Transport Triathlon

Transport Triathlon

Three ways substances cross a membrane. **Diffusion**, **osmosis** and **active transport** are about to race head-to-head. The exam almost never asks about them one at a time; it asks you to **tell them apart**. This is the sprint that makes the differences stick.

Line them up

The whole topic comes down to three questions asked of each process: does it need energy, what actually moves, and which way does it go relative to the gradient?

Spot the passive two

Pick the TWO that happen **passively**, needing **no** energy from respiration.

• Diffusion
• Osmosis
• Active transport
• Mineral ion uptake by a root hair cell from dilute soil
• Sugar absorption from the gut when blood sugar is already higher
• Moving a substance from a low concentration to a higher one

Define the odd one out

Active transport moves substances _____ the concentration gradient, so it needs _____ released by _____.

Why active transport costs energy

Diffusion and osmosis are like rolling **downhill**: particles spread from where there are lots to where there are few, all on their own. Active transport goes **uphill**: it drags substances from a **low** concentration to a **higher** one, against the natural flow. Working against the gradient is why it **must** use energy from **respiration**.

Mark the error

This student answer about active transport contains ONE statement that is wrong. Tap it.

• Active transport
• moves dissolved substances against the concentration gradient
• ,
• using energy released by respiration
• .
• Like diffusion, it happens on its own without the cell doing any work
• .
• Root hair cells use it to take up mineral ions

Define osmosis precisely

Osmosis is the movement of _____ from a dilute solution to a more _____ solution, through a _____ membrane.

Water or any particle?

Water moves into a plant root from the surrounding soil water. A student calls this diffusion. Why would an examiner want the word osmosis instead?

• Osmosis is the specific term for water moving through a partially permeable membrane
• Diffusion needs energy from respiration, but osmosis does not
• Osmosis moves water against the gradient, while diffusion moves it down
• Osmosis is simply a faster form of diffusion

Real examples worth knowing

Exam questions almost always arrive as an example rather than a name, so learn the process through these.

Match the real example

The mineral puzzle

Soil water is very **dilute** in mineral ions, yet root hair cells still pull those ions **in**: building up an even higher concentration inside. Which process must the root hair cell be using?

• Active transport
• Diffusion
• Osmosis
• No process can do that

Run the triathlon

A single root hair cell is doing several jobs at the same time. Name the process behind each one.

• Water is moving from the dilute soil water into the cell, through its partially permeable membrane. Which process is that?
• The soil water is very dilute in mineral ions, yet the cell keeps taking them in. Which process, and how can you tell?
• The cell respires hard to fuel that job. If the soil flooded and oxygen ran short, what would happen to mineral uptake?
• Meanwhile the carbon dioxide made by that respiration leaves the cell. Which process moves it out, and why is there no energy cost?

Finish line

Triathlon done. Carry these three lines into the exam: • **Diffusion**: any particle, **down** the gradient, **passive**. • **Osmosis**: **water** only, across a partially permeable membrane, **down** the gradient, **passive**. • **Active transport**: **against** the gradient, so it **needs energy** from respiration (root-hair mineral uptake, gut sugar absorption).

Compare all three

Compare diffusion, osmosis and active transport. For each, say whether energy is needed, what moves, and the direction relative to the concentration gradient. Give one real example of each.

• Take each of the three questions in turn and answer it for ALL three processes.
• For osmosis be precise: water, and a partially permeable membrane.
• Give the direction of each using the words down and against.
• Name where the energy for active transport comes from.
• Finish with one real example of each, not three examples of the same process.