cell membranes

what are cell membranes

• Selectively permeable barriers

• Maintain constant internal environment

• Enclose cell contents

Describe why the cell membrane structure is called a fluid mosaic model

• fluid - molecules free to move laterally in phospholipid bilayer - dynamic structure

• mosaic - many components: phospholipids, proteins, glycoproteins, glycolipids

explain the phospholipid bi layer structure (5)

• Amphiphilic – both hydrophilic and hydrophobic parts

• hydrophilic phosphate heads - polar head; negatively charged phosphate group - attracted to water

• hydrophobic fatty acid non polar tail; no charge - repelled from water

• Phospholipids ‘self-assemble’ to create the bilayer - arranges itself in a way that the phosphate heads are facing water/outwards and fatty acid tails cluster on the interior away from the water

• which allows it to be selectively permeable

describe the saturatedness of the fatty acid tails

• can either be saturated or unsaturated

• saturated = single bonds only between the Cs in the hydrocarbon chain

• unsaturated = at least one double bond C=C between carbons in the hydrocarbon chain

• the double bond creates a kink/bend in the fatty acid chain of the tail

explain how the saturated/unsaturated nature of the fatty acid tails affects membrane permeability

• Higher concentrations of unsaturated acids promote
  membrane fluidity = kinks from the double bonds = looser packing = large gaps in between = more fluid membrane

• Higher concentration of saturated fatty acids reduce cell
  membrane fluidity = tighter/neat packing = fewer gaps = less fluid membrane

what are the 2 main types of carbohydrates in the phospholipid bilayer

• glycoproteins

• glycolipids

3 main roles of glycoproteins and glycolipids

• Maintain membrane stability and cell protection (glycocalyx)

• Aid cell-cell adhesion (cell adhesion molecules)

• Facilitate cell recognition/signaling

• xtra: act as receptors/antigens // allow it to respond to chemicals (hormones)

explain the structure of cholesterol (4)

• Is a lipid - approximately 20% of membrane lipid is cholesterol

• Also has polar and non polar regions

• Wedges itself between the phospholipid tails - bonds to phospholipid hydrophobic fatty acid tails

• Can migrate and even ‘flip’ between the membrane layers

name some of the different functions proteins in the bi layer serve

enzymes, carrier proteins, channel proteins, receptors, cell adhesion, recognition

what are the 2 types of proteins in the bi layer

• integral/intrinsic

• peripheral/extrinsic

explain integral proteins (5)

• Firmly inserted into the membrane

• Span the bilayer of the membrane

• Transmembrane portion is hydrophobic

• Extracellular and cytosolic portions are hydrophilic

• Carrier proteins and channels are
  integral proteins

explain peripheral proteins (2)

• Loosely attached to membrane/on surface of membrane

• May be removed easily from the membrane with minimal disruption

what are 3 factors that affect membrane fluidity

• Extreme temperatures (hot or cold)

• Concentration of unsaturated fatty acids

• Cholesterol levels

explain how membrane fluidity is affected by high AND low temperature

• Low temp: less kinetic energy = phospholipids pack together = membrane less fluid

• High temp: more kinetic energy = phospholipids tend to move further away = membrane more fluid

explain how cholesterol levels affects membrane fluidity

• Maintains fluidity by preventing lipids from getting too close or too far apart from each other

• restricts the movement of other molecules making up the membrane (phospholipids) - decreases fluidity/increase rigidity

Are the proportions of the individual components the same in
membranes of different cell-types?

• No - Different cells have different functions, so their membranes are customized - for example:

• Red Blood cell – equal amounts of lipid and protein and a small amount of carbohydrate

• Nerve cells – higher amounts of lipid in cell membranes (80% lipid) - for insulation (myelin sheath)