Edexcel IGCSE Biology Revision Notes (Part 2)

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  • Created on: 19-06-11 12:56
Preview of Edexcel IGCSE Biology Revision Notes (Part 2)

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Nervous and Chemical Co-ordination
Nervous Chemical
Nature of Message Electrochemical impulse Hormones
Route of Transmission Specific Nerve Cells General blood system
Type of effects Rapid but short term Usually slower but generally
longer lasting
The Nervous System
Follows a similar pattern of response to the endocrine system:
Stimulus ­ Receptor ­ Processing ­ Effector ­ Response
But it is much faster and uses electrical signals more than chemical ones
3 main types of Nerve Cell/Neurone:
1. Sensory Neurone: carry impulses from receptors to the CNS
2. Motor Neurone: carries impulses from the CNS to the effectors
3. Relay Neurone: are found in the CNS and connect sensory and motor neurons
Receptors include the: Olfactory cells in the nose, pressure receptors in the skin and ear,
light-sensitive cells in the retina of the eye
Effectors are usually muscles or glands
Reflex Arc:
The Synapse

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Neurones may only form one synapse
with another neurone, but relay neurones in the brain will have thousand of synaptic
connections. Some neurones may influence or be influenced by others. Therefore synapses
are very important for integrating information in the CNS. Other chemicals in the body often
affect neurotransmitters. Alcohol is known as a depressant because it slows down synaptic
transmission.…read more

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Visual Defects:
Both natural and artificial
selection can only arise because organisms are different from one another.
Genetic Variation:
Much of this is due to genes:
Many different alleles or different version of the same gene exist
Genes can also mutate
The way that homologous chromosomes line up at the equator during meiosis is
completely random. You inherit random halves of your parents genes
Environmental Variation:
Factors in an organism's environment strongly affect it, particularly during early stages of
development.…read more

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The new nucleotides are joined together by the enzyme: DNA polymerase
4. The two new identical daughter strands are formed containing one parent strand and
one new polynucleotide strand
DNA replication has to occur before a cell can divide either by mitosis or meiosis. It means
that until cell division occurs each chromosome is attached to an identical copy of itself
Cell Division
There are two kinds of cell division.…read more

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Diseases include:
Cystic Fibrosis
Huntington's chorea
Hemophilia ­ this is an inherited sex-linked disease and only men can have it because the
gene is only found on the Y chromosome
Patterns of Inheritance
Genetic Cross Diagram:
This diagram works with dominant
and recessive alleles
Codominance: Sometimes both alleles have an equal effect on the phenotype of an
individual. When this happens, the alleles are said to be codominant.…read more

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Cell Division:
Characteristic Meiosis Mitosis
Role in Living Things Sexual Reproduction Growth and Repair, Asexual
Number of Chromosomes in 46 46
Parent Cells
Number of Chromosomes in 23 46
Daughter Cells
Number of Daughter Cells 4 2
Described as... HAPLOID DIPLOID
Daughter Cells are...…read more

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N.B Cancer results from uncontrolled cell division. The extra cells invade adjacent tissues,
compete for nutrients and may eventually cause death.
Selective Breeding
Also called artificial selection:
1. From your existing stock select the ones, which have the best characteristics.
2. Breed them with each other.
3. Select the BEST of the offspring and combine them with the best you already have
4. Continue this process over SEVERAL GENERATIONS to develop the desired traits.…read more



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