• Created by: aysha09
  • Created on: 17-03-15 18:21


Photosynthesis can only be carried out by green plants and algae.

Carbon Dioxide + Water --> Glucose + Oxygen

The process of photosynthesis:

1) Carbon dioxide is taken in by the leaves and water is taken up by the roots

2) Chlorophyll traps the light energy needed for photosynthesis

3) The energy is used to turn the carbon dioxide and water into glucose

By testing leaves with iodine solution, we can see whether they are photosynthesising. The iodine turns black in the presence of starch which is what plants store glucose as. 

Variegated leaves have patches of green (with chlorophyll) and white (without chlorophyll) so the iodine solution will only turn black in the green parts.

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How do plants use glucose?

The uses of soluble glucose:

  • Converted into insoluble starch for storage
  • Used for respiration
  • Converted into fats and oils for storage
  • Used to produce cellulose which strengthens cell walls
  • Used to produce proteins

Plant and algal cells alson need mineral ions and nitrate ions to produce protein which they get from the soil or water they live in.

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Limiting Factors

Limiting Factor: anything that puts a cap on the rate of photosynthesis


Light is a limiting factor because a lack of light would slow down the rate of photosynthesis.

Light may be limited for plants in the shade or living in dark climates.


Temperature is a limiting factor becasue if the temperature is low then the rate of photsynthesis will decrease.

Carbon Dioxide concentration-

If there is too little carbon dioxide in the air then the rate of photosynthesis will decrease.

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Factors affecting the distribution of Organisms

Temperature: temeprature is a limiting factor so not many plants can grow in extremely cold climates, thus reducing the amount of fauna species

Availability of Nutrients: plants need a readily availbale source of nutrients, if there is none then plants cannot survive, therefore neither can animals

Amount of Light: plants need light to photosynthesise, if there is a lack of light in an area, not many plants can survive so not many animals can survive

Availability of Water: limits the plant growth and animal distribution

Availability of Oxygen: both plants and animals need oxygen to respire

Availability of Carbon Dioxide: plants need carbon dioxide to photosynthesise, a lack of this in the atmosphere would limit plant growth and make it hard for animals to survive

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Measuring the Distribution of Organisms

Quantative data can be used to describe how physical factors might be affecting the distribution of organisms in a specific habitat.

Quantative data can be obtained by:

  • random quantative sampling using a quadrat
  • sampling along a transect

Transect: a line marked between two points

It is also important to consider sample size, scale and an average to use.

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Cell division and Growth

Mitosis results in two identical cells being produced from the original cell.

1) A copy of each chromosome in the cell is made

2) The cell then divides in two to form two daughter cells, both clones of the original parent cell

Mitosis is used to make most body cells.


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Cell division in sexual reproduction

Meiosis forms gametes. (sperm and ova in humans)

Gametes have a haploid number of chromosomes, one chromosome from each original pair.


1) A copy of each chromosome is made

2) The cell divides to form two identical daughter cells

3) These cells divide again to form four daughter nuclei, each with a haploid number of chromosomes (23 in humans) 

This process results in variation as the gametes from each parents fuse during fertilisation so half the genetic information is from each parents.

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Stem Cells


In early ebryonic development stages, animal and human cells are unspecialised and can differntiate to become any necessary type of cell. 

Most animals cells differentiate early in development and mitosis is mostly for repair.

Plant cells can differntiate throughout the plant's life.

Stem cells:

  • Unspecialised
  • Found in human embryos and adult bone marrow
  • They can differentiate
  • Layers of cells in the embryo differentiate into all the cells the body needs
  • It is hoped that human stem cells can be made to differentiate into many types of cell and used to treat conditions such as paralysis
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Mendel the Monk & Inheritance

Gregor Mendel was a monk who worked out how characteristics were inherited.

It took a long time for his ideas to be accepted because scientists did not know about chromosomes and genes until after he died.

Mendel's 'factors' are now called genes. Mendel also found the 3:1 ratio of dominant and recessive alleles.

Genetic Code: each gene codes for a particular combination of amino acids which makes a specific protein.

Genes controlling a particular characteristic are called alleles.

-->These can be dominant (mask the effect of another allele) or recessive (can be masked by the other type of allele)

Phenotype- physical appearence of the characteristic (e.g. dimples or no dimples)

Genotype- the genetic make up- which alleles does the individual inherit? (e.g. DD/Dd/dd)


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Inherited Conditions

Cystic Fibrosis:

  • Caused by a recessive allele- the allele affects cell membranes and causes the production of thick mucus
  • The recessive allele must be inherited from both parents who carry it. They are called carriers.
  • Symptoms: mucus in lungs cause breathing problems, lack of apetite, bowel problems, constant cough
  • Treatments: physiotherapy for shifting mucus, taking digestive enzymes to aid digestion


  • Caused by a dominant allele so only has to be inherited from one parent
  • Symptoms: extra fingers/toes/thumbs
  • Treatments: Surgery (then follow up surgery)
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Fossils are found in rocks, which can be dated, so we can find out when organisms were alive.

Fossis may be formed in many ways:

  • from the hard parts of animals that do not decay easily
  • from parts of organisms that have not decayed due to the conditions e.g. in ice
  • when parts of the organism are replaced by other materials e.g. minerals, when they decay
  • as preserved tracesof organisms e.g. footprints, burrows

Most organisms that die do not leave fossil evidence because the conditions are not exactly right.

Many early life forms did not leave fossil evidence because they were soft bodied, some traces were destroyed in earthquakes and other geological events.

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Extinction: when a species that once existed has completely died out

Causes of Extinction:

New disease

Environment changes over geological time

New predator evolves or is introduced

New competitor evolves or is introduced

Single catastrophic event

Natural changes in species over time

Climate change

Destruction of habitat

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Isolation and Speciation

A new species can arise from an existing speices if a group becomes isolated from the rest.

Geographical isolation: can occur due to an island, river, mountain range, crater etc. 


  • A group within a species becomes isolated from the rest
  • Each population has a range of alleles that controls their characteristics (this is genetic variation)
  • Natural selection will occur in each population
  • Each population will evolve differently
  • Speciation has occured when the two populations can no longer interbreed becuase they have evolved differently

isolation --> genetic variation --> alleles selected --> interbreeding no longer possible (speciation)

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