The cell is the structural and functional unit of life. The human body is composed of specialized tissues which in turn consist of vast clusters of cells. Two general types of cells are recognized in nature. They are prokaryotes and eukaryotes.
- The word Prokaryote is derived from a Greek word (Pro – before; karyon – nucleus). They lack a well defined nucleus and possess simple structure. Present mainly in the bacteria, blue green algae and certain types of microorganisms.
- The word Eukaryote is also derived from the Greek word (Eu – true ; karyon – nucleus). They possess a well defined nucleus and are more complex in their structure and function. Present in all higher organisms, (both plant and animal) as well as in fungi, protozoa, etc.
The differences between a prokaryotic and eukaryotic cell are given in Table 1.1.
EUKARYOTIC CELL
In humans, eukaryotic cells exist in a variety of sizes and shapes that allow adaptation to their special functions. Despite their diversity in function they have certain features in common. The nucleus is separated from the rest of the cell constituents by an envelope called the nuclear membrane and contains the DNA (Fig. 1.1).
- The cell units are separated from their surroundings by a plasma membrane. The plasma membrane is 7–10 nm thick.
- It is primarily composed of a lipid bilayer, in which is dispersed a considerable amount of protein. A small amount of carbohydrate is present in membranes as glycoproteins and glycolipids.
- Both protein and lipid molecules have freedom of movement within the framework of the membrane which is made possible by the fluid nature of the bilayer.2
- Movement of proteins is maintained in a lateral direction. This important property of fluidity facilitates the conformational changes that occur in the protein molecules of the membrane during their functional activity.
- The plasma membrane performs the following functions.
- It holds the cells together and makes it a functional unit.
- Serves as a selective barrier—permitting the entrance of essential nutrients into the cell.
- Secretes the waste products and thus keeps out toxic materials.
- Binds certain regulatory substances, particularly hormones in a location that is most advantageous to the performance of their function.
SUBCELLULAR ORGANELLES
The various subcellular organelles present in the cell include the following:
- Nucleus
- Mitochondria
- Endoplasmic reticulum
- Golgi apparatus
- Lysosomes
- Peroxisomes
- Cytosol and cytoskeleton
The subcellular organelles are also surrounded by membranes. A brief note on the structure and function of various subcellular organelles are mentioned.
Nucleus
It is the largest cellular organelle surrounded by a double layered membrane known as nuclear envelope. The ground material present in the nucleus is known as nucleoplasm. Each of the membranes is a typical bilayer composed of phospholipids and cholesterol. The outer membrane is usually covered with ribosomes and is continuous with the endoplasmic reticulum. At certain intervals the two nuclear membranes have 3nuclear pores with a diameter of about 90 nm. These pores permit the free passage of the products synthesized in the nucleus into the surrounding cytoplasm.
Nucleolus
Nucleus contains a dense body known as the nucleolus. This consists of that portion of DNA which codes for ribosomal RNA. The nucleolus is therefore rich in ribosomal RNA which enters the cytosol through the nuclear pores.
Function: Nucleus contains DNA, the repository of genetic information. Approximately 90% of the cells DNA is found concentrated in the organelle as part of structures called chromosomes.
- Human cells contain the genetic information distributed among 46 such chromosomes.
- Eukaryotic DNA is associated with basic proteins (histones) in the ratio of 1:1 to form nucleosomes. An assembly of nucleosomes constitutes the chromatin-fibres of the chromosomes.
- The nucleoplasm is rich in enzymes such as DNA polymerases and RNA polymerases which are involved in the synthesis of DNA and RNA and also possess the enzymes for DNA repair.
Mitochondria
Are the second largest organelles in the cell measuring 0.5–1 µm in diameter and 1–10 µm in length. There are usually 1–1000 mitochondria per cell depending on the nature and function of the cell. They are composed of a double membrane system. The outer membrane is smooth and composed of a phospholipid and cholesterol rich bilayer with an equal amount of protein. The inner membrane is invaginated for forming shelves or cristae which extends into the matrix.
Function: Mitochondria are the site of respiration and concomitant production of ATP which is the energy currency of the cell.
- ATP generated in the mitochondria is exported to all the parts of the cell to provide energy for cellular work, such as motility, active transport of nutrients and certain energy requiring metabolic reactions. Mitochondria are often termed power houses of the cell.
- Their number and size may reflect the need for energy and the nature of metabolic activity occurring in the tissue. For example, in cardiac muscle, where metabolism is essentially aerobic, the mitochondria are large and numerous, whereas in skeletal muscle which can function anaerobically as well, the mitochondria are smaller and fewer.
- The components of electron transport chain (ETC) and oxidative phosphorylation concerned with ATP production are buried in the inner mitochondrial membrane.
- The matrix contains several enzymes concerned with catabolism of carbohydrates, lipids and amino acids. The matrix enzymes also participate in the synthesis of heme and urea.
- Mitochondria also contain a certain amount of DNA and ribosomes and are capable of synthesizing some of its own proteins.
Eukaryotic cells possess an extensive network of internal membranes pervading the cytoplasm. They are termed the cytomembranes and are usually subdivided into endoplasmic reticulum and Golgi apparatus.
Endoplasmic Reticulum
Consists of a complicated array of vesicular spaces separated from the cytosol by a system of membrane (7nm). The endoplasmic 4reticulum is divided into rough and smooth types.
Rough Endoplasmic Reticulum (RER)
Is lined on the cytoplasmic surface with multitude particles called ribosomes. These are responsible for the rough appearance of this type of endoplasmic reticulum under the electron microscope. Cells that produce proteins for secretion or export, e.g., pancreatic acinar cells and intestinal goblet cells, are rich in membrane bound ribosomes.
Function: The rough endoplasmic reticulum is the site of protein synthesis.
Smooth Endoplasmic Reticulum (SER)
Is in continuation with the rough endoplasmic reticulum. It is distinguishable from the rough by the absence of ribosomes.
Function:
- Mainly concerned with the synthesis of lipids.
- It is also the location of glycosyl transferases involved in the synthesis of glycoproteins and glycolipids.
- It is responsible for the detoxification of xenobiotics.
Golgi Apparatus
Appear as flattened sacs or vesicles and are continuous with the endoplasmic reticulum. They function in modifying and sorting out the proteins.
- The newly synthesized proteins are transported and temporarily stored in the Golgi apparatus (e.g., zymogen granules found in the pancreatic acinar cells prior to the release of proteins from the cells).
- Secretory proteins as well as proteins of lysosomes and plasma membrane are synthesized on the membrane bound polyribosomes. They are then transported to the Golgi apparatus where they are suitably modified and sent to appropriate destinations.
- Secretory proteins are enveloped in vesicles. Secretion involves the fusion of the vesicles with the plasma membrane followed by discharge of the contents into the extracellular space. This process is called exocytosis.
- In doing so they must successively pass through each of the following cytomembrane systems.
RER ♦ SER ♦ Golgi Apparatus ♦ Vesicles ♦ ECF
Function: Primarily concerned with the modification (sorting) and export of proteins.
Lysosomes
Are spherical vesicles enveloped by a single membrane (0.4–0.9 µm in diameter). They contain a variety of hydrolytic degradation enzymes and are regarded as the digestive tract of the cell. The enzymes attack the various substrates and are as follows:
Glycosidase | – Glycogen |
Arylsulfatase | – Glycolipids |
Lipase | – Lipids |
Phospholipase | – Phospholipids |
Cathepsins | – Proteins |
Phosphatase | – Phosphorylated substrate |
- The lysosomes (primary lysosomes) fuse with membrane bound vesicles containing the substrates to be partially or wholly digested. The fused lysosome is called secondary lysosome.
- The pH of the lysosomal matrix is more acidic (pH < 5) than the cytosol (pH > 7) and this facilitates the degradation of different compounds.
- The digestive enzymes of the cell are confined to the lysosomes in the best interest of the cell. Escape of these enzymes into the cytosol will destroy the functional macromolecules of the cell and result in many complications.
- Occurrence of diseases such as arthritis and some of the muscular disorders have been partly attributed to the release of lysosomal enzymes.
Function: The lysosomal enzymes are responsible for maintaining the cellular compounds in a dynamic state by their degradation and recycling.
Peroxisomes
Are also known as microbodies and are spherical with a diameter of 0.8–1.5 µm. They are distinguishable by their content of catalase, an enzyme that degrades hydrogen peroxide (H2O2).
Function:
- Catalase protects the cell from the toxic effects of H2O2 by converting it to H2O and O2.
- Peroxisomes are also involved in the oxidation of long chain fatty acids (C>18 and synthesis of plasmalogens and glycolipids.
Cytosol
The extranuclear cell content that possesses both soluble and insoluble constituents is termed as cytosol. The cytosol makes up 70–75% of the cellular weight. Many proteins, enzymes, metabolic intermediates and inorganic salts are found in this gel like medium.
Function:
- Important metabolic processes such as glycolysis, gluconeogenesis, HMP shunt pathway and fatty acid synthesis occur in the cytosol.
- Recent studies indicate that cytoplasm contains a complex network of protein filaments spread throughout and constitutes the cytoskeleton. The cytoplasmic filaments are of three types, viz
- Microfilaments
- Actin filaments
- Intermediate filaments.
The filaments are polymers of proteins which are responsible for the structure, shape, and intracellular movement of organelles and also provide support to the cell.