Diffusion, active transport, osmosis Flashcards
Define diffusion
The net movement of particles down the concentration gradient from an area of high concentration to an area of low concentration. It is a passive process and continues until and equilibrium is achieved.
What is facilitated diffusion
The transport of polar molecules, ions, large molecules and non-lipid soluble molecules across the cell-surface membrane in and out of the cell. It uses carrier and channel proteins to achieve this.
Describe in detail how channel proteins carry out facilitated diffusion
Channel proteins form water-filled, hydrophilic channels across the membrane.
They allow specific water-soluble ions to pass through.
The channels are selective- each opens only in the presence of a specific ion.
If the specific ion is not present, the channel remains closed.
Describe how carrier proteins carry out facilitated diffusion
When the molecule that is specific to the protein is present, it binds to the protein. This causes the carrier protein to change shape in such a way that the molecule is released to the inside of the membrane.
Define osmosis
The passage of water from an area of high water concentration to an area of low water concentration (down the concentration gradient) through a partially permeable membrane.
What is water potential measure in
KiloPascals (kPa)
What symbol is used to represent water potential
The Greek leet psi- Ψ
What is water potential
The pressure created by water molecules in a cell/against a membrane
Under standard temperature and pressure conditions, what is the water potential of pure water
Zero
What do we count as standard conditions of temperature and pressure
25 degrees Celsius and 100kPa
What happens to the water potential of the solution is a solute is added to pure water
The water potential decreases
Why is the water potential of a solution (water and solute) always a negative number
The form of water with the maximum water potential that can be achieved is pure water and that has a water potential of zero so therefore a solution will always have a lower water potential and so the value for water potential will always be a negative number (less than zero) .
What happens to the water potential of a solution the more concentrated it becomes
The more concentrated a solution, the low (more negative) the water potential is.
Describe osmosis in terms of water potential
In osmosis, water will move from a region of higher water potential (less negative) to an area with a lower (more negative) water potential.
When does osmosis stop
When the solutions either side of the partially permeable membrane have the same water potential- a dynamic equilibrium has been reached.
What is a hypotonic solution.
A hypotonic solution is a solution that has a higher (less negative) water potential than the cell and therefore has a higher concentration of water molecules in it than the cell does.
What happens to a cell when placed in a hypotonic solution
Water moves into the cell via osmosis which causes the cell to swell, and possibly even burst. The cell is cytolysed and turgid.
What is an isotonic solution
A solution that has the same water potential as the cell- it has the same concentration of water molecules.
What happens when a cell is placed in an isotonic solution
There is no net movement of water molecules. The cell does not change- it is normal and flaccid.
What is a hypertonic solution
A hypertonic solution is a solution with a lower (more negative) water potential than the cell meaning it has a lower concentration of water modules in it than the solution in the cell does.
What happens to a cell when it is placed in a hypertonic solution
Water moves from the cell into the solution via osmosis. This causes the cell to shrink- it becomes crenated and plasmolyzed.
Define active transport
The movement of particles from an area of low concentration to an area of high concentration (against the concentration gradient). It is an active process to requires energy-in the form of ATP. It also requires carrier proteins.
What two things does active transport need to occur
Carrier proteins and ATP
State the tow things that ATP is used for in active transport
- to directly move molecules
- to individually move molecules using a concentration that has already been set up by direct active transport. This is known as co-transport.
When drawing a graph to represent the movement of water in/out of a tissue by osmosis at different concentrations of solutions, what is on the x axis
Concentration of solutions.
When drawing a graph to represent the movement of water in/out of a tissue by osmosis at different concentrations of solutions, what is on the y axis
Percentage change in mass
When looking at a graph that represents the movement of water in and out of a tissue by osmosis at different concentrations of solutions, how do you find the value for the concentration of a solution that is isotonic (has the same water potential) as the tissue
The x intercept is the concentration at which the solution is isotonic to the cells within the tissue.
List four ways in which active transport differs from passive forms of transport
- metabolic energy in the form of ATP is needed
- substances are moved against the concentration gradient
- carrier protein molecules which act ‘pumps’ are involved (also seen in facilitated diffusion)
- the process is very selective, with only specific substances being transported.
Describe the process of direct active transport
1) the carrier proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it.
2) the molecule or ion binds to receptor sites on the carrier protein.
3) On the inside of the cell/organelle, ATP binds to the protein, causing it to split into ADP and a phosphate molecule. This causes the protein molecule to change shape and open to the opposite side of the membrane.
4) the molecule or ion is then released to the other side of the membrane.
5) The phosphate molecule is released from the protein which causes the protein to revert back to its original shape.
6) The phosphate molecule then recombines with the ADP to form ATP during respiration.
What is the sodium-potassium pump
A process where one carrier protein removes sodium ions from the cell/organelle whilst simultaneously taking in potassium ions from the surroundings.
List the four main factors that affect the rate of diffusion
The size of concentration gradient
The surface area over which diffusion takes place
The thickness of the exchange surface
The temperature
Describe how the size of the concentration gradient affects the rate of diffusion
The larger the difference between concentrations, the faster the rate of diffusion.
Describe how the size of the surface area for diffusion affects its rate
The larger the surface area, the faster the rate of diffusion.
Describe how the thickness of the exchange surface affects the rate of diffusion
The thinner the exchange surface, the faster the rate of diffusion.
Describe how temperature affects the rate of diffusion
The higher the temperature is, the higher the kinetic energy of the particles is and therefore the faster the rate of diffusion.
What is the ileum
The small intestine
What are microvilli
Finger like projections of the cell surface membrane possessed by the epithelial cells that line the ileum.
How do microvilli increase the rate of transport
They provide a larger surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport can take place.
Apart from microvilli, what is another way that movement of substances in general can be increased across any membrane
Increase the number of protein channels and carrier proteins in any given area of membrane
Is the concentration of carbohydrates and proteins higher in the ileum or in the blood
Higher in the ileum
What helps to maintain the concentration gradient between the ileum and the blood
The fact that blood is constantly being pumped around your body at a high pressure- so any substances that diffused into the blood would be quickly moved away from the membrane as the blood moves rapidly.
What is the mechanism by which glucose and amino acids are absorbed into the blood
Co-transport
Describe how glucose and amino acids are absorbed into the blood by co-transport with sodium ions
1) Sodium ions are actively transported out of epithelial cells, by the sodium potassium pump, into the blood. This takes place in one type of protein-carrier molecule found in the cell surface membrane of epithelial cells.
2) This maintains a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells.
3) Sodium ions diffuse into the epithelial cells down this concentration gradient through a different type of protein carrier in the cell-surface membrane.
4) As the sodium ions diffuse in through this second carrier protein, they carry either amino acid molecules or glucose molecules into the cell with them.
5) The glucose/amino acid molecules then pass into the blood plasma via facilitated diffusion using another type of carrier.
What makes the transport of glucose and amino acids into the blood indirect active transport rather than direct active transport
It is the sodium ion concentration gradient, rather than ATP directly that powers the movement of glucose and amino acids into the cells.
In terms of their concentration gradients, how do sodium ions and glucose/amino acid molecules move into cells via co-transport
The sodium ions move down their concentration gradient whereas the the glucose/amino acid molecules move against their concentration gradient.
