Biology for Form 3 Pupil

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CHAPTER 1
Cell Structure and Organisation
Cells
* All living organisms are made up of cells
* The simplest living organisms have one cell only
* His name is unicellular-e.g. bacteria.
* Most other living organisms have many cells- his name is multicellular- e.g. Man, plants, animals. * Cells can be seen only under microscope
Cell structure
All cells have the following structure in common:

a) Cell membrane- a very thin layer of protein and fat, surrounding the cell. It is partially permeable; it allows some substances to pass through it but not others. The function of the cell membrane is to control the passage of substances into and out of the cell. b) Cytoplasm- it is a jelly-like fluid consisting mainly of water plus dissolved substances. It also contains tiny structure called organelles where reactions of the cell occur. c) Nucleus- a round or oval body which contains thread-like structures called chromosomes/ these chromosomes are made up of DNA (deoxyribo Nucleic Acid), which determines all the characteristics of the organism. The nucleus controls cell division and all the characteristics of the organism. The nucleus controls cell division and all chemical reactions within the cell. * DNA is the genetic material of the cell.

* Chromosomes contain genes, that is, sections of DNA, which are responsible to make specific proteins, such as enzymes. * The cytoplasm and the nucleus make-up of the protoplasm of the cell. Plant cells structures:

(i) Cell wall- it is found outside the cell membrane, it is made up of cellulose. It protects and gives shape to the cell. It is freely permeable to water, gases and dissolved substances. (ii) A large central vacuole- it is a space filled with cell sap. The vacuole is often called sap vacuole and is surrounded by a membrane. It helps to maintain the shape of the cell. (iii) Chloroplasts- these are small structures lying in the cytoplasm. They contain the green pigment...
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