What Are Cells?

All living cells consist of two parts:  The inside of the cell is called the "protoplast". The protoplast is then enclosed by a "cell membrane". In plant cells this membrane is usually surrounded by non-living, relatively rigid shell, called the "cell wall".

 

The most important components to be found inside a cell are:

1. The Nucleus

  • Each cell only has one nucleus.
  • The nucleus is where the genetic material (ie. genes and chromosomes) are found.
  • The genes occur scattered along the length of threadlike chromosomes.
  • The genes determine the characteristics of the organism.
  • The nucleus is enclosed inside a double layer nuclear membrane.
  • The "nucleolus" in the centre of the nucleus is a tiny part of the nucleus rich in RNA and protein (there may be more than one nucleolus inside a nucleus).

 

2. Chloroplasts

  • These are tiny organs containing chlorophyll and other pigments.
  • Photosynthesis occurs inside the chloroplasts.

 

3. Mitochondria

  • These are small organs in which respiration occurs.
  • Mitochondria are usually identified by their spherical or rod-like shape.

 

4. Ribosomes

  • These are small particles containing RNA (ribonucleic acid) which float freely in the cytoplasm.
  • Proteins are made in the ribosomes.

 

5. Endoplasmic Reticulum

  • Strands or tubes of colourless cytoplasm.

 

6. Vacuole

  • A space or cavity inside the cytoplasm of a cell.
  • Products and by‑products of the plant are stored in the vacuole.

 

7. Plasma Membrane

  • A thin membrane located between the cell wall and the cytoplasm.
  • It is a fatty or oily layer.

THE CELL WALL

This protects the inside of the cell, and in some tissues (particularly wood and bark), the cell wall provides most of the strength or structural support to the cell.

The cell wall is not alive. It is made up of cellulose together with pectic material and lignin. Lignin provides strength. Pectin is a complex carbohydrate (The same as the substance used to make jelly).

Cellulose which is the main constituent can stretch and contract, but is strong.

Individual cells in tissue are connected together by strands of cytoplasm that can actually extend through the cell walls joining the cytoplasm of one cell to the cytoplasm of the next. Water and chemicals can move from cell to cell along these connecting channels.

 

THE NUCLEUS

The nucleus is the control centre of the cell. All cells have a nucleus at some stage of their life though in some cases the nucleus can disappear at some point in time (eg. Red Blood Cells in animals loose their nucleus as they grow older). The nucleus controls and directs the development of the cell.

The chromosomes are long lengths of chemicals made up of DNA (deoxyribonucleic acid) and proteins. The chromosomes contain the genetic information that directs:

a/ All cell functions

b/ Differentiation of the organism (eg. telling it what should be leaf, flower, stem, root etc)

c/ Plant reproduction.

 

Study Cell Biology -Click to see our course