1.4 Membrane transport

Question 1

Two key qualities of cellular membranes:

Answer 1

1. They are semi-permeable - only certain materials may freely cross (large and charged substances are typically blocked)
2. There are selective - proteins in the membrane regulate the passage of materials

Question 2

Diffusion

Definition

Answer 2

The movement of particles from a region of high concentration to a region of low concentration, as a result of the random motion of particles.

A type of passive transport.

Question 3

Types of diffusion

Answer 3

• Simple diffusion
• Facilitated diffusion

Question 4

Factors that affect diffusion

Answer 4

• Temperature: molecules diffuse faster if the temperature is higher
• Surface area of the membrane: an increase in surface area allows more molecules to diffuse
• Size of particles: smaller particles diffuse faster than larger ones
• Concentration gradient of diffusing particles: the bigger the gradient the faster the diffusion

Question 5

Passive transport

Definition

Answer 5

The movement of a solid liquid or gas along a concentration gradient. It does not require energy.
- Simple diffusion
- Facilitated diffusion
- Osmosis

Question 6

Simple diffusion

Definition

Answer 6

The movement of small molecules along (down) a concentration gradient.

Question 7

Facilitated diffusion

Definition

Answer 7

The movement of large and charged molecules via proteins in the membrane. It occurs along (down) a concentration gradient and does not require any energy.
Proteins involved in facilitated diffusion can be either carrier proteins of channel proteins
- Carrier proteins: form a bond with the molecules and then change shape to transport the molecule to the other side of the membrane
- Channel proteins: span the cell membrane and open and close to let substances pass through

Question 8

Example of facilitated diffusion: Potassium channels in axons (nerve cells)

Application

Answer 8

• During the generation of an action potential in neurons, potassium diffuses via protein channels
• These protein channels are voltage gated meaning they open and close with changes in electrical potential to control the movement of K⁺ ions.

Question 9

Osmosis

Definition

Answer 9

The passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration along a partially permeable membrane.
- Water always moves by osmosis from the hypotonic solution to the hypertonic solution.

Question 10

Osmosis: tissues and organs used in medical procedures

Application

Answer 10

• In medical procedures, such as transplant operations, tissues/organs need to be kept in a saline solution for storage.
• This is because it is essential that the osmolarity (a measure of the solute concentration) of the saline solution is the same as that in the cytoplasm of the cells of the tissue to prevent any osmosis that would damage the cell.

Question 11

Osmolarity

Answer 11

A measure of solute concentration, as defined by the number of osmoles of a solute per litre of solution (osmol/L)
- Solution that has a relatively higher osmolarity - hypertonic solution
- Solution that has a relatively lower osmolarity - hypotonic solution
- Solitions that have the same osmolarity - isotonic

Question 12

How to estimate osmolarity by bathing samples in hypotonic and hypertonic solutions

Skill

Answer 12

The osmolarity if a tissue can be determined by bathing a sample in solutions with known osmolarities
- The tissue will gain water when placed in hypotonic solutions and lose water when placed in hypertonic solutions
- Water loss/gain can be determined by weighing the sample before and after bathuing it in the solution
- Tissue osmolarity may be inferred by identifying the concentration of solution in which there is no weight change (where the solution is siotonic)

Question 13

Hypertonic

Definition

Answer 13

Used to describe the solution with a greater concentration of solutes (lower water potential)
- If cells are placed in a hypertonic solution they shrink

Question 14

Hypotonic

Definition

Answer 14

Used to describe the solution with a lower concentration of solutes (higher water potential)
- If cells are placed in a hypotonic solution they will swell up and may burst

Question 15

Isotonic

Definition

Answer 15

When both solutions have equal solute concentrations/water potential
- If cells are placed in an isotonic solution there is no net movement of water/no osmosis.

Question 16

Active transport

and methods of active transport

Answer 16

The movement of particles across membranes, requiring energy in the form of ATP. The energy is used to move substances AGAINST a concentration gradient, from a region of low concentration to a region of high concentration.

Methods of active transport:
- Protein pumps (a type of carrier protein)
- Vesicular transport: endocytosis and exocytosis

Question 17

Example of active transport: sodium-potassium pumps

Answer 17

The concentration of sodium needs to be relatively higher outside the neuron than inside and the concentration of potassium has to be relatively higher inside than outside.
Summary of active transport by the sodium-potassium pumps:
1. The pump opens to the inside of the axon where three sodium ions bind to the pump.
2. ATP attatches to the protein and releases energy by breaking down into ADP + P and transferring one of its phosphate groups (the P) to the pump.
3. This causes the protein to change shape, expelling the sodium ions to the outside.
4. Two potassium ions from the outside bind to the pump which leads to the release of the phosphate which causes the pump to change shape again so that its only open to the inside of the axon.
5. The potassium ions are released inside
6. Sodium can now enter and bind to the pump again. The cycle continues.

Each cycle of the pimp causes 3 sodium ions to move out of the cell and 2 potassium ions to move into the cell.

Question 18

Vesicular transport (endocytosis and exocytosis)

Method of active transport

Answer 18

• This type of transports involves vesicles, which are sacs with a membrane made up of phospholipids
• Ocurrs when cells have to secrete or absorb larger molecules, much larger quantities of a substance or even take up other cells
• Uses energy
• Dependent on flexibility of plasma membrane

Question 19

Endocytosis

Answer 19

When cells take in (large) molecules or substances from outside the cell by engulfing them in the a cell membrane to form a vesicle.
- A portion of the cell membrane is pinched off to enclose the molecule(s)

Question 20

Phagocytosis

Endocytosis

Answer 20

When certain living cells called phagocytes ingest or engulf others cells or particles.
Example: when a white blood cell/phagocyte engulfs a pathogenic microbe.
- Then its transported to the lysosomes so the bacterium is digested.

Question 21

Pinocytosis

Not in syllabus?

Answer 21

The absorption of liquid substances.
- Allows faster entry than via protein channels

Question 22

Exocytosis

Answer 22

When the cell ejects (large) molecules or substances by fusing vesicles with the plasma membrane, expelling its contents.
Example: when the pancreas produces insulin and secretes it into the bloodstream to regulate blood sugar levels

Question 23

Excretion

Exocytosis

Answer 23

Following phagocytosis, any undigested remains of the microbe that are not useful to the cell are excreted outside the cell.

Question 24

The role of vesicles within cells

Answer 24

• Vesicles can move materials within cells.
• Many organelles in a cell are membrane bound meaning they can make their own vesicles to transport what they synthesise to another organelle and eventually outside the cell.
• Example: The rough ER transports proteins via vesicles to the Golgi apparatus and then that will be sent towards the plasma membrane to exit the cell by exocytosis.