Question

1. You’re riding your bike in the bike lane through Golden Gate Park. Suddenly, you drift out of the bike lane and into automobile traffic. Fortunately, you quickly move back into the bike lane and continue toward Ocean Beach. This scenario is a metaphor for homeostasis, where the controlled condition (physiologic variable) is the position of the bike on the road (e.g., inside or outside the bike lane). Identify: (a) The established set point for the controlled condition (b) The receptor (c) The control center (integration center) (d) The effector There’s no need to explain the physiology of vision or muscle contraction. Rather, demonstrate your understanding of feedback systems by mapping the components of a feedback system onto this scenario.

2. The three-dimensional shape of a protein determines its function. Briefly explain these terms as they relate to protein shape and provide a supporting example for each: denature, conformational change, genetic mutation. Each example must include a specific protein.

3.Compare and contrast simple diffusion and facilitated diffusion. In other words, how are they similar and how are they different? Provide supporting examples for each.

4.(a) What is the osmolarity of a solution containing 85 mM C6H12O6, 120 mM KCl, and 24 mM CaCl2? Show your calculations. (b) What would happen to human blood cells put in the solution above? Explain.

Answer 1

1 (a) The lines marking the bike lane are the set point.

(b) Photoreceptors of the eye (in the rods and cones) are involved in perception. Light causes disinhibition of optic nerve, a process that involves glutamate neurotransmitter receptors, to convey the message to the brain. Finally, the message from the brain is relayed to the muscles controlling the bike through the acetylcholine receptors binding to acetylcholine.

(c) The brain is the integration center (explained just above)

(d) Voluntary skeletal muscles, particularly in the arms and shoulders (explained in b)

2. Denaturation is the change in structure and shape of a protein. Normally, non-polar amino acids are buried inside the protein and polar amino acids remain on the surface after a protein folds into its native/functional form. But denaturation alters this arrangement by disrupting the non-covalent interactions within the protein through the supply of heat or change in pH (adding strong acid or base). Sometimes denaturation is reversible. Denatured proteins become insoluble and non-functional. A common example is boiling of egg, which results in denaturation and hardening of the proteins.

3. Diffusion (both simple and facilitated), involves the passive movement of molecules from a region of higher concentration to a region of lower concentration, along the concentration gradient. (eg): the smell from an open perfume bottle spreads through the whole room by simple diffusion. A drop of ink spreads throughout a glass of water due to simple diffusion.

While this is all that happens in diffusion, facilitated diffusion is slightly more complex: it requires something (usually a protein) to facilitate/mediate the diffusion process. This is because, there exists a non-permeable barrier between the region of higher concentration and the region of lower concentration that does not permit the molecule to pass through. An example is water (which is polar) and the cell membrane (which is a lipid bilayer; non-polar). In this case, a channel protein called aquaporin allows water to move from a region of higher concentration to a region of lower concentration.

4 a) CaCl₂ will dissociate into 3 particles (Ca²⁺, Cl⁻ and Cl⁻) in water. KCl will dissociate into 2 particles. Glucose will not dissociate. Osmolarity is the total number of particles in the solution, which in this case, is: (120 × 2) + (24 × 3) + (85 × 1) = 397 mosmol.

(b) The osmolarity of RBC is around 300 mosmol. The above solution is hypertonic with respect to RBC. So, water from RBC will leave RBC and enter the solution by exosmosis. As a result, the RBCs will shrink.