Science GCSE OCR B4

Structure of a leaf-

-A pallisade cell contains many chloroplasts. Photosynthesis occurs at a high rate in these cells and lots of sugars and starch are produced.

-Carbon dioxide enters and oxygen exits a leaf through the stomata by diffusion.

Arrangement of cells-
-The epidermis is thin and transparent allowing light through to inner cells
-The pallisade cells contain large numbers of chloroplasts to absorb lots of light energy
-The chloroplasts are arranged mainly down the sides of the palisade cells, allowing some light to reach the mesophyll cells
-Air spaces between mesophyll cells allow gases to diffuse easily and reach all cells.
-The mesophyll cells are small and irregular. This incraeses their surface area to volume ratio so large amounts of gases can enter and exit. 
-Few stomata which are mainly in lower epidermis to reduce water loss 
The leaf-
-Broad- has a large surface area to absorb light
-Thin- gases dont have far to travel and light can reach all the way through
-chloroplasts in most of its cells
-Network of specialised cells in veins to support it and carry water and sugars to different parts of the plant
-Stomata to allow gas exchange  

Osmosis- The diffusion of water across a partially permeable membrane from an area of high water concentration to an area of low water concentration 
Water will move into cells by osmosis when they're placed in water. Cells will lose water by osmosis if they're placed in strong sugar solution.
Flaccid- e.g. potato chips become flaccid because there's a higher concentration of water molecules inside the cells than outside.
Plasmolysed- When water leaves a plant cell by osmosis, the cell contents shrink and there's less water pressure against the cell wall. The cell contents are plasmolysed and the cell collapses. 
Turgid- When water enters a plant cell it swells up and there's an increase in water pressure against the cell wall. This turgor pressure causes the cell to become turgid. 
If a blood cell of an animal is placed in pure or salt water, it loses or gains water by osmosis. Since it has no cell wall for support, it shrinks (becomes crenate) or bursts (lysis).   

Transpiration- Process by which water that is absorbed by plants, usually through the roots, is evaporated into the atmosphere from the plant surface, such as leaf pores.
 

Root hairs are long and thin and have a large surface area to absorb lots of water from the soil by osmosis.
The evaporation of water from leaves is called transpiration. It's useful because-
Transpiration brings water to the leaves for photosynthesis
A cell full of water gives support
The water moving up the stem carries useful dissolved minerals

To prevent too much water evaporating from the leaf it has a waxy covering called the cuticle, and stomata mainly on its shaded underside.

The inelastic cell walls of a plant also help to support it.  

A high rate happens when-

Light instensity Increases- The stomata open allowing more water to escape

Temperature increases- Random movement of water molecules increase and more water escapes

air movement (wind) increases- Wind causes more water molecules near stomata to be removed. This increases evaporation and diffusion of water from inside the leaf.

humidity (amount of water in the atmosphere) falls- in dry conditions there's a very low concentration of water molecules outside the leaf. This causes more diffusion of water from inside the leaf to the outside. 

Xylem and phloem are specialised cells that form the transport system in a plant. They form continuous vascular bundles from the roots to the stems and leaves.

Xylem cells- carry water and minerals from the roots to the leaves for photosynthesis. Some water evaporated and escapes by transpiration from the leaves.

They have a thick strong cell wall that contains a chemical called lignun.

Xylem vessels are made of dead xylem cells joined to form a long hollow tube. The hole in the middle is called the lumen.

Phloem cells- carry dissolved food such as sugars from the leaves to other parts of the plant. This movement is called translocation. The sugars can be used for growth or stored as starch. 

Make long thin columns but the cells are still alive. 

Nitrates- Needed to make proteins for growth. Through fertilisers, nitrogen makes amino acids to make different proteins such as enzymes. 

Deficiency- poor growth and yellow leaves

Phosphates- respiration and growth (especially in roots). Phosphorus is used to make cell membranes and DNA when used as fertilisers. 

Deficiency- Poor root growth and discoloured leaves

Potassium compounds- used in respiration and photosynthesis. Potassium is used to make enzymes to speed up chemical reactions such as photosynthesis and respiration when used as fertilisers. 

Deficiency- Poor fruit and flowers and discoloured leaves

Magnesium compounds- photosynthesis. Magnesium is used to make chlorophyll molecules in fertilisers. 

Deficiency- yellow leaves

An increase in the uptake of minerals by a plant is matched by an increase in its respiration rate. This shows that energy is necessary for the uptake of minerals. The minerals are absorbed against a concentration gradient.

A plant absorbs different minerals in different amounts and selects the minerals it needs.

Special carrier molecules take the mineral ions across the plant cell membranes by a process called active transport. Different carriers take different minerals. 

The energy from biomass can be usde as alternatives to fossil fuels.

Fast growing trees such as willow can be burnt in power stations. They're always a good supply.

Brazil produces a lot of sugar cane which is fermented using yeast to make alcohol. The process uses anaerobic respiration. The alcohol is mixed with petrol to make gasohol, a fuel for cars.

Advantages of biofuels-

They're renewable by growing more plants or collection methane

Contribute less to air pollution

Biofuels help towards a country's energy self-reliance 

• Produces as much food as possible in the space available. Although it's an efficient method, it raises ethical issues such as curelty to animals and environmental concerns as the pesticides can pollute the land.
• Pesticides can build up to lethal doces in the food chain.

Pesticides from farmland get into lakes (0.02ppm).

Microscopic life absorbs small amounts of the pesticde (5ppm).

Fish eat large amounts of the microscopic life and the pesticides build up inside of them (200ppm). 

Grebes eat fish and the pesticide builds up, killing them.

• Intensive farming is extremely efficient as more energy is usefully transferred. This is because there are fewer weeds and pests in crops.
• Less heat is lost from animals kept in sheds and their movement is restricted