34 terms in 3.2
All cells arise from other cells
In a multicellular organism, most cells lose the ability to divide once they specialise. Only certain cell types keep that ability throughout life.
Cells
All cells arise from other cells
Some eukaryotic cells — cells that contain a nucleus — repeatedly follow a fixed sequence of stages called the cell cycle. Each cycle ends with the cell dividing to produce new cells.
Cells
All cells arise from other cells
Before a cell divides, it must copy all of its DNA. This copying happens during interphase — the preparation stage of the cell cycle.
Cells
All cells arise from other cells
Mitosis is a type of cell division. It splits one cell into two new cells, and each new cell carries an exact copy of the original cell's DNA.
Cells
All cells arise from other cells
Mitosis splits one cell into two identical cells. Chromosomes go through five named stages as this happens. Each stage has a distinct, testable behaviour.
Cells
All cells arise from other cells
During cell division, protein fibres called spindle fibres attach to chromosomes and pull the two identical copies apart. Each copy moves to opposite ends of the cell.
Cells
All cells arise from other cells
After a cell copies and separates its DNA, cytokinesis splits the cytoplasm to produce two new daughter cells. The word 'usually' matters — the process can occasionally fail.
Cells
All cells arise from other cells
Normally, cells divide in a tightly controlled way. When that control breaks down, cells divide without stopping — forming a tumour, which can become cancer.
Cells
All cells arise from other cells
Cancer cells divide too fast and without control. Many treatments work by targeting specific stages of cell division to slow or stop this process.
Cells
All cells arise from other cells
Bacteria divide by splitting in two — a process called binary fission. The cell copies its DNA first, then splits its cytoplasm to make two new daughter cells.
Cells
All cells arise from other cells
Viruses are not alive, so they cannot divide like cells do. Instead, a virus injects its genetic material into a host cell and forces that cell to make new virus copies.
Cells
All cells arise from other cells
Meiosis is a different type of cell division from mitosis. AQA covers it separately in section 3.4.3, not here.
Cells
All cells arise from other cells
New cells can only come from pre-existing cells, and in eukaryotes (organisms whose cells have a nucleus) this happens through a tightly regulated sequence of events called the cell cycle, which culminates in mitosis — a type of cell division that produces two genetically identical daughter cells. Prokaryotes (cells without a nucleus, such as bacteria) divide by a simpler process called binary fission, while viruses bypass cell division entirely by hijacking a host cell's machinery to copy themselves. Understanding how cell division is controlled matters beyond the basics: when that control breaks down, cells can divide uncontrollably, leading to tumours and cancer — which is why many cancer treatments specifically target the cell cycle.
Cells
Cell recognition and the immune system
The surface of every cell is studded with molecules — mostly glycoproteins (proteins with sugar chains attached) and glycolipids — that act as chemical identity markers. Your immune system continuously scans these markers and compares them against a reference set of "self" molecules established early in development. This scanning system allows the immune system to recognise four distinct categori
Cells
Cell recognition and the immune system
An antigen is a molecule on a cell's surface that the immune system can recognise as foreign. When antigens change shape, the immune system can no longer recognise them, making diseases harder to prevent.
Cells
Cell recognition and the immune system
Phagocytes are white blood cells that engulf pathogens whole. They then use digestive enzymes called lysozymes to break the pathogen apart and destroy it.
Cells
Cell recognition and the immune system
When a pathogen invades, specialised white blood cells called helper T cells act as coordinators. They receive a signal from antigen-presenting cells and then activate the immune system's other defenders.
Cells
Cell recognition and the immune system
The humoral response is the antibody-based branch of the adaptive immune system. It targets antigens (foreign molecules, usually proteins) that are free in body fluids or on pathogen surfaces. The sequence of events runs as follows: 1. A B lymphocyte with a complementary receptor binds to a specific foreign antigen. Helper T cells (TH) release cytokines that activate this B cell. 2. Clonal selec
Cells
Cell recognition and the immune system
A vaccine trains your immune system to fight a specific pathogen without you getting ill. When enough people in a population are vaccinated, even unvaccinated individuals gain protection — this is herd immunity.
Cells
Cell recognition and the immune system
Active immunity means your body makes its own antibodies after meeting an antigen. Passive immunity means your body receives ready-made antibodies from an outside source.
Cells
Cell recognition and the immune system
HIV is a virus that invades helper T cells — the white blood cells that coordinate your immune response. It hijacks those cells to copy itself, gradually destroying your immune system.
Cells
Cell recognition and the immune system
HIV destroys the immune cells that coordinate your body's defences. Over time, this leaves the body unable to fight off infections that a healthy person would easily survive.
Cells
Cell recognition and the immune system
Scientists engineer identical antibodies that bind to one specific target on a cell. Doctors use these monoclonal antibodies to deliver drugs directly to diseased cells or to detect specific substances in the body.
Cells
Cell recognition and the immune system
AQA does not ask you how factories or laboratories make monoclonal antibodies. You only need to know what they do and how they are used.
Cells
Cell recognition and the immune system
Both vaccines and monoclonal antibodies raise ethical debates about safety, consent, animal use, and fair access. Scientists and society must weigh these benefits against the risks and concerns.
Cells
Cell recognition and the immune system
The ELISA test uses antibodies to detect whether a specific protein or pathogen is present in a sample. A colour change signals a positive result.
Cells
Cell recognition and the immune system
Your body constantly distinguishes between its own healthy cells and anything foreign or dangerous, using surface molecules called antigens — proteins or glycoproteins on a cell's surface that the immune system can recognise. When a pathogen (a disease-causing organism) enters the body, a coordinated immune response is triggered, involving specialised white blood cells such as T lymphocytes and B lymphocytes, which work together to destroy the threat and build immunological memory. Understanding this system explains how vaccines protect populations, why HIV is so damaging, and how monoclonal antibodies — laboratory-produced proteins designed to bind to a single specific target — are used in medicine and diagnosis.
Cells
Cell structure
All living organisms are built from cells, and understanding how those cells are organised internally is the foundation of the entire A-level course. Eukaryotic cells — cells that contain a membrane-bound nucleus, found in animals, plants and fungi — contain a range of specialised organelles (small structures within the cell, each with a specific function), whereas prokaryotic cells, such as bacteria, are structurally simpler and lack a true nucleus. Knowing what each organelle does, and how eukaryotic and prokaryotic cells differ, directly underpins everything that follows: how cells divide, how substances move in and out, and how the immune system recognises threats.
Cells
Transport across cell membranes
Every cell membrane — whether surrounding a cell or enclosing an organelle — shares the same basic structure. A double layer of fat-based molecules forms the core, with proteins embedded throughout.
Cells
Transport across cell membranes
The fluid-mosaic model describes the cell membrane as a flexible, constantly moving double layer of fat-based molecules. Proteins, glycoproteins, and glycolipids sit within or on this layer, each doing a different job.
Cells
Transport across cell membranes
Cholesterol molecules sit between the fatty acid tails of phospholipids in the membrane. They hold the tails together and reduce how freely the membrane can move.
Cells
Transport across cell membranes
The five transport mechanisms each solve a different problem. **Simple diffusion** — small, non-polar (uncharged, fat-soluble) molecules such as oxygen and CO₂ dissolve directly through the phospholipid bilayer, moving down their concentration gradient. Larger or charged molecules cannot cross this way; the hydrophobic (water-repelling) core of the bilayer blocks them. **Facilitated diffusion**
Cells
Transport across cell membranes
Some cells need to move substances very quickly. They do this by having a larger membrane surface area or more transport proteins embedded in the membrane.
Cells
Transport across cell membranes
Cell membranes are selectively permeable barriers — meaning they control which substances can enter and leave the cell — and understanding how molecules cross them is central to almost every process in biology. The fluid-mosaic model describes the membrane as a flexible phospholipid bilayer (a double layer of fat-based molecules) embedded with proteins, glycoproteins, and cholesterol, each playing a specific role in regulating transport. Substances move across membranes by several distinct mechanisms — including simple diffusion, facilitated diffusion, osmosis, active transport, and co-transport — each suited to different molecules and conditions, and cells can be structurally adapted to make these processes faster or more efficient.
Cells