paper 1: Transport Across Membranes Flashcards
what is the cell surface membrane sometimes known as
plasma membrane
what are the functions of the cell membrane
- it controls the substances that enter and leave the cells
- it holds the contents of the cell/ organelle in
- it allows different conditions to be established between the inside and outside of the cell/organelle
- the cell surface membrane only allows certain molecules through it, for this reason it is known as partially permeable
describe and explain how phospholipids are arranged in a cell membrane
the hydrophilic phosphate heads face inside and outside of the cell to stay in contact with water
the hydrophobic fatty acid tails in the middle of the membrane away from the water
what properties do phospholipids give a cell membrane
- make the membrane flexible
- allow lipid soluble substances to cross the membane
- prevent water soluble substances from crossing the membrane
why is the plasma membrane described as the fluid-mosaic model
fluid: molecules can move around within the layers of the membrane
mosaic: due to proteins embedded throughout the membrane
how does cholesterol effect a cell membrane
- it prevents the membrane being too fluid by restricting movement of other molecules
- this increases the rigidity of plasma membranes
what are the functions of proteins in cell surface membranes
- transport proteins
- enzymes
- antigens
- receptors
what do channel proteins do
form a channel to allow ions and polar molecules and large molecules to pass through the bilayer
what is the role of carrier proteins
- molecules bind specifically to carrier protein binding site
- protein changes shape to allow molecule through
- returns to normal once molecule has passed through
- normally needs energy from ATP to change shape
what kind of binding site do receptors have
specific and complimentary to the substance they respond to
describe antigens
- proteins which stick out of the cell membrane
- if they are proteins that your body does not make then they can be recognised as non self
- this triggers an immune response
what is the difference between a glycolipid and a glycoprotein
glycoprotein: carbohydrate bound to a protein
glycolipid: carbohydrate bound to a phospholipid
what are glycoproteins and glycolipids used for
receptors used in cell recognition
how does temperature effect cell membranes
- low temperatures can make membranes less fluid so membrane permeability becomes lower
- high temperatures make lipids more fluid so increase the membrane permeability
- the heat makes the molecules of the membrane vibrate as they have more kinetic energy. the higher the temp. the higher the vibration
- if the temperature is too high it causes too many vibrations which causes the membrane to rupture
define diffusion
the passive net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient
what is passive transport
a transport process that does not require any extra energy from the hydrolysis of ATP
which molecules can diffuse across the phospholipid bilayer and how
- small molecules as they can pass between phospholipids
- lipid soluble (non-polar) substances because they can dissolve in the phospholipid bilayer
which molecules can’t diffuse directly across the phospholipid bilayer and why
- large molecules because they cannot pass between the phospholipids
- water soluble (polar) substances because they are repelled by the hydrophobic fatty acid tails in the phospholipid bilayer
define facilitated diffusion
the passive movement of particles down a concentration gradient, using channel or carrier proteins
how do channel proteins allow molecules through the phospholipid bilayer
pores in the membrane for molecules to diffuse through
draw a graph to show what happens to rate of entry as the external concentration of substance increases during simple diffusion
sketch a graph to show what happens to rate of entry as the external concentration of substance increases during facilitated diffusion
define active transport
the movement of particles from a region pf lower concentration to a region of higher concentration using carrier proteins and energy from the hydrolysis of ATP
describe the stages of active transport
- the molecule and ATP binds to the specific carrier protein
- the carrier protein changes shape and transports the molecule across the membrane against its concentration gradient. this requires energy from the hydrolysis of TAP
- as the molecule is released the carrier goes back to its original shape
what factors effect the rate of diffusion
- size of concentration gradient
- surface area the diffusion is occurring across
- the thickness of the diffusion pathway
- the number of carrier/ channel proteins for facilitated diffusion
define osmosis
the passive net movement of water molecules from a region of higher water potential to a region of lower water potential down a water potential gradient across an partially permeable membrane
what is water potential measured in
KPa
what is a hypertonic solution
the solution has a lower water potential than inside the cell so H2O moves out of the cell
what is a hypotonic solution
he solution has a higher water potential than inside the cell so H2O moves into the cell
what is an isotonic solution
the solution has the same water potential as inside the cell so there is no net movement of H2O
what happens to an animal cell if it is placed in a hypotonic solution
- too much water had transported into the cell by osmosis because the solution has a higher water potential than inside the cell.
- the cell cannot hold anymore water so the cell membrane bursts (osmotic lysis)
what happens to an animal cell if it is in an isotonic solution
the water potential of the cell and the solution are the same so there is no net movement of water and the rate of osmosis has reached an equilibrium so the cell remains normal
what happens to an animal cell is it is placed in a hypertonic solution
- the water potential outside the cell is lower than inside the cell
- so water transports out of the cell by osmosis
- if too much water leaves the cell it becomes shrivelled (crenated)
what happens to a plant cell if it is in a hypotonic solution
- higher water potential outside than inside the cell
- water moves into cell via osmosis
- cell becomes turgid due to the cell wall
what happens to a plant cell if it is in an isotonic solution
the water potential is the same inside and outside the cell
so there is no net movement of water by osmosis
the cell stays the same
what happens if a plant cell is placed in a hypertonic solution
- lower water potential outside cell than inside
- water moves out of cell by osmosis
- cell becomes flaccid and plasmolyses
describe the process of co transport of glucose
- sodium ions are actively transported from the epithelial cells into the blood. this maintains the sodium concentration gradient between the lumen of the small intestine and the epithelial cell
- glucose and sodium ions are co-transported from the lumen of the small intestine, into epithelial cell via a carrier protein, this is facilitated diffusion, because sodium ions move down their concentration gradient
- the glucose molecules move into the blood via facilitated diffusion down the concentration gradient
why do people with lactose intolerance get diarrhoea when they consume lactose
- some people do not produce the enzyme lactase
- this means they cannot hydrolase lactase into glucose and galactose
- lactose remains in the lumen of the small intestine
- the lactose lowers the water potential in the lumen of the small intestine
- water would move into the lumen of the small intestine by osmosis
- this causes diarrhoea
what is co transport
a form of facilitated diffusion where the concentration gradient of one molecule is sued to move the other molecule against its own concentration gradient
how does alcohol affect the permeability of membranes
- makes membrane more permeable
- dissolves the phospholipid bilayer
how does acid affect the permeability of a membrane
- makes it more permeable
- protein channels/ carrier proteins denature so are non- selective allowing any molecule through
how to increase the rate of diffusion across a membrane
- higher temp: increases kinetic energy so faster diffusion
- larger surface area: more phospholipids for simple diffusion and more carrier/channel proteins for facilitated diffusion
- high conc. gradient: as concentration increases so does rate of diffusion
- more carrier/ channel proteins: more proteins for molecules to pass through
