Cells and tissues

The human body is built in levels. Atoms form molecules, molecules form organelles, organelles work together inside cells, similar cells and their surrounding material form tissues, two or more tissues combine into an organ, and organs that cooperate form an organ system. No system works alone: the respiratory system supplies oxygen that the circulatory system delivers, the digestive system supplies nutrients, and the excretory system removes wastes. Together they keep the internal environment suitable for every cell.

Inside each cell, membrane-bound and non-membrane-bound organelles divide up the work. The nucleus holds DNA and directs protein production. Ribosomes, free in the cytosol or attached to rough endoplasmic reticulum, assemble proteins. The smooth endoplasmic reticulum makes lipids and helps detoxify some substances. The Golgi body modifies, sorts and packages proteins into vesicles. Mitochondria carry out the aerobic stages of cellular respiration and release most of the cell's usable energy as ATP.

Lysosomes contain digestive enzymes that break down worn-out organelles and engulfed material. The cytoskeleton gives shape and allows movement of organelles, and centrioles help organise the microtubules that separate chromosomes during division.

These processes only continue if the cell is supplied with raw materials (oxygen, glucose, amino acids, fatty acids, water, ions) and if wastes such as carbon dioxide, excess heat and nitrogenous compounds are removed. If wastes accumulate, pH and other conditions drift and enzymes work less efficiently. This is why exchange at the cell membrane is so important.