Transport In Cells
Overzicht
Nothing gets into a cell by being told to. Substances arrive because they are more concentrated somewhere else, or because the cell spends energy dragging them in against the flow. Those two ideas, one free and one expensive, explain how you absorb a meal, how a root drinks from soil that is barely damp, and why a plant wilts.
In this lesson you will separate diffusion, osmosis and active transport cleanly, learn the vocabulary that surrounds osmosis, and then use all three to explain why lungs, intestines, roots and leaves are shaped the way they are. Get this topic right and several later ones stop needing to be learned separately.
Doelstellingen
- Diffusion is the spreading of the 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. The greater the difference in concentration, the faster the rate of diffusion.
- Dissolved substances can move into and out of cells by diffusion.
- Oxygen required for respiration passes through cell membranes by diffusion.
- Osmosis is the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane that allows the passage of water molecules.
- Differences in the concentrations of the solutions inside and outside a cell cause water to diffuse into or out of the cell by osmosis. Students should be familiar with experiments related to diffusion and osmosis as well as the terms isotonic, hypotonic, hypertonic, turgor and plasmolysis. Required practical: Investigate the effect of different concentrations of solutions separated by a partially permeable membrane.
- Substances are sometimes absorbed against a concentration gradient. This requires the use of energy from respiration. The process is called active transport.
- Active transport enables plants to absorb ions from very dilute solutions, eg by root hair cells. In animals, sugar may be absorbed from low concentrations in the intestine and from low concentrations in the kidney tubules.
- A single-celled organism has a relatively large surface area to volume ratio. All the necessary exchanges occur across its surface membrane. The increased size and complexity of an organism increases the difficulty of exchanging materials.
- In multicellular organisms many organ systems are specialised for exchanging materials. The effectiveness of an exchange surface is increased by: having a large surface area that is thin, to provide a short diffusion path; (in animals) having an efficient blood supply; (in animals, for gaseous exchange) being ventilated. Students should be able to explain how the small intestine and lungs in mammals, and the roots and leaves in plants, are adapted for exchanging materials.
- Gas and solute exchange surfaces in humans and other organisms are adapted to maximise effectiveness. Students should be able to explain how gas and solute exchange surfaces are adapted to maximise effectiveness.
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- In which direction does net movement occur during diffusion? A. From lower to higher concentration B. From higher to lower concentration C. In both directions equally D. Only across a partially permeable membrane Answer: B
- Osmosis is the movement of water from: A. a more concentrated to a more dilute solution B. a dilute to a more concentrated solution C. a cell to the air D. a gas to a liquid Answer: B
- Which process requires energy from respiration? A. Diffusion B. Osmosis C. Active transport D. Evaporation Answer: C
- A plant cell is placed in a hypertonic solution. What happens? A. It becomes turgid B. Water enters the cell C. Water leaves the cell D. There is no net movement of water Answer: C
- Which feature does NOT increase the effectiveness of an exchange surface? A. A large surface area B. A thick barrier C. An efficient blood supply D. Ventilation Answer: B
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