Revision:Gcse biology: cell division and structure

Animal and Plant cells

Although plants and animals have many things in common, there are four main differences:

  • Plant cells have a strong cell wall made of cellulose, on the contrary, animal cells do not
  • Plant cells have a large permanent vacuole containing cell sap, vacuoles in animal cells are small and temporary
  • Plant cells many contain chloroplasts containing chlorophyll for photosynthesis. Animal cells never contain chloroplasts
  • Animal cells store energy as granules of glycogen but plants store starch

Plant and animal cells have many smaller structures in the cytoplasm. These can be seen by using an electron microscope.

Even as said, plant cells contain chloroplasts, some types of plant cells don't have them, e.g. root cells, as roots are in the ground and sunlight in not needed.

Also, many people think that only animals respire, however plants also respire as respiration is one of the seven traits of a living organism. In addition, this shows that plant cells also have mitochondria. Mitochondria are examples of these structures and are the site of respiration in the cell.

Nuclei, Chromosomes and Genes

Most cells contain a nucleus that controls all of the chemical reactions that go on in the cell. Nuclei can do this because they contain the genetic material. Genetic material controls thec haracteristics of an organism and is passed on from one generation to the next. The genetic material is made up of structures called chromosomes. They are made up of a chemical called Deoxyribonucleic Acid, or DNA. The DNA controls the cell by coding for the making of proteins, such as enzymes. The enzymes will control all the chemical reactions taking place in the cell.

A gene is a part of a chromosome that codes for one particular protein.

By controlling cells, genes therefore control all the characteristics of an organism. Different organisms have different numbers of genes and different numbers of chromosomes. In most organisms that reproduce by reproduction, the chromosomes can be arranged in pairs. This is because one of each pair comes from each parent.

Cell Division

There seems to be a limit to how large one cell can become. If organisms are to grow, ells must split or divide. Cells also need to divide to make special cells called gametes for reproduction.

There are two types of cell division, one for each of these two reasions:

  • Mitosis - used for growth
  • Meiosis - used for gametes

Both of these two types of cell division have certain things in common. The DNA of the chromosomes has to be copied first to make new chromosomes. The chromosomes are then organised into new nuclei and the cytoplasm then divides into new cells.

In mitosis, two cells are produced from one. As long as the chromosomes have been copied correctly, each new cell will have the same number of chromosomes and the same information. Cancer is caused by abnormal cell division.

The process of mitosis is:

  • A cell has a pair of chromosomes that have divided
  • The two strands of chromosomes are separating
  • The chromosomes have now separated
  • New nuclei are forming
  • The cytoplasm is dividing to make two cells each with two chromosomes

In meiosis, the chromosomes are also copied once by the cell divides twice. This makes four cells each with half the number of chromosomes, one from each pair.

The process of meiosis is:

  • A cell has a pair of chromosomes each of which have divided
  • The two chromosomes are separating
  • Two cells are formed by the cytoplasm dividing
  • Each double stranded chromosome is now split up
  • Four new cells are formed each with one chromosome

Cell Specialisation

By the process of mitosis, a large number of cells can be produced. This enables organisms to grow or repair damaged tissue. The different cells all contain the same genes but developd differently.

Cells become adapted for different functions. This is called specialisation.

Specialisation allows cells to become more efficient at carrying out their jobs.

The disadvantage of being specialised is that the cells lose the ability to take over the jobs of other cells if they are lost.

A example of a specialised cell is a nerve cell or neurone. Its features are:

  • Nucleus
  • Cell membrane
  • Cytoplasm
  • Man projections - can receive impluse from many other neurones
  • Sheath made of fatty material called myelin. This insulates the neurone and speeds up the impulse.
  • Long distance so one neurone can send impluses a long way in the body
  • Neurone divides so that is can pass impluses onto many other cells
  • Similar cells that do similar jobs are gathered together into tissues
  • More complicated organisms have organs that are made up of a number of tissues
  • Groups of organs work together in systems to carry out certain functions

Cells > Tissues > Organs > Systems

NB: bone and muscle are tissue but *a* bone or *a* muscle is an organ