Transport In Cells
Aperçu
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. It is also the topic where biology first collides with arithmetic: percentage change in mass, and the surface area to volume ratio that decides how big a single cell can afford to be. Get this right and several later topics stop needing to be learned separately.
Objectifs
- 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 selectively 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, hypertonic, turgor and plasmolysis.
- 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. Similarly, 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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Note de cours
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Évaluation de la leçon
Félicitations, vous avez terminé la leçon sur Transport In Cells. Maintenant que vous avez exploré le concepts et idées clés, il est temps de mettre vos connaissances à lépreuve. Cette section propose une variété de pratiques des questions conçues pour renforcer votre compréhension et vous aider à évaluer votre compréhension de la matière.
Vous rencontrerez un mélange de types de questions, y compris des questions à choix multiple, des questions à réponse courte et des questions de rédaction. Chaque question est soigneusement conçue pour évaluer différents aspects de vos connaissances et de vos compétences en pensée critique.
Utilisez cette section d'évaluation comme une occasion de renforcer votre compréhension du sujet et d'identifier les domaines où vous pourriez avoir besoin d'étudier davantage. Ne soyez pas découragé par les défis que vous rencontrez ; considérez-les plutôt comme des opportunités de croissance et d'amélioration.
- 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 selectively 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
- A cube of tissue has sides of 2 cm. What is its surface area to volume ratio? A. 6 to 1 B. 3 to 1 C. 1.5 to 1 D. 24 to 1 Answer: B
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