العلوم
Diffusion: Why Particles Spread Out on Their Own
Diffusion is particles spreading from crowded to less crowded until evenly mixed. Nothing pushes them. Compare it with osmosis and active transport here.
Diffusion
Diffusion is the spreading out of particles from where they are crowded to where they are not, until they are evenly mixed.
- أين يقابله الطلاب
- Grade 7 science, when the particle model is used to explain a smell crossing a room, and again in Grade 9 for gas exchange in the lungs.
الإجابة باختصار
Diffusion is the net movement of particles from a region where they are more concentrated to one where they are less concentrated, until they are evenly spread. It happens because particles move randomly, it needs no energy, and it goes faster with a steeper concentration gradient, a higher temperature or a larger surface area.
مثال
Put one drop of ink into a beaker of still water and leave it alone. Within minutes the colour has spread; within an hour the whole beaker is faintly coloured. Nobody stirred it and nothing pushed the ink. The ink particles were simply moving in all directions already, and because they all began crowded in one spot, far more of them happened to move outwards than inwards. Once the colour is even, the particles keep moving, but there is no longer any direction with more of them to leave.
Nothing pushes the particles
Diffusion is easy to picture wrongly, as though crowded particles were shoving each other apart. They are not. Particles in a liquid or a gas are already moving constantly in random directions, and that motion is all diffusion has to work with.
What creates the direction is arithmetic. If one side of a space holds a thousand particles of something and the other side holds ten, then random movement sends more of them one way than the other simply because there are more of them there to move. The gradient does not cause the movement; it decides how the movement adds up.
That also explains what happens at the end. When the particles are evenly spread, they do not stop. Crossings in both directions continue and merely cancel out, which is why the definition says net movement rather than movement.
Four things that change the rate
Everything that speeds diffusion up does so by increasing the number of successful crossings per second, and the four factors below are the ones a question will ask about.
Living things exploit all four. Lungs are divided into millions of alveoli to make an enormous surface area, the alveolus wall and the capillary wall are each one cell thick to keep the distance tiny, and a constant blood flow carries oxygen away as fast as it arrives so the gradient never flattens out.
- A steeper concentration gradient — a bigger difference between the two sides
- A higher temperature — particles have more kinetic energy and move faster
- A larger surface area — more places for particles to cross at once
- A shorter distance — a thinner membrane is crossed more quickly
Diffusion, osmosis and active transport side by side
These three are examined together and the distinguishing features are few enough to hold in your head. What moves, which way it moves, and whether it costs anything answers almost every question that asks you to name the process.
One further separator settles most borderline cases: only active transport stops when oxygen is removed. If an experiment reports that uptake fell away once a root was deprived of oxygen, the process being described is active transport, whatever the concentrations happen to be.
- Diffusion — any small particle, including gases and dissolved substances; moves down the concentration gradient; no energy needed; no membrane required
- Osmosis — water only; moves through a partially permeable membrane towards the more concentrated solution; no energy needed
- Active transport — dissolved substances; moves up the concentration gradient, from low to high; requires carrier proteins and energy from respiration
أسئلة شائعة
What is diffusion in simple terms?
It is particles spreading out from where there are many of them to where there are few, without anything pushing them. Open a bottle of perfume at one end of a room and it can be smelled at the other end a few minutes later. The particles were moving anyway; spreading out is just what random movement adds up to.
Does diffusion need energy?
Not from the organism. Diffusion runs on the kinetic energy particles already have, so a cell spends nothing on it and it continues in non-living systems such as ink in water. That is the sharpest contrast with active transport, which needs energy released by respiration and stops when respiration does.
Is osmosis just a type of diffusion?
In mechanism, yes: it is the same passive spreading, applied to water across a partially permeable membrane. In an exam, keep the words apart. If water and a membrane are involved, the expected answer is osmosis, and writing diffusion instead will often lose the mark even though the physics is the same.
Why is diffusion faster when it is warmer?
Because temperature is a measure of how much kinetic energy the particles have. Warmer particles move faster, so they cover more ground per second and cross a given space more often. The direction of the net movement is unchanged; only the rate goes up. This is why sugar dissolves and spreads through hot tea faster than through cold water.
Where does diffusion happen in the body?
Wherever something has to move a short distance down a gradient. Oxygen diffuses from the alveoli into the blood and carbon dioxide diffuses the other way; digested food molecules diffuse from the small intestine into the blood; urea diffuses out of cells into the blood plasma for the kidneys to remove.
آخر تحديث
معرفة الكلمة ليست كاستخدامها
يستطيع المعلّم أن يرى الطالب وهو يستعمله في سؤال، فيتبيّن أين يتوقف الفهم بالضبط. الحصة الأولى مجانية.