1. © 2014 Pearson Education, Inc. Chapter 7 Membrane Structure and Function

2. © 2014 Pearson Education, Inc. CELLs are chemical machines. They acquire materials and energy from the environment. These materials are transformed within the cell by the chemical activities which collectively is called, METABOLISM (an organisms chemical reactions consisting of anabolic and catabolic processes and pathways of protoplasm). Finally, cells return some of the end products of these metabolic activities to the environment.

3. © 2014 Pearson Education, Inc. What a machine!

4. © 2014 Pearson Education, Inc. The environment of cells is always liquid. Whether we consider an amoeba, an oak tree or cells of the human body, all cells are bathed in a fluid – if they are living. Only the dead cells on the outside of the skin, which are exposed to air, are not in fluid. The fluid that bathes cells is called EXTRACELLULAR FLUID or ECF. The major component of the ECF is water (solvent). Solutes can be: a.Gases such as O 2, CO 2 b.Inorganic ions such as sodium, chloride, potassium, calcium, bicarbonate (HCO 3 - ), phosphate (PO 4 ), and trace elements c.Organic compounds such as foods (source of energy) and vitamins. d.Hormones which are released into the ECF by certain cells and that affect the metabolic activities of other cells. The ECF also carry away wastes from the cells. The most important wastes are those from protein and nucleic acid metabolism. The wastes contain nitrogen, ammonia and urea and these are very toxic. The hydrogen ion concentration (pH) of the extracellular fluid and its temperature are critical to the metabolism of the cell. For example, if the pH of the human blood should move out of the range of 7.34-7.44 or temperature is not at 37.5 C (98.6 F), chemical imbalance can occur. In order to keep the extracellular fluid within normal ranges, five known mechanisms occur in the body. These are diffusion, osmosis, active transport, endocytosis and exocytosis.

5. © 2014 Pearson Education, Inc. Glyco- protein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Microfilaments of cytoskeleton Fibers of extra- cellular matrix (ECM) Cholesterol Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE The Fluid Mosaic Model states that a membrane is a “fluid” bilayer structure of phospholipids with a “mosaic” of various proteins embedded in the membrane.

6. © 2014 Pearson Education, Inc. FLUID (Phospholipids) Phospholipids in the plasma membrane move within the bilayer. The lipids can drift laterally. Sometimes, lipids can move transversely across the membrane. With cool temperatures, membranes switch from a fluid state to a solid state. The temperature at which a membrane solidifies depends on the types of lipids. Membranes rich in unsaturated (double bonds cause kinks in chain) fatty acids are more fluid than those rich in saturated(straight chains) fatty acids. Membranes must be fluid to work properly. The steroid cholesterol has an important role in the cell’s fluidity at varying temperatures. At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids. At cool temperatures, it maintains fluidity by preventing tight packing. Lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions. Plants adapted to harsh winters have a higher percentage of unsaturated phospholipids.

7. © 2014 Pearson Education, Inc. FluidViscous Unsaturated tails prevent packing. Saturated tails pack together. Cholesterol Cholesterol reduces membrane fluidity at moderate temperatures, but at low temperatures hinders solidification. (a) Unsaturated versus saturated hydrocarbon tails (b) Cholesterol within the animal cell membrane

8. © 2014 Pearson Education, Inc. Mosaic (Proteins) The cell membrane is a collage of different proteins that determine most of the membrane’s specific functions. Proteins are not randomly distributed in the membrane. Peripheral proteins are bound to the surface of the membrane. Integral proteins penetrate the hydrophobic core. Remember that the membrane is made of phospholipids. The bilayer creates an amphipathic environment with both hydrophobic(tails) and hydrophilic(heads) regions. Hydrophobic molecules like gases can easily dissolve in the lipid bilayer and cross it, but hydrophilic molecules are impeded by the hydrophobic core. Membrane proteins help!

9. © 2014 Pearson Education, Inc. Hydrophilic head Hydrophobic tail WATER

10. © 2014 Pearson Education, Inc. There are six major functions of membrane proteins: a) Transport Proteins allow passage of hydrophilic substances across the membrane. Channel proteins have a hydrophilic channel that certain molecules or ions can use. Transport proteins utilize ATP to pump across the channel. A transport protein is specific for the substance it moves. Channel proteins called aquaporins facilitate the passage of water. b) Enzymatic proteins built into the membrane with the active site exposed to substances in the adjacent solution. c) Signal Proteins have a binding site that fits the shape of a chemical messenger, such as a hormone. The messenger can cause a shape change in the protein that sends a message inside the cell. d) Intercellular joining proteins allow cells to attach at junctions. e) Cell-cell recognition glycoproteins serve as identification tags that are specifically recognized by other cells. Distinguishes species to species and even blood types. f) Attachment to the Cytoskeleton and Extracellular Matrix (ECM) proteins serve as bonding points for microfilaments of the cytoskeleton to maintain shape. Others adhere to the ECM to coordinate extracellular and intracellular changes.

11. © 2014 Pearson Education, Inc. (a) Transport (b) Enzymatic activity (c) Signal transduction (d) Cell-cell recognition (e)Intercellular joining (f)Attachment to the cytoskeleton and extracellular matrix (ECM) Enzymes ATP Signaling molecule Receptor Signal transduction Glyco- protein

12. © 2014 Pearson Education, Inc. Transmembrane glycoproteins Secretory protein Golgi apparatus Vesicle Attached carbohydrate ER lumen Glycolipid Transmembrane glycoprotein Plasma membrane: Cytoplasmic face Extracellular face Membrane glycolipid Secreted protein Membranes have distinct inside and outside faces that are determined when the membrane is built by the ER and Golgi apparatus.

13. © 2014 Pearson Education, Inc. Mechanisms for exchange of matter across the Membrane include: Passive transport: diffusion of a substance across a membrane with no energy investment Facilitated Diffusion: passive transport aided by proteins Active Transport: the movement of ions and molecules against a concentration gradient with the use of energy The rate at which materials will diffuse through the cell membrane is dependent not only on the concentration gradient but on the size, charge, and lipid solubility of the particles in question. Lipid-soluble (hydrophobic) molecules diffuse faster than hydrophilic molecules. Cells are less permeable to ions than to small non charged molecules. Smaller molecules travel across membranes faster than large molecules.

14. © 2014 Pearson Education, Inc. ATP Passive transportActive transport Diffusion Facilitated diffusion

15. © 2014 Pearson Education, Inc. Diffusion occurs when molecules or ions travel in the direction of their concentration gradient. Molecules travel from greater to lesser concentration. The molecules are attempting to reach equilibrium. Diffusion is the tendency for molecules to spread out evenly into an available space. At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other. Passive Transport : The diffusion of a substance across a biological membrane with no energy expended by the cell to make it happen.

16. © 2014 Pearson Education, Inc. Molecules of dye Membrane (cross section) WATER (a) Diffusion of one solute (b) Diffusion of two solutes Net diffusion Equilibrium

17. © 2014 Pearson Education, Inc. Osmosis is a type of diffusion in which water is moving across a selectively permeable membrane. Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides. Water can exert pressure on the outside or inside of the cell membrane. This pressure is called OSMOTIC PRESSURE. The greater the pressure on either side of the membrane, the greater the tendency for osmosis to occur and the greater the osmotic pressure. When the rate at which the filtration process occurs becomes equal to the rate at which water molecules are coming in because of the difference in concentration, osmosis ceases.

18. © 2014 Pearson Education, Inc. Lower concentration of solute (sugar) Higher concentration of solute More similar concentrations of solute Sugar molecule H2OH2O Selectively permeable membrane Osmosis

19. © 2014 Pearson Education, Inc. A HYPOTONIC SOLUTION occurs when the solution contains more water and lower solute concentration than another solution. For example, human red blood cells are placed in pure water (100%), water will diffuse into the blood cell and cause them to burst (animal = lysis; plant = turgor). If blood cells are placed into sea water (mostly salt and very little water), the water from the blood cell will leave the cell and it will shrink (animal = crenation; plant = plasmolysis). The solution around the blood cell is called HYPERTONIC SOLUTION. Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water. When blood cells are placed in a solution with the same concentration as the blood cell (blood plasma or a 0.9% solution of NaCl), they neither gain nor lose water by osmosis. This solution is ISOTONIC.

20. © 2014 Pearson Education, Inc. Cell walls help maintain water balance. A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm).If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp). In plants when a freshwater plant is placed in sea water, its cells quickly lose turgor (water pressure) and the plant wilts. This is because the sea water is HYPERTONIC to the cytoplasm. As water continues to diffuse from the cytoplasm into the sea water, the cells shrink (PLASMOLYSIS).

21. © 2014 Pearson Education, Inc. HypotonicIsotonic Hypertonic H2OH2O LysedNormal Shriveled Plasma membrane Cell wall Turgid (normal) Flaccid Plasmolyzed (a) Animal cell (b) Plant cell Plasma membrane H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O

22. © 2014 Pearson Education, Inc. Hypertonic or hypotonic environments create osmotic problems for organisms. Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump. Animals which live in salty water and drink the water must constantly desalt the water and send the salt back to their environment. Fish use their gills to desalt the water they drink. Marine birds use two salt glands in their heads to desalt the water. Marine snakes use a similar device. Contractile vacuole 50 μm

23. © 2014 Pearson Education, Inc. Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane; Aquaporins facilitate the diffusion of water. Ion channels facilitate the diffusion of ions. Some ion channels, called gated channels, open or close in response to a stimulus. Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane. If the metabolism of a cell is slowed down because of the appearance of a hydrophilic fluid around the cell, an enzyme can be produced that will provide a pathway for the substance to cross. E. coli (intestinal bacterium) is transferred from a glucose medium to a lactose medium, metabolism is slowed. The cell membrane is impermeable to lactose. After a few minutes, lactose begins to enter the bacterium. This is due to the production of the enzyme permease, which carries the lactose across the hydrophobic lipid layer of the cell membrane.

24. © 2014 Pearson Education, Inc. (a) A channel protein (b) A carrier protein Carrier protein Channel protein Solute EXTRACELLULAR FLUID CYTOPLASM

25. © 2014 Pearson Education, Inc. The result of active transport is that cells may contain concentrations of substances greater or lesser than concentrations in the cell’s environment. Blood cells contain a concentration of potassium ions 30 times greater than the plasma; plasma contains a concentration of sodium ions 11 times greater than the blood cell. The actual mechanism is not clearly understood. The sodium-potassium pump is one of the best known examples.

26. © 2014 Pearson Education, Inc. CYTOPLASM 2 [Na + ] low [K + ] high [Na + ] high [K + ] low Na + ATP ADP P EXTRACELLULAR FLUID 1 Cytoplasmic Na + binds to the sodium- potassium pump. The affinity for Na + is high when the protein has this shape. Na + binding stimulates phosphorylation by ATP.

27. © 2014 Pearson Education, Inc. 4 3 Phosphorylation leads to a change in protein shape, reducing its affinity for Na +, which is released outside. The new shape has a high affinity for K +, which binds on the extracellular side and triggers release of the phosphate group. Na + P K+K+ K+K+ P P i

28. © 2014 Pearson Education, Inc. 6 K + is released; affinity for Na + is high again, and the cycle repeats. Loss of the phosphate group restores the protein’s original shape, which has a lower affinity for K +. 5 K+K+ K+K+ K+K+ K+K+

29. © 2014 Pearson Education, Inc. Membrane potential - voltage difference across a membrane created by differences in the positive and negative ions across a membrane. The electrochemical gradient drives the diffusion of ions across a membrane. The chemical force is the ion’s concentration gradient while an electrical force is the effect of the membrane potential on the ion’s movement. An electrogenic pump is a transport protein that generates voltage across a membrane. The sodium-potassium pump is the major electrogenic pump in animal cells. In plants, fungi, and bacteria, it is a proton pump. Electrogenic pumps help store energy that can be used for cellular work. EXTRACELLULAR FLUID ATP CYTOPLASM H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Proton pump − − − − + + + +

30. © 2014 Pearson Education, Inc. Sucrose ATP H+H+ H+H+ H+H+ H+H+ H+H+ Proton pump − − − − + + + + Sucrose-H+ cotransporter H+H+ H+H+ H+H+ H+H+ Sucrose Diffusion of H + Cotransport is a type of active transport in which a solute indirectly drives transport of other substances. Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

31. © 2014 Pearson Education, Inc. Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins. Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles. Bulk transport requires energy. In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane. Then, enzymes from the lysosomes break down the products into amino acids and sugars that can pass into the cytoplasm. There are three types of endocytosis: Phagocytosis (“cellular eating”) - a cell engulfs a particle in a vacuole. The vacuole fuses with a lysosome to digest the particle. Amoebas eat solid food this way. White blood cells use this method to eat bacteria. Pinocytosis (“cellular drinking”) - molecules dissolved in droplets are taken up when extracellular fluid is “gulped” into tiny vesicles Receptor-mediated endocytosis - binding of ligands to receptors triggers vesicle formation. A ligand is any molecule that binds specifically to a receptor site of another molecule.

32. © 2014 Pearson Education, Inc. Phagocytosis Pinocytosis Receptor-Mediated Endocytosis Solutes Pseudopodium “Food” or other particle Food vacuole CYTOPLASM EXTRACELLULAR FLUID Plasma membrane Coated pit Coated vesicle Coat protein Receptor

33. © 2014 Pearson Education, Inc. Exocytosis is the reverse of endocytosis. In cells that secrete large amounts of protein, the protein first accumulates in a Golgi body. It moves to the surface of the cell, where its membrane fuses with the cell membrane, and it discharges its contents to the outside. Fat molecules from the intestine are given off this way.