Diffusion, Osmosis, Active Transport and Co-Transport

Question 1

what is the definition of diffusion?

Answer 1

the net movement as a result of the random motion of its molecules or ions of a substance from a region of higher concertation to a region of its lower concertation

Question 2

do they move up or down a concentration gradient?

Answer 2

molecules or ions move down the concertation gradient

Question 3

what causes diffusion?

Answer 3

the natural kinetic energy of the molecules or ions

Question 4

what happens as a result of diffusion?

Answer 4

molecules or ions tend to reach equilibrium

Question 5

what what factors affect the rate of diffusion?

Answer 5

steepness of gradient, temperature, surface area, properties of molecules or ions

Question 6

what certain substances can’t diffuse through the phospholipid bilayer?

Answer 6

large polar molecules e.g. glucose and amino acids
long e.g. sodium and chloride ions

Question 7

channel proteins:

Answer 7

• channel proteins are water filled pores
• they allow ions to diffuse through the cell membranes
• the diffusion os these ions doesn’t occur freely most channel proteins are gated meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
• this allows channel proteins to control the exchange of ions

Question 7

what is facilitated diffusion?

Answer 7

large polar molecules and long e.g. chloride ions can only cross the phospholipid bilayer with the help of certain proteins

Question 8

what proteins help facilitated diffusion?

Answer 8

channel and carrier proteins which are highly specific to one molecule or ion

Question 9

carrier proteins:

Answer 9

• unlike channel proteins which have a fixed shape carrier proteins can switch between two shapes
• this causes the binding site of the carrier protein to be open on one side of the membrane first and then open the other side of the membrane when the carrier protein switches shape
• the direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
• net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient

Question 9

what is the definition of osmosis?

Answer 9

is the diffusion of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane - in doing this water is moving down its concentration gradient
osmosis is the net movement of water molecules from a region of high water potential to a region of low water potential through a partially permeable membrane

Question 10

what does water potential describe?

Answer 10

the tendency of water to move out of a solution
a dilute solution has a high water potential and a concentrated solution has a low water potential

Question 11

what happens when plant cells are placed in pure water or a dilute solution?

Answer 11

-water will enter the plant cell through the partially permeable cell surface membrane by osmosis as the pure water or dilute solution has a higher water potential than the plant cell
- as water enters the vacuole of the plant cell the volume of the plant cell increases
- the expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell wall - the inelastic cell wall prevents the cell from bursting
- the pressure created by the cell wall also stops too much water entering and this also helps to prevent the cell from bursting
- when a plant cell is fully inflated with water and has become rigid and firm it’s described as fully turgid
- this turgidity is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plat - making the plant stand upright with its leaves held out to catch sunlight
- if plants don’t have enough water the cells can’t remain rigid and turgid and the plant wilts

Question 12

what happens when a plant cell is placed in a solution with a lower water potential than the plant cell?

Answer 12

• water will leave the plant cell through the partially permeable cell surface membrane by osmosis
• as water leaves the plant cell the volume of the plant cell decreases
• the protoplast gradually shrinks and no longer exerts pressure on the cell wall
• as the protoplast continues to shrink it begins to pull away from the cell wall
• this process is known as plasmolysis - the plant cell is plasmolysed

Question 13

what do animal cells not have that make loss or gain of water more server?

Answer 13

a cell wall

Question 14

what happens to an animal cell went it’s placed in a hypertonic solution?

Answer 14

• e.g. an animal cell is placed in a solution with a lower water potential than the cell water will leave the cell through its partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up
• this occurs when the cell is in a hypertonic environment (the solution outside of the cell has a higher solute concentration than inside the cell)

Question 15

what happens if an animal cell is placed in in pure water or a dilute solution?

Answer 15

• water will enter the cell through the partially permeable cell surface membrane by osmosis the pure water or dilute solution has a higher water potential
• the cell will continue to gain water by osmosis until the cell membrane is stretched too far and the cell bursts (cytolysis) as it has no cell wall to withstand the increased pressure created
• this occurs when the cell is in a hypotonic environment (the solution outside of the cell has a lower solute concentration than the inside of the cell)
• this is why a constant water potential must be maintained inside the bodies of animals

Question 16

what happens if animal cells are put in an isotonic solution?

Answer 16

if an animal cell is in an isotonic environment (the solution outside the cell has the same solute concentration as the inside of the cell) the movement of water molecules into and out of the cell occurs at the same rate (no net movement of water) and there is no change to the cells

Question 17

what is the definition of active transport?

Answer 17

it’s the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration

Question 18

what is the energy in active transport used for?

Answer 18

the energy it requires to make the carrier protein change shape allowing it to transfer the molecules or ions across the membrane

Question 19

what provides the energy for active transport?

Answer 19

the energy is provided by adenosine triphosphate (ATP) produced during respiration, the ATP is hydrolysed to release energy

Question 20

why is active transport important?

Answer 20

• reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules
• absorption of some products of digestion from the digestive tract
• loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around for plant
• loading inorganic ions from the soil into root hairs

Question 20

what is co-transport?

Answer 20

co-transport is the coupled movement of substances across a cell membrane via a carrier protein. this means that two types of molecules are moved across the membrane at the same time the movement of one is dependent on the movement of the other. It involves a combination of facilitated diffusion and active transport.

Question 21

how does the co-transport protein absorb glucose and sodium ions?

Answer 21

• the active transport of sodium ions from the epithelial cell into the blood lowers the sodium ion concentration inside the cell and generates a sodium ion concentration gradient between the ileum and the epithelial cells
• sodium ions move into the cell from the ileum by facilitated diffusion carrying glucose molecules along with them via a co-transport protein
• the glucose concentration inside the epithelial cell increases and glucose molecules enter the blood via facilitated diffusion
• active transport generates the concentration gradient needed for co-transport to occur

Question 22

The H+ pump:

Answer 22

• the H+ pump is an example of an ion-coupled pump
• the active transport of H+ ions across the membrane results in a build-up of H+ ions on one side of the membrane
• the H+ ions then bind to a carrier protein along with substances being transported (e.g. sucrose) and both get moved to the outer side of the membrane (e.g. sucrose) loading in the phloem

Question 23

the Na+/K+ pump:

Answer 23

• the sodium pump is another coupled pump
• the Na+/K+ pump involves the transport of the two ions in opposite directions across the membrane
• the Na+/K+ pump is also used to transport glucose against the concentration gradient. this uses a second transport protein which, binds Na+ and glucose; the Na+ goes back across the membrane taking the glucose with it
• e.g. glucose and galactose loading in the intestinal epithelial cells