Cells as the basic units of living organisms

Cells are the fundamental units of all organisms, whether they be unicellular or multicellular.
All living things, from the smallest bacterium to the largest whale, are made up of cells.
Different types of cells have different structures and functions but share some common features.

Cells are responsible for a myriad of functions vital for the organism’s survival, such as energy production, nutrient absorption, waste removal, and reproduction.
Each perform specific tasks as part of an interconnected whole. For example, nerve cells transmit signals, while red blood cells transport oxygen.

All cells are bound by a cell membrane, which controls the movement of substances in and out of the cell.
Inside the cell membrane is the cytoplasm, a jelly-like substance where the cell’s metabolic reactions occur.
Every cell contains genetic material, either in the nucleus (in eukaryotes) or in the nucleoid region (in prokaryotes).

All organisms are made of either prokaryotic or eukaryotic cells, the two main types of cell.
Prokaryotic cells (which include bacteria and archaea) are simpler and smaller than eukaryotic cells. They lack a nucleus and have no membrane-bound organelles.
Eukaryotic cells (which make up animals, plants, fungi, and protists) have a nucleus enclosing their genetic material and contain membrane-bound organelles, such as mitochondria and chloroplasts.

In multicellular organisms, cells undergo differentiation to become specialised for particular functions.
This involves changes in the cell’s shape, size, and organelles to adapt it for a specific role. For example, muscle cells developing more mitochondria to better handle the demands of contraction.
This allows for a high level of efficiency and organisation within the organism.

Cells reproduce by a cycle of growing and dividing called the cell cycle.
The cell cycle includes phases of intense activity, including DNA replication and cell division (mitosis or meiosis, depending on the cell type).
The regular cycling of cells ensures continued growth, repair, and renewal within organisms.