GCSE Biology Unit 2 - Homeostasis & genetics.

Fairly comprehensive notes on;

  • homeostasis
  • genetic inheritance
  • cell division
  • ethical debates surrounding genetics.

 Be aware that the gaps in the notes where diagrams would fit are designed to be hand-written diagrams, furthering your own knowledge and familiarity with the processes.

Created for the 2011 AQA Unit 2 exam. Any changes to the specification since then are not included.

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  • Created on: 03-01-12 18:21
Preview of GCSE Biology Unit 2 - Homeostasis & genetics.

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For your body to function properly, the conditions must stay as constant as
possible. This is done through homeostasis keeping internal conditions constant
within a very narrow range.
Waste Products
The cells of your body are constantly creating waste products due to chemical
reactions ­ there are two main products, urea and carbon dioxide.
Carbon Dioxide
o Produced through cellular respiration
o If left, it can dissolve into cell cytoplasm and affect pH
o pH change would denature enzymes in cytoplasm
o CO² moves into bloodstream and is carried to lungs
o Lungs exchange it for oxygen
o Produced in liver when excess amino acids are broken down
o Urea passes into the bloodstream
o It is poisonous and can cause damage in the blood
o Kidneys filter out urea from the blood
o Urea removed via urine with excess water and salts
Water and Ions
We get water and ions from eating and drinking, and
their levels are carefully monitored by the body to prevent
damage. This is because if the water or ion content is wrong, too
much water can move in or out of your cells [osmosis].
Water is removed through
o Sweating
o Urine
o Breathing
Ions are removed through
o Sweating
o Urine

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Temperature Control
Temperature receptors in the skin detect changes in temperature, whereas the
blood temperature is assessed by the thermoregulatory centre in the
hypothalamus of the brain.
When the body is too cold ...
o Vasoconstriction occurs so less heat can radiate from the skin's surface.
o Muscles contract to generate heat.
o Hair stands on end to create a layer of insulating air.
o Sweat production ceases.
When the body is too hot ...…read more

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The Diabetes Debate
Two scientists named Banting and Best discovered in the 1920s how animal
insulin could be used to treat human diabetes. Ever since diabetes sufferers have
used either genetically engineered insulin or insulin from cows, pigs and dogs to carry
on life as normal.…read more

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The `father of genetics' was a monk, Gregor Mendel, in the 1800s who saw the
patterns in breeding green and yellow peas and deduced the concept of dominant and
recessive characteristics. However, his work was not recognised as it was too
complex for scientists of the day to understand, while it was published in an obscure
journal. By 1900 we could clarify his
theory.…read more

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Mitosis divides cells to make copies identical to the original, with the same
number of chromosomes. It's used in growth and repair, or to replace damaged cells.
Mitosis is used primarily in asexual reproduction for some plants' reproduction, like the
strawberry plant. The offspring are clones of the parent, so there is no variation.
Mitosis is when a cell reproduces itself by splitting to form two identical offspring.…read more

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During sexual reproduction, two gametes containing half sets of
chromosomes fuse to form an embryo with a full set of genetic information. To make
these gametes, cells must divide by meiosis to ensure that the new cells have only half
the original number of chromosomes. In humans it only occurs in the reproductive
organs [testes and ovaries].
Meiosis produces cells which have half the normal number of chromosomes.…read more

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X and Y Chromosomes
The 23rd pair of chromosomes is the X and Y chromosomes, and dictates
gender. All men have an XY pair ­ the Y causes male characteristics. All women have
an XX pair ­ the XX combination causes female characteristics.
When making sperm, the X and Y chromosomes are drawn apart in meiosis,
thus there is a 50% chance the sperm cell gets an X chromosome, and a 50%
chance it gets a Y chromosome.…read more

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Genetic Diagram
Alleles are different versions of the same gene aka. alleles for blue eyes and
alleles for brown eyes. Generally you receive two, one from mother and one from father,
but only one expresses itself.
Bb : this is a dominant brown eyes allele paired with a recessive blue eyes
allele. The child will have brown eyes as dominant masks the recessive .
The pairing is heterozygous.
BB : these are two dominant brown eyes alleles.…read more

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Aa denotes mild form of `sickle cell trait' which ironically protects against
malaria, so in malarial areas the allele is passed onto children ­ in nonmalarial
areas it does not persist.
Genetic Screening
Embryos can be screened for a certain genetic disorder the egg is fertilized
through IVF and one cell is slipped out while the embryo is at a very early stage of
division. The cell's DNA is then analysed to see if the foetus will inherit the disease.…read more

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Many people suffer and die because parts of their body do not work properly. In
1998, two scientists managed to culture human embryonic stem cells, which we
now hope can be encouraged to grow into new differentiated cells we could
potentially use them to grow new organs for transplant surgery and to cure people
of paralysis and other diseases.
Unfortunately we don't currently know how the cells are switched on and off, our
progress is very slow, expensive and difficult.…read more


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