BASIC UNIT OF LIFE: CELL
Just like a wall is made up of building blocks called bricks, our bodies are also made up of building blocks called as the cell.
A cell is the basic structural and functional unit of life.
A living thing whether made of one cell or many cells is called an organism. When cells of one kind are interconnected and perform a shared function, they form a tissue. And when such tissues function together, they form an organ and in turn, these organs together constitute an organ system.
Thus, it can be said that an organism is composed of various organ systems that coordinate and support life.
A cell is an independent unit which is capable of its independent existence, such as the single-celled organisms. A cell can perform all its functions as it has the distribution of labor in the form of cell organelles.
The cell can be divided into two types on the basis of evolutionary development; it can be prokaryotic and eukaryotic.
PROKARYOTIC AND EUKARYOTIC CELLS
The prokaryotic cells are the primitive cells that do not have the membrane-bound organelles and a true nucleus. Organisms with prokaryotic cells are unicellular, like bacteria.
Their genetic material lies in the nucleoid region that contains DNA and devoid of any membranes.
The prokaryotic cells are simpler when compared to a eukaryotic cell.
The eukaryotic cell has all the membrane-bound organelles and is more advanced. Their organelles perform a specific function. Organisms having eukaryotic cells are protozoa, fungi, plants, and animals. They have the true nucleus, which contains the genetic material DNA and is surrounded by a membrane.
CELL ORGANELLES AND THEIR FUNCTION
The cell and its components are bounded by a cell membrane. In plants, there is an additional layer called the cell wall around the cell membrane. A cell is composed of various cell organelles such as a nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, etc.
A nucleus is a double membrane structure; the outer membrane is continuous with the membrane of the endoplasmic reticulum. The two membranes have pores at certain intervals that allow the passage of molecules across it. The nucleus contains DNA, which is the genetic material. The ground material of nucleus is called as the nucleoplasm, which is rich in certain enzymes. The nucleus can be regarded as the controlling unit of the cell.
The mitochondria are called as the powerhouse of the cell, as it is responsible for generating the energy required for all cellular functions. They are the centers for cellular respiration and energy metabolism. They are rod-like structures with a double membrane around them. The outer membrane is smooth; while the inner membrane has infoldings that form ‘cristae’. The internal chamber of mitochondria is also called as the matrix. The electron transport chain and oxidative phosphorylation occur in the mitochondria.
The mitochondria also have their own DNA, RNA, and independent protein synthesis machinery.
ER is a network of membrane-enclosed spaces that extend throughout the cytoplasm. ER is of two types depending on the presence of ribosomes; it can be Smooth Endoplasmic Reticulum when there are no ribosomes and Rough Endoplasmic Reticulum when ribosomes are present on its surface.
The RER is responsible for the synthesis of proteins.
The SER is involved in the synthesis of lipids, metabolism of drugs and supplying calcium ions for cellular functions.
The ER may extend from the nuclear membrane to the cell membrane.
The Golgi apparatus is a cluster of membrane vesicles. The proteins synthesized from the ER are sent to Golgi apparatus for packing and transport. It also catalyzes the addition of carbohydrates, lipids and sulfate moieties to the protein. The protein is enclosed in vesicles and secreted from the Golgi apparatus when signaled.
Lysosomes are single membrane-bound structures that are also known as the suicidal bags of the cell. Their main function is the digestion of cellular substance such as proteins, lipids, carbohydrates and nucleic acids. They have enzymes called hydrolases that carry out the function. The degraded products leave the lysosomes by diffusion. The release of these digestive enzymes into the cytosol will cause a destruction of cellular functions.
The cellular matrix is referred to as the cytosol. It contains the enzymes, metabolites, and salts in an aqueous gel like a medium. The cytoplasm contains the complex network of proteins that form the cytoskeleton and provide shape to it.
The cell wall is a characteristic component of a plant cell. It is also found in other organisms as well, like the fungi, the bacteria.
The cell wall is rigid and non-permeable. It has a protective function and also provides shape to the cell.
It is made up of different chemicals in different organisms, like chitin in fungi and cellulose in plants.
Vacuoles are membrane-bound liquid filled vesicles. They may contain a wide range of dissolved molecules and various pigments. Characteristically found in plants and lower organisms.
There are certain organelles that are found in plants and not animal cells, like chloroplast.
These cell organelles make the cell a self-sustaining unit and when these cells function together, they sustain the life of a living thing.