CellThe smallest unit of life. Your body is made of about 37 trillion cells, each performing specific functions.TissueA group of similar cells working together to perform a function. The four main types are epithelial, connective, muscle, and nervous tissue.OrganA structure made of two or more tissue types that performs a specific job. The heart, brain, and lungs are examples of organs.SystemA group of organs working together to carry out a major body function, like the digestive system or nervous system.DNADeoxyribonucleic acid — the molecule that carries your genetic instructions. If uncoiled, the DNA in one cell would stretch about 2 metres long.GeneA segment of DNA that contains the instructions for making a specific protein. Humans have about 20,000–25,000 genes.ChromosomeA tightly coiled package of DNA. Humans have 23 pairs (46 total) of chromosomes in each cell.EnzymeA protein that speeds up chemical reactions in the body. Without enzymes, digestion and metabolism would be far too slow to sustain life.HormoneA chemical messenger produced by glands and carried by the blood. Hormones regulate growth, mood, metabolism, and reproduction.AntibodyA Y-shaped protein produced by immune cells that recognises and binds to a specific invader (antigen) to neutralise it.MetabolismAll the chemical reactions in your body that convert food into energy and building materials. It includes both breaking down and building up molecules.HomeostasisThe body's ability to maintain stable internal conditions (temperature, pH, blood sugar) despite changes in the external environment.PathogenAny microorganism that can cause disease — including bacteria, viruses, fungi, and parasites.PlasmaThe liquid part of blood (about 55% of blood volume). It carries blood cells, nutrients, hormones, and waste products.HaemoglobinThe iron-containing protein in red blood cells that binds to oxygen and gives blood its red colour.CollagenThe most abundant protein in the human body. It provides structure and strength to skin, bones, tendons, and cartilage.MitochondriaTiny structures inside cells that produce energy (ATP) from food. Often called the 'powerhouses of the cell.'NeurotransmitterA chemical substance released at nerve endings to transmit signals across a synapse to the next neuron or target cell.CartilageA firm but flexible connective tissue that cushions joints, shapes the nose and ears, and supports other structures.TendonA tough band of fibrous tissue that connects muscle to bone, transmitting the force of muscle contraction to move the skeleton.LigamentA short band of tough, flexible tissue that connects bones to each other at a joint, providing stability. 