Biology GCSE AQA year 9: Transport in Cells

Give 2 examples of diffusion in the body

. Oxygen and carbon dioxide in gas exchange

. The waste product urea diffusing from cells into the blood plasma for excretion in the kidney

How does diffusion happen in a single-celled organism?

. Has a relatively large surface area to volume ratio

. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism

How does diffusion happen in multicellular organisms?

. Have mechanisms to increase surface area proportionately, such as additional absorbing areas or adaptations of shape as diffusion across their surface is too slow

. Have transport systems and keep distances to a minimum for diffusion

Give an example of active transport in animals

. Allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration

. Sugar molecules are used for cell respiration

Which factors affect the rate of diffusion?

. Differences in concentrations (concentration gradient)

. Temperature

. Surface area of the membrane

What is the effectiveness of an exchange surface increased by?

. Large surface area

. Thin membrane

. Efficient blood supply (in animals) to provide a short diffusion path

. Being ventilated (in animals for gaseous exchange)

What is osmosis?

The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane

What is active transport?

. Moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient)

. Requires energy from respiration

Give an example of active transport in plants

. Allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil

. Plants require ions for healthy growth

What is a solvent?

The substance that something else can dissolve in. (e.g water)

What is a solute?

The substance that dissolves (e.g squash)

What does hypertonic mean?

The concentration of solutes in the solution outside the cell is higher than the internal concentration so the solution is hypertonic to the cell

What does isotonic mean?

The concentration of solutes in the solution outside the cell is the same as the internal concentration so the solution is isotonic to the cell

What does hypotonic mean?

The concentration of solutes in the solution outside the cell is lower than the internal concentration so the solution is hypotonic to the cell

What happens to hypertonic animal cells?

If the water conc outside a cell falls, water will move out of the cell, by osmosis, down a conc gradient, through a partially permeable membrane, causing it to shrivel up.

What happens to isotonic animal cells?

Ideal concentration for animal cells to be in.

What is diffusion?

The spreading out of particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration

What happens to hypotonic animal cells?

If the water conc outside a cell increases, water will move into the cell, by osmosis, down a conc gradient, through a partially permeable membrane, causing it to become lysed and burst.

What happens to hypertonic plant cells?

The cell becomes plasmolysed: Water moves out of the cell by osmosis, down a conc gradient, through a partially permeable membrane, causing the vacuole and cytoplasm to shrink which makes the cell membrane pull away from the cell wall.

What happens to isotonic plant cells?

They are flaccid: Water moves into and out of the cell by osmosis, down a conc gradient, through a partially permeable membrane, so the cells are no longer firm and swollen.

What happens to hypotonic plant cells?

They become turgid: Water moves into the cells by osmosis, down a conc gradient, through a partially permeable membrane, causing the vacuole to swell which presses the cytoplasm against the cell wall. The cell will not burst because of the cell wall. Fully turgid cells support the stems of non-woody plants.

Why does concentration gradient affect the rate of diffusion?

The greater the difference in concentration, the quicker the rate of diffusion.

Why does temperature affect the rate of diffusion?

The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly.

Why does the surface area of the cell membrane separating the different regions affect the rate of diffusion?

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

How do you work out the surface area: volume ratio of a cube?

Surface area = length x length x 6

Volume = length x length x length

Simplify the ratio.

If one object has a higher surface area to volume ratio than another is it bigger or smaller?

Smaller (more surface area for each cm cubed of volume)

Why might organisms reduce their surface area? Give an example.

Organisms living in harsh environmental conditions may reduce their surface area, e.g cacti, to reduce loss of water.

How do you measure the rate of water uptake in an hour?

Water uptake in 1 hour = change in mass x (60 mins / period of time in mins)

How do you measure percentage change in mass?

(mass at the end - mass at the start/mass at the start) x 100

What does active transport need?

. Transport protein embedded in cell membrane

. Energy from respiration

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