Biomolecules group together to create cells: the smallest structures that can be considered living things.

Cells are the functional and structural units of all living things. 

All living things, including human beings, are made of cells. This is why cells are structural units. At the same time, cells are the smallest structures capable of performing the three vital functions of nutrition, interaction and reproduction. This is why cells are also known as functional units.

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As you can see, diatom algae and elephants are very different, so we divide living things into two categories:

• Unicellular: an organism consisting of a single cell, such as bacteria, fungi and algae.
• Multicellular: an organism consisting of more than one cell. All plants and animals are multicellular living things.

THE DISCOVERY OF CELLS: CELL THEORY

In 1665, the English scientist Robert Hooke first described a cell. He observed it using a rudimentary microscope which he had built himself. He observed a polygonal structure that was replicated multiple times inside thin sheets of cork. Hooke called this structure cella which means ‘cell’ or ‘gap’ in Latin.

In 1838, Matthias Schleiden and Theodor Schwann were the first scientists to formulate cell theory. This theory was later extended by other scientists, and nowadays it states that:

• All organisms are made of cells.
• Cells are the structural and functional units of living things. 
• Every cell is the result of the division of a pre-existing cell.
• Cells are the simplest independent life units which we know of.

CELL STRUCTURE

Bacteria, skin cells and plant cells all have a basic structure, just like all cells found in nature. A cell consists of a plasma membrane, cytoplasm and at least one nucleic acid molecule.

• The plasma membrane is the envelope around the cell. It separates the inside of the cell from the exterior. It allows substances to enter and exit the cell. In addition to this membrane, some cells have another rigid outer casing which is called the cell wall.
• Cytoplasm is a watery substance found in the interiors of cells. In some cells, we can find organelles, which are specialised structures that perform specific functions. For example, a mitochondrion performs cellular respiration.
• The nucleic acid contains genetic information which is essential for the functioning of the cell. It can be dispersed within the cytoplasm, or surrounded by a membrane, which is called the nuclear membrane.

Cell size

Cells exist in different sizes, although generally they are microscopic. To measure cells we use micrometres (µm):

1 µm = 0.001 mm = 0.000001 m

1m = 1000 mm = 1000000 µm

CELL TYPES

Different species of living things have different amounts of cells as well as different cell structures. We can classify cells into two types according to their structures: prokaryotes and eukaryotes. Eukaryote cells differ according to whether they are found in animals or plants.

THE ENDOSYMBIOTIC THEORY

This theory was formulated by Lynn Margulis in 1967. It states that eukaryote cells originated from the interaction of several primitive prokaryote cells 500 million years ago.

Margulis proved that mitochondria and chloroplasts originated from primitive bacteria. Her theory is widely accepted by the scientific community.

CELL SPECIALISATION

Multicellular living things are made up of many cells. All cells in living things have a common characteristic: they have the same genetic material (DNA).

However, within the same organism, we can find many different types of cell. Each of them is specialised to perform a particular job.

Every type of cell uses one part of the information found in the DNA, as if the cell was reading a chapter of an instruction manual. This is known as cell specialisation.

Cell specialisation gives multicellular living things many advantages:

• Division of labour: Each type of cell performs a different job so living things can multitask.
• Greater efficiency: Because they do specific jobs, they do them very efficiently.
• Longevity: Damaged cells can be replaced by other cells. This means that multicellular organisms are more likely to survive.

However, multicellular living things need more energy to allow the different structures and functions to work at the same time.

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