4. TRANSPORT ACROSS MEMBRANES Flashcards
State the function of the cell-surface membrane
Separates two aqueous environments. Controls what goes into and out of the cell.
Describe the functions of membranes within cells
Membranes around organelles divide the cell into different compartments. They act as a barrier between organelles and the cytoplasm.
Define the term hydrophobic
Dissolves in organic solvents (does not dissolve in aqueous solutions)
Define the term hydrophilic
Dissolves in aqueous solutions
Explain why the cell-surface membrane is referred to as the fluid-mosaic model
Mosaic because they contain phospholipids and proteins, and fluid because the proteins move within the bilayer.
Describe the structure of the phospholipid bilayer
The hydrophillic head face outwards, and the hydrophobic tails face towards each other
Describe the function of cholesterol
Cholesterol affects the fluidity and increases the stability of the CSM.
Name the two types of proteins that exist within the CSM
Intrinsic and extrinsic
How would you recognise extrinsic proteins on a diagram of the CSM?
Extrinsic proteins are embedded in the outside of the bilayer
Describe the two functions of extrinsic proteins
Receptors and antigens
What are glycoproteins?
A protein with a carbohydrate group attached
What are glycolipids?
A lipid with a carbohydrate group attached
How would you recognise intrinsic proteins on a diagram of the CSM?
Intrinsic proteins are embedded within and span the CSM
Give two examples of intrinsic proteins
Carrier and channel proteins
How would you recognise a channel protein on a diagram of the CSM
It has a continuous pore all the way through
How do channel proteins transport substances?
They provide a hydrophilic passageway through the CSM
Why are channel proteins specific?
Because they are different sizes, charges and shapes, depending on the size and shape and charge of the ion or molecule
How would you recognise a carrier protein on a diagram of the CSM
It will be closed at one end
How do carrier proteins transport substances?
The molecule first binds with the carrier protein. This causes the carrier protein to change shape, transporting the molecule across the membrane.
Why are carrier proteins specific?
Each carrier protein is specific to one molecule or ion because it has a specific tertiary structure, which means it can bind to molecules with a specific shape and charge.
Name the channel protein water moves through by osmosis
Aquaporins
Name the two types of diffusion
Simple diffusion and facilitated diffusion
Why is diffusion referred to as a passive process?
It does not require ATP
Define simple diffusion
Diffusion is the net movement of molecules or ions from an area of high concentration to an area of low concentration
How will lipid-soluble substances travel through the CSM?
Simple diffusion through the phospholipid bilayer
Define facilitated diffusion
Diffusion is the net movement of molecules or ions from an area of high concentration to an area of low concentration through channel or carrier proteins
Why can small polar molecules and ions not pass through the phospholipid bilayer by simple diffusion?
They are charged
How will small polar molecules and ions travel through the CSM?
Facilitated diffusion through channel proteins
Why can large molecules not travel through the phosphoilipid bilayer by simple diffusion?
They are too large
How will large molecules and ions travel through the CSM?
Facilitated diffusion through carrier proteins
State the factors that affect the rate of simple diffusion
Surface area, concentration gradient, temperature
State the factors that affect the rate of facilitated diffusion
Surface area, concentration gradient, temperature, number of channel / carrier proteins
Describe the effect of concentration gradient on rate of diffusion
The higher the concentration gradient, the faster the rate of diffusion
Describe the effect of temperature on rate of diffusion
As temperature increases, rate of diffusion increases
Describe the effect of surface area on rate of diffusion
The larger the surface area (e.g of the cell-surface membrane), the faster the rate of diffusion
When is the rate of facilitated diffusion directly proportional to the concentration gradient?
If the number of channel / carrier proteins is not limiting
How can you tell from a graph that the number of channel / carrier proteins has become a limiting factor of facilitated diffusion?
The line (rate of transport) starts to plateau
Describe the other reason a graph would plateau
If it’s a graph of concentration, and equilibrium has been reached
Define solute
The component in a solution that is dissolved in the solvent.
Define solvent
A liquid that solutes are dissolved in.
Define solution
A solute dissolved in a solution.
Define water potential
The potential of water to move from one area to another
Describe the effect of solute on water potential
As solute increases, water potential decreases / it becomes more negative.
State what an isotonic solution is
Two solutions have the same water potential (same concentration)
State what a hypotonic solution is
A solution has a lower concentration, and therefore a higher water potential
State what a hypertonic solution is
A solution has a higher concentration, and therefore a lower water potential
State the water potential of pure water
Zero
Water will always move down a water potential gradient. What does this mean?
It moves from an area of higher water potential to an area of low water potential
Describe what selectively permeable mean
Allows some substances through, but not others
Define osmosis
The movement of water down a water potential gradient over a selectively permeable membrane
Give 4 factors that affect the rate of osmosis
Water potential gradient, temperature, surface area, number of aquaporins
Describe what can happen to an animal cell if too much water enters it by osmosis
The cell can swell and burst (lysis)
Describe what can happen to an animal cell if too much water leaves it by osmosis
The cell can shrink and crenate
Describe what can happen to a plant cell if too much water enters it by osmosis
It swells and becomes too turgid
Describe what can happen to a plant cell if too much water leaves it by osmosis
It will shrink and become plasmolysed. The cell will completely pull away from the cell wall.
Explain why animal cells placed in pure water bursts while a plant cell does not
Animal cell only have cell membranes, whereas plant cells have a cell wall made of cellulose which gives strength and support.
State what turgid means
Swollen
State what plasmolysed mean
When cells lose water in hypertonic solutions, the cytoplasm shrinks and the cell membrane pulls away from the cell wall.
State how to find the point at which the water potential of the cell is equal to the water potential of the solution
The point where the curve intersects the x axis
How are the cells of aquatic organisms adapted to their surroundings?
They contain solutes to make them isotonic with their surroundings
Define active transport
The movement of molecules or ions from a region of a lower concentration to a region of higher concentration using ATP through carrier proteins.
State why active transport is an active process
Because it requires energy in the form of ATP
State one way in which active transport is similar to facilitated diffusion
It uses carrier proteins
Describe the two main differences between active transport and facilitated diffusion
Active transport moves solutes from low to high concentration, whereas facilitated diffusion moves solutes from high to low concentration.
Active transport uses ATP as a form of energy source, whereas facilitated diffusion is a passive process.
Give an example of a type of carrier protein
Co-transporters
State the factors affecting the rate of active transport
The number of carrier proteins.
The rate of respiration in the cell and the availability of ATP. Temperature.
Describe three ways you can recognise graphs of active transport
The rate of uptake will be faster, total uptake will be higher, the concentration on one side can fall to zero (shows uptake against a concentration gradient)
Describe how glucose is absorbed in the ileum
Glucose enters the ileum epithelium with sodium ions by co-transport
State the molecule that provides energy for active transport
ATP
How are cells adapted to carry out active transport
They have a lot of mitochondria to produce more ATP
By which method of molecular transport is glucose transported into the blood
Co-transport (into the ileum epithelial cells. Facilitated diffusion into the blood)
What is the only active stage of the co-transport of glucose
Na+ are actively transported out of the epithelial cells and into the blood
What is the purpose of actively transporting Na+ from the epithelial cells into the blood?
It reduces the concentration of Na+ in the epithelial cell so it is lower than the lumen
What occurs because of the Na+ concentration gradient between the lumen and the epithelial cells?
Na+ and glucose are co-transported from the ileum lumen into the epithelial cell by facilitated diffusion down a concentration gradient
In the co-transport of Na+ and glucose, which one travels down a concentration gradient, and which one travels up a concentration gradient?
Na+ travels down, and glucose travels up
Describe the difference between villi and microvilli
Villi are projections of the epithelial tissue or an organ, whereas microvilli are projections of the cell membrane
How does the presence of villi and microvilli in the ileum affect the rate of co-transport?
Villi and microvilli increase the rate of co-transport, because the surface area is increased
What is the function of the colon (large intestine)?
To absorb water
Name two conditions that affect the uptake of water by the colon
Diarrhea and constipation
Diarrhea is treated by drinking a salt solution. These salts are absorbed by the colon cells. Describe the effect on the uptake of water by the colon.
The salts would decrease the water potential of the colon cells. More water would move by osmosis from the faeces into the colon cells, down a water potential gradient.
Constipation is treated by drinking a salt solution. These salts remain in the lumen of the colon. Describe the effect on the uptake of water by the colon.
The salts would decrease the water potential of the faeces. More water would move by osmosis from the colon cells into the faeces, down a water potential gradient.
What effect would this salt solution have on the faeces?
It would soften them.
