2.5 Biological Membranes Flashcards
Biological Membranes
what do membranes do
control what passes through them
what is the roles of membranes on the SURFACE of cells - cell surface PLASMA membrane
PLASMA membranes:
- barrier between cell and the environment
- control which substances enter and leave the cell
- partially permeable, letting some molecules through but not others (through diffusion, active transport and osmosis)
- allow recognition by other cells, e.g. cells of the immune system
- allow cell communication ( cell signalling)
what are the roles of membranes WITHIN cells
- compartmentalisation: divide and separate organelles into compartments, separate from the cytoplasm (makes different functions more efficient, like substances needed for respiration are kept inside mitochondria, and specific conditions needed are maintained)
- form vesicles to transport substances between different areas of the cell
- control which substances enter and leave the organelle (e.g. RNA)
- also partially permeable
- can also get membranes within organelles, acting as a barrier between membrane contents and rest of organelle
- site of chemical reactions (e.g. inner membrane of mitochondria= enzymes for respiration)
what is the name for the structure of membranes / scientific name
Fluid Mosaic Model (1972)
- phospholipid bilayer
explain the name fluid mosaic model
fluid: phospholipid molecules are constantly moving
mosaic: protein molecules are scattered and embedded throughout bilayer, like a mosaic
explain the structure of the phospholipid bilayer
-PHOSPHOLIPIDS= form the two layers of the bilayer, with the heads facing outwards and the tails facing inwards
-CHOLESTEROL= found embedded throughout the bilayer, between the tails
- PROTEIN= molecules scattered throughout the bilayer (intrinsic which are embedded through both layers via hydrophobic R-groups on external surface, and extrinsic which are present on one side, but can move in between via hydrophilic R-groups on external surface)
- GLYCOPROTEINS= proteins with polysaccharide chains attached
- GLYCOLIPIDS= lipids with polysaccharide chains attached
what is the function of the phospholipids
- have a hydrophilic head (attracts water)
- have a hydrophobic tail ( repels water)
- automatically arranged into a bilayer, with heads facing out towards the water on either side of the membrane
- the centre is hydrophobic, so doesn’t allow water-soluble substances (e.g.) ions through
- acts as a barrier to dissolved substances
- fat-soluble substances like vitamins can still dissolve and pass straight through
what is the role of cholesterol in the membrane
controls FLUIDITY of the bilayer:
- cholesterol is a type of lipid present in all cell membranes (except bacterial)
- fit between the phospholipid molecules
- at HIGH temp, they bind to the hydrophobic tails, causing them to pack more closely, making it less fluid and more rigid
- at LOW temp, cholesterol prevents the phospholipids from packing too close, increasing fluidity of the membrane
role of proteins in cell membranes
- 2 types
-CHANNEL proteins: allow small and charged particles through the bilayer
-CARRIER proteins: transport molecules and ions across the membrane by active transport and facilitated diffusion - also act as receptors in cell signalling (when molecule binds to protein, chemical reaction is triggered inside the cell)
role of glycolipids and glycoproteins in the cell membrane
- stabilise the membrane by forming hydrogen bonds with surrounding water molecules (cell adhesion)
- sites for drugs, hormones and antibodies to bind to
- act as receptors for cell signalling
- also antigens, cell surface molecules involved in immune response (cell-markers)
how do cells communicate with each other
- cell signalling
- how they control processes inside the body
- and respond to changes within the environment
outline how cells use messenger molecules
- one cell releases a messenger molecules (hormone)
- molecule travels (in the blood) to another cell
- messenger molecule is detected by new cell because it binds to a receptor on its cell membrane
what is a membrane bound receptor
proteins in the cell membrane which act as receptors for messenger molecules
how do membrane bound receptors work
- receptor proteins have a specific shape which only complimentary messenger molecules can bind to
- different cells have different types of receptors, so respond to difference messenger molecules
what is the name for the cell which responds to a specific messenger molecule
target cell
give an example of receptors in use of human body
GLUCAGON:
- hormone released when not enough glucose in the blood
- binds to receptors on liver cells
- causes cell to break down glycogen stores into glucose
how can drugs bind to cell membrane receptors
- bind to the receptors
- trigger a response in cell
OR - block the receptor and prevent it from working
example of drug interacting with receptors in human body
ANTIHISTAMINES:
- cell damage causes the release of histamines
- they bind to receptors of other cells and cause inflammation
- antihistamines block these histamine receptors on cell surfaces
- prevents a histamine from binding
- stops inflammation
what effects the permeability of a membrane
- temperature
- solvent type
- solvent concentration
explain why beetroot cells can be used to investigate permeability if the membrane
- contain a coloured pigment which leaks out
- higher the permeability, the more pigment leaks out of the cell
PAG: explain how to investigate temperatures effect on membrane permeability
1) cut 5 equal sized beetroot pieces and rinse them to remove any pigment released from cutting
2) place the 5 pieces into different test tubes, each with 5cm^3 of water
3) place each test tube into a water bath at a different temp (10,20,30…)
4) keep the pieces in for same amount of time
5) removed the beetroot pieces from the test tubes, leaving just the coloured liquid
6) use a colorimeter (machine that passed light through liquid and measures how much light is absorbed)
7) higher permeability= more pigment released=higher absorbable of liquid
explain the permeability of the cell membrane below 0 degrees
- phospholipids don’t have much energy, so can’t move much
- packed closely and the membrane is rigid
-BUT channel and carrier proteins in the membrane deform, increasing the permeability of the membrane - ice crystals may form and pierce membrane, making it highly permeable when it thaws
explain the permeability of a membrane between 0 to 45 degrees
- phospholipids can move and aren’t as tightly packed, so partially permeable
- as temp increases, the phospholipids move more because they have more energy
- increases the permeability of the membrane
explain the permeability of a cell membrane over 45 degrees
- phospholipids start to melt (break down)
- membrane becomes more permeable
- water inside cell expands, putting pressure on membrane
- channel and carrier proteins deform so can’t control what enters or leaves the cell
- increases the permeability of the cell
how do solvents effect membrane permeability
- due to surrounding cells in solvents, such as ethanol
- solvents dissolve the lipids in a cell membrane, so the membrane loses its structure
- some more than others (ethanol>methanol)
what is diffusion
the net movement of particles (molecules or ions) from an area of higher to lower concentration
describe the movement of particles in diffusion
- molecules diffuse both ways
- the net movement will be to an area of lower concentration
- movement continues until particles are evenly distributed throughout the gas or liquid
-DOWN the concentration gradient (path from an area of higher to lower concentration)
what type of process is diffusion
passive process
what is meant by passive transport, and give examples
- no energy required for it to happen
- simple, facilitated diffusion
- osmosis
what type of molecules can diffuse through a partially permeable membrane
- small
- non-polar
- oxygen and carbon dioxide
- water as well as small enough, even though not non-polar
-directly through phospholipid bilayer
name the 4 factors which affect the rate of diffusion
- concentration gradient
- thickness of exchange surface
- surface area
- temperature
effect of concentration gradient on diffusion
- higher the gradient, faster the rate
effect of thickness of exchange surface on rate of diffusion
- thinner the exchange surface, faster the rate
- reduces the distance the particles have to travel
effect of surface area on rate of diffusion
- larger the surface area
- the faster the rate
effect of temperature on rate of diffusion
- warmer it is, faster the rate
- particles have more kinetic energy and move faster
what is phenolphthalein
- a pH indicator
- pink in alkaline
- colourless in acidic solution
PAG: explain the overall investigation for effect of diffusion in model cells
1) make agar jelly with phenolphthalein and sodium hydroxide (alkali) = pink shade
2) fill beaker with dilute hydrochloric acid
3) use a scalpel to cut out cubes from the jelly and put them in beaker of acid
4) leave cubes to turn colourless (acid diffuses into the agar jelly and neutralises the sodium hydroxide)
PAG: effect of surface area
- cut the agar jelly into different sized cubes and work out their SA:V ratio
- time how long it takes for each cube to turn colourless
- place in SAME CONCENTRATION of hydrochloric acid each time
- larger SA:V = turn colourless the fastest
- in multiple choice, use the shortest pathway
PAG: effect of concentration gradient
- prepare test tubes containing different concentrations of hydrochloric acid
- put equal sized cubes of agar jelly into each test tube and time how long it takes to turn colourless
- expect the cubes in highest concentration to turn colourless first
PAG: effect of temperature
- equal sized cubes and equal concentration of hydrochloric acid into each boiling tube
- put each tube into water bath of varying temperature
- time how long it takes to turn colourless
- highest temperature = colourless fastes
- don’t increase temp over 65 degrees as agar will start to melt
which particles diffuse through facilitated diffusion
- larger molecules ( amino acids, glucose)
- ions
- polar molecules
what is facilitated diffusion
- diffusion through use of channel and carrier proteins in the cell membrane
- move down concentration gradient
- passive process (don’t use energy)
how do channel proteins help with facilitated diffusion
- form pores in the membrane for charged particles to diffuse through, down the concentration gradient
- through providing a hydrophilic channel
- different channel proteins facilitate the diffusion of different charged particles
how do carrier proteins help with facilitated diffusion
- move large molecules into or out of a cell, down the concentration gradient
- different carrier proteins facilitate the diffusion of different molecules
1) large molecule attaches to a carrier protein in the membrane on binding site
2) then, protein changes shape
3) this releases the molecule on the opposite side of the membrane
what is active transport
- movement of molecules and ions across plasma membranes using energy, against a concentration gradient
- uses carrier proteins
- active process
how does active transport take place
1) molecule attaches to carrier protein
2) protein changes shape - due to phosphate group from the hydrolysis of ATP binding to the inside of the carrier protein
3) moves the molecule across the membrane, releasing it onto other side
4) energy is used from ATP to move solute against its concentration gradient
5) e.g. calcium ion, which attaches to calcium binding site
what is endocytosis/ exocytosis
- bulk transport of materials into/out of the cell
which molecules move into the cell by endocytosis
- too large to be taken into the cell by carrier proteins
- proteins, lipid, some carbohydrates
- some MUCH LARGER:
- white blood cells (phagocytes) take in microorganisms and dead cells so that they can destroy them
explain endocytosis
- cell surrounds a substance with a section of its plasma membrane
- membrane then pinches off to form a vesicle inside the cell containing the ingested substance
- uses ATP for energy
which molecules move out of the cell by exocytosis
-substances produced by the cell which need to be released
- digestive enzymes
- hormones
- lipids
- some aren’t released outside cell, but rather inserted straight into plasma membrane (membrane proteins)
explain exocytosis
- vesicles containing substance pinch off from sacs of the golgi apparatus
- move towards the plasma membrane
- vesicles fuse with the plasma membrane and release their contents outside the cell
- uses ATP as an energy store
what is osmosis
diffusion of water molecules (move from area of high concentration to lower concentration of water molecules) across a partially permeable membrane down a water potential gradient
what is water potential
measure of the quantity of water molecules compared to solutes in a solution, measured as the amount of pressure created by water molecules in kPa
which substance has the highest water potential
- pure water (0)
- the rest are lower negative numbers)
what is hydrostatic pressure
the pressure of water in an enclosed system (increases if water enters cell via osmosis)
what is hypotonic solution
solution with a higher water potential than cell
what is isotonic solution
solution with the same water potential as the cell
what is hypertonic solution
solution with a lower water potential than the cell
what happens when a red blood cell is placed in a hypotonic solution
- net movement of water molecules into the cell
- cell bursts (cytolysis)
what happens when a red blood cell is placed into a isotonic solution
- water molecules pass into and out of the cell in equal amounts
- cell stays the same
what happens when a red blood cell is placed into a hypertonic solution
- net movement of water molecules out of the cell
- cell shrinks
what happens when a plant cell gets placed in a hypotonic solution
- net movement of water into cell
- the vacuole swells
- the vacuole and cytoplasm press against the cell wall
- the cell becomes turgid (swollen)
what happens to a plant cell in isotonic solution
- mater molecules move in and out of the cell in equal amounts
- the cell stays the same
what happens to plant cell in hypertonic solution
- net movement of water is out of the cell
- cell becomes flaccid (limp)
- the cytoplasm and membrane pull away from the cell wall
- called plasmolysis
PAG: explain experiment for investigating water potential
1) prepare sucrose solutions of 0.0 to 1.0 M (higher the sucrose concentration, lower the water potential (more negative))
2) use cork borer to cut potatoes into same sized pieces (about 1cm in diameter)
3) divide the chips into groups of three and use a mass balance to measure the mass of each group
4) place one group into each solution
5) leave the chips in the solution for as long as possible (SAME AMOUNT), at least 20 minutes
6) remove the chips and pat gently with a paper towel
7) reweigh each group and record your results
8) calculate the % change in mass, and plot on graph
PAG: explain the graph for water potential experiment
- as concentration increases, mass change decreases
- when line crosses x-axis, this is the point where the water potential of the sucrose solution is the same as in the potato cells
PAG: explain another way to investigate water potential other than potatoes
use eggs:
- with their shells dissolved
- remaining membrane is partially permeable, so good model for osmosis on animal tissue
what is the width of the cell surface plasma membrane
7 micrometres
