Topic 2B - Cell Membranes - complete

- all cells are surrounded by membranes
- in eukaryotic cells, many of the organelles are surrounded by membranes too
- cell-surface membranes surround cells
- they are a barrier between the cell and its environment and control which substances enter and leave the cell
- the membranes are partially permeable meaning they let some molecules through but not others
- substances can move across the cell-surface membrane by diffusion, osmosis or active transport
- the membranes around organelles divide the cell into different compartments and they act as a barrier between the organelle and the cytoplasm
- they are also partially permeable and control what substances enter and leave the organelle

- the basic structure of all cell membranes is very similar
- they're composed of lipids (mainly phospholipids), proteins and carbohydrates (attached to proteins or lipids)
- in 1972, the fluid mosaic model was suggested to describe the arrangement of molecules in the membrane
- in the model, phospholipid molecules form a continuous bilayer (about 7nm thick)
- the bilayer is 'fluid' because the phospholipids are constantly moving
- cholesterol molecules are present within the bilayer
- proteins are scattered through the bilayer, like tiles in a mosaic
- these include channel proteins and carrier proteins, which allow large molecules and ions to pass through the membrane
- receptor proteins on the cell-surface membrane allow the cell to detect chemicals released from other cells
- the chemicals signal to the cell to respond in some way e.g. the hormone insulin binds to receptor proteins on liver cells, which tells the cells to absorb glucose

- some proteins are able to move sideways through the bilayer, while others are fixed in position
- some proteins have a polysaccharide (carbohydrate) chain attached ~ these are called glycoproteins
- some lipids also have a polysaccharide chain attached called glycolipids

- phospholipids molecules have a 'head' and a 'tail'
- the head is hydrophilic so it attracts water
- the tail is hydrophobic so it repels water
- the molecules automatically arrange themselves into a bilayer
- the heads face out towards the water on either side of the membrane
- the centre of the bilayer is hydrophobic so the membrane doesn't allow water-soluble substances (like ions) through it
- it acts as a barrier to these dissolved substances

- cholesterol is a type of lipid
- it is present in all cell membranes except bacterial cell membranes
- cholesterol molecules fit between the phospholipids 
- they bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together 
- this restricts the movement of the phospholipids, making the membrane less fluid and more rigid
- cholesterol helps to maintain the shape of animal cells (which don't have cell walls)
- this is particularly important for cells that aren't supported by other cells e.g. red blood cells, which float free in the blood

- the permeability of cell membranes is affected by different conditions e.g. temperature and solvent concentration
- you can investigate how these things affect permeability by doing an experiment using beetroot
- beetroot cells contain a coloured pigment that leaks out and the higher the permeability of the membrane, the more pigment leaks out of the cell
1) use a scalpel to carefully cut five equal sized pieces of beetroot
- rinse the pieces to remove any pigment released during cutting
2) add the five pieces to five different test tubes, each containing 5cm³ of water
- use a measuring cylinder or pipette to measure the water
3) place each test tube in a water bath at a different temperature (e.g. 10^oC, 20^oC, 30^oC, 40^oC and 50^oC) for the same length of time (measured using a stopwatch)
4) remove the pieces of beetroot from the tubes, leaving just the coloured liquid
5) now you need to use a colorimeter which is a machine that passes light through the liquid and measures how much of that light is absorbed
- the higher the absorbance, the more pigment released, so the higher the permeability of the membrane

Temperatures below 0
- the phospholipids dont have much energy, so they cant move a lot
- theyre packed closely together and the membrane is rigid
- channel proteins and carrier proteins in the membrane deform, increasing the permeability of the membrane
- ice crystals may form and pierce and the membrane making it highly permeable when it thaws
Temperatures between 0 and 45
- the phospholipids can move around and arent packed as tightly together 
- the membrane is partially permeable 
- as the temperature increases the phospholipids move more because they have more energy
- this increases the permeability of the membrane
Temperatures above 45
- the phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable 
- water inside the cell expands, putting pressure on the membrane
- channel proteins and carrier proteins deform so they can't control what enters or leaves the cell
- this increases the permeability of the membrane

- diffusion is the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration
- molecules will diffuse both ways, but the net movement will be to the area of lower concentration
- this continues until particles are evenly distributed throughout the liquid or gas
- the concentration gradient is the path from an area of higher concentration to an area of lower concentration
- particles diffuse down a concentration gradient
- diffusion is a passive process meaning no energy is needed for it to happen
- particles can diffuse across cell membranes, as long as they can move freely through the membrane
- e.g. oxygen and carbon dioxide can diffuse easily through cell membranes because they're small, so they can pass through spaces between the phospholipids
- they're also non-polar, which makes them soluble in lipids, so they can dissolve in the hydrophobic bilayer 
- when molecules diffuse directly through a cell membrane, its also known as simple diffusion

- some larger molecules (e.g. amino acids, glucose) would diffuse extremely slowly through the phospholipid bilayer because they're so big
- charged particles, e.g. ions and polar molecules, would also diffuse slowly
- this is because they're water soluble, and the centre of the bilayer is hydrophobic
- to speed things up, large or charged particles diffuse through carrier proteins or channel proteins in the membrane instead
- this is called facilitated diffusion
- like diffusion, facilitated diffusion moves particles down a concentration gradient, from a higher to a lower concentration
- it is also a passive process which means it doesnt use energy

Carrier Proteins
- move large molecules across membranes down their concentration gradient
- different carrier proteins facilitate the diffusion of different molecules
1) first, a large molecule attaches to a carrier protein in the membrane
2) then, the protein changes shape
3) this releases the molecule on the opposite side of the membrane

Channel Proteins
- form pores in the membrane for charges particles to diffuse through (down their concentration gradient)
- different channel proteins facilitate the diffusion of different charged particles

The Concentration Gradient:
- the higher it is, the faster the rate of diffusion
- as diffusion takes place, the difference in concentration between the two sides of the membrane decreases until it reaches an equilibrium 
- this means that diffusion slows down over time
Thickness of the Exchange Surface:
- the thinner the exchange surface (i.e. the shorter the distance the particles have to travel), the faster the rate of diffusion
Surface Area:
- the larger the surface area (e.g. of the cell-surface membrane), the faster the rate of diffusion

- some cells (e.g. epithelial cells in the small intestine) have microvilli which are projections formed by the cell-surface membrane folding up on itself
- microvili give the cell a larger surface area and in human cells, microvili can increase the surface area by about 600 times
- a larger surface area means that more particles can be exchanged in the same amount of time, increasing the rate of diffusion

Concentration Gradient
- the higher the concentration gradient, the faster the rate of facilitated diffusion (up to a point)
- as equilibrium is reached, the rate of facilitated diffusion will level off

Number of Channel or Carrier Proteins
- once all the proteins in a membrane are in use, facilitated diffusion cant happen any faster, even if you increase the concentration gradient
- so the greater the number of channel or carrier proteins in the cell membrane, the faster the rate of facilitated diffusion

- aquaporins are special channel proteins that allow the facilitated diffusion of water through cell membranes
- some kidney cells are adapted to have lots of aquaporins
- the aquaporins allow the cells to reabsorb a lot of the water that would otherwise be excreted by the body (about 180 litres need re-absorbing every day)

- osmosis is the diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential
- water potential is th potential of water molecules to diffuse into or out of a solution
- pure water has the highest water potential and all solutions have a lower water potential than pure water
- if two solutions have the same water potential, they're said to be isotonic

The Water Potential Gradient
- the higher the water potential gradient, the faster the rate of osmosis
- as osmosis takes place, the difference in water potential on either side of the membrane decreased, so the rate of osmosis levels off over time

Thickness of Exchange Surface
- the thinner the exchange surface, the faster the rate of osmosis

Surface Area of Exchange Surface
- the larger the surface area, the faster the rate of osmosis

- a simple experiment using potato cylinders will tell you the water potential of plant tissue
- you nees to make up several solutions of different, known concentrations to test the potato cylinders in
- to make 5 serial dilutions of a sucrose solution starting with an initial sucrose concentration of 2 M and diluting each solution by a factor of 2
1) line up 5 test tubes in a rack
2) add 10cm³ of the initial 2 M sucrose solution to the first test tube and add 5cm³ of distilled waer to the other four test tubes
3) then, using a pipette, draw 5cm³ of the solution from the first test tube, add it to the distilled water in the second test tube and mix thoroughly. Now you have 10cm³ of solution thats half as concentrated as the the solution in the first test tube (1 M)
4) repeat this process three more times to create solutions of 0.5 M, 0.25 M and 0.125 M

- you can make sucrose solutions of any concentration by finding the scale factor
EXAMPLE: if you want to make 15cm³ of 0.4 M sucrose solution
1) start with a solution of a known concentration e.g. 1 M
2) find the scale factor by dividing the concentration of this solution by the concentration of the solution you want to make (1 M / 0.4 M = 2.5)
3) this means the solution you want to make is 2.5 times weaker than the one you have.
- to make this solution 2.5 times weaker, use 2.5 times less of it, i.e. 15cm³ / 2.5 = 6cm³
- transfer this amount to a clean test tube
4) top up the rest of the test tube with distilled water to get the volume you want to make (add 9cm³ of distilled water)

1) use a cork borer to cut potatoes into identically sized chips, about 1cm in diameter
2) divide the chips into groups of three and measure the mass of each group using a mass balance
3) place one group into each different sucrose solutions
4) leave the chips in the solution for a set time (at least 20 minutes) 
5) remove the chips and dry gently with a paper towel
6) weigh each group again and record the results
7) calculate the percentage change in mass for each group
8) use the results to make a calibration curve, showing % change in mass against sucrose concentration

- the potato chips will gain water (and mass) in solutions with higher water potential and lose water in solutions with a lower water potential
- the point at which the curve crosses the x-axis (where % change = 0) is the point at which the water potential of the solution and the potato cells are the same
- find the concentration at this point, then look up the water potential for that concentration of sucrose solution

- this process is similar to facilitated diffusion
- 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
- there are two main differences between active transport and facilitated diffusion:
1) active transport usually moves solutes from a low to high concentration
- in facilitated diffusion, they always move from a high to a low concentration
2) active transport requires energy
- ATP is a common source of energy in the cell and is produced by respiration
- ATP undergoes a hydolysis reaction, splitting into ADP and P_i 

Co-Transporters
- this releases energy so that the solutes can be transported
- they bind two molecules at a time
- the concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient
EXAMPLE
- sodium ions move into the cell down their concentration gradient, which moves glucose into the cell as well (against its concentration gradient)

- when active transport moves molecules and ions against theur concentration gradient, a decreasing concentration gradient doesn't affect the rate of active transport

Speed of Individual Carrier Proteins
- the faster they work, the faster the rate of active transport

Number of Carrier Proteins Present
- the more proteins there are, the faster the rate of active transport

The Rate of Respiration in the Cell and the Availability of ATP
- if respiration is inhibited, active transport can't take place

- glucose is absorbed into the bloodstream in the small intestine
- in the ileum (the final part of the small intestine) the concentration of glucose is too low for glucose to diffuse into the blood
- glucose is absorbed from the lumen to the ileum by co-transport
1) sodium ions are actively transported out of the ileum epithelial cells, into the blood, by the sodium-potassium pump
- this creates a concentration gradient (there is now a higher concentration of sodium ions in the lumen of ileum inside the cell)
2) this causes sodium ions to diffuse from the lumen of the ileum into the epithelial cell, down their concentration gradient
- they do this via the sodium-glucose co-transporter protein
3) the co-transporter carries glucose into the cell with the sodium
- as a result the concentration of glucose inside the cell increases
4) glucose diffuses out of the cell, into the blood, down its concentration gradient through a protein channel, by facilitate diffusion