B2.1.5 cell membranes

Question 1

functions of cell membranes (5)

Answer 1

1. selective permeability
2. cell signalling
3. electrical signals
4. membranes around organelles to form different compartments
5. specific receptors for hormones + enzymes/ attachment sites

Question 2

fluid - mosaic model (1972)

Answer 2

• the phospholipid molecules move freely and make the membrane fluid
• proteins are distributed unevenly in a mosaic pattern

Question 3

phospholipid bilayer

Answer 3

• made of a hydrophilic head + hydrophobic tails
• arranged so that hydropholic heads are facing out towards the water + hydrophobic tails are facing in away from the water

Question 4

what can pass through the phospholipid bilayer

Answer 4

• hydrophobic in the centre so polar substances cannot pass through
• but fat-soluble substances can dissolve in the bilayer and pass directly through

Question 5

how polar molecules pass through the membrane

Answer 5

they bounce off the hydrophilic heads and go through a protein channel

Question 6

how non polar molecules pass through the membrane

Answer 6

they can diffuse directly through the membrane of the bilayer

Question 7

function of glycoproteins + glycolipids

Answer 7

• cell adhesion
• act as receptors for cell signalling
• act as cell identity markers/ antigens
• also from H bonds with water molecules surrounding the cell to stabilise the membrane structure

Question 8

cholesterol

Answer 8

positioned between phospholipids + regulates fluidity of the cell membrane

Question 9

channel protein

Answer 9

• pores in the membrane
• allows movement of large/ hydrophilic molecules
• often shaped to allow only one type of ion through
• may be gated i.e. can open + close

Question 10

carrier protein

Answer 10

• often have a particular shape so only specific molecules fit, then changes shape to allow molecules through
• often actively move substances across membranes i.e. against conc gradient using ATP energy

Question 11

intrinsic proteins

Answer 11

• embedded through both sides of the bilayer
• include channel + carrier proteins

Question 12

extrinsic proteins

Answer 12

• only on one side of the bilayer
• provide support to the membrane/ may be involved in cell signalling