Plant organ system

Epidermal tissue

Epidermus is thin and tranparent to allow light to enter below in palisade mesophyll

Palisade mesophyll

Column shaped calls tigly packed with chloroplasts to absorb more light, maximising photosynthesis

Spongy mesophyll

Contains internal air spaces that increase the surface area to volume ratio for the diffusion of gases.

Xylem

transports water into the leaves for mesophyll cells to use in photosynthesis and for transpiration from stomata.

Phloem

transports surcose and amino acids around the plants

psotion of the xylem and phloem in a diagram

xylem is always on the inside and phloem is always on the outside.

how is the xylem adapted to its function?

* Xylem is adapted in many ways:
* A substance called **lignin** is deposited in the cell walls which causes the xylem cells to die
* These cells then become **hollow** (as they lose all their organelles and cytoplasm) and join end-to-end to form a **continuous tube** for water and mineral ions to travel through from the roots
* **Lignin** strengthens the plant to help it withstand the pressure of the water movement

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* Movement in xylem **only takes place in one direction** - from **roots to leaves**

Definition of transpiration

it is the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata

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**(Water** travels up **xylem** from the roots into the leaves of the plant to replace the water that has been lost due to **transpiration)**

Effect of temperature in the rate of transpiration

high temp more trans

because particles have higher kintetic energy so transpiration occurs at a faster rate as water molecules evaporate from the mesophyll and diffuse away faster than at lower temperatures.

Effect of humidity in the rate of transpiration

high hum less trans

* humiditidy is a measure of moisture (water vapor) in the air.
* when the air is saturated with water vapor the concentration gradient is weaker so less water is loss.

Effect of light intensity in the rate of transpiration

high light high trans

guard cells are responsive to light intensity.

So when its high they are turgid and the stomata opens allowing water to be lost.

Effect of air movement in the rate of transpiration

hish air hgh tran

good airflow removes water vapours from the air surrounding the leaf which sets up a concentration gradient between the leaf and the air, increase water loss

translocation

The transport of sucrose and amino acids in phloem, from regions of production to regions of storage or use

how is the phloem adapted to its function?

have pores in the end walls (so) dissolved sugars / food / contents can move from cell to cell

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or

no nucleus or few / no subcellular structures

maximises space for movement of dissolved sugars

Why is it important for dissolved sugars to move both upwards and downwards?

* sugars are made in the leaves by photosynthesis
* all cells need sugar for respiration
* sugars transported to meristems for growth / cell division / mitosis /for storage as starch / fat / oi