Lysosomes are the main digestive compartment of the cell. As such, they contain a variety of enzymes capable of degrading different types of biological material including nucleic acids, lipids and proteins among others.
They can be found in animal cells and some plant cells (occurring as vacuoles) and are capable of breaking down various types of macromolecules brought in to the cell to be degraded. Most of these are either damaged or have completed their life cycle and are no longer useful.
In addition to these macromolecules, lysosomes also serve to break down cells once they die. While they can be found in almost all cells in animals (except red blood cells) they are particularly abundant in tissues/organs that are involved in high enzymatic reactions.
These include such tissues/organs as the liver, kidney, macrophages and pancreas among a few others. Cells of these tissues/organs contain abundant lysosomes.
* The name lysosome originated from Greek words Lysis (meaning destroy/dissolve) and Soma (meaning body).
* Animal cells may contain numerous lysosomes (several hundred) plant and yeast cells typically have a single, large lysosome (vacuole).
There are two main types, these include:
Primary lysosomes - are formed from Golgi apparatus appearing as small vesicles. Although primary lysosomes are popular on Golgi apparatus, they also occur as granulocytes and monocytes. These lysosomes are surrounded by a single phospholipid layer and contain acid hydrolases.
The pH value of the acid in these vesicles is important in that its changes activate or deactivate the enzymes. Ultimately, most of the primary granules will fuse with phagosomes, which results in the formation of secondary lysosomes.
Secondary lysosomes - are formed when primary lysosomes fuse with phagosomes/pinosome (they are also referred to a endosomes). The fusion also causes the previously inactive enzymes to be activated and capable of digesting such biomolecules as nucleic acids and lipids among others.
Compared to primary lysosomes, secondary are larger in size and capable of releasing their content (enzymes) outside the cells where they degrade foreign material.
A majority of lysosomal enzymes function inside the acidic environment, which is why they are referred to a acid hydrolases.
They contain about 45 enzymes that are grouped in to six main categories:
* Lysosomes cannot digest themselves - Most of the proteins present in its membrane contain high amounts of carbohydrate-sugar groups. Because of the present of these groups, digestive enzymes are unable to digest the proteins present on the membrane.
Lysosomes are membrane-delimited organelles. This means that they are surrounded by a membrane that prevents its components from being released. This is particularly important given that uncontrolled release of the acidic fluid and enzymes can cause damage to the components of the cell.
They also have a high concentration of protons, which results in pH value of less than 5.
The surrounding membrane is composed of integral proteins as well as a vacuolar-type H+ ATPase, highly glycosylated proteins and a number of transporters. Depending on the type of lysosome and their function, they also greatly vary in size (between 1 micrometer and several microns) and general shape.
They are less defined compared to other types of organelles. When viewed, they appear as cytoplasmic dense bodies that may be ovoid, spheric or tubular on occasion.
The manner in which lysosomes function highly depends on the way the enzymes affect other materials outside and inside the cell. There are a number of processes through which lysosomes digest material.
Endocytosis is one of the most popular phenomenon exhibited by cells. In endocytosis, invagination of the plasma membrane of the cell results in the creation of an endocytic vesicle that engulfs different types of extracellular molecules.
In phagocytosis (a type of endocytosis) large molecules/microorganisms like bacteria are engulfed in a vacuole (phagosome). However, in pinosomes (another type of endocytosis) a small amount of the surrounding fluid and solute molecules are pinched off as pinosomes (pinocytic vesicle).
Once these vesicles fuse with primary lysosomes, secondary lysosomes are formed. Enzymes are then activated and act on the molecules. This process is commonly referred to as the "Endosomal Pathway"
See also: Endocytosis Vs Exocytosis
* Digested material is typically passed into cellular component while undigested ones are excreted.
Material uptake- formation of endosome - formation of the phagolysosome - lysis- diffusion of digested material and exocytosis.
Apart from endocytosis, lysosomes are also involved in another process referred to as autophagocytosis. This process helps in the degradation of various cell components that are either worn out or malfunctioning.
In addition to simply breaking down cellular components, this process helps in recycling of these material. Here, autophagic lysosomes (secondary) release enzymes that digest various cell components as soon as the cell dies. This is also referred to as autolysis.
Autophagy also takes place during starvation. During starvation periods, lysosomes will start hydrolyzing organic foods that are stored in cells so as to produce energy.
In plants, vacuoles serve many functions, which make them multifunctional organelles. However, they also have basic properties similar to lysosomes found in the cells of animal. As such, they have an acidic nature and contain various hydrolic enzymes capable of breaking down different types of molecules.
There are different types of vacuoles that serve different functions. These include:
Lysosomes are also important in that they act as recycling centers. Whenever different types of molecules or cell components are broken down (for instance proteins broken down to amino acids) the amino acids are then used as building blocks of new proteins. This ensures that some of the byproducts are re-used in the body.
On the other hand, they help recycle material that are not easy to excrete. A good example of this is iron. When iron is released from the breakdown of various cells or cell components (such as red blood cells), iron is recycled and used for the construction of new organelles. This allows for a minimized excretion of the by-products and retention of others to be used in the body.
The following is a procedure to view plant vacuole:
Apart from viewing many irregular cells and a cell nucleus, students will clearly see a large vacuole at the center of the cell.
Vinay Kumar (2012) Complete Biology for Medical College Entrance Examination.