Biology Revision

Biology IGCSE HT Revision

In Green is Triple Stuff


DNA, Genes and Chromosomes

A cell contains a nucleus whci contains chromosomes. A single chromosome contains long lengths of DNA and a gene is a short length of DNA.

DNA is a long list of instructions on how to put an organism together and make it work. Each separate gene in a DNA molecule is a chemical instruction that codes for a particular protien. Proteins are important because they control most processes in the body. They also determine inherited characteristics e.g. eye colour. By controlling the production of proteins, genes also control our inherited characteristics. There can be different versions of the same gene, which give different versions of a charateristic- like blue or green eyes. The different versions of the same gene are called alleles.

A DNA molecule has two strands coiled together in the shape of a double helix. The two stands are held together by chemicals called bases. There are four different bases- Adenine, Cytosine, Guanine and Thymine. The bases are paired and they always pair up in the same way- A-T and C-G. This is called Complementary Base-Pairing. DNA is a type of nucleic acid.

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Asexual Reproduction and Mitosis

Asexual Reproduction involves Mitosis

A ordinary cell can make a new cell by simplt dividing in two. Both new cells are genetically identical to the original cell-they both contain exactly the same genetic information. This type of cell division is known as mitosis. Some organisms produce offspring using mitosis. this is known as asexual reproduction. Organisms which reproduce asexually include bacteria and some plants.

Asexual reproduction involves only one parent. The offspring have identical genes to the parent- so there's no variation between parent and offspring.

Mitosis Produces Gentically Identical Cells

Mitosis is when a cell reproduces itself be splitting to form two cells with identical sets of chromosomes

So when a diploid cell divides by mitosis, you get two cells that are both diploid.

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Asexual Reproduction and Mitosis 2

Heres how mitosis works:

  • In a cell that's not dividing, the DNA is all spread out in long strings
  • If the cell gets a signal to divide, it needs to duplicate its DNA- so there's one copy for each new cell. The DNA forms X-shaped chromosomes. Each 'arm' of the chromosome is an exact duplicate of the other.
  • The chromosomes then line up at the centre of the cell and cell fibres pull them apart. The two arms of each chromosome go in opposite ends of the cell.
  • Membranes form around each of the sets of chromosomes. These become the nuclei of the two new cells.
  • Lastly, the cytoplasm divides
  • You now have two new cells containing exactly the same DNA- they're genetically identical.

Mitosis also makes New Cells for Growth and Repair

Mitosis isn't just used during asexual reproduction- it's how all plants and animals grow and repair damaged tissue, Cloning also involves mitosis.

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Sexual Reproduction and Meiosis

Sexual Reproduction Produces Genetically Different Cells

Sexual reproduction is where genetic information from two organisms (a father and a mother) is combined to produce offspring which are genetically different to either parent. In sexual reproduction the mother and father produce gametes. Gametes are sperm cells and egg cells. Gametes are haploid- this means they have half the number of chromosomes in a normal cell. In humans, each gamete contains 23 chromosomes. At fertilisation, a male gamete fuses with a female gamete to form a zygote (fertilised egg). The zygote ends up with the full set of chromosomes. The zygote the undergoes cell division and develops into an embryo. The embryo inherits features from both parents- its received a mixture of chromosomes from its mum and its dad (and its the chromosomes that decide how you turn out). The fertilisation of gametes is random- this produces genetic variation in the offspring.

Sexual Reproduction involves the fusion of mine and female gametes. Because there are two parents, the offspring contain a mixture of their parents' genes.

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Sexual Reproduction and Meiosis 2

Gametes are Produced by Meiosis

Meiosis is another type of cell division. Its different to mitosis because it doesn't produce identical cells. In humans, meiosis only happens in the reproductive organs.

Meiosis produces four haploid cells whose chromosomes are not identical

Meiosis- Division 1

  • Before the cell starts to divde, it duplicates its DNA. One arm of each chromosome is an exact copy of the other arm.
  • In the first division in meiosis the chromosomes line up in pairs in the centre if the cell.
  • The pairs are then pulled apart, so each new cell only has one copy of each chromosome. Some of the father's chromosomes and some of the mother's chromosomes go into each new cell.
  • Each new cell will have a mixture of the mother's and father's chromosomes. Mixing up the genes like this creates variation in the offspring.
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Sexual Reproduction and Meiosis 3

Meiosis- Division 2

  • In the second division the chromosomes line up again in the centre of the cell. It's a lot like mitosis. The arms of the chromosomes are pulled apart.
  • You get four haploid gametes- each only has a single set of chromosomes. The gametes are all genetically different.
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Sexual Reproduction in Plants

The Flower Contains both Male and Female Gametes

Flowering plants have both male and female structures- they're contanined in the flower:

The Stamen is the Male Reproductive Part 

The stamen consists of the anther and filament:

  • The ANTHER contains pollen grains- these produce the male gametes (sperm).
  • The FILAMENT is the stalk that supports the anther

The Carpel is the Female Reproductive Part

The carpel consists of the ovary, style and stigma.

  • The STIGMA is the end bit that the pollen grains attach to.
  • The STYLE is the rod-like section that supports the stigma.
  • The OVARY contains the female gametes (eggs).
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Sexual Reproduction in Plants 2

  • Polination is the transfer of pollen from an anther to a stigma, so that the male gametes can fertilise the female gametes
  • In sexual reproduction, pollen is transferred from the anther of one plant to the stigma of another plant. This is known as cross-pollination.
  • Plants that reporduce sexually rely on things like insects or the wind to help them pollinate.

Some Plants are Adapted for Insect Pollination

Here's how some plants can be adapted for pollination by insects...

  • They have brightly coloured petals to attract insects
  • They also have scented flowers and nectaries (glands that secrete nectar) to attract insects.
  • They make big, sticky pollen grains- the grains stick to insects as they go from plant to plant.
  • The stigma is also sticky so that any pollen picked up by insects on other plants will stick to the stigma.
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Sexual Reproduction in Plants 3

Other Plants are Adapted for Wind Pollination

Features of plants that are adapted for pollination by wind include...

  • Small, dull petals on the flower
  • No nectaries or strong scents
  • A lot of pollen grains- they're small and light so that they can easily be carried by the wind
  • Long filaments that hang the anthers outside the flower, so that a lot of the pollen gets blown away by the wind.
  • A large and feathery stigma to catch pollen as it's carried past by the wind. The stigma often hangs outside the flower too.
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Fertilisation and Germination in Plants

Fertilisation is the Fusion of Gametes

  • A pollen grain lands on the stigma of a flower, usually with help from insects or the wind
  • A pollen tube grows out of the pollen grain and down through the style to the ovary
  • A nucleus from the male gamete moves down the tube to join with a female gamete in the ovary. Fertilisation is when the two nuclei fuse together to make a zygote. This divides by mitosis to form an embryo.
  • Each fertilised female gamete forms a seed. The ovary develops into a fruit around the seed.

Germination is when Seeds Start to Grow

A seed will often lie dormant until the conditions around it are right for germination. Seeds need the right conditions to start germinating: 

  • Water- to activate the enzymes that break down the food reserves in the seed
  • Oxygen- for respiration, which provides the energy for growth.
  • A suitable temperature- for the enzymes inside the seed to work. This depends on what type of seed it is.
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Fertilisation and Germination in Plants 2

The seed takes in water and starts to grow using its store of energy >>> The first root starts to grow down into the soil >>> The shoot gows up >>> Finally, extra roots and the first green leaves appear.

Germinating Seeds get Energy from Food Stores

  • A developed seed contains an embryo and a store of food reserves, wrapped in a hard seed coat.
  • When a seed starts to germinate, it gets glucose for respiration from its own food store. This gives it the energy it needs to grow.
  • Once the plant has grown enough to produce green leaves, it can get its own food for energy from photosynthesis.
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Asexual Reproduction in Plants

Plants can Reproduce Asexually Using Natural Methods...

Plants have several different ways of reproducing asexually. Some plants do so by growing new plants from their stems...

E.g Strawberry Plants

  • The parent strawberry plant sends out runners- fast-growing stems that grow out sideways, just above the ground.
  • The runners take root at various points and new plants start to grow.
  •  The new plants are clones of the parent strawberry plant, so there's no genetic variation between them.

...Or We Can Clone Them Using Artificial Methods

Asexual reproduction can be used to clone plants.

  • Gardeners can take cuttings from good parent plants and then plant them to produce genetically identical copies.
  • These plants can e produced quickly and cheaply
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Human Reproductive Systems

The Male Reproductive System Makes Sperm

  • Sperm are male gametes. They're made in the testes, all the time after puberty.
  • Sperm mix with a liquid to make semen, which is ejaculated from the penis into the vagina of the female during sexual intercourse.

Urethra- A tube which carries sperm through the penis during ***********. Urine also passes through the urethra to exit the body.

Erectile tissue- Swells when filled with blood to make penis erect

Testis- Where sperm is made

Glands- Produce the liquid added to sperm to make semen

Vas Deferens- Muscular tube that carries sperm from testis towards the urethra

Scrotal Sac (scrotum)- Hangs behind the penis and contains the testes.

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Human Reproductive Systems 2

The Female Reproductive System Makes Ova (Eggs)

  • Ova are female gametes. An ovum is produced every 28 days from one of the two ovaries.
  • It then passes into the Fallopian tube- this is where it might meet sperm that have entered the vagina during sexual intercourse
  • If it isn't fertilised by sperm, the ovum will break up and pass out of the vagina.
  • If it is fertilised, the ovum starts to divide. The new cells will travel down the Fallopian tube to the uterus and attach to the endometrium. A fertilised ovum develops into an embryo.

Fallopian Tube- A muscular tube that carries the ovum from the ovary to the uterus

Uterus- The organ where an embryo grows

Vagina- Where the sperm is deposited

Ovary- The organ that produces ova and sex hormones

Endometrium- Has good blood supply for implantation of an embryo

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Human Reproductive Systems 3

Hormones Promote Sexual Characteristics at Puberty

At puberty, your body starts releasing sex hormones- testosterone in men and oestrogen in women. These trigger off secondary characteristics:

Oestrogen in women causes...

  • Extra hair on underarms and pubic area
  • Hips to widen
  • Devopment of breasts
  • Ovum realease and start of periods

Testosterone in men causes...

  • Extra hair on face and body
  • Muscles to develop
  • Penis and testicles to enlarge
  • Sperm production
  • Deepening of voice
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The Menstrual Cycle and Pregnancy

The Menstrual Cycle has Four Stages

Stage 1- Day 1 is when bleeding starts.

The uterus lining breaks down for about four days.

Stage 2- The uterus lining builds up again,

From day 4 to day 14, into a thick spongy layer full of blood vessels, ready to recieve a fertilised ovum.

Stage 3- An ovum develops and is released

From the ovary at day 14.

Stage 4- The wall is then maintained 

For about 14 days until day 28. If no fertilised ovum has landed on the uterus wall by day 28, the spongy lining starts to break down and the whole cycle starts again.

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The Menstrual Cycle and Pregnancy 2

Oestrogen and Progesterone

These two hormones are produced in the ovaries, and they control the main events of the cycle:

  • Oestrogen: 1) Causes the lining of the uterus to thicken and grow. Stimulates the release of an ovum at day 14.
  • Progesterone: Maintains the lining of the uterus. When the level of progesterone falls, the lining breaks down.

The Embryo Develops During Pregnancy

Once an ovum has been fertilised, it develops into an embryo and implants in the uterus. In later stages of pregnancy the embryo is called a fetus.

Placenta Once the embryo has been implanted, the placenta develops- this lets the blood of the embryo and the mother get very close to allow the exchange of oxygen, food and waste.

Amniotic Fluid The amnion membrane forms- this surrounds the baby and protects it.

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Genetic Diagrams

Alleles are Different Versions of the Same Gene

Most of the time you have two copies of each gene- one from each parent. If the alleles are different, you have instructions for two different versions of a characteristic but you only show one version of the two. The version of the characteristic that appears is caused by the dominant allele. The other allele is said to be recessive. The characteristic caused by the recessive allele only appears if both alleles are recessive. In genetic diagrams, letters are used to represent genes. Dominant alleles are always shown with a capital letter and recessive alleles with a small letter. If you're heterozygous for a trait you have two different alleles for that particular gene. Your genotype is the alleles that you have. Your phenotype is the characteristics the alleles produce. Some characteristics are caused by codominant alleles. Neither allele is recessive, so you show characteristics from both alleles group A or B, but blood group AB.

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Genetic Diagrams 2

Genetic Diagrams show the Possible Alleles in the Offspring

Parents Phenotypes: Normal and Boring Normal and Boring

Parents Genotypes: Bb Bb

Gametes Genotypes: B b B b Offsprings Genotypes: BB Bb Bb bb Offsprinds Phenotypes: Normal Normal Normal Crazy Another Way to Draw Genetic Diagrams Another way is drawing a punnett square. Then you fill it in like this:

  • Put possible gamets from one parent down the side and those from the other parent across the top
  • In each middle square fill in the letters from the top and side that line up with that square. The pairs of letters in the middle show the possible combinations.
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Genetic Diagrams 3

You Can Draw Genetic Diagrams for Codominant Inheritance too

Codominant Inheritance of Blood Groups

  • Your blood type is determined by two codominant alleles (A and B) and one recessive (O)
  • Blood can be type A (AA or AO genotype), type B (BB or BO genotype), type AB (AB genotype) or type O (OO genotype)
  • For two people with type AB blood there's a 50% chance their children will be type AB, a 25% chance they'll be type A and a 25% chance they will be type B.
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Family Pedigrees and Sex Determination

You Need to Understand Family Pedigrees

Using a family tree of genetic disorders e.g. cystic fibrosis

  • The allele which causes cystic fibrosis is a recessive allele, 'f', carried by about 1 person in 30.
  • Because it's recessive, people with only one copy of the allele won't have the disorder- they're known as carriers.
  • For a child to have a chance of inheriting the disorder, both parents must be either carriers or sufferers.
  • There's a 1 in 4 chance of a child having the disorder if both parents are carriers.

Your Chromosomes Control Whether You're Male or Female

There are 23 matched pairs of chromosomes in every human body cell. The 23rd pair is labelled ** or XY. They're two chromosomes that decide whether you turn out male or female. There is an equal chance of having a boy or a girl

All Men have XY chromosomes. The Y chromosome causes male characteristics

All Females have ** chromosomes which gives them female characteristics

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Genetic Variation is Caused by... Genes

All animals are bound to be slightly different from each other because their genes are slightly different. We all end up with a slightly different set of genes. The exceptions to this rule are identical twins, because their genes are exactly the same.

Most Variation in Animals is Due to Genes AND Enviroment

Most variation in animals is caused by a mixture of genetic and enviromental factors. Almost every single aspect of human is affected by our enviroment in some way, however small. In fact it's a lot easier to list the factors which aren't affected in any way by enviroment:

  • Eye colour
  • Hair colour
  • Inherited disorders
  • Blood group

Enviroment can have a large effect on human growth even before someone's born. And having a poor diet whilst you're growing up can stunt your growth.

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Variation 2

For some characteristics, it's hard to say which factor is more important- genes or enviroment...

  • Health- Some people are more likely to get certain diseases because of their genes. But lifestyle also affects the risk
  • Intelligence- One theory is that although your maximum possible IQ might be determined by your genes, whether you get to it depends on your enviroment
  • Sporting ability- Again, genes probably determine your potential, but training is important too

Enviromental Variation in Plants is Much Greater

Plants are strongly affected by:

1) sunlight 2) moisture levels 3) temperature 4) the mineral content of the soil

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Evolution and Natural Selection

Theory of Evolution: Life began as simple organisms from which more complex organisms evolved (rather than just popping into existence).

Natural Selection Means the 'Survival of the Fittest

Natual selection is one of the key processes that causes evolution. It works like this:

  • Living things show variation- they're not all the same.
  • The resources living things need to survive are limited. Individuals must compete for these resources to survive- only some of the individuals will survive
  • Some of these varieties of a particular species will have a better chance of survival. Those varieties will then have an increased chance of breeding and passing on their genes
  • This means that a greater proportion of individuals in the next generation will have the better alleles, and so the characteristics, that help survival.
  • Over many generations, the species becomes better and better able to survive. The 'best' features are natually selected and the species become more and more adapted to its enviroment.
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Evolution and Natural Selection 2

The Best Genes for a Particular Enviroment Tend to Survive

The individuals who are less suited to an enviroment are less likely to survive than those that are better suited, and so have less chance to pass their alleles on. Gradually, over time, this results in a population which is extremely well suited to the enviroment in which it lives.

Rememer- Variations are caused by the enviroment itself are involved in natural selection. Variations in a species can have either enviromental or genetic causes, but only the genetic ones are passed on to the next generation and influence the evolution of the species.

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Mutations and Antibiotic Resistance

Mutations are Changes to the Genetic Code

Occasionally a gene may mutate. A mutation is a rare, random change in an organism's DNA that can be inherited. Mutations change the sequence of the DNA bases. This could stop the production of a protein, or it might mean a different protein is produced instead. This can lead to new characteristics increasing variation. Mutations can happen spontaneously- when a chromosome doesn't quite copy itself properly. However, the chance of mutation is increased by exposing yourself to:

  • Ionising raidiation e.g. Xrays, Gamma rats or ultraviolet light
  • Chemicals called mutagens e.g. chemicals in tobacco

Mutations are usually harmful. If a mutation occurs in reproductive cells, the offspring might develop abnormally or die. If a mutation occurs in body cells, the mutant cells may start to multiply in an uncontrolled way and invade other parts of the body (which is cancer). Some mutations have no effect at all, for example they occur in an unimportant part of the DNA- these mutations are said to be neutral. Very occationally, mutations are beneficial and give an organism a survival advantage, so it can live on in conditions where the others die. This is natural selection as work. 

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Mutations and Antibiotic Resistance 2

Bacteria can Evolve and Become Antibiotic-Resistant

Like all organisms, bacteria sometimes develop random mutations in their DNA. This can lead to changes in the bacteria's characteristics. Sometimes, they mean that a bacterium is less affected by a particular antibiotic. For the bacterium, this ability to resist antibiotics is a big advantage. Its better able to survive, even in a host who's being treated to get rid of the infection, and so it lives for longer and reporduces many more times. This leads to the gene for resistance being passed on to lots of offspring- its just natural selection. This is how it spreads and becomes more common in a population of bacteria over time. This problem for people who become infected with these bacteria, because you can't easily get rid of them with antibiotics. Sometimes drug companies can come up with a new antibiotic that's effective, but 'superbugs' that are resistant to most known antibiotics are becoming more common.

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Habitat- The place where an organism lives

Population- All the organisms of one species in a habitat

Community- All the different species in a habitat

Ecosystem- All the organisms living in a particular area and all the non-living conditions

You Can Estimate Population Sizes in Different Areas Using a Quadrat

A quadrat is a square frame enclosing a known area e.g. 1m2. You just place it on the ground, and look whats inside it. 

Two Important Points About This Kind of Counting Method

The sample may not be representative of the population. The sample size affects the accuracy of the estimate- the bigger your sample, the more accurate your estimate of the total population is likely to be. So it's better to use a quadrat at several points, get an average value for the number of organisms in a 1m2 quadrat, then multiply that by the total area.

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Ecosystems 2

You can use Quadrats to Investigate the Distribution of Organisms too

You can use quadrats to help find out how organisms (like plants) are distributed across their habitat. The quadrats are laid out along a line called a transect.

Here's what to do:

  • Mark out a line in the area you want to study
  • Then collect data along the line using quadrats placed next to each other
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Pyramids of Numbers, Biomass and Energy

Food Chains Show Whats Eaten by What in an Ecosystem

Food chains always start with a producer. Producers make their own food using energy from the Sun. Producers are eaten by primary consumers. Primary consumers are eaten by secondary consumers and secondary consumers are eaten by tertiary consumers. All these organisms eventually die and get eaten by decomposers. Decomposers break down dead material and waste. Each stage is called a trophic level.

You Need to Understand Pyramids of Numbers

Each bar on a pyramid of numbers shows the number of organisms at that stage of the food chain. So the dandelion bar would be longer that the fox bar. Dandelions go at the bottom because they're at the bottom of the food chain. This is a typical pyramid of numbers, where every time you go up a trophic level, the number of organisms goes down. This is because it takes a lot of food from the level below to keep one animal alive. There are cases where a number pyramid is not a pyramid at all. E.g 1 fox may feed 500 fleas.

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Pyramids of Numbers, Biomass and Energy 2

You Have to Understand Pyramids of Biomass Too

Ear bar on a pyramid of biomass shows the mass of living material of that stage of the food chain- basically how much all the organisms at each level would 'weigh' if you put them all together. So the fox would have a big biomasss and the hundreds of fleas would have a very small boimass. Biomass pyramids are practically always the right shape.

Pyramids of Energy Transfer Are Always Pyramid-Shaped

Pyramids of energy show the energy transferred to each trophic level in a food chain. Pyramids of energy transfer are always the right shape.

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Energy Transfer and Food Webs

Energy is Transferred Along a Food Chain

Energy from the Sun is the source of energy for nearly all life on Earth. Plants use light energy from the Sun to make food during photosynthesis. This energy then workf its way though the food chain as animals eat the plants and each other. Not all the energy that's availiable to the organisms in a trophic level is passed on to the next trophic level- around 90% of the energy is lost in various ways. Some parts of food aren't eaten by organisms so the enery isn't taken in. Some parts of food are indigestible so pass through organisms and come out as waste. A lot of the energy that does get taken in is used for staying alive which powers all life processes. Most of this energy is eventually lost to the surroundings as heat. Only around 10% of the total energy availiable becomes biomass. This is the energy that's transferred from one trophic level to the next.

Food Webs Show How Food Chains are Linked

There are many different species within an enviroment- which means lots of different possible food chains. You can draw a food web to show them. All species in a food web are interdependent, which means if one species changes, it affects all the others

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The Water Cycle and The Carbon Cycle

The Water Cycle Means Water is Endlessly Recycled

The water here on planet Eath is constantly recycled. Heat from the Sun makes water evaporate from the land and sea, turning it into water vapour. Water also evaporates from plants- this is known as transpiration. The warm water vapour is carried upwards. When it gets higher up it cools and condenses to form clouds. Water falls from the clouds as precipitation and is returned to the land and sea.

The Carbon Cycle Shows How Carbon is Recycled

Carbon is an important element in the material that living things are made from. But there's only a fixed amount of carbon in the world. This means it's constantly recycled.

The whole thing is powered by photosynethesis. Green plants use the carbon from CO2 in the air to make carbohydrates, fats and proteins. Eating passes the carbon compounds in the plant along to animals in a food chain or web. Both plant and animal respiration while the organisms are alive releases CO2 back into the air. Plants and animals eventually die and decompose, or are killed and turned into useful products. When plants and animals decompose they're broken down by bacteria and fungi. These decomposers release CO2 back into the air. Some useful plant and animal products also release CO2 back into the air

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The Nitrogen Cycle

Nitrogen is Also Recycled in the Nitrogen Cycle

The atmosphere contains 78% nitrogen gas. This is very unreactive and so it can't be used directly by plants or animals. Nitrogen is needed for making proteins for growth, so living organisms have to get it somehow. Plants get their nitrogen from the soil, so nitrogen in the air has to be turned into nitrogen compounds before plants can use it. Animals can only get proteins by eating plants. Nitrogen fixation isn't an obsession with nitrogen- it's the process of turning N2 from the air into nitrogen compounds in the soil which plants can use. There are two main ways that this happens:

  • Lightning- there's so much energy in a bolt of lightning that it's enough to make nitrogen react with oxygen in the air to give nitrates.
  • Nitrogen-Fixing Bacteria in roots and soil

There are four different types of bacteria involved in the nitrogen cycle:

  • Decomposers- Break down proteins and urea and turn them into ammonia
  • Nitrifying Bacteria- Turn ammonia in decaying matter into nitrates
  • Nitrogen- fixing bacteria- turn N2 into nitrogen compounds that plants can use
  • Denitrifying bacteria- turn nitrates back into N2 gas. This is of no benefit to living organisms.
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Air Pollution

Carbon Monoxide is Poisonous

When fossil fuels are burnt without enough air supply they produce the gas carbon monoxide. Its a poisonous gas. If it combines with red blood cells it prevents them from carrying oxygen. 

Acid Rain is Caused by Sulfur Dioxide

Burning fossil fuels releases harmful gases like CO2. Sulphur dioxide comes from sulfur impurities in the fossil fuels. When this gas mixes with rain clouds it forms dilute sulfuric acid. This then falls as acid rain. Internal combustion engines in cars and power stations are the main causes of acid rain.

Acid rain can cause a lake to become more acidic. This has a severe effect on the lake's ecosystem. Many organisms are sensitive to changes in pH and can't survive in more acidic conditions. Many plants and animals die. Acid rain can kill trees. The acid damages leaves and releases toxic substances from the soil, making it hard for the trees to take up nutrients.

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The Greenhouse Effect

Greenhouse Gases Trap Heat from the Sun

The temperature of the Earth is a balance between the heat it gets from the Sun and the heat it radiates back out into space. Gases in the atmosphere absorb most of the heat that would normally be raidiated out into space, and re-radiates it in all directions. If this didn't happen, then at night we would get cold very quickly. 

Human Activity Produces Lots of Greenhouse Gases

Carbon Dioxide- Humans release CO2 into the atmosphere all the time as part of our everyday lives- in car exhausts, as we burn fossil fuels ect. People around the world are also cutting down trees for timber and to clear land for farming. 

Methane- Methane gas is also produced naturally from various sources. However, two 'man-made' sources of methane are on the increase: rice growing and cattle rearing.

Nitrous Oxide- Nitrous oxide is released naturally by bacteria in soils and the ocean. A lot more is released from soils after fertiliser is used. It's also released from vehicle engines.

CFCs- CFCs are man made chemicals from aerosol sprays and fridges. They're really powerful greenhouse gases.

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Water Pollution and Deforestation

Fertilisers can Leach into Water and Cause Eutrophication

Nitrates and phosphates are put onto fields as mineral fertilisers. If too much fertiliser is applied and it rains afterwards, nitrates are easily leached into rivers and lakes. The result is eutrophication, which can cause serious damage to rivers and lakes:

  • Fertilisers enter the water, adding extra nutrienrts.
  • The extra nutrients cause algae to grow fast and block out light.
  • Plants can't photosynthesise due to lack of light and start to die.
  • With more food available, microorganisms that feed on dead plants increase in munber and deplete all the oxygen in the water.
  • Organisms that need oxygen die

Another cause of euthrophication is pollution by sewage. Sewage contains lots of phosphates from detergents. It also contains nitrates from urine and faeces. These extra nutrients cause eutrophication in the same way that fertilisers do.

Deforestation Affects The Soil, Water Cycle and Carbon Cycle

Leeching- Nutrients leached away. Soil erosion- soil wash away creating infertile land. Dist-urbing the water cycle- less water in air. Disturbing balance of CO2 &O2

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Increasing Crop Yields

You Can Artifically Creat the Ideal Conditions for Photosynthesis

Keeping plants enclosed keeps them pest free. Artificial light means more photosynthesis. Heat keeps plants are optimun temperature. Increase CO2 levels means more photosynthesis can happen. 

Fertilisers Are Used to Ensure the Crops Have Enough Nutrients

Plants need certain elements, so they can make important compounds like proteins. If plants don't get enough of these elements, their growth and life processes are affected. Sometimes these elements are missing from the soil because they've been used up by a previous crop. Farmers use fertilisers to replace these missing elements or provide more of them. This helps to increase the crop yield.

Pest Control Stops Pests Eating Crops

Pests include microorganisms, insects and mammals. Pests that feed on crops are killed using various methods of pest control. This means fewer plants are damaged or destroyed, increasing crop yield. Pesticides are a form of chemical pest control. They're often poisonous to humans so must be used in small amounts. Biological control is an alternative which is a helful organism eating predators and they have a longer effect.

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Bacteria and Making Yoghurt

Bacteria Ferment Milk to Produce Yoghurt

Fermentation is when microorganisms break sugars down to release energy- usually by anaerobic respiration. Yoghurt is basically fermented milk.

Microrganisms are Grown in Fermenters

In industry microorganisms are grown in fermenters. The fermenter is full of liquid 'culture medium' in which microorganisms can grow and reproduce. The conditions inside the fermentation vessels are kept at the optimun levels for growth- this means the yield of products from the microorganisms can be as big as possible.

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Yeast and Making Beer

We Use Yeast for Brewing Beer

Firstly you need to get the sugar out of the grain. Beer is made from barley. The barley grains are allowed to germinate for a few days, during which the starch in the grains is broken down into sugar by enzymes. Then the grains are dried in a kiln. This is called malting. The malted grain is mashed up and water is added to produce a sugary solution with lots of bits in it. This is then sieved to remove the bits. Hops are added to the mixture to give the beer its bitter flavour. Yeast is added and the mixture is incubated. The yeast ferments the sugar into alcohol. The fermenting vessels are designed to stop unwanted microorganisms and air getting in. The rising conc. of alcohol in the fermenter to anaerobic respiration eventually starts to kill the yeast. As the yeast dies, fermentation slows down. Different species of yeast can tolerate different levels of alcohol. some species can be used to produce strong beer with a high conc. of alcohol. The beer is drawn off through a tap. Sometimes chemicals called clarifying agents are added to remove particles and make it clearer. The beer is then pasteurised and casked ready for sale.

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Yeast and Making Beer 2

The Respiration Rate of Yeast Depends on Its Conditions

You can do this experiment to investigate how the rate of CO2 production by yeast changes under different conditions. Here's how to measure the effect of changing temperature:

  • Mix together some sugar, yeast and distilled water, then add the mixture to a test tube
  • Attach a bung with a tube leading to a second test tube of water.
  • Place the tube containing the yeast mixture in a water bath at a certain temperature.
  • Leave the tube to warm up a bit and then count how many bubbles are produced in a given period of time. Use this to calculate the rate of CO2 production
  • Repeat the experiment with the water bath set at different temperatures
  • Respiration is controlled by enzymes- so as the temperature increases, so should the rate of respiration

The example looks at how temperature can affect the rate of CO2 production but we could change it to a different variable e.g. concentration of sugar.

You can also make this experiment more reliable by using a gas syringe instead of a second tube so you can accurately measure how much CO2 is produced.

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Selective Breeding

Selective Breeding is Mating the Best Organisms to Get Good Offspring

Organisms are selectively bred to develop the best features, which are things like:

  • Maximum yield of meat, milk, grain, etc.
  • Good health and disease resistance.
  • In animals, other qualities like temperament, speed, fertility, good mothering skills, etc.
  • In plants, other qualities like attractive flowers, nice smell, etc.

This is the basic process involved in selective breeding:

  • From you existing stock select the ones which have the best characteristics
  • Breed them with each other
  • Select the best of the offspring, and breed them together
  • Continue this process over several generations, and the desirable trait get stronger and stronger. In farming, this will give the farmer gradually better and better yields/
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Selective Breeding 2

Selective Breeding is Very Useful

Selective breeding can increase the productivity of cows

Cows can be selectively bred to produce offspring with, e.g. high meat yield. First, the animals with characteristics that will increase meat yield are selected and bred together. Next, the offspring with the best characteristics are selected and bred together. If this is continued, cows with very large meat yields can be produced.

Selective breeding can increase the number of offspring in sheep

Farmers can selectively breed sheep to increase the number of lambs born. Female sheep who produce large numbers of offspring are bred with rams whose mothers had large numbers of offspring. The characterisitc is passed on to the next generation.

Selective breeding can increase crop yield

Tall wheat plants have good grain yield but are easily damaged by wind and rain. Dwarf wheat plants can resist wind and rain but have a lower grain yield. These two types are cross-bred. This resulted in a new variety of wheat combining the characteristics.

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Fish Farming

Fish Can Be Farmed In Cages In The Sea

Salmon farming in Scotland is a good example:

  • Fish are kept in cages in the sea to stop them using too much energy swimming about
  • The cage also protects them from interspecific predation (being eaten by other animals like birds or seals)
  • They're fed a diet of food pellets that's carefully controlled to maximise the amount of energy they get. The better the quality the food is, the quicker and bigger the fish will grow
  • Young fish are reared in special tanks to ensure as many survive as possible
  • It's important to keep younger fish separate and to provide regular food- this makes sure the big fish don't eat the little fish. 
  • Fish kept in cages are more prone to disease and parasites. One pest is sea lice, which can be treated with pesticides which kill them. To avoid pollution from chemical pesticides, biological pest control can be used instead
  • Fish can be selectively bred to produce less aggressive, faster-growing fish
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Fish Farming 2

Fish Can Be Farmed In Tanks Too

Freshwater fish can be farmed in ponds or indoors in tanks where conditions can be controlled. This is especially useful for controlling the water quality.

  • The water can be monitored to check the temperature, pH and oxygen level is okay
  • It's easy to control how much food is supplied and give exactly the right sort of food
  • Water can be removed and filtered to get rid of waste food and fish poo. This keeps the water clean for the fish and avoids pollution wherever the water ends up
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Genetic Engineering

Enzymes Can Be Used To Cut Up DNA or Join DNA Pieces Together

  • Rectriction enzymes recognise specific sequences of DNA and cut the DNA at these points
  • Lipase enzymes are used to join two pieces of DNA together
  • Two different bits of DNA stuck together are known as recombinant DNA

Vectors Can Be Used To Insert DNA Into Other Organisms

A vector is something that's used to transfer DNA into a cell. There are two sorts- plasmids and viruses:

  • Plasmids are small, circular molecules of DNA that can be transferred between bacteria
  • Viruses insert DNA into organisms they infect
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Genetic Engineering 2

Here's how genetic engineering works:

- The DNA you wnat to insert is cut out with a resitriction enzyme. The vector DNA is then cut open using the same restriction enzyme.

- The vector DNA and the DNA you're inserting are mixed together with ligase enzyme.

- The ligases join the two pieces of DNA together to produce recombinant DNA

- The recombinant DNA is inserted into other cells

- These cells can now use the gene you inserted to make the protien you want

Bacteria that contain the gene for human insulin are transgenic- this means that they contain genes transferred for another species. You can get transgenic animals and plants too.

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Genetic Engineering 3

Genetically Modified Plants Can Improve Food Production

Crops can be genetically modified to increase food production in lots of different ways. Making crops insect-repellent means farmer dont have to spray as many pesticides. It also increases crop yield, making more food. Making crops herbicide-resistant means farmers can spray their crops to kill weeds, without affecting the crop itself. This also increases crop yield. 

There are concerns about growing genetically modified crops. One is that transplanted genes may get out into the enviroment. Another concern is that genetically modified crops could adversely affect food chains- or even human health. Some people are against genetic engineering altogther- they worry that changing an organism's genes might create unforseen problems- which could then get passed on to future generations.

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Micropropagation is Used to Clone Plants

Plants can be cloned from existing plants using a technique  called micropropagation:

  • A plant with desirable characteristics is selected to be cloned. Small pieces are taken from the tips of the stems and the side shoots of this plant.
  • The explants are sterilised to kill any microorganisms.
  • The explants are then grown in vitro- this means that they're placed in a petri dish containing a nutrient medium. The medium has all the nutrients the explants need to grow. It also contains growth hormones
  • Cells in the explants divide and grow into a small plant. If large quantities of plants are required, further explants can be take from these plants and so on until enough small plants are produced.
  • The small plants are take out of the medium, planted in soil and put into glasshouses- they'll develop into plants that are genetically identical to the original plant- so they share the same characteristics
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Cloning 2

Cloning an Adult Mammal is Done by Transplanting a Cell Nucleus

The first mammal to be successfully cloned from an adult cell was a sheep called "Dolly" in 1996. This is the method that was used:

  • The nucleus of the sheep's egg cell was removed, creating an enucleated cell
  • A diploid nucleus was inserted in its place. This was a nucleus from a mature udder cell of a different sheep
  • The cell was stimulated so that it started dividing by mitosis, as if it was a normal fertilised egg
  • The dividing cell was implanted into the uterus of another sheep to develop until it was ready to be born
  • The result was Dolly, a clone of the sheep that the udder cell came from

Other animals can also be cloned using this method.

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Cloning 3

There Are Advantages and Disadvantages to Cloning There are many possibe uses for cloned transgenic animals: Animals that can produce medicines in thier milk could be cloned. Reasearchers have managed to transfer human genes that produce useful proteins into sheep and cows e.g. human antibodies used in therapy for illnesses like arthritis, some types of cancer and multiple sclerosis Animals that have organs suitable for organ transplantation into humans could be developed by genetic engineering and then cloned in the same way The main benefits of cloning are that the useful genetic characteristic are always passed on- this doesn't always happen with breeding. Farmers also don't have to wait until the breeding season, and infertile animals can be cloned But there are risks too. There's evidence that cloned animals might not be as healthy as normal ones. Embryos formed by cloned from animal cells often don't develop normally Cloning is also a new science and it might have consequences that we're not yet aware of. At the moment it's also difficult, time-consuming and expensive.

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