All living things are made up of cells. So understanding what a cell is and what it is made up of is very important in Biology.
Cells are rather tiny – you could fit about 100 on a pin head. They contain even tinier little pieces called organelles that each have a specific job to do within the cell.
The cell is something that has a function to carry out – for example a muscle cell has to shorten to allow your muscles to contract. What cells do always requires energy and they get this energy from a burning reaction called respiration.
In the cell, most of the work of respiration is done within little organelles called mitochondria. They are like the cell’s power stations, churning out a chemical called ATP that the cell can use when it needs energy.
The cell is surrounded by a membrane that controls what enters or leaves the cell. The cell obviously needs to hold its contents inside and to select what enters. Substances such as glucose continually enter the cell through the action of transport proteins that grab hold of the glucose and transport it across the membrane. Water is also moving in and out of the cell all the time through pores in the membrane.
Water is a really important part of the cell’s chemistry because it can dissolve most biological molecules and therefore acts as the solute for the cell’s chemical solution – the cell’s cytoplasm is at least 70% water – in fact 70% of our overall mass is water. Water also takes part in important reactions that build up and break down biological molecules such as starch and proteins.
The nucleus can be described as the cell’s equivalent of a brain and holds the instructions that tell the cell how to perform its functions and how to build up its structure. These instructions are genes and are made of DNA – which is coiled up into the chromosomes that are found in the nucleus. The human cell contains 46 of these chromosomes – 23 from the father and 23 from the mother.
The cell specialises by using a certain set of the DNA instructions. For example a pancreatic cell will use the gene that instructs it how to produce insulin because that is part of its function – a liver cell won’t use this gene at all – but will use other genes. Cells have to specialise because they each have different types of function to perform.
All cells: in plants, animals and fungi, share the common features of the nucleus, cytoplasm and the cell membrane. Plants and fungi also have a cell wall to give them rigidity plus plant cells have chloroplasts to allow them to photosynthesise and a large vacuole to help bring water into the cell.