TOPIC 5 : BIOLOGICAL MEMBRANE Flashcards
2.5.1 - Cell membrane
Membrane at the cell surface ( plasma membrane )
- Plasma membranes are a barrier between the cell and its environment, controlling, which substances enter and leave the cell.
- They’re particailly permeable –> they let some molecules through but not others
- Substances can move across the plasma memebrane by diffusion, osmosis or by active transport
2.5.1 Cell membrane
What is the function of phospholipids ?
- Form a barrier to dissolve substances
- HEAD –> is HYDROPHILIC –> attracts water
- TAIL –> is HYDROPHOBIC –> repels water
- The molecules automatically arrange themselves into a bilayer
- The centre of the bilayer is hydrophobic so the membrane doesn’t allow water - soluable substances to diffuse through it - it acts as a barrier to these dissolved substances
2.5.1 Cell membrane
What is the function of glycoprotein and glycolipids?
- Stabilise the membrane by forming hydrogen bonds with surrounding water molecules
- Acts as receptors for messenger molecules in cell signalling and are sites where drugs, hormones and antibodies bind
- They’re also antigens- cell surface molecules involved in self - recognition and the immune response
2.5.1 Cell membrane
What is the function of cholesterol ?
- Gives the membrane stability
- Channel proteins –> movement of water - soluable substances
- Carrier proteins –> Transports specific substances which cannot pass directly across the membrane
- Enzymes and co-enzymes –> some reactions take place on the membrane
2.5.1 Cell membrane
What is the function of proteins?
- Controls what enters and leaves the cell
- Some form channels in the membrane, allowing small charged particles through
- Other proteins transports larger molecules and charged particles across the membrane by active transport and facilitated diffusion.
- Proteins also act as receptors for molecules in cell signalling
- When a molecule binds to the protein, a chemical reaction is triggered inside the cell
2.5.1 Cell membrane
What is the function of membranes within cells?
- They act as a barrier between the organelle and the cytoplasm
- Membranes can form vesicles to transport substances between different areas of the cell
- Membranes within cells are also particially permeable so they can control which substances enter and leave the organelle
- You can also get membranes within organelles - these act as barriers between the membrane contents and the rest of the organelle
- Memebranes within cells can be the site of chemical reactions. The membranes of some organelles are folded, increasing their surface are and making chemical reactions more efficient.
2.5.1 Cell membrane
How does solvents affect membrane permeability?
- The permeability of cell membranes depends on the solvent surrounding them
- This is because some solvents dissolve the lipids in a cell membrane, so the membrane loses its structure
- Some solvents increase membrane permeability more than others
- Increasing the concentration of the solvent will also increase membrane permeability
2.5.1 Cell membrane
How does temperature belowe 0’C affect membrane permeability?
- The phospholipds don’t have much energy, so they can’t move very much
- They’re packed closely together and the membrane is rigid
- But channel proteins and carrier proteins in the membrane denature, increasing the permeabilty of the membrane
- Ice crystals may form and pierce the membrane, making it highly permeable when it thaws
2.5.1 Cell membrane
How does temperature between 0 - 45’C affect membrane permeability?
- The phospholipds can move around and aren’t packed as tightly together - the membrane is particially permeable
- As the temperature increases the phospholipds move more because they have more energy - this increase permeability of the membrane
2.5.1 Cell membrane
How does temperature above 45’C affect membrane permeability?
- The phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable
- Water inside the cell expands, putting pressure on the membrane denature
- Channel proteins and carrier proteins in the membrane denature so they can’t control what enter or leaves the cell - this increases the permeability of the membrane
2.5.1 Cell membrane
2.5.1 Cell membrane
How does cholesterol and temperature link?
Cholesterol regulates the fluidity of membranes
At low temperatures, phospholipds cluster together more due to lower kinetic energy so fluidity decreases
However, cholesterol is inserted between the phospholipids, so distance between the phospholipids increases which means fluidity increases at low temperatures
2.5.1 Cell membrane
How does cholesterol and high temperatures link?
At high temperatures, the phospholipids have more kinetic energy so move farther apart and the membrane
However, cholesterol pulls the phospholipds closer together so decreases membrane fluidity
2.5.2 Cell membrane and signalling
What is cell signalling?
- Communicate with each other to control processes inside the body and to respond to changes in the environment
- Cell communicate with each other by cell signalling, which uses messenger molecules
- Cell signalling starts when one cell release a messenger molecule
- This molecule travels to another cell
- The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane
- The binding then triggers a change in the cell
2.5.2 Cell membrane and signalling
What is membrane receptors?
- Proteins in the cells membrane act as receptors for messenger molecules
- These receptor proteins are called membrane-bound receptors
- Receptor proteins have specific shapes - only messenger molecules with complementary shape can bind to them
- Different cells have different types of receptors - they repond to different messenger molecules
- A cell that responds to a particular messenger molecule is called a target cell
2.5.2 Cell membrane and signalling
What is the role of drugs in cell membranes?
They either trigger a reponse in the cell, or block the receptor and prevent it from working
2.5.3 Diffusion and Osmosis
What is diffusion?
Diffusion is the net movement of molecules or ions from a region where they are at high concentration to a region where they are at a lower concentration.
Passive process - does not require ATP
Down the concentration gradient
2.5.3 Diffusion and Osmosis
What factors affect the rate of diffusion?
- The concentration gradient - the highest it is, the faster the rate of diffusion
- The thickness of the exchange surface - the thinner the exchange surface
- The surface area - the larger surface area, the faster the rate of diffusion
- The temperature - the warmer it is, the faster the rate of diffusion because the particles have more kinetic energy so they move faster
2.5.3 Diffusion and Osmosis
What is osmosis?
The net movement of water molecules from an area of higher water potential to an area of lower water potentail across a partially permeable membrane
2.5.3 Diffusion and Osmosis
What happens to osmosis when has a water potential of zero?
Pure water has a water potential of zero
Adding solutes to pure water lowers its water potential- so the water potential of any solution is always negative
The more negative the water potential, the stronger the concentration of solutes in the solution
EXAMPLE:
Glass A contains pure water its’s got a water potential of zero
Glasss B contains a solution of orange squash
The orange squash molecules are a solute
They lower the concentration of the water molecules
This means that the water potential of the orange squash is lower than the water potential of pure water
2.5.3 Diffusion and Osmosis
What happens in isotonic solutions?
Cells in a isotonic solution won’t lose or gain any water - there’s no net movement of water molecules because there’s no difference in water potential between the cell and the surrounding solution.
2.5.3 Diffusion and Osmosis
What happens in hypotonic solutions?
If the cell is placed in a solution that has a higher water potential, water will move into the cell by osmosis
Solutions with a higher water potential compared with the inside of the cell are called hypotonic
An ANIMAL cell in a hypotonic solution will swell and could eventually burst
IN A PLANT CELL:
If in a hypotonic solution, the vacuole will swell and the contents and the vacuole and cytoplasm will push against the cell wall
This causes the cell to become turgid
The cell won’t burst because the inelastic cell wall is able to withstand the increase in pressure
2.5.3 Diffusion and Osmosis
What happens in hypertonic solutions?
If a cell is placed in a solution that has a lower water potential, water will move out of the cell by osmosis
Solutions with a lower water potential than the cell are called hypertonic
If an ANIMAL cell is placed in a hypertonic solution it will shrink
If a PLANT cell is placed in a hypertonic solution it will become flaccid (limp)
The cytoplasm and plasma membrane will eventually pull away from the cell wall –> called plasmolysis
2.5.3 Diffusion and Osmosis
How is water potential measured?
kiloPascals (kPa)
