Transport across Cell Membranes

Question 1

Give the functions of: (in the cell-surface membrane)
- Phospholipids
- Proteins

Answer 1

Phospholipids;
- allow lipid soluble molecules to enter and leave the cell.
- prevent water soluble substances entering and leaving the cell.
- make the membrane flexible and self-sealing.

Proteins;
> Protein channels -
- form water filled tubes to allow water soluble ions to diffuse across the membrane.
> Carrier proteins -
- binds to ions/molecules, then changes shape to allow these ions/molecules to move across the membrane.
> Overall -
- provide structural support.
- act as channels for water soluble substances.
- allow active transport through carrier proteins
- help cells adhere together, and act as receptors.

Question 2

Give the function of: (in cell surface membrane)
- Cholesterol
- Glycolipids
- Glycoproteins

Answer 2

Cholesterol;
- reduce lateral movement of other molecules.
- makes membrane less fluid at higher temperatures.
- prevent leakage of water and dissolved ions from the cell.

Glycolipids/ Glycoproteins;
- act as recognition sites.
- help maintain stability of membrane.
- help cells attach to one another and form tissues.

Question 3

Define:
- Simple diffusion
- Facilitated diffusion

Answer 3

Simple;
- The net movement of small, non-polar molecules or ions from a region of higher concentration to a region of lower concentration.
Facilitated;
- The net movement of large, polar molecules across a membrane using carrier proteins from a region of higher concentration to a region of lower concentration.

Question 4

Give three differences between Carrier proteins and Protein channels.

Answer 4

Carrier proteins;
- they are glycoproteins.
- relatively slow transport.
- transport water soluble and insoluble molecules down or against a concentration gradient.

Protein channels;
- they are lipoproteins.
- much faster transport.
- they only transport water soluble molecules down a concentration gradient.

Question 5

Define:
- Osmosis
- Active Transport

Answer 5

Osmosis;
- the net movement of water molecules from a region of higher water potential to a region of water potential.
Active Transport;
- the net movement of molecules from an area of lower concentration to an area of higher concentration.

Question 6

Describe how Active Transport takes place.

Answer 6

• the molecule or ion needing to be transported binds to the receptor sites on the carrier proteins spanning across the plasma membranes.
• on the inside of the cell, ATP binds to the carrier protein, breaking down into ADP and Pi, the Pi attaching to the protein.
• the attached Pi causes the carrier protein to change shape and open to the inside of the cell while closing to the outside, allowing the molecule/ion to be released into the cell.
• the Pi is released from the protein, causing the protein to change shape again, opening back up to the outside of the cell. The Pi recombines with ADP to reform ATP during respiration.

Question 7

Describe the process by which glucose/amino acids are absorbed into the bloodstream.

Answer 7

Co-transport;
- Sodium ions actively transport out of the epithelial cell due to the sodium-potassium pump, producing a downward concentration of sodium ions into the cell.
- Sodium ions bind with glucose/amino acids molecules, which are then co-transported into the epithelial cell by facilitated diffusion.
- The glucose/amino acids and sodium ions then diffuse through respective specific channel proteins into the capillaries.