English narration · English + 中文 subtitles burned in · 英語ナレーション・英語＋中文字幕 burning-in
3.1
Diffusion · 拡散
Syllabus · シラバス
English
Core
Supplement
1 Describe diffusion as the net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement
2 State that the energy for diffusion comes from the kinetic energy of random movement of molecules and ions
3 State that some substances move into and out of cells by diffusion through the cell membrane
4 Describe the importance of diffusion of gases and solutes in living organisms
5 Investigate the factors that influence diffusion, limited to: surface area, temperature, concentration gradient and distance
Source: Cambridge International syllabus · 出典: Cambridge International シラバス
English
Diffusion: random motion, one-way flow
Diffusion 扩散 is the net movement 净移动 of particles 粒子 from a region of their higher concentration 浓度 to a region of their lower concentration. We say the particles move down a concentration gradient 浓度梯度.
The particles spread out because of their own random movement 随机运动. The energy 能量 for diffusion comes from the kinetic energy 动能 of the random movement of molecules 分子 and ions 离子 — so diffusion needs no extra energy.
Some substances pass into and out of cells by diffusion through the cell membrane 细胞膜.
Why diffusion matters. Many gases 气体 and dissolved substances (solutes 溶质) move by diffusion in living things. For example, oxygen 氧气 diffuses into cells for respiration, and carbon dioxide 二氧化碳 diffuses out.
The rate of diffusion changes with four factors:
Factor
Diffusion is faster when…
surface area 表面积
the surface is larger
temperature 温度
it is hotter (particles move faster)
concentration gradient
the difference in concentration is bigger
distance
the distance to travel is shorter (a thinner barrier)
1 Describe the role of water as a solvent in organisms with reference to digestion, excretion and transport
7 Describe osmosis as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
2 State that water diffuses through partially permeable membranes by osmosis
3 State that water moves into and out of cells by osmosis through the cell membrane
4 Investigate osmosis using materials such as dialysis tubing
5 Investigate and describe the effects on plant tissues of immersing them in solutions of different concentrations
8 Explain the effects on plant cells of immersing them in solutions of different concentrations by using the terms: turgid, turgor pressure, plasmolysis, flaccid
9 Explain the importance of water potential and osmosis in the uptake and loss of water by organisms
6 State that plants are supported by the pressure of water inside the cells pressing outwards on the cell wall
Source: Cambridge International syllabus · 出典: Cambridge International シラバス
English
Osmosis: water crosses the membrane
Water as a solvent
Water is a very good solvent 溶剂 — many substances dissolve 溶解 in it to make a solution 溶液. This matters for digestion 消化 (food must dissolve before it can be used), excretion 排泄 (wastes are carried away dissolved in water) and transport 运输 (substances travel around the body dissolved in blood and sap).
What osmosis is
Osmosis 渗透 is a special kind of diffusion: the net movement of water across a partially permeable membrane 半透膜. This kind of membrane has tiny holes that let small water molecules through but hold back larger solute particles. Water moves into and out of cells by osmosis through the cell membrane.
A concentrated solution has a lot of dissolved solute and little water. A dilute solution has little solute and a lot of water.
(Supplement) Osmosis is the net movement of water molecules from a region of higher water potential 水势 (a dilute solution — more water) to a region of lower water potential (a concentrated solution — less water), through a partially permeable membrane.
Osmosis and plant cells
When you put plant tissue into different solutions, water moves by osmosis:
In pure water or a dilute solution, water moves into the cells. The cells swell and become firm, or turgid 膨胀. The water presses outward on the cell wall; this outward push is the turgor pressure 膨压. Turgid cells make a plant stand up straight — this is how plants are supported.
In a concentrated solution, water moves out of the cells. The cells lose their firmness and become soft, or flaccid 松软.
(Supplement) If even more water leaves, the cell membrane pulls away from the cell wall. This is plasmolysis 质壁分离.
You can investigate osmosis using dialysis tubing 透析管 (an artificial partially permeable membrane), or using cylinders cut from a potato. Measure their length or mass before and after soaking. Cylinders in pure water or a dilute solution gain length and mass; cylinders in a concentrated solution lose length and mass; in a solution of equal concentration there is no change.
Worked example. A potato cylinder has a mass of 5.0 g before soaking and 5.6 g after soaking in a dilute solution. Find the percentage change in mass.
A positive answer means the cylinder gained water, so the solution was more dilute than the cell contents. Always work out the percentage change, not just the change in grams — it lets you compare cylinders that started at different masses fairly.
1 Describe active transport as the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (i.e. against a concentration gradient), using energy from respiration
2 Explain the importance of active transport as a process for movement of molecules or ions across membranes, including ion uptake by root hairs
3 State that protein carriers move molecules or ions across a membrane during active transport
日本語
コア
サプリメント
1 能動輸送とは、呼吸から得たエネルギーを用いて、粒子を濃度の低い領域から濃度の高い領域（すなわち濃度勾配に逆らって）へ細胞膜を通過させる movement であることを説明する
Source: Cambridge International syllabus · 出典: Cambridge International シラバス
English
Active transport vs diffusion
Active transport 主动运输 is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration — that is, against the concentration gradient. Because this is "uphill", it needs energy from respiration 呼吸作用.
(Supplement) Active transport lets a cell take in useful molecules or ions even when they are already more concentrated inside the cell. For example, root hairs 根毛 take up mineral ions from the soil by active transport. Protein carriers 载体蛋白 in the membrane pick up the molecules or ions and carry them across, using energy.
Active transport step by step · 能動的輸送のステップバイステップ
Step the carrier protein through its cycle: it uses energy from ATP to move a particle against the gradient, from low to high. · キャリアタンパク質をサイクルを通じて一歩ずつ操作します：ATP由来のエネルギーを使って、粒子を濃度勾配に逆らって、低濃度から高濃度へ移動させます。
Explore · 探索
Active transport — pumping uphill · 能動的輸送 — 上り坂へのポンピング
Unlike diffusion, active transport moves substances AGAINST the concentration gradient, so it needs energy. · 拡散と異なり、能動的輸送は物質を濃度勾配に逆らって移動させるため、エネルギーを必要とする。
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