Movement across membranes

Question 1

What is diffusion?

Answer 1

The net movement of molecules from a region of higher concentration to a region of lower concentration down a concentration gradient.

Question 2

What is passive transport?

Answer 2

Transport of molecules or ions from one location to another purely dependent on the natural kinetic energy of the molecules and requires not additional energy input.

Question 3

What factors affect the rate of diffusion?

Answer 3

1. Temperature: Increasing temperature gives molecules more energy, thus increasing the rate of diffusion.
2. Concentration gradient: More molecules on one side maintains high concentration gradient, increasing rate of diffusion.
3. Stirring/air movement: Gives particles more kinetic energy, increasing rate of diffusion.
4. Surface area: Increasing surface area increases rate of diffusion.
5. Diffusion distance: Decreasing thickness of membrane increases rate of diffusion.
6. Size of molecule: Smaller molecules diffuse quicker.

Question 4

What molecules are able to cross the plasma membrane?

Answer 4

1. Uncharged molecules like other lipids, oxygen and carbon dioxide since they are fat soluble.
2. Small charged molecules like water are able to fit through gaps between the phospholipid molecules in the phospholipid bilayer.

Question 5

What molecules are unable to cross the plasma membrane?

Answer 5

Polar molecules and charged ions since they are repelled by the hydrophobic tails in the centre of the phospholipid bilayer.

Question 6

What is facilitated diffusion?

Answer 6

Certain proteins embedded in the phospholipid bilayer allow polar and charged particles to pass through the plasma membrane by a passive process. However, this only happens in their presence so the process is called facilitated diffusion.

Question 7

What are channel proteins?

Answer 7

Gated pores in the membrane shaped specifically to allow a certain type of molecule to diffuse across. They are often gated to control diffusion.

Question 8

What are carrier proteins?

Answer 8

These proteins have a complementary shape to the molecule they carry. The binding on of the molecule changes the shape of the protein, allowing the molecule to pass through the membrane. This is usually know as ‘flip-flop’ action.

Question 9

What is active transport?

Answer 9

Movement of particles across a membrane against the concentration gradient with the use of energy from ATP.

Question 10

What is active transport carried out by?

Answer 10

Carrier proteins.

Question 11

How do active transport carrier proteins differ form facilitated diffusion carrier proteins?

Answer 11

1. They only move a molecule one way.
2. The molecule binding on doesn’t induce shape change, energy is required.
3. They carry particles against concentration gradient.

Question 12

What is the process of active transport?

Answer 12

1. Energy is used to change the shape of the carrier protein so that the molecule only fits on one side.
2. The molecule is transported across the membrane to the other side.
3. Energy is used once again to change the shape of the protein so molecule no longer fits.
4. Molecule leaves carrier protein on the other side.

Question 13

What is endocytosis?

Answer 13

Movement of a substance in bulk into the cell.

Question 14

What is exocytosis?

Answer 14

Movement of a substance in bulk out of the cell.

Question 15

What is the process of endocytosis?

Answer 15

1. The substance accumulates outside the plasma membrane.
2. Plasma membrane wraps around substance and pinches off, leaving substance in a vesicle inside the cell.
3. The vesicle is then moved by microtubules around the inside of the cell.
4. The pinching of the membrane and the movement of the vesicle requires ATP.

Question 16

What is the process of exocytosis?

Answer 16

1. The substance is formed and packaged into a vesicle by the golgi body.
2. The vesicle is moved to the plasma membrane by microtubules.
3. Membrane around vesicle fuses with plasma membrane.
4. Substance is released (excreted) outside of cells.

Question 17

What is osmosis?

Answer 17

The passive net movement water across a partially permeable membrane from an area of higher water potential to an area of lower water potential, down the water potential gradient.

Question 18

What is water potential?

Answer 18

A measure of the the total kinetic energy water molecules have in a system. The higher the water potential, the more water molecules there is.

Question 19

What happens when a cell is put into a hypotonic solution?

Answer 19

Water moves into the cell from the surroundings by osmosis, down the water potential gradient, from an area of higher water potential to an area of lower water potential. Cells expand; animal cells eventually burst (haemolysed) and plant cells become turgid.

Question 20

What happens when a cell is put into a hypertonic solution?

Answer 20

Water moves out of the cell into the surroundings by osmosis, down a water potential gradient, from an area of higher water potential to an area of lower water potential. Cells shrink; animal cells wrinkle (crenated) and plasma membrane shrinks away from cell wall (plasmolysed).

Question 21

What factors affect rate of osmosis?

Answer 21

1. Amount of water present.
2. Solute concentration.
3. Presence of aquaporins.
4. (Plants) turgor pressure.

Question 22

Which processes are overall passive?

Answer 22

1. Diffusion.
2. Facilitated diffusion.
3. Osmosis.

Question 23

Which processes are overall active?

Answer 23

1. Active transport.
2. Endocytosis.
3. Exocytosis.