Cell membrane structure and transport

Question 1

What is the structure of the phospholipid bilayer?

Answer 1

Question 2

What are the components of the cell membrane?

Answer 2

Proteins: 55%
Lipids: 45%
- Phospholipids: 75%
- Cholesterol: 20%
- Glycolipids: 5%

Question 3

What are the functions of membrane lipids?

Answer 3

Forms a protective fence around the cell.
Serves as a selective barrier to passage of substances between the ICF & ECF.
Gives the flexibility to the membrane.

Question 4

What are the properties of each lipid component?

Answer 4

Cholesterol: Between the phospholipid molecules. It is needed for normal flexibility of the membrane.
Glycolipids: It is a phospholipids bound with short oligosaccharide chains. These are found only on the ECF face of the membrane.

Question 5

What are the classifications of membrane proteins?

Answer 5

Integral proteins, Peripheral proteins, Glycoproteins

Question 6

What are integral proteins?

Answer 6

Integral proteins: Protrude all the way through the membrane.
Functions include: channels, carriers, pumps, enzymes and receptors

Question 7

What are peripheral proteins?

Answer 7

Peripheral proteins: Loosely attached to intercellular face of the membranes. Function: enzymes

Question 8

What are glycoproteins?

Answer 8

Glycoproteins: Integral proteins bound with short oligosaccharide chains. Found only on the extracellular face of the membrane.
Functions include: self-identity markers (cell-recognition marker), antigen-like (e.g. blood group markers), cellular attachment and communication, receptors (for hormones) and involved in immune reactions.

Question 9

What is the glycocalyx?

Answer 9

The entire dense, gel-like outer surface of the cell which forms a physical barrier, often has a loose carbohydrate coat that is called glycocalyx.

Question 10

What is passive transport?

Answer 10

Movement of substances from a region of high concentration to a region of low concentration.
Does not require energy
Types of passive transport:
- Simple diffusion
- Facilitated diffusion
- Osmosis
- Filtration

Question 11

What is active transport?

Answer 11

Movement of substances from a region of low concentration to a region of high concentration.
Requires energy
Types of active transport:
- Primary active transport
- Secondary active transport
- Endocytosis
- Exocytosis

Question 12

What is the mechanism of simple diffusion?

Answer 12

By random kinetic movement; each particle is moving in its own separate way.
It occurs through 2 ways:
- Simple diffusion through the lipid bilayer.
- Simple diffusion through protein channels.

Question 13

What substances can/cannot pass through the lipid bilayer by simple diffusion?

Answer 13

Lipid-soluble substances: can cross cell membranes easily e.g., N2 & O2., alcohol.
Water: Because of its small size and high kinetic energy, they can penetrate the membranes despite not being lipid soluble
Small uncharged water-soluble molecules: e.g., CO2 crosses the membrane with water.
Large uncharged water-soluble molecules: e.g., glucose & urea, their diffusion is more slower.
Charged molecules (ions): pass membrane extremely slow, so it needs ion channels.

Question 14

Describe passive transport through protein ion channels.

Answer 14

Question 15

What are the characteristics of facilitated diffusion/carrier mediated transport?

Answer 15

Facilitated diffusion needs a carrier protein
The rate of facilitated diffusion has a transport maximum that is determined by the number & activity of the carrier.
The number and activity of the carrier can be controlled by hormones e.g., insulin hormone controls carrier mediated-transport of glucose to most of the body cells

Question 15

What are the characteristics of facilitated diffusion/carrier mediated transport?

Answer 15

Facilitated diffusion needs a carrier protein
The rate of facilitated diffusion has a transport maximum that is determined by the number & activity of the carrier.
The number and activity of the carrier can be controlled by hormones e.g., insulin hormone controls carrier mediated-transport of glucose to most of the body cells

Question 16

What are the factors affecting diffusion?

Answer 16

Concentration difference: Higher concentration gradient = higher rate of diffusion
Surface area of membrane: Higher surface area = higher rate of diffusion
Permeability of the membrane: Increase membrane permeability = higher rate of diffusion

Question 17

What can affect the permeability of the membrane?

Answer 17

Directly with the temperature, lipid solubility & number of protein channels.
Indirectly with the thickness of the membrane & molecular weight of the diffusing substances.

Question 18

Formula for Fick’s Law of diffusion

Answer 18

Diffusion rate (amount of the substance moved/unit time) = concentration gradient (conc. gradient) x surface area (A) x permeability coefficient (P)

Question 19

What is the mechanism of osmosis?

Answer 19

Passive movement of water across a partially permeable membrane from a region of low solute concentration to a region of higher solute concentration.

Question 20

What is osmotic pressure?

Answer 20

Osmotic pressure of the solution is the pressure required to be applied on the concentrated solution to prevent water movement from the diluted side; i.e. It is the pressure that stops water migration.

Question 21

What is tonicity and its types?

Answer 21

Question 22

What impact does vasopressin (ADH) have on collecting duct cells?

Answer 22

As vasopressin acts on principal cells of the CD, it inserts protein water channels (=aquaporin water channels).
This increases water permeability of CD (water moves from tubules to medullary interstitium to the venous blood)

Question 23

What is filtration?

Answer 23

It is a process in which fluid is forced to pass through a porous membrane due to the difference in hydrostatic pressure on the 2 sides of the membrane. e.g., along capillary membrane in different tissues.

Question 24

What is primary active transport?

Answer 24

Movement of 3Na+ ions out of the cell and 2K+ ions into the cell using the Na+/K+ pump

Question 25

What is secondary active transport?

Answer 25

Uses the energy of Na+ gradient, produced by the primary active transport of Na+, to transfer other molecules (glucose & amino acids) against their concentration gradient with no further need of energy.

Question 26

What are the types of secondary transport?

Answer 26

Co-transport and Counter-trasport

Question 27

Co-transport

Answer 27

The transport of glucose, into the GIT cell or renal cells is in association (co-transport) with Na+ entry is an example of secondary active transport. Similarly, amino acids could be co-transported with Na+ by another specific carrier proteins.

Question 28

Counter-transport

Answer 28

Exchange of ions in the opposite direction of Na+ movement (counter-transport) is another example of secondary active transport. The binding of Na+ ions to the carrier binding sites at the cell outer surface is simultaneously accompanied by binding of Ca++ or H+ ions to the carrier at the cell interior. Movement of Na+ along its concentration gradient causes the other substance to be transported to the other site against their electrochemical gradient. e.g., Na-Ca exchanger (heart), Na-H exchanger (kidney)

Question 29

Compare uniport, symport and antiport

Answer 29

Question 30

What factors affect the maximum rate of active transport?

Answer 30

Carrier number
Carrier activity
Presence of carrier inhibitors
Presence of competing substance (competitive inhibition)

Question 31

What are the types and purposes of vesicular transport?

Answer 31

For large molecules, e.g. secretory products, neurotransmitters, bacteria, etc.)
Exocytosis: Out of the cell
Endocytosis: Into the cell
Phagocytosis: Engulfing large particles
Pinocytosis: Taking in extracellular fluids