Week 3: Membrane Transport

Question 1

What type of ions can pass through the lipid bilayer and which cant without help?

Answer 1

Most hydrophilic ions cannot pass through lipid bilayer. But the passage of many hydrophilic, water-soluble molecules ie. inorganic ions, sugars, amino acids, nucleotides and other cell metabolites occurs - crossing by simple diffusion - passage must be accelerated by facilitated transport.

Question 2

How well do small, nonpolar molecules diffuse?

Answer 2

Readily and rapidly

Question 3

How well do uncharged polar molecules diffuse?

Answer 3

Readily but only if small enough ie. water and ethanol. Larger molecules can hardly cross

Question 4

How well do charged substances cross?

Answer 4

Charges and strong electrical attraction to water molecules inhibit their entry into inner, hydrocarbon phase of bilayer.

Question 5

Describe the internal vs. external ion concentrations for Ca2+. Na+, H+, K+, Cl- and fixed anions/

Answer 5

Ca2+, Na+ and Cl- are more concentrated extracellularly and H+. K+ and fixed anions are more concentrated intracellularly.

Question 6

What is membrane potential? What does it do?

Answer 6

Caused by electrical imbalances When a cell is unstimulated, the movement of anions and cations across membrane well be balanced - the resting membrane potential - holds steady between -20 and -200 mV (dep. organism and cell type).

allows cells to power transport of metabolites and provides cells that are excitable with way to communicate.

Question 7

What are transport proteins? What do they do? What are the two classes of transport proteins?

Answer 7

Have polypeptide chains that traverse lipid bilayer multiple times - multipass transmembrane proteins - when they cluster they establish a protein-lined path for small, hydrophilic molecules to cross. Channels and transporters.

Question 8

What is passive transport and which transport proteins use this?

Answer 8

Small hydrophilic molecules use transporters and channels but their transport depends on concentrations of solute - moving down concentration gradient - passive transport.

Question 9

What are some influences on passive transport? For uncharged substances? For charged substances?

Answer 9

For uncharged molecule - direction is determined only by concentration gradient

For charged substances - membrane potential exerts force - the cytosolic side is usually negative to extracellular side, so it pulls positive changed ions and molecules into cell and drives negative solutes out. A charged solute will also ten to move down concentration gradient. The net driving force between concentration gradient and membrane potential is electrochemical gradient.

Question 10

What happens when voltage and concentration gradients work in same direction? In opposing directions?

Answer 10

Steep electrochemical gradient - ie. Na+.

If they have opposing effects the electrochemical gradient is small, ie. K+.

Question 11

What is osmosis? What is it facilitated by?

Answer 11

Movement of water down concentration gradient - area of low solute concentration to high solute concentration - osmosis

Water molecules are small and uncharged - diffuse directly. Is facilitated by aquaporin channels.

The total concentration of solute particles inside the cell - osmolarity - generally exceeds the solute concentration outside the cell.

Question 12

What happens if osmosis occurs without constraint? How do cells cope?

Answer 12

Can cause cell swelling if occurs without constraint. Diff cells cope with this in different ways. Some protozoans use contractile vacuoles. Plant cells have cell walls - use osmotic swelling (turgor pressure) to keep cell walls tense. Animal cells use transmembrane pumps to expel solute.

Question 13

What is active transport?

Answer 13

To move solute against gradient - requires input of energy from ATP and pumps

Question 14

What is a transporter?

Answer 14

responsible for movement of most small, water soluble, organic molecules and some inorganic ions. Highly selective. Each cell has characteristic set

Question 15

What are passive transporters? Describe glucose transporter

Answer 15

Move solute along its electrochemical gradient + are highly selective. ie. glucose transporter - consists of polypep chains that cross membrane 12 times - can adopt several conformations and switches between the. In one, it exposes binding sites for glucose to exterior. The direction it is transported is determined by its concentration gradient alone.

Question 16

What are the 3 types of pumps (active transporters)?

Answer 16

i) gradient-driven pumps - link uphill transport of one solute to downhill transport of another ii) ATP driven pumps - drive uphill transport iii) light-driven pumps - use energy from light to drive uphill transport.

Question 17

What is a symport? Antiport? Uniport?

Answer 17

Symports and antiports are coupled transport by gradient-driven pumps. Symports diffuse two molecules in same direction via co-transported ions. An antiport transports molecules in opposite directions via anti-transported ion. Uniports transport one type of molecule in one direction.

Question 18

Describe ATP driven Na+ pump.

Answer 18

Na+ pump - transporter/carrier protein - - transports Na+ out and K+ in - Na+-K+ ATPase. The pump keeps Na+ concentration in cytosol lower than extracellular fluid and K+ higher. Useful for gradient driven pumps fueled by downhill flow of Na+. See images

Question 19

Describe gradient driven pumps

Answer 19

Gradients of solutes can be used to drive active transport of second molecule. The downhill movement of first down its gradient gives energy to power uphill transport of second. If pump moves both solutes in same direction - symport. If opposite directions - antiport. If it ferries only one type of solute across membrane down concentration gradient (not a pump)- uniport - ie. glucose transporter.

Question 20

What is a symport? Describe glucose-Na+ symport

Answer 20

Some make use of inward flow of Na+ down steep electrochemical gradient. There are glucose-Na+ symports. See images.

Question 21

What is Na+-H+ exchanger (antiport)?

Answer 21

ie. Na+-H+ exchanger - uses downhill influx of Na+ to pump H+ out of cell - control pH. see images

Question 22

How do plants, bacteria and fungi create H+ gradients?

Answer 22

Plant cells, bacteria and fungi do not have Na+ pumps, they create a gradient with H+ pumps - pump out of cell. The import of many sugars and amino acids is mediated by them.

In some photosynthetic bacteria, the H+ gradient is created by activity of light-driven H+ pumps. In other bacteria, fungi and plants it is generated by ATP.

Some intracellular organelles 0 ie. lysosomes and central vacuole transport H+ out helping to keep pH of cytosol neutral and pH of interior of organelle acidic. see images.

Question 23

What are channels?

Answer 23

🌉 Channel - allow small, water-soluble substance to cross membrane to other - form transmembrane pores

A few form large, aqueous pores - ie. proteins that form gap junctions and porins the form pores in mitochondria and bacterial outer membranes.

Question 24

What is an aquaporin?

Answer 24

Facilitate flow of water across plasma membrane. Allow passive diffusion while prohibiting ion movement.

Question 25

What are two important properties of ion channels?

Answer 25

Two important properties 1) ion selectivity - dependent on shape of ion channel and distribution of charged amino acids lining it. 2) not continuously open - most are gated.

Question 26

How do ion channels have advantages over transporters?

Answer 26

Ion channels do not undergo conformational changes for each ion it passes - advantage over transporter in rate. Simply make membrane transiently permeable to select inorganic ions - mainly Na+, K+, Ca2+ or Cl-

Question 27

How are electrical charges mediated?

Answer 27

alterations in permeability of membranes to ions. Resting animal cells have negative charges on inside, balanced by K+ (continuously imported into cell by Na+ pump, generates K+ gradient).

Question 28

What are K+ leak channels?

Answer 28

allow K+ to move freely. These are the main ion channels open in resting cell. The charge imbalance will eventually oppose any further movement of K+ out of cell. This makes the electrochemical gradient for K+ 0 even though there is a higher concentration of K+ inside than out.

Question 29

What is resting membrane potential?

Answer 29

Resting membrane potential - flow of positive and negative ions is precisely balanced. Nernst equation expresses it. In animal cells, -20 to -200 mV - largely reflection of electrochemical K+ gradient.

Question 30

What happens when cell is stimulated?

Answer 30

closing of ion channels is important for cell signaling.

Question 31

How do ion channels change?

Answer 31

Even when conditions are constant, channels snap back and forth between open and closed as it is knocked by random thermal movements. The activity of each channel is all or none. When appropriate conditions change, the random behaviour continues but with changed bias, may spend a greater percent of time open.

Question 32

What is a ligand-gated channel? Mechanically-gated channel?

Answer 32

opening is controlled by binding of molecule.

opening is controlled by a force, ie. auditory hair cells.

Question 33

What are voltage-gated channels?

Answer 33

Voltage-gated play large role in nerve cells. They have domains called voltage sensors that are sensitive to changes in membrane potential - above a certain threshold and the channel will open/close. Membrane potential affects the probability they are open. When one type of voltage-gated ion channel opens, the membrane potential can change - activating or deactivating other voltage-gated channels.

Question 34

How do hair cells work?

Answer 34

Hair cells - ends have stereocilia. Sound causes underlying basal membrane to vibrate. Cilia move relative to each other - ion channels open - mechanically-gated. When basilar membrane vibrates up and down, stereocilia tilt causing stretching of linking filament - forcing ion channels open - K+ rushes in a depolarizes cell - neurotransmitter passes signal to auditory nerve. (see images)