All living things are made of cells. Animal cells have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells have all of these plus a cell wall (made of cellulose), a permanent vacuole (filled with cell sap) and chloroplasts (for photosynthesis).
Bacterial cells are prokaryotic: no nucleus, instead a single circular loop of DNA plus small rings called plasmids. They are much smaller than animal or plant (eukaryotic) cells.
Cells differentiate to become specialised for a job, e.g. sperm cells (tail for swimming), root hair cells (large surface area for water uptake), nerve cells (long, for fast signals), muscle cells (packed with mitochondria for energy).
Light microscopes magnify up to about x2000; electron microscopes give much higher magnification and resolution (down to nanometre scale), so they reveal smaller structures like ribosomes and mitochondria detail.
Mitosis produces two genetically identical body cells for growth and repair. The cell cycle has growth (DNA duplicates), then division into two daughter cells. Stem cells (embryonic and adult) can differentiate into different cell types - used in treating conditions like paralysis and diabetes, but embryonic stem cell use raises ethical debate.
Three key processes move substances across cell membranes:
Cells are organised into a hierarchy: cells > tissues > organs > organ systems > organism. A tissue is a group of similar cells with the same function (eg muscular tissue, glandular tissue, epithelial tissue). An organ is a group of different tissues working together (eg the stomach contains muscular, glandular and epithelial tissue). Organ systems work together for the whole organism, eg the digestive system.
It is one long tube (the alimentary canal) plus glands that add digestive juices. Key organs and their jobs:
Enzymes are biological catalysts made of protein. Each has an active site that fits one specific substrate (the 'lock and key' model). High temperature or wrong pH changes the active site shape - the enzyme is denatured and stops working.
Bile is made in the liver, stored in the gall bladder, and released into the small intestine. It is alkaline, so it neutralises stomach acid to give the right pH for intestinal enzymes to work. It also emulsifies fat - breaking it into tiny droplets to increase surface area for lipase to act faster.
Don't say the stomach 'digests food with acid' - the acid itself doesn't digest, it creates the right pH for protease and kills microbes. Don't confuse bile with an enzyme - it is not an enzyme, it never breaks bonds, it only emulsifies fat and neutralises acid. Always give units and specifics in exam answers, eg name the actual enzyme and organ, not just 'digestive juice'.
A pathogen is a microorganism that causes disease. The four types are bacteria, viruses, fungi and protists. Bacteria and viruses reproduce rapidly inside the body and make you feel ill by damaging cells or producing toxins.
Know these named examples: measles (virus, red skin rash, spread by droplets), HIV (virus, attacks the immune system, spread by sexual contact or exchange of body fluids, can lead to AIDS), tobacco mosaic virus (virus, discolours leaves in plants), salmonella (bacteria, food poisoning, spread by eating contaminated food, controlled by vaccinating poultry), gonorrhoea (bacteria, STI, spread by sexual contact, causes pain and discharge, treated with antibiotics but resistance is increasing), rose black spot (fungus, purple/black spots on leaves, spread by water and wind, treated with fungicide and removing affected leaves), and malaria (protist, spread by mosquito vectors, causes fevers, controlled by preventing mosquito bites and destroying breeding sites).
White blood cells defend the body in three main ways: phagocytosis (engulfing and digesting pathogens), producing antibodies (specific to each antigen, causing pathogens to clump so they can be destroyed), and producing antitoxins (neutralise toxins produced by pathogens).
A vaccine contains small quantities of dead or inactive pathogen. This stimulates white blood cells to produce antibodies. If the same pathogen enters again, memory cells trigger a faster, stronger secondary response before you get ill. Widespread vaccination gives herd immunity, reducing disease spread even among unvaccinated people.
New drugs are tested for toxicity, efficacy and dosage. Preclinical testing happens on cells, tissues and live animals; clinical trials then test on healthy volunteers at low doses before testing on patients. Double-blind trials use a placebo so neither patient nor doctor knows who received the real drug, removing bias.
Photosynthesis is an endothermic reaction (it takes in energy) where light energy is transformed into chemical energy stored in glucose.
The word equation is: carbon dioxide + water --light--> glucose + oxygen. Learn this equation exactly - it comes up constantly.
It happens in chloroplasts, which contain chlorophyll (the green pigment that absorbs light energy).
Four things can limit the rate of photosynthesis: light intensity, carbon dioxide concentration, temperature, and the amount of chlorophyll.
Glucose made in photosynthesis is used for: respiration; making cellulose for cell walls; making amino acids (combined with nitrate ions) for proteins; stored as insoluble starch; and converted to lipids for storage in seeds.
Respiration is exothermic (releases energy) and happens continuously in every living cell.
Aerobic respiration (needs oxygen): glucose + oxygen --> carbon dioxide + water. This releases far more energy per glucose molecule than anaerobic respiration.
Anaerobic respiration in animals (no oxygen, e.g. during vigorous exercise): glucose --> lactic acid. No carbon dioxide is produced. This causes oxygen debt and muscle fatigue.
Anaerobic respiration in plants and yeast (fermentation): glucose --> ethanol + carbon dioxide. Yeast fermentation is used commercially in brewing and bread-making.
Metabolism is the sum of all the reactions in a cell or body, and respiration is central to it because it releases the energy needed to drive all other reactions, including building larger molecules from smaller ones.
Homeostasis is the regulation of internal conditions to maintain a stable internal environment, in response to internal and external changes.
It keeps things like body temperature, blood glucose, water levels and blood pH within a narrow range so cells work properly.
The nervous system uses electrical impulses for very fast responses.
A synapse is the tiny gap between two neurones. Common mistake: signals cross a synapse using a chemical (neurotransmitter) not an electrical spark.
The neurotransmitter diffuses across the gap and sets off a new electrical impulse in the next neurone.
A reflex is an automatic, rapid response that does not involve conscious thought, protecting the body from harm.
Required practical: use the ruler-drop test to measure human reaction time.
One person drops a ruler through a partner's fingers without warning; the distance it falls before being caught converts to a reaction time.
Most characteristics are controlled by genes, found on chromosomes in the nucleus. A gene is a small section of DNA that codes for a particular protein. Different forms of the same gene are called alleles.
Humans have 23 pairs of chromosomes (46 in total) in most body cells. One chromosome in each pair comes from each parent. Gametes (sperm and egg) contain only 23 chromosomes, produced by meiosis.
Use Punnett squares to predict the ratio of offspring genotypes and phenotypes from a cross. A common mistake is forgetting that ratios are probabilities, not guarantees, for each individual offspring.
Biological sex is determined by one pair of chromosomes: XX in females, XY in males. There is a 50:50 chance of having a boy or girl at each pregnancy.
Polydactyly (extra fingers or toes) is caused by a dominant allele. Cystic fibrosis (thick mucus in lungs and pancreas) is caused by a recessive allele, so both parents can be unaffected carriers.
Embryo screening can test for genetic disorders before implantation (IVF) or during pregnancy, but raises ethical issues around cost, choice and 'designer babies'.
Variation comes from genetic causes (differences inherited from parents), environmental causes (e.g. diet, climate), or a combination of both. Mutations are rare, random changes to DNA; most have no effect, some are harmful, and very occasionally one is beneficial.
Evolution by natural selection: individuals show variation, those with characteristics best suited to the environment are more likely to survive and reproduce, passing on the advantageous alleles. Over many generations, this can lead to a new species.
Antibiotic resistance in bacteria is a key example of natural selection observed today, which is why finishing a full course of antibiotics matters.
A community is all the populations of different species living in a habitat. Organisms are adapted (structurally, behaviourally, functionally) to survive in their environment. Interdependence means organisms in a community rely on each other for food, shelter and pollination, forming stable communities.
Abiotic factors (light, temperature, water, soil) and biotic factors (predators, food, disease) both affect distribution of organisms. Quadrats and transects are used to sample and estimate population size and distribution.
Energy is lost at each stage of a food chain (as heat, movement, waste), so pyramids of biomass get smaller at each trophic level, and food chains rarely have more than 4-5 links.