Week 1: Homeostasis and Membrane Transport

Question 1

Internal Environment

Answer 1

Body systems (i.e. circulatory system). Does not include cells of the body. Some systems are both external and internal (i.e. digestive, respiratory, renal system)

Question 2

External Environment

Answer 2

Outside our body (generally) but are continuous with the internal environment (i.e. digestive, renal, and respiratory systems). Does not include cells of the body.

Question 3

Homeostasis

Answer 3

Maintenance of relatively stable conditions in the internal environment. This is vital - every single organ system works to maintain homeostasis EXCEPT the reproductive system, which maintains the species and not the internal environment!

The body relies on 2 control systems to maintain homeostasis:
1. Nervous systems (fast, short duration)
2. Endocrine system (slow, long duration)
Both control systems use negative feedback

Question 4

Chemical Composition of Body Fluids

Answer 4

Salty banana
Bicarbonate = extracellular 
Protein = intracellular
Calcium = extracellular
In the ECF, there is more in plasma because the proteins can’t leave capillaries as easily

Question 5

Fick’s First Law of Diffusion

Answer 5

Rate of diffusion can be predicted by Fick’s First Law of Diffusion

J = -KT/6(pi)nr * dc/dx * A

J = net rate of diffusion
k = Boltzmann constant
T = absolute temp
r = molecular radius
n = viscosity of the medium
A = total SA of the membrane for diffusion
dc/dx = concentration gradient of solute

Question 6

Factors Affecting the Rate of Diffusion

Answer 6

1. Molecule size (< or = 0.8 nm)
2. Charge on the molecule and channel (+, -)
3. Electrochemical gradient
   a) If the ion is positive and channel is negative, it would have a strong pull for the cell
4. Pressure gradient (increase KE)
5. Hydration energy (water shell that surrounds ions)
   a) Hille’s theory of closest fit
   b) Polar molecules attract other polar molecules (i.e. water)
   c) Because water is a polar molecule, ions float around in aqueous solutions in association with a cloud or shell of water molecules; that is, ions in aqueous solutions are hydrated.
   d) Water molecules have to be stripped off in order for the ion to get through by itself - the water shell must be removed for the ion to be energetically stable
   e) Only Na+ channel can remove the water shell from a Na+ ion (same for K+ and K+ channels)

*NOT CHEMICAL SPECIFICITY

Question 7

Membrane Permeability

Answer 7

A typical cell at REST has the following ion permeabilities:

1) Na+ - 2e-8

Question 8

Characteristics of Carrier-Mediated Transport

Answer 8

1. Chemical specificity - only shuttles a specific chemical because it needs to have a specific shape to bind to the carrier
2. Competitively inhibited - this can occur by structurally similar molecules (i.e. size, shape) which partially bind to shut down the carrier (but not get shuttled through)
   The “inhibiting” molecule will reversibly bind to the active site of the carrier protein and the protein will not undergo a conformational change to shuttle the molecule across the membrane. This will “shutdown” the carrier and essentially inhibiting facilitated diffusion.
3. Saturation kinetics - transport rate is limited by the number of carriers and the speed of the conformational change
   Saturation is possible because there are only a few carriers that can only change shapes so fast
   Rate of diffusion will reach a maximum, regardless of the concentration of the substrate

Question 9

Active Transport: Na+/K+ pump

Answer 9

1. Protein is phosphorylated by ATP - opens it to the outside
   No affinity for Na+, now has affinity for K+ ions
2. Na+ is released to outside of the cell, K+ is pumped inside
3. The protein is dephosphorylated - no longer has the changed affinities

Question 10

Inhibitors of Na+/K+ pump

Answer 10

1. Ouabain (from poison arrow tree) - high dose can kill a hippo
   A metabolic inhibitor
   Can inhibit the Na+/K+ ATPase at high concentrations by binding to the extracellular side of the ion pathway of the pump before K+ binds (between steps 3 and 4).
   This binding of ouabain blocks the carrier.
2. Digoxin (foxglove plant)
   Also a metabolic inhibitor of the Na+/K+ pump – particularly in the heart

Question 11

Functions of the Na+/K+ pump

Answer 11

1. Helps maintain the conc gradients for Na+ and K+ across the cell membrane (especially if they leak for some reason)
2. Causes slight increased negativity inside the cell (more positive charge is removed than replaced - more Na+ leaves)
   Contributes to RMP
3. Keeps cell from swelling and bursting due to osmosis
   Cells contain large numbers of proteins and other organic compounds to which the cell membrane is impermeable.
   Most of these carry many negative charges and hence attract large numbers of positive ions around them.
   This increases the number of particles inside the cell which would cause substantial osmosis of water into the cell.
   Since the sodium- /potassium pump pumps out three sodium ions for each two potassium ions it pumps in, it causes a net reduction in particles inside the cell.
   This causes osmosis of water out of the cell and offsets the osmosis into the cell and cell volume is kept constant.
   Cell swelling, by a mechanism which is not fully understood, increases the activity of the sodium/potassium pump.