B4 - Transport across cell membranes

Question 1

What do membranes do?

Answer 1

Separate different areas
Control the exchange of material across them
Acts as an interface for communication
Are partially permeable
Use diffusion, osmosis & active transport to move substances across them
Formed from a bilayer of phospholipids

Question 2

What happens when phospholipids mix with water?

Answer 2

They form a micelle
The hydrophilic phosphate heads facing out towards the water
The hydrophobic fatty acid tails facing in towards each other

Question 3

What is an example of a 2 membrane organelle?

Answer 3

Mitochodria

Question 4

What is an example of a 1 membrane organelle?

Answer 4

Lysosome

Question 5

What is an example of a 0 membrane organelle?

Answer 5

Ribosomes

Question 6

Why are cell membranes fluid?

Answer 6

Phospholipids and proteins move around (mostly sideways) via diffusion

Question 7

Why are cell membranes mosaic?

Answer 7

Because of the patterns produced by the distribution of proteins

Question 8

What is an extrinsic protein?

Answer 8

A protein that spans 1 layer

Question 9

What is an intrinsic protein?

Answer 9

A protein that spans 2 layers

Question 10

What is glycoprotein?

Answer 10

Protein with carbohydrate chains attached, that project outwards into the extracellular space

Question 11

What is a glycolipid?

Answer 11

Lipids with carbohydrate chains attached

Question 12

What is the role of cholesterol in a bilayer?

Answer 12

Regulates the fluidity of the membrane by sitting between phosphlipids
Prevents them from packing too closely together
Increase strength and stability

Question 13

When are membranes less fluid?

Answer 13

When there are more saturated fatty acids chain
When there are lower temperatures

Question 14

When are membranes more fluid?

Answer 14

More unsaturated fatty acids chain
When there are higher temperatures

Question 15

What is the role of glycoproteins and glycolipids?

Answer 15

Act as receptor molecules, by binding to substances at the cell’s surface

Question 16

What is the role of transport proteins?

Answer 16

They create hydrophilic channels, which allow ions & polar molecules to pass through

Question 17

What are the 2 types of transport proteins?

Answer 17

Channel proteins
Carrier proteins

Question 18

What is the definition of diffusion?

Answer 18

The net movement of a substances from a region of its higher concentration to a region of its lower concentration

Question 19

How does the presence of carrier / channel proteins affect the rate of diffusion?

Answer 19

More carrier/channels proteins present in a membrane, faster rate of diffusion (until all proteins are full)

Question 20

What types of substances cannot diffuse through the phospholipid bilayer of cell membranes?

Answer 20

Large polar molecules such as glucose and amino acids
Ions such as sodium ions (Na+) and chloride ions (Cl-)

Question 21

How do substances (that cannot naturally) cross the phospholipid bilayer?

Answer 21

With the help of highly specific proteins through facilitated diffusion

Question 22

How do channel proteins work?
(faciliated diffusion)

Answer 22

Allow charged substances (e.g., ions) to diffuse through the cell membrane
Most are gated - they have to open and close to let ions through, therefore controlling their exchange

Question 23

How do carrier proteins work?
(facilitated diffusion)

Answer 23

They can switch between two shapes:
Molecule/ion binds to carrier protein on one side
Conformational change within the protein
Protein opens up on the other side, letting the molecule/ion through

Question 24

What is the definition of osmosis?

Answer 24

Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane

Question 25

What is water potential?

Answer 25

Tendency of water to move out of a solution

Question 26

What happens when a plant is placed in a dilute solution?

Answer 26

Water enters the plant cells’ cytoplasm & vacuole by osmosis (high to low w.p.) → volume of cell increases which increases pressure → cells become turgid
Turgidity provides support and strength for a plant - e.g., allows them to stand upright

Question 27

What happens when a plant is placed in a concentrated solution?

Answer 27

Water leaves the plant cells’ cytoplasm & vacuole by osmosis (high to low w.p.) → volume of cell decreases, which decreases pressure → cell eventually becomes plasmolysed

Question 28

Why do animal cells feel the effect of water more severely?

Answer 28

As they don’t have a cell wall

Question 29

What is a hypertonic solution?

Answer 29

When the outside solution is more concentrated with solutes, so water leaves the cell, leaving the cell shrivelled

Question 30

What is a hypotonic solution?

Answer 30

When the outside solution is more diluted with water, so water enters the cell, causing it to swell and eventually lysing (bursting)

Question 31

What is an isotonic solution?

Answer 31

When the outside solution & cytoplasm a similar solute concentration so the cells remain normal

Question 32

What is active transport?

Answer 32

The movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration, also requiring specific carrier proteins

Question 33

What is co-transport?

Answer 33

The coupled movement of substances across a cell membrane via a carrier protein (basically facilitated diffusion & active transport combined)

Question 34

What is the process of co-transport?

Answer 34

1. Na+ & glucose molecules are transported into epithelial cells via f. Diffusion
2. Na+ is then actively transported out of the cell, into the blood (which helps maintain a conc. Gradient for sodium
3. Glucose exits the cell and enters the blood, again via f. diffusion