Cells

CELLS

• Living things are made of cells
• 1 celled organisms are unicellular
• 2 or more celled organisms are multi cellular
• All multi cellular organisms are made of eukaryotic cells

Organelles

Microscopes

•  2 types of microscope
• Light microscope and electron microscope
• Microscopes work by refracting the light that has passed through an object
• The light microscope refracts the light rays passing through an object. It has an eyepiece and objective lenses. The light microscope can see individual cells but not structural detail.

Nucleus

• Found in both plant and animal cells
• Surrounded by a nuclear envelope. This consists of 2 unit membranes that are perforated by nuclear pores. This allows the exchange of substances between the nucleus and cytoplasm. 
• Contains chromatin which is composed of coils of DNA bound to proteins called histones.
• Contains a nucleoplasm which contains a variety of chemical substances such as ions, proteins and nucleotides.
• Contains a nucleolus which produces rRNA from which ribosomes are assembled.

Endoplasmic Reticulum

• A complex network of membrane bound cavities called cisternae.
• May have vesicles, tubules, sacs and layers.
• Rough ER is covered with ribosomes. These are the sites of protein synthesis. The rough ER temporarily stores and transports the proteins that are made by the ribosomes. 
• Smooth ER has no ribosomes attached. They are tubular and they synthesise lipids including steroids. 

Ribosomes

• Small, dense granules scattered through the cytoplasm and on the rough ER's surface.
• Consist of 2 sub-units each containing RNA and protein
• The two sub-units combine with each other in the presence of mRNA.
• The ribosome moves alone the mRNA in translation during which a polypeptide chain is formed
• Free ribosomes produce proteins for use in the cytoplasm whereas ribosomes bound to the rough ER synthesise proteins which are exported out of the cell

Centrioles

• Small, hollow cylinders
• Found in animal cells and in lower plant cells
• Occur in pairs and the two units lie at right angles to each other
• They are found in the centrosome in the cytoplasm.
• Each centriole is made up of 9 groups of microtubules arranged in triplets.
• They play a part in cells division and are involved in the formation of the nuclear…