2.1.5 Biological Membranes

Question 1

What are the 5 roles of membranes within cells and at the surface of cells?

Answer 1

1. compartmentalisation
2. control what enters/exits cell
3. control what enters/exits organelles
4. cell communication/signalling
5. site of chemical reactions

Question 2

Define compartmentalisation

Answer 2

• formation of separate membrane bound areas in a cell

Question 3

Why is compartmentalisation useful to cells?

Answer 3

• keeps incompatible reactions separate
• maintains necessary chemical conditions for different reactions + protects other organelles

Question 4

Define partially permeable

Answer 4

membrane that allows some substances to cross but not others

Question 5

Define cell signalling

Answer 5

complex systm of intercellular communication

Question 6

Define phospholipid bilayer

Answer 6

• phospholipid bilayer forms membranes
• hydrophilic phosphate heads of phospholipids form inner and outersurface of membrane
• hydrophobic fatty acid tails form hydrophobic core

Question 7

What does ‘fluid’ and ‘mosaic’ in the fluid mosiac model of membrane structure mean?

Answer 7

fluid - phospholipids within the bilayer are free to move
mosaic - proteins embedded in bilayer vary in shape size and position

Question 8

Define glycoprotein

Answer 8

• intrinsic protein embedded in plasma membrane
• have attatched carbohydrate chains

Question 9

What role do glycoproteins have?

Answer 9

• in cell adhesion (cells join together)
• in cell signalling (chemical binds to receptor and sets of cascade of events)
• as a receptor for chemical signals

Question 10

Define phospholipid

Answer 10

• phosphate head with 2 fatty acid tails
• hydrophobic head and hydrophilic tails

Question 11

Define cholesterol

Answer 11

• lipid with hydrophilic end and hydrophobic end
• It regulates membrane fluidity

Question 12

Define glycolipid (and its role)

Answer 12

• lipids with attatched carbohydrate chains
• acts as cell markers/antigens to be recognised by immune system as self/non-self

Question 13

Define channel protein

Answer 13

• intrinsic protein
• provides hydrophobic channel through membrane so polar molecules and ions can diffuse through

Question 14

Define carrier protein

Answer 14

• involved in passive (down conc gradient) and active (against conc gradient) transport
  -active transport often changes shape of protein

Question 15

Define intrinsic protein

Answer 15

proteins embedded through both layers of a membrane

Question 16

Define extrinsic protein

Answer 16

proteins present on one side of the bilayer only

Question 17

Draw a diagram of a phospholipid
on paper flashcard

Answer 17

• hydrophilic phosphate head
• hydrophobic fatty acid tails (2)

Question 18

Draw a diagram of the fluid mosaic model
on paper flashcard

Answer 18

• fluid - phospholipids move in bilayer
• mosaic - proteins different shapes/sizes/positions

Question 19

What is the role of phospholipids?

Answer 19

• form the phospholipid bilayer which forms plasma/ cell surface membrane

Question 20

What is the role of cholesterol?

Answer 20

• regulate membrane fluidity

Question 21

Define cell adhesion

Answer 21

cells join together to form tight junctions in certain tissues

Question 22

How does cholesterol affect the fluidity and stability of membranes?

Answer 22

• cholesterol is positioned between phospholipids
• hydrophilic ends interact with heads and hydrophobic end interact with tails which pull them together
• cholesterol molecules prevent crystalising by stopping phospholipid molecules coming too close

Question 23

What is the importance of membrane bound proteins in chemical reactions?

Answer 23

• have to be held in specific positions for reactions to take place

Question 24

How does the binding of a molecule to a membrane bound glycoprotein cause effects within a cell?

Answer 24

• when chemical binds to receptor (glycoprotein) triggers cascade of events

Question 25

What are the effects of changing temperature on the permeability of cell membranes?
(describe+explain)

Answer 25

increased temp:
- increases permeability of membrane
- phospholipids have more kinetic energy and move more
-makes membrane more fluid and loses structure
- if temp high enough membrane breaks down completely

Question 26

What is the effect of solvent concentration on the permeability of cell membranes?
(describe and explain)

Answer 26

solvents e.g. ethanol dissolve membranes and make them more permeable as structure is lost
- increasing conc of solvent increases membrane permeability
(organic solvents are less polar than water)

Question 27

Describe a method using a colorimeter to investigate the effect of temperature (or ethanol concentration) on the permeability of cell membranes

Answer 27

1) 5 small equal sized pieces of betroot cut and rinse with water
2) 5 betroot placed into test tubes with 5cm3 distilled water
3) label test tubes with 5 temps 10 degrees intervals and place in designated waterbaths
4) leave for 5 mins until test tubes reach waterbath temp
5) remove betroot
6) calibrate colorimeter and use blue filter
7) transfer liquid from test tube into cuvette and into colorimeter
8) record absorbance of solution and repeat for all test tubes
- higher absorbance more pigment released so higher permeability of membrane