2.3 Transport Across Membranes

Question 1

Role of membrane in rough ER?

Answer 1

Holds ribosomes in place
Forms vesicles to separate protein products from cytoplasm

Question 2

Role of membrane in mitochondria?

Answer 2

Holds protein/ enzymes in place required for aerobic respiration
Organelle - inner membranes folded into cristae

Question 3

Role of membrane in lysosome?

Answer 3

Separates hydrolytic enzymes from the cytoplasm (preventing autolysis)
Organelle - vesicle containing hydrolytic enzymes

Question 4

Role of membrane in nucleus?

Answer 4

Organelle - nuclear pores & nuclear envelope
Control the movement of molecules into and out the nucleus

Question 5

What are membranes needed for?

Answer 5

Control entry and exit of substances
Contain cell contents and organelles
Compartmentalise individual metabolic processes
Communication with external environment
Site of chemical reactions
Allows cell to change shape
Transport and packaging within a cell
Isolate enzyme that might damage the cell (lysosomes)

Question 6

What is a phospholipid made up of?

Answer 6

2 fatty acids + glycerol + phosphate
Hydrophilic head + hydrophobic tail

Question 7

Function of phospholipid?

Answer 7

Allow lipid soluble substances to cross the membrane
Prevent water soluble substances crossing
Fluid layer that allows flexibility

Question 8

Phospholipid properties?

Answer 8

1) micelles - phospholipids in water, hydrophilic heads attracted, hydrophobic tails repelled so sphere with head pointing out and tails inside

2) liposome - water inside and outside structure, spherical belayer, forms with hydrophilic heads facing outwards and hydrophobic tails inwards

3) bilayer sheet - cytoplasm contains lots of water, tissue fluid surrounding the cell is also mostly water, hydrophilic heads attracted to both and hydrophobic tails repelled by both so bilayer forms

Question 9

Cholesterol?

Answer 9

Fatty substance
Hydrophobic so pulls together phospholipid tails
Reduces fluidity of membrane (particularly at high temperatures)
Prevents leakage of molecules
Adds strength and some rigidity to membrane (increases stability)

Question 10

Intrinsic proteins?

Answer 10

Span the membrane and act as a channel and carrier proteins

Question 11

Extrinsic ?

Answer 11

On the surface of the membrane
Acts as receptor or enzymes
May have carbohydrate bound to them

Question 12

Glycoproteins?

Answer 12

Carbohydrates bonded to protein
Recognition
Attach to other cells to form tissues
Cell surface receptor (hormones + neurotransmitters)

Question 13

Glycolipids?

Answer 13

Carbohydrates bonded to lipids
Carbohydrate part extends into extra cellular fluid
Cell surface receptor (i.e. blood group)
Maintains stability of membrane
Attach to other cells to form tissues

Question 14

Define diffusion

Answer 14

The net movement of molecules or ions from an area of high concentration to an area of low concentration, down its concentration gradient

Question 15

Does diffusion require energy? What type of process is it?

Answer 15

Does not require energy (other than heat)
Passive process

Question 16

What is diffusion caused by?

Answer 16

Random movement of particles due to kinetic energy

Question 17

What are the 4 factors that affect diffusion?

Answer 17

Temperature
Concentration gradient
Surface area
Distance

Question 18

What is Fick’s law? What do we assume?

Answer 18

Rate of diffusion is proportional to surface area x conc gradient / distance
Assuming temp remains constant

Question 19

What molecules are affected by facilitated diffusion?

Answer 19

Larger hydrophilic molecules e.g. glucose, amino acids, ions
Repelled by hydrophobic tails of phospholipids
Pass through intrinsic proteins instead

Question 20

Protein channels?

Answer 20

Water filled hydrophilic channels, allow water soluble ions to pass through

Question 21

Carrier proteins?

Answer 21

Specific molecules (i.e. glucose) bind to protein, change shape, no external energy required

Question 22

What type of molecules does simple diffusion diffuse?

Answer 22

Small, lipid soluble, unchanged molecule
E.g. O2, CO2

Question 23

What type of molecules does facilitated diffusion diffuse?

Answer 23

Channel proteins: small, polar molecules + ions
E.g. Na+, H+
Carrier proteins: larger, lipid insoluble molecules (water soluble)
E.g. glucose, amino acids

Question 24

How does bilayer formed by phospholipid affect entry of substances in and out of a cell?

Answer 24

Allows movement of lipid soluble/ non polar molecules
Prevents movements of water soluble/ polar
Pass through partially permeable membrane

Question 25

Define osmosis

Answer 25

The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a partially permeable membrane

Question 26

How can osmosis be quantified?

Answer 26

By using water potential - measured in kPa
100% pure water has water potential = 0
All solitons have water potential < 0
Cannot get water potential > 0

Question 27

Hypotonic ?

Answer 27

Solute conc outside cell is lower
Water potential outside cell is higher
Net movement of water - in to cell

Question 28

Isotonic?

Answer 28

Solute conc outside cell is the same
Water potential outside cell is the dame
Net movement of water - no net movement of water

Question 29

Hypertonic?

Answer 29

Solute conc outside cell is higher
Water potential outside cell is lower
Net movement of water - out of cell

Question 30

What happens in a hypertonic environment in plants?

Answer 30

Vacuole lose water, cytoplasm shrinks (plasmolysis)
Chloroplasts are seen in the centre of the cell

Question 31

What happens in a hypotonic environment in plants?

Answer 31

Vacuoles fill with water
Turgor pressure develops
Chloroplasts are seen next to cell wall

Question 32

What happens in a hypotonic environment? (Not in plants)

Answer 32

Water enters the cell which may burst (lysis)

Question 33

What happens in a hypertonic environment? (Not in plants)

Answer 33

Water leaves the cell which shrives (crenation)

Question 34

Define active transport

Answer 34

The movement of a substance against the concentration gradient across a membrane by a protein carrier
Requires energy (supplied by ATP)
Proteins are highly specific

Question 35

Two types of bulk transport?

Answer 35

Endocytosis (out to in)
Exocytosis (in to out)
Active process (requires energy)

Question 36

Exocytosis of chylomicrons?

Answer 36

Chylomicrons are packaged into vesicles
Vesicles fuse with the cell surface membrane
Chylomicrons are released out of the cell

Question 37

Co-transport?

Answer 37

Carrier proteins can sometimes transport two particles at once
Both must be present for transport to take place
Active and passive process as one particle is moved with the concentration gradient and the other is moved against it
Substances can be moved in the same (symport) or opposite (antiport) direction

Question 38

Factors that affect active transport?

Answer 38

Surface area
ATP availability
Number of proteins present

Question 39

What does absorption in the gut require ?

Answer 39

Facilitated diffusion
Active transport
Co transport

Question 40

Epithelial cells

Answer 40

Microvilli (+ whole surface folded into villi) - increases SA
Many transport proteins in cell membrane - transport digested food products
Many RER, Golgi + mitochondria - synthesise transport proteins
Close proximity to capillaries - short distance for diffusion
Mitochondria - makes ATP for active transport

Question 41

Absorption: active transport

Answer 41

Sodium/potassium pump moved Na+ from epithelial cell into blood and K+ from blood into epithelial cell - this lowers the concentration of Na+ in cell

Question 42

Absorption: co transport

Answer 42

Low concentration of Na+ in cell allows Na+ / glucose cotransporter to move Na+ from lumen into cell by diffusion (down conc gradient) and glucose against conc gradient

Question 43

Absorption: facilitated diffusion

Answer 43

High concentration of glucose in epithelial cell results in facilitated diffusion into the blood through channel protein