Forces driving movement across cell membrane 2.1 and 2.2

Question 1

How is an electrical gradient formed across cell membrane?

Answer 1

Because the ions creating the concentration gradients are charged particles there is also a difference in charge across the membrane. This creates an electrical gradient.

Question 2

What is an electrochemical gradient?

Answer 2

The net effect of these two forces create an electrochemical gradient which ultimately drives the direction of passive movement.

Question 3

Characteristics of diffusion?

Answer 3

A. Passive diffusion directly through the lipid bilayer or;
B. Facilitated diffusion via either:
i) protein channels or;
ii) protein transporters

Question 3

What do we need to move against electrochemical gradient?

Answer 3

Energy - active transport

Question 4

Characteristics of active transport?

Answer 4

To move against electrochemical gradient , supply energy to transporter protein and pump against gradient.

Question 5

Characteristics of osmosis?

Answer 5

Movement of water determined by force of osmosis.

Question 6

Characteristics of filtration?

Answer 6

Movement of items across capillary wall. ( Between plasma and ISF)

Question 7

What is a brief description of endocytosis and exocytosis?

Answer 7

These are mechanisms for moving macromolecules across membranes without disrupting them.

Question 8

What happens in endocytosis?

Answer 8

In endocytosis, there is invagination of the cell membrane to form a vesicle which eventually disintegrates on the cytoplasmic (inside) surface of the membrane, releasing contents which then migrate within the cell to their destination.

Question 9

What happens in exocytosis?

Answer 9

Exocytosis involves the reverse process. Many proteins manufactured in cells are released from those cells by the process of exocytosis.

Question 10

When is endocytosis of membrane receptors used?

Answer 10

common mechanism of terminating signals from extracellular ligands

Question 11

What is the process of diffusion?

Answer 11

Process by which a gas or substance in solution expands to fill all the available volume. Therefore molecules spread from regions of high concentration to regions of low concentration, until the concentration is uniform throughout the volume

Question 12

What are cell membranes effectively impermeable to?

Answer 12

Intracellular proteins and organic anions. These items cannot diffuse in any capacity so they stay inside the cell.

Question 13

What is passive diffusion?

Answer 13

substance moves directly through lipid bi layer

Question 14

What is facilitate diffusion?

Answer 14

substance requires assistance from membrane proteins to cross lipid bi layer

Question 15

For a molecule to pass through lipid bilayer, describe the characteristics that they must be?

Answer 15

1. SMALL
2. UNCHARGED
3. Lipophilic (hydrophobic)
   Being small and uncharged is most important.

Question 16

Which gases can pass through lipid bi layer?

Answer 16

Gases (small and uncharged) pass readily though the lipid bilayer, whether lipophilic or lipophobic, although lipophilic ones like O2 and N2 pass faster than lipophobic gases like CO2.

Question 17

What is facilitated diffusion?

Answer 17

Movement of molecules through transport proteins down an electrochemical gradient. Substances that can cross membrane but cannot diffuse directly , like ions and glucose.
Involves membrane proteins.

Question 18

What are the two types of proteins that help nutrients and ions cross lipid membrane?

Answer 18

Channel or mediated transport

Question 19

Describe channel proteins and their role?

Answer 19

These are transmembrane proteins that provide an aqueous route through the membrane for the simultaneous diffusion of water and ions.

Question 20

Which ions are able to get through with the help of channel proteins?

Answer 20

Only allow the passage of mineral ions such as Na+ and K+, Cl-, Ca2+ and H+ accompanied by H2O.
Molecules such as glucose are too large to go through.

Question 21

What is the main route for water to pass between compartments?

Answer 21

H2O passes freely through aquaporins, specific family of channels, that are ubiquitously distributed and permanently open to water only.
There are NO barriers to water movement in the body.

Question 22

Describe the physical characteristics of ion channels?

Answer 22

-made up of multiple protein subunits
-subunits can have different roles ( receptor, ligand binding site)
-integral proteins
-when open , continuous pore

Question 23

When does a typical ion channel open?

Answer 23

They remain closed until a stimulus (chemical or change in electrical charge across the membrane) causes them to open.

Question 24

How do voltage gated channels work?

Answer 24

open/close in response to alterations in the membrane electrical potential (charge difference either side of the membrane).

Question 25

Where are voltage gated channels found?

Answer 25

Found extensively in muscle and nerve cells.

Question 26

How do ligand gated channels work?

Answer 26

open/close when they bind a chemical such as neurotransmitter or hormone to a receptor binding site on the channel protein.

Question 27

What are multi-tasking membrane proteins?

Answer 27

Act as both receptor and transporter.

Question 28

What happens after stimulus has reached channel?

Answer 28

In both cases the stimulus, either electrical or chemical, causes a conformational change in the configuration of the channel proteins causing them to open or close their channel as appropriate.

Question 29

What is the direction of diffusion dependent on?

Answer 29

The direction of diffusion is dependent on the concentration gradient and, in the case of charged particles, the electrical gradient. Sometimes these forces co-operate, sometimes they oppose each either. Net direction of movement depends on dominant force

Question 30

What is the membrane potential?

Answer 30

There is a separation of charge across most cell membranes so that the inside of the cell carries a negative charge with respect to the outside. The magnitude of this difference is called the membrane potential (it creates a potential gradient down which ions will flow)

Question 31

What is the electrochemical gradient?

Answer 31

The combined gradient (chemical and electrical) down which ions will flow is  called the electrochemical gradient.

Question 32

When is electrochemical equillibirum reached?

Answer 32

Electrochemical equilibrium is reached when the chemical and electrical gradients are in balance.

Question 33

Why do we need carrier-mediated transport proteins?

Answer 33

Molecules too big to move through channel proteins use carrier-mediated transport proteins or “transporters”.

Question 34

How do carrier mediated transport proteins work?

Answer 34

These proteins have binding sites for the solutes they transport such as glucose. When they bind the solute, the transporter undergoes a change in shape which exposes the binding site on the other side of the membrane. The solute moves away from the transporter and the transporter returns to its original configuration.

Question 35

Describe the way in which transporter proteins work?

Answer 35

-glucose mol in higher conc in ECF than ICF
-when mol is open , binding site of protein exposed
-protein undergoes conformational change
-closes to ECF and opens to ICF
-Attraction of glucose to receptor is reduced and glucose is dropped into cell

Question 36

What if electrochemical gradient favours direction of movement?

Answer 36

movement occurs by diffusion and no energy in put is required

Question 37

What if electrochemical gradient opposes movement?

Answer 37

energy in the form of ATP is required to move the molecule against this gradient. In these cases the carrier protein also functions an enzyme (ATPase) as it hydrolyses ATP to release energy. To power movement of transporter.

Question 38

When is active transport needed?

Answer 38

When molecules are moved against their concentration gradient

Question 39

Mechanism of the Na+ / K+ pump?

Answer 39

-Pump open to inside of membrane and has 3 active sites for sodium
- 3 sodium ions in ICF attracted to binding sites as high affinity
-ATPase closed on ICF with Pi from ATP
-ATPase opens on ICF
-sodium released out (against conc gradient)
-2 potassium ions attracted to ATPase on ECF
-conformational change and ATPase opens on ICF.
In doing so it produces net movement of positive charge out of the cell = electrogenic pump (creates an charge difference across the membrane)