Diffusion, osmosis and active transport, surface area to volume ratio, and how exchange surfaces are adapted.
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1.Define diffusion.
The spreading out of the particles of a gas, or of any substance in solution, resulting in a net movement from an area of higher concentration to an area of lower concentration.
2.Does diffusion require energy from the cell?
No. Diffusion is a passive process driven by the random movement of particles.
3.State three factors that affect the rate of diffusion.
The concentration gradient, the temperature, and the surface area of the membrane.
4.How does a steeper concentration gradient affect the rate of diffusion?
The greater the difference in concentration, the faster the rate of diffusion.
5.How does temperature affect the rate of diffusion?
A higher temperature gives particles more kinetic energy so they move faster, increasing the rate of diffusion.
6.How does surface area affect the rate of diffusion?
A larger surface area means more particles can cross at once, so the rate is faster.
7.Give an example of diffusion in the lungs.
Oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli.
8.Give an example of diffusion in the small intestine.
Small digested food molecules such as glucose diffuse from the intestine into the blood.
9.Which waste product diffuses out of body cells?
Urea, which diffuses from cells into the blood plasma and is removed by the kidneys.
10.Define osmosis.
The movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.
11.Is osmosis active or passive?
Passive. It requires no energy from the cell.
12.What does partially permeable mean?
The membrane allows some molecules through, such as water, but not larger solute molecules.
13.What happens to an animal cell placed in pure water?
Water enters by osmosis, the cell swells, and it may burst because there is no cell wall to resist the pressure.
14.What happens to an animal cell placed in a concentrated solution?
Water leaves the cell by osmosis and the cell shrinks and shrivels.
15.What happens to a plant cell placed in pure water?
Water enters by osmosis, the vacuole swells and the cell becomes turgid. The cell wall stops it bursting.
16.What happens to a plant cell placed in a concentrated solution?
Water leaves by osmosis, the cell becomes flaccid, and eventually the membrane pulls away from the cell wall, which is called plasmolysis.
17.Why does a plant cell not burst in pure water?
The rigid cellulose cell wall resists further expansion once the cell is turgid.
18.Define active transport.
The movement of substances from a more dilute solution to a more concentrated solution, against the concentration gradient, using energy released by respiration.
19.Where does the energy for active transport come from?
Respiration, which takes place in the mitochondria.
20.Give an example of active transport in plants.
Root hair cells absorb mineral ions from the very dilute solution in the soil, where the concentration is lower than inside the cell.
21.Give an example of active transport in humans.
Glucose is absorbed from the gut into the blood even when the concentration of glucose in the gut is lower than in the blood.
22.Why is active transport important for digestion?
It allows all the glucose to be absorbed from the gut so it can be used for respiration, rather than being lost in waste.
23.Which two transport processes are passive?
Diffusion and osmosis.
24.Which transport process needs energy?
Active transport.
25.Why do root hair cells and gut lining cells contain many mitochondria?
They carry out a lot of active transport, which needs energy released by respiration in the mitochondria.
26.What is meant by surface area to volume ratio?
The surface area of an organism divided by its volume, usually written as a ratio such as 3 to 1.
27.How does the size of an organism affect its surface area to volume ratio?
The larger the organism, the smaller its surface area to volume ratio.
28.Why do single-celled organisms not need a transport system?
They have a large surface area to volume ratio, so diffusion across the surface is fast enough to meet their needs.
29.Why do multicellular organisms need exchange surfaces and transport systems?
They have a small surface area to volume ratio and a high demand, so diffusion across the outer surface alone would be far too slow.
30.State four features of an effective exchange surface.
A large surface area, a membrane that is thin to give a short diffusion path, an efficient blood supply in animals, and ventilation for gas exchange surfaces.
31.How are alveoli adapted for gas exchange?
There are millions of them giving a large surface area, their walls are one cell thick, they have a moist lining, and a good blood supply and ventilation maintain a steep concentration gradient.
32.How are villi adapted for absorption?
They give the small intestine a large surface area, have walls one cell thick, have a good blood supply, and microvilli increase the surface area further.
33.How are fish gills adapted for gas exchange?
Many thin filaments covered in lamellae give a very large surface area, the surface is thin, and a good blood supply maintains the concentration gradient.
34.How are the leaves of a plant adapted for gas exchange?
They are flat and thin for a short diffusion path, contain air spaces to increase the internal surface area, and have stomata that let gases in and out.
35.Calculate the surface area to volume ratio of a cube of side 2 cm.
Surface area = 6 x 2 x 2 = 24 cm squared. Volume = 2 x 2 x 2 = 8 cm cubed. The ratio is 24 to 8, which simplifies to 3 to 1.
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