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4.6 Tissues and Organs in Plants

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

A free GCSE Biology lesson for AQA (8461) from OBA School: plant tissues, the leaf as an organ and how its tissues are adapted for photosynthesis. 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

  • Name the main plant organs and the plant tissues found in a leaf
  • Describe the function of epidermal tissue, palisade and spongy mesophyll, xylem, phloem and meristem
  • Explain how the structure of the tissues in a leaf relates to photosynthesis and gas exchange
  • Calculate a mean and a magnification from data on plant cells

Key words

  • Plant organ: A structure made of several tissues working together, such as a root, stem, leaf or flower.
  • Epidermis: The outer layer of cells that covers and protects the plant.
  • Palisade mesophyll: Tall, closely packed cells just under the upper epidermis, full of chloroplasts.
  • Spongy mesophyll: Loosely packed, rounded cells with air spaces between them, for gas exchange.
  • Xylem and phloem: Transport tissues: xylem carries water and mineral ions, phloem carries dissolved sugars.
  • Meristem: Tissue at the tips of roots and shoots where cells divide and can become any type of plant cell.
  • Stomata: Pores, mostly in the lower epidermis, that let gases in and out. Each is controlled by two guard cells.

Plant organs and organ systems

Plants are organised just like animals: cells → tissues → organs → organ systems. The main plant organs are the roots, stem and leaves (and flowers for reproduction). Roots, stem and leaves work together as the plant transport system, an organ system that moves water, mineral ions and sugars around the plant.

  • Root; Main job anchors the plant; absorbs water and mineral ions from the soil
  • Stem; Main job holds leaves up towards the light; carries substances between roots and leaves
  • Leaf; Main job the main site of photosynthesis; gas exchange

Key fact: A leaf is an organ because it is made of several different tissues (epidermis, mesophyll, xylem, phloem) working together to carry out photosynthesis.

The tissues of a leaf

  • Upper epidermis; Function protects the leaf; How it is adapted cells are transparent so light passes through; covered with a waxy cuticle that reduces water loss
  • Palisade mesophyll; Function most of the photosynthesis; How it is adapted near the top surface, tall and closely packed, with many chloroplasts to absorb as much light as possible
  • Spongy mesophyll; Function gas exchange (and some photosynthesis); How it is adapted rounded cells with large air spaces so carbon dioxide and oxygen diffuse easily
  • Xylem; Function carries water and mineral ions to the leaf; How it is adapted tubes of dead cells, strengthened with lignin
  • Phloem; Function carries dissolved sugars away from the leaf; How it is adapted columns of living cells with pores in their end walls
  • Lower epidermis; Function protection and gas exchange; How it is adapted contains many stomata, each opened and closed by two guard cells

Exam tip: If asked how a leaf is adapted, link each feature to light (palisade, transparent epidermis), carbon dioxide (stomata, air spaces) or water (xylem, cuticle).

Meristem tissue

Meristem tissue is found at the growing tips of shoots and roots. Its cells divide by mitosis and can differentiate into any type of plant cell, all through the life of the plant. This is why gardeners can grow new plants from cuttings.

Key fact: Meristem cells act like stem cells. Unlike most animal cells, many plant cells keep the ability to differentiate throughout life.

Working with data on plant cells

Worked example: mean number of chloroplasts

A student counts the chloroplasts in five palisade cells: 36, 42, 39, 45, 38. Calculate the mean.
mean = total ÷ number of values = (36 + 42 + 39 + 45 + 38) ÷ 5 = 200 ÷ 5 = 40

Answer: 40 chloroplasts per cell

Worked example: magnification

A palisade cell is 75 µm long. In a photo it is 30 mm long. Calculate the magnification.
30 mm = 30 000 µm. magnification = image size ÷ actual size = 30 000 ÷ 75 = 400

Answer: × 400

Common mistakes

  • Saying the epidermis does photosynthesis. Epidermis cells (apart from guard cells) have no chloroplasts.
  • Mixing up xylem and phloem. Xylem carries water and minerals up; phloem carries sugars.
  • Saying spongy mesophyll is for "storing air". Its air spaces allow gases to diffuse to and from the cells.
  • Saying the waxy cuticle stops all water loss. It reduces water loss; most water is lost through the stomata.

Revision questions

Try these, then check your answers below each one.

Question 1. Which leaf tissue carries out most of the photosynthesis? (Choose from: Upper epidermis, Spongy mesophyll, Palisade mesophyll, Xylem.)

Answer: Palisade mesophyll

Working: The palisade mesophyll cells are packed with chloroplasts near the top of the leaf.

Question 2. True or false? A leaf is an organ.

Answer: True

Working: True. A leaf is made of several tissues working together to carry out photosynthesis.

Question 3. Name the waxy layer on the upper surface of a leaf that reduces water loss.

Answer: cuticle

Working: The waxy cuticle reduces evaporation from the leaf surface.

Related lessons

Continue in the full course

This lesson is section 4.6 (AGCBH0406) 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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