2.1.5 Biological Membranes

Question 1

Where are membranes found?

Answer 1

• Around prokaryotic and eukaryotic cells
• Around organelles
• Double membranes around some organelles
• Ingestion and secretion from cell

Question 2

Functions of biological membranes at the surface of the cell

Answer 2

• Keep cell contents together and separate from other cells
• Communication between cells by protein receptors for hormones and other signalling molecules
• allows body to determine foreign bodies (antigens)
• Control the movement of substances
• Active transport
• Phagocytosis
• Cell attachment

Question 3

Functions of biological membranes within the cell

Answer 3

• To isolate the nucleus
• To separate organelle components from cell
• Prevent disruptions of pathways and reactions
• Allows attachment of ribosomes
• Internal transport
• Increase the SA

Question 4

What is compartmentalisation and why is it important?

Answer 4

• Containing reactions within cell
• Vital as metabolism includes many different and often incompatible reactions
• allows specific conditions needed for reactions to be contained

Question 5

What makes up the majority of the cell surface membrane

Answer 5

Phospholipid molecules

Question 6

How thick is a phospholipid bilayer

Answer 6

7-10 nm

Question 7

What three formations can the phospholipid membrane form in

Answer 7

• Micelle - individual units are wedge shaped
• Bilayer - Individual units are cylindrical
• Liposome - aqueous cavity inside

Question 8

Why does a phospholipid membrane form in a bilayer

Answer 8

• Phosphate group is charged and therefore interacts with water
• Fatty acid tail is not charged and is hydrophobic

Question 9

Structure of a phospholipid molecule

Answer 9

One molecule of glycerol, 2 fatty acid chains and one phosphate group

Question 10

How do Carbon Carbon double bonds in the fatty acid chains affect the Phospholipid bilayer

Answer 10

Forms a kink in the fatty acid chain which therefore means molecules can pack less tightly and membrane is more fluid

Question 11

What are intrinsic proteins

Answer 11

They have amino acids with hydrophobic R groups on their external surfaces which interact with the hydrophobic core of the membrane - keeping them in place

Question 12

Channel Proteins

Answer 12

• Intrinsic Protein
• hydrophilic channel allows for passive movement of polar molecules and ion down a concentration gradient through membranes
• held in place by the interactions with hydrophobic core

Question 13

Carrier proteins

Answer 13

• Intrinsic protein
• Both passive transport and active transport into cells
• often involves the shape of the protein changing

Question 14

Glycoproteins

Answer 14

• intrinsic protein
• Embedded in the cell surface membrane with attached carbohydrate chains
• They play a role in cell adhesion and receptors for chemical signals

Question 15

Cell signalling

Answer 15

When the chemical binds to the receptor it elicits a response from the cell which may cause a response

Question 16

Receptors for neurotransmitters

Answer 16

Binding to neurotransmitters triggers or prevents and impulse in the next neurone

Question 17

Receptors for peptide hormones

Answer 17

Affect uptake and storage of glucose molecules

Question 18

Glycolipid

Answer 18

• Intrinsic protein
• Lipids attached to carbohydrate chains
• Called cell markers / antigens
• recognised by the immune system

Question 19

Extrinsic proteins

Answer 19

• Present on one side of the bilayer
• Often have hydrophilic R groups on their outer surfaces and interact with the polar heads of the phospholipids or the intrinsic proteins.
• Can be present in either layer or some move between layers

Question 20

Cholesterol

Answer 20

• Extrinsic protein
• One hydrophilic and one hydrophobic end
  Regulates fluidity of membrane
• Positioned between phospholipid molecules in bilayer
• Prevent membranes becoming too fluid by adding stability
• Prevent membranes becoming too solid by stopping phospholipid molecules grouping too closely and crystallising

Question 21

Factors affecting membrane structure

Answer 21

• Temp - higher temp, higher ke, more fluid, permeability increases
• Solvents - Organic solvents will dissolve membranes

Question 22

Definition of Diffusion

Answer 22

The net movement of particles from a region of higher concentration to lower concentration down a concentration gradient until equilibrium is reached

Question 23

Why is diffusion faster when the diffusion distance is smaller

Answer 23

• particles move at high speeds and are constantly colliding which slows overall movement, shorter diffusion distance means less collisions take place

Question 24

What is the link between cells being microscopic and diffusion

Answer 24

• much larger cell would slow rates of diffusion, reactions would not get to substrates they need or ATP quick enough for energy requiring processes

Question 25

factors affecting diffusion

Answer 25

- Temp - higher temp = greater rate of diffusion - Concentration difference - greater difference = greater rate of diffusion - Thickness of membrane - SA of membrane - Size and type of membrane

Question 26

Why can’t hydrophilic molecules diffuse across membranes? What’s an exception to this?

Answer 26

- hydrophilic molecules such as ions and polar molecules due to the hydrophobic centre (hydrophobic molecules can diffuse rapidly across membrane) - Water is the exception to this as it is very small

Question 27

Facilitated diffusion

Answer 27

Diffusion across membranes of hydrophilic molecules via protein molecules - no interruption of Hydrophobic centre

Question 28

Carrier Proteins

Answer 28

- Have a binding site for a specific chemical - When chemical binds, tertiary structure of protein changes bringing chemical across membrane where it is released

Question 29

Protein channels

Answer 29

- Protein with a central pore lined with hydrophilic amino acids and contains water - Hydrophilic substances can pass through

Question 30

Why can protein channels not let all substances through?

Answer 30

- Selective to only some substances - Need a trigger in order to open (chemical binding to protein channel or change in voltage)

Question 31

What is active transport

Answer 31

The movement of molecules/ ions in or out of a cell from a region of lower concentration to an e area of higher concentration - Requires energy supplied by ATP and carrier proteins

Question 32

Process of Active Transport

Answer 32

- On the inside of the cell, ATP binds to the carrier protein whilst the molecule / ion binds to the receptors in the channel of carrier protein on the outside - ATP is hydrolysed into ADP and Phosphate - Binding of the phosphate molecule opens up the inside of the cell releasing the molecule/ ion - Phosphate molecule recombines with ADP to form ATP and carrier protein returns to original shape

Question 33

Bulk transport

Answer 33

- Form of active transport for large molecules too big for channel proteins

Question 34

Bulk transport examples

Answer 34

- Endocytosis - bulk transport into the cell - Pinocytosis - bulk transport of liquids into cell - Exocytosis - bulk transport out of cell

Question 35

Endocytosis

Answer 35

Membrane bends inwards and pinches off to form a vesicle which can then travel through cytoplasm

Question 36

Exocytosis

Answer 36

Vesicle from eg: Golgi body moves and fuses with cell surface membrane which is then released outside of cell

Question 37

What is Osmosis

Answer 37

The net diffusion of water through a partially permeable membrane from a lower concentration of solutes and higher water potential to an area with a higher concentration of solutes and lower water potential passively

Question 38

What is water pressure

Answer 38

The pressure exerted by water molecules as they collide with a membrane or container - measured in Pa or kPa

Question 39

Water pressure of pure water

Answer 39

0kPa - this is the highest possible value and all solutions have a negative water potential

Question 40

Relation of kinetic energy to water potential

Answer 40

The more pure the water and so the higher the water pressure, the higher the kinetic energy in the water

Question 41

Effects of osmosis in Animal cells

Answer 41

haemolysis - When water moves into cell by osmosis: cell swells and bursts Crenation - when water moves out of cell by osmosis: cell shrinks and shrivels

Question 42

Effects of osmosis in plant cells

Answer 42

Turgid: When water flows into a plant cell by osmosis: Hydrostatic pressure increases pushing cell surface membrane up to cell wall Plasmolysis: When water moves out of a plant cell by osmosis: Reduction in the volume of the cytoplasm pulling cell surface membrane away from plant cell wall