31. If we don’t get to this, you won’t be tested on it yet.) What are the major

Question

31. If we don’t get to this, you won’t be tested on it yet.)

What are the major ways that compounds can move across the cell membrane? What determines which of these transport methods will be used?

What are some of the similarities and some of the differences between facilitated diffusion and simple diffusion? Between facilitated diffusion and active transport? (Be sure you understand whether each type of transport can be inhibited/saturated)

What characteristics of a compound and/or cell membrane determines whether a compound will move (via either simple diffusion or facilitated diffusion)?

What is the major difference between a primary active transporter and a secondary active transporter?

What is the difference between a symport system and an antiport system?

What is oemolarity? How do we determine osmolarity? What is the normal osmolarity inside the cell?

What is tonicity? How does it differ from osmolarity? What is the concentration of an isotonic salt solution?

What is a gated channel? What are the possible gating mechanisms?

What is meant by the membrane potential? What determines this potential? Why is it normally negative?

Understand the three "water compartments" - intracellular, interstitial, and extracellular. How and why do their compositions differ?

Be sure you know what ions are more concentrated inside vs. outside the cell; know this for Na+, K+, Cl-, and other major ions.

What is the normal resting membrane potential for a cell? What contributes to this potential? What would happen to the membrane potential if you opened a K+ channel? A Ca2+ channel? Be able to explain the changes.

Explanation / Answer

answers:

Cell membrane is basically a phospholipid bilayer with a polar or hydrophilic head and a hydrophobic tail. The permeability of substances depends upon their lipid solubility rather than their particle size. Water soluble polar compounds are generally impermeable and require carrier-mediated transport.whereas nonpolar substances are easily transported.This is a very important property of the cell membrane as this can block unwanted substances from moving in and out of the cell.

Various mechanisms of transport are:

2. Active transport

3. Pumps.

Passive transport:

Simple diffusion:

The method by which passive entry of substances takes place driven by a concentration gradient,i.e, from an area of higher concentration to lower concentration, without the expenditure of energy. But is a slow process.

Facilitated diffusion:

Although passive, differs from simple diffusion in that structurally similar solutes can competitively inhibit one another.It occurs faster than simple diffusion and can occur in both the directions than one.It takes the help of carrier molecules which occur in ping and pong states.In pong states, the active sites are exposed to the exterior surface taking the solutes in and in ping state, the protein opens inside to release the solute.

An example of facilitated diffusion is by the uptake of glucose by cells by GLUT family transporters under the regulation of hormone insulin.

Aquaporins are another group of channels through which water can enter or exit cells. These channels play a role in reabsorption of water by the nephrons.

Ion channels:

Specialized protein molecules found in the membrane which help in quick transport of electrolytes such as calcium, potassium, sodium and chloride ions.They are often referred to as gated channels since their opening is followed by rushing of ions through it along a concentration gradient. They are important in nerve impulse propagation, secretion of biologically active substances from the cells.

a.Ligand-gated channels: They open when 'effectors' bind to a receptor site of the ligand.

e.g.In neuromuscular junction, Acetylcholine binds to the receptor in the postsynaptic region, opening the channel and influx of sodium ion causing generation of an action potential.

b. Voltage-gated channels: They are opened by membrane depolarization.In ground state , the channel is normally closed but when there is a membrane potential change, it switches on the channel to open. E.G. voltage gated sodium and potassium channels seen in nerve cells.

Active transport:

It differs from facilitated diffusion in that here there is an expenditure of energy since the solutes are moving against a concentration gradient.Unlike facilitated, active transport occurs only unidirectionally and requires special integral proteins called transporters.

There are two types of active transport:

Primary active and secondary active.

Primary active : The sodium pump( Na+K+ATPase pump) is the classic example. This pump takes in sodium fro outside and gives out potassium by energy acquired directly from the ATP.

Secondary active: This uses an electrochemical gradient already generated by moving substances, to move other substances against their respective gradients.e.g The ATP dependent calcium pump regulating muscular contraction.

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