Transport across membranes

Question 1

What is the cell surface membrane referred to as and why is it called that

Answer 1

referred to as the fluid-mosaic model because:
-Molecules within membrane can move laterally (fluid) e.g. phospholipids

• Mixture of phospholipids, proteins, glycoproteins and glycolipids

Question 2

structure of a cell membrane

Answer 2

its a Phospholipid bilayer:
- Phosphate heads are hydrophilic so attracted to water – orientate to the aqueous environment either side of the membrane
- Fatty acid tails are hydrophobic so repelled by water – orientate to the
inside/interior of the membrane

It also has proteins embedded into it which can be extrinsic or intrinsic.

It consists of carrier and channel protein. these are intrinsic
Consists of glycoprotein and glycolipids( act as antigens) (also they are extrinsic proteins)

And cholesterol which regulates fluidity

Question 3

How do molecules move inside the cell through the cell surface membrane

Answer 3

Phospholipid bilayer:
- Allows movement of non-polar small/lipid soluble molecules e.g. oxygen or water, down a concentration gradient (simple diffusion)
- Restricts the movement of larger/polar molecules

Channel proteins and carrier proteins
- Allows movement of water-soluble/polar molecules / ions, down a concentration gradient (facilitated diffusion)

Carrier proteins
- Allows the movement of molecules against a concentration gradient using
ATP (active transport)

Question 4

Describe movement through simple diffusion

Answer 4

Net movement of small, non-polar molecules e.g. oxygen or carbon dioxide, across a selectively permeable membrane, down a
concentration gradient

• Passive as no ATP / energy required
• Factors affecting rate – surface area, concentration gradient,
  thickness of surface / diffusion distance (Fick’s law)

Question 5

Describe movement through facilitated diffusion

Answer 5

• Net movement of larger/polar molecules e.g. glucose, across a
  selectively permeable membrane, down a concentration gradient
• Through a channel/carrier protein

Carrier proteins transport large molecules, the protein changes
shape when molecule attaches

• Channel proteins transport charged/polar molecules through its
  pore (some are gated so can open/close e.g. Voltage-gated sodium
  ion channels)

Question 6

Importance of diffusion across membranes

Answer 6

Useful as diffusion is crucial for the Pacinian Corpuscle in generating a generator potential as sodium ion channel proteins are required to open so NA+ can diffuse into the sensory neurone to cause depolarisation and trigger an action potential. So these channel proteins are extremely important in diffusing NA+ into the sensory neurone so that it can trigger action potential. Without channel proteins the NA+ would not undergo facilitated diffusion and therefore it affects the reflex arc as pressure needs to be exerted on the Pacinian corpuscle to allow the facilitated diffusion of NA+ and only channel proteins can transport these ions across cell membrane.