Transport Across Membranes Flashcards
Structure of cell membranes
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What are plasma/cell membranes?
All cells are surrounded by one - allow substances to enter and exit the cell
What is the basic structure of cell membranes the same as?
It is the same for cell-surface membranes and membranes around cell organelles of eukaryotes
What cells inside the cell have single membranes?
- Endoplasmic reticulum
- Golgi
- Lysosomes
- Vesicles
- Vacuoles
What cells inside the cell have double membranes?
- Mitochondria
- Nucleus
What functions do the membranes inside the cell have?
- Separate organelles form the cytoplasm so that specific metabolic reactions can happen in them (Respiration in mitochondria and photosynthesis in chloroplasts)
- Control the entry and exit of materials organelles (Mitochondria)
- Provide an internal support system (Endoplasmic reticulum)
- Isolates enzymes that might damage the cell (Lysosomes)
- Provides surfaces for reactions (Protein synthesis on RER)
What type of permeability are plasma membranes?
Partially permeable and can control which substances can enter and leave the cell - allow small molecules but not large ones
What do the plasma membranes act as?
Act as barriers that create separate environments
What functions do plasma membranes have?
- Can keep the components of the cell isolated from the external environment
- Compartmentalize cellular functions by separating the inside of organelles from the cytoplasm
- Communication with external environments
- Allows the cell to change shape
What is the plasma membrane made up of?
Made up of a bi-layer of phospholipids
What is the structure of a phospholipid bilayer?
- The phosphate group is hydrophilic (attracts water)
- The fatty acid chains are hydrophobic (repel water)
How have the structure and function of the cell membrane?
Have been defined by over half a century of research using biochemical, physiological, cellular and molecular techniques
What is embedded within the phospholipid bilayer?
Has integrated proteins, glycoproteins and glycolipids
What are phospholipid bilayers called fluid mosaic model?
‘fluid’ - because the phospholipids are constantly moving
‘mosaic’ - proteins are scattered through the bilayer like tiles in a mosaic
What is the function of the phospholipid bilayer?
- Form a barrier to dissolved substances * The centre of the bilayer is hydrophobic so doesn’t allow water soluble substances (like ions and polar molecules) through it but does allow small, non-polar molecules like CO, to diffuse through
What is the function of intrinsic proteins?
Channel proteins and carrier proteins allow large molecules, polar molecules and ions to pass through the membrane
What is the function of peripheral proteins?
Can act as receptors and allow the cell to detect chemicals released from other cells so that they can respond
What is the function of glycoproteins?
Proteins with carbohydrates attached
What is the function of glycolipids?
Lipids with a carbohydrate attached
What is the function of cholesterol?
- A type of lipid which fits between the phospholipids
- Restricts the movement of other molecules in the membrane and gives it stability by binding to the tails of phospholipids and causing them to pack together more closely
- Reduces their movement and makes the membrane less rigid
- Has hydrophobic region so acts as a barrier to polar substances moving through the membranes
Factors affecting membranes
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What are plasma membranes described as fluid?
Because the phospholipids and proteins can move around
The amount of movement and gaps (and therefore permeability) can be affected by temperature and solvents
How does temperature effect the phospholipid bilayer?
As temperature increases, phospholipids will have more kinetic energy, so they move faster making the bilayer less gel-like and more fluid
How are the phospholipid bilayer affected below 0°C?
- Phospholipids are packed closely together and the membrane is rigid
- Channel and carrier proteins can denature which increases the permeability of the membrane
- Ice crystals can also form and pierce the membrane making it more permeable when it thaws.
How are the phospholipid bilayer affected from 0-45°C?
- Phospholipids can move around freely and the membrane is partially permeable
- As the temperature increases the kinetic energy increases and makes the membrane more permeable
How are the phospholipid bilayer affected above 45°C?
- The phospholipids start to melt and the membrane becomes more permeable * Water inside the cell expands putting pressure on the membrane
- Channel proteins and carrier proteins in the membrane denatures
- This increases the permeability of the membrane.
Can non-polar solvents insert themselves in the bilayer?
Non-polar solvents such as alcohols or acetone can insert themselves into the bilayer
How do ethanol molecules insert themselves in the bilayer?
- Ethanol, for example, can form hydrogen bonds with a phospholipid molecule near the ester bonds
- Pushes the phospholipids out of their orderly placement and increases their movement
- This disrupts the membrane structure increases its permeability, solvents can denature proteins by distrupting bonds
- This is how alcohol works to breakdown bacterial cell membranes and kill the cell
How can you investigate how different variables affect cell membrane permeability?
Can investigate how different variables that affect the cell membrane permeability by doing experiments using beetroot
What do the beetroot cells contain?
They contain red pigments called betalains located within the cell vacuole
What do the pigments in the beetroot cell normally do?
Normally the pigments can’t pass through membrane, but they leak out when the beetroot is cooked
What happens to the pigments in the beetroot cell when at high temperatures?
The high temperatures increase membrane fluidity and cause the pigment to leak out of the cell, and the amount of pigment can be measured by using a colorimeter
What is a colorimeter?
It is a machine that passes light of a specific wavelength through a liquid and measures how much of that light is absorbed
If the concentration of the solution is greater, more light will be absorbed, which can be identified by measuring the difference between the amount of light at the start and that after passing through the solution
What is the filter in the colorimeter used to do?
The filter in the colorimeter is used to select the colour of light which the solute absorbs the most to maximise the accuracy of the experiment
Colour of the absorbed light is the ‘opposite’ of the colour of the specimen, so a blue filter would work for an orange substance
How do you determine the concentration of an unknown solution?
Several sample solutions of a known concentration are first prepared and tested
The concentration are plotted on a graph against absorbance, thereby generating calibration curve. The results of the unknown sample are compared to that of the known sample of the curve to measure the concentration
Diffusion
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What is the definition of diffusion?
- Diffusion is net movement of molecules or ions from an area of a high concentration to an area of a low concentration
- Although all particles move in both directions there will always be an overall (net) movement of particles towards the lower concentration
What does diffusion continue until and what type of process is it?
- It continues until equilibrium is reached
- This is a passive process (does not require any energy as the particles are moving down their concentration gradient.
Where does diffusion occur across?
- Diffusion often happens across a partially permeable membrane
- Molecules can diffuse simply through the membrane if they are not too large or polar e.g oxygen and carbon dioxide.
How does the concentration gradient affect the rate of diffusion and why?
- The greater the concentration difference the faster the rate of diffusion
- A good blood or air supply helps to maintain concentration gradients by transporting substances to and from the membrane. Otherwise diffusion slows down over time as equilibrium is reached.
How does the membrane thickness affect the rate of diffusion and why?
- The thinner the membrane the faster the rate of diffusion
- Thin membranes reduce the distance the particles have to travel
How does the surface area affect the rate of diffusion and why?
- The larger the surface area the faster the rate of diffusion
- Increasing the surface area of a membrane (e.g by folding) means more particles can be exchanged in the same amount of time, increasing the rate of diffusion
How does the temperature affect the rate of diffusion and why?
- The higher the temperature the faster the rate of diffusion
- At higher temperatures, particles will have more kinetic energy so they will be able to move around faster
What is facilitated diffusion?
- Larger molecules (e. amino acids, glucose) and charged particles (eg ions and polar molecules would move very slowly through the membrane so they need to diffuse through specialised carrier or channel proteins instead - this is facilitated diffusion
- The proteins help to “facilitate” the movement of the particles through the membrane
What type of process is facilitated diffusion?
- This is still diffusion as particles are still moving down their concentration gradient (from high to low concentration) and it is still a passive process as no energy is involved
- It is much faster than simple diffusion
What is a carrier protein?
- Move large molecules across the membrane
- Different proteins facilitate the diffusion of different large molecules
- Large molecules attach and then the protein changes hape releasing the molecule on the other side of the membrane
What is a channel protein?
- Form pores for charged particles to diffuse through
- Different protein channels facilitate the diffusion of different charged particles
- No changing shape, simple “tunnel” which allows the charged particles to cross the membrane
How does the concentration gradient affect the rate of facilitated diffusion and why?
- The greater the concentration
difference the faster the rate of facilitated diffusion - As particles are still moving down the concentration gradient.
- This is true until all the proteins are in use or once equilibrium is reached, then the rate will level off/slow down.
How does the number of channel or carrier proteins affect the rate of facilitated diffusion and why?
- The greater the number of channel or carrier proteins in the plasma membrane the faster the rate of facilitated diffusion
- This is true up to a certain point, once all the proteins in the membrane are in use facilitated diffusion can’t happen any faster even if you increase the concentration gradient
- So, it becomes a limiting factor.
- The line at the bottom represents simple diffusion.
Osmosis
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What is the definition of osmosis?
Osmosis is the net diffusion of water molecules from a region of high water potential to an area
of lower water potental across a partially permeable membrane
In relation to water potential in what direction will water move?
Moves from less negative to more negative
How is water concentration described as?
- Describe water concentration using the term water potential (ψ - kPa)
- Water potential is the potential or likelihood of water molecules to diffuse out of or into a solution
What is the water potential of pure water?
Pure water has a water potential of zero (O) because no water will diffuse into this solution but there is a high chance that water will leave
What makes the water potential lower?
Any solution that contains a solute dissolved in water will have a lower (more negative) water potential than the pure water
What is the scale of water potential?
Weak salt solution
0
-1
-2
-3
-4
-5
Strong salt solution
What does isotonic mean?
Two solutions with the same water potential - no net movement of water
How does a hypotonic solution affect water potential?
Less negative water potential (less solute)
How does a hypertonic solution affect water potential?
More negative water potential (more solute)
How does water move from a hypertonic solution (dilute solution, pure water) with a higher water potential move in relation to a cell?
The solution has a higher water potential than the inside of the cell so water will move into the cell by osmosis
How does a solution with a higher water potential that in the cell (dilute solution, pure water) affect an animal cell?
- Animal cells swell up and will eventually burst as their cell membrane is not strong enough to withstand the pressure
- Lysed
How does a solution with a higher water potential that in the cell (dilute solution, pure water) affect a plant cell?
Plant cells swell up but do not burst because their cell wall protects them - their vacuoles expand, and they become turgid (swollen) as their cell membrane cell membrane pushes on the cell wall
How does water move from an isotonic solution move in relation to a cell?
There will be no net movement of water (no osmosis) because there is no difference in water potential between the cell and the surrounding solution
How does an isotonic solution affect an animal cell?
It remains normal - unaffected
How does an isotonic solution affect a plant cell?
Remains flaccid - normal
How does water move from a hypotonic solution (concentrated solution) with a lower water potential move in relation to a cell?
The solution has a lower water potential than the inside of the cell so water will move out of the cell by osmosis
How does a solution with a lower water potential that in the cell (hypotonic solution, concentrated solution) affect an animal cell?
Animal cells will shrivel (shrink) as they los3 water from their cytoplasm
How does a solution with a lower water potential that in the cell (hypotonic solution, concentrated solution) affect a plant cell?
Plant cells are protected from shrinking, but their vacuoles shrink and eventually their cell membranes pull away from the cell walls, this is known as plasmolysis
How does the water potential gradient affect the rate of osmosis and why?
- The higher the water potential gradient
(difference) the faster the rate of osmosis - As osmosis takes place the difference in water potential either side of the membrane decreases as the solutions become closer to being isotonic so the rate of osmosis levels off over time
How does the membrane thickness affect the rate of osmosis and why?
- The thinner the membrane the faster the rate of osmosis
- Thin membranes reduce the distance the molecules have to travel
How does surface area affect the rate of osmosis and why?
- The larger the surface area the faster the rate of osmosis
- Increasing the surface area of a membrane (e.g by folding) means more molecules can move across in the same amount of time, increasing the rate of osmosis
Serial dilutions
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What is a serial dilution?
A serial dilution is any dilution in which the concentration decreases by the same factor in each successive step
If you add a 1cm³ to 9ml of water what dilution factor do you have?
This is a 1 in 20 dilution or the solution has been diluted by a factor of 10, 10x more dilute than the original
What is the first step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2
Fill a test tube with 10 cm’ of the 2M sucrose solution
How do you find the dilution factor?
The dilution factor can be calculated using the initial volume of stock solution (Vi) divided by the final solution volume (Vt).
DF = Vi/Vf
What is the second step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2
Fill 5 test tubes with 5 cm’ of distilled water.
What is the third step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2
Using a pipette transfer 5 cm³ of of the sucrose solution from the test tube to the first tube of 5 cm³ of water
What is the fouth step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2
- Mix the solution thoroughly. You now have 10 cm³ of sucrose solution that is half as concentrated as the solution in the stock solution (1M)
- Dilution factor is 5ml/10ml = 1/2 or 1:2 dilution
What is the fifth step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2
Repeat the process by transferring 5 cm³ from the 1M solution and adding it to the next tube containing distilled water to make a 10 cm³ solution which is now half as concentrated again (0.5M)
What is the sixth step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2
Repeat to create 0.25M and 0.125M solutions
What are the units for making solutions?
Lor dm³ = units of volume (1 dm⁻³ = 1 Litre)
ml or cm³ = units of volume (1 cm⁻³) = 1ml)
M= the unit for concentration (A 0.1M solution = 0.1 mol dm⁻³)
0.0974 M means 0.0974 mol dm⁻³ or 0.0974 mmol cm⁻³
mol dm⁻³ = unit of concentration units of moles per cubic decimetre (or
per L)
mol cm⁻³ = unit of concentration units of moles per cubic centimetre (or per ml) - To convert mol dm⁻³ 0.0974 mol dm⁻³ = 97.4 mol cm⁻³
What does the method of finding a solution rely on?
You can apply this method to making any solution as long as you knowthe volume of the solution you want to make and the concentration of the final and original/stock solution
How would you make 15cm³ of 0.4 mol dm⁻³ sucrose solution using a stock of 1M
First you find the dilution factor
DF = 1M/0.4M = 2.5
So we need to make a solution which is 2.5 times weaker than the stock solution
If we had 15 cm³ of 1M to make it 2.5x weaker we would need to use 15/2.5 = 6 cm³
Then we would need to dilute it with 9 cm³ of water to keep the volume 15 cm³
What is a calibration curve?
- Calibration curves are graphs used to determine an unknown concentration of a sample by comparing the unknown to a set of standard samples with known concentrations - they are also known as standard curves
- A dilution series can be used to create a set of samples with known concentrations
What can a calibration curve be used to work out?
- A calibration curve can be used to determine an unknown water potential in a potato sample
- Water potential is the tendency of water to diffuse from one area to another
- Water molecules move from areas of high water potential to areas of low water potential by osmosis
- The water potential is determined by the concentration of solutes
- The movement of water in and out of cells is related to the relative concentration of solutes either side of the cell membrane
Investigating water potential practical
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What is the equipment list for the investigating investigating water potential practical?
• Potato tuber
• Cork borer
• Scalpel
• Ruler
• Distilled water
• Sucrose solution (1M)
• Boiling tubes
• Boiling tube rack
• Timer
• Digital balance
• Paper towels
What is step 1 of the investigating water potential practical?
Make a series of dilutions of 1M sucrose solution. These should be at 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0M sucrose. Dilute using distilled water.
What is step 2 of the investigating water potential practical?
Measure 5cm³ of each dilution into separate test tubes
What is step 3 of the investigating water potential practical?
- Use a cork borer to cut out six potato chips and cut down the sections into identically sized chips
(Ensure all skin is removed as it could affect results) - Dry each chip using a paper towel to remove excess water but do not squeeze
(Helps remove any excess water that would affect starting mass)
What is step 4 of the investigating water potential practical?
Weigh each before the start of the experiment
What is step 5 of the investigating water potential practical?
Place a potato chip in each test tube (one per sucrose concentration) and leave for 20 minutes
(Must know which solution is which)
What is step 6 of the investigating water potential practical?
- Remove each potato chip
- Dry gently using paper towel
(To remove excess solution on the outside of the chip that would affect mass) - Weigh them in turn
(If they have gained water by osmosis, they will have increased in mass. If water has left by osmosis, they would have decreased in mass)
What is step 7 of the investigating water potential practical?
- Calculate the change in mass for each sucrose solution
(Done by subtracting the final mass from the initial mass - those that have decreased in mass will have negative results) - Calculate the percentage change in mass for each sucrose solution
(Means you can compare the effect on chips that don’t have the same initial mass - change in mass/starting mass = % change)
Why is a scalpel a hazard?
There is a risk of there being cuts from sharp object
What are the safety precautions for using a scalpel?
Cut away from fingers; use forceps to hold sample whilst cutting, keep away from the edge of the desk
What is the hazard of broken glass?
There is a risk of there being cuts from sharp object
What are the safety precautions for having broken glass?
Take care when handling slides and coverslips; keep glassware away from edge of desk
How do you plot the graph for the investigating water potential practical?
- Plot a graph of change in mass against concentration of sucrose solution
- The point at which the line of best fit crosses the x axis (zero change in mass) indicates the point at which the solution is isotonic. This is when the water potential of sucrose solution is the same as the water potential of the potato tissue, so there is no net movement of water in or out of the potato
What is the conclusion for the investigating water potential practical?
- Potato chips in lower concentrations of glucose solution will increase in mass, whilst those in the higher concentrations of glucose solution will decrease in mass
Active and Co-transport
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What is active transport?
The movement of a substance from a region where it is in a low concentration to a region where it is in a high concentration
The process requires the expenditure of metabolic energy
Is active transport a passive or an active process?
Active transport is an active process because it moves molecules across plasma membranes from an area of a low concentration to an area of a high concentration (against the concentration
gradient).
* This process requires energy from ATP
• ATP is produced by aerobic respiration in the mitochondria.
How is active transport similar to facilitated diffusion?
Like facilitated diffusion active transport requires carrier proteins which change shape and move a molecule across the membrane, but they move molecules from low to high concentration and they require energy to change shape.
A molecule of ATP is used (it is hydrolysed into
ADP and a phosphate molecule) which releases energy so that the molecule can be transported.
What does active transport not use?
Unlike facilitated diffusion active transport does not use channel proteins but instead uses special carrier proteins called co-transporters
What are co-transporters?
- Special carrier proteins
- They bind two molecules at a time, one moves down its concentration gradient and this is used to move the other molecule across the membrane against its concentration gradient
What is an example of co-transporters being used?
- The best example of this is the co-transport of glucose in the small intestine (specifically the final part called the ileum)
- Sodium ions move across the membrane down their concentration gradient by binding to co-transporters
- Glucose binds alone with the sodium so it is moved across the membrane too, against its concentration gradient.
What is the process of sodium ions leaving epithelial cells by active transport?
(1) Na+ ions are actively transported out of the epithelial cells in the ileum into the blood by the sodium-potassium (Na+/K+) pump. This creates a concentration gradient as there are more sodium ions in the lumen of the ileum than inside the
epithelial cell.
(2) The concentration gradient causes Na+ ions to diffuse from the lumen of the ileum into the epithelial cell down their concentration gradient.
They do this through the Na+ -glucose cotransporter protein. Glucose enters the cell with the sodium.
(3) There is a higher concentration of glucose inside the epithelial cell than in the blood so glucose diffuses into the blood down its concentration gradient through a protein channel by facilitated diffusion
diffusion.
How does the speed of carrier proteins affect the rate of active transport and why?
- The faster they work the faster the rate of active transport
- Decreasing the concentration gradient will have no effect on the rate of active transport but proteins moving slowly can
How does the number of carrier proteins affect the rate of active transport and why?
- The greater the number of carrier proteins in the plasma membrane the faster the rate of active transport
- Cells which are specially adapted to carry out a lot of active transport e.g ileum epithelial cells or root hair cells in plants will have large numbers of carrier proteins to help speed up the rate of active transport
How does the rate of respiration affect the rate of active transport and why?
- The greater the rate of respiration the more ATP there will be available for active transport to take place
- ATP is produced by aerobic respiration in the mitochondria. The rate of active transport therefore depends on the rate of respiration. Oxygen is therefore also required for active transport to occur. If respiration is inhibited, e.g due to lack of oxygen then active transport cannot take place.
Bulk transport - Endocytosis and Exocytosis
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What does endocytosis and exocytosis require?
- This is another type of transport that requires energy
- It involves the transport of large amounts of small partices at once or the movement of very large particles across the cell membrane
- Lots of ATP is required to move the vesicles
What is endocytosis?
It is the term used for the various types of active transport that move particles into a cel
by enclosing them in a vesicle made out of plasma membrane.
First, the plasma membrane of the cell invaginates (folds inward), forming a pocket around the target particle or particles. The pocket then pinches off with specialized proteins, leaving the particle trapped in a newly created vesicle or vacuole inside the cell.
What is exocytosis?
It is the term used to describe how cells transport signalling chemicals or waste products from inside of the cell to the outside of the cell in vesicles which fuse with the plasma membrane
What are examples of endocytosis?
- Pinocytosis = “cell drinking”
- Receptor-mediated endocytosis
What is pinocytosis?
Receptor-mediated endocytosis = uses receptor proteins on the cell surface to capture a specific target molecule. When the receptor binds to the target molecule endocytosis is triggered, usually to take up large molecules which are in low concentration outside cells but this can be abused by pathogens such as flu viruses and cholera toxins to gain cell entry.
What is receptor-mediated endocytosis?
Receptor-mediated endocytosis = uses receptor proteins on the cell surface to capture a specific target molecule. When the receptor binds to the target molecule endocytosis is triggered, usually to take up large molecules which are in low concentration outside cells but this can be abused by pathogens such as flu viruses and cholera toxins to gain cell entry.
What are examples of exocytosis?
Most vesicles come from the golgi and contain proteins made to be released outside such as signalling molecules or proteins which need to be embedded in the plasma membrane.
Other vesicles contain waste products that cells need to remove e.g leftovers of digested pathogens after phagocytosis.
Signalling molecules (like neurotransmitters from neurones) are not constantly released, exocytosis is triggered by chemicals binding to receptors. This causes the release of Ca2+ ions which trigger the movement of vesicles towards the membrane
