2.5 - Cell Membranes

Question 1

Roles of cell surface membranes

Answer 1

• Partially permeable to control which substances enter and leave the cell.
• Site of cell signalling.

Question 2

Roles of membranes within cells

Answer 2

• Forms vesicles to transport substances around cells.
• Provide barrier between cytoplasm and organelles.

Question 3

Describe fluid-mosaic model

Answer 3

• Fluid - components free to move in phospholipid bilayer.
• Mosaic - components such as cholesterol, phospholipids, glycoproteins, glycolipids scattered throughout.

Question 4

How thick is cell membrane?

Answer 4

5-10 nm

Question 5

List components of cell surface membrane

Answer 5

• Proteins - peripheral, integral, glycoprotein
• Phospholipids
• Cholesterol (animal cells)

Question 6

Why is phosphate head hydrophilic?

Answer 6

It is polar - faces outwards

Question 7

Why are fatty acid tails hydrophobic?

Answer 7

They are non-polar

Question 8

Role of cholesterol

Answer 8

• Restricts movement of phospholipids, which prevents membrane being too fluid when warm.
• Regulates fluidity, so prevents membrane becoming too solid when cold.

Question 9

Function of membrane proteins

Answer 9

• Receptors
• Channels for facilitated diffusion
• Pumps for active transport

Question 10

Role of phospholipid bilayer

Answer 10

• Allows lipid soluble substances to enter/leave cell.
• Doesn’t allow water soluble/polar substances to enter/leave cell.

Question 11

Role of glycoLIPIDS

Answer 11

• Act as recognition sites.
• Adhesion - helps cells attach to each other to form tissues.

Question 12

Role of glycoPROTEINS

Answer 12

• Act as recognition sites
• Receptors for cell signalling
• Adhesion - helps cells stick together to form tissues.
• Act as antigens - so cell can be detected

Question 13

Cell signalling process

Answer 13

• Signal molecule released by exocytosis.
• Glycoproteins act as receptors
• Receptors are specific site for signal molecules - Shape of receptor and signal molecule are complementary.
• Attachment of signal molecule to receptor causes change inside cell.
• Cell surface membrane allows entry of some lipid-soluble signal molecules.

Question 14

Effects of high temperature on membrane structure

Answer 14

• KE of phospholipids increase - Phospholipids move more.
• Bilayer becomes more fluid and begins to lose structure - Increases number of gaps in membrane (becomes more permeable).
• Proteins in membrane could denature.

Question 15

Effects of organic solvent on membrane

Answer 15

Organic solvents dissolve membranes and can disrupt and break down cells

Question 16

What investigation looks into the effects of temperature on membrane permeability?

Answer 16

Beetroot PAG

Question 17

How could beetroot PAG investigate the effects of different solvents on membrane permeability?

Answer 17

Keep temp constant.

Use range of organic solvents

Question 18

Diffusion definition

Answer 18

Net movement
High to low conc
Down concentration gradient

Question 19

Why do some substances diffuse through cell membrane at different rate?

Answer 19

centre of cell membrane is hydrophobic due to fatty acid tails.

Question 20

What type of substances can diffuse rapidly across membrane?

Answer 20

Hydrophobic substances e.g. steroid hormones

Question 21

What type of molecules cannot diffuse across membrane?

Answer 21

Hydrophilic molecules e.g. polar molecules, ions

Question 22

Why can water diffuse across membrane, even though it’s polar?

Answer 22

It’s a very small molecule

Question 23

How do polar molecules and ions pass through membrane?

Answer 23

Facilitated diffusion

Question 24

What type of proteins are involved in facilitated diffusion?

Answer 24

Carrier & channel proteins

Both intrinsic proteins

Question 25

What do carrier proteins do?

Answer 25

Have binding site for specific chemical. When chemical binds, causes tertiary structure of carrier protein to change. This change brings chemical across membrane, where it is released.

Question 26

What do channel proteins do?

Answer 26

Protein with central pore. Central pore lined with hydrophilic amino acids and contains water. Hydrophilic substances can pass through one side of membrane to the other. They are selective about type of chemical that can pass through. Some types are always open, some only open when triggered e.g. type of chemical binds to channel i.e. neurotransmitter.

Question 27

Describe facilitated diffusion

Answer 27

Net movement High to low concentration Through protein channels or using carrier proteins that change shape when molecules bind.

Question 28

Name of protein channels for water

Answer 28

Aquaporins

Question 29

Define water potential

Answer 29

Pressure exerted by molecules as they collide with a membrane or container. Measured in kPa

Question 30

Define osmosis

Answer 30

Net movement of water molecules From area of high to low water potential Across partially permeable membrane

Question 31

Effect of putting animal tissue in conc solution

Answer 31

Solution has higher water potential than tissue. Water moves out via osmosis. Cell becomes crenated (wrinkled)

Question 32

Effect of putting plant tissue in conc solution

Answer 32

Solution has lower water potential than plant cells. Water moves out of cell via osmosis. Cells become flaccid Cells become more plasmolysed

Question 33

Effects of putting animal cells in dilute solution

Answer 33

Water moves into animal cells via osmosis | Since no cell wall, cells swell and burst

Question 34

Effect of putting plant tissue in dilute solution

Answer 34

Water moves into cells via osmosis. | Since cell wall is strong, cell becomes turgid

Question 35

Define active transport

Answer 35

Net movement Low to high concentration Requires energy from ATP

Question 36

Explain how fluidity of cell membrane can be used to transport substances

Answer 36

By endocytosis and exocytosis

Question 37

What is endocytosis?

Answer 37

Plasma membrane encloses solid particles. Membrane pinches off (requires ATP). Vesicles formed. Vesicle breaks away from plasma membrane and is free to move inside cell.

Question 38

What is exocytosis?

Answer 38

Vesicles move to plasma membrane (requires ATP) Vesicles fuse with plasma membrane. Vesicles release contents to outside of the cell. Membrane returns to original shape.

Question 39

How are vesicles moved around in cell?

Answer 39

Microtubules of cytoskeleton. Provide pathways for movement. Vesicles along microtubules. Process requires ATP

Question 40

How do larger substances enter and leave cell?

Answer 40

Endocytosis and exocytosis (bulk transport)

Question 41

How do polar substances enter cell?

Answer 41

Facilitated diffusion through channel proteins and carrier proteins Can also be active transport through carrier proteins.