Biological membranes Flashcards
What do intracellular membranes do?
form compartments within the cell, such as organelles (including the nucleus, mitochondria and RER) and vacuoles
Why are membranes partially permeable?
because they control the exchange of materials passing through them
do membranes play roles in cell signalling by acting as an interface for communication between cells?
yes
What different factors affect cell membrane permeability?
temperature and organic colvent concentration
what does the fluid mosaic model help to explain?
-Passive and active movement between cells and their surroundings
-Cell-to-cell interactions
-Cell signalling
how can substances cross memebranes?
by diffusion, facilitated diffusion, osmosis and active transport
Why does the fluid mosaic model describe cell membranes as mosaics?
because the scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
Why does the fluid mosaic model describe cell membranes as fluids?
because…
-The phospholipids and proteins can move around via diffusion
-The phospholipids mainly move sideways, within their own layers
-The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
What are the 4 main components of the fluid mosaic model?
-Phospholipids
-Cholesterol
-Glycoproteins and glycolipids
-Transport proteins
are the heads of phospholipids hydrophilic or hydrophobic?
hydrophillic
are the tails of phospholipids hydrophilic or hydrophobic?
hydrophobic
what molecules do phospholipids act as a barrier to?
to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane).This ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in
how can phospholipids be chemically modified to act as signalling molecules?
by….
-Moving within the bilayer to activate other molecules (eg. enzymes)
-Being hydrolysed, which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm
what are phospholipids made up of?
2 fatty acid molecules bonded to a glycerol molecule. The glycerol is also bonded to a highly charged phosphate group
What are the functions of membranes?
-acting as barriers between the internal contents of the cell and the external environment
-barriers between the contents of an organelle and the cytoplasm
-separation of one part of an an organelle from another
-location for chemical reactions e.g some stages of respiration
-cell signalling
why can hydrophobic non-polar molecules pass through the cell membrane?
because the phospholipid bilayer has a hydrophobic centre
why can water molecules pass through the cell membrane despite being polar?
because water molecules are extremely small
does cholesterol have a polar hydrophilic group at one end?
yes and this group can attract the polar head groups of the phospholipid molecules
what can the non-polar hydrophobic part of a cholesterol molecule attract?
it can attract the non-polar fatty acid tails in the phospholipid
How does cholesterol control the fluidity of the cell membrane?
-it can prevent the cell surface membrane from becoming too fluid at high temperatures and stop the cell membrane from becoming too rigid at low temperatures.
-This occurs because cholesterol can make the phospholipid tails pack closer together or less close together
what are the 2 categories of proteins in the cell membrane?
intrinsic/integral and extrinsic/peripheral
describe intrinsic proteins
they’re fully embedded in the membrane from one side to the other. Because they pass right through the bilayer, intrinsic proteins have hydrophobic amino acids on the outside surface of the protein. These can interact with the hydrophobic fatty acid tails in the phospholipid bilayer.
what are 2 types of intrinsic protein?
carrier proteins and protein channels
describe protein channels
they run through the centre of the phospholipid bilayer. This channel is lined with hydrophilic amino acids and is filled with water molecules. They allow water -soluble molecules and ions to diffuse through (hydrophilic substances)
describe carrier proteins
they have a binding site for certain chemicals . When that chemical binds, it causes the tertiary structure of the carrier protein to change. This change in tertiary structure brings the chemical across the membrane where the chemical is now released
describe extrinsic proteins
they don’t span the membrane and are found on one side of the membrane or the other. They can sometimes be attached to intrinsic proteins
describe the functions of extrinsic proteins
- some play a structural role within the membrane
- some act as enzymes
- ## some are receptors for other molecules, such as hormones
what are glycoproteins?
membrane proteins attached to a carbohydrate molecule
what are the functions of glycoproteins?
- allowing cells to attach to each other to form tissues such as nervous tissues
- presenting antigens to T cells
- ## acting as receptors for hormones
what are glycolipids?
carbohydrates attached to phospholipid molecules
what are glycolipids used for?
-contacting other cells. the glycolipids of one cell can be recognised by another cell and this can determine whether cells come into contact .
- they act as antigens e.g in determining blood groups
what 2 factors affect the permeability of the cell membrane?
temperature and solvent concentration
what happens to the lipids when the temperature of the phospholipid bilayer increases?
-The lipids become more fluid and so the cell membrane becomes a less effective barrier to polar molecules and so they can easily pass through.
- Diffusion takes place at a higher speed due to increased kinetic energy. So, more molecules pass through
what happens to the proteins in the cell membrane at really high temperatures (temperatures exceeding 40 degrees)?
the proteins in the cell membrane denature, which means that it can no longer form an effective barrier and so substances can freely pass through the disrupted membrane
why do organic solvents increase membrane permeability
because they dissolve the lipids in the membrane, causing the membrane to lose its structure
what can be used to investigate how different factors affect membrane structure and permeability?
beetroot
the higher the permeability of the beetroot cell membrane…
the more pigment leaks out of the cell
summarise the practical that investigates the affects of temperature on membrane permeability
Test tubes containing beetroot are put into water baths of different temperatures. A colorimeter will consequently be used to measure the absorbance of light. The higher the absorbance of light, the greater the membrane permeability- as it means more pigment was released. The general trend would’ve been the higher the temperature, the greater the increase in membrane permeability. This is due to the increased fluidity and increased diffusion rate at higher temperatures.
why does membrane permeability also increase below 0 degrees Celsius?
-Increased permeability can be caused by channel or carrier proteins deforming at these low temperatures
-Ice crystals that form can also pierce the cell membrane, making it highly permeable
what are some limitations to the practical that investigates the affects of temperature on membrane permeability
-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
what is a solution to the fact that cuvettes may differ in thickness ?
use the same cuvette for every reading, or repeat the investigation many times and find a mean
what is a solution the fact that the beetroot pieces may not be identical in size and shape, meaning some test tubes could contain slightly more beetroot tissue than others?
cut the discs as accurately as possible using a scalpel and ruler, and repeat each investigation several times to find a mean
what is a solution the fact that some parts of beetroot tissue have more pigment in their cells than others?
conduct several repeats, using different parts of the beetroot and find a mean
what is diffusion?
The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration.
what factors is the rate of diffusion dependant on?
concentration gradient, surface area, the distance over which diffusion occurs
how does the steepness of the concentration gradient affect the rate of diffusion?
The greater the difference in concentration between 2 areas, the greater the rate of diffusion
how does the surface area affect the rate of diffusion?
The greater the surface area, the greater the rate of diffusion
how does the distance over which diffusion occurs affect the rate of diffusion ?
The shorter the diffusion path, the greater the rate of diffusion