Each cell contains millions of ribosomes. They are very tiny, non-membrane bounded organelles made of protein and RNA which consist of two sub-units that come together during protein synthesis. Ribosomes are distributed throughout the cell. Some of them are embedded in rough endoplasmic reticulum to translate a protein secreted into it while some flows freely in the cytoplasm for making protein that stays in the cell.
Fused to the nuclear membrane is the endoplasmic reticulum. There are two types of endoplasmic reticulum: the smooth endoplasmic reticulum (sER) and the rough endoplasmic reticulum (rER). As implied by the name, they are basically the same organelle with different appearance. The sER is a complex network of fluid-filled space enclosed by a system of membrane. The rER are similar with ribosomes adhere on the cytoplasmic surface. However, they have different functions. The sER is the site where carbohydrates, steroids, lipids and other non-protein products are synthesized, secreted and stored; whereas the rough synthesis protein. The coded information in nucleus moved to ribosomes on the rER and into it through cisternae. New protein are synthesised in the rER and buds-off at the other end into the cytoplasm.
Part of the rER containing cell products moved to the Golgi apparatus (also known as Golgi body). The Golgi apparatus is layers of fluid-filled flattened sacs with smaller vacuoles or vesicles at both ends of the main structure and roughly parallel to each other. It serves as a packaging department to deliver protein products made in the cell. Molecules from rER are processed and packaged in the sacs to form other substances such as carbohydrates and enzymes so that they are well isolated. When the products are excreted from the Golgi apparatus, they appear as vacuoles at both ends of the body.
Lysosomes are also made in the Golgi apparatus. It is a small membrane-bounded sac vesicles in the cytoplasm filled with digestive enzymes to break down and remove worn out parts of the cell. They bud off from the Golgi apparatus and fuse with the food vacuoles.
The products packaged from golgi apparatus are then released out of the cell by exocytosis through the plasma membrane. It is a phospholipid bilayer – the hydrophilic heads face out towards the water on both sides of the membrane while the hydrophobic tails face inwards – which makes a good barrier against most water-soluble molecules. Extrinsic protein partly embedded in the bilayer to provide mechanical support while intrinsic proteins pass all the way through the bilayer to provide a passage for large or charged water-soluble substances. Glycoprotein and glycolipids are connected to the bilayer to serve as recognition sites. Microvilli are folds of the membrane and are found on cells involving processes like absorption. They are finger-like projections which increase the surface area for diffusion to take place.
Cells often require energy for their activities and which is provided by the mitochondria. It is a double membrane bounded organelle in oval shape and sometimes branched. The highly-folded inner membrane forms cristae and contains matrix which store enzymes involved in respiration. Hydrogen ions in the matrix give an electrical potential to serve as a 'power house'.
A vacuole is a general term for any membrane bounded sac with little or no internal structure. It is a compartment containing cell sap to cope with the function of the cell. In a plant cell, it plays an important role in water balance and storing minerals and wastes. In an animal cell, it is just a tiny waste storage space.
A plant cell has all the same organelles with similar functions as animal cells but a few extras. Cell wall and chloroplast are organelles found only in plant cells and other eukaryotic organisms.
Cell wall is a rigid layer with plasmodesmata. This allows the exchange of substances with adjacent cells. The cell wall is mainly made of carbohydrate cellulose. Cellulose is a disaccharide with more complex bonding which requires more energy to break the bonds. Thus it can provide mechanical support and protection.
Chloroplasts have similar functions as the mitochondria. It is a small, flattened discs surrounded by a double membrane. It contains internal membranes called thylakoids which form coin-like stacks called grana and linked by thin, flat pieces of thylakoids membranes called lamellae. Photosynthesis takes place in the thylakoids membranes to provide the cell with glucose. There is also a thick fluid called stroma in the chloroplast where photosynthesis takes place there sometimes.
In general, the structures and the functions of organelles have a close linkage. With the above information, it is shown that all organelles have their unique way of operation which none of it is dispensable.
