endocytosis.

Cell Size and Shape Exchange affects cell size and shape As a spherical cell enlarges, its innermost parts get farther away from the plasma membrane Also, its volume increases more rapidly than its surface area A larger cell has a relatively smaller area of membrane for nutrition exchange than a small cell

FIGURE 5-17 Surface area and volume relationships

Section 5.3 Outline 5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate? Desmosomes Attach Cells Together Tight Junctions Make Cell Attachments Leakproof Gap junctions and Plasmodesmata Allow Direct Communication Between Cells

Desomosomes Desmosomes attach cells together Found where cells need to adhere tightly together under the stresses of movement (e.g. the skin)

FIGURE 5-18a (part 1) Cell attachment structures (a) Cells lining the small intestine are attached by desmosomes. Protein filaments bound to the inside surface of each desmosome extend into the cytosol and attach to other filaments inside the cell, strengthening the connection between cells.

FIGURE 5-18a (part 2) Cell attachment structures (a) Cells lining the small intestine are attached by desmosomes. Protein filaments bound to the inside surface of each desmosome extend into the cytosol and attach to other filaments inside the cell, strengthening the connection between cells.

Tight Junctions Tight junctions make the cell leakproof Found where tubes and sacs must hold contents without leaking (e.g. the urinary bladder)

FIGURE 5-18b (part 1) Cell attachment structures (b) Tight junctions prevent leakage between cells, as between cells of the urinary bladder.

FIGURE 5-18b (part 2) Cell attachment structures (b) Tight junctions prevent leakage between cells, as between cells of the urinary bladder.

Gap Junctions and Plasmodesmata Gap junctions and plasmodesmata allow for communication Cell-to-cell channels allowing for passage of hormones, nutrients, and ions in animal cells are gap junctions Plant cells have cytoplasmic connections called plasmodesmata

FIGURE 5-19 Cell communication structures (a) Gap junctions, such as those between cells of the liver, contain cell-to-cell channels that interconnect the cytosol of adjacent cells. (b) Plant cells are interconnected by plasmodesmata, which form cytosolic connections through the walls of adjacent cells.

FIGURE 5-19a (part 1) Cell communication structures (a) Gap junctions, such as those between cells of the liver, contain cell-to-cell channels that interconnect the cytosol of adjacent cells.

FIGURE 5-19a (part 2) Cell communication structures (a) Gap junctions, such as those between cells of the liver, contain cell-to-cell channels that interconnect the cytosol of adjacent cells.

FIGURE 5-19b (part 1) Cell communication structures (b) Plant cells are interconnected by plasmodesmata, which form cytosolic connections through the walls of adjacent cells.

FIGURE 5-19b (part 2) Cell communication structures (b) Plant cells are interconnected by plasmodesmata, which form cytosolic connections through the walls of adjacent cells.

Caribous Legs and Membranes Membrane function varies between organisms

Caribous Legs and Membranes Plasma membrane phospholipids in caribous legs adapted for cold Cold areas near hooves have more membrane unsaturated fatty acids to keep cell membranes fluid

FIGURE 5-20 Caribou browse on the frozen Alaskan tundra The lipid composition of the membranes in the cells of a caribou's legs varies with distance from the animal's trunk. Unsaturated phospholipids predominate in the lower leg; more saturated phospholipids are found in the upper leg.

Viscous Venoms Snake and spider venoms contain phospholipases, enzymes that break down phospholipids Venoms attack cell membranes, causing cells to rupture and die Cell death destroys tissue around bite

FIGURE 5-21a Phospholipases in venoms can destroy cells (a) A brown recluse spider bite on a person's forearm.

FIGURE 5-21b Phospholipases in venoms can destroy cells (b) A rattlesnake bite on the forearm. Both show extensive tissue destruction caused by phospholipases.