Biology GCSE Notes

Cell Structure, Division and Transport

Animal and plant cells have many features in common, such as a nucleus, cytoplasm and cell membrane.
Plant cells have a cell wall made of cellulose, a large vacuole containing cell sap, and may contain chloroplasts for photosynthesis. Animal cells store carbohydrates as glycogen while plant store starch.
The nucleus is the store of genetic information, made of DNA which are arranged as chromosomes. Humans have 46 chromosomes in every cell. Genes are parts of chromosomes that code for one protein.
Mitosis is used for growth and repair, and is the only method of reproduction in asexually reproducing organisms such as bacteria. The parent cell divides into two genetically identical (clones) daughter cells.
Meiosis is used to make gametes (sex cells) in sexually reproducing organisms. The parent cell divides twice to give four non-identical daughter cells with half the number of chromosomes as the parent cell.
Cell specialisation is important in higher organisms because of the varying of different functions that have to be performed. All cells contain the same genes but develop differently. Groups of specialised cells form tissues, which join together to form organs. Groups of organs work together in organ systems.
Diffusion is the net movement of a substance from a high to low concentration, or down a concentration gradient. Diffusion increases with temperature and with smaller particles, which move faster.
Osmosis is a special kind of diffusion, involving the movement of water from a dilute solution of a substance to a concentrated solution across a partially permeable membrane, which only allows water molecules to pass through but not the dissolved substance.
Active transport is used to move substances from a low to a high concentration. Active transport requires special membrane proteins and energy from respiration.

Nutrition

Enzymes are biological catalysts and control nearly all reactions in the body. Enzymes are very specific for their substrate due to their shape. Enzymes usually work best at body temperature and neutral pH.
Enzymes are responsible for digestion of food. Amylases break down starch, proteases break down protein, and lipases break down fat.
Iodine solution turns blue-black in the presence of starch, Benedict’s solution turns orange-red in the presence of simple sugars, and the Biuret reagent (sodium hydroxide and copper sulphate) turns purple in the presence of protein. To test for fats, shake the substance in ethanol and then add a few drops of water. A milky white emulsion forms in the water if fat is present.
Digestion occurs throughout the alimentary canal. In the mouth, food is chewed into small pieces and saliva containing amylase is released to soften food and digest starch.
The stomach releases gastric juice which contains protease, to digest proteins, and hydrochloric acid, to kill microbes. The liver makes bile that contains bile salts, that break down fat into smaller droplets (emulsification). Bile is stored in the gall bladder. The pancreas makes digestive enzymes including proteases, amylases and lipases, and these are released into the small intestine.
The first part of the small intestine is called the duodenum. The duodenum is relatively short and it is where the digestive enzymes from the pancreas are secreted.
The second part of the small intestine is called the ileum, which is specialized for the absorption of digested food. The ileum is long and has a very large surface area. The inside of the ileum is folded and the folds have thousands of finger-like projections called villi. Furthermore, the cells on the villi have small projections called microvilli. Nutrients are absorbed by active transport.

The Circulatory System

Arteries carry oxygenated blood away from the heart. They have a thick wall with plenty of elastic and muscle tissue to withstand the high pressure. Blood is pumped at high pressure so valves are not needed.
Veins carry deoxygenated blood towards the heart, at low pressure. Valves are needed to prevent the back-flow of blood. Veins have thinner walls.
Capillaries join the arteries to the veins. They are very thin (one cell thick) and are responsible for delivering nutrients to and removing wastes from all tissues in the body. Plasma leaks out of the capillaries into the tissues, which is known as tissue fluid.
The tissue fluid carries glucose, amino acids, and other useful substances to the cells. Oxygen diffuses out of the capillaries into the tissues, while carbon dioxide diffuses into the blood. Most tissue fluid returns by osmosis, and excess tissue fluid enters the lymphatic system through the lymph vessels.
Humans have a double circulation, which means that blood passes the heart twice in every circuit. Thus, blood is returned to the heart to gain high enough pressure to get through the capillaries of the body.
The heart is a muscular pump made of a special type of tissue called cardiac muscle. The cardiac muscle stimulates itself by electrical impulses to produce a regular beat.
The heart is composed of four chambers: the left atrium, the right atrium, the left ventricle, and the right ventricle. The top two chambers, the atria, receive the blood from the veins and then pump them into the arteries. The lower two chambers, the ventricles, pump blood out to the arteries.
Atrioventricular (AV) valves (also known as the bicuspid valve on the left and the tricuspid valve on the right) close when the ventricles are pumping, to prevent blood from returning into the atria. Semi-lunar valves close after blood is pumped out of the ventricles to prevent blood from returning to the ventricles.
The right side of the heart carries deoxygenated blood that has returned from the body and is pumped to the lungs. Blood enters the right atrium through the vena cavae, and is pumped to the lungs through the pulmonary artery. The left side of the heart carries oxygenated blood that has returned from the lungs and is pumped to the body. Blood enters the left atrium through the pulmonary vein, and is pumped to the body through the aorta.

The Respiratory System

Breathing (ventilation) is a set of muscular movements that draw air in and out of the lungs. The lungs help the body take in oxygen and remove carbon dioxide.
The pleural membrane in the thorax forms an airtight pleural cavity. When the diaphragm contracts and the intercostal muscles contract, the volume of the pleural cavity increases, reducing the pressure. Air moves into the lungs and the lungs expand (inhalation). The opposite occurs during exhalation.
Gaseous exchange occurs in the millions of alveoli in the lungs, which are highly adapted for efficient gas exchange. The walls of the alveoli are only one cell thick, and are surrounded by capillaries. The surface of the alveoli are moist so that gases can dissolve. Carbon dioxide diffuses from the blood into the alveoli while oxygen dissolves into the capillaries.
Respiration is the breakdown of food to generate energy. It is not the same as ventilation!
In aerobic respiration, glucose and oxygen combine to make carbon dioxide and water, and energy.
- C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
When not enough energy is available, glucose can be broken down by anaerobic respiration (or fermentation). In humans, this produces lactic acid and energy. In yeast, this produces carbon dioxide, ethanol and energy. Anaerobic respiration by yeast is how breweries make beer and wine.
Anaerobic respiration in humans releases much less energy than aerobic respiration. The build up of lactic acid in the muscles causes muscle fatigue and results in an oxygen “debt”. When the vigorous exercise stops, excess oxygen is breathed in to break down the lactic acid.
A calorimeter can be used to measure the energy content of food. Food is burnt in a closed container, and the temperature rise in the surrounding water is measured.

The Nervous System

All living organisms can respond to their environment (sensitivity). Plants usually respond more slowly than animals. However, the order of events is always the same:
- stimulus → receptor → coordination → effector → response
Receptors detect changes and pass on the information to the central nervous system (the brain and the spinal cord). This coordinates all the information and sends a message to the effectors to bring about a response. These messages are sent by nerves (faster) or hormones (slower).
Different receptors in the body respond to different stimuli. Receptors are often gathered together into sense organs, that have other various structures that help the receptors gain more information.
The eye is a sense organ for detecting light. Light enters through the pupil, and is focused onto the retina by the cornea and the lens. The size of the pupil can be reduced by the muscles of the iris in bright light, by contracting the radial muscles and relaxing the circular muscle.
When looking at close objects, the ciliary muscles contract and the lens becomes more rounded and powerful. The ciliary muscles relax when viewing distant objects.
Rods and cones are the receptor cells that are concentrated in the fovea of the retina. They detect light and send messages to the brain along the optic nerve. The cones require bright light and can detect colour in detail. The rods can work in dim light, but can only detect in black and white, in low detail.
Neurones are specialized cells that carry messages around the body in the form of electrical charges. Sensory neurones carry electrical messages from the sense organs to the CNS. Motor neurones carry electrical signals from the CNS to the effectors, such as muscles and glands. Relay neurons relay messages between neurones in the CNS. Nerves are collections of thousands of neurones.
Neurones connect to each other through a synapse, which is a small junction where the message is transmitted through chemical signals (neurotransmitters) instead of electrical. The neurotransmitter is secreted from the synaptic knob of the first neurone and diffuses across the junction, where it stimulates a new impulse in the second neurone, before quickly being destroyed by enzymes.
In a reflex action, the message is passed straight from the sensory neurone to the motor neurone via a relay neurone. This does not require conscious thought and is very quick, which helps protect the body from damage. In a voluntary action, the message is relayed into the higher centres of the brain.

The Endocrine System

The endocrine system is a system of glands that secretes hormones into the bloodstream. Hormones are chemical messengers that travel in the bloodstream to their target organ.
The pituitary gland in the brain secretes anti-diuretic hormone (ADH), luteinising hormone (LH) and follicle stimulating hormone (FSH). The thyroid gland secretes thyroxine to control the metabolic rate. The adrenal gland secretes adrenaline for the “fight or flight” response. The pancreas secretes insulin to control blood glucose level.
Sex organs develop during puberty, and they start to produce sex cells. The ovary secretes oestrogen to control the menstrual cycle in females, and progesterone to maintain pregnancy. The testis secretes testosterone in males. These result in secondary sexual characteristics, such body hair.
The stimulation and release of egg cells is described by the 28-day menstrual cycle, which is regulated by FSH, LH, oestrogen and progesterone. FSH stimulates the ovum to ripen, which causes the release of oestrogen. Oestrogen promotes the growth of the uterus lining and stops the release of FSH, preventing more than one egg from being released at once. LH is released which stimulates ovulation, and the corpus luteum secretes progesterone to maintain the thick uterus lining.
After ovulation, if the egg is not fertilized, the corpus luteum breaks down and menstruation occurs. If the egg is fertilised, the embryo sends a message to the ovary stopping the corpus luteum from breaking down. Progesterone production carries on and menstruation does not occur.
Hormones can be used to increase or decrease fertility. The contraceptive pill contains oestrogen and progesterone to stop FSH from being released by the pituitary gland. On the other hand, FSH or similar hormones can be injected to try to stimulate the ovary to produce eggs (this may lead to multiple births).
Male sex hormones increase muscle growth and aggression. Athletes may illegally use these hormones, known as anabolic steroids, to improve their strength and performance.

Homeostasis

Homeostasis is maintaining a constant internal environment. This is regulated by both the nervous system and the endocrine system.
Insulin converts excess glucose into glycogen, to be stored in the liver. Diabetics do not produce enough insulin naturally, and require insulin injections in order to control the level of glucose in their blood.
Kidneys control waste and the amount of water in our bodies. The main waste product is urea, the waste material produced from the breakdown of proteins.
The kidney has thousands of fine tubules, called nephrons. Inside the Bowman’s capsule, small molecules such as water, glucose, salt and urea are squeezed out of the glomerulus (a network of capillaries) into the tubules by ultrafiltration. Glucose and most water is later reabsorbed, leaving behind a concentrated solution of salt and urea called urine. When we are thirsty, our body conserves water by secreting ADH from the pituitary gland. This hormone causes more water to be reabsorbed.
Our body has a constant temperature of 37 °C, the temperature at which our enzymes work best. We can produce heat from respiration. Core body temperature is monitored and controlled by the brain. Temperature receptors send nerve impulses to the skin, which help to regulate our body temperature.
When we feel hot, sweating cools the body because water evaporating from the skin absorbs heat energy from the body. Vasodilation is the expansion of blood capillaries near the skin surface, which allows more blood to flow near the surface of the skin, increasing the rate of heat loss.
When we feel cold, shivering (increases the rate of respiration and more energy is released as heat. Vasoconstriction narrows the blood vessels near the skin and the rate of heat loss is reduced.

Microbes, Food and Disease

Microbes reproduce quickly when they have sufficient food, water, and the right temperature and pH. Aerobic bacteria need oxygen, anaerobic bacteria do not.
Some microbes are very useful for the production of food. Yeast is used to make bread and wine, and bacteria is used to make yoghurt, cheese, vinegar.
Fermenters are used to grow a single type of microbe under carefully controlled conditions. The conditions must be sterile to kill unwanted microbes, with the ideal amount of temperature and food. Fermenters can be used to make mycoprotein, enzymes or drugs such as insulin.
Microbes can also be used to treat sewage to break down organic substances. Methane and carbon dioxide produced can be used for fuel or used to power machinery.
Most microbes are harmless, however, some microbes cause disease and are termed pathogens.
The HIV virus causes AIDS by infecting white blood cells. It is spread by bodily fluids.
Parasites are larger than microbes. The round worm lives in the gut of cats and dogs. The malaria parasite lives in the blood of mosquitoes. An organism that transmits a disease is called a vector.
Penicillin is an antibiotic, a drug that can be swallowed to kill bacteria. Antiseptics are chemicals that kill microbes on the skin’s surface. Painkillers help relieve pain, but they do not kill microbes.
Bacteria resistance to antibiotics spreads quickly due to their high rate of reproduction. It is important to avoid overuse of antibiotics to reduce the antibiotic resistance.
Bacteriophages are viruses that attack bacteria. They can potentially be used to kill pathogenic bacteria.
When a microbe (antigen) enters our body, our white blood cells make antibodies to destroy the antigen. Memory cells remain in the blood to produce more antibodies if the same antigen enters the body again.
Vaccines are dead or weakened bacteria or viruses that are introduced into our bodies. Our body makes antibodies to target the harmless antigen, so that when the real virulent pathogen enters our body, we have the memory cells and antibodies ready to destroy it before it can multiply and cause disease.
Active immunity is when we make our own antibodies to fight disease. Passive immunity is when antibodies are injected into a person or provided from mother to baby. This provides instant protection but is temporary because once the antibodies have gone, the person does not have memory cells for that antibody.
Vaccines do not always give long-term protection. Some viruses such as influenza and the common cold mutate rapidly and change their outer coat shape, so the old antibodies do not fit the shape of the virus.

Plant Biology

Photosynthesis is the process where plants make the food glucose, from carbon dioxide and water. The green pigment chlorophyll, found in chloroplasts, absorbs light energy for photosynthesis.
- 6CO2 + 6H2O → C6H12O6 + 6O2
Photosynthesis takes place mainly in the mesophyll (photosynthetic tissue) of the leaf. Chloroplasts are found mainly in the palisade layer, and the spongy layer contains air spaces to allow gas exchange.
Stomata are found on the lower surface of the leaf, and allow gas exchange. The vascular bundles contain xylem vessels, which transport water, and phloem vessels, which transport glucose.
Plants respire all the time, even during the day. Although photosynthesis does not occur during the night, it takes place much faster than respiration during the day.
Photosynthesis is controlled by light, the concentration of carbon dioxide and temperature. As light or carbon dioxide increases, so does the rate of photosynthesis, until the rate levels out at a new optimum level. The rate is then stable until the new limiting factor is removed. For temperature, any increase above the optimum level causes the rate to slow, as high temperature denatures the enzymes.
Glucose made in the chloroplasts can be used for respiration, converted into starch for storage, converted into cellulose for cell walls, used to make more chlorophyll, or converted into fats and protein.
For healthy growth, plants need minerals. Without nitrates for proteins, plants show stunted growth and yellow older leaves. Without phosphates for make DNA, cell membranes and chemical reactions, plants show purple younger leaves. Without potassium to help enzymes work, plants show yellow leaves with dead spots. Without magnesium for chlorophyll, plants show stunted growth with pale yellow leaves.
Plant hormones called auxins mediate plant responses. Phototropism – shoots grow towards light. Geotropism – shoots grow away from gravity, roots towards gravity. Hydrotropism – roots grow towards water. Auxins make cells grow longer. In shoots, auxin accumulates on the dark side of the soot. This causes the cells on the dark side to lengthen and the shoot bends toward the light.
Hormone rooting powder promotes the growth of roots in shoot cuttings. Unpollinated flowers can be treated to produce seedless fruits. Ripening of fruits can be slowed down in order to keep them fresh. Some weedkillers contain a synthetic hormone which causes broad leaf plants to overgrow and die.
Transpiration is the continuous movement of water up from the roots of the plants to the leaves. Water enters the roots through the root hairs by osmosis, and travels up the stem through the xylem. Xylem vessels are dead, hollow tubular cells joined together that have had the ends of the cells removed. Water reaches the leaf cells and evaporates, passing out through the stomata by diffusion.
Higher temperature increases the movement of water molecules out of the leaf. Higher humidity reduces the concentration gradient so water molecules leave the leaf slower. Wind blows away the water molecules near the stomata so that a large diffusion gradient is maintained.
Transpiration can pull water up trees 100 metres tall because of the cohesive and adhesive properties of water – water molecules stick to themselves and to the walls of the xylem. The evaporation of water from the leaves pulls the continuous column of water up from the roots of the plant.
Plants want to avoid losing excess water by transpiration. Stomata close at night to conserve water, and they are located on the underside of the leaf to protect them from sunlight and the wind. The top surface of the leaf is often covered with a water-impermeable, waxy layer.
Plants uptake minerals through the roots through diffusion or by active transport, which requires energy.
Plant cells are enclosed in a rigid cell wall. When water enters the plant by osmosis, the cell membrane is pushed hard against the cellulose wall, and the cell becomes turgid. This gives the plant support.
If water leaves a plant cell by osmosis, the cytoplasm shrinks and the cell becomes flaccid. Eventually, the plasma membrane detaches from the cell wall (plasmolysis). The cell collapses and the plant wilts.
The phloem is made of living cells that are joined to each other by holes that connect the cytoplasm together, forming a continuous system of living material to transport sugar and other nutrients.

Variation, Inheritance and Evolution

There is variation in members of a species. Some variation is genetic or inherited from our parents, while some is a result of our environment. Some traits, like weight or intelligence, are affected by both.
Variation can be continuous (showing a range of phenotypes, such as height) or discontinuous (phenotypes fall into distinct categories, such as blood group).
Sexual reproduction involves the fusion of gametes. Body cells have 46 chromosomes, or 23 pairs (diploid), while gametes have 23 chromosomes (haploid). A baby can receive either one of the 23 pairs from its mother and either one of the 23 pairs from its father, producing genetic diversity.
Mitosis produces genetically identical cells, or clones. Bacteria use mitosis as a form of reproduction (asexual reproduction). Only one parent is required and the offspring are clones of the parent.
The sex chromosomes are the 45th and 46th chromosomes in humans. Females are XX, males are XY.
Alleles are different versions of the same gene. Humans have two copies of each gene in their cells. During meiosis, the alleles of a gene separate into different gametes (law of segregation). During fertilization, different traits are inherited independently of each other (law of independent assortment). These laws were formulated by an Augustinian monk, Gregor Mendel.
The genotype of an organism is its particular combination of alleles. The phenotype is its appearance.
Since there can be more than one allele for a gene, an organism can have two of the same allele (homozygous) or have two different alleles (heterozygous). In the case of a heterozygous individual, the dominant allele is always expressed. A recessive allele is only expressed if no dominant allele is present. Dominant alleles are usually represented with a capital letter, and recessive alleles with small letters.
Monohybrid inheritance can be represented using a Punnett square. Two heterozygous individuals would produce offspring with a 3:1 ratio of phenotypes.
Some diseases can be inherited. Huntington’s chorea is a dominant disease of the nervous system. Cystic fibrosis is a recessive disease that affects the lungs and digestive system.
Down’s syndrome occurs when the ovum that is fertilised has an extra chromosome, because they did not divide properly during meiosis. Down’s syndrome individuals have 47 chromosomes.
Some plants can reproduce asexually, using runners (e.g. strawberry), bulbs (e.g. daffodil) or tubers (e.g. potato). Commercial growers can clone plants by taking cuttings. A modern way of cloning plants very quickly is to use micropropagation, which involves tissue culture of plant cells in a laboratory.
Selective breeding involves breeding individuals who have the desired characteristics. Farmers do this to produce larger chicken eggs, different breeds of dog, or crops with resistance to disease.
Charles Darwin developed the theory of evolution by natural selection. Organisms produce more offspring than can survive, and only those with characteristics beneficial to their environment will pass on their traits to their offspring. Evolution is supported by circumstantial evidence, like the fossil record.

Genetics and genetic engineering

DNA is a genetic code that consists of four bases – adenine, thymine, guanine and cytosine. Every three bases code for one amino acid, which join together to make proteins. One gene codes for one protein.
The structure of DNA is a double helix. The two strands are held together by A-T and G-C base pairing. The bases are held together by a ribose sugar attached to a phosphate molecule.
DNA copies itself just before the cell divides, so each new cell has an exact copy.
Instructions sent from DNA in the nucleus into the cytoplasm are carried by messenger RNA (mRNA). It is a single-stranded with thymine replaced by uracil. Each mRNA is a copy of a single gene.
Once the mRNA leaves the nucleus, it enters a ribosome where a protein is made from the encoded message. Transfer RNA (tRNA) is responsible for bringing the correct amino acid to the ribosome.
Scientists have decoded all the bases in the human DNA, so we now have a genetic map of all genes.
Mutation is a change in DNA, that can be caused by radiation (including X-rays and UV light from the sun) and chemical mutagens (as in cigarettes). If a mutation occurs in a gamete, the offspring may develop abnormally and pass on the mutation to their own offspring. If the mutation occurs in a body cell, it could start to multiply uncontrollably, known as cancer.
Using in vitro fertilisation (IVF), we can identify genetic diseases in the embryo before the baby is born. A cell is removed from the embryo for genetic screening. A gene probe is attached to the marker, and the probe attaches itself to the faulty gene if present, indicating that the embryo has the genetic disease.
The genetic code is universal in all living organisms, so a gene from one organism can be placed into another where it will continue to carry out its function. The gene is cut open using restriction enzymes and then inserted into the new host organism, then DNA ligase is used to join the DNA back together.
While genetic engineering has benefits, like better food and cure for diseases, some people think it is “against God and Nature” and potentially dangerous.
90% of all our DNA is “junk” DNA that does not code for proteins, and it usually consists of repetitive short sequences. However, junk DNA is useful for making genetic fingerprints, as each individual has a unique pattern of junk DNA. DNA from an individual can be cut into small fragments using restriction enzymes, and the fragments can be analyzed using a gel with an electric field.
Animals can be cloned by splitting apart cells from an embryo and implanting the new embryos into different host mothers (embryo transplants) or replacing the nucleus of an ovum with a nucleus from another sheep, and then implanting into a host mother (nuclear transplant).
Cloning can be used to save species that are close to extinction.

Ecology

A habitat is a place where an organism lives. All the organisms in a habitat form a community.
Plants compete for light, water and minerals. Animals compete for food, water, mates and territory.
Predators kill and eat other animals called prey. Parasites feed off a living organism called the host.
Organism are adapted to their environment. For example, polar bears have a large body, thick insulating fur and a thick layer of fat under the skin to conserve heat.
Mutualism is when two organisms cooperate together. Lichens consist of algae and fungi.
The human population has undergone a population explosion in the last century. There has been a greater demand on land for space, resources, and food. Many other organisms have decreased in number.
Carbon dioxide and methane causes the greenhouse effect. Fertilisers and sewage cause eutrophication. Heavy metals, herbicides and carbon monoxide are poisonous. Sulphur dioxide and nitrogen oxides cause acid rain. Smoke causes smog and lung problems.
The greenhouse effect due to gases in the atmosphere that trap heat rays that are emitted from the earth.
Acid rain decreases pH in soil and natural waters, killing fish and trees.
Eutrophication caused by fertilisers or sewage that promote excess growth and death of algae and plants. Decomposing bacteria then use up all the oxygen in the water, causing all other aquatic life to die.
Overexploitation can happen to resources, plants or animals that are consumed for human uses. Many animals have become extinct forever. Deforestation leads to loss of habitat, and increased soil erosion. Fossil fuels are non-renewable and will eventually run out.
Loss of organisms can have unexpected effects on the environment, such as deforestation or increased numbers of pests. Some organisms may prove to be useful in the future for drugs or breeding.
Conservation is trying to keep all species of organisms alive. Sustainable growth means using resources in an environmentally friendly way.
A food chain shows how food passes through a community of organisms. Pyramids of numbers, biomass or energy can be used to quantify a particular food chain. These are shaped like pyramids because energy is lost at each level due to incomplete consumption or digestion, waste and heat loss from respiration.
Fertilisers and pesticides are used in intensive farming to get maximum food production. Animals are kept in warm conditions indoors with little space, so less energy is lost by respiration.
Decomposes break down dead animal and plant material, releasing nutrients back into the ecosystem.
In the nitrogen cycle, nitrifying bacteria convert ammonium compounds to nitrates using oxygen and denitrifying bacteria convert nitrates into nitrogen gas without oxygen. Nitrogen fixing bacteria live in the soil or in the roots of pea and bean plants, and convert nitrogen gas into useful nitrogen compounds.