Transport Across Cell Membranes

Question 1

Why are the bilayer of phospholipids and sphingolipids and cholesterol present in the cell membrane?

Answer 1

They are there for membrane integrity and fluidity.

Question 2

The meaning of Amphipathic is?

Answer 2

hydrophilic phosphatidyl head group and hydrophobic tails [two]

Question 3

What do we know about the eukaryotic membrane?

Answer 3

It consists of a lipid bilayer that is not miscible with extracellular fluid or intracellular fluid

• It, therefore, forms a barrier against the movement of mainly water-soluble substances between the two compartments

• Lipid soluble substances are able to penetrate the
bilayer

• Lipid bilayer also contain protein attached to it or
passing through it [proteins that span the lipid membrane]

• Importantly protein function to i) transport substances
through the membrane, ii) act as enzymes and receptors,
or iii) form attachments to the cell matrix.

Question 4

Types of diffusion?

Answer 4

Passive diffusion and Facilitated diffusion.

Question 5

Aquaporins

Answer 5

allow water to pass through in a single file. and ions? use the potassium ion channel. but not sodium cause its smaller?

Question 6

Selective permeability

Answer 6

integral proteins form pores that allow substances to flow through [permeases]. shape size and electric charge provide selectivity that permits only certain molecules to pass.

Question 7

Gating of protein channels

Answer 7

Voltage gating and Ligand gating

Question 8

Facilitated diffusion

Answer 8

similar to passive, substances move down their concentration gradient [ from high to low] But here a transporter is required also called Carrier mediated diffusion. - a protein facilitates the diffusion.

Question 9

voltage gating:

Answer 9

action potentials in nerves. the molecular conformation of the gate changes when there is a change in the electric potential across the membrane.

– E.g sodium transport through a sodium channel – strongly negative inside the cell keeps gate closed. When the voltage goes up (becomes more positive) gate opens allowing a flux of Na+ ions into
cell.
[negative inside of cell keeps Na+ channels closed when it becomes positivegates open allowing an influx of Na ions to get inside of the cell.]

Question 10

Ligand gating:

Answer 10

occurs when a substance binds to the membrane protein which causes a conformational change in the protein that opens the gate. e.g Acetylcholine channel that when acetylcholine binds to it, it allows the opening of negatively charged pore that will allow influx of all positively or uncharged particles less than 0.65nm to enter. very NB in nerve to nerve impulse transmission and between muscle and nerve cells that causes muscle to contract.

Question 11

The potassium channel allows only potassium, why?

Answer 11

• protein pore is tetrameric, pores form a filter that is lined by carbonyl oxygen. k is hydrated in solution by 4H2O. as k enters the pore, its dimensions is the right size to exchange water for oxygen in the filter. all water is then substituted and the potassium moves down the conc. gradient into the cell [ dehydrated]. sodium wont be able to do this.

Question 12

How does the sodium channel work?

Answer 12

The Sodium-ion channel uses electronegativity and a conformational gate to move the ions through the channel.

Question 13

Passive diffusion

Answer 13

small molecule, high to low conc. [water, oxygen, carbon dioxide] fat soluble molecules - simple diffusion.

Question 14

Difference between simple diffusion and facilitated diffusion

Answer 14

• in simple diffusion, the rate increases proportionally with the concentration of the diffusing substance.
• but with facilitated diffusion there exists a point when the rate of diffusion reaches a maximum.[where protein facilitating the process she’s done. =because substance need to interact with the protein, and when they do, the protein changes shape, the rate of facilitated diffusion depends on the time it takes the protein to go back to its normal shape to then take up another substance ] transport maximum??
  e. g glucose and amino acids. GLUT 4 is activated by insulin. GLUT 4 has a maximum amount of glucose that it can transport per time.

Question 15

Factors affecting diffusion.

Answer 15

1. Amount of substance that diffuses through membrane.
2. Concentration difference across membrane.
3. Membrane electrical potential.
4. Pressure difference. [from high pressure to low pressure.]

Question 16

Active transport types.

Answer 16

1. Primary [Na/k pump], H+ pump and Ca2+ ions and

2. Secondary [co-transport and counter transport]

Question 17

Define Active transport.

Answer 17

From low to high concentration… when a large concentration of a substance is required in the ICF even though the ECF has a low concentration of that substance. e.g K+ the opposite is true e.g Na+ [more outside the cell]

Question 18

Why can’t active transport occur by diffusion?

Answer 18

Active transport cannot occur by diffusion, it leads to equilibrium and we don’t want that here. need to use energy to cause excess k movement into the cell and excess Na out of the cell. Active transport is when cell membranes move substances #against# its conc. gradient (uphill) [low to high]

Question 19

Primary active transport

Answer 19

Energy derived directly from breakdown of ATP which is the energy currency of the cell . requires carrier proteins. But here the carrier protein imparts energy to the transported substance to move it up its conc. gradient. The Na/K pump– very NB in all cells. it pumps potassium in and sodium out [ Na you’re out , K you’re in] this pump is NB for maintaining sodium potassium differences across the membrane and establish a negative electrical voltage inside cell. NB in transmission of nerve signals in nervous system. larger alpha and smaller beta unit.

Question 20

PRIMARY ACTIVE TRANSPORT OF CALCIUM IONS

Answer 20

Two important calcium pumps maintain very low conc. of ca in cell. This pump pumps calcium out of the cell and into sarcoplasmic reticulum in muscle cells.

Question 21

PRIMARY ACTIVE TRANSPORT OF HYDROGEN IONS

Answer 21

: Transport of hydrogen in gastric glands of the stomach . late distal tubules and cortical ducts of kidney/ from blood to urine.

Question 22

Why the cell membrane is important?

Answer 22

to have compartments, shape, and form, Selectively permeable. surface for proteins to anchor on, cell adhesion, cell communication. lets things in or out by polarity, charge, size.

-The cell membrane is selectively permeable: it allows necessary proteins in and attempts to keep unnecessary proteins and pathogens out. It also allows small molecules in (water) and
prevents large molecules from getting in (eg. glucose, that’s why it needs to be taken in via proteins)

Question 23

Ficks Law

Answer 23

J = - DA Δc/Δx

Where D is diffusion coefficient, A is the membrane area, Δc is concentration and Δx is the thickness of the membrane

Question 24

Simple diffusion occurs by two pathways

Answer 24

1. Through the interstices of the lipid bilayer if the substance is lipid
   soluble
2. Through watery channels

Question 25

Diffusion Through watery channels

Answer 25

– integral membrane proteins that penetrate through the membrane may have a watery channel through which water readily diffuses

– Other lipid insoluble substances pass through protein channels if they are small enough. As their size increases their diffusion decrease e.g. urea 20% greater than H20 but 1000x less
penetration

Question 26

Through the interstices of the lipid bilayer if the substance is lipid-soluble

Answer 26

Lipid solubility is very important in determining the rate of
diffusion of these substances

– Lipid solubility of oxygen, nitrogen, carbon dioxide, and alcohol
are high and facilitates very fast diffusion of these substances through the cell

Question 27

Diffusion across a membrane. AKA osmosis

Answer 27

When there is a concentration difference across a membrane, net movement occurs. = osmosis.

Question 28

Secondary active transport. Discuss. [co-transport and Counter transport]

Answer 28

• when Na is transported out of the cell by primary active transport, a large concentration gradient [difference] is formed.
  The gradient in itself is a source of energy that can be used to transport substances.
• When sodium diffuses into the cell, it can pull other molecules with it. = co-transport. this occurs via a carrier protein that has an attachment for sodium and another molecule, for example, glucose.
• Both molecules attach to the protein at specific binding sites. The conformational changes cause both molecules to enter the cell down the concentration gradient of sodium. This is called a Symport.
• In counter transport sodium ions still try to diffuse into cells via conc. gradient but the substance to be transported is on the inside of the cell. = Antiport system.

Question 29

Who uses active transport?

Answer 29

Potassium, sodium, calcium iron, hydrogen, chloride, iodide,
urate ions and several sugars as well as most amino acids

Question 30

Regarding the Sodium potassium channel. [remember 3Na+ /2K+]

Answer 30

Carrier protein made up of two globular proteins –larger α
subunit and smaller β unit
• Smaller unit may be used for anchoring
• Larger unit has three important features
– It has 3 receptor sites for binding sodium ions inside the cell
– It has two receptor sites for binding potassium ions on the outside
– Inside portion near the sodium binding site has ATPase activity

Question 31

The important thing to note about the counter transport system.

Answer 31

• counter transport system mechanisms is sodium-calcium, which occurs in all cell membranes in +tion to primary active transport of calcium. And sodium-hydrogen counter transport, occurs in the proximal tubules of the kidneys - not as effective as primary transport.