3.2.3 transport across all cell membranes

Question 1

carrier proteins

Answer 1

span the plasma membrane

eg glucose = binds with the protein, causing a change shape in such a way that the molecule is released to the inside of the membrane
no external energy is needed
molecules move from a region where they are highly concentrated to one of lower concentration, usually only the kinetic energy of the molecules them selves

Question 2

what are protein channels?

Answer 2

proteins form water filled hydrophilic channel

allow specific water soluable ions to pass through = channels are selective

if particular ion is not present, channel remains closed

control over entry and exit of ions

ions bind with protein causing it to change shape which closes it to the other side of the membrane and opens the other side

Question 3

describe facilitated diffusion

Answer 3

charged ions and polar molecules dont easily diffuse across the CSM, due to hydrophobic nature of fatty acids of the biophospholipid layer

passive process = no energy needed

relies on inbuilt motion of particles, not ATP

occurs down a concentration gradient and specific points in CSM, where there are special protein molecules

2 mechanisms = protein channels or carrier proteins

Question 4

what is the equation for ficks law of diffusion?

Answer 4

measure of the rate of diffusion: surface area x difference in concentration / length of diffusion path

Question 5

describe simple diffusion

Answer 5

net movement of molecules of ions from a region where they are more highly concentrated to one where concentration is lower

molecules that can diffuse across the CSM are small, non-polar molecules like oxygen

Question 6

describe the structure of the CSM?

Answer 6

hydrophilic heads of both phospholipid layers point to the outside of the CSM attracted by water on both sides

hydrophobic tails of both ohospholipid layers point into the centre of the CSM, repelling water

Question 7

what are the functions of phospholipids?

Answer 7

to allow lipid soluble substances to enter and leave the cell

prevent water soluble substances entering and leaving the cell

make the membrane flexible and self-sealing

Question 8

where are proteins embedded into the CSM?

Answer 8

occur on the surface of the bilayer and never extend completely across it

carrier proteins

protein channels

Question 9

what are the functions of the proteins in the membrane?

Answer 9

provide structural support

act as channels transporting water soluble substances across the membrane

allow active transport across the membrane through carrier proteins

form cell surface receptors for identifying cells

helps cells adhere together

act as receptors

Question 10

what is the function of cholesterol?

Answer 10

reduce lateral movement of other molecules including phospholipids

make the membrane less fluid at high temperatures

prevent leakage of water and dissolved ions from the cell

Question 11

describe cholesterol in the CSM

Answer 11

cholesterol occur within the phospholipid bilayer of the CSM
add strength to the membranes

very hydrophobic and therefore are important in preventing loss of water and dissolved ions from the cell

pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules but without making the membrane as a whole too rigid

Question 12

describe glycolipids in the CSM

Answer 12

carbohydrate covalently bonded with a lipid

carbohydrate portion extends from the bilayer into the watery environment outside the cell where it acts a CSM for specific chemicals

Question 13

what is the function of glycolipids?

Answer 13

acts as recognition sites

helps maintain the stability of the membrane

helps cells to attach to one another and so forms tissues

Question 14

describe glycoproteins in the CSM

Answer 14

carbohydrate chains are attached to many extrinsic proteins on the outer CSM

act as cell surface receptors for hormones and neurotransmitters

Question 15

what is the function of glycoproteins?

Answer 15

act as recognition sites

helps cells to attach to one another and so forms tissues

allows cells to recognise one another

Question 16

what is the permeability of the CSM?

Answer 16

most molecules are not soluble in lipids and cannot pass through the phospholipid bilayer

most molecules are too large to pass through channels in the membrane

molecules of the same charge as the charge on channel proteins are repelled

electrically charged and therefore have difficulty passing through the non polar hydrophobic heads

Question 17

describe the arrangement in the fluid-mosaic model

Answer 17

fluid = individual phospholipid molecules can move relative to one another. gives the membrane a flexible structure that is constantly changing in shape

mosaic = proteins embedded vary in shape, size and pattern in the same way as the stone or tiles of a mosaic

Question 18

what are the three concentrations of osmosis?

Answer 18

1. isotonic solution = a solution of equal water potential to cell (same strength)
2. hypertonic solution = a solution of lower water potential than a cell (high strength)
3. hypotonic solution = a solution of higher water potential than a cell ( low strength)

effects: diagram on notes

Question 19

how does osmosis take place in plant cells?

Answer 19

plant cells placed in pure water, osmosis causes water to move into planrt cell until it is turged, the cell membrane is pushed up against the cell wall

cell prevents the cell from bursting and as the pressure increases as it reaches a point where no more water can enter the cell due to water pressure inside the cell

Question 20

animal cells and osmosis

Answer 20

CSM approx 7nm, cannot stretch to a great extent

red blood cells placed in pure water will burst (haemolysis)

to prevent this animal cells live in a liquid which has the same water potential as cells eg blood plasma

if a red blood cell is placed in a solution with a lower water potential, cell shrinks and becomes shriveled

table in textbook

Question 21

water potential and solutions

Answer 21

water potention is represented by greek letter psi

measured in units of pasacls, usually kilopascals (Kpa)

water potential is created by water molecules
higher the water potential the greater the energy of the water molecules and the most likely to move by osmosis

at standard room temp (25*) and pressure (100KPA) the water potential of pure water is 0

addition of a solute to pure water will lower its water potential

water potential of a solute must always have a negative value
potential of water + solute must always be less than 0

water will move by osmosis from a region of higher water to one of lower water potential

Question 22

osmosis defintion

Answer 22

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 permable membrane

Question 23

sodium potassium pump

Answer 23

sometimes more then one molecule/ion may be moved

in this pump = sodium ions are actively removed from the cell/organelle whilst potassium ions are actively taken in from the surroundings

important in many processes eg creation of nerve impulses

potassium pump diagram on notes

Question 24

difference between active transport and facilitated diffusion

Answer 24

both use carrier proteins however…

facilitated diffusion occurs down a concentration gradient

active transport occurs against a concentration gradient

facilitated diffusion does not require metabolic energy where as active transport does

metabolic energy = ATP

Question 25

direct active transport of a single molecule/ion description

Answer 25

1. carrier proteins span the plasma membrane and bind to the molecule or ion to be transporred on one side of it
2. molecule/ion binds to receptor sites on the. carrier protein
3. inside of the cell/organelle, ATP binds to the protein causing it to split into ADP and a phosphate molecule
   as a result the protein molecule changes shape and opens up to the opposite side of the membrane
4. molecule/ion is then released to the other side of the membrane
5. phosophate molecule is released from protein which causes protein to revert back to its orginal shape, ready for the process to be repeated
   phosphate molecule then recombines with the ADP to form ATP during respiration

Question 26

describe active transport

Answer 26

metabolic energy (ATP) is required

substances are moved against a concentration gradient, from a lower to higher concentration

carrier protein molecules act as pumps

process is selective with specific substances being transported = molecules/ions must be correct shape and size to fit the tertiary structure

Question 27

what is active transport?

Answer 27

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

involves the use of metabolic energy to transport molecules in and out of cells

Question 28

how is rate of movement increased across membranes in the ileum?

Answer 28

epithelial cells lining the ileum possess microvilli
microvilli provide more SA for the insertion of carrier proteins through diffusion, active transport and osmosis

increased number of protein channels and carrier proteins in any given area
increases the density

Question 29

what is the role of diffusion in the absorption of glucose in the ileum?

Answer 29

normally a greater concentration of glucose and amino acids within the ileum than in the blood
concentration gradient down moves glucose by facilitated diffusion from inside the ileum into the blood

glucose absorbed is continuously moved by cells as they use it for respiration.
this helps to maintains the concentration gradient between the inside of the ileum and the blood
rate of movement by facilitated diffusion across CSM is increased

Question 30

what is the role of active transport in the absorption of glucose in the ileum?

Answer 30

diffusion only results in the concentrations either side of the intestinal epithelium becoming equal.
not all available glucose and amino acids can be absorbed in this way and some is passed out through the body

glucose and amino acids are also absorbed by active transport
all should be absorbed into the blood

Question 31

describe the manner of cotransport of glucose in the ileum

Answer 31

CO TRANSPORT

1. sodium ions are actively transported out of epithelial cells, by the sodium potassium pump into the blood. takes place by one type of protein carrier molecule found in the CSM
2. maintains a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells
3. sodium ions diffuse into the epithelial cells down this concentration gradient through a different type of protein carrier in the CSM.
   as the sodium ions diffuse in through the second carrier protein, they carry either amino acid molecules or glucose molecules into the cell
4. glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier