biology paper 2

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes.

Homeostasis maintains optimal conditions for enzyme action and all cell functions.

In the human body, these include control of:

• blood glucose concentration

• body temperature

• water levels

These automatic control systems may involve nervous responses or chemical responses.

All control systems include:

• cells called receptors, which detect stimuli (changes in the environment)

• coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from receptors

• effectors, muscles or glands,

The structure of the nervous system is adapted to its functions.

The nervous system enables humans to react to their surroundings and to coordinate their behaviour.

Information from receptors passes along cells (neurones) as electrical impulses to the central nervous system (CNS). The CNS is the brain and spinal cord. The CNS coordinates the response of effectors which may be muscles contracting or glands secreting hormones.

            Stimulus →    receptor →     coordinator  →     effector  →     response

The various structures in a reflex arc – including the sensory neuron, synapse, relay neuron and motor neurons – relate to their function.

Reflex actions are important. Reflex actions are automatic and rapid; they do not involve the conscious part of the brain.

The data from graphs, charts and tables can be extracted to make interpretations about the functioning of the nervous system.

The endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream. The blood carries the hormone to a target organ where it produces an effect. Compared to the nervous system the effects are slower but act for longer.

The pituitary gland in the brain is a ‘master gland’ which secretes several hormones into the blood in response to body conditions. These hormones in turn act on other glands to stimulate other hormones to be released to bring about effects.

The endocrine glands and their function

The pituitary gland: lies under the base of the skull.

It secretes eight hormones, some of which are responsible for controlling the other endocrine glands of the body.

The thyroid gland: produces thyroxine that controls the speed at which oxygen and food products are burned up to produce energy.

The pancreas secretes digestive juices. It also secretes insulin that regulates the amount of sugar in the blood.

The ovaries: in females, secretes oestrogen the hormone that controls the development of secondary sexual characteristics and plays an important part during pregnancy.

The testes: in males secretes testosterone the hormone that controls the development of secondary sexual characteristics.

The adrenal glands: lie just in front of each kidney. They secrete the hormones adrenalin and noradrenalin times of stress.

The number of chromosomes is halved during meiosis and then combined with new genes from the sexual partner to produce unique offspring. Gene mutations occur continuously and on rare occasions can affect the functioning of the animal or plant. These mutations may be damaging and lead to a number of genetic disorders or death. Very rarely a new mutation can be beneficial and consequently, lead to increased fitness in the individual. Variation generated by mutations and sexual reproduction is the basis for natural selection; this is how species evolve.

An understanding of these processes has allowed scientists to intervene through selective breeding to produce livestock with favoured characteristics. Once new varieties of plants or animals have been produced it is possible to clone individuals to produce larger numbers of identical individuals all carrying the favourable characteristic.

Scientists have now discovered how to take genes from one species and introduce them into the genome of another by a process called genetic engineering. In spite of the huge potential benefits that this technology can offer, genetic modification still remains highly controversial.

Sexual and asexual reproduction

Meiosis leads to non-identical cells being formed while mitosis leads to identical cells being formed.

Sexual reproduction involves the joining (fusion) of male and female gametes:

• sperm and egg cells in animals

• pollen and egg cells in flowering plants

In sexual reproduction, there is a mixing of genetic information which leads to variations in the offspring. The formation of gametes involves meiosis.

Asexual reproduction involves only one parent and no fusion of gametes. There is no mixing of genetic information. This leads to genetically identical offspring (clones). The only mitosis is involved.

Advantages and disadvantages of sexual and asexual reproduction (biology only)

Advantages of sexual reproduction:

• produces variation in the offspring

• if the environment changes variation gives a survival advantage by natural selection

• natural selection can be speeded up by humans in selective breeding to increase food production.

Advantages of asexual reproduction:

• only one parent needed

• more time and energy efficient as do not need to find a mate

• faster than sexual reproduction

• many identical offspring can be produced when conditions are favourable.

Some organisms reproduce by both methods depending on the circumstances.

• Malarial parasites reproduce asexually in the human host, but sexually in the mosquito.

• Many fungi reproduce asexually by spores but also reproduce sexually to give variation.

• Many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils.

Meiosis

Meiosis halves the number of chromosomes in gametes and fertilisation restores the full number of chromosomes.

Cells in reproductive organs divide by meiosis to form gametes.

When a cell divides to form gametes:

• copies of the genetic information are made

• the cell divides twice to form four gametes, each with a single set of chromosomes

• all gametes are genetically different from each other.

Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases. As the embryo develops cells differentiate.

Genetic inheritance

An allele is a variation of a gene.

Genotype is the genetic makeup of organisms which determines one’s physical characteristic (phenotype).

The phenotype is the physical characteristic observed in an organism, for example, black fur or brown eyes.

A dominant allele is always expressed in the phenotype, even if only one copy is present (for example, Cc or CC).

A recessive allele is only expressed if two copies are present, for example, cc.

Homozygous: both alleles for a particular characteristic are the same.

Heterozygous: the individual has two different alleles for a particular characteristic.  

Some characteristics are controlled by a single gene, such as fur colour in mice; and red-green colour blindness in humans. Each gene may have different forms called alleles.

The alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as a phenotype.

A dominant allele is always expressed, even if only one copy is present.

A recessive allele is only expressed if two copies are present (therefore no dominant allele present).

If the two alleles present are the same the organism is homozygous for that trait, but if the alleles are different they are heterozygous.

Ordinary human body cells contain 23 pairs of chromosomes.

22 pairs of control characteristics only, but one of the pairs carries the genes that determine sex.

• In females, the sex chromosomes are the same (**).

• In males the chromosomes are different (XY).

Parents’ sex:male          X        female  

Parents sex chromosomes:              XY           X**

Possible chromosomes in gametes:                X and Y           XX  and  X

At fertilisation:

X

Y

X

**

XY

X

**

XY

There is a huge impact of selective breeding of food plants and domesticated animals.

Selective breeding (artificial selection) is the process by which humans breed plants and animals for have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals.

Selective breeding involves choosing parents with the desired characteristic from a mixed population. They are bred together. From the offspring, those with the desired characteristic are bred together. This continues over many generations until all the offspring show the desired characteristic.

The characteristic can be chosen for usefulness or appearance:

• Disease resistance in food crops.

• Animals which produce more meat or milk.

• Domestic dogs with a gentle nature.

• Large or unusual flowers.

Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects.

Genetic engineering is a process which involves modifying the genome of an organism by introducing a gene from another organism to give the desired characteristic.

Plant crops have been genetically engineered to be resistant to diseases or to produce bigger better fruits.

Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes. There are potential benefits and risks of genetic engineering in agriculture and in medicine and that some people have objections. In genetic engineering, genes from the chromosomes of humans and other organisms can be ‘cut out’ and transferred to cells of other organisms.

Crops that have had their genes modified in this way are called genetically modified (GM) crops. GM crops include ones that are resistant to insect attack or to herbicides. GM crops generally show increased yields.

Concerns about GM crops include the effect on populations of wildflowers and insects. Some people feel the effects of eating GM crops on human health have not been fully explored.

Modern medical research is exploring the possibility of genetic modification to overcome some inherited disorders.

The main steps in the process of genetic engineering are:

• enzymes are used to isolate the required gene; this gene is inserted into a vector, usually a bacterial plasmid or a virus

• the vector is used to insert the gene into the required cells

• genes are transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristics.

Abiotic factors

The change in an abiotic factor would affect a given community. Abiotic (non-living) factors which can affect a community are:

• light intensity

• temperature

• moisture levels

• soil pH and mineral content

• wind intensity and direction

• carbon dioxide levels for plants

• oxygen levels for aquatic animals

Biotic factors

A change in the abiotic factor might affect a given community.

Biotic (living) factors which can affect a community are:

• availability of food

• new predators arriving

• new pathogens

• one species outcompeting another so the numbers are no longer sufficient to breed

Organisms are adapted to live in their natural environment. They have features (adaptations) that enable them to survive in the conditions in which they normally live. These adaptations may be structural, behavioural or functional.

Some organisms live in environments that are very extreme, such as at high temperature, pressure, or salt concentration. These organisms are called extremophiles. Bacteria living in deep sea vents are extremophiles.

Animals and plants may be adapted for survival in the conditions where they normally live, eg deserts, the Arctic.

Animals may be adapted for survival in dry and arctic environments by means of:

• Changes to surface area

• The thickness of the insulating coat

• Amount of body fat

• Camouflage.

Plants may be adapted to survive in dry environments by means of:

• Changes to surface area, particularly of the leaves

• Water-storage tissues

• Extensive root systems.

Animals and plants may be adapted to cope with specific features of their environment, eg thorns, poisons and warning colours to deter predators.

Camel

• The camel can go without food and water for 3 to 4 days.

• Fat stored in their humps provides long-term food reserve and a supply of metabolic water. The fat is not distributed around the body; this reduces insulation, allowing more heat loss.

• They are tall and thin, increasing their surface area to volume ratio, increasing heat loss by radiation.

Polar Bear

• A polar bear has thick fur and fat beneath its skin to insulate it.

• Their large, furry feet help to distribute their weight as they walk on a thin ice.

• They are white which camouflages them against the snow.  This helps them to hunt.

• They are compact in shape, reducing their surface area to volume ratio; this reduces heat loss by radiation.

Desert plants

• Eg the cactus, require very little water to survive

• Leaves are spines. Spines guard against most browsing herbivorous animals.

• Spines also reduce their surface area, reducing water loss by evaporation

• A thick waxy coating surrounds the plant to reduce evaporation.

• Fewer 'stomata', reducing water loss

• Roots tend to spread sideways to catch rainwater.

  Arctic plants

• Many of the plants are small, growing close to the ground and very close together to avoid the wind and conserve heat.

• Some possess a light, fuzzy covering to insulate the buds so they can grow.

• Many are dark colours of blue and purple to absorb the heat from the sunlight even during the winter months.

• Because of the cold and short growing seasons, arctic plants grow very slowly.

• Some grow for ten years before they produce any buds for reproduction.

Quadrats

These are square frames, used to mark off specific areas of ground. They are typically 0.5m x 0.5m with a grid of 10cm X 10 cm.  They can be used to survey: which species are present, numbers of each species, or percentage cover of a species.

Random Sampling:

• Construct a regular grid using tape across the area.

• Generate random numbers using a calculator or computer.

• Use these to determine coordinates.

• This ensures that there is no bias by the investigator.

• It ensures the results are valid.

• Investigate the population of the species in the quadrat.

• Repeat many times.

Biodiversity

Biodiversity is the variety of all the different species of organisms on earth, or within an ecosystem.

A great biodiversity ensures the stability of ecosystems by reducing the dependence of one species on another for food, shelter and the maintenance of the physical environment. The future of the human species on Earth relies on us maintaining a good level of biodiversity. Many human activities are reducing biodiversity and only recently have measures been taken to try to stop this reduction.

Waste management

Rapid growth in the human population and an increase in the standard

of living mean that increasingly more resources are used and more waste is produced. Unless waste and chemical materials are properly handled, more pollution will be caused.

Pollution can occur:

• in water, from sewage, fertiliser or toxic chemicals

• in the air, from smoke and acidic gases

• on land, from landfill and from toxic chemicals.

Pollution kills plants and animals which can reduce biodiversity

Land use

Humans reduce the amount of land available for other animals and plants by building, quarrying, farming and dumping waste. The destruction of peat bogs and other areas of peat to produce garden compost reduces the area of this habitat and thus the variety of different plant, animal and microorganism species that live there (biodiversity). The decay or burning of the peat releases carbon dioxide into the atmosphere.

Deforestation

Large-scale deforestation in tropical areas has occurred to:

• provide land for cattle and rice fields

• grow crops for biofuels

Large-scale deforestation in tropical areas, for timber and to provide land for agriculture, has:

• increased the release of carbon dioxide into the atmosphere (because of burning and the activities of microorganisms)

• reduced the rate at which carbon dioxide is removed from the atmosphere and ‘locked up’ for many years as the wood

• reduction in biodiversity

Air pollution  

Pollution can occur in the air from smoke and from acidic gases. Acidic gases are released into the atmosphere and spread around by the wind.

Air pollution kills plants and animals, which can reduce biodiversity.

Acid rain forms when sulfur dioxide and nitrogen oxides dissolve into rain and snow.

1. Acid rain directly damages plant life by falling on plants and by soaking into the soil and being taken up by roots.

2. Acid rain contaminates soil and watercourses, making them more acidic and eventually unable to sustain life. Increasing sulfur dioxide levels threaten to reduce global biodiversity as whole ecosystems can be destroyed.  

Global warming

Carbon dioxide and methane in the atmosphere absorb most of the energy radiated by the Earth.

Some of this energy is reradiated back to the Earth and so keeps the Earth warmer than it would otherwise be.

The greenhouse effect: when energy transferred from Sun to Earth. Much of this heat is reflected back into space, but some is absorbed by greenhouse gases in the atmosphere and reradiated back to Earth. Earth's surface and atmosphere are warmed (greenhouse effect), maintaining conditions ideal for life.

The impact of global warming:

• loss of habitat – reducing biodiversity

• changes in distribution – some organisms may disappear from some areas as habitat changes

• changes in migration patterns – caused by changes in climates and seasons

• reduced biodiversity – some organisms will become extinct as climate changes

Maintaining biodiversity

There are both positive and negative human interactions in an ecosystem and have an impact on biodiversity. Scientists and concerned citizens have put in place programmes to reduce the negative effects of humans on ecosystems and biodiversity.

These include:

• breeding programmes for endangered species

• protection and regeneration of rare habitats