B4.6 Tissues and Organs in Plants

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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 (roots, stem, leaves) 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
• Describe how to look at stomata using a leaf peel, and calculate a mean and a magnification

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; root hair cells absorb water by osmosis and mineral ions by active transport; may store starch; usually white because there are no chloroplasts underground
• 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

A leaf is thin and flat, giving a large surface area to absorb light and a short distance for gases to diffuse. Inside, several tissues are arranged in layers, each with its own job.

• 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 (the only epidermis cells with chloroplasts)

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.

Looking at stomata and working with data

To see stomata, paint a thin layer of clear nail varnish on a small patch of the upper and lower surfaces of a leaf. When it is dry, lift it off with clear sticky tape, stick it on a labelled slide and look at it under a microscope. You will see the outlines of epidermis cells and pairs of guard cells. Count the stomata in several fields of view on each surface.

Worked example: mean number of chloroplasts

A student counts the chloroplasts in five palisade cells: 44, 37, 41, 46, 42. Calculate the mean.
mean = total ÷ number of values = (44 + 37 + 41 + 46 + 42) ÷ 5 = 210 ÷ 5 = 42

Answer: 42 chloroplasts per cell

Worked example: magnification

A palisade cell is 64 µm long. In a photo it is 16 mm long. Calculate the magnification.
16 mm = 16 000 µm. magnification = image size ÷ actual size = 16 000 ÷ 64 = 250

Answer: × 250

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. The diagram shows a section through a leaf. Which letter shows the tissue where most photosynthesis happens? (Choose from: A, B, C, F.)

Answer: B

Working: B is the palisade mesophyll. A is the upper epidermis, C spongy mesophyll and F the lower epidermis.

Question 2. True or false? Guard cells are the only cells of the leaf epidermis that contain chloroplasts.

Answer: True

Working: True. Other epidermis cells are transparent; guard cells have chloroplasts.

Question 3. Name the tissue at the tips of roots and shoots where cells divide to make new cells.

Answer: meristem

Working: Meristem tissue makes new cells that can become any type of plant cell.

Related lessons

• Previous lesson: B4.5 Breathing and Gas Exchange
• Next lesson: B4.7 Transport Systems in Plants
• In this chapter: B4.1 The Blood
• In this chapter: B4.2 The Blood Vessels
• In this chapter: B4.3 The Heart

Continue in the full course

This lesson is section B4.6 (AGCSC0406) of our free GCSE Combined Science 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 Combined Science course.