Cell Membranes And Membrane Transport 2.2-2.4 and CP3

Question 1

Describe the fluid mosaic model

Answer 1

• Phospholipid belayer is constantly moving, therefore it’s fluid
• Protein molecules are scattered through the bilayer , like tiles in a mosaic, which can also move

Question 2

Describe a phospholipid molecule

Answer 2

• Phosphate head is hydrophilic so attracts water

- Fatty acid (x2) tail is hydrophobic

Question 3

How is the bilayer orientated and what does this prevent?

Answer 3

• The hydrophilic heads face outwards towards the water

- The hydrophobic tails face the centre which prevents water-soluble substances, such as ions, from passing through

Question 4

What does it mean that the membrane is ‘partially permeable’?

Answer 4

• Small molecules move through gaps between the phospholipids
• Large molecules and ions can only pass through channel proteins and carrier proteins

Question 5

What did scientists think of the cell membrane pre-1970s?

Answer 5

Originally thought there were two protein layers with a phospholipid layer in the middle = 3 layers

Question 6

What experiment did scientists do to prove the model is ‘fluid’?

Answer 6

Fused a mouse cell with a human cell = membrane proteins of both organisms had intermixed throughout the cell membrane

Question 7

What types of molecule are found in a membrane?

Answer 7

• Glycoproteins
• Glycolipids
• Cholesterol

Question 8

What are glycoproteins?

Answer 8

Proteins with a polysaccharide chain

Question 9

What are glycolipids?

Answer 9

Lipids with a polysaccharide chain attached

Question 10

Where does cholesterol attach? What does it provide?

Answer 10

Fits in between the phospholipids to form bonds with them
It allows the membrane to be more rigid
At lower temperatures cholesterol makes the membrane more fluid

Question 11

What is osmosis?

Answer 11

The net movement of water molecules across a partially permeable membrane from an area of high water potential to low water potential.
(Water molecules diffuse both ways)

Question 12

What kind of process is osmosis, diffusion and facilitated diffusion?

Answer 12

Passive process

Question 13

When would facilitated diffusion be used?

Answer 13

When large molecules or ions that can’t diffuse through the phospholipid bilayer want to get across the membrane

Question 14

What is facilitated diffusion?

Answer 14

The movement of molecules down a concentration gradient using carrier or channel proteins and not requiring energy

Question 15

What size molecules do carrier proteins transport?

Answer 15

Large molecules

Question 16

How do carrier proteins work in facilitated diffusion?

Answer 16

1) A large molecules attached to the carrier protein
2) The protein changes shape
3) The molecules is release on the opposite side of the membrane

Question 17

What type of molecules do channel proteins allow through?

Answer 17

Charged particles

Question 18

How do channel proteins work in facilitated diffusion?

Answer 18

They form pores in the membrane

Question 19

What is active transport?

Answer 19

Uses energy and carrier proteins to move molecules and ions against a concentration gradient

Question 20

What’s the difference between active transport and facilitated diffusion?

Answer 20

• Active transport requires energy from the hydrolysis of ATP
• Active transport only uses carrier proteins, not channel proteins

Question 21

What size of molecules are transported by endocytosis?

Answer 21

Very large molecules eg. Proteins , lipids and carbohydrates

Question 22

How does endocytosis work?

Answer 22

• Cell membrane surrounds the molecule(s)

- The membrane pinched off to form a vesicle containing the ingested substance

Question 23

Do endocytosis and exocytosis require energy?

Answer 23

Yes, through the hydrolysis of ATP

Question 24

Give an example of a cell that uses endocytosis

Answer 24

White blood cell (phagocytes) inject microorganisms and dead cells

Question 25

What type of substances are removed by exocytosis?

Answer 25

Digestive enzymes, hormones, lipids

Question 26

How does exocytosis work?

Answer 26

• Vesicles which contain the substance pinch off from the sacs of Golgi apparatus and move towards cell membrane
• Vesicle fuses with csm and release contents out of cell

Question 27

Describe an experiment to investigate how temperature affects beetroot membrane permeability:

Answer 27

1) use a scalpel to cut 5 equal sized pieces of beetroot and rinse to remove any pigment released during cutting
2) Use a pipette to measure 5cm ³ of water into 5 different test tubes
3) Place the test tubes into water baths at different temperatures (10, 20, 30, 40, 50) and leave for 5 mins to acclimatise
4) Place the pieces of beetroot into the test tubes and record for a period of time
5) Remove pieces of beetroot from tubes to leave behind coloured liquid
6) Measure absorbency of coloured liquid with a colorimeter (see separate card)

Question 28

How does a colorimeter work?

Answer 28

Machine passes light of a specific wavelength through a liquid and measures how much light is absorbed

Question 29

Describe the steps in taking a colorimeter reading?

Answer 29

1) Switch on and allow colorimeter to stabilise for 5 mins
2) Select blue filter (wavelength 470nm) and use pure water to calibrate machine to zero
3) Use a pipette to transfer sample of beetroot liquid to cuvette until it’s three-quarters full
4) Put cuvette in colorimeter and record absorbance
5) Repeat for remaining 4 test tubes

Question 30

What’s the relationship between the absorbance and permeability?

Answer 30

The higher the absorbance the less light is passing through the solution which means more pigment has been released so the higher the permeability of the membrane

Question 31

Describe membrane permeability at temperatures below 0 degrees Celsius:

Answer 31

• Phospholipids have limited energy so can’t move as much
• Membrane is rigid as packed close together
• HOWEVER Channel proteins and carrier proteins in membrane deform increasing the permeability
• Ice crystals pierce membrane making it highly permeable when it thaws

Question 32

Describe membrane permeability at temperatures between 0-45 degrees Celsius:

Answer 32

• Phospholipids can move around and aren’t as tightly packed together (partially permeable)
• As temp increases the phospholipids move as they have more energy which increases permeability

Question 33

Describe membrane permeability at temperatures above 45 degrees Celsius:

Answer 33

• Phospholipid bilayer starts to melt and membrane becomes more permeable
• Water inside the cell expands putting pressure on the membrane
• Channel and carrier proteins deform so can’t control what enters or leaves the cell

Question 34

Describe relationship between alcohol concentration and membrane permeability + why:

Answer 34

As alcohol concentration increases, the permeability go through cell membrane increases because alcohol dissolves the lipids in the cell membrane so loses its structure