Transport across cell membrane

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 control of movements

Answer 6

Diffusion allows certain molecules to move into the cell by allowing it to diffuse straight through (simple diffusion) or diffuse through intrinsic proteins( carrier/channel through facilitated diffusion) Channel proteins allow for the control of movement of sodium ion such as the ones involved in generating a generator potential in the Pacinian corpuscle.
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.