Animal and plant cells share a nucleus, cytoplasm and cell membrane. Plant cells add a cell wall (made of cellulose), a permanent vacuole and chloroplasts. Bacterial cells are much simpler: no nucleus (instead a single loop of DNA plus small rings called plasmids), no membrane-bound organelles, and a cell wall not made of cellulose.
Magnification = image size divided by actual size. Learn the equation triangle and always convert units to the same size first (1mm = 1000 micrometres). Light microscopes magnify up to about x1500 with a resolution around 200nm. Electron microscopes magnify up to about x2000000 with a much finer resolution, around 0.1nm, which is why they reveal internal organelle detail light microscopes cannot.
Stem cells are undifferentiated and can divide to become specialised cells. Embryonic stem cells can become almost any cell type; adult stem cells (e.g. bone marrow) are more limited. Specialised cells include sperm cells (tail for swimming, many mitochondria for energy), root hair cells (large surface area for water uptake) and nerve cells (long axon, branched endings).
The cell cycle has growth/DNA-copying stages followed by mitosis, which produces two genetically identical daughter cells for growth and repair. Chromosomes are copied before the cell splits, so each new cell gets a full identical set.
There are three key processes and common mistakes trip people up on all three.
As an organism or cell gets bigger, its surface area to volume ratio gets smaller, making diffusion alone too slow to supply the whole volume. This is why larger organisms need specialised exchange surfaces (like alveoli, gills, root hair cells and villi) which all share features: large surface area, thin walls (short diffusion distance), good blood supply or ventilation to maintain a steep concentration gradient.
Common mistakes: mixing up diffusion and osmosis (osmosis is only about water); saying active transport happens 'because there is a gradient' rather than against one; forgetting active transport needs energy from respiration; and confusing magnification with resolution (magnification makes things look bigger, resolution is about seeing fine detail).
The digestive system is a muscular tube (alimentary canal) plus glands that break down food into small, soluble molecules the body can absorb. Organs work together as a system: mouth, oesophagus, stomach, small intestine, large intestine, plus the liver, gall bladder and pancreas.
Cells form tissues (e.g. muscular tissue, glandular tissue, epithelial tissue). Tissues form organs (e.g. the stomach). Organs form organ systems (the digestive system) which work together in the organism.
Digestion uses enzymes to speed up the breakdown of large insoluble molecules into small soluble ones.
Enzymes work by a 'lock and key' mechanism: the substrate fits the enzyme's active site exactly. Each enzyme has an optimum temperature and pH. Above the optimum temperature the enzyme denatures - its active site changes shape and the substrate no longer fits, so the reaction stops permanently. This is a common exam trap: denatured enzymes do NOT work again on cooling.
The small intestine is lined with villi, adapted for efficient absorption:
Enzyme reaction rate can be investigated using amylase and starch with iodine, timing how long it takes for the iodine to stop turning blue-black.
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 can produce toxins that damage tissues and make you feel ill.
Common mistake: pathogens spread in different ways, so prevention methods differ. Learn them by transmission route:
The body has non-specific barriers that stop pathogens entering:
If a pathogen gets past these, white blood cells respond by phagocytosis (engulfing pathogens), producing antibodies (specific to the antigens on that pathogen) and producing antitoxins (neutralise toxins produced by pathogens).
Vaccination involves injecting small quantities of dead or inactive pathogen. This carries antigens that cause white blood cells to produce antibodies. If the same live pathogen appears later, white blood cells respond quickly to produce antibodies fast enough to prevent infection, because memory cells recognise the antigen. Herd immunity happens when enough of a population is vaccinated to protect unvaccinated individuals too.
Common mistake: confusing preclinical and clinical trial stages. Preclinical testing uses cells, tissues and live animals in a laboratory. Clinical trials then use healthy volunteers and patients, starting with very low doses. In double-blind trials neither doctor nor patient knows who has the drug or a placebo, to remove bias.
Photosynthesis is an endothermic reaction where light energy is transferred to plants and algae to build glucose.
Three factors can limit the rate of photosynthesis: light intensity, carbon dioxide concentration and temperature.
Glucose made in photosynthesis is used for respiration, converted to insoluble starch for storage, built into cellulose for cell walls, or turned into lipids and proteins for growth (using nitrate ions from the soil).
Respiration is an exothermic reaction that goes on continuously in every living cell, releasing energy for movement, warmth, growth and chemical reactions.
Metabolism is the sum of all reactions in a cell or body, including respiration, protein synthesis and breakdown of excess proteins into urea.
Homeostasis is the regulation of internal conditions to maintain a stable internal environment, in response to internal and external changes. This keeps enzymes and cells working properly.
The gap between two neurones is a synapse. Chemical transmitter substances diffuse across the gap, which triggers a new electrical impulse in the next neurone. This is why nerve signals only travel one way.
Reflexes are rapid, automatic responses that do not involve conscious thought, protecting the body from harm.
Order: stimulus - receptor - sensory neurone - relay neurone (spinal cord) - motor neurone - effector - response
Common mistake: students often say the signal goes to the brain first. In a reflex it does NOT - it bypasses conscious brain processing via the spinal cord, which is why reflexes are so fast.
The thermoregulatory centre in the brain monitors blood temperature and receives input from skin temperature receptors. Normal core body temperature is about 37 degrees C.
DNA is a polymer made of two strands forming a double helix, held together by complementary base pairing: A-T and C-G.
A gene is a small section of DNA that codes for a specific protein (a sequence of amino acids).
Most human cells contain 23 pairs of chromosomes (46 total) in the nucleus. Gametes (sperm and egg) contain 23 chromosomes, one from each pair.
An allele is a different version of a gene. Genotype is the genetic makeup (e.g. Bb); phenotype is the physical characteristic shown.
Dominant alleles are shown with a capital letter and always show up in the phenotype if present. Recessive alleles use a lowercase letter and only show if two copies are present (homozygous recessive).
Homozygous means two identical alleles (BB or bb); heterozygous means two different alleles (Bb).
Use a Punnett square to predict offspring ratios, e.g. Bb x Bb gives a 3:1 ratio of dominant to recessive phenotype, and Bb x bb gives 1:1.
Sex is determined by chromosomes: XX is female, XY is male. There is a 50% chance of each at each pregnancy, independent of previous births.
Common mistake: ratios from a Punnett square are probabilities, not guarantees for a small number of offspring.
Variation between individuals can be genetic (inherited), environmental (caused by conditions), or a combination of both.
Mutations are random changes to the DNA base sequence. Most have no effect on the protein made, some are harmful, and rarely one may be beneficial.
Continuous variation (e.g. height, mass) shows a range of values and is usually controlled by many genes plus environment. Discontinuous variation (e.g. blood group) falls into distinct categories.
Natural selection: organisms with characteristics best suited to the environment are more likely to survive and reproduce, passing on their alleles.
Over many generations this can lead to a new species forming (speciation) if populations become isolated and can no longer interbreed.
Selective breeding is when humans choose parents with desired characteristics to breed together, repeated over generations, reducing the gene pool and variation.
An ecosystem is all the organisms living in an area plus the non-living (abiotic) conditions, such as light, temperature, and pH.
A community is all the different species living in a habitat. A population is all the organisms of one species in a habitat.
Organisms compete for resources: plants compete for light, water, space and mineral ions; animals compete for food, mates and territory.
Energy flows through a food chain from producers (plants, via photosynthesis) to primary, secondary and tertiary consumers. Only around 10% of energy transfers to the next trophic level; the rest is lost as heat, movement, egestion and excretion.
Use quadrats to estimate population size and distribution, and transects to study how species distribution changes across an area (e.g. with distance from a factor like light or a shoreline).
Don't confuse gene and allele: a gene is the section of DNA; an allele is a version of that gene.
Don't say a trait is caused only by genes or only by environment when the question implies both (continuous variation).
Always show full genetic diagrams (alleles for parents, gametes, and offspring grid) for full marks.