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Keeping a constant internal enviroment is called homeostasis

Homeostatis involves balancing bodily inputs and outputs.

Automatc control systems keep the levels of temperature, water and carbon dioxide steady. This makes sure all cells can work at their optimum level.

Negative feedback controls are uzsed in homeostasis. Negative feeback systems act to cancel out a change such as decreasing temperature level.

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Temperature control

The body temperature of 37 degrees c is linked to the optimum temperature for many enzymes.

A high temperature can cause; Heat stroke( skin becomes cold and clammy and pulse is rapid and weak), Dehydration( loss of too much water) , both heat stroke and dehydration can be fatal if not treated.

To avoid overheating, sweating increases heat transfer from the body to the enviroment. The evaporation of sweat requires body heat to change the liquid sweat into water vapour.

A very low temperature can cause hypothermia ( slow pulse rate, violent shivering), which can be fatal if not treated,

Blood temperature is monitored by the hypothalamus gland in the brain,. Reaction to temperature extremes are controlled by the nervous and hormonal systems, which trigger vasoconstriction or vasodilation.

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more on temperature control

Vasoconstriction is the constriction (narrowing) of small blood vessels in the skin. This causes less blood flow and less heat transfer

Vasodilation is the dilation ( widening) of small blood vessels in the skin. This causes more blood flow near the skin surface resulting in more heat transfer.

in vasoconstriction and vasodilation the size of the blood vessels is altered, the blood vessels do not move.

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Control of blood sugar levels

A hormone called insulin controls blood sugar levels.

Hormone action is slower than the nervous reactions as the hormones travel in the blood.

Type 1 diabetes is caused by the pancreas not producing any insulin,so must be treated by doses of insulin. Type 2 diabetes, which is caused either by the body producing too little insulin or the body not reacting to it.

Insulin converts excess glucose into glycogen, which is stored in the liver. This regulates the blood sugar level.

The insulin dosage in type 1 diabetes needs to vary according to the persons diet and activity. Strenuous exercise needs more glucose to be present in the blood, so a lower insulin dose is required.

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plant respones

Phototropism is a plants growth response to light . Geotropism is a plants growth response to gravity.

Parts of a plant respond in different ways;

-Shoots are positively phototropic (they grow towards light) and negatively geotropic( grow away from the pull of gravity).

-Roots are negatively phototropic ( they grow away from the light) and positively geotropic( they grow with the pull of gravity)

A positive reaction means that the root or shoot grows towards the stimulus.

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Plant hormones

Auxins are a group of plant hormones. They move through the plant in solution.

Auxins are involved in phototropism and geotropism.

Auxins are made in the root and shoot tip.

Different amounts of auxin are found in different parts of the shoot when the tip is exposed to light.

More auxin is found in the shady parts of the shoots.

A higher amount of auxin will increase the length of the cells. Therefore the increase in cell length on the shady side of the shoot causes curvature of the shoot towards the light.

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Commercial uses of plant hormones

Plant hormones have many commercial uses, they are used;

- as selective weedkillers, which kill specific weeds and increase crop yields.

- as rooting powder to increase root growth of cuttings.

-to delay or accelerate fruit ripening to meet market demands.

-to control dormancy in seeds.

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inherited characteristics

Some human characteristics, such as facial features, can be inherited. They can be dominant or recessive.

Alleles are different versions of the same gene.

There is debate over how much 'nature or nurture' (genetic or enviromental factors) affect intelligence, sporting ability and health.

Dominant and recessive characteristics depend on dominant and recessive alleles.

Dominant alleles are expressed when present but recessive alleles are expressed only in the absence of the dominant allele,

A gene can have two different alleles, one dominant and the other recessive.

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Most body cells have the same number of chromosomes. The number depends on the species of organism. Human cells have 23 pairs.

Sex chromosomes determine sex in mammals. Females have identical sex chromosomes called **, males have different sex chromosomes called XY.

A sperm will carry either an X or Y chromosome, All eggs will carry an X chromosome.

There is a random chance of which sperm fertilises an egg, There is therefore an equal chance of the offspring being male or female.

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Genetic variation and a monohybrid cross

Genetic variation is caused by;

-mutations, which are random changes in genes or chromosomes, rearrangement of genes during the formation of gametes or fertilisation, which results in a zygote with alleles from the father or mother.

A monohybrid cross involves only one pair of characteristics controlled by a single gene, one allele being dominant and one recessive.

Homozygous means having identical alleles, heterozygous means having different alleles,

A persons genotype is their genetic make-up. Their phenotype is which alleles are expressed.

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Inherited disorders

Inherited disorders are caused by faulty genes.

Many personal and ethical issues are raised:

in deciding to have a genetic test ( a positive result could alter lifestyle, career, insurance)

by knowing the risks of passing on an inherited disorder (whether to marry/have a a family)

Inherited disorders are caused by faulty alleles, most of which are recessive.

It is impossible to predict the probability of inheriting such disorders by interpreting genetic diagrams.

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