2.1.5 Biological membranes Flashcards
Role of membranes at surface of cell (plasma membranes)
barrier between the cell and its environment
- control which substances enter and leave the cell
partially permeable, let some molecules through but not others
- can more across membrane by diffusion, osmosis or active transport
cell recognition eg. the cells of the immune system
cell communication (cell signalling)
Role of membranes within cells
compartmentalise
- membranes around organelles divide them into different compartments
- this makes functions more efficient
eg. lysosomes contain digestive enzymes within their membrane
can form vesicles
- to transport substances between different areas of the cell
control which substances enter and leave the organelle
- partially permeable
membranes within organelles
- act as barriers between membrane and contents of the rest of the organelle
eg. thylakoid membrane in chloroplasts
site of chemical reactions
eg. inner membrane of a mitochondrion contains enzymes needed for respiration
Describe the fluid mosaic model of membrane structure
- phospholipid molecules from a continuous double layer (bilayer)
- this bilayer is fluid as phospholipids constantly moving
- cholesterol molecules are present within the bilayer
- protein molecules are scattered through the bilayer
- glycoproteins have a polysaccharide chain attached - glycolipid molecules - lipids with a polysaccharide chain attached
Roles of components in membrane - phospholipids
Have a head and a tail
- head is hydrophilic it attracts water
- tail is hydrophobic it repels water
the molecules automatically arrange themselves in a bilayer (heads face outwards)
the centre of the bilayer is hydrophobic
- doesn’t allow water soluble substances (ions) though it, its acts as a barrier
- fat soluble substances can dissolve in the bilayer and pass directly through the membrane
Roles of components in membrane - cholesterol
a type of lipid
in all cell membranes (except bacteria)
fit between phospholipids
- they bind to the hydrophobic tails of the phospholipids
- this causes them to pack more closely together
this makes the membrane less fluid and more rigid
Roles of components in membrane - intrinsic proteins
control what enters and leaves the cell
channel proteins
- allow small or charged particles through (diffusion)
carrier proteins and transport molecules
- active transport
- facilitated diffusion
Roles of components in membrane - extrinsic proteins
glycolipids and glycoproteins
stabalise membrane by forming hydrogen bonds with surrounding water molecules
the sites where drugs hormones and antibodies bind
act as receptors for cell signaling
antigens - cell surface molecules involves in the immune system
how do cells communicate with each other?
by cell signalling
one cell releases a messenger molecule (eg. a hormone)
this molecule travels (eg. in the blood) to another cell
the messenger molecule is detected by the cell because it binds to a receptor on its cell membrane
Why do cells need to communicate with each other?
to control processes inside the body and to respond to changes in the environment
How are plasma membranes involved in cell signalling?
proteins in the membrane act as receptors for messenger molecules
receptor proteins have specific shapes, only a messenger molecule with a complementary shape can bind to them (target cells)
different cells have different types of receptors, they respond to different messenger molecules
How do drugs work?
by binding to receptors in cell membranes
they either trigger a response or block the receptor and prevent it from working
Factors affecting membrane permeability
temperature
solvents
How does temperature affect membrane affect solubility? - below 0degrees
phospholipids dont not have much energy so cannot move very much
packed closely together and the membrane is rigid
channel proteins and carrier proteins deform - INCREASING PERMEABILITY
ice crystals may form
- this pierces the membrane making it highly permeable when it thaws
How does temperature affect membrane affect solubility? - between 0 and 45 degrees
the phospholipids can move around and arent as tightly packed
- the membrane is partially permeable
as temp. increases the phospholipids move more (have more kinetic energy)
- this INCREASES PERMEABILITY of the membrane
How does temperature affect membrane affect solubility? - above 45degrees
the phospholipid bilayer starts to melt (break down)
- membrane MORE PERMEABLE
water inside the cell expands putting pressure on the membrane
channel and carrier proteins deform (denature) so cannot control when enters or leaves the cell - INCREASING PERMEABILITY
How does changing the solvent affect membrane permeability?
surrounding cells in a solvent INCREASES PERMEABILITY of the membrane
- solvents dissolve the lipids in the membrane so it loses its structure
- some solvents increase permeability more than others
- INCREASING SOLVENT CONC. INCREASES PERMEABILITY
Experiment to measure permeability of the membrane
using beetroot
1. cut equal-sized pieces of beetroot and rinse them (removed pigment released during cutting)
- place in five different test tubes
- temp –> same vol. of water and put in water baths for the same length of time
- solvent –> same vol. of different concs. same length of time (poss. same temp. water bath) - remove pieces of beetroot from the tubes leaving just the coloured liquid
- use a colorimeter to measure absorbance
- the higher the permeability of the membrane the more pigment is released so the higher the absorbance of the liquid
Transport across membranes - passive movement
doesn’t require ATP (energy from metabolic processes)
- diffusion
- facilitated diffusion
Transport across membranes - active transport?
requires ATP (from metabolic processes)
- active transport
What is diffusion?
the net movements of particles (molecules or ions) from an area of higher concentration to an area of lower concentration. It is a passive process
molecules diffuse both ways (net movement high –> low) this continues until particles are evenly distributed throughout the gas or liquid (equilibrium)
What molecules diffuse through membranes, why?
small, non-polar molecules
- oxygen
- carbon dioxide
can fit through spaces in the phospholipid bilayer
Factors that affect the rate of diffusion
concentration gradient
diffusion distance
surface area
temperature
how does the concentration gradient affect the rate of diffusion?
the higher it is, the faster the rate of diffusion
how does the thickness of exchange surface affect the rate of diffusion?
the thinner the exchange surface (the shorter the distance the particles have to travel), the faster the rate of diffusion
how does the surface area affect the rate of diffusion?
the larger the surface area (eg. of a cell membrane) the faster the rate of diffusion
how does temperature affect the rate of diffusion?
the warmer it is, the faster the rate of diffusion because the particles have more kinetic energy so they move faster
Investigating diffusion in model cells - agar jelly
- make up some agar jelly with phenolphthalein (pink in alkaline, colourless in acidic) and dilute sodium hydroxide
- this will make the jelly a shade of pink - then fill a beaker with some dilute hydrochloric acid
- using a scalpel, cut out a few cubes from the jelly and put them in the beaker of acid - of you leave the ecubes for a while they’ll eventually turn colourless as the acid diffuses into the agar jelly and neutralises the sodium hydroxide
Investigating diffusion in model cells (agar jelly) - surface area
cut the agar jelly into different sized cubes and work out their surface area to volume ratio
time how long it takes each cube to go colourless when placed into THE SAME CONC. of hydrochloric acid
expect the cubes with the highest surface area to vol. ratio to go colourless fastest
Investigating diffusion in model cells (agar jelly) - conc. gradient
prepare test tubes containing different concentrations of hydrochloric acid
put an EQUAL-SIZED cube of the agar jelly in each test tube and time how long it takes each one to turn colourless
expect the cubes in the highest conc. of hydrochloric acid to go colourless
Investigating diffusion in model cells (agar jelly) - temperature
prepare several boiling tubes conatining the SAME CONCENTRATION of hydrochloric acid and put tubes into water baths of VARYING TEMPERATURES
put an EQUAL SIZED tube of agar jelly into each boling tube and time how long it takes for each cube to go colourless
expect the cubes in the highest temperature to go colourless fast
What is facilitated diffusion?
some larger molecules (amino acids, glucose), ions and polar molecules don’t diffuse directly through phospholipid bilayer of the cell membrane
instead they diffuse through CARRIER OR CHANNEL PROTIENS in the cell membrane
particles move down a concentration gradient (high —> low)
PASSIVE PROCESS
Facilitated diffusion - carrier proteins
move large molecules into or out of the cell down their concentration gradient
different carrier protiens facilitate the diffusion of different molecules
- first a large molecule attaches to a carrier protein in the membrane
- then the eprotien changes shape
- this releases the molecule on the opposite side of the membrane
Facilitated diffusion - channel proteins
form pores in the membrane for charged particles to diffuse through down their concentration gradient
different channel protiens facilitate the diffusion of different charged particles
selectively permeable
what is active transport?
active transport uses energy to move molecules and ions across plasma membranes. against a concentration gradient. This process involves carrier protiens
- a molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side
- ATP is needed to move the solute against the conc. gradient
What is endocytosis?
taking in substances
some molecules too large to be taken into a cell by carrier protiens (protiens, lipids, carbohydrates)
a cell can surround a substance with a section of its plasma membrane
the membrane then pinches off to form a vesicel inside the cell containing the ingested substance
requires ATP (energy)
example of endocytosis
some white blood cells (phagocytes) use endocutosis to take in things like microorganisms and dead cells so they can destroy them
what is exocytosis?x
secreting substances
some substances produced by the cell (eg. digestive enzymes, hormones, lipids) need to be released from the cell
vesicles containing these substances pinch off from the sacs of the golgi apparatus and move towards the plasma membrane
the vesicles fuse with the plasma membrane and release their contents outside the cell
some substances (like membrane protiens) aren’t released outside the cell - instead they are inserted straight into the plasma membrane
requires ATP
What is osmosis?
the movement of water molecules across a partially permeable membrane down a water potential gradient (from a high water conc. —> low water conc.)
What is the water potential of pure water?
0
all solutions have a lower water potential than pure water
How are cells affected by the water potential of the surrounding solution?
how much water moves in or out of the cell depends on the water potential of the surrounding solution
animal cells affected by water potential? - hypotonic solution
when a animal cell is in a solution with a higher water potential
net movement of water into cell, cell bursts
animal cells affected by water potential? - isotonic solution
when an animal cell is in a solution with the same water potential
water molecules pass into and out of the cell in equal amounts
the cell stays the same
animal cells affected by water potential? - hypertonic solution
when an animal cell is in a solution with a lower water potential
net movement of water out of cell, cell shrinks
plant cells affected by water potential? - hypotonic solution
when a plant cell in a solution with a higher water potential
the net movement of water is into the cell.
the vacuole swells
the vacuole and cytoplasm push against the cell wall
the cell becomes turgid (swollen)
plant cells affected by water potential? - isotonic solution
when a plant cell is in a solution with the same water potential
water molecules move into and out of the cell in equal amounts
the cell stays the same
plant cells affected by water potential? - hypertonic solution
when a plant cell is in a solution with a lower water potential
net movement of water is out of the cell
the cell becomes flaccid (limp)
the cytoplasm and the membrane pulls away from the cell wall
- plasmolysis
experiments investigating water potential
using potato cylinders in different concentrations of sucrose solution to find the water potential of plant tissue
- prepare sucrose solutions, - 0.0M to 1.0M
- use a cork borer or chip maker to cut potatoes into the same size (1cm in diameter)
- divide the chips into groups of 3 and measure the mass of each using a mass balance
- place one group in each solution
- leave the chip for as long as possible - 20mins (same amount of time)
- remove the chips and pat dry with a paper towel
- weigh each group again and record your results
- calculate % change in mass for each group
- plot results on a graph
