Biology - Unit 3: Movement into and out of Cells

what is the role of water?

a solvent in organisms within the following situations:

• dissolved substances can be easily transported around organisms

• digested food molecules are in the lumen of intestines but need to be moved to cells all over the body — absence of water as a solvent would not allow this to occur

• toxic substances such as urea and substances in excess of requirements such as salts can dissolve in water which makes them easy to remove from the body in urine.

• water is also an important part of the cytoplasm and plays a role in ensuring metabolic reactions can happen as necessary in cells.

where does the energy for diffusion and osmosis come from?

kinetic energy of random movement of molecules and ions.

what is diffusion?

net movement of molecules or ions from a region of their higher concentration to a region of their lower concentration down a concentration gradient as a result of their random movement.

what factors influence diffusion?

• surface area to volume ratio

• distance

• temperature

• concentration gradient

how does the surface area to volume ratio influence diffusion?

• the bigger a cell or structure is, the smaller its surface area to volume ratio. thus, the rate at which substances can move across its surface slows down.

• many cells which are adapted for diffusion have increased surface area in some way.

how does distance influence diffusion?

the shorter the distance molecules have to travel, the faster diffusion will occur.

how does temperature influence diffusion?

• the higher the temperature, the faster molecules move

• this is because they have more kinetic energy

• this results in more collisions against the cell membrane and therefore a faster rate of movement across them

how does concentration gradient influence diffusion?

• the greater the difference in concentration on either side of the membrane, the faster movement across it will occur

• this is because on the side with the higher concentration, more random collisions against the membrane will occur

what is osmosis?

the net movement of water molecules from a region of higher water potential to a region of lower water potential down the concentration through a partially permeable membrane.

how are plants supported?

plants are supported by the pressure of water inside the cells pressing outwards on the cell wall

what happens if a plant cell is placed in a dilute solution?

• water moves into the plant cells via osmosis

• water molecules push the cell membrane against cell wall, increasing turgor pressure in the cells which make them turgid

what happens when a plant cell is placed in a concentrated solution?

• water molecules will move out of the plant cells by osmosis, making them flaccid.

• if plant cells become flaccid it can negatively affect the plant’s ability to support itself.

• if looked under a microscope, the plant cells might be plasmolysed. this means that the cell membrane has pulled away from the cell wall

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

• cell will gain water by osmosis

• due to no cell wall to create turgor pressure, water will continue to enter cell until cell membrane is stretched too far and bursts

what happens if an animal cell is placed in a concentrated solution?

• cell will lose water by osmosis

• cell will become crenated (shrivelled up)

how can osmosis be investigated using dialysis tubing?

• fill a section of dialysis tubing with concentrated sucrose solution

• suspend the tubing in a boiling tube of distilled water for a set period of time

• note whether the water level outside the tubing decreases as water moves into tubing via osmosis (as water moves from a dilute to concentrated concentration down the water potential gradient through a partially permeable membrane in osmosis).

how can osmosis be investigated by placing plant tissues in solutions of different concentrations?

• cut cylinders of root vegetables and place them into distilled water and sucrose solutions of increasing concentration.

• cylinders of root vegetables are weighed before placing into the solutions.

• they are then left into solutions for 20-30 minutes and then removed, dried to remove excess liquid and reweighed.

• if the plant tissue gains mass: water moves into the plant tissue from the solution surrounding it via osmosis as the solution surrounding the tissue has a higher water potential than the plant tissue (which is more concentrated). this results in the plant tissue becoming turgid as the water molecules inside the cell push the cell membrane against the cell wall, increasing turgor pressure in cells which make them turgid.

• if the plant tissue loses mass: water moves out of the plant tissue into the solution surrounding it via osmosis as solution surrounding tissue is more concentrated and has a lower water potential than plant tissue (which is more dilute). the plant tissue will become flaccid as the cell membrane is pulled away from the cell wall and the cell can no longer support itself; the said cell is said to be plasmolysed.

• if there’s no overall change in mass: there’s no net movement of water as the concentration in both the plant tissue and solution surrounding it must be equal. however, water may move into and out of the plant tissue but there wouldn’t be any net movement in this case.

what is active transport?

the movement of molecules or ions into or out of a cell through the cell membrane from a region of their lower concentration to a region of their higher concentration against the concentration gradient using energy released during respiration

what is the importance of active transport?

vital process for the movement of molecules or ions across membranes like the uptake of mineral ions from soil water by root hair cells in plants.