Cell wall Cell membrane Nucleus Cytoplasm Organelles

The cell is the smallest independently-functioning unit in plants, as it is in all living things.

Plant cells can vary in size, shape, composition and function. A photosynthetic cell from a leaf differs from a phloem cell or a pollen cell, but they all have features in common which are characteristic of plant cells.

Viewed through a light microscope, the typical plant cell is composed of four main structures. Three of these are the same as in animal cells: a thin cell membrane on the outside, the jelly-like cytoplasm and the cell nucleus (these are discussed in greater detail in Animal cells).

The fourth observable feature, the cell wall, is the main characteristic distinguishing plant cells from animal cells.

Most plant cells also differ from animal cells in their ability to convert light energy from the Sun into chemical energy (sugars) by the process of photosynthesis. The sunlight energy is trapped by a pigment, usually green-coloured chlorophyll. Without plants supplying this renewable food source there would not be enough food to sustain the range of life that we have on Earth. However, some plant cells (e.g. the roots) do not contain the green pigment chlorophyll because they cannot photosynthesise as they are not exposed to light.

Cell wall
The cell wall is made of cellulose, a complex carbohydrate known to dietitians as "fibre". It maintains the cell's shape at all times and provides strength. Because of this, plant cells do not expand greatly or distort when under pressure as animal cells do.

Without cell walls the entire plant would collapse under its own weight.

The cell wall allows free passage of water and dissolved substances.

Cell membrane
The cell membrane holds the cell together. It holds the contents of the cell inside and determines what passes in and out of the cell.

Nucleus
The nucleus is the control centre of the cell and is responsible for all the processes that occur in the cell.

Cytoplasm
The cytoplasm is the jelly-like substance in which the functioning organelles are located.

Organelles
There are seven major organelles within the cytoplasm which need an electron microscope to be seen clearly. The chloroplasts are found only in green plant cells. The others are all common to plant and animal cells and have been covered in detail in Animal Cells.

• Chloroplasts are membrane-bound organelles containing the green pigment chlorophyll which is used in photosynthesis to convert the Sun's radiant energy into chemical energy as sugar. This energy is then converted by the mitochondria into useable energy. Chloroplasts only appear in the parts of the plant receiving sunlight, like the surface cells in leaves. Their presence gives that part of the plant an overall green appearance. It would be inefficient to have them underground in root cells or anywhere else that didn't receive light. Therefore not all plant cells have chloroplasts so this cannot be considered a distinguishing feature between plant and animal cells.
  
  
• Mitochondria are the site of cellular respiration which uses the by-product of photosynthesis (oxygen) and sugar to produce the energy required by the plant. Animals also carry out respiration, but plants both produce their own food by photosynthesis and then consume it in respiration.
  
  
• Ribosomes are the site of protein production.
  
  
• The Endoplasmic Reticulum is used for intracellular transport of proteins.
  
  
• Golgi Bodies are used for packaging of proteins to be released from the cell. Carbohydrates are also be made here to store food for the plant cell (starch).
  
  
• The Centriole is involved in chromosome separation during cell division and is only visible at that time.
  
  
• The Vacuole is a watery, fluid filled sac. It is used to store water and some food particles. It may also contain some of the cell's waste materials. It occupies a large proportion of a plant cell's total volume and, when full, pushes against the cell wall making it stiff (turgid). This also helps support the plant.
  

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