C4- Plasma Membranes Flashcards
4 functions of membranes
Site of chemical reactions
Partially permeable barrier
Cell communication
Compartmentalisation
–> not plasma/ cell surface membranes
Compartmentalisation
The formation of separate membrane bound areas in a cell
Allows for specific conditions required for cellular reactions
maintains chemical gradients
Membrane general structure
Phospholipid bilayer
hydrophilic heads form inner and outer surfaces of the membranes
Fatty acid tails form the core
Fluid mosaic model
Phospholipids are free to move within the layer, giving it flexibility
Proteins in the layer form a mosaic like pattern
Extrinsic protiens
embedded on one side of the bilayer or between layers
hydrophobic R group interacts with the hydrophilic heads
Cholesterol
Membranes
Extrinsic protein
Regulates fluidity of membranes
–> adds stability to membranes without making them too rigid
Positioned between phospholipids
intrinsic proteins
with example
Transmembrane proteins embedded in both layers of proteins
channel and carrier proteins
Glycoproteins- used for cell signalling
Channel vs carrier protiens
channel= allows passive movement of polar molecules and ions down a concentration gradient though membranes
carrier proteins= move against the concentration gradient
–> both passive and active transport
membranes
Site of chemical reactions
membranes have to be in a particular position for chemical reactions to take place
What happens when a membrane looses its structure
If they loose their structure, the cell’s processes will be disrupted
Effect of high temperature on membranes
increased kinetic energy
Membrane become more fluid and looses its structure.
–> increasing its permeability
Carrier and channel proteins denature
Effect of Solvent on membranes
water
water is polar and keeps bilayer intact
Effect of alcohol on membranes
general, strong conc, weak conc
Less polar than water so can dissolve membranes, disrupting cells
Strong concentrations
–> dissolves membranes of bacterium
destroy cells in the body
less concentrated
–>alcohol enters the membrane but cannot dissolve it
Diffusion
Passive net movement of particles from a high concentration to a low concentration, down a concentration gradient
Will continue to equilibrium
Factors affecting rate of diffusion
Temperature
–> higher temp means particles have more kinetic energy
Concentration difference
–> greater difference= greater rate
Across membranes
- surface area, greater sa greater rate
- thickness of membrane, thin = greater rate
Simple diffusion
Diffusion in the absence of a membrane or barrier
Facilitated diffusion
Diffusion across a membrane using protein channels
Selective permeability
The effect of temperature on membrane permeability PAG
Method
Beetroot PAG
Using a scalpel, cut five equal-sized cubes of beetroot
Rinse the beetroot pieces
Add the beetroot pieces to five different test tubes, each containing the same volume of distilled water
Put each test tube in a water bath at a different temperature (e.g. 10℃, 20℃, 30℃, 40℃, 50℃) for the same length of time
Remove the beetroot pieces, leaving just the coloured liquid in the five test tubes
Use a colorimeter to measure how much light is absorbed as it passes through each of the five samples of coloured liquid
–> higher the value, greater absorbance so more colour/pigment
The effect of temperature on membrane permeability PAG
Limitations
Cuvettes may differ in thickness (very slightly). A thicker (or scratched) cuvette will absorb slightly more light than a thinner unscratched cuvette
The beetroot pieces may not be identical in size and shape, meaning some test tubes could contain slightly more beetroot tissue than others
Some parts of beetroot tissue have more pigment in their cells than others
Active transport
The movement of molecules or ions into or out of a cell against the concentration gradient
Active process that requires energy provided by ATP and carrier proteins
Selective process- specific substances transported by specific carrier proteins
Active transport
Process
Shape of carrier protein is complimentary to molecule they can carry
One way flow can only fit into carrier protein on one side of the membrane
Molecule binds
ATP binds to carrier protein inside cells
ATP- ADP which changes shape of a protein
Molecule transported across membranes
Phosphate detached and =recombines with ADP
–> protein reverts back original shape
Why do cells performing active transport contain numerous mitochondria
To generate lots of ATP in aerobic respiration
Why can glucose not pass through membrane by passive diffusion
Large molecule
therefore phospholipid bilayer acts as a barrier
Water Potential
The pressure exerted by water as they collide with a membrane or container
The greater the amount of water (the more dilute the solution) the greater the water potential
Water potential
Values
pure water = 0kpa
–> highest possible value
All solutions have negative water potential, the more concentrated the solution the more negative the water potential
Hydrostatic pressure
Increased pressure in a closed system such as a cell, due to diffusion of water.
Animal cell in different water concentrations
Hypotonic= cytolysis
isotonic= normal
Hypertonic= crenated
Plant cell in different water concentrations
Hypotonic= turgid
isotonic= flaccid
Hypertonic= plasmolysis
–> cell membrane pulls away from cell wall, which keeps its structure. Solution fills the gaps made
Osmosis
Osmosis is the net movement of water molecules through a partially-permeable membrane, from a region of high water potential to a region of low water potential.
Roles of proteins in a cell surface membrane
glycoproteins- cell signalling and adhesion
channel proteins- transport by facilitated diffusion;
carrier proteins- active transport
Membrane permeability PAG
Why was it reliable and valid
valid- use the same beetroot, use cubes of the same size, same volume of water
Reliable- repeat (at least) three times at each temperature, identify anomalies, calculate mean
Explain why rate of diffusion increases as temperature increases
Increased temperature increases the kinetic energy of particles
causing the particles to move at an increased speed
State two changes to a cell surface membrane that would increase the rate that polar molecules diffuse into a cell
Increased surface area
Decreased thickness
Explain why facilitated diffusion is not a form of active transport
Diffusion is always a passive process, it does not require a metabolic energy source
particle shave their own kinetic energy, moving down the concentration gradient
In facilitated diffusion a channel/co-transport protein aids diffusion
Why is it impossible to have a water potential above zero
The water potential of pure water is zero
addition of solute decreases water potential
therefore all solutions have negative water potential.
Explain why its important to maintain concentration of electrolytes in the body to ensure proper hydration
Electrolytes/solutes/minerals are necessary for many body processes
and help prevent excess water loss by osmosis
To help maintain correct fluid balance for reactions
Explain why glucose cannot pass through a cell membrane by simple diffusion
Phospholipids act as a barrier
Molecule is too large
Polar molecule due to polar OH groups- insoluble in bilayer
Bulk transport
Movement of large molecules such as enzymes or hormones in or out of the cell
Exocytosis
Endocytosis
Co transport
transport of one soluté against its concentration gradient in tandem with another that is diffusing down it’s concentration gradient
e. g H+ ions active transport via Proton pump alongside diffusion of sucrose
