2.3 Plants, tissues, organs and systems

2.3.1 Plant tissues

Keywords: Choloroplasts, spongy mesophyll, lower epidermis, cuticle, upper epidermis, guard cells, xylem, palisade layer, stomata, pholem
FSL: GCSE Biology Revision "Plant Tissues"

- A leaf is an example of an organ. This is how it is comprised: Leaf Structure _{Source, under CC-BY-SA 3.0, added white background}

Choloroplasts	Contains chlorophyll, which is used to make glucose from carbon dioxide and water using sunlight.
Spongy mesophyll	Contains air spaces that increase the surface area for gas exchange.
Lower epidermis	Contains guard cells and stomata.
Cuticle	Waxy layer on top of the leaf, prevents water from evaporating.
Upper epidermis	Thin and transparent layer that allows light to enter the palisade layer underneath; produces the waxy cuticle.
Guard cells	Absorbs and loses water to open the stomata during the day and close it during the night to reduce water loss.
Xylem	Transports water into the leaf from the roots for mesophyll cells to use in photosynthesis and transpiration in stomata.
Palisade layer	Column shaped cells close together that contain chloroplasts to absorb more light and maximise photosynthesis.
Stomata	Where gas exchange and evaporation of water takes place. Found in higher concentrations on the underside of the leaf.
Phloem	Transports sucrose and amino acids from the leaf to other plants of the plant.

Transport Systems in Plants

Keywords: Phloem, Xylem

- Plants require mineral ions and water from the soil.
- Water enters root hair cells using osmosis.
- Mineral ions enter root hair cells using active transport.
- Mineral ions dissolve in water which moves through the xylem vessel.
- Products of photosynthesis are glucose and oxygen.
- These products are used for aerobic respiration.
- Products of photosynthesis (collectively cell sap) reach all parts of the plant via the phloem vessel.

Structure of Xylem and Phloem

Xylem	Phloem
Transports water and mineral ions	Transports glucose only
Transpiration stream	Translocation
Dead cells - no cell content	Alive cells
No cytoplasm	Thin layer of cytoplasm
Lignin in cell walls give strength (makes it waterproof)	No lignin
No end walls	Sieve plates with pores instead of end walls
Movement is upwards only (from roots to leaves)	Movement is in all directions

_{Source, unmodified}

- Xylem and ploem tissues form vascular bundles. These are in:

Root: Vascular bundles in the centre of the root. This resists forces that could pull the plant ot of the ground.
Stem: Vasular bundles near the edge of the stem. Phloem is on the outside and xylem on the inside. Resists bending caused by the wind.

Stem Vascular Bundle

_{Source, unmodified}

Root Vascular Bundle

_{Source, unmodified}

Transpiration (part of 2.3.2 Plant organ system)

Keywords: Transpiration, Transpiration Stream, Evaporation, Xylem, Osmosis, Root Hair Cell
FSL: GCSE Biology Revision "Transpiration"

Transpiration: The process by which water is lost from the plant as water vapour at the surfaces of the mesophyll cells is lost. This is followed by diffusion of water vapour via the stomata.
Transpiration Stream: The transpiration stream is when water is drawn up through the xylem from the roots via osmosis and is then transported to the leaves, causing transpiration.

Transpiration as a process

The guard cells open the stomata to let in carbon dioxide.
Air around the plant usually contains less water than the cells of the plant, so water evaporates. This means that when it is really humid, transpiration is greatly reduced.
Water evaporates from the leaf underside causing more water to be pulled up through the xylem to the leaves.
No energy is required.

How guard cells open the stomata

When opening:

Water moves into guard cells by osmosis.
Guard cells to plump and turgid.
Stomata open.

When closing:

Water moves out of guard cells by osmosis.
Guard cells to flat and flaccid.
Stomata close.

Measuring water loss: Potometer

- A potometer measures the uptake of water in different conditions.
- A bubble moves along a tube depending on how much water the plant has taken up (the plant gets its water from the tube).

Controlling water loss

Waxy Cuticle: Prevents uncontrolled water loss.
Stomata on Underside of Leaf: Not in direct sunloght to minimise evaporation.
When a plant loses more water than the roots can replace:

Plant will wilt.
Vacuole shrinks.
Cells become floppy causing plant to collapse.

_{(plasmolysis)}

Factors that effect transpiration

Temperature: Higher temperatures cause transpiration to be increased as the air can hold more water and there is more energy for water molecules to evaporate.
Humidity: Higher humidity causes transpiration to be reduced as the air already has more water and so diffusion is slower.
Light Intensity: Higher light intensity causes transpiration to be increased as more stomata are open (for more photosynthesis).
Wind: If it is more windy, transpiration is increased by maintaining a steep concentration gradient.