GCSE · Biology · AQA · Spec 8461

Plant organ system (xylem, phloem, transpiration, translocation)

A tree has no heart, yet water from its roots reaches leaves far above the ground. The secret isn't a push from below. It's a pull from above.

One plant, one transport system

Follow the water from soil to air

Follow one drop of water on its way through a plant. Tap each zone in turn. The line climbs as the water climbs. It's a route map, not a drawing to scale.

Zone 1 of 5

Root hair cell

The journey starts here. A root hair cell has a long, thin extension that pushes out between the soil particles, giving it a large surface area for taking things in. Water moves into it by osmosis. Mineral ions are usually more concentrated inside the cell than in the soil, so they can't just drift in. They are moved in by active transport, which uses energy from respiration.

  • Water in by osmosis
  • Mineral ions in by active transport
  • Large surface area

Roots take water in, the stem carries it up, and the leaves use some of it and lose the rest through their stomata. Three organs, one route: that is the plant organ system for transport.

Two tubes, two different loads

XylemvsPhloem

Start with the top row. It's the one people swap.

Focus

What it carries

Xylem

Water and dissolved mineral ions

Phloem

Dissolved sugars (food molecules)

The insight

Here's the trick: phloem sounds like 'flow-em', and it flows the food. Xylem is the water pipe.

Which way

Xylem

Upwards, from the roots to the stems and leaves

Phloem

From the leaves to the rest of the plant, where the sugars are used straight away or stored

Name of the movement

Xylem

The transpiration stream

Phloem

Translocation

What it's made of

Xylem

Hollow tubes whose walls are strengthened by lignin

Phloem

Tubes of elongated cells, with pores in the end walls between one cell and the next

How the structure fits the job

Xylem

Being hollow gives water an open path all the way up; lignin makes the walls strong enough to hold their shape

Phloem

The pores in the end walls let cell sap pass from one phloem cell to the next

The transpiration stream

?

Reason it through

With no pump, why does water keep moving up a plant?

Link 1 of 4

First link · your turn

The stomata are open so carbon dioxide can get into the leaf. What else can get through an open stoma?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

What changes the rate?

Four factors, one grid

Picture a healthy, well-watered plant. Increase one factor at a time and keep everything else the same. For each row, tick every statement that is true.

Higher temperature
Higher humidity
More air movement
Brighter light

Read the rate off the graph

01.534.5603.5710.514Time (hours)Total water lost (g)(3, 3)

Time (hours): 3. Total water lost (g): 3

Drag the point along the line and read off the values.

Watch out: Don't confuse height with steepness. At 6 hours the line is at its highest: that's the most water lost in total. The rate is how steep the line is, and the line gets steeper once the fan goes on. Try it: read the values at 3 hours and 6 hours, then work out the rate once the fan is on.

Your turn

Calculate a rate, then scale a graph

Illustrative results: a plant had lost 6 g of water after 2 hours and 30 g after 8 hours. Calculate its rate of transpiration between 2 and 8 hours. Then choose a y-axis scale for plotting the results on graph paper that is 8 large squares tall.

  1. Rate of transpiration = change in water lost ÷ time takenA rate always means 'how much per unit of time'.
  2. missing step
Which line is step 2?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Two sets of tubes, some tiny pores in the leaves, and a pull that starts at the top.

What you need to know

  • Roots, stem and leaves form one plant organ system for transporting substances.
  • Root hair cells absorb water by osmosis and mineral ions by active transport.
  • Xylem carries water and mineral ions up from the roots; phloem carries dissolved sugars from the leaves.
  • The transpiration stream is pulled up the plant by water evaporating from the leaves and escaping through the stomata.
  • Temperature, humidity, air movement and light intensity all change the rate of transpiration.

The big picture

Roots, stem and leaves work together as one organ system for transport. Root hair cells take in water by osmosis and mineral ions by active transport. Xylem carries water and mineral ions up to the leaves in the transpiration stream, which is pulled along by water evaporating and escaping through the stomata. Phloem carries dissolved sugars from the leaves to the rest of the plant, a movement called translocation. Temperature, humidity, air movement and light intensity all change how fast a plant loses water.

Key points

1The roots, stem and leaves form a plant organ system for transport.
2Root hair cells have a large surface area; water enters them by osmosis, and mineral ions by active transport, which needs energy.
3Xylem is made of hollow tubes strengthened by lignin. It carries water and mineral ions from the roots to the stems and leaves in the transpiration stream.
4Phloem is made of tubes of elongated cells with pores in their end walls. It carries dissolved sugars from the leaves to the rest of the plant, where they are used straight away or stored. This is translocation.
5Stomata let gases in and out of the leaf; guard cells open and close them, controlling gas exchange and water loss.
6Transpiration speeds up with higher temperature, more air movement and brighter light, and slows down with higher humidity.
7Rate of transpiration = change in water lost ÷ time taken, in units such as g per hour. On a graph of water lost against time, a steeper line means a faster rate.

Worked example

Problem

Illustrative data: plant A lost 12 g of water in 4 hours. Plant B lost 15 g of water in 6 hours. Which plant had the higher rate of transpiration?

⚠ Watch out

Saying the roots pump water up the plant. The transpiration stream is pulled up by water evaporating from the leaves and escaping through the stomata. The other classic slip is swapping the tissues: xylem carries water and mineral ions, phloem carries dissolved sugars.

🧠

Memory hook

Phloem sounds like 'flow-em': it flows the food. TranspIRATION is the water that 'perspires' out of the leaves; transLOCATION sends sugar to a new location.

✓

Check yourself

Cover the page. Name the tissue that carries sugars and the one that carries water. Then explain why a plant on a breezy day loses water faster than one in still air.

Flashcards

(14)
Which organs make up the plant organ system for transport?
The roots, the stem and the leaves, working together to move substances around the plant.
How is a root hair cell adapted for taking up water and mineral ions?
It has a long, thin extension that gives it a large surface area in contact with the soil water.
How do water and mineral ions get into a root hair cell?
Water enters by osmosis. Mineral ions enter by active transport, which uses energy from respiration.
What does xylem carry, and in which direction?
Water and dissolved mineral ions, from the roots up to the stems and leaves.
How is xylem adapted to transport water?
It is made of hollow tubes, giving water an open path, and its walls are strengthened by lignin.
What is translocation?
The movement of dissolved sugars (food molecules) through the phloem, from the leaves to the rest of the plant for immediate use or storage.
What is phloem made of?
Tubes of elongated cells. Cell sap moves from one cell to the next through pores in the end walls.
What is transpiration?
The loss of water vapour from the leaves, mostly through the stomata, which draws the transpiration stream up the xylem.
What do stomata and guard cells do?
Stomata are pores that let gases in and out of the leaf. Guard cells open and close them, controlling gas exchange and water loss.
How does a rise in temperature affect the rate of transpiration?
It increases the rate: water evaporates and diffuses out of the leaf faster.
How does higher humidity affect the rate of transpiration?
It decreases the rate: the air already holds more water vapour, so vapour diffuses out of the leaf more slowly.
How does more air movement affect the rate of transpiration?
It increases the rate: moving air carries water vapour away from the leaf, so more keeps diffusing out.
How does brighter light affect the rate of transpiration?
It increases the rate: stomata open wider in bright light, so more water vapour can escape.
How do you work out a rate of transpiration?
Divide the change in the amount of water lost by the time taken. Give a compound unit, such as g per hour.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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