A Closer Look at the Cell

A typical nucleated human cell contains nearly two metres of DNA packed into a nucleus smaller than the width of a human hair. That DNA sits inside a cell where membranes and specialised structures divide up the work of keeping the cell functioning.

The body is organised in levels, and cells are the smallest living units in that hierarchy. Tissues are built from cells, organs from tissues, and organ systems from organs. Here we zoom in on a typical human cell and the structures that keep it working. Human cells contain specialised internal structures. Many are called organelles, meaning structures within the cell that perform particular functions. Thinking of a cell as a small workplace can help because different parts share different tasks. It is only an analogy. Organelles are not miniature body organs, and different cell types do not all contain the same structures in the same amounts. The cell membrane: a selective boundary Every living human cell is enclosed by a cell membrane. It separates the cell's contents from the surrounding environment and regulates movement in and out. Some small molecules, including oxygen, can cross the membrane directly. Many other substances use protein channels, carriers, pumps or vesicles. The membrane also contains receptors and other proteins that let cells respond to signals. Many medicines act on receptors, channels or transporters in cell membranes. The nucleus: where most DNA is stored Many human cell types contain a nucleus. The nucleus holds most of the cell's DNA, packaged into chromosomes. A small amount of DNA is also found inside mitochondria. DNA contains genetic information. Most nucleated body cells contain broadly the same DNA sequence, but different cell types use different sets of genes. This difference in gene expression helps a muscle cell behave differently from a skin cell. Not every human cell has one nucleus. Mature red blood cells have no nucleus, while skeletal muscle fibres contain many. Mitochondria: making ATP Mitochondria generate most of the ATP used to power cellular reactions. ATP is a small molecule that transfers usable chemical energy to processes inside the cell. In aerobic metabolism, mitochondrial ATP production depends on oxygen and on molecules derived from nutrients. Cells with high energy demands, including muscle cells, often contain many mitochondria. Ribosomes and the endoplasmic reticulum: making and processing proteins Ribosomes assemble proteins from amino acids using genetic instructions that have been copied from DNA into RNA. Some ribosomes are free in the cytoplasm. Others are attached to the rough endoplasmic reticulum, or rough ER. The rough ER, together with its attached ribosomes, helps make and begin processing many proteins that will be inserted into membranes, sent to other parts of the cell or released outside it. The smooth ER has different roles, including lipid production. The Golgi apparatus: sorting and packaging Many proteins and lipids from the ER move to the Golgi apparatus. The Golgi modifies, sorts and packages them into vesicles that carry them to their next destination. Lysosomes: breaking down and recycling Lysosomes contain enzymes that break down worn out cell components and some material taken into the cell. Useful building blocks can then be recycled. Lysosomes are one of several systems cells use to handle damaged or unwanted material. A cell therefore works through coordinated processes rather than one control centre doing everything. Structure and function are closely linked: the way a cell part is built helps it perform its role. Cell biology also gives a useful foundation for understanding disease. A problem may begin inside a cell, or something outside the cell, such as reduced blood supply or inflammation, may disrupt how it functions. Knowing the parts helps explain possible mechanisms, but it does not let a symptom identify one organelle or one diagnosis.

A human cell contains specialised structures that share different tasks. The membrane regulates exchange, the nucleus stores most DNA, mitochondria generate most ATP, ribosomes make proteins, the ER and Golgi process and route materials, and lysosomes help break down and recycle cell components.