Unit 6: Cell Organelle
Prokaryotic cell
Ribosome
A small round organelle that is use for protein synthesis during RNA translation. The RNA translation refers to the action of synthesizing an amino acid chain according to the sequence of mRNA. This done by the ribosome binding to the corresponding region of the mRNA chain and draw in the amino acid. Thus, it is regarded as a molecular machine as it takes in a command and synthesize a product according to it.
A ribosome is made from complexes of RNAs and proteins and is therefore a ribonucleoprotein. The structure of ribosome comprises of 2 subunits:
- smaller subunit (SSU) binds to a LSU and the mRNA
- larger subunit (LSU) binds to the tRNA, the amino acids, and the SSU.
For prokaryotes, ribosome is free-lying and directly translated from the mRNA. However, Prokaryotic ribosomes are smaller and have a slightly different shape and composition than those found in eukaryotic cells. Some of the processes involved may also be different.
Plasmids
A plasmid is a small circular DNA molecule that is physically separated from the chromosome and can replicate independently. It maybe an extracellular genetic material from Horizontal gene transfer or artificially induced via transformation. The information contained in the plasmid is usually less important and does not directly relate the survival of the organism. In fact, the plasmid controls the additional traits that can help the cell to better survive.
Eukaryotic cell
The table below gives an overview of the major eukaryotic organelles.
Chloroplast
This organelle is extremely important as it is responsible of carrying photosynthesis, which provides the organisms with food.
The disc-like structure shown is known as thylakoids, and together in a stack is known as granum. The other remaining space in the chloroplast is filled with stroma, a gel like fluid that contains the medium and all the enzymes for the biochemical reactions of photosynthesis to take place.
However, as photosynthesis will not be an emphasis on this curriculum, we will not spend that much time in this.
Endoplasmic Reticulum (ER)
ER can be classified into Rough ER (RER) and Smooth ER (SER), by the presence of ribosome. The ER appear as a flattened sac that spans out in the cytoplasm,but actually it is a network of membrane (known as cisternae) that encloses some phospholipids molecule it.
RER has ribosome attached to it via a translocon, and is responsible for the synthesis of protein by translation (will be discussed in. The peptide produce maybe transported to the Golgi apparatus for further processing by a transport vesicle.
SER has no ribosome attached to it and and is responsible for the lipids production. However as this is not related to this curriculum so we won't discuss about it here.
Golgi apparatus
The Golgi appears to be a stack of disc-like membrane but it does vary a lot in size. It mainly consist of 3 compartments: cis, medial, and trans. It function as a major collection and dispatch station of protein products received from the RER. It modifies the protein using various enzymes that is packed in it and release its product via outgoing transport vesicle.
Mitochondrion
This is an extremely important organelle for a cell as the mitochondria provides energy to the cell by respiration. It’s structure is rather uncommon as it has a double membrane instead of a single membrane like most other organelles does. The Kreb cycle and oxidative phosphorylation will take place in the mitochondria for aerobic respiration.
The highly folded inner membrane provided a large surface area for the various biochemical reactions to take place. The mitochondrial matrix also provides a suitable medium for various reactions to happen.
Nucleus
The nucleus houses all the genetic material (DNA) that is present in the cell. The DNA are stored as chromosome an it is enclosed by the nucleus membrane. The mRNA transcripted in the nucleus will be transported out of the nucleus membrane. This process will be further discussed in course 2.
Vacuole
It is a single membrane bounded compartments which are filled with water containing inorganic and organic molecules like enzymes in solution. It can vary greatly in size and shape, but the size is related to the water potential.
Cytoskeleton
It is a giant network of filaments and tubules that is dynamic in nature and is capable of rapid growth and disassemble upon the command of the cell. Its primary function is to give the cell its shape and mechanical resistance to deformation. Another function of it is to facilitate intracellular transport (the transport of substance within a cell). You can picture the cytoskeleton as the highway and different vehicles (transport vesicle) is travelling on it.
