Module 2.5 - Biological Membranes

Question 1

Role of membranes

Answer 1

Separate cell components from cytoplasm
Separate cell contents from outside environment
Hold components of some metabolic pathways in place
Cell recognition and signalling
Regulate transport of materials in and out of cells

Question 2

Describe the structure of phospholipids

Answer 2

Hydrophilic phosphate head

2 fatty acid/lipid hydrophobic tails

Question 3

Why is the phospholipid head hydrophilic?

Answer 3

Charges are unevenly distributed so it can interact with water more easily

Question 4

Why are the phospholipid tails hydrophobic?

Answer 4

Charges are evenly distributed so will repel water molecules

Question 5

Why is the membrane fluid?

Answer 5

Lack of bonds between phospholipids means they can slide around each other, like a fluid

Question 6

Why is the membrane mosaic?

Answer 6

Embedded proteins create a mosaic pattern

Question 7

Thickness of phospholipid bilayers

Answer 7

7-10nm thick

Question 8

How are membranes partially permeable?

Answer 8

Water diffuses through the bilayer

Some membranes contain aquaporins (protein channels which allow water through) which make them more water permeable

Question 9

Functions of glycoproteins

Answer 9

Cell signalling (allow recognition from immune system)
Receptors for hormones/drugs
On surface of pathogens are antigens, immune system can recognise them as foreign
Bind cells together in tissues

Question 10

Functions of glycolipids

Answer 10

Cell signalling (allow recognition from immune system)
Receptors for hormones/drugs
On surface of pathogens are antigens, immune system can recognise them as foreign

Question 11

Functions of intrinsic proteins

Answer 11

Transport for small water-soluble molecules/ions and larger molecules

Question 12

Functions of cholesterol

Answer 12

Stability
Plugs gaps between fatty acid tails and makes membrane less permeable to water molecules and ions
Restricts too much movement within phospholipid layer

Question 13

Function of actin microfilaments

Answer 13

Anchors proteins and stops them moving around too much

Question 14

Effect of high temperature on membranes

Answer 14

More kinetic energy for molecules so they move faster

Membrane becomes more fluid so more permeable and more substances can enter and exit the cell

Question 15

How do hormones bind to cells?

Answer 15

Hormone receptors
Cells with these specific hormone receptors are target cells
Hormone molecules bind to receptors on target cell surface membrane as they are complementary in shape
Binding stimulates a response in the target cell

Question 16

How can medicinal drugs work?

Answer 16

Can be made complementary to receptor molecules so they block them
Some act like natural neurotransmitters that some people can’t make
Beta-blockers stop increasing heart rate for people at risk of a heart attack

Question 17

How do poisons work?

Answer 17

Bind with receptors

Blocks effect of natural neurotransmitters or hormones

Question 18

Substances that can simply diffuse across a membrane

Answer 18

Lipid-based molecules (fat soluble so dissolve and diffuse readily across the bilayer e.g. steroid hormones)
Very small molecules (O2 and CO2 are small enough to pass between phospholipid molecules, water and urea are polar so they cross much slower)

Question 19

Main points about channel proteins

Answer 19

Form pores in membranes
Hydrophilic inside pore
Specific to certain small water-soluble molecules or ions (e.g. sodium, calcium)
Can be gated, so can be opened or closed by a signal or voltage change across the membrane

Question 20

Main points about carrier proteins

Answer 20

Carry larger molecules (e.g. glucose, amino acids)
Shaped for specific molecules to bind to
Molecule binding changes shape of protein
Causes molecule to be carried to and released on other side of membrane
No energy required

Question 21

Examples of active transport

Answer 21

Mineral ions from soil t root

Glucose and amino acids from intestines to blood

Question 22

How does active transport work?

Answer 22

Carrier proteins act as pumps
Complementary shaped molecule binds
Molecule can’t diffuse due to gradient
ATP provides energy needed for the protein to change shape and transport the molecule across the membrane
Shape change only allows the molecule to fit in the protein on one side of the membrane - won’t fit other side - stopping transport in wrong direction
Faster than diffusion

Question 23

Phago

Answer 23

Solids

Question 24

Pino

Answer 24

Liquids

Question 25

Process of exocytosis

Answer 25

Vesicle moves towards cell surface membrane
On microtubules
Uses ATP
Vehicle fuses with plasma membrane - requires ATP
Molecules ejected from the cell via exocytosis

Question 26

Process of endocytosis

Answer 26

Molecule binds to receptor
Causes cell surface to invaginate (fold in)
Requires ATP
Plasma membrane fuses with itself
Forms a vesicle
Vesicle moves through cytoplasm to where it’s needed in the cell

Question 27

Cells in higher water potential

Answer 27

Water moves into cell by osmosis
Cells swell
Animal plasma membrane will burst (cell is haemolysed)
Plant cell cytoplasm and vacuole push membrane against cell wall and osmosis will stop regardless of water potential gradient (cell is turgid)

Question 28

Cells in more negative water potential

Answer 28

Water moves out of cells by osmosis
Animal cells shrink and membrane wrinkles (cell is crenated)
Plant cells’ cytoplasms and vacuoles shrink and plasma membrane pulls away from cell wall (cell is plasmolysed)

Question 29

Point where the membrane is just about to pull away from the cell wall

Answer 29

Incipient plasmolysis

Question 30

Effect of long, saturated lipids on membrane

Answer 30

High melting point so decrease fluidity

Straight fatty acid tails mean that the lipids can pack together more tightly

Question 31

Effect of short, unsaturated lipids on membrane

Answer 31

Low melting point, so increases fluidity

Double bonds in unsaturated fatty acids create kinks so the molecules are less tightly packed