Topic 2B: Cell membranes Flashcards
What do cell membranes do?
- Surround cell and individual organelles
- Control exchange of substances
- Help communication with the environment with receptors
- Help organelle function (cristae in mitochondria, thylakoid in chloroplasts)
What is the cell membrane bi layer?
- Phospholipid bilayer
- Hydrophilic glycerol heads and hydrophobic fatty acid tails
- Tails face in to create the bi layer and be shielded
What substances can easily pass through the bi layer?
- small
- uncharged
- lipid soluble
What is the cell membrane model called?
- fluid mosaic
What are glycolipids?
lipid with carbohydrate chain attached
What are glycoproteins?
proteins with a carbohydrate chain attached
What do glycolipids and glycoproteins do?
- Act as receptors (glycoproteins)
- Help to stabilise the membrane
What does cholesterol do?
- Type of lipid
- Help regulate fluidity of the membrane
- Bind to hydrophobic tails so they pack more closely together –> restricts the movement of phospholipids and decreases fluidity
- Helps maintain the shape of animal cells (have no cell wall)
- Also have hydrophobic regions to act as a further barrier
What do the extrinsic proteins do?
- Channel proteins (facilitated diffusion) and carrier proteins (active transport) help to move substances across the membrane
What is membrane permeability like below 0°C?
- Little energy in molecules so little movement (of phospholipids)
-Closely packed so rigid membrane - Channel and carrier proteins can denature to increase permeability
- Ice crystals can form and pierce the membrane when it thaws –> inc permeability
What is membrane permeability like 0-45°C?
- Less tightly packed, partially permeable
- As temperature inc, phospholipids have more energy so move more and inc permeabiity
What is membrane permeability like above 45°C?
- Bi-layer begins to melt –> more permeable
- Water inside expands, inc pressure on the membrane
- Channel and carrier proteins denature, inc permeability
Describe the experiment to investigate membrane permeability.
- Solvents disrupt the membrane by inserting themselves into it and increase the permeability
- Beetroot has a red pigment in the vacuole that normally cannot fit through the membrane
- Put beetroot in different solvent concentrations in a water bath and measure the pigment released using a colorimeter and a calibration curve
- Can use different temperature instead of solvent concentrations
What is simple diffusion?
- Net movement of particles from high to low concentration
- Continues until molecules are equally distributed
- Passive
- Can occur across a cell membrane (directly)
What factors affect the rate of simple diffusion?
- Concentration gradient –> higher gradient = faster diffusion that slows over time as the gradient gets more shallow
- Thickness of the exchange surface –> thinner = shorter diffusion pathway
- Surface area –> higher = faster
What is facilitated diffusion?
- Helped diffusion of large, polar or water soluble molecules (glucose, ions etc)
- Move from high to low concentration
- Passive
- Use transport proteins
How do carrier proteins work?
- Move large molecules
- Molecule attaches
- Protein changes shape
- Molecule released on the other side of the membrane
How do channel proteins work?
- Move charged particles
- Form pores in the membrane
- Molecules diffuse through the pore
- Different channel proteins help different substances move
What factors affect the rate of facilitated diffusion?
- Concentration gradient –> higher = faster but slows over time as gradient gets more shallow
- Number of transport proteins –> Once all proteins in use, diffusion cannot happen any faster
What is osmosis?
- Diffusion of water from high to low water potential across a partially permeable membrane
What is water potential?
- Pure water = 0 water potential
- More negative = higher solute concentration
What is isotonic?
- Solutions have equal concentrations
What is hypertonic?
- In a higher water concentration (low solute conc)
What is hypotonic?
- In a lower water concentration (high solute conc)
What factors affect rate of osmosis?
- Water potential gradient –> higher = faster but slows over time as potential gets lower
- Thickness of exchange surface –> thinner = shorter diffusion pathway
- Surface area –> higher = faster
What happens to plant cells in different solutions?
- Outside = lower water potential –> water moves out, cytoplasm pulls away from cell wall = plasmolysed
- Outside = higher water potential –> water moves in, cytoplasm pushes against cell wall = turgud
What happens to animal cells in different solutions?
- Outside = higher water potential –> water moves in, no cell wall so cell eventually bursts = lysis
- Outside = lower water potential –> water moves out, cytoplasm pulls away from membrane = crenated
Describe the experiment to investigate the water potential of potato
- Put potato in different sucrose solutions and in a water bath and leave them
- Measure mass before and after to find change in mass ans then % change in mass
- Plot the results and where the line crosses the x-axis is the sucrose concentration of the potato
What is the calculation to make solutions from stock solution?
(Conc required / Conc stock solution) x volume solution required = volume of stock solution to use
What is active transport?
- Movement of molecules from low to high concentration (against conc gradient)
- Happens through carrier proteins specific to the substance
- Requires energy
How do carrier proteins work in active transport?
- Molecule attaches to the protein
- The protein changes shape
- The molecule is released on the other side of the membrane
- ATP is hydrolysed to ADP + Pi to release energy for this
What factors affect the rate of active transport?
Speed of individual carrier proteins –> faster they work = faster rate
- Number of carrier proteins present –> more = faster
- Rate of respiration in the cell and ATP availability
What are co-transporters?
- Type of carrier protein that bind to 2 molecules at a time
- The conc gradient of one is used to help move the other one against their conc gradient
- e.g. sodium ions move high –> low conc moving glucose with them (low–>high)
How is glucose co-transported and absorbed from the ileum to the blood?
- Sodium ions (Na+) are moved out of epithelium cell and into blood via active transport
- This creates a conc gradient (lower in cell than ileum) so Na+ now diffuse into the cell using the sodium glucose co-transporter from the ileum
- Co-transporter moves glucose with the Na+ against its conc gradient
- Conc of glucose then becomes higher in the cel than the blood so glucose moves out of the cell via facilitated diffusion using a carrier protein into the blood
