Nitrogen cycle

Nitrogen gas in the atmosphere is turned into nitrogen compounds that a plant can use. The plant is eaten or decayed by decomposers forming ammonia compounds either from decay or the urea of animals. The ammonia is then turned into nitrates by nitrifying bacteria. Nitrates are then converted back into nitrogen gas in the atmosphere by de-nitrifying bacteria.

Human influences on the environment

CO2 and other greenhouse gases: CO2, Methane, water vapour

1. Released from deforestation
2. Respiration
3. Burning of fossil fuels

The greenhouse effect is the trapping of heat within the atmosphere which increases global warming. It then radiates the heat back down towards earth.

SO2 is released from the burning of fossil fuels and released from sewage treatment plants. Can cause acid rain and erosion of rock i.e. limestone.

Carbon monoxide is released from incomplete combustion. It can bind irreversibly to haemoglobin thus preventing oxygen from being transported around the body.

Ammonia [from fertilisers] can be leached into rivers and streams.

Leaching is when excess dissolved nutrients and minerals from fertilisers run into streams and rivers and pollute water. This causes algae to bloom in immense quantities. Algae prevents sunlight from reaching plants on the river bed hence prevents plants from photosynthesising. The plants would die and the quantity of dead matter in the water. This pollutes the water even more. Bacteria respire aerobically to decompose the matter. This uses up all the oxygen in the water hence the organisms cannot respire. Nitrates and phosphates.

Natural selection

Natural selection occurs when a mutation arises in members of a species.

If the mutation is beneficial to the organism, it will live longer and reproduce more.
More of its offspring would have inherited the characteristic and too will survive.
This will continue over generations until the mutation becomes a common gene in the species in which those without will not be able to compete and will die.

Mutations

Occur when a gene has been copied incorrectly. This is not done on purpose and this faulty gene can be passed down through generations.

Stages of mitosis

1. Prophase is when the dna begins to condense and become more visible. The nucleus has also disappeared.
2. Metaphase is when the spindle of fibres have formed across the length of the cell and are prominent.
3. Anaphase is when the fibres have split in half and are pulled to the opposite sides of the cell.
4. Telophase is when the chromosomes have reached the opposite sides of the cell and the nuclear membrane has begun to reform. Cytokinesis has occurred and two identical daughter cells have been created.

Cytokinesis is the split of the cell physically.

Meiosis and Mitosis [haploid and diploid]

Meiosis occurs in the ovaries and in the testes. It is when one cell divides to produce four haploid cells i.e. gametes. A haploid cell is one with one chromosome i.e. half the genetic makeup. As a result, gametes are genetically different.

Mitosis occurs during growth and development. It is when one cell divides to produce two diploid cells. Diploid cells are those with a pair of chromosomes i.e. the full genetic makeup.

Offspring are genetically different to their parents as they have different genotypes as a result of the receipt of different chromosomes.

Note that it is at the stage of a foetus where cells begin to specialise.

DNA structure and purpose

DNA is found in the nucleus of any cell [except red blood cells which have none]. They must be able to do two things:

1. Contain instructions which control protein production hence which control the characteristics of an organism.
2. Be able to replicate in a way that preserves all the genetic information required. It should be able to be transferred from a parent to a child.

DNA is in the form of a double helix, with two strands forming the shape of a twisted ladder. Each 'rung' of the ladder consists of a base, Adenine, Thymine, Cytosine and Guanine. Bases are held together by intermolecular forces.

A sequence of bases contains a code that informs the cell of what protein to make. There are millions of sequences and a vast number of proteins that can be produced. A section of DNA that contains the instructions to create on complete protein is called a Gene.

DNA = sentence
Gene= Paragraph
Chromosome= Chapter

We have a pair of each chromosome so that there we have two copies of each gene which results in more variation within our species. A gene can come in different forms such as alleles. Alleles can be either dominant or recessive. We may have a chromosome from our mother who has the gene for brown hair and a chromosome from our father who has a gene for blue eyes. We have characteristics from both parents. A child who looks very much like their parent means their alleles are most likely dominant.

Source: BBC Bitesize

A pair of chromosomes carry the same genes in the same place, on each chromosome within the pair. However, there are different versions of a gene called alleles. These alleles may be the same (homozygous) on each pair of chromosomes, or different (heterozygous), for example, to give blue eyes or brown eyes.

Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father.

Which chromosome we get from each pair is completely random. This means different children in the same family will each get a different combination. This is why children in the same family look a little like each other and a little like each parent, but are not identical to them.

A dominant allele is the one that will be made.
A recessive allele is one that will be masked.
A homozygous genotype is one that contains two of the same allele.
A heterozygous genotype is one that contains two different alleles, one dominant and one recessive.
Phenotype: A set of characteristics in an organism that originate from a combination of a genotype and interaction with the environment.
Genotype: Set of genes in DNA responsible for a particular trait.

Placenta and amniotic fluid

The placenta is significant for the transfer of molecules from the mother's blood to the embryo's blood stream. This is imperative as the embryo cannot respire, digest or excrete.

The placenta is spread out over a large surface area so that the diffusion of molecules occurs quickly.

Molecules transferred include:

1. Oxygen
2. Carbon dioxide
3. Glucose
4. Urea
5. ANTIBODIES [provides specific immunity against various diseases].

The amniotic fluid provides support, protection and allows movement. It also regulates body temperature and absorbs pressure to the uterus hence protecting the embryo.

Coagulation is the mix of different blood types. Although the placenta must be situated close to the embryo to ensure diffusion occurs quickly, it is important that the mother's blood and embryo's blood do not mix.

Menstrual cycle

The overall function of the menstrual cycle is to release an ovum into the reproductive system and for it to be successfully fertilised, implant and grow to term.

There are three main stages of the menstrual cycle:

1. Preparation: When the egg matures in the ovaries and the uterus wall lining has an increasing blood supply.
2. Ovulation: The release of an ovum from the ovaries to fallopian tubes.
3. Maintenance: When the blood supply of the uterus wall lining is maintained so that the egg may be fertilised and implant into the wall.

FSH [follicle stimulating hormone] is released from the pituitary gland when there are no significantly high levels of hormones in the bloodstream. It causes an egg to mature within a follicle in the ovaries.
Oestrogen is released from the ovaries as the egg begins to mature. It increases the blood supply to the uterus thus thickening the wall lining. It causes FSH to cease and LH to be released from the brain. LH causes the egg to be released from the follicle and causes the follicle to change structure into a corpus luteum. The corpus luteum is yellow. The corpus luteum produces progesterone. This maintains the blood supply to the uterus until the corpus luteum decays.

Oestrogen: develops secondary sexual characteristics in females.
Testosterone: ' in males.

1. Breast development
2. Increased body hair + pubic hair
3. Regulation of body mass - redistributed to hips and breasts
4. Development of sexual organs
5. Voice deepens

1. Development of sexual organs
2. Voice deepens
3. Pubic hair
4. Increased muscle mass

Structure of human reproductive systems

Female:

Ovaries: Where eggs are produced along with oestrogen [follicles] and progesterone [corpus luteum].
Fallopian tubes: Where ovum[a] are fertilised. They contain cilia which beat rhythmically to move the egg or zygote along into the uterus.
Uterus: This is where a zygote would implant itself and develop during the gestation period [pregnant period].
Cervix: This is a powerful muscle that prevents bacteria from entering the uterus. It produces mucus and several alkaline solutions.
Vagina: The walls secret a fluid that acts as a lubricant. Its muscular walls contract during childbirth.

Male:

Penis: Contains a vast number of nerve endings which allows it to be stimulated. It also allows muscle contraction for when the sperm is released.
Testicle: This is where sperm are produced. They are kept outside of the body so that the sperm can develop under cooler conditions.
Epididymis: This is where sperm is stored. As it is coiled, the area in which the sperm can be kept increases hence a larger volume of sperm can be stored.
Vas Deferens: A tube with muscular walls that contracts to push sperm.
Prostate gland: Along with the seminal vesicle, this gland produces a liquid called semen which sperm can swim in and gain nourishment from.
Bladder: Where urine is stored.
Urethra: The muscular tube in the penis that contracts to push sperm [in semen] out of the penis.

Cloning and selective breeding

Cloning can be viewed as artificial asexual reproduction. It is the process by which genetically identical copies of organisms are made. The characteristics of the cloned organism are identical to that of the original natural organism as the DNA in both is the same.

Micropropagation: 

The process of growing plantlets in tissue culture then planting them out.

1. Clippings of a plant are placed in a growth medium to produce shoots.
2. The plantlets are then transferred to a root medium to encourage the growth of roots.
3. The plant will then be transferred into soil where it will grow into a plant with identical characteristics as the natural plant.

Humans have been able to clone plants for thousands of years through their own natural process of asexual reproduction. However, humans have started to clone animals. This is done by splitting an embryo into several embryos hence artificially recreating the formation of twins.

Selective breeding: 

The breeding of two organisms with the desired characteristics for human use.

Organisms with the desired characteristics are selected.
These organisms are bred.
The offspring with the desired characteristics are selected.
These specific offspring are bred.
The process is repeated over several generations.

Advantages:

A large number of organisms with the desired characteristics and domesticated organisms.

Disadvantages:

There is no variation and as a result, disease would affect all of these organisms, potentially killing all of them.

Plants:

1. Big fruits
2. Nice colour
3. Desirable scents and flavours
4. Tolerant to a specific temperature
5. Resistant to a particular disease.

Animals:

1. Large muscle mass
2. Domesticated [lack of aggression]
3. Certain animals may produce more fur or wool

Natural methods of asexual reproduction in plants

Runners from plants such as strawberries extend towards the ground. They then specialise into specific cells such as root cells and a whole new plant develops.

There are artificial methods also such as cuttings:

A branch is cut off from a plant and the leaves are removed. It is then placed in soil and covered in a CLEAR plastic bag to retain the warm temperature and keep it moist. After a few weeks, new roots develop and a new plant begins to grow.

Seed germination

After fertilisation, seeds need to be removed from the plant and grow elsewhere so they are not in direct competition for resources such as minerals, water, sunlight etc. Therefore there needs to be an effective dispersal method of the seeds.

1. Through fruit. An animal may eat the fruit and the seed, inside, is resistant to the digestive juices hence it passes out through faeces.
2. Some seeds have small hooks which stick to the fur of animals.
3. Other seeds are very light so that they can be carried by the wind.

Germination is the process by which a plant emerges from the seed and begins growth. There are various conditions required for this to happen at a good rate:

1. Air supply
2. Water supply
3. Correct temperature to ensure the area is suitable for growth.

The food store in a seed is large. It is called the cotyledon. It contains starch which is a store of glucose. The glucose is utilised in respiration which enables mitosis to occur. [mitosis is the division of cells].This continues until the plant is large enough to start producing glucose from photosynthesis.

The embryo is where the cells of the seed are situated. These multiply through mitosis hence the seed will grow.

The testa is the seed coat. Water causes the testa to split which enables the radicle to grow downwards to form roots and the plumule to grow upwards to form the plant.

Fertilisation in a plant

When the pollen grain has landed on the stigma, it sticks as the stigma produces two chemicals. The stigma also produces a hormone which enables the pollen to produce an enzyme. This enzyme allows the pollen to digest part of the style and form a tube leading down to the ovaries. When the pollen grain reaches the ovaries, it fuses with a ovum. The ovum produces a hormone which directs the pollen grain in the direction to go during its journey down the style.

Fertilisation occurs as the pollen fuses with the ovum to form a zygote. After this happens, the zygote begins to divide by mitosis. As it divides, it produces a hormone which catalyses various changes in the structure of the plant.

1. The sepal and petals may wither away and fall off as no more insects need to be attracted to the plant.
2. Similarly, the stamen, style and stigma would wither away and fall off.
3. The carpel [stigma, style and ovary] may harden and form a fruit.

The resulting fruit has one job: to ensure the dispersal of the seeds is effective.

The carpel is the female reproductive organ of the plant.

Note that meiosis does not occur in asexual plants to form gametes. Mitosis only occurs to produce genetically identical cells and so all genetic data is copied.

Structure of a plant [reproduction]

Pollination is the transfer of pollen from an anther to a stigma.

Cross-pollination is the transfer of pollen from the anther of one plant to the stigma of another plant.

Self pollination is the transfer of pollen from the anther to the stigma of the same plant.

Sepals: Strong modified leaves that wrap around the flower as it is developing and forming.
Petals: Modified colourful leaves that are typically found on insect pollinated plants as they attract insects. They vary in size and colour.
Stamens: Consist of the anther and the filament.
Anthers: Male part of the plant where the male gametes, pollen are produced through meiosis.
Filament: This is the stalk that supports the anthers and holds them up. They are typically longer and grow out of the plant in wind pollinated plants.
Stigma: This is the female part of the plant. It produces a two chemical substances which enable pollen to stick to it. It also contains a hormone which enables the pollen to release an enzyme which can allow it to digest part of the style and form a tube down to the ovary of the plant. It has a large surface area in wind pollinated plants.
Style: The style is the large stalk that supports the stigma. It is longer in wind pollinated plants so that it is more accessible and easier for the pollen to stick to it.
Ovary: This is where the ovules are found.
Nectaries: Can be located anywhere on a plant yet they provide glucose to insects in insect pollinated plants who need it for respiration.

Insect pollinated:

1. Large petals
2. Nectaries
3. Powerful scents

Wind pollinated:

1. Stigma with large surface area.
2. Stamens typically stick out of the plant.

Sometimes it is beneficial for the male parts of the flower to develop prior to the female parts. Self-pollination is prevented and the flower will have to do cross pollination. This results in more genetic variation in the species hence a greater chance of it surviving.

An advantage of self-pollination: less energy is required as no petals or nectaries have to be created.

Types of reproduction

Asexual/ sexual reproduction

Sexual reproduction is when two organisms from the same species are required to produce  non-idential offspring where characteristics are acquired from both parents. This is typical in most animals.

Asexual reproduction on the other hand is when one organism is required to produce genetically identical offspring. This occurs in some plants and in bacteria [binary fission].

A cell is diploid if it contains two copies of each chromosome [sexual].
A cell is haploid if it contains one copy of each chromosome [asexual].

Gametes are sex cells and and they are only utilised in sexual reproduction.
Fertilisation is the fusion of two gametes to produce a zygote which undergoes cell division and forms an embryo.

Structure of circulatory system

There are several significant arteries and veins leading away from important organs in the body such as:

Heart - Aorta/ Vena Cava
Lungs - pulmonary
Liver - hepatic
Kidneys - renal
Stomach - gastric

Effect of exercise on the heart

How the heart rate changes during exercise:

When you exercise, your muscles are moving more and at a greater rate hence the muscle cells respire more. Respiration requires a continuous flow of oxygen in order to release the maximum amount of energy as possible from glucose. Therefore the heart rate needs to increase so that the oxygen is pumped at a greater rate around the body to all the respiring cells primarily the muscle cells. More oxygen is breathed in and blood is pumped at a greater rate to and from the lungs where it is oxygenated. More carbon dioxide needs to be removed from cells also. As a result the heart rate increases.

Adrenaline

Adrenaline is produced in the adrenal glands above the kidneys. It stimulates receptors in the heart which increases the rate at which it works at. 

Immune system

Pathogens are harmful bacteria/ viruses that disrupt the normal functioning of tissues or organs or an entire organism. However, if the rate of their reproduction is less than the rate at which they are killed, one does not feel the effects of it and they show no symptoms.

If the rate of reproduction is higher than the rate at which they are killed, symptoms arise and one would feel ill.

Pathogens are destroyed by white blood cells. There are two different types:

1. Phagocytes
2. Lymphocytes

Phagocytes kill pathogens by engulfing them. However, this does not necessarily occur at a high rate and most of the time, lymphocytes are involved.

There are two types of lymphocytes: T cells and B cells.

Sequence of specific immunity

Phagocytes encounter a pathogen and begin to engulf it. Whilst doing so, it displays the pathogen's antibodies which alerts a t cell.
The t cell in turn alerts a b cell with the specific antibodies needed to target this pathogen.
The b cell begins to divide and produce antibodies in huge quantities.
The antibodies neutralise the toxins produced and cause the pathogens to clump together so that the phagocyte may engulf them all at the same time.
Some of the b cells are stored in the lymph nodes incase this pathogen is encountered again.

This sequence of events is called the specific immune response as it is targeting specific pathogens through the use of antibodies.

Vaccinations

Vaccinations work by injecting a weak version of a pathogen into the body. The antibodies specific to these pathogens are then produced by b cells. These increase the rate of destruction of these pathogen then they are stored in the lymph nodes where they are kept until the same pathogen is encountered again and a faster response is required. Future antibody production would be quicker and in a larger quantity thus targeting the pathogen faster.

Structure of the heart

There are four chambers in the heart:

1. Right atrium
2. Right ventricle
3. Left atrium
4. Left ventricle

The atria are where blood flows into the heart from [lungs/ body] i.e. from veins.
The ventricles have thicker walls as they generate the pressure to pump blood away from the heart. The left ventricle generates more pressure to pump blood around the entire body.

The atrio-ventricular valves are valves situated between the atria and ventricles to prevent blood from flowing in the wrong direction.

Tendinous chords prevent the valves from turning inside out in the event of high blood pressure.

The main blood vessels are:

1. Aorta
2. Vena Cava
3. Pulmonery artery
4. Pulmonery vein
___________

1. Arteries
2. Veins
3. Capillaries

1. The Aorta is the main artery taking oxygenated blood away from the heart to the rest of the body. It has thick muscular walls and a narrow lumen to deliver blood under a high pressure.

2. The Vena Cava is the main vein taking deoxygenated blood to the heart from the rest of the body. It has thin walls and a wide lumen. It delivers blood under a low pressure but contains valves to prevent blood from flowing the wrong way.

3. The pulmonary artery is the blood vessel that takes deoxygenated blood away from the heart to the lungs. It has thick muscular walls and a narrow lumen to increase the blood pressure.

4. The pulmonary vein takes oxygenated blood away from the lungs to the heart under low pressure. This is a result of the blood passing through small capillaries where it loses its pressure.

5. Arteries in general transfer oxygenated blood to tissues and organs all over the body. They have thick muscular walls and a narrow lumen to increase the blood pressure. Therefore they have no valves - the blood already flows at a high pressure. They have elastic walls to allow the artery to expand and recoil.

6. Capillaries are have walls one cell thick to decrease the diffusion distance for gases during gas exchange. They allow the exchange of substances into and out of the blood.

7. Veins have thin walls and a large lumen. They contain blood under a low pressure therefore they have valves to prevent it from flowing in the wrong directions. They contain deoxygenated blood except the pulmonary vein which transfers oxygenated blood from the lungs to the heart.


Note that smoking can severely increase blood pressure resulting in blood vessels from bursting. The bad cholesterol levels are raised due to the chemicals in tobacco smoke and they block the lumen.