synoptic essays for AQA A2 biology unit 5

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  • Created on: 26-05-10 11:29
Preview of synoptic essays for AQA A2 biology unit 5

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The relationship between cell organelles and their function
Cell organelles are important because they enable a cell to carry out metabolism. They
perform the same functions as organs in a human: digestion (lysosomes), ATP synthesis
(mitochondria), control of cell processes (nucleus), protection and regulation of diffusion of
substances in and out of a cell (cell membrane). They enable a cell to carry out its job.
There are many different kinds of cell organelles which have been specialized according to
their function. An example of one is mitochondria. Mitochondria are rod ­ shaped cell
organelles, they are made up of a double membrane which surrounds the organelle, the outer
one controls materials coming in and going out. The inner membrane is folded to form
extensions known as cristae. Cristae are shelf like extensions of the inner membrane. These
provide a large surface area for the attachment of an enzyme called ATPase which is
involved in respiration. Enzyme ATPase is used to join an inorganic phosphate molecule to
an ADP molecule to produce ATP. The matrix in the mitochondria contains proteins and
lipids. Mitochondria are the sites of respiration. Some stages of respiration, such as the link
reaction, Krebs cycle and the electron transport chain take place in the mitochondria. As
mitochondria plays such a vital role in respiration and the release of ATP, they occur in great
numbers in metabolically active cells.
The Endoplasmic Reticulum (ER) organelle is a network of disklike tubules, sacks and
vesicles found in eukaryotic cells. Its main function is to operate as a transport system. It is
found in the cytoplasm of cells. The ER is folded and stacked layer upon layer within the cell
and is connected to the cell's nuclear membrane. There are two types of ER, Rough and
Smooth. Rough ER contains ribosomes on its surface, small circular structures that control
protein synthesis. Rough endoplasmic reticulum branches out and expands as protein
synthesis increases, providing more surface area for ribosomes to spread out and create
more proteins. During protein synthesis, the ribosomes on the rough ER create new proteins
and the ER then folds them properly and sorts them according to function and destination.
Smooth endoplasmic reticulum (SER) lacks ribosomes on its surface and is often more
tubular in appearance. The function of SER is to synthesise, store and transport lipids,
synthesise, store and transport carbohydrates. Another function of SER is to control the
movement of newly synthesized proteins to their proper locations in the cell or to the
membrane to be sent outside the cell.
The Golgi apparatus occurs in almost all eukaryotic cells and is similar to the SER in structure
except that it is more compact. It consists of a stack of membranes that make up flattened
sacks (cisternae) with small rounded structures called vesicles. The proteins and lipids
produced by the ER are passed through the Golgi apparatus. The Golgi modifies these
proteins often adding nonprotein components to them. It also sorts out the lipids and
proteins, allowing them to be accurately sent to their correct destinations. Once sorted, the
modified proteins are transported into the vesicles which are regularly pinched off and
moved to the cell surface, where they release their contents.

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