a full summary of everything you need to know for AQA A level biology unit 2, from a variety of sources including textbooks, class notes and above all the exact AQA specification and past mark schemes to give you the information you need to get the marks.
Each cell is often adapted to its function so contains a certain internal structure known as the ultrastructure of the
cell. Using an electron microscope allows us to study the structure of organelles to understand their function.
tHE NUCLEus
Structure - the nucleus is usually spherical and between 10 and 20
micrometres in diameter. It has smaller parts inside which allow it to carry out
its wider functions:
- The nuclear envelope is a double membrane surrounding the cell
which is continuous with the rough endoplasmic reticulum in places.
Its main function is to control entry and exit of substances into and
out of the cell and to control the reactions within it.
- Nuclear pores allow molecules such as rRNA and mRNA to pass
through the cell. Each nucleus has around 3000 pores.
- The nucleoplasm is the granular jelly like substance found inside the nucleus, imagine the cytoplasm of the
nucleus
- Chromatin is a coloured substance which is sometimes used to attract pollinators. Chromosomes are histone
proteins surrounded by DNA
- The nucleolus makes ribosomal RNA subunits and lines up ribosomes
Function -
- Production of mRNA and rRNA, hence protein synthesis. This means that it is often referred to as the ‘control
centre’.
- To retain genetic information in chromosomes
- To produce ribosomal RNA and ribosomes
, Mitochondria
Structure - usually rod shaped and 1-10 micrometres in
length. Similar to the nucleus, different structural aspects
allow different functions in the organelle:
- They are made of a double membrane which
controls entry and exit of substances
- The inner membrane is folded to increase the
surface area for attachment of enzymes and other proteins
required for aerobic respiration. These folds are cristae.
- The matrix of the mitochondrion is the middle of
the inner membrane, which contains ribosomes, DNA, proteins and lipids that mean mitochondria have the
ability to produce their own proteins.
Function -
- Mitochondria are the site of aerobic respiration
- Due to this, they require large amounts of energy, which the mitochondria are able to produce in form of ATP
Endoplasmic reticulum
Structure - folded membranes containing tubules and
flattened sacs called cisternae. The rough endoplasmic
reticulum is attached to the nucleus and is coated with
ribosomes. The smooth endoplasmic reticulum spreads
throughout the cytoplasm.
Function -
- The rough endoplasmic reticulum has a large surface area for
synthesis of proteins and glycoproteins
- The RER also provides a pathway out of cells for molecules eg
protein
- The smooth endoplasmic reticulum is used for synthesis,
storage and transport of lipid and carbohydrate molecules.
, Golgi body / golgi apparatus
Structure - a stack of membranes and flattened
sacs called cisternae with round, hollow vesicles.
Once sorted and labelled, these vesicles carry
proteins and lipids to the membrane, sent from the
golgi.
Function -
1. Adding carbohydrates to proteins to form
glycoproteins
2. Produce secretory enzymes, for example in
the pancreas
3. Secrete carbohydrates, for example for use
in making plant cell walls
4. Transporting, modifying and storage of lipid molecules
5. Formation of lysosomes
Lysosomes
Structure - lysosomes are formed when golgi vesicles contain proteases and lipases, as well as lysozymes. Lysozymes
are used to hydrolyse cell walls of some bacteria. Up to 50 enzymes can be in a single lysosome.
Function -
1. Hydrolysis of ingested materials from phagocytic cells for
example white blood cells and bacteria
2. Releasing enzymes to the outside of the cell
3. Digestion of worn out organelles and conversion into
useful chemicals to be recycled
4. Completely digest dead cells (autolysis)
Ribosomes
Structure - there are two types, depending on the cell.
1. 80s - eukaryotic cells, around 25nm diameter
2. 70s - prokaryotic cells, mitochondria and chloroplasts, slightly smaller
All mitochondria contain 2 structural subunits each containing rRNA and protein
Function - the site of protein synthesis
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