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4.7 Transport Systems in Plants

GCSE Biology · AQA · Chapter 4: Organising Animals and Plants

A free GCSE Biology lesson for AQA (8461) from OBA School: xylem and phloem: structure, the substances they carry and translocation. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the structure of xylem and phloem tissue
  • Explain how xylem carries water and mineral ions from the roots to the leaves
  • Describe translocation: the movement of dissolved sugars in the phloem
  • Interpret simple experiments on plant transport and calculate rates of movement

Key words

  • Xylem: Tissue made of hollow tubes of dead cells that carries water and mineral ions up from the roots.
  • Lignin: A tough substance that strengthens the walls of xylem vessels.
  • Phloem: Tissue made of columns of living cells that carries dissolved sugars around the plant.
  • Sieve plate: An end wall between phloem cells with pores in it, so cell sap can pass through.
  • Translocation: The movement of dissolved sugars (made in the leaves) through the phloem to the rest of the plant.
  • Transpiration stream: The flow of water from the roots, up the xylem and out of the leaves.

Two transport tissues

Plants need to move water and mineral ions up from the roots, and sugars made by photosynthesis away from the leaves. They have two separate transport tissues, found together in vascular bundles in the roots, stem and leaves (the veins of a leaf).

  • Cells; Xylem dead cells joined end to end; Phloem living cells
  • End walls; Xylem broken down, forming a continuous hollow tube; Phloem sieve plates with pores, so cell sap moves from cell to cell
  • Walls; Xylem thick and strengthened with lignin; Phloem thin, no lignin
  • Carries; Xylem water and mineral ions; Phloem dissolved sugars (mainly sucrose)
  • Direction; Xylem upwards only: roots → stem → leaves; Phloem both up and down

Memory aid: Xylem carries water up; Phloem carries food (sugars) both ways.

Xylem and the transpiration stream

Root hair cells absorb water by osmosis and mineral ions by active transport. The water and ions pass into the xylem and travel up the stem to the leaves. As water evaporates from the leaves, more is pulled up behind it: this flow is the transpiration stream (see section 4.8).

Lignin makes xylem walls strong enough not to collapse as water is pulled up. Xylem also helps to support the plant: wood is mostly xylem.

Worked example: rate of movement in xylem

A celery stalk is stood in coloured dye. After 25 minutes the dye has moved 9.0 cm up the xylem. Calculate the mean rate of movement in cm/min.
rate = distance ÷ time = 9.0 ÷ 25 = 0.36

Answer: 0.36 cm/min

Phloem and translocation

Sugars made in the leaves are turned into sucrose and carried in the phloem to wherever they are needed: to growing regions (shoot and root tips) for respiration and growth, and to storage organs such as potato tubers, bulbs and fruits, where they may be stored as starch. This movement is called translocation.

Because sugars go to wherever they are needed, translocation happens both up and down the plant.

Key fact: If a ring of bark is removed from a tree trunk, the phloem is removed but the xylem in the wood is left. The leaves stay healthy (water still reaches them), but sugars build up above the ring and it swells, while the roots below eventually starve.

Common mistakes

  • Saying xylem carries sugars or that phloem carries water and minerals. Learn which carries which.
  • Saying xylem cells are alive. Mature xylem cells are dead; phloem cells are living.
  • Writing that translocation is the movement of water. Translocation is the movement of dissolved sugars in the phloem.
  • Saying phloem only carries sugar downwards. Sugars move both ways, to growing tips and to storage organs.

Revision questions

Try these, then check your answers below each one.

Question 1. What does phloem transport? (Choose from: Dissolved sugars, Water and mineral ions, Oxygen, Carbon dioxide.)

Answer: Dissolved sugars

Working: Phloem carries dissolved sugars from the leaves to the rest of the plant.

Question 2. True or false? Mature xylem cells are dead.

Answer: True

Working: True. Xylem is made of dead cells that form hollow tubes.

Question 3. What is the movement of dissolved sugars through the phloem called?

Answer: translocation

Working: This is translocation.

Related lessons

Continue in the full course

This lesson is section 4.7 (AGCBH0407) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.

 

 

 

 

 

 

 

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