Plant tissues, organs and translocation – GCSE Combined Science Biology

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A plant looks simple, but it is built in layers, just like your body. Small parts join up to make bigger parts. Each part has a job.

This matters because plants make the food and oxygen that we rely on. To do that, they need to take in light and gases and move water and sugar around.

By the end you will be able to name the tissues in a leaf and say how each is adapted. You will compare xylem and phloem. You will also explain what translocation is.

Plant organs and tissues

Cells are the smallest living parts. A tissue is a group of similar cells that do the same job. An organ is made of several tissues working together.

The leaf is an organ for photosynthesis. The stem supports the plant and carries substances. The root anchors the plant and takes in water and minerals.

The roots and stem form one transport system. The leaves, stem and flowers form the shoot system.

Tissues in a leaf

A leaf is thin and flat, like a sandwich with layers. The top layer is the upper epidermis. It is thin and transparent, and it has a waxy cuticle on top. The cuticle is a waterproof layer. Light passes through.

Below it is the palisade mesophyll. Its cells are packed with chloroplasts, so it does most of the photosynthesis. Under that is the spongy mesophyll, which has air spaces so gases can move through.

The lower epidermis has stomata. These are small holes that let gases in and out. Xylem and phloem run through the leaf in the veins.

How leaf tissues are adapted

Adapted means shaped to do a job well. Palisade cells are tall and packed tightly near the top. They catch a lot of light where it is strongest.

The spongy mesophyll cells are loosely packed. The air spaces give a large surface area for gases to diffuse. Diffuse means spread out from a high to a low concentration.

Guard cells surround each stoma. They open and close it. The leaf is thin, so gases only have a short way to go. The waxy cuticle reduces water loss from the surface.

Xylem

Xylem is the tissue that carries water and mineral ions from the roots up to the leaves. Think of it as the water pipes of the plant.

Xylem cells are dead. They are hollow tubes joined end to end, so water can flow straight through. The walls are strengthened with lignin, a tough substance that stops the tubes collapsing.

Water in the xylem only travels one way, which is upwards. The xylem also helps to support the plant.

Phloem

Phloem is the tissue that carries dissolved sugars around the plant. The main sugar is sucrose. Think of the phloem as a set of delivery vans.

Phloem cells are living. They have small holes in their end walls, so the sugar solution can pass from cell to cell. Next to them are companion cells, which provide energy for the phloem cells.

Unlike xylem, phloem can carry sugar up or down the plant. It goes wherever the sugar is needed.

Translocation

Translocation is the movement of dissolved sugars in the phloem. The sugars go from the leaves, where they are made, to the parts of the plant that need them.

A part that makes sugar is called a source. A leaf is a source. A part that uses or stores sugar is called a sink. Roots, fruits and growing shoots are sinks. Sugars are used for respiration and growth, or stored as starch.

Translocation needs energy, so it is an active process. Phloem cells must respire to supply it. If the phloem is damaged, a sink cannot get sugar and stops growing.

Recap

Leaf tissues include the epidermis, palisade mesophyll, spongy mesophyll, xylem and phloem.

Xylem carries water and minerals up. It is made of dead, hollow cells with lignin.

Phloem carries sucrose up and down. It is made of living cells.

Translocation moves sugar from sources to sinks and needs energy from respiration.