Plant organs and transport

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Big idea: a plant is a system of organs

Big idea (key concept): Systems. A plant is made of parts that work together. The roots, stem and leaves are linked by transport tissues, so a change in one part affects the others.
Related concept: Function. Each organ and each tissue has a structure that suits its job. Asking “what is this part for?” helps you understand its shape.
Global context: Scientific and technical innovation. Farmers and plant scientists use what we know about plant transport to design irrigation, greenhouses and drought-resistant crops.
A plant has four main organs: roots (anchor the plant and take in water and minerals), stems (hold leaves up to the light and carry substances), leaves (make food by photosynthesis) and flowers (reproduction).
Each organ is made of tissues, such as xylem and phloem, and each tissue is made of specialised cells.
Plants have a transport system because their parts are far apart. Roots cannot photosynthesise, and leaves cannot take up water, so each needs the other.

Roots and root hair cells

Roots anchor the plant in the soil and absorb water and dissolved mineral ions such as nitrate and magnesium.
Near the tip of a root, many root hair cells grow out between the soil particles. Each root hair is a long thin extension of one cell on the surface of the root.
The long, thin shape gives the cell a very large surface area, so more water and minerals can be absorbed in the same time.
Water enters by osmosis: it moves from where there is more water, in the soil, to where there is less, inside the root hair cell. Mineral ions are taken in by active transport, which uses energy.
Root hair cells have no chloroplasts, as they are underground and get no light. They do have many mitochondria to release the energy needed.
Root hairs are fragile. If a plant is dug up and moved carelessly, many are torn off, and the plant may wilt until new ones grow.

A root hair cell and where root hairs grow

Xylem and phloem: the transport tissues

Xylem carries water and mineral ions from the roots up to the stem and leaves. It moves in one direction only.
Xylem vessels are hollow tubes made of dead cells joined end to end, with no end walls, so water flows freely. Their walls are strengthened with a tough substance called lignin, which also helps to support the plant.
Phloem carries sucrose and amino acids from where they are made or stored to where they are needed. Substances can move up or down, so we say phloem moves things in both directions.
Phloem is made of living cells called sieve tubes, with small holes in their end walls, and companion cells that provide the energy.
Xylem and phloem lie side by side in vascular bundles. In a leaf the xylem is on the upper side of each vein. In a stem the bundles form a ring. In a root the xylem is in the centre.
Do not mix them up: xylem takes water up; phloem moves products of photosynthesis (sugars) around.

Where xylem and phloem sit in a leaf, stem and root

The journey of water: transpiration

Water travels from the soil through a root hair cell, across the root cortex cells by osmosis, into a xylem vessel, up the stem, and into the leaf.
In the leaf, water evaporates from the surface of the spongy mesophyll cells into the air spaces. The water vapour then diffuses out through the open stomata.
Transpiration is the loss of water vapour from a plant, mainly through the stomata of the leaves.
As water leaves the leaf, more is pulled up the xylem behind it. This continuous flow is called the transpiration stream. Water molecules stick together, so the water is pulled up like a chain.
Transpiration is not just waste. It brings mineral ions up to the leaves, keeps the leaves cool as the water evaporates, and keeps the cells full of water so the plant stays firm.
Each stoma is controlled by two guard cells. When they take in water they swell and open the stoma. When they lose water they go limp and the stoma closes, which saves water but stops carbon dioxide getting in.

The path of water from soil to xylem

What changes the rate of transpiration, and what goes wrong

Higher temperature: water evaporates faster and diffuses faster, so transpiration speeds up.
More light: stomata open wider in the light so that carbon dioxide can get in, so more water vapour escapes. At night they are mostly closed.
More wind: moving air sweeps water vapour away from the leaf, so the gradient stays steep and transpiration is faster.
Higher humidity: the air already holds a lot of water vapour, so there is less difference between the leaf and the air, and transpiration slows down.
If a plant loses water faster than the roots can replace it, its cells lose water and are no longer firm. The leaves and stem droop: this is wilting. A wilted plant usually recovers when watered.
Plants cut their water loss in dry places with a thick waxy cuticle, fewer stomata, hairy leaves or leaves rolled up. These are adaptations to the environment.

Four factors that change the rate of transpiration

Think like a scientist: measuring water loss with a potometer

A potometer measures how fast a cut shoot takes up water. Most of the water taken up is lost by transpiration, so water uptake is a good estimate of the transpiration rate.
Method: cut the shoot under water so no air gets in, fit it into the apparatus with a watertight seal, let one air bubble into the capillary tube, and record how far the bubble moves in a fixed time.
It is only an estimate, because the plant also uses a little water for photosynthesis, and some water stays in its cells.
Inquiry task: plan an investigation into whether wind speed changes the rate of transpiration. The independent variable is the wind speed (for example a fan at different distances); the dependent variable is the distance the bubble moves in 5 minutes.
Control the other variables: the same shoot or plant, the same temperature, the same light, the same humidity (as far as you can) and the same time.
Evaluate: how could you make the results more reliable (repeat each speed and take a mean)? Which variable is hardest to control? Would a different species of plant give the same results?

Using a bubble potometer

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Câu hỏi luyện tập

Xem trước miễn phí — 8 trên 47 câu hỏi. Đăng ký để xem tất cả.

1.Which plant organ takes in water and minerals from the soil?
Easy
- AThe roots
- BThe leaves
- CThe flowers
- DThe stem
2.Which plant organ is the main site of photosynthesis?
Easy
- AThe stem
- BThe root
- CThe leaf
- DThe flower
3.What is one job of the stem?
Easy
- ATo absorb water from the soil
- BTo hold leaves up to the light
- CTo make new seeds for the plant
- DTo take in carbon dioxide for the plant
4.What is the main job of a flower?
Easy
- ATo make glucose
- BTo anchor the plant
- CReproduction
- DTo absorb water
5.Xylem vessels carry water and minerals up the plant.
Easy
True or false?
6.Phloem carries dissolved sugars to the parts of the plant that need them.
Easy
True or false?
7.What is a root hair cell?
Easy
- AA leaf cell that makes a lot of sugar
- BA root surface cell with a thin extension
- CA stem cell that carries water upwards
- DA tiny hair that protects the root tip
8.Why do root hair cells have a long, thin extension?
Easy
- AIt makes the whole root grow straight downwards
- BIt lets the cell make sugar in the light
- CIt gives a large surface area
- DIt stops the cell from drying out in the soil

Thêm câu hỏi

9.Match each part of a plant to its job.
Easy
- Root hair cell
- Xylem
- Phloem
- Guard cells
- Carries sugars around the plant
- Open and close the stomata
- Takes in water and minerals
- Carries water up the plant
10.What is transpiration?
Easy
- ATaking in water through the roots
- BMaking sugar in the leaves
- CWater vapour lost from leaves
- DMoving sugar down to the roots
11.Through which structures does water vapour leave a leaf?
Easy
- ARoot hairs
- BXylem vessels
- CThe waxy cuticle
- DStomata
12.Complete the sentence.
Easy
The tissue that carries water up a plant is the ____.
13.Complete the sentence.
Easy
The loss of water vapour from the leaves of a plant is called ____.
14.A plant wilts when its cells lose water and are no longer firm.
Easy
True or false?
15.On which kind of day would a plant lose water fastest?
Easy
- AA cool, rainy, windless day
- BA cool, calm, damp day
- CA cold, still, foggy day
- DA hot, windy, dry day
16.Which of these are plant organs? (select all that apply)
Easy
- ARoot
- BStem
- CLeaf
- DXylem
- EGuard cell
17.Put these in order from the smallest to the largest level of organisation.
Easy
- Leaf
- Whole plant
- Palisade cell
- Xylem tissue
18.What do guard cells do?
Easy
- AOpen and close the stomata
- BCarry water up to the leaf
- CAbsorb minerals from the soil
- DMake glucose for the plant
19.Where are most stomata found on a leaf?
Easy
- AOn the upper surface
- BOnly on the stem
- COn the underside
- DOnly on the veins
20.Why are most stomata on the underside of a leaf rather than the top?
Medium
- AThe top is cooler and shaded all day
- BThe underside is cooler, so less water is lost
- CLight cannot reach the top surface of a leaf
- DRoots only ever grow towards the underside
21.In which direction does water move in the xylem?
Medium
- ABoth up and down the plant
- BUpwards only, from the roots to the leaves
- CDownwards only, from the leaves to the roots
- DSideways, between the vascular bundles
22.In which direction do substances move in the phloem?
Medium
- AUpwards only, as in the xylem
- BDownwards only, towards the roots
- CBoth up and down, as needed
- DOnly from the roots up to the flowers
23.What is the transpiration stream?
Medium
- AThe flow of sugar from the leaves to the roots
- BThe flow of water from the leaf into the soil
- CThe flow of carbon dioxide into the leaf
- DThe flow of water up the xylem
24.Which feature of xylem vessels helps them to carry water?
Medium
- AThey are very short, thin cells
- BThey have living contents that pump water
- CThey have small holes in their side walls
- DThey are hollow tubes with no end walls
25.Which statement correctly compares xylem and phloem?
Medium
- ABoth carry only water and nothing else
- BXylem carries sugars and phloem carries water
- CPhloem is living, but xylem vessels are dead
- DXylem is found only in the roots of a plant
26.Why does a higher temperature increase the rate of transpiration?
Medium
- AMore light reaches the leaf
- BWater evaporates and diffuses faster
- CThe stomata close completely
- DThe roots absorb a lot less water
27.Why does high humidity reduce the rate of transpiration?
Medium
- AThe air is already full of vapour, so less leaves
- BThe roots stop taking in any water at all
- CThe leaf becomes much warmer than the air
- DThe xylem vessels become blocked with sugar
28.Why does wind increase the rate of transpiration?
Medium
- AIt cools the leaf so more water evaporates
- BIt pushes water back down the xylem
- CIt opens the roots up to the air
- DIt sweeps vapour away from the leaf
29.Why does a plant wilt when it does not have enough water?
Medium
- AIts xylem is completely full of sugar
- BIts cells lose water and go soft
- CIts leaves make far too much glucose
- DIts stomata are blocked by soil particles
30.Why can a plant not keep its stomata closed all the time?
Medium
- AIt would lose far too much water
- BIt could not take in carbon dioxide
- CIts guard cells would swell and burst open
- DThe roots would stop growing downwards
31.Why does water move from the soil into a root hair cell?
Medium
- AThe soil has less water than the cell has
- BThe root hair cell pumps water in using energy
- CThere is more water in the soil, so it moves in by osmosis
- DGravity pulls the water down into the root
32.Which of these would increase the rate of transpiration? (select all that apply)
Medium
- AA rise in temperature
- BStronger wind
- CBrighter light
- DHigher humidity
- EClosing the stomata
33.Match each change to its effect on transpiration.
Medium
- A rise in temperature
- Higher humidity
- Stronger wind
- Smaller vapour gradient, so the rate falls
- Faster evaporation, so the rate rises
- Vapour is swept away, so the rate rises
34.Put these in the order water moves through a plant.
Medium
- Xylem in the stem
- Soil
- Stomata in the leaf
- Root hair cell
35.In a potometer experiment, an air bubble moves 45 mm in 3 minutes. How far does it move per minute, on average?
Medium
36.Complete the sentence.
Medium
Root hair cells have a large surface ____ so they can absorb water quickly.
37.Plants absorb their food, in the form of sugar, from the soil through their roots.
Medium
True or false?
38.
A student measures how far a bubble moves in a potometer in 5 minutes.
Conditions Distance moved (mm)
Still air 12
Fan on low 21
Fan on high 30
What is the best conclusion from these results?
Hard
- AWind speed made no difference to the water uptake
- BA faster wind made the bubble move much less
- CFaster wind gave faster water uptake
- DThe plant stopped transpiring altogether in still air
39.In the wind investigation above, what is the independent variable?
Hard
- AThe distance the bubble moves in the tube
- BThe temperature of the room in the lab
- CThe speed of the air from the fan
- DThe length of the shoot that is used
40.Which of these must be kept the same in the wind investigation to make it a fair test?
Hard
- AThe fan speed in each trial
- BThe temperature, light and shoot
- CThe distance the bubble moves each time
- DThe results from each of the trials
41.Why is the water uptake measured by a potometer only an estimate of the transpiration rate?
Hard
- AThe shoot has been cut off the plant
- BSome water is used up or stays in the cells
- CThe bubble always moves at a changing speed
- DWater is taken up through the leaves instead
42.A student smears petroleum jelly over the lower surface of one leaf and over the upper surface of another, identical leaf. Which leaf loses less water, and why?
Hard
- AThe leaf sealed on top, as the top has most stomata
- BBoth lose the same, as petroleum jelly is waterproof
- CThe leaf sealed on top, as it loses water through the cuticle
- DThe leaf sealed underneath, as most stomata are there
43.On a hot afternoon the leaves of a plant droop, even though the soil is wet. What is the best explanation?
Hard
- AThe leaves lose water faster than roots supply it
- BThe roots have stopped taking in any minerals at all
- CThe phloem has stopped moving sugar to the roots
- DThe soil is too wet for the plant to be able to use
44.If a ring of bark containing the phloem is cut right round a tree trunk, sugar can no longer travel down past the cut.
Hard
True or false?
45.Explain why a plant needs both xylem and phloem.
Hard
46.Explain how the structure of a root hair cell makes it well suited to taking in water and minerals.
Hard
47.Plan an investigation to find out whether the wind speed affects the rate of transpiration. Include what you change, what you measure, two things you keep the same and how you make your results reliable.
Hard

Ghi chú ôn tập

## The big idea - A plant is a **system**: **roots**, **stem**, **leaves** and **flowers** are organs made of tissues, linked by **xylem** and **phloem**. Each structure suits its **function**. ## Roots - **Root hair cells** have a long, thin extension, giving a large **surface area**. Water enters by **osmosis**; mineral ions by **active transport**. ## Transport tissues - **Xylem**: dead hollow tubes with lignin; water and minerals **up**. - **Phloem**: living sieve tubes and companion cells; **sucrose and amino acids**, up or down (**translocation**) from **sources** to **sinks**. ## Transpiration - Water: soil → root hair → cortex → xylem → leaf → evaporates → diffuses out of the **stomata**. This pulls the **transpiration stream** up. - Faster with more **heat**, **light** and **wind**; slower with more **humidity**. - Too much water loss causes **wilting**. ## Investigating - A **potometer** estimates water uptake. Change one variable, measure one, control the rest, repeat and take a mean.

Organ: A part of a plant made of different tissues that does one main job, such as a leaf or a root.
Tissue: A group of similar cells that work together, such as xylem.
Root hair cell: A root surface cell with a long thin extension that absorbs water and minerals.
Xylem: The tissue that carries water and mineral ions up from the roots.
Phloem: The tissue that carries sucrose and amino acids around the plant.
Vascular bundle: A group of xylem and phloem that runs through the root, stem and leaf.
Lignin: A tough substance that strengthens the walls of xylem vessels.
Stoma: A tiny pore in the leaf surface that lets gases and water vapour pass.
Guard cells: A pair of cells that open and close a stoma.
Transpiration: The loss of water vapour from a plant, mainly through the stomata.
Transpiration stream: The flow of water up the xylem as water is lost from the leaves.
Translocation: The movement of sucrose and amino acids in the phloem.
Wilting: Drooping of a plant when its cells lose water and are no longer firm.
Potometer: Apparatus that measures water uptake by a cut shoot, to estimate transpiration.

Thẻ ghi nhớ

Name the four main organs of a flowering plant.

Roots, stem, leaves and flowers.

What does a root hair cell do?

It absorbs water and mineral ions from the soil.

How is a root hair cell adapted for its job?

A long, thin extension gives a large surface area; it has many mitochondria for energy.

How does water enter a root hair cell?

By osmosis, from the soil where there is more water into the cell where there is less.

What does xylem carry, and in which direction?

Water and mineral ions, upwards only, from the roots to the leaves.

What does phloem carry, and in which direction?

Sucrose and amino acids, in both directions, from sources to sinks.

How are xylem vessels adapted?

Hollow tubes of dead cells with no end walls and walls strengthened with lignin.

What is a vascular bundle?

A group of xylem and phloem running together through the root, stem and leaf.

What is transpiration?

The loss of water vapour from a plant, mainly through the stomata.

Describe the path of water through a plant.

Soil, root hair cell, root cortex, xylem, leaf cells, evaporates, and diffuses out of the stomata.

What is the transpiration stream?

The flow of water up the xylem pulled by water evaporating from the leaves.

What do guard cells do?

Open and close the stomata.

Why do most stomata sit on the underside of a leaf?

It is cooler and out of the sun, so less water is lost.

Which conditions speed up transpiration?

Higher temperature, brighter light and more wind.

Why does high humidity slow transpiration?

The air already holds a lot of water vapour, so there is less difference to drive diffusion.

What is translocation?

The movement of sucrose and amino acids in the phloem from sources to sinks.

Why does a plant wilt?

It loses water faster than it takes it up, so its cells go soft and no longer support it.

What does a potometer measure?

The rate of water uptake of a cut shoot, which estimates the rate of transpiration.

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Conditions	Distance moved (mm)
Still air	12
Fan on low	21
Fan on high	30