Transport Across Membranes

Question 1

Explain the orientation of phospholipids in the membrane

Answer 1

The phosphate group has a negative charge, so hydrophilic
So orientate outwards towards other polar molecules such as aqueous cytoplasm or extracellular fluid

Hydrophobic fatty acid tail orientated toward the middle away from water

Forming a self sealing double membrane (bilayer)

Question 2

Why is it called the fluid mosaic model

Answer 2

Model = representation of a structure
Fluid = phospholipid bilayer is constantly moving
Mosaic = proteins unevenly distributed throughout so look like a mosaic

Question 3

What does selectively permeable mean

Answer 3

Only allows the passage of certain substances

Question 4

What determines selective permeability

Answer 4

The type, number and distribution of specific proteins in the membrane

Question 5

What is the function of the phospholipids in membrane

Answer 5

Allows lipid soluble substances to enter/leave cell
Prevents water soluble substances entering/leaving cell
Makes the membrane flexible and self sealing

Question 6

What is the function of the phospholipid bilayer

Answer 6

Keeps organelles in cell
Protects organelles
Controls what enters/leaves cell
Transports toxic substances out of cell

Question 7

What is found in the cell membrane

Answer 7

Proteins (carrier/channel)
Phospholipids
Glycoproteins
Glycolipids
Cholesterol
Receptor molecules
Aquaporins
Enzymes

Question 8

Cholesterol function in membrane

Answer 8

Decreases permeability and increases stability
More = less fluid
~25% (20-30 in most cells)
Different cells have different amounts because different functions

Question 9

Channel proteins in cell membrane

Answer 9

Pore like structures
Only allow specific charged ions or small molecules to move across membrane by facilitated diffusion
Open or closed
Intrinsic
Proteins so have specific tertiary structure so complementary to one type of molecule so only transport one type of molecule

Question 10

Channel proteins transport…

Answer 10

Small molecules
Charged ions
By facilitated diffusion

Question 11

Carrier protein function in cell membrane

Answer 11

Aid transport of ions/polar molecules and large molecules like glucose and amino acids
By active transport of facilitated diffusion
Proteins so have specific tertiary structure so complementary to one type of molecule so only transport one type of molecule

Question 12

Carrier proteins transport…

Answer 12

Ions/polar molecules
Large molecules
By active transport or facilitated diffusion

Question 13

Receptor protein function in cell membrane

Answer 13

Proteins so have specific tertiary structure so complementary to one type of molecule so only one type of molecule can bind
Specific cells have specific receptors based on function
Hormones can bind to receptors
Allows the cell to respond (eg by increasing permeability)

Question 14

Enzyme function in cell membrane

Answer 14

Active site complementary to substrate so can only form an ESC with one that type of molecule
Embedded within membrane

Question 15

Glycoprotein function in cell membrane

Answer 15

Carbohydrates and proteins on the surface of the cell membrane important for cell recognition
Often act as antigens
Immune cells detect specific shapes of glycoproteins to identify whether self/non-self
Produced in the golgi body by the cell displaying them
All cells have them

Question 16

Glycolipid function in cell membrane

Answer 16

Carbohydrates and lipids on the surface of the cell membrane important for cell recognition
Often act as antigens
Immune cells detect specific shapes of glycolipids to identify whether self/non-self
Produced in the golgi body by the cell displaying them
All cells have them

Question 17

Types of transport across membranes

Answer 17

Diffusion
Facilitated diffusion
Active transport
Osmosis
Bulk transport (exocytosis/endocytosis)

Question 18

How can proteins be integrated into phospholipids

Answer 18

Proteins are charged in parts and so attracted to hydrophilic phosphate head and aqueous solutions in/out of cell
Non-polar parts associate with hydrophobic fatty acid tails

Question 19

What is diffusion

Answer 19

A passive process
Not requiring ATP
The net movement of a substance from a higher concentration to a lower concentration across a partially permeable membrane

Question 20

Diffusion transports…

Answer 20

Small, lipid soluble molecules

Oxygen, Carbon Dioxide, Oestrogen

Question 21

When will net diffusion stop

Answer 21

When number of molecules either side of a partially permeable membrane reaches equilibrium
Equal number on either side
Net stays 0 but diffusion can still occur

Question 22

Ficks Law

Answer 22

Rate of diffusion = (concentration gradient x surface area)/diffusion distance

Question 23

Factors that affect the rate of diffusion

Answer 23

Temperature
Surface area
Concentration gradient
Diffusion distance

Question 24

How does temperature affect rate of diffusion

Answer 24

Increased temperature means increased kinetic energy of molecules
Faster rate of diffusion

Question 25

How does surface area affect the rate of diffusion

Answer 25

Larger surface area means more space for molecules to pass through So more can pass through at once More space for proteins for active transport or facilitated diffusion Microvilli

Question 26

How does concentration gradient affect the rate of diffusion

Answer 26

As difference increases so does the rate of diffusion

Question 27

How does diffusion distance affect the rate of diffusion

Answer 27

Shorter means faster because shorter distance to diffuse through Time to cross membrane decreases Determined by length of fatty acid tails

Question 28

What is facilitated diffusion

Answer 28

Passive process Does not require ATP Uses channel or carrier proteins to transport water soluble, charged or large molecules that are unable to pass between the hydrophobic fatty acid tails across the membrane Transports molecules from an area of higher concentration to an area of lower concentration across a partially permeable membrane

Question 29

Facilitated diffusion transports...

Answer 29

Water soluble molecules Charged molecules Large molecules

Question 30

Why can't hydrophilic substances pass through membrane

Answer 30

They can't pass through the hydrophobic tails of phospholipid bilayer Because the fatty acid tails repel them due to being non-polar

Question 31

When does facilitated diffusion stop

Answer 31

When all carrier proteins are saturated (binding sites all occupied) Substance concentration has reached equilibrium (isotonic)

Question 32

What is osmosis

Answer 32

The net movement of water from an area of higher water potential to an area of lower water potential Across a selectively permeable membrane Via aquaporins

Question 33

What is water potential

Answer 33

Free water molecules are able to move around and collide with membrane Exerting a pressure on it This is known as water potential in kPa More free molecules means more collisions and so greater potential and higher water potential

Question 34

Explain a lower water potential

Answer 34

More solute dissolved in water, more negative the water potential Because water is a dipole molecule its attracted to the solutes molecules charges and so the number of free moving water molecules decreases

Question 35

Explain higher water potential

Answer 35

Less solute dissolved in water, less negative water potential More free moving water molecules so more collisions and more pressure means higher water potential

Question 36

Explain pure water

Answer 36

0kPa Nothing dissolved in it and so every water molecule is free moving Highest water potential

Question 37

What happens when water potential is higher than in a cell

Answer 37

Water moves in to cell via osmosis down a water potential gradient Causing swelling and osmotic lysis Destroying cells contents Cellulose cell wall prevents lysis in plants

Question 38

What happens when water potential is lower than in a cell

Answer 38

Water moves out of cell via osmosis down a water potential gradient Animals = crenation = shrivelling Plant = plasmolysed = cell membrane pulls away from cell wall

Question 39

Describe isotonic

Answer 39

No net movement of water in or out of cell | No water potential gradient so no gain or loss in mass

Question 40

What is active transport

Answer 40

Transport of molecules across a membrane against a concentration gradient from an area of lower concentration to an area of higher concentration By carrier proteins with specific complementary shapes Active process so requires ATP

Question 41

How is ATP used in active transport

Answer 41

Carrier proteins require energy source, supplied by ATP which is produced during aerobic respiration via condensation reaction The hydrolysis of ATP into ADP and Pi provides a small amount of energy/phosphorylates the carrier protein Causing a conformational change in shape so it can transport the molecule into an area of higher concentration against a gradient

Question 42

Explain exocytosis

Answer 42

Method of bulk transport that uses golgi vesicle to move very large quantities of molecule from inside to outside of cell Used to move enzymes and glycoproteins from the golgi apparatus to the cell surface membrane to secrete proteins ATP is required to move the vesicles to the cell surface Vesicle fuses with cell membrane

Question 43

Why is ATP required in exocytosis

Answer 43

Requires the energy provided by breakdown of ATP into ADP and Pi to move the vesicles to the cell surface membrane so it can fuse with the membrane

Question 44

Explain endocytosis

Answer 44

Method of bulk transport in which the cell surface membrane is pulled inwards to create a vesicle Any molecules next to that part of cell membrane are enclosed in the vesicle Requires breakdown of ATP into ADP and Pi Part of reason why membrane must be flexible

Question 45

Why do cells using active transport have lots of mitochondria

Answer 45

Mitochondria are the site of aerobic respiration and therefore ATP production Active transport needs the small amount of energy and inorganic phosphate group (Pi) produced from the breakdown of ATP during aerobic respiration So there is enough energy and Pi for the carrier protein

Question 46

What happens if the tertiary structure of a channel/carrier protein changes

Answer 46

``` The binding site will change shape No longer complementary to its molecule Won't bind No ESC Can't be transported and so facilitated diffusion or active transport can't occur ```

Question 47

How does a respiratory inhibitor affect active transport

Answer 47

A respiratory inhibitor prevents mitochondria from making ATP When the inhibitor is added, the rate of uptake will decrease rapidly, indicating energy is needed Graph will also plateau when all carrier proteins saturated

Question 48

How does a respiratory inhibitor affect diffusion

Answer 48

As concentration gradient across membrane increases so does the rate of uptake Uptake not affected by respiratory inhibitor because does not require energy provided by ATP since it is a passive process Curve won't plateau because no proteins to be saturated and concentration keeps increasing

Question 49

How does a respiratory inhibitor affect facilitated diffusion

Answer 49

Prevents mitochondria making ATP When inhibitor added no change on rate of uptake since its passive Curve plateaus because all carrier proteins saturated

Question 50

Define intrinsic and extrinsic

Answer 50

``` Intrinsic = spans both sides of membrane Extrinsic = spans only one side of the membrane ```