Plant tissues, transpiration and translocation · GCSE Biology

Plant tissues in a leaf

Epidermal tissue covers the leaf; the upper epidermis is transparent and covered by a waxy cuticle that reduces water loss.
Palisade mesophyll has many chloroplasts and is near the top of the leaf, so it absorbs lots of light for photosynthesis.
Spongy mesophyll has air spaces that let gases diffuse in and out of cells.
Xylem and phloem transport substances around the plant. Meristem tissue at the growing tips makes new cells.

Stomata are tiny pores, mostly on the underside of leaves, that let carbon dioxide in and oxygen and water vapour out.
Each stoma is surrounded by two guard cells that open and close it, controlling gas exchange and water loss.
Stomata usually close at night or when the plant is short of water.

Xylem tissue is made of dead cells joined end to end, strengthened with lignin, forming hollow tubes.
Xylem carries water and mineral ions from the roots to the stems and leaves.
Transpiration is the loss of water vapour from the leaves, mostly through the stomata. It pulls a continuous stream of water up the xylem: the transpiration stream.
Root hair cells absorb water by osmosis and mineral ions by active transport.

A higher temperature increases the rate: water evaporates and diffuses faster.
Lower humidity increases the rate: there is a steeper concentration gradient of water vapour.
More air flow (wind) increases the rate by blowing away water vapour near the leaf.
Brighter light increases the rate because more stomata open.
A potometer measures water uptake, which is used to estimate the rate of transpiration.

Phloem tissue is made of elongated living cells with pores in the end walls (sieve plates) that let cell sap move from one cell to the next.
Phloem carries dissolved sugars from the leaves to the rest of the plant, for immediate use or storage. This is called translocation.
Unlike xylem, phloem transports substances in both directions.

Xylem: Tissue of dead, hollow cells strengthened with lignin that carries water and mineral ions up from the roots.
Phloem: Tissue of living cells with sieve plates that carries dissolved sugars around the plant.
Transpiration: The loss of water vapour from the leaves of a plant.
Translocation: The movement of dissolved sugars through the phloem.
Stomata: Tiny pores in a leaf that allow gases to diffuse in and out.
Guard cells: Pairs of cells that open and close stomata.
Palisade mesophyll: Leaf tissue with many chloroplasts where most photosynthesis happens.
Spongy mesophyll: Leaf tissue with air spaces that allow gases to diffuse.
Potometer: Apparatus used to measure water uptake by a plant shoot.
Waxy cuticle: A waterproof layer on the surface of a leaf that reduces water loss.
Lignin: A strong substance in the walls of xylem cells.