2.1.5 Biological Membranes

Question 1

What model and why is the plasma membrane described as?

Answer 1

Fluid mosaic model
- ‘Fluid’ = phospholipids are constantly moving around
- ‘Mosaic’ = proteins are scattered throughout
- ‘Model’ = best representaion based on evidence available - may be updated if new information is discovered

Question 2

What components make up the plasma membrane?

Answer 2

• phospholipids
• glycoproteins
• glycolipids
• cholestrol
• intrinsic proteins
• extrinsic proteins

Question 3

Describe the structure and function of phospholipids

Answer 3

• form a bilayer - hydrophobic fatty acid tails face inwards, hydrophilic phopshate heads face outwards
• allows lipid-soluble and non polar substances to enter + leave the cell
• prevents large molecules, water-soluble molecules, and polar substances from entering + leaving the cell

Question 4

Describe the structure and function of glycoproteins

Answer 4

• proteins with sugar molecules attached
• cell adhesion + receptors for cell signalling (neurotransmitters and hormones)

Question 5

Describe the structure and function of glycolipids

Answer 5

• phospholipids with sugar molecules attached
• act as recognition sites and antigens - allows the body to detect which cells are self or non-self

Question 6

Describe the structure and function of cholesterol

Answer 6

• lipid which slots in between the phospholipid tails
• increases packing of the membrane - regulating stability + fluidity

Question 7

Describe the structure and function of intrinsic proteins

Answer 7

• proteins which span both bilayers of the membrane
• act as channels or carrier proteins & transport water-soluble molecules

Question 8

Describe the structure and function of extrinsic proteins

Answer 8

• proteins found on the surface of the membrane
• examples are enzymes

Question 9

Describe the structure and function of channel and carrier proteins

Answer 9

Channel Proteins
- transport of ions by facilitated diffusion
- act as “pores” in the membrane + allow specific ions through
- Can be gated so they only open/close in specific circumstances

Carrier Proteins
- facilitated diffusion of large molecules
- only allow specific large molecule to pass through
- changes shape and transfers the molecule to the other side of the membrane
- active transport of water soluble molecules and charged ions using ATP

Question 10

What is the thickness of the plasma membrane?

Answer 10

7nm

Question 11

Jobs of membranes outside + inside cells

Answer 11

Inside
- produce different compartmenmts in cells
- sites of chemical reactions
- provide attachment sites for enzymes
- provide attachment sites for pigments
- form vesicles for the transport of proteins
- allow cellular compartments to have different conditions
Outside
- seperates cell contents from the external environment
- cell signalling
- cell rocignition

Question 12

Define cell signalling

Answer 12

Processes that lead to communication and coordination between cells, so that they can work together to trigger a response

Question 13

Explain cell signalling

Answer 13

• Receptor in plasma membrane picks up signals - in form of a hormone or chemical mediator binding to the receptor
• signals secreted from cell and travel in blood
• binding brings about actions within cell

Question 14

Explain key aspects of hormone receptors

Answer 14

• protein and glycoprotein receptors stick out target cells
• specific shape
• shape is complementory to shape of specific signalling molecule
• hormone binds to receptor - cell responds in particular way

Question 15

What does cell signalling allow?

Answer 15

• communication between cells
• cells to coordinate actions
• trigger reponses inside cells
• cell recognition+ identification

Question 16

Explain cell signalling in the context of controlling high blood glucose levels

Answer 16

• insulin hormoner releasedwhen blood sugar increases
• insulin receptors on liver + muscle cells bind insulin - cells take in more glucose to reduce blood glucose concentration
• insulin receptors have specific shape - complementory to shape of insulin molecule

Question 17

Define and explain key aspects of diffusion

Answer 17

The net passive movement of molecules down a concentration gradient from a higher to lower concentration
- passive - doesn’t require ATP
- will continue until equilibrium between two areas
-faster over short distances - slower over long
- happens in gases + liquids - particles have kinetic energy

Question 18

What molecules diffuse accross the phospholipid bilayer?

Answer 18

• lipid soluble e.g. steroid hormones
• small + non polar e.g. CO2, O2
• small + polar e.g. H20

Question 19

What factors affect the rate of diffusion and why?

Answer 19

• temperature = more kinetic energy
• concentration gradient = steeper - faster rate of diffusion
• stirring motion = more kinetic energy
• surface area = more space for molecules to move accross
• diffusion distance - smaller distance to move
• size of molecule - larger molecules diffuse smaller than smaller ones

Question 20

Why can’t large or polar membranes, as well as charged ions cross a membrane?

Answer 20

Can’t pass througfh phospholipid bilayer - require carrier + channel proteins for facilitated diffusion

Question 21

Key aspects of facilitated diffusion

Answer 21

• there is a concentration gradient
• no ATP required
• protein + carrier proteins present
• polar molecules + ions have kinetic energy

Question 22

Explain facilitated diffusion in channel + carrier proteins

Answer 22

Channel Proteins
- act as pores
- some only allow specific ions through e.g. Na+ Ca2+
- can be gated

Carrier Proteins
- only allow specific large molecules through e.g. glucose
- when molecule binds, it changes shape and transfers it to the other side

Question 23

Define and explain key aspects of active transport

Answer 23

The movement of molecules or ions against a concentration gradient, from a lower to high concentration. using ATP and a transport/carrier protein
- ATP required
- only carrier proteins required
- moves substances against concentration gradient
- selective process

Question 24

How is one way flow ensured in active transport?

Answer 24

• ATP changes shape of carrier proteins
• energy changes shape and releases molecule

Question 25

Define and explain key aspects of bulk transport using endocytosis

Answer 25

The bulk movement of substances into the cell with ATP. Substances have part of the membrane surrounding them, which pinces off to form a vesicle inside the cell
- into the cell
- cell surface membrane bends inwards
- membrane engulfs material
- membranes fuse together + vesicle formed
- ATP required

Question 26

Define and explain key aspects of bulk transport using exocytosis

Answer 26

The bulk movement of substances out of the cell with ATP. Inside the cell, vesicles containing substances fuse with the plasma membrane releasing material outside the cell.
- out of the cell
- vesicle fuses with the membrane
- contents of vesicle released outside the cell
- vesicle usually formed by golgi apparatus
- ATP required

Question 27

Names for when solids vs lipids transported

Answer 27

solids = phagocytosis
liquids = pinocytosis

Question 28

Define and explain key aspects of bulk water potential

Answer 28

A measure of the tendency of water molecules to diffuse from one place to another
- uses the units kilo Pascals kPa - ψ
- pure water has a water potential of 0kPa
- the more solute dissolved the lower the water potential - less free water molecules to move
- solutions always have negative water potential

Question 29

Define and explain key aspects of osmosis

Answer 29

The net movement of water molecules accross a partially-permeable membrane from a less negative water potential to a more negative water potential down a water potential gradient

Question 30

Explain hypotonic, hypertonic and isotonic solutions

Answer 30

• isotonic = the solution surrounding the cell has the same water potential as the cell
• hypertonic = the solution surrounding the cell has a lower water potential than the cell (e.g. concentrated sugar solution)
• hypotonic= the solution surrounding the cell has a higher water potential than the cell (e.g. pure water)

Question 31

List factors that affect the fluidity of the plasma membrane

Answer 31

• temperature
• presence of alcohol
• number of cholesterol molecules = more cholesterol between fatty acid chains means membrane becomes less fluid, vise versa
• short chains in fatty acids = shorter chains = more fluid membrane
• number of double bonds between carbon atoms in fatty acid chains - more kinky fatty acid - can’t pack as closely together