1.5 Specialisation in Plant Cells

A free GCSE Biology lesson for AQA (8461) from OBA School: how root hair cells, photosynthetic cells, xylem and phloem are adapted to their functions. 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 how root hair cells are adapted to absorb water and mineral ions
• Describe how leaf cells are adapted for photosynthesis
• Describe how xylem and phloem cells are adapted to transport substances
• Calculate how much a feature such as root hairs increases surface area

Key words

• Root hair cell: A cell on the surface of a root with a long, thin extension that grows between soil particles.
• Xylem: Tissue made of hollow tubes of dead cells that carry water and mineral ions from the roots up to the leaves.
• Phloem: Tissue made of living cells that carries dissolved sugars around the plant (translocation).
• Lignin: A tough chemical that strengthens the walls of xylem cells.
• Sieve plate: An end wall between phloem cells, full of holes so the dissolved sugars can flow through.

Root hair cells

Plants absorb water by osmosis and mineral ions (such as nitrate and magnesium) by active transport through their root hair cells.

• Long, thin projection (root hair): gives a very large surface area, so more water and mineral ions can be absorbed.
• Large permanent vacuole: speeds up the movement of water into the cell by osmosis.
• Many mitochondria: release energy for the active transport of mineral ions from the soil.
• No chloroplasts: roots are underground in the dark, so they cannot photosynthesise.

Worked example: surface area

A 1 mm length of root has a surface area of 1.2 mm² without hairs. Its root hairs add another 8.4 mm².
Total surface area = 1.2 + 8.4 = 9.6 mm²
Times bigger = 9.6 ÷ 1.2 = 8
Percentage increase = (8.4 ÷ 1.2) × 100 = 700%

Answer: the root hairs make the surface area 8 times bigger (a 700% increase)

Photosynthetic cells

Most photosynthesis happens in palisade cells near the top surface of a leaf.

• Packed with chloroplasts: more chlorophyll absorbs more light for photosynthesis.
• Tall, thin shape, close to the upper surface and packed side by side: lots of cells receive light.
• Permanent vacuole: keeps the cell rigid, which helps hold the leaf flat towards the light.

Xylem and phloem: the transport cells

• Carries; Xylem water and dissolved mineral ions; Phloem dissolved sugars (made in photosynthesis)
• Direction; Xylem up only, from roots to leaves; Phloem up and down, to where sugars are used or stored
• Cells; Xylem dead cells; end walls broken down to make a continuous hollow tube; Phloem living sieve tube cells joined end to end by sieve plates
• Key adaptations; Xylem lignin spirals or rings strengthen the walls so tubes do not collapse; also support the plant; Phloem few sub-cellular structures so sugars flow easily; companion cells with many mitochondria supply energy

Memory tip: Xylem = water going up. Phloem = food (sugars) going up and down. Both are found together in the vascular bundles of roots, stems and leaves.

Common mistakes

• Saying root hair cells have chloroplasts. They are underground and have none.
• Saying xylem carries sugars or that phloem carries water as its main job. Xylem: water and mineral ions. Phloem: dissolved sugars.
• Saying xylem cells are alive. Mature xylem cells are dead and hollow.
• Writing “root hairs increase surface area” without saying what this does: more water and mineral ions are absorbed.
• Saying minerals are absorbed by osmosis. Water moves by osmosis; mineral ions mostly by active transport.

Revision questions

Try these, then check your answers below each one.

Question 1. What does xylem carry? (Choose from: Dissolved sugars, up and down the plant, Water and mineral ions, from roots to leaves, Oxygen, from leaves to roots, Starch, from leaves to roots.)

Answer: Water and mineral ions, from roots to leaves

Working: Xylem carries water and mineral ions up from the roots to the leaves.

Question 2. True or false? Mature xylem vessels are made of living cells.

Answer: False

Working: False. Xylem cells are dead; their end walls break down to form a continuous hollow tube.

Question 3. Name the substance that strengthens the walls of xylem vessels.

Answer: lignin

Working: Lignin strengthens xylem walls so the tubes do not collapse.

Related lessons

• Previous lesson: 1.4 Specialisation in Animal Cells
• Next lesson: 1.6 Diffusion
• In this chapter: 1.1 The World of the Microscope
• In this chapter: 1.2 Animal and Plant Cells
• In this chapter: 1.3 Eukaryotic and Prokaryotic Cells

Continue in the full course

This lesson is section 1.5 (AGCBH0105) 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.