Molecules and cells - Edexcel GCE Biology Revision Notes

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Cells and Organelles

  • The nucleus is a large organelle surrounded by a nuclear envelope – which is double membrane. The membrane contains many nuclear pores. The nucleus contains chromatin and nucleolus (which makes ribosome)

Chromatin is made of proteins and DNA. The pore allows substances (I.e. mRNA) to move between the nucleus and the cytoplasm.

  • Lysosome is a round organelle surrounded by a membrane with no clear internal structure. It contains hydrolytic digestive enzyme. It can be used to digest invading cells / break down old organelles.

  • Vesicle is a small fluid – filled sac in the cytoplasm, it transports substances in and out of the cells. Most are formed by the Golgi apparatus and the cell surface.

  • Ribosomes are very small organelles that are free in the cytoplasm or are attached to the rough endoplasmic reticulum – it’s the site of protein synthesis.

  • There are 2 types of endoplasmic reticulum, the smooth ER is a system of membrane enclosing a fluid filled space – it synthesized and processes lipid.

The rough ER is similar to sER but is covered in ribosome. They both contain a network of flattened, membrane-enclosed sacs / tubes known as cisternae. rER folds (folds the polypeptide chain into its 3D shape) and processes proteins.

  • Golgi apparatus is a group of fluid filled sac (consists of flattened membrane disks - cisternae) and vesicles are often seen at the end of the sacs. It processes and packages new lipids and proteins i.e. add carbohydrates to the protein chain to make glycoprotein. Lysosome is also produced here

  • Centrioles are hollow cylinders containing a ring of microtubules (protein cylinders) they are normally seen in pairs arranged perpendicular to each other. They’re involved in separation of chromosomes during cell division (spindle fibre formed)

  • Mitochondria are usually oval shaped with an inner and outer membrane with spaces in between called inter-membrane space. The inner membrane is folded into structure called cristae – inside is a space called the matrix, which contains enzymes involved in aerobic respiration (to produce ATP)


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  • rER, vesicles and the Golgi apparatus are involved un protein transport. Proteins are made at the ribosome on the surface of rER. The protein chain is passed through the rER’s cisternae to be folded into their 3D shape – by forming bonds.

  • New proteins are then transported to the Golgi apparatus in vesicles, the vesicle fused with the Golgi’s membrane. The protein then undergo further processing i.e. carbohydrate groups are added.

The protein than enters more vesicles to be transported around the cell. Extra cellular enzymes such as digestive enzymes moves to the cell surface and are secreted ...

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