Diffusion, active transport, osmosis

Question 1

Define diffusion

Answer 1

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.

Question 2

What is facilitated diffusion

Answer 2

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.

Question 3

Describe in detail how channel proteins carry out facilitated diffusion

Answer 3

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.

Question 4

Describe how carrier proteins carry out facilitated diffusion

Answer 4

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.

Question 5

Define osmosis

Answer 5

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.

Question 6

What is water potential measure in

Answer 6

KiloPascals (kPa)

Question 7

What symbol is used to represent water potential

Answer 7

The Greek leet psi- Ψ

Question 8

What is water potential

Answer 8

The pressure created by water molecules in a cell/against a membrane

Question 9

Under standard temperature and pressure conditions, what is the water potential of pure water

Answer 9

Zero

Question 10

What do we count as standard conditions of temperature and pressure

Answer 10

25 degrees Celsius and 100kPa

Question 11

What happens to the water potential of the solution is a solute is added to pure water

Answer 11

The water potential decreases

Question 12

Why is the water potential of a solution (water and solute) always a negative number

Answer 12

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) .

Question 13

What happens to the water potential of a solution the more concentrated it becomes

Answer 13

The more concentrated a solution, the low (more negative) the water potential is.

Question 14

Describe osmosis in terms of water potential

Answer 14

In osmosis, water will move from a region of higher water potential (less negative) to an area with a lower (more negative) water potential.

Question 15

When does osmosis stop

Answer 15

When the solutions either side of the partially permeable membrane have the same water potential- a dynamic equilibrium has been reached.

Question 16

What is a hypotonic solution.

Answer 16

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.

Question 17

What happens to a cell when placed in a hypotonic solution

Answer 17

Water moves into the cell via osmosis which causes the cell to swell, and possibly even burst. The cell is cytolysed and turgid.

Question 18

What is an isotonic solution

Answer 18

A solution that has the same water potential as the cell- it has the same concentration of water molecules.

Question 19

What happens when a cell is placed in an isotonic solution

Answer 19

There is no net movement of water molecules. The cell does not change- it is normal and flaccid.

Question 20

What is a hypertonic solution

Answer 20

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.

Question 21

What happens to a cell when it is placed in a hypertonic solution

Answer 21

Water moves from the cell into the solution via osmosis. This causes the cell to shrink- it becomes crenated and plasmolyzed.

Question 22

Define active transport

Answer 22

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.

Question 23

What two things does active transport need to occur

Answer 23

Carrier proteins and ATP

Question 24

State the tow things that ATP is used for in active transport

Answer 24

• 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.

Question 25

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

Answer 25

Concentration of solutions.

Question 26

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

Answer 26

Percentage change in mass

Question 27

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

Answer 27

The x intercept is the concentration at which the solution is isotonic to the cells within the tissue.

Question 28

List four ways in which active transport differs from passive forms of transport

Answer 28

• 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.

Question 29

Describe the process of direct active transport

Answer 29

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.

Question 30

What is the sodium-potassium pump

Answer 30

A process where one carrier protein removes sodium ions from the cell/organelle whilst simultaneously taking in potassium ions from the surroundings.

Question 31

List the four main factors that affect the rate of diffusion

Answer 31

The size of concentration gradient
The surface area over which diffusion takes place
The thickness of the exchange surface
The temperature

Question 32

Describe how the size of the concentration gradient affects the rate of diffusion

Answer 32

The larger the difference between concentrations, the faster the rate of diffusion.

Question 33

Describe how the size of the surface area for diffusion affects its rate

Answer 33

The larger the surface area, the faster the rate of diffusion.

Question 34

Describe how the thickness of the exchange surface affects the rate of diffusion

Answer 34

The thinner the exchange surface, the faster the rate of diffusion.

Question 35

Describe how temperature affects the rate of diffusion

Answer 35

The higher the temperature is, the higher the kinetic energy of the particles is and therefore the faster the rate of diffusion.

Question 36

What is the ileum

Answer 36

The small intestine

Question 37

What are microvilli

Answer 37

Finger like projections of the cell surface membrane possessed by the epithelial cells that line the ileum.

Question 38

How do microvilli increase the rate of transport

Answer 38

They provide a larger surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport can take place.

Question 39

Apart from microvilli, what is another way that movement of substances in general can be increased across any membrane

Answer 39

Increase the number of protein channels and carrier proteins in any given area of membrane

Question 40

Is the concentration of carbohydrates and proteins higher in the ileum or in the blood

Answer 40

Higher in the ileum

Question 41

What helps to maintain the concentration gradient between the ileum and the blood

Answer 41

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.

Question 42

What is the mechanism by which glucose and amino acids are absorbed into the blood

Answer 42

Co-transport

Question 43

Describe how glucose and amino acids are absorbed into the blood by co-transport with sodium ions

Answer 43

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.

Question 44

What makes the transport of glucose and amino acids into the blood indirect active transport rather than direct active transport

Answer 44

It is the sodium ion concentration gradient, rather than ATP directly that powers the movement of glucose and amino acids into the cells.

Question 45

In terms of their concentration gradients, how do sodium ions and glucose/amino acid molecules move into cells via co-transport

Answer 45

The sodium ions move down their concentration gradient whereas the the glucose/amino acid molecules move against their concentration gradient.