Passive Membrane Diffusion

Question 1

Plasma Membrane Structure

Answer 1

Question 2

Phopholipids Structure

Answer 2

Question 3

Function of Cholesterol in the PM

Answer 3

Determinant of membrane stiffness and fluidity

Question 4

Fluid Mosiac Model

Answer 4

the classic bi- molecular lipid layer structure (first postulated by Singer & Nicholson  1970). The first major component of the plasma membrane of the cell is the bimolecular lipid component of the membrane consisting of a continuous double layer of lipids about 5 nm thick

Question 5

Membrane Embedded Proteins

Answer 5

The second major component of all cell membranes—membrane-embedded proteins (Fig. 1- (B) & (C). These proteins constitute about 30% of the membrane mass. Some of these proteins partially span the membrane layer and some fully span the membrane bilayer. Importantly, membrane-bound proteins are essential for the major portion of known membrane functions underlying cell homeostasis. However, membrane lipids also carry out important biochemical functions in the maintenance of cell homeostasis – in addition to their structural role in the membrane—for example in smooth muscle hydrolysis a specific membrane phospholipid (phosphotidylinositol) releases inositol trisphosphate—which releases Ca2+ ions from the sarcoplasmic reticulum to cause contraction.

Importantly, the normal function of both these two cell membrane components is essential for maintenance and control of normal function of each organ system in the body (as will be described in every section of this physiology course).

Question 6

Net Flux of Noncharged solute across the cell membrane

Answer 6

Question 7

Net Flux of Charged solute across the cell membrane - the electrochemical gradient

Answer 7

Question 8

Simple Equilibrium Model

Answer 8

Question 9

Membrane Pore (non-gated channel)

Answer 9

Ions and other charged hydrophilic solutes preferentially pass through specialized transmembrane integral protein structures because charged hydrophilic solutes are not very soluble in the lipid bilayer (i.e. they are “lipophobic” and thus have a very small partition coefficient “”).

First type of transmembrane pathway - transmembrane (integral) protein “pores” (Fig. 6 a.): This transmembrane pathway is a protein conduit with no regulatory barriers or gates (i.e. it is a non-gated, straight tube that is always open). Physiological examples of pores include pores

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in the outer membrane of mitochondria and Cl channels in skeletal

muscle (but not neurons, smooth muscle cells, and many others). This is a non-gated channel—classic example is Cl- channel in skeletal muscle cell.

Question 10

Channel (Gated Pore)

Answer 10

Second type of transmembrane pathway - transmembrane protein “channels (Fig. 6 b.): This transmembrane protein pathway is a special pore that is “open” or “closed” by a controllable “gate” – located on the intracellular side of the channel. Gating results from small reversible step movements of a specific group of amino acids within the channel’s polypeptide chain. The best known physiological examples of transmembrane channels are the many specific gated ion channels for passage of Na+, K+, Cl- and Ca2+. They are present in almost all cell membranes – to be described in greater detail in a later lecture.

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With a carrier protein, the solute particle enters at one open end of the conduit and causes closure of the gate to form a compartment with binding sites. The particles are transported along the binding sites in a stepwise fashion to the other side of the compartment where they are released upon opening of the exit gate, like a relay throw from the outfield in a baseball game RF-2B-Catcher (9-2-4) to cut down a runner at the plate.