Biology · 30 pages
Biology explainers
Plain-language biology for students and the curious — the cell and its parts, how living things release and capture energy, the basics of inheritance, and how organisms fit together in ecosystems. Each page opens with a concise definition, then sets out key facts and clears up the misconceptions people most often meet. Where a topic has a deeper molecular definition, we link across to the CASRAI life-sciences pillar and the standards dictionary.
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Mitosis vs meiosis
The difference is that mitosis is cell division that makes two genetically identical body cells, used for growth, repair and replacing worn-out cells. Meiosis is cell division that makes four genetically varied sex cells (gametes), each with half the usual number of chromosomes. Mitosis keeps the chromosome number the same; meiosis halves it so that fertilisation restores the full set.
ComparisonGenotype vs phenotype
The difference is that the genotype is the set of genes an organism carries for a trait. The phenotype is the observable characteristic that results — what you can actually see, measure or test, such as eye colour or height. The phenotype comes from the genotype interacting with the environment.
ComparisonOsmosis vs diffusion
The difference is that diffusion is the net movement of any particles — gases or dissolved substances — from a higher to a lower concentration. Osmosis is a special kind of diffusion: the movement of water across a partially permeable membrane, from a dilute solution to a more concentrated one. Both are passive.
ComparisonDominant vs recessive
The difference is that a dominant allele produces its trait whenever it is present, even as a single copy, masking the other allele. A recessive allele only produces its trait when both copies are recessive, because a single dominant allele would override it. Dominant alleles are usually written with a capital letter (B) and recessive with a lower-case letter (b).
ComparisonPlant vs animal cell
The difference is that plant cells have three extra structures that animal cells lack: a rigid cell wall for support, chloroplasts for photosynthesis, and a large permanent vacuole that keeps the cell firm. Both are eukaryotic and share a nucleus, cell membrane, cytoplasm and mitochondria as their common core.
ComparisonProkaryotic vs eukaryotic
The difference is that prokaryotic cells, such as bacteria, have no nucleus — their genetic material floats freely in the cytoplasm — and they lack membrane-bound organelles. Eukaryotic cells, found in animals, plants, fungi and protists, keep their DNA inside a nucleus and contain organelles like mitochondria. Prokaryotic cells are also much smaller and simpler than eukaryotic cells.
ComparisonDNA vs RNA
The difference is that DNA is the long-term store of genetic instructions, made of two strands twisted into a double helix and kept in the nucleus. RNA is a single-stranded copy that carries those instructions out to where proteins are made. DNA uses thymine and deoxyribose; RNA uses uracil and ribose.
DefinitionMitochondria
Mitochondria are tiny organelles inside cells where most aerobic respiration takes place, releasing energy from glucose for the cell to use. Often called the powerhouse of the cell, they transfer energy into a usable form (ATP). Cells that need a lot of energy, such as muscle and nerve cells, contain many mitochondria. Each one is enclosed by a double membrane.
DefinitionAnimal cell
An animal cell is the basic eukaryotic cell that makes up the bodies of animals. It is surrounded by a flexible cell membrane and contains a nucleus that holds the genetic material, cytoplasm where reactions occur, ribosomes that build proteins, and mitochondria that release energy. Unlike plant cells, animal cells have no cell wall, no chloroplasts and no large vacuole.
DefinitionPlant cell
A plant cell is the basic eukaryotic cell that makes up plants. As well as the nucleus, cell membrane, cytoplasm and mitochondria found in animal cells, it has three extra parts: a rigid cell wall for support, chloroplasts that carry out photosynthesis, and a large permanent vacuole filled with cell sap that keeps the cell firm.
DefinitionCellular respiration
Cellular respiration is the process by which cells release energy from glucose so the organism can use it for movement, growth and warmth. Aerobic respiration uses oxygen and releases the most energy, producing carbon dioxide and water; anaerobic respiration works without oxygen and releases less. It is not the same as breathing.
DefinitionPhotosynthesis
Photosynthesis is the process that green plants and algae use to make their own food. They use light energy, usually from the Sun, to turn carbon dioxide and water into glucose, releasing oxygen as a by-product. The word equation is: carbon dioxide + water → glucose + oxygen. It happens in chloroplasts.
DefinitionFood chain
A food chain is a simple diagram showing how energy passes from one organism to the next as food. It starts with a producer, usually a green plant that makes food by photosynthesis, and moves along to consumers that eat it. The arrows point the way energy flows — from the organism eaten to the one eating it.
DefinitionFood web
A food web is a network of many connected food chains, showing how the organisms in an ecosystem feed on one another. Because most animals eat more than one food and are eaten by more than one predator, their food chains overlap into a web. This gives a fuller, more realistic picture than a single food chain.
DefinitionMeiosis
Meiosis is a type of cell division that makes sex cells, or gametes, for sexual reproduction. It produces four cells, each genetically different and each with half the chromosome number of the parent. Halving the chromosomes means the full number is restored at fertilisation. Meiosis also creates genetic variation.
DefinitionMitosis
Mitosis is the type of cell division that produces two new cells genetically identical to the parent cell, each with the full set of chromosomes. It is used for growth, for repairing damaged tissue, for replacing old or worn-out cells, and for asexual reproduction. Before mitosis, the cell copies its DNA so that each new cell receives a complete copy.
DefinitionProkaryotic cell
A prokaryotic cell is a simple type of cell that has no nucleus, so its genetic material floats freely in the cytoplasm, and no membrane-bound organelles such as mitochondria. Bacteria and archaea are prokaryotic. These cells are much smaller and simpler than eukaryotic cells, and they are almost always single-celled. Their DNA is usually a single circular loop.
DefinitionEukaryotic cell
A eukaryotic cell is a complex cell that holds its genetic material inside a membrane-bound nucleus and contains organelles such as mitochondria. Animals, plants, fungi and protists are all made of eukaryotic cells. They are larger and more complex than prokaryotic cells, and they can form single-celled organisms or multicellular bodies like a plant or a human.
DefinitionTrophic levels
Trophic levels are the steps, or feeding positions, in a food chain. The first level is the producers (plants), which make their own food. Above them come primary consumers (herbivores), then secondary consumers and top predators. Only a small part of the energy passes from one level to the next, so they form a pyramid.
DefinitionGenotype
A genotype is the set of alleles, or genetic instructions, that an organism carries for a particular trait. It is the underlying genetic make-up, inherited from the parents, that helps determine an organism’s characteristics. Genotypes are usually written with letters, such as BB, Bb or bb. With the environment, it produces the phenotype.
DefinitionPhenotype
A phenotype is an organism’s observable characteristics — the traits you can actually see, measure or test, such as eye colour, height or blood group. It is produced by the genotype (the genes an organism carries) interacting with the environment. Because of this, the same genotype can give different phenotypes.
DefinitionKrebs cycle
The Krebs cycle is a stage of aerobic respiration that takes place in the mitochondria. It is a cycle of chemical reactions that breaks down products of glucose, releasing carbon dioxide and energy-carrying molecules used to make most of the cell’s ATP. Also called the citric acid cycle, it follows glycolysis and needs oxygen.
DefinitionGlycolysis
Glycolysis is the first stage of cellular respiration, in which a glucose molecule is split into two smaller molecules in the cytoplasm. It releases a small amount of energy and does not need oxygen, so it is the starting point for both aerobic and anaerobic respiration. Its products then pass into the mitochondria.
DefinitionOsmosis
Osmosis is the movement of water molecules across a partially permeable membrane, from a region with more water (a dilute solution) to a region with less water (a concentrated solution). It is passive, needing no energy, and continues until the concentrations are balanced. It is how plant roots absorb water.
DefinitionDiffusion
Diffusion is the net movement of particles — such as gases or dissolved substances — from a region where they are more concentrated to one where they are less concentrated, until they are evenly spread. It is a passive process that needs no energy. It is how oxygen reaches cells and carbon dioxide leaves them.
DefinitionNucleus
The nucleus is the control centre of a eukaryotic cell. It holds the cell’s genetic material, the DNA, organised into chromosomes, and directs the cell’s activities by controlling which proteins are made. It is surrounded by a membrane. Animal and plant cells have a nucleus; prokaryotic cells, such as bacteria, do not.
DefinitionCell membrane
The cell membrane is the thin, flexible layer that surrounds every cell and forms its boundary. It controls what enters and leaves, letting some substances through while blocking others — which is why it is described as partially permeable. All cells have a membrane; in plant cells it lies just inside the cell wall.
DefinitionEcosystem
An ecosystem is all the living organisms in an area together with the non-living parts of their environment — such as water, air, soil, light and temperature — interacting as a system. The living and non-living parts depend on one another. Ecosystems range in size from a single pond to a whole forest.
DefinitionNatural selection
Natural selection is the process by which organisms with characteristics that suit their environment are more likely to survive and reproduce, passing those helpful characteristics on to their offspring. Over many generations this gradually changes a species. It is the main mechanism of evolution — "survival of the fittest".
DefinitionChloroplast
A chloroplast is an organelle found in plant cells and algae where photosynthesis takes place. It contains chlorophyll, the green pigment that absorbs light energy and gives plants their colour. Using that light energy, chloroplasts turn carbon dioxide and water into glucose. Chloroplasts are found mainly in the cells of leaves and green stems, not in animal cells.
