Cell Structure and Organisation

Mitochondria

Mitochondria are roughly 3 - 5 micrometres in diameter. They are comprised of:

• A double membrane, separated by an inter membrane space. The inner membrane is folded inwards to form cristae. 
• An organic matrix, which contains compounds such as lipid droplets and proteins.
• There are also 70s ribosomes and circular DNA in the matrix. These allow mitochondria to self-replicate and produce some of their own proteins. 

The function of the mitochondria is to synthesise ATP. They are also the site of aerobic respiration. Some reactions occur in the matrix whilst some occur on the inner membrane. The cristae provide a larger surface area for the attachment of enzymes involved in respiration. 

Metabolically active cells, such as muscle cells, require a plentiful supply of ATP and so have a large number of mitochondria. 

As mitochondria are cylindrical, they have a larger surface area: volume ratio. The diffusion pathway for oxygen from outside the mitochondria to their centre is shorter than that of a cylindrical organelle, increasing efficiency. 

Rough Endoplasmic Reticulum

The RER consists of a membrane forming flattened sacs, of which the outer membrane is encrusted with ribosomes. The membranes are separated by fluid-filled spaces called cisternae. The RER is involved in the transport of proteins.

Smooth Endoplasmic Reticulum

The SER consists of a membrane forming flattened sacs. The membranes are separated by fluid-filled spaces called cisternae. The SER is involved in the synthesis and transport of lipids. 

Ribosomes

Ribosomes have two units: a large subunit and a small subunit. Ribosomes also vary in size: in prokaryotes, they are size 70s (smaller) and are found free in the cytoplasm, whilst in eukaryotes, they are 80s (larger) and are found either attached to the RER or in the cytoplasm. Ribosomes are assembled in the nucleolus from rRNA and protein. They are the site of translation, where an mRNA strand is used to assemble a polypeptide chain. 

Golgi Body

The Golgi Body is formed from the RER being pinched off at either end to form vesicles. A number of these vesicles then fuse together to form the Golgi apparatus. The Golgi body is…