A Closer Look at the Cell

Nearly two metres of DNA can fit inside the nucleus of a single human cell. Around it, specialised structures share the work of keeping that cell alive and functioning.

Cells are tiny, but they are organised living systems. The diagram in this lesson shows a generalised human cell. Real cell types vary, and not every cell contains every structure shown. Many specialised structures inside cells are called organelles . Together they help the cell control exchange, store genetic information, make usable energy, build and move proteins, and recycle material. The boundary around the cell Every living human cell is surrounded by a cell membrane . It separates the cell's contents from its surroundings and helps control what moves in and out. Proteins in the membrane also help cells receive signals and move substances that cannot simply pass through the membrane. The nucleus and genetic information In many human cells, the nucleus contains most of the cell's DNA. DNA carries genetic information used to build and maintain the cell. Different cell types can use different parts of that information. This helps explain how cells in the same body can specialise for very different jobs. The diagram is a generalised model rather than a rule for every cell. Mature red blood cells, for example, do not have a nucleus. Making usable energy In many human cells, mitochondria make much of the cell's ATP . ATP is a small molecule that transfers usable chemical energy to processes inside the cell. Cells with high energy demands, such as muscle cells, often contain many mitochondria. Building and moving proteins Ribosomes build proteins. Proteins do many different jobs. For example: Haemoglobin carries oxygen, antibodies help defend against infection, insulin helps regulate blood glucose, and proteins such as collagen and keratin help give tissues their structure. Some newly made proteins enter a membrane network called the endoplasmic reticulum (ER) , where they begin to be processed. Many then pass through the Golgi apparatus , which modifies, sorts and packages them for use elsewhere in the cell or for release outside it. Breaking down and recycling Cells constantly maintain and renew their internal parts. One way they do these is with the help of organelles called lysosomes . Lysosomes contain enzymes that help break down worn out cell parts and some material brought into the cell. Useful components can then be reused. No organelle works in isolation. Cell function depends on these structures working together, and different cell types emphasise different structures according to the jobs they perform. Disease can disturb a cell from inside or change the environment around it. Cell biology helps explain how that can affect tissues and organs, but a symptom by itself does not identify one organelle or one diagnosis.

A cell is a coordinated living system. Different structures share different jobs, including controlling exchange, storing genetic information, making usable energy, building and routing proteins, and recycling material.