Boundaries & membranes

Question 1

What do cell boundaries allow for?

Answer 1

• Formation of suitable semi-fluid matrix (protoplasm) for cell functions (life)
• Formation of specialised microenvironements where concrete solutes dedicated cell processes happen
• Segregation of harmful activities

Question 2

What is the principle boundary for a cell?

Answer 2

The exterior plasma membrane

Question 3

How do membranes help eukaryotic cells?

Answer 3

They have internal boundaries which allow compartmentalisation

Question 4

What are membranes composed of?

Answer 4

They are sheet-like structures composed of lipids & proteins in varying ratios

Also contains carbohydrates

Question 5

What is the structure of a membrane called?

Answer 5

lipid-bilayer

Composed of two closely touching lipid sheets - very thin 5-8nm

Question 6

What are membrane lipids like?

Answer 6

Amphipathic

Cont hydrophilic & hydrophobic parts

Effects how molecules orientate in an aqueous environment - leads to formation of lipid bilayer

Question 7

What is the most common phospholipid in eukaryotic cell membranes?

Answer 7

Phosphatidyl choline

Question 8

Name 3 types of membrane lipids

Answer 8

• Phosphatidylseriene (a phospholipid)
• Cholesterol (a sterol)
• Galactocerebroside (a glycolipid)

Question 9

What role does cholesterol play in the membrane?

Answer 9

It inserts itself in the membrane and stiffens it

Question 10

What is the structure of a cholesterol molecule?

Answer 10

• Polar head group
• Rigid planar steroid ring structure
• Nonpolar hydrocarbon tail

Question 11

What replaces cholesterol in fungi membranes?

Answer 11

Ergosterol

Question 12

What is the ‘fluid-mosaic’ model?

Answer 12

Contains INTEGRAL proteins penetrating lipid bilayer floating in ‘lipid sea’

Also PERIPHERAL proteins outside lipid bilayer linked by non covalent bonds

Question 13

What are the key structural features of a lipid bilayer?

Answer 13

• Self assembling
• Lipid layers are fluid
• Asymmetry
• Integral proteins pass through the lipid bilayer

Question 14

What does it mean that lipid bilayers are ‘self-assembling’?

Answer 14

They can repair tears in the membrane

Question 15

What does it mean that lipid bilayers are fluid?

Answer 15

Lipids may move 2um in 1 sec!

However, proteins have more restricted movement & some held in fixed positions by cytoskeleton, ‘rafts’ & membrane ‘domains’

Question 16

What does it mean that lipid bilayers are asymmetrical?

Answer 16

Inner & outer lipid layers have different components & lipid composition

Lipids rarely cross monolayers unless under action of ‘scrambles’ or ‘flippase’ enzymes

Question 17

How is the structure of a lipid bilayer linked to membrane functions?

Answer 17

• ‘Semi-permeable’ barrier to movement of solutes/gases
• Transporting solutes
• Flexibility for cell movement & division, membrane fusion
• Respond to external signals
• Intercellular interaction

Question 18

Describe how the ‘semi-permeable’ membrane works?

Answer 18

• Barrier to movement of solute/gases
• Small non-polar molecules (O2, N2, CO2) & lipid soluble pass thru EASILY
• Ions & larger polar molecules (amino acids, sugars) FAIL
• Water fairly rapid diffusion, ASSISTED by ‘aquaporin’ channels

Allows maintenance of inteerior & compartmentalisation

Question 19

Describe how the bilayer’s structure allows for transporting solutes?

Answer 19

Integral proteins provide means of accumulating sugars/amino acids/ions –> act as channels often via active processes

Question 20

Describe how the bilayer’s structure allows for it to respond to external signals?

Answer 20

Protein receptors in membrane respond to external simuli

Question 21

Describe how the bilayer’s structure allows for intercellular interation?

Answer 21

Carbohydrates on membrane surface provide cell identity, used in self recognition

Question 22

How can cells adjust their membrane fluidity?

Answer 22

By adjusting their lipid composition

e.g. cells grown at lower temperature have more lipid with double bonds in tails to maintain fluidity

Question 23

How can the membrane compostiion vary according to its biological function?

Answer 23

Inner mitochondrial membrane has high protein content –> due to transport processes

Whereas nerve myelin sheath has high lipid content –> needed for electrical insulation

Question 24

Why are cells walls sometimes needed?

Answer 24

Plasma membrae is very thin –> often needs additional support/protection

–> cell walls MUST have mechanisms to allow permeability

Question 25

What are bacteria cell walls made from?

Answer 25

Have PEPTIDOGLYCAN cell wall –> conferrs rigidity

Question 26

What are fungi cell walls made from?

Answer 26

Have CHITIN cell walls for strength

Question 27

What are plant cell walls made from?

Answer 27

Have CELLULOSE cell walls

Question 28

What is the cell cortex?

Answer 28

A framework or proteins under membrane called ‘cell cotex’

Attached ti the PM via transmembrane proteins

Question 29

What is the best example of a cell cortex?

Answer 29

Best understood in RBCs in animals - a protein called spectrin forms a meshwork providing support and matitains cell shape

Connected to PM via actin and anchro protiens

Provides elasticity

Question 30

What are 2 examples of extracellular components?

Answer 30

• Glycocalyx
• Extracellular matrix

Question 31

What is the glycocalyx?

Answer 31

An extracellular component

An extensive exterior cell coat based on sugar molecules (the slimy ‘glycocalyx’)

Attched to membrane proteins and/or glycolipids

Question 32

What is the extracellular matrix?

Answer 32

A more complex extracellular component = organised extrcellular material

Question 33

What is the extracellular matrix made up of?

Answer 33

Collagen (most abundant glycoprotein in animal ECM) - forms strong fibres outside of cells

Protects cell surface from mechanical & chemical damage

Also has important role in cell-cell recognition, provides cell ‘clothing’