Transport across cell membranes

Question 1

structure of cell-surface membrane

Answer 1

-phospholipid bilayer where molecules have hydrophobic phosphate head = attracted to water and hydrophobic fatty acid tail = repelled from water
-many components - phospholipids, proteins, cholesterol, glycolipids. glycoproteins

Question 2

functions of phospholipids in cell-surface membranes?

Answer 2

-Lipid soluble substances to pass through
-Flexibility and self-sealing
-Stop water soluble substance entering and leaving cells

Question 3

Describe the bilayer

Answer 3

-formed from two layers of phospholipids.
-hydrophilic heads which face out of the bilayer
-hydrophobic tails which face into the bilayer, so they don’t come in to contact with water .

Question 4

2 ways in which proteins are emebedded in phospholipid bilayer

Answer 4

-surface of bilayer and never extend completely across it > mechanical support to membrane or in conjunction with glycolipids, as cell receptors for molecules like hormones

-channel proteins or carrier proteins

Question 5

What are 2 types of proteins found in cell membranes?

Answer 5

-integral; span the whole of the membrane (e.g transport proteins)
-extrinsic; only one side of the bilayer
(give support or act with glycolipids as cell receptors)

Question 6

Function of channel proteins

Answer 6

-form water-filled tubes > allow water soluble-ions to diffuse across the membrane

Question 7

function of carrier proteins

Answer 7

-bind to ions/ molecules eg glucose and a.a then change shape in order to move these molecules across the membrane

Question 8

6 functions of proteins in cell-surface membranes?

Answer 8

• structural support
• act as protein channels > transport water soluble substances
• allow active transport across membrane through carrier proteins
• form cell-surface receptors for identifying cells
• help cells adhere together
• act as receptors

Question 9

Describe cholesterol

Answer 9

-occur within phospholipid bilayer of cell-surface membrane
-add strength to membranes
-very hydrophobic > preventing water loss and dissolved ions from cell.
-pull together fatty acid tails of phospholipid molecules > limiting their movement and other molecules without making whole membrane too rigid

Question 10

Function of cholesterol in membrane

Answer 10

-reduce lateral movement of other molecules including phospholipids
-make membrane less fluid at high temperature
-prevent leakage of water and dissolved ions from cell

Question 11

Describe glycolipids

Answer 11

-made of carbohydrate covalently bonded with a lipid
-carbohydrate portion extends from phospholipid bilayer into watery environment outside cell where it acts as a cell-surface receptor for specific chemicals eg human ABO blood system

Question 12

function of glycolipids

Answer 12

-act as recognition sites
-help maintain the stability of the membrane
-help cells to attach to one other and so form tissues

Question 13

Describe glycoproteins

Answer 13

-carbohydrate chains attached to many extrinsic proteins on outer surface on cell membrane
-also act as cell-surface receptors for hormones and neurotransmitters

Question 14

Function of glycoproteins

Answer 14

-act as recognition site
-help cells to attach to one another and so form tissues
-allows cells to recognise one another eg lymphocytes recgonise own cells

Question 15

permeability of cell surface membranes

Answer 15

cell surface membrane controls movement of substances in and out cell. Most molecules don’t freely diffuse across because many are:

• not soluble in lipids > can’t pass through phospholipid layer
  -too large to pass through channels in membrane
  -of same charge as charge on protein channels > repel
  -polar > have difficulty passing through non-polar hydrophobic tails in phospholipid bilayer

Question 16

Explain fluid mosaic structure

Answer 16

fluid- individual phospholipid molecules can move relative to one another > gives membrane a flexible structure and is constantly changing shape
mosaic- proteins that are embedded in bilayer vary in shape, size and pattern

Question 17

How does temperature impact cell surface membranes?

Answer 17

an increase in temperature increases the fluidity of the membrane, as a result increasing it’s permeability

Question 18

How does pH impact cell surface membranes?

Answer 18

it impacts the protein structure of the proteins in the cell membrane

Question 19

How does solute concentration impact cell surface membranes?

Answer 19

Phospholipid bilayer is dissolved more easily, increasing its permeability

Question 20

How does colour indicate the permeability of membranes when investigating using beetroot?

Answer 20

The darker the solution of beetroot, the more pigment has been released because the membrane is more permeable. A darker solution will have a higher reading for absorbance

Question 21

Why is fluidity an important characteristic of cell membranes?

Answer 21

It allows flexibility for the cell, increased motion needed in transport

Question 22

Define simple diffusion

Answer 22

net movement of molecules/ions from a region where there more highly concentrated to one where their concentration is lower, until they’re evenly distributed

Question 23

Explain how simple diffusion occurs

Answer 23

• Particles have kinetic energy and move randomly
  -They will distribute themselves evenly . - Net movement from a higher concentration to a lower
  concentration

Question 24

Describe molecules that travel by simple diffusion and explain why they can

Answer 24

-Small, non polar molecules such as oxygen and carbon dioxide. They are small enough
-not charged, so can pass through the phospholipid bilayer.
-Lipid soluble substances can also diffuse through the phospholipid bilayer,

-net movement of lipid-soluble and non polar or very small substances eg co2, o2 , steroid hormones
-from area of high conc to area of a lower conc down a conc gradient
-across phospholipid bilayer

Question 25

Define facilitated diffusion

Answer 25

Diffusion involving the presence of protein carrier and channel molecules to
allow the passive movement of charged and larger molecules, charged ions and polar molecules across plasma
membranes

Question 26

Why does facilitated diffusion occur

Answer 26

charged ions and polar molecules don’t diffuse easily due to hydrophobic nature of fatty acid tails of phospholipids in membrane

Question 27

Explain why facilitated diffusion is a passive process?

Answer 27

There is no ATP from respiration used in the process, the only energy used is the built-in (kinetic) energy of the molecules themselves

Question 28

Explain how facilitated
diffusion occurs including
the roles of carrier and
channel proteins

Answer 28

-Facilitated diffusion is a passive process that relies on the kinetic energy of the diffusing molecules.
-Diffusion down a concentration gradient can only occur where there are specific carrier of channel proteins.

Question 29

Describe the difference
between carrier proteins
and channel proteins

Answer 29

• Channel proteins - water filled hydrophilic channels across the membrane which only open to allow specific ions across.
• Carrier proteins - when a specific molecule eg glucose binds and
  conformational change occurs which releases the molecule to the other side of the membrane

Question 30

Describe protein channel (water-soluble -polar- substances)

Answer 30

-water-filled hydrophobic channels across membrane
-allow specific water-soluble ions to pass through
-channels =selective, so open in presence of specific ion and remain closed if not present (so controls in and exit of ions)
-ions bind with protein causing it to change shape in a way that closes it to one side of the membrane and opens it to the other side

Question 31

Describe carrier proteins (slightly larger substances)

Answer 31

-molecule like glucose that’s specific to protein is present, it bind with protein
-causes change in shape so molecule is releases to inside of membrane
-no external energy needed
-molecules move from high conc to lower conc using kinetic energy of the molecules

Question 32

Describe the molecules
that travel by facilitated
diffusion

Answer 32

-Ions and polar molecules cannot pass through the phospholipid bilayer because of the hydrophobic fatty acid tails.
-Larger molecules cannot pass through the phospholipid bilayer because of their size.
-Transmembrane protein channels
and carriers are required to facilitate (help) diffusion.

Question 33

Define osmosis

Answer 33

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 selectively permeable
membrane

Question 34

What is meant by the term ‘water potential’?

Answer 34

a measure of the pressure caused by water molecules, and the likelihood of them moving

Question 35

way of finding of water potential of cells have taken place

Answer 35

-place in series of solutions of different water potentials
-where there’s no net gain/loss of water from cells/tissues
-water potential inside cells/tissues must be same as that of external solution

Question 36

Explain how osmosis
occurs in terms of water
potential

Answer 36

-Water potential is the tendency for water molecules to move, it represents the
pressure created by the water molecules.
-An area of high water potential has a lower solute concentration, the solute cannot move through the partially permeable membrane unlike water.
-The water moves from a higher water potential
(less negative number) to a lower water potential (more negative number) through the partially permeable membrane until dynamic equilibrium is reached.

Question 37

lower conc in left and higher conc on right. explain osmosis

Answer 37

-solution on left has lower conc of solute molecules while solution on right has higher conc of solute molecules
-both solute and water molecules are in random motion due to their kinetic energy
-selectively permeable plasma membrane only allows water molecules across it and not solute molecules
-water molecules diffuse from left (higher water potential) to right (lower water potential) down a water potential gradient
-at point where water potentials on either side of plasma membrane are equal, dynamic eqm is established and there’s no net movement of water

Question 38

highest value of water potential (pure water) is what

Answer 38

0
-the more negative the value the lower the water potential

Question 39

Describe and explain the
effect of solutions of
different water potentials
on cells

Answer 39

Animal cells:
-external solution higher water potential - cells bursts as net movement of
water in to cell
-external solution equal water potential - dynamic equilibrium, no net
movement of water
-external solution lower water potential - cells shrink as net movement of
water out of cell

Plant cells:
-external solution higher water potential - TURGID net movement of water in
to cell - cell does not burst due to cell wall - protoplast pushed against cell
wall
-external solution equal water potential - INCIPIENT PLASMOLYSIS dynamic
equilibrium, no net movement of water
-external solution lower water potential - PLASMOLYSIS net movement of water out of cell - protoplast pulled away from cell wall

Question 40

RBC and osmosis

Answer 40

-absorb water by osmosis due to having a lower water potential
-cell surface membranes are thin and can’t stretch to great extent
-cell surface membrane will break
-bursting cell and releasing content

prevent:
-animals cells normally live in a liquid which has the same water potential as the cells

-if rbc placed in a solution with water potential lower than its own , water leaves via osmosis and cell shrinks and becomes shrivelled

Question 41

What is a selectively permeable membrane?

Question 42

How might a cell be adapted to increase the rate of osmosis?

Answer 42

Larger surface area would increase the rate of osmo

Question 43

Explain how K+ ions are involved in the opening and closing of stomata

Answer 43

-K+ ion concentration in guard cells increases
-lowers water potential inside guard cell
-water moves from higher water potential into the lower water potential outside of the cell, by osmosis
-cells become turgid and change shape, opening the stomata

Question 44

Define active
transport

Answer 44

-The movement of molecules or ions into or out of a cell from a region of lower
concentration to a region of higher concentration using ATP and carrier proteins
-occurs AGAINST a conc gradient
-requires metabolic energy by ATP

Question 45

uses of active transport

Answer 45

-directly move molecules
-individually move molecules using conc gradient which has already been set up y active transport (co-transport)

Question 46

Explain how active transport occurs inc’ importance of carrier proteins and hydrolysis of atp (direct active transport of a single molecule /ion)

Answer 46

-Specific carrier protein for the molecule/ion
spans the membrane
Molecule/ion binds to receptor on the carrier
protein
-On the inside of the membrane ATP binds to
the carrier protein and splits into ADP and Pi
-Energy from ATP causes carrier protein to
change shape and open on opposite side of
membrane
-Molecule/ion released to other side of membrane
Phosphate released from carrier protein causing it to revert to original shape
-ATP reformed from ADP and P during respiration

Question 47

What is the role of ATP in active transport?

Answer 47

ATP provides the energy needed, it undergoes ATP hydrolysis, forming ADP and Pi

Question 48

How does an increase in the number of carrier proteins impact the rate of active transport?

Answer 48

more carrier proteins will increase the rate of active transport, as more molecules can be moved at the same time

Question 49

Give three differences between active transport and facilitated diffusion

Answer 49

-Active transport requires energy, facilitated diffusion does not
-active transport moves from low concentration to high, facilitated diffusion moves from a high concentration to low
-Active transport only uses carrier proteins, facilitated diffusion used channel and carrier proteins

Question 50

why is sodium potassium pump essensial

Answer 50

essential to number of processes in organism , inc’ nerve impulse

Question 51

Define co-transport

Answer 51

Transport of one substance coupled with the transport of another across
the plasma membrane in the same direction through the same carrier
protein

Question 52

Explain how co-transport
occurs using absorption
of sodium ions and
glucose by cells lining
the mammalian ileum as
an example

Answer 52

-Sodium ions actively transported out of epithelial cells by sodium potassium pump (carrier protein) into the blood
-This maintains a higher concentration of sodium ions in the lumen of the ileum compared with the inside of the epithelial cells
-Sodium ions diffuse into the epithelial cell through a co-transport protein
(carrier protein)
-The movement of sodium ions carries glucose/amino acids through the
same protein into the epithelial cell
-Glucose/amino acids move into the blood by facilitated diffusion (through
another carrier protein)

Question 53

Describe how cells are
adapted for rapid
exchange across
membranes

Answer 53

-Microvilli on epithelial lining > finger-like projections on the cell surface membrane provide a larger surface area for carrier proteins involved in facilitation diffusion,
active transport and co transport
-High density of carrier proteins and protein channels on the membrane

Question 54

Role of diffusion in absorption

Answer 54

-carbohydrates and proteins being digested continuously, and there’s normally a greater conc of glucose and a.a within ileum than in blood
-so there’s conc gradient down which glucose moves by facilitated diffusion from inside ileum into blood
-blood constantly being circulated by heart , glucose absorbed into it is constantly being removed by cells as they use it up during respiration
-maintain conc gradient between inside of ileum and blood
-so rate of movement by facilitated diffusion across epithelial cell-surface membrane is increased

Question 55

Role of active transport in absorption

Answer 55

-diffusion only results in conc of either side of intestinal epithelium becoming equal
-means not all available glucose and a.a can be absorbed in this way + some may pass out the body > not happening as glucose and a.a being absorbed by active transport > glucose and a.a should be absorbed into blood

Question 56

Why would the rate of active transport eventually level off, even if the extracellular concentration of a solute increased?

Answer 56

As all the carrier proteins would be fully saturated

Question 56

What carrier proteins are involved in the transport of glucose

Answer 56

Na-K pump, transports Na+ ions into the blood (lowers conc. inside the cell)
Na-glucose, moves glucose against conc. gradient, as Na+ ions are moved down conc. gradient