2B - Cell Membranes Flashcards
What are the different types of membrane?
- Cell membranes
* Organelle membranes
Do all cells have a cell membrane?
Yes
What type of cells also contains organelle membranes?
Eukaryotic cells
What is the function of cell surface membranes?
Control what goes in and out of cells.
What term is used to describe the way in which cell membranes control what goes in and out of cells?
Partially permeable
What are the types of molecule in cell membranes?
- Lipids (mainly phospholipids)
- Proteins
- Carbohydrates
How do carbohydrates fit in the cell membrane?
They attach to proteins or lipids (glycoproteins and glycolipids).
What is the model used to describe cell membranes?
Fluid mosaic model
Describe the fluid mosaic model.
- Continuous phospholipid bilayer -> These are constantly moving (thus “fluid”)
- Cholesterol molecules are inside the bilayer
- Channel, carrier and receptor proteins are either fixed or can move through the bilayer
- Polysaccharides attach to some proteins and lipids to form glycoproteins and glycolipids
Name all of the individual components of a cell membrane.
- Phospholipids
- Cholesterol
- Proteins -> Channel, Carrier and Receptor
- Glycoproteins
- Glycolipids
What is the role of receptor proteins in the cell-surface membrane?
- Allow the cell to detect chemicals released from other cells.
- These chemicals signal for the cell to respond in some way.
Give an example of how receptor proteins work.
Insulin binds to receptor proteins on liver cells, which tells them to absorb glucose.
Explain why the cell-surface membrane can be described as having a fluid mosaic structure.
- “Fluid” -> The phospholipids (and some other molecules) in the bilayer can move around freely.
- “Mosaic” -> The phospholipids, proteins and other molecules are arranged in a mosaic-like pattern.
What is the role of phospholipids in cell-surface membranes?
Arrange themselves in a bilayer, which acts as a barrier for water-soluble substances.
Why do phospholipids arrange themselves in a bilayer in cell-surface membranes?
- Have a hydrophilic head and hydrophobic tail
* In water, a bilayer forms with the heads facing out and the tails facing in
Why does the phospholipid bilayer in cell-surface membranes not allow water-soluble substances to pass through it?
The centre of the bilayer is hydrophobic (that’s where the hydrophobic tails are).
Which parts of a phospholipid molecule are hydrophobic and hydrophilic?
- Hydrophobic - Tail
* Hydrophilic - Head
What type of molecule is cholesterol?
Lipid
In what type of cell membrane is cholesterol found?
All types, except bacterial cell membranes.
Where in the cell-surface membrane is cholesterol found and what does this do?
- Between phospholipid molecules
* This restricts the movements of the phospholipids, making the membrane less fluid and more rigid
What is the role of cholesterol in cell-surface membranes?
- The cholesterol between phospholipids restricts their movement and makes the membrane more rigid
- This maintains the shape of cells
In which cells is cholesterol in the membrane especially important?
Cells that aren’t supported by other cells (e.g. RBCs floating freely in the blood)
What experiment can be used to investigate how temperature affects cell membrane permeability?
Beetroot in water experiment
Describe an experiment to investigate the effect of temperature on cell membrane permeability (beetroot).
1) Cut 5 equal sized pieces of beetroot and rinse.
2) Add the pieces to 5 test tubes and add 5cm3 of water to each.
3) Place each tube in a water bath at a different temperature (e.g. 10C, 20C, 30*C, etc.) and for the same length of time.
4) Remove the beetroot and keep only the coloured water.
5) Use a colorimeter to measure the absorption of each sample - The higher the absorbance, the more pigment is released, so the higher the membrane permeability.
6) Draw a graph of the results.
What is a colorimeter and what must be done when using it?
• A device that passes light through liquid and measures how much is absorbed.• First, it must be calibrated at 0 by taking a measurement with pure water
Describe how cell membrane permeability changes with temperature.
• There is a minimum permeability at a certain temperature (e.g. 0C).
• Below this temperature (below 0C), permeability increases rapidly.
• Above that temperature (0 - 45C), permeability increases gradually. Eventually (above 45C), it increases rapidly.
(See diagram pg 37 of revision guide)
Explain membrane permeability below 0*C.
• Permeability increases rapidly as temperature decreases.
BECAUSE:
• Channel and carrier protein deform
• Ice crystals may form, piercing the membrane
Explain membrane permeability 0 - 45*C.
• Permeability increases as temperature increases.
BECAUSE:
• Phospholipids gain energy, move around faster and are less tightly packed
Explain membrane permeability above 45*C.
• Permeability increases rapidly as temperature increases.
BECAUSE:
• Phospholipid bilayer starts to melt
• Water inside the cell expands, putting pressure on the membrane
• Channel and carrier proteins denature
Name 2 factors that may affect the permeability of cell membranes.
- Temperature
* Solvent concentration
What is diffusion?
- The net movement of particles from an area of higher concentration to an area of lower concentration.
- It is a passive process.
In diffusion, do particles only move from high to low concentration?
No, they move both ways, but the NET movement is from high to low concentration.
For how long does diffusion happen?
Until the particles are evenly distributed throughout the liquid or gas.
When answering a diffusion question, what is it important to mention?
It is a PASSIVE process.
Does diffusion / facilitated diffusion require energy?
No, it is a passive process.
When can diffusion happen across a membrane?
When:
• Going from high to low concentration
• The particles can move freely through the membrane
Why can oxygen and CO2 easily diffuse through cell membranes?
- Small -> Can pass between phospholipids
* Non-polar -> Soluble in lipids + can dissolve in the hydrophobic bilayer
What are the two types of diffusion?
- Simple diffusion
* Facilitated diffusion
What is the difference between simple and facilitated diffusion?
Simple diffusion doesn’t require any carrier proteins or channel proteins, while facilitated diffusion does.
Why is there a need for facilitated diffusion through cell membranes?
- Larger molecules -> Would diffuse extremely slowly
* Charged particles -> Are water soluble and the bilayer is hydrophobic
What is facilitated diffusion?
- When carrier and channel proteins move particles across a membrane from an area of higher concentration to an area of lower concentration.
- It is a passive process.
What are the two types of protein used in facilitated diffusion?
- Carrier proteins
* Channel proteins
What type of particle do carrier and channel proteins move across cell membranes?
- Carrier proteins -> Large molecules
* Channel proteins -> Charged particles
Can one carrier/channel protein facilitate the diffusion of every molecule?
No, different carrier/channel proteins facilitate the diffusion of different molecules/charged particles.
Do diffusion and facilitated diffusion happen up or down a concentration gradient?
Down
How does a carrier protein work?
1) A large molecule attaches to the protein.
2) Protein changes shape.
3) This releases the molecule on the other side of the membrane.
(See diagram pg 38)
How does a channel protein work?
1) Proteins form pores in the membrane.
2) Charged particles can diffuse through these.
(See diagram pg 38)
Remember to revise facilitated diffusion diagrams.
Pg 38 of revision guide.
What is another set of terms for cell-surface membranes and organelle membranes?
External and internal membranes
What factors affect the rate of diffusion?
1) Concentration gradient
2) Thickness of exchange surface
3) Surface area
How does the rate of diffusion change over time and why?
- As diffusion happens, the difference in concentration between both sides decreases until it reaches equilibrium.
- So the rate slows down over time.
Give an example of an exchange surface adapted for diffusion.
- Epithelial cells in the small intestines -> Have microvilli (projections formed by the cell-surface membrane folding)
- Increase the surface area
What factors affect the rate of facilitated diffusion?
1) Concentration gradient
2) Number of carrier/channel proteins
Why does the number of carrier/channel proteins affect the rate of facilitated diffusion?
Once all of the proteins in the membrane are in use, facilitated diffusion cannot happen any faster, even if you increase the concentration gradient. So only increasing their number will speed up facilitated diffusion.
Give an example of an exchange surface adapted for facilitated diffusion.
- Kidney cells -> Have lots of aquaporins (channel proteins that allow the facilitated diffusion of water)
- These allow the kidneys to reabsorb lots of water
What is osmosis?
The diffusion of water molecules across a partially permeable membrane, from high to low water potential.
What is water potential?
A measure of how likely water molecules are to diffuse out of a solution.
What does a high water potential mean?
Water is very willing to move away from that solution.
What solution has the highest water potential?
Pure water (0KPa)
What is the unit for water potential?
Kilopascals (KPa)
What is unusual about water potential?
It is usually negative, so the water potential of 0KPa is actually the highest.
What is the term for two solutions having the same water potential?
Isotonic
In terms of water potential, what do hypotonic, hypertonic and isotonic mean?
- Hypertonic - Lower water potential
- Hypotonic - Higher water potential
- Isotonic - Same water potential
Which water potential is higher, -200 or -400KPa?
-200KPa
What does hypertonic mean?
Having a higher solute concentration.
What does hypotonic mean?
Having a lower solute concentration.
What factors affect the rate of osmosis?
1) Water potential gradient
2) Thickness of exchange surface
3) Surface area of exchange surface
What is a serial dilution?
When samples of several different concentrations of a solution are made by taking a sample of the previous and adding water to make the next.
Describe how you would obtain solutions of 2M, 1M 0.5M, 0.25M and 0.125M sucrose solution when given some 2M sucrose solution.
1) Line up 5 test tubes.
2) Add 10cm3 of the 2M sucrose solution to the first.
3) Add 5cm3 from this one and add it to the next tube. Then add 5cm3 of distilled water to make 1M.
4) Repeat step 3 another 3 times to get solutions of 0.5M, 0.25M and 0.125M.
Describe how you can make a solution of a particular concentration when given a solution of known concentration.
1) Start with a solution of known concentration.
2) Find the scale factor by dividing this concentration by the target concentration.
3) You now know that the target solution must be that many times weaker than the original.
4) Look at the volume that you need and divide it by the SF to find the amount of the original solution you need.
5) Fill the rest of the volume required with distilled water.
Use the scale factor method to make 15cm3 of 0.4M of sucrose solution when given a 1M sucrose solution.
- SF = 1M / 0.4M = 2.5
- 15cm3 / 2.5 = 6cm3
- Therefore, use 6cm3 of the 1M solution and 9cm3 of distilled water.
Describe an experiment to find the water potential of some potato cells.
1) Use a cork borer to cut potatoes into identical length cylinders
2) Place the cylinders into groups of 3 and measure the mass of each group using a mass balance
3) Put each group into a different concentration of sucrose solution
4) Leave for 20 minutes
5) Remove the potato and dry each piece with a paper towel
6) Weigh each group and calculate % change in mass
7) Draw a calibration curve of sucrose concentration (x) against % change in mass (y)
8) Read at which concentration there is no change in mass
REMEMBER: This is a concentration, not the water potential. You will need to look up the water potential value that corresponds to this sucrose concentration.
What is the graph draw when calculating the water potential of some potato cells called?
Calibration curve
In the experiment to find the water potential of some potato cells, is it enough to read off the concentration at which the graph crosses the x-axis?
No, this is the CONCENTRATION of the cells, not their WATER POTENTIAL. You need to look up the water potential that corresponds to that sucrose solution concentration.
What is active transport?
The movement of molecules and ions across a membrane from low to high concentration requiring energy.
What molecules are required in active transport?
Carrier proteins
In what types of transport are channel and carrier proteins used?
- Carrier - Facilitated diffusion + Active transport
* Channel - Facilitated diffusion
Are channel proteins involved in active transport?
No, only carrier proteins.
What are the 2 main differences between facilitated diffusion and active transport?
1) AT moves solutes from low to high concentration, while FD moves them from high to low
2) AT requires energy, while FD is passive.
Where does the energy used in active transport come from?
- ATP is produced by respiration.
* It undergoes hydrolysis to split into ADP and Pi, releasing energy for active transport.
What is it called when two different molecules are moved by a protein across a membrane, using the concentration gradient of one to move the other?
Co-transport
What type of molecule is a co-transporter?
Carrier protein
What is co-transport?
When two different molecules are moved by a protein across a membrane, using the downward concentration gradient of one to move the other against its concentration gradient.
Describe the process of co-transport.
1) 2 different molecules bind to the co-transporter.
2) One goes from high to low concentration, and this is used to move the other from low to high concentration.
Give an example of co-transport.
- Sodium ions and glucose
* Sodium ions move down their concentration gradient, moving glucose against its concentration gradient.
What factors affect the rate of active transport?
1) Speed of carrier proteins
2) Number of carrier proteins
3) The rate of respiration + Availability of ATP
Is the rate of active transport affected by the concentration gradient?
No
What type of transport is co-transport most closely related to?
Active transport
What example of co-tranport do you need to know about?
The absorption of glucose in the mammalian ileum.
What is the ileum?
The final part of the small intestine.
Where is glucose absorbed into the bloodstream?
The small intestine.
Why is the process of absorbing glucose into the bloodstream different in the ileum?
- The ileum is the final part of the small intestine.
- The concentration of glucose is too low for glucose to diffuse into the blood (since most has been absorbed by then).
- Therefore, co-transport is used instead.
Describe the absorption of glucose into the blood in the ileum.
1) Na+ ions move from ileum epithelial cells into the blood by active transport -> Using a sodium-potassium pump.
2) This creates a concentration gradient between the ileum and the cells.
3) Na+ ions can diffuse from the ileum into the cells via sodium-glucose co-transporter proteins.
4) The co-transporter takes glucose with it from low to high concentration.
5) This increases the glucose concentration of the cells.
6) Glucose can now move into the blood by facilitated diffusion from high to low concentration -> Using a channel protein.
Remember to practice drawing out the diagram for absorption of glucose in the ileum.
Pg 43 of revision guide.
