b3 ocr gcse

Year 11 Biology Revision Notes. Module B3 Living and Growing.

Molecules of life

What’s in a cell?

Nucleus – contains genetic information, this is carried on chromosomes

Cell membrane – controls movement of substances in and out of the cell

Cytoplasm – where many chemical reactions happen

Mitochondria – cell respiration is carried out here. Energy is released from glucose in the presence of oxygen.

DNA code

Found in the nucleus. Forms structures called chromosomes. A section of a chromosome is called a gene. Each gene is a code for making proteins. Our bodies need proteins to grow and make proteins. Everyone has his or her unique DNA code.

DNA – double helix; complimentary base pairs (adenine – thymine; cytosine – guanine)

Each time a cell divides the DNA copies itself. This is called DNA REPLICATION. The base pairs separate and the strand unzips to form two single strands. New bases pair up by complimentary base pairing to form two new double strands.

The order of bases found in a section of DNA is called the base code. Each THREE bases code for an amino acid.

E.g. AAA TAT CTC CCC TCA ACC GGG CGG TAA ATG (10 amino acids are coded for here)

The complimentary base pairs would be:

TTT ATA GAG GGG AGT TGG CCC GCC ATT TAC

DNA fingerprinting

1. Isolate blood of cell sample2. Extract DNA3. Use restriction enzymes to fragment the DNA4. Place DNA fragments on gel5. Apply and separate fragments using an electric current (electrophoresis)6. Banding of DNA fingerprint can be matched.

Enzymes

Enzymes are biological catalysts – they speed up a biological reaction.Each enzyme is specific to a substrate. The substrate molecules are changed into product molecules.Enzyme controlled reactions are affected by pH and temperature.

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Optimum pH or temperature – the pH / temperature where the reaction works best.

Lock and key theory- each enzyme has a unique sequence of amino acids – therefore each enzyme has a different shape. Within this shape is a structure called an active site. Only one type of substrate can fit into the active site, this makes enzymes specific to a reaction. Once the substrate is attached to the active site it is turned into a product. The enzyme is like a lock and the substrate is like the key.

Denaturing enzymes

If the shape of an enzyme changes it can no longer catalyse a reaction because the substrate can no longer fit into the active site. The enzyme has become denatured.

Enzymes can be denatured by:

Extremes of pH

High temperatures

Diffusion

Diffusion – the movement of a substance from a region of high concentration to a region of low concentration.

Moving substances in the body

Different substances diffuse in and out of the cells across the cell membranes.

Oxygen – moves from the lungs into the red blood cells. It then moves from the red blood cells into the body tissue.

Carbon dioxide – moves from the body tissue into the blood, then from the blood into the lungs.

After eating – digested food molecules move from the small intestine into the blood. They then leave the blood and go into body tissue.

Changing the rate of diffusion

It can be increased by:

Increasing the surface area

Decreasing the diffusion distance

A greater concentration difference

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Surfaces adapted for diffusion

Diffusion takes place in the villi in the small intestine and the alveoli in the lungs. Both are adapted to increase the rate of diffusion.

Villus – produce a large surface area, villi wall have folds (microvilli). Surface area of small intestine is approx 9 m2.

One cell thick, food does not have to far to diffuse into the blood

Good blood supply – means digested food is quickly taken away from villus so more can diffuse across to replace it

Membrane of villi is permeable, this means food molecules can pass through the membrane

Alveoli – in the lungs

Breathing makes sure there is always a high concentration of oxygen in the alveoli.

Good blood supply makes sure as oxygen diffuses into the blood it is replaced with blood containing very little oxygen.

Alveolus is only one cell thick so the gases do not have far to travel

Large numbers of alveoli – this helps to increase the surface area, so more molecules can move across at any time.

Alveoli membrane is permeable to gases and is also moist; this helps to speed up diffusion.

Other substances adapted for diffusion

Placenta

To move substances across the placenta as quickly as possible. To speed up movement, the placenta has:

A very large surface area

A very thin wall so substances only have a short distance to diffuse

The leaf

To increase the rate of gas exchange, the leaf has a large surface area. The under-surface of the leaf also has many stomata through which gases can diffuse.

Synapses

The gap between two neurones (nerve cells). The synapse releases a chemical that can diffuse across the gap between the two neurones. A large surface area and short diffusion distance is important.

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Keep it moving

Blood

Comprised of:

Plasma – a yellow liquid, adapted to transport dissolved substances such as water, hormones, antibodies and waste products

Red blood cells – transport oxygen around the body. Red colour comes from haemoglobin. Oxygen joins to the haemoglobin to form oxyhaemoglobin, which allows it to be transported around the body. They do not have a nucleus – this leaves more room to carry oxygen. They are disc-shaped and have a dent on both sides – this allows them to absorb a lot of oxygen. They are very small so they can carry oxygen to all parts of the body.

White blood cells – defend the body against disease. They are adapted to change shape, they can wrap around microbes and engulf them. They can squeeze through capillary walls to reach microbes.

Platelets – help to clot the blood if we cut ourselves.

Blood vessels

Blood is carried around the body in three different blood vessels.

Artery – thick muscular and elastic wall to help it withstand high blood pressure as the blood leaves the heart.

Vein – large lumen (hole) to help blood flow at low pressure; valves stop blood from flowing the wrong way.

Capillary – thin, permeable wall to allow exchange of material with body tissue

The Heart

Structure

Four chambers

Two atria – receive blood from the veins

Two ventricles – pump blood into arteries

Valves – bicuspid, tricuspid, semi-lunar valves – prevent the blood flowing backwards when the heart relaxes and so maintain blood pressure.

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Four key vessels

Right hand side:

o vena cava, pulmonary artery

Left hand side:

o pulmonary vein, aorta

Function

The heart pumps blood around the body. There are two sides to a heart.

The right side pumps blood to the lungs

The left side pumps blood to the rest of the body

The blood leaves the heart in arteries where the pressure is high.

The blood returns to the heart at low pressure in the veins.

Heart problems

Blood flows through arteries at high pressure. Saturated animal fats such as cholesterol can stick to the walls of arteries. This can slow down or block the flow of blood. If this happens in a main blood vessel it can cause a heart attack or a stroke.

Mending the heart

Mechanical – heart valves

Biological – heart transplant

Divide and rule

Every day new cells are made. To do this the body carries out cell division. Cells divide whenever the body needs to:

Grow

Replace worn out cells

Repair damaged tissue

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Inside the nucleus of a human body cell there are 46 chromosomes. Humans have 23 pairs of chromosomes. The chromosomes in a pair look the same and carry similar information. They are called homologous. When a cell has pairs of chromosomes it is called a diploid cell. During growth a type of cell division called mitosis makes new cells. The new cells are exact copies and contain 23 pairs of chromosomes.

Mitosis

1. Cell resting

2. Each chromosome is copied, the single strand forms double-stranded “X” shape

3. Spindle forms, chromosomes arranged along equator

4. Chromosome single strands move to poles of cell

5. Two genetically identical cells are produced.

Human eggs and sperm

These are our sex cells or gametes. Gametes join during fertilisation. They are adapted to carry out their jobs.

Egg

Much larger than the sperm because it contains food for the developing embryo

The nucleus of an egg contains genes which hold the instructions to make new cells

Sperm

Much smaller than the egg

Has a tail to help it move

Male releases millions of sperm to increase the chance of one reaching the egg

The nucleus of a sperm contains genes which are the instructions to make new cells

Sperm have to travel along way and then get inside an egg – they have large numbers of mitochondria to release energy for motion.

Sperm also have a structure called an acrosome – this releases enzymes that digest the cell membrane of an egg and allows the sperm inside.

Gametes have half a set of chromosomes called the haploid number. During fertilisation the gametes join to form a zygote. The zygote is diploid and can develop into an embryo.

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When fertilisation takes place gametes from a male and female join. The resulting offspring have genes from both parents. They are different to their parents – they are new individuals. Reproduction using meiosis results in a lot of genetic variation within a species.

Meiosis

A special type of cell division that produces gametes. Gametes are made when diploid cells divide by meiosis to produce haploid cells. Meiosis involves two cell divisions. First the chromosomes separate then the chromosomes divide in the same way as mitosis

Genetic disorders

Haemophilia: an inherited condition , affects blood clotting. It is sex linked, this means it is carried on an X chromosome. Haemophilia is a recessive disorder. This means if a woman has inherited a healthy X chromosome from one parent and a damaged “haemophilia” chromosome from the other parent, the healthy chromosome dominates and she does not suffer from haemophilia – she does carry it and may pass it on though. A man has only one X chromosome (XY), if he inherits a haemophilia X chromosome he will suffer from it.

e.g.

Carrier female and unaffected male

X Y

X (h) X(h)X (carrier) X(h)Y (affected male)

X X X (unaffected female) XY (unaffected male)

Carrier female and affected female (very very rare!)

X (h) Y

X (h) X(h)X (h) (affected female) X(h)Y (affected male)

X X X (h) (carrier female) XY (unaffected male)

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Growing up

Plant cells

Plant cells have the following components:

Nucleus

Cytoplasm

Cell membrane

Vacuole – contains cell sap and provides support

Cell wall – provides support

Chloroplasts – absorb light energy for photosynthesis

Animal cells DO NOT have a cell wall, chloroplasts, they may have a small vacuole but they DO NOT have cell sap.

Animal cell and plant cell growth

For a fertilised egg to grow into an embryo and a foetus the cells need to divide and change so they can carry out different jobs. Some cells turn into nerve cells and other cells may turn into bone cells. This is called cell differentiation.

The cells of animals and plants cause them to grow in different ways.

Plant Animal

Most growth is due to cells elongating (growing longer) not dividing

Growth is due to cells dividing

Cell division only normally occurs at the tips of shoots and roots

Cell division occurs all over the body

Many cells never lose the ability to differentiate Most animal cells lose the ability to differentiate very early on

A few days after an egg is fertilised it contains a group of cells called stem cells. These all have the same simple cell structure. They divide and then differentiate to form all the different specialised cells in the body. As the embryo grows all the specialised cells form tissues and organs.

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Some stem cells are found in the adult body. Bone marrow contains stem cells that turn into different types of blood cells.

Scientists have found ways of making stem cells develop into other specialised cells in the hope of replacing damaged cells. Ethically this can cause problems. People may object to stem cell research because it can involve human embryos. Scientists use embryo stem cells because they are easier to grow than adult stem cells.

Human growth

There are five main stages:

1. Infancy

2. Childhood

3. Adolescence (puberty)

4. Adulthood (maturity)

5. Old age

Gestation

The length of time from fertilisation to birth. The larger the animal the longer the gestation tends to be. An elephant has a gestation of 700 days, a rat has a gestation of 22 days. This is because the animal needs time to develop enough to survive outside the uterus.

Growth of a baby

Different parts of the foetus and baby grow at different rates. The brain and head develop quickly to coordinate the complex human structure and chemical activity.

Growth curves

After a baby is born it has regular growth checks. The baby’s weight and head size are recorded. The measurements will show if the baby is growing at a normal rate.

Poor weight gain – can indicate problems with a baby’s digestive system

Larger than normal head size – can indicate fluid collection on the brain or the separate skull bones not fusing together

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Controlling plant growth

Plant hormones

Plants make chemical called hormone. They control different processes in a plant.

Growth of shoots towards the light

Growth of roots downwards in the soil in response to gravity

Growth of flowers

Ripening of fruit.

A plant is sensitive and responds to different stimuli. A plant’s response may be:

Negative – it grows away from a stimulus

Positive – it grows towards a stimulus.

A hormone called AUXIN controls the response. It is made in the tips of roots and shoots of a plant. It travels through a plant in solution.

Phototropism

o When a plant responds to light it is called phototropism.

o Plant shoots grow towards light = positive phototropism

o Plant roots grow away from light = negative phototropism

Geotropism

When a plant responds to gravity it is called geotropism

Plant shoots grow away from the pull of gravity = negative geotropism

Plant roots grow with the pull of gravity – positive geotropism

How auxin works

When the tip of a shoot is cut off it stops growing. If the tip is replaced on the stem it starts to grow again. Removing the tip removes the source of auxin and stops growth.

Auxin is made in the tip

Auxin moves away from light and collects on the shady side of a shoot.

Auxin causes cells on the shady side to elongate (grow longer) more than cells on the light side

The shady side becomes longer and causes the shoot to bend10


Artificial use of hormones

Farmers can spray their crops with hormones to make the plants grow fruit. They can also use hormones to slow down growth – this stops the fruit from falling off the tree before the harvest.

The use a man made hormone called synthetic auxin.

Selective weed killers

Synthetic auxin is sprayed on crops to kill weeds. The hormones makes the weeds grow too fast and they die. The concentration only affects broad-leaved weeds, leaving the narrow-leaved crops unaffected.

Growing roots

A cutting is dipped into a hormone based rooting powder. The synthetic auxin stimulates roots to grow from the shoot.

Seedless grapes

Unfertilised plants can be sprayed with synthetic auxin. This causes the fruits to grow without the flowers being fertilised. The fruits have no pips, e.g. seedless grapes

Transporting bananas

Bananas are harvested before they are ripe. During transport they are sprayed with ethane, this causes them to ripen ready for sale.

New genes for old

Selective breeding

Choose the animals and plants with the best characteristic and breed them to produce offspring that have those characteristics. The selection and breeding process needs to be repeated for a number of generations.

Mutations

This is when there is a change in the animal or plants genes. This usually causes harm to the organism, although some mutations can be an advantage to an organism giving it a better chance of survival.

Mutations can be caused by:

Radiation such as x rays

Chemicals such as those found in cigarette smoke

Chance

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Transferring genes

Scientists can take genes from one organism and put them into a different organism. These changes are called genetic engineering or genetic modification (GM)

How genetic engineering works

1. Select the characteristic

2. Identify and isolate the gene

3. Insert the gene into the chromosome of a different organism

4. Replicate (copy) the gene in the organism and produce the protein.

Enzymes are used to cut a gene out of an original chromosome and put it into a new chromosome.

Examples:

Insulin – a bacterium called E coli has been genetically engineered to make human insulin.

Vitamin A rice – rice is the main diet for many people living in Asian countries, it does not contain vitamin A which is needed to prevent night blindness. Scientists have added the gene to make beta-carotene from carrots to the rice plants. Humans eating the rice can then convert the beta-carotene into vitamin A.

Some people are opposed to GM due to ethical reasons and have concerns of “playing God “with nature.

More of the same

Cloning

Clones are genetically identical. They have the same DNA as the original animal or plant. Identical twins are natural clones – they have the same DNA.

Types of cloning:

Embryo transplantation

Sperm is collected from the prize bull; a prize cow is artificially inseminated with the sperm. When the fertilised egg divides into an eight-cell embryo it is collected and split into four two-celled embryos. Each embryo is implanted into a surrogate cow where it grows into a calf. All the calves will be genetically identical to each other but not to their parents.

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Nuclear transfer

Egg cell taken from sheep A and the nucleus is removed.

An udder cell is taken from sheep B and the nucleus is removed.

The nucleus from sheep B is put into the egg cell of sheep A.

The egg cell is put into a sheep to grow.

The cell grows into a clone of sheep B (where the nucleus containing the genetic information came from)

The importance of cloning

Organ supply for humans

Cloning of human embryos to provide stem cells

Risks

Low success rate

Moral / ethical issues

Complications / early death of clones

Benefits

Cloned pigs could make up a shortage in transplant organs

Diseases could be cured using embryonic stem cells

Asexual reproduction in plants

Plants that can do it naturally:

Potato plants – tuber

Strawberries – runner

Spider plants – plantlets

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New plants from old

How? – take cuttings

Advantages of cloning plants

All the plants are genetically identical

Cloning is quite. a quick process in comparison to growing plants from seeds

Cloning enables growers to produce plants that are difficult to grow from seed such as bananas

Disadvantages

The plants are all genetically identical. If the environment changes or a disease breaks out it is unlikely any of the plants will survive.

Cloning plants over the years has resulted in very little genetic variation

Tissue culture

Plants can be cloned using tissue culture. This must be carried out using aseptic technique

1. Plants with desired characteristics are chosen

2. A large number of small pieces of tissue are taken from the parent plant

3. They are put into sterile test tubes that contain the growth medium

4. The tissue pieces are left in suitable conditions to grow into plants

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