Chapter 7 Ap Bio 721y4f

Chapter 7 – Membrane Structure and Function – Homework 1. The most accepted model of membrane structure as of this time is the fluid mosaic model. Explain the meaning behind the “fluid” and “mosaic”. The name of this model refers to the way the phosolipids and proteins behave in a membrane. It is fluid because the molecules in the phospholipids bilayer are not bonded together; it must be this way for the membranes to function. Mosaic describes how the proteins float in a sea of phospholipids and form a collage pattern that shifts over time. 2. Describe examples of experimental evidence that the hypothesis that membrane proteins drift or move. There is evidence that s this hypothesis such as Tem pictures of fractured membranes, cell fusion studies, and tagging of membrane proteins by antibodies. 3. Are both sides of a membrane the same? Explain and give evidence to your answer. No, because the proteins in the phospholipid bilayer float around and shift over time. 4. Explain why integral proteins stay in place in the phospholipid bilayer. Integral proteins stick to the membrane because of the solubility of their amino acids. 5. Discuss what types of molecules have difficulty crossing the plasma membrane and explain why. The phosopholipid bilayer is hydrophobic, so hydrophilic materials don’t cross easily. Large molecules also don’t cross easily because they are too big to get through the membrane. 6. Discuss what aquaporins are and why are they important. Aquaporins are integral membrane proteins that form pore in the membrane of biological cells. They are important because they regulate the flow of water through the cell. 7. Contrast and compare ive and active transport. How are they different? What kind of molecules are moved by each? Which direction do the materials move? ive is movement across a membrane that does not require cellular energy. The goal is to reach equilibrium, which happens when the concentration is equal on both sides. There are 3 types of ive transport: diffusion, osmosis, and facilitated diffusion. Diffusion means that the molecules go from high to low concentration. Because of this, they don’t require any extra energy. The diffusion of one solute into water is an example of ive diffusion. Active transport is movement across cell membranes that does require cellular energy. There are 3 types of active transport: carrier-mediated, endocytosis, and exocytosis. The movement is from low to high. For example, electrogenic of H+ pumps and cotransport. The biggest difference between these two is that ive transport doesn’t require energy while active transport does. In ive transport, the molecules also go from high to low concentration while in active transport they go from low to high. 8. Predict what will happen to one of your body cells if they are placed in the following solutions? a) hypertonic: The cell would shrivel up because of the decrease in water.

b) isotonic: There would be no change. c) hypotonic: The cell would swell up because of the increase in water. 9. Predict what will happen to a cell with a rigid cell wall (e.g. bacteria or plant) if they a are placed in the following solutions? a) hypertonic: The plant cell would become plasmolyzed. b) isotonic: The plant cell would become flaccid. c) hypotonic: The plant cell would be turgid (normal). 10. A writer for a TV soap opera consults with you about their idea for killing off one of the characters. Their idea is to have the villain replace the IV drip being give to the character while they are unconscious in the hospital with a bag of distilled water. Could murder most foul be accomplished this way? Explain. Yes, because if the cells were presented with distilled water they would all shrivel up trying to reach equilibrium with the water. The would try to balance out the molecules inside the cell with the molecules outside the cell, and that would kill someone. 11. The Na+ - K+ pump, the major electrogenic pump in animal cells, exchanges sodium ions for potassium ions, both of which are cations. Describe how this exchange generates a membrane potential or difference in electrical charge since both ions have the same charge. The binding of cytoplasmic Na+ to the protein stimulates phosphorylation by ATP. This causes the protein to change its conformation. This change expels Na+ to the outside and extracellular K+ binds. K+ binding triggers release of a phosphate group. The loss of phosphate restores original conformation. K+ is released and Na+ sites are receptive again; the cell repeats. 12. One of the first signs of cell death is that ions such as Na+ and K+ start moving down their concentration gradients. Explain why this is a good indicator that the cell is no long viable. This is a good indicator because is means that the cell is getting weaker and can’t hold as many molecules. It also means that its cell membrane is getting weaker.