Biology B4


Local Environment

Habitat = Where a plant or animal lives is its habitat.

Community = All the plants and animals which live in an area.

Population = The number of a particular plant or animal present in a community.

Artificial ecosystems = created by humans to protect an organism for humans benefit.

Population size can be estimated by obtaining data from a small sample and scaling it up.

( Population Size

Using a capture-recapture method assumes:

  • There are no deaths or births, or no movement of the organisms in and out the area.
  • Identical sampling methods are used for both samples.
  • The markings do not affect the survival of the organism.
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Photosynthesis is a two stage process:

  • Water is split up by light energy releasing oxygen has and hydrogen ions.
  • Carbon dioxide gas combines with the hydrogen ions producing glucose and water.

6CO2 + 6H2Reaction (light energy) ( C6H12O6 + 6O2

The rate of photosynthesis can be increased by:

1. More carbon dioxide.

2. More light.

3. Higher temperature which increases enzyme action.

Since photosynthesis depends on light, temperature and carbon dioxide, a lack of one of these factors will limit the rate of photosynthesis. They are called limiting factors.

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Leaf Structure & Adaptions


Outer epidermis - lacks chloroplasts so there are no barriers to the entry of light.

Upper palisade - contains most of the leaf's chloroplasts because they receive most of the light.

Spongy mesophyll cells - spaced so that diffusion of gases can take place between cells and atmosphere.


  • Large surface area to get as much light as possible.
  • They are usually thin so that gases can diffuse easily and light can get to the cells.
  • They have specilised guard cells which control what enters and leaves the cells.
  • They contain vascular bundles for support and transport of chemicals such as water and glucose.

By having many pigments the plant cells can maximise the use of the Sun's energy. Each pigment absorbs light of different wavelengths.

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Diffusion & Osmosis

Diffusion = The net movement of particles in a gas or liquid from an area of high concentration to an area of low concentration, resulting from the random movement of the particles.

Leaves are adapted to increase the rate of diffusion of carbon doixide and oxygen by having:

  • a large surface area
  • specilised openings called stomata, which are spaced out
  • gaps between the spongy mesophyll cells

Osmosis = Is a type of diffusion, where water is moved across a partially-permeable membrane from an area of high concentration to an area of low concentration.

- Knowing the different concentrations of water inside and outside cells makes it possible to predict the net movement of water molecules.

Turgid = plants cells are full of water, which make it rigid and not elastic.

Flaccid = When the plant cells shrink and become plasmolysed due to lots of water being lost.

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Transpiration is the evaporation and diffusion of water from inside leaves. This loss of water from leaves helps to create a continous flow of water from the roots to the leaves in xylem cells.

The structure of a leaf is adapted to prevent too much water loss, which could cause wilting. Water loss is reduced by having a waxy cuticle covering the outer epidermal cells and by most stomatal openings being situated on the shaded lower surface.

Further reduction in water loss is brought about by having fewer stomata, smaller stomata, the position of satmata, and their distribution.

The rate of transpiration can be increeased by:

  • an increase in light intensity, which results in the opening of stomata.
  • an increase in temperature, causing an increase in the evaporation of water.
  • an increase in air movement, blowing air away which contains a lot of evaporated water.
  • a decrease in humidity, allowing more water to evaporate.

- Xylem cells are involved in transpiration.

- Phloem cells are involved in translocation.

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Plants need minerals, such as:

  • Nitrates - to make proteins for cell growth and nitrogen from nitrates produce amino acids. Lack of nitrates leads to poor growth and yellow leaves.
  • Phosphates - is involved in respiration and growth and phosphorus is used to make DNA. Lack of phosphates causes poor root growth and discoloured leaves.
  • Potassium - is involved in respiration and photosynthesis. Potassium is also used to help enzyme action. Lack of potassium causes poor flower and root growth and discoloured leaves.
  • Magnesium - is involved in photosynthesis because magnesium is used to make chlorophyll. Lack of magnesium leads to yellow leaves.

Mineral uptake:

Minerals are taken up by root hair cells by active transport, rather than by diffusion or osmosis. A system of carriers transport selected minerals across the cell membrance.

Active transport enables minerals, present in the soil only in low concentrations, to enter root hairs already containing higher amounts of minerals.

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Detritivores = organisms which feed on other dead organisms and decaying material (e.g. maggots and earthworms). Detritivors increase the rate of decay by breaking up the detritus and so increasing the surface area for further microbial breakdown.

The rate of decay can also be increased by:

  • increasing the temperature to an optimum of 37oC for bacteria or 25oC for fungi will increase their rate of respiration.
  • increasing the amount of oxygen, bacteria will use aerobic respiration to grow and reproduce faster.
  • increasing the amount of water will allow for material to be digested and absorbed more efficiently and increase growth and reproduction of bacteria and fungi.

Food preservation:

Food preservation methods reduce that rate of decay. Canning is done by placing the food in a vacuum to prevent the entry of oxygen and bacteria. Freezing food also prevents decay as it kills some bacteria and slows down their growth and reproduction.

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Organic farming:

Organic farming does not use artificial fertilisers or pesticides. It uses animal manure and compost and crop rotation to avoid expensive farming. However, the crops are smaller and more expensive. On the other hand, many people believe they taste better and are healthier.


Hydroponics is an extreme form of growing crops in glasshouses. Soil is replaced by a mineral solution pumped around the plant roots. Removing the soil means there's no risk of soil organisms causing disease but the plants have to be supported.

Monitoring and adjusting the concentrations of minerals in the solution enable the grower to control growth.It allows crops to be grown in regions where there's no soil. Due to the costs involved, hydroponics is used only for high-value crops.

Biological control - uses living organisms to control pests. Examples are using lady birds to eat aphids, which damage plants.

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