Plant tissues, organs and systems

2. Organisation

Plant cells are organised into tissues, tissues into organs such as the leaf, and organs into organ systems. The leaf is the one to know inside out: five tissues, each with a structure that fits its job.

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What plant tissues, organs and systems covers

Every subtopic below has its own questions, mark schemes and notes.

4.2.3.1Plant tissues and the leaf

4.2.3.2Transport in plants

Two separate sets of tubes run through every part of a plant. Xylem carries water upwards from the roots; phloem carries dissolved sugars in both directions. Water is lost from the leaves by transpiration, and the guard cells decide how much.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Tissue — A group of similar cells working together to carry out a particular function.

Organ — A group of different tissues working together to perform a function.

Organ system — A group of organs working together to perform a particular job.

Mesophyll — The tissue in the middle of a leaf, made up of the palisade layer and the spongy layer.

Stomata — Tiny pores, mostly in the lower epidermis, that let gases move in and out of the leaf. One of them is a stoma.

Meristem — Plant tissue at the growing tips of shoots and roots whose cells can differentiate into any plant cell type.

Transpiration — The loss of water from a plant, mostly as water vapour diffusing out of the stomata in the leaves.

Transpiration stream — The constant movement of water from the roots, up through the xylem, and out of the leaves.

Translocation — The transport of dissolved sugars through the phloem, from the leaves to the rest of the plant.

Lignin — The strengthening material in xylem walls that supports the plant and keeps the vessels open.

Cell sap — The water and dissolved substances being carried along inside phloem tubes.

Turgid — Swollen and firm because the cell is full of water.

Potometer — Apparatus that estimates the rate of transpiration by measuring how much water a shoot takes up.

Where marks get lost

The mistakes that come up again and again in this topic.

Palisade mesophyll and spongy mesophyll are different tissues with different jobs. Palisade is packed with chloroplasts for photosynthesis; spongy has air spaces for gas exchange.
The waxy cuticle and the upper epidermis are not the same thing. The cuticle is the waxy waterproof covering; the epidermis is the layer of cells beneath it.
Do not say the upper epidermis is transparent 'so it can photosynthesise'. It is transparent so light can reach the palisade layer, which does the photosynthesising.
If a question asks why a leaf structure is useful, name the structure and then say what it lets the plant do. A description on its own rarely gets the explain mark.
Xylem carries water upwards only; phloem carries sugars in both directions. Mixing these up is the single most common way to lose marks in this topic.
Xylem cells are dead and hollow. Phloem cells are alive. Saying 'both are dead tubes' costs the mark.
Transpiration is water loss from the plant; translocation is sugar transport in the phloem. They are different processes in different tubes.
When explaining humidity or air movement, mention the concentration gradient between the inside of the leaf and the air outside. 'The air is already wet' on its own is not enough.
A potometer measures water uptake, not water loss directly. Say that uptake is assumed to equal transpiration if you are asked about its limitations.
Always give the unit with a rate, for example mm per minute or grams per hour. An unlabelled number often gets no mark.

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