Cell: The Basic Unit of Life (With Diagram)

Read this article to learn about the cell-structural and functional unit of life and also about the silent features of animal cell and plant cell.

Cell:

The cell is the structural and functional unit of life. It may be also regarded as the basic unit of biological activity.

The concept of cell originated from the contributions of Schleiden and Schwann (1838). However, it was only after 1940, the complexities of cell structure were exposed.

Prokaryotic and Eukaryotic Cells:

The cells of the living kingdom may be divided into two categories:

1. Prokaryotes (Greek: pro-before; karyon – nucleus) lack a well-defined nucleus and possess relatively simple structure. These include the various bacteria.

2. Eukaryotes (Greek: eu-true; karyon-nucleus) possess a well-defined nucleus and are more complex in their structure and function. The higher organisms (animals and plants) are composed of eukaryotic cells.

A comparison of the characteristics between prokaryotes and eukaryotes is listed in Table 62.1.

Eukaryotic Cell:

The salient features of an animal cell and a plant cell are briefly described:

Animal Cell:

The human body is composed of about 10¹⁴ cells. An eukaryotic cell is generally 10 to 100µm in diameter. A diagrammatic representation of a typical rat liver cell is depicted in Fig. 62.1.

The cell consists of well-defined subcellular organelles, enveloped by a plasma membrane. By differential centrifugation of tissue homogenate, it is possible to isolate each cellular organelle in a relatively pure form.

Nucleus:

Nucleus is the largest cellular organelle, surrounded by a double membrane nuclear envelope. The outer membrane is continuous with the membranes of endoplasmic reticulum. At certain intervals, the two nuclear membranes have nuclear 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.

Nucleus contains DNA, the repository of genetic information. Eukaryotic DNA is associated with basic protein (histones) in the ratio of 1: 1, to form nucleosomes. An assembly of nucleosomes constitutes chromatin fibres of chromosomes (Greek: chroma-colour; soma-body). Thus, a single human chromosome is composed of about a million nucleosomes. The number of chromosomes is a characteristic feature of the species. Humans have 46 chromosomes, compactly packed in the nucleus.

The nucleus of the eukaryotic cell contains a dense body known as nucleolus. It is rich in RNA, particularly the ribosomal RNA which enters the cytosol through nuclear pores. The ground material of the nucleus is often referred to as nucleoplasm. It is rich in enzymes such as DNA polymerases and RNA polymerases. To the surprise of biochemists, the enzymes of glycolysis, citric acid cycle and hexose monophosphate shunt have also been detected in the nucleoplasm.

Mitochondria:

The mitochondria (Greek: mitos-thread; chondros-granule) are the centres for the cellular respiration and energy metabolism. They are regarded as the power houses of the cell with variable size and shape. Mitochondria are rod-like or filamentous bodies, usually with dimensions of 1.0 × 3 nm. About 2,000 mitochondria, occupying about ¹/₅th of the total cell volume, are present in a typical cell.

The mitochondria are composed of a double membrane system. The outer membrane is smooth and completely envelops the organelle. The inner membrane is folded to form cristae (Latin-crests) which occupy a larger surface area. The internal chamber of mitochondria is referred to as matrix.

The components of electron transport chain and oxidative phosphorylation (flavoprotein, cytochromes b, c_1; c, a and a₃ and coupling factors) are buried in the inner mitochondrial membrane. The matrix contains several enzymes concerned with the energy metabolism of carbohydrates, lipids and amino acids (e.g., citric acid cycle, β-oxidation).

The matrix enzymes also participate in the synthesis of heme and urea. Mitochondria are the principal producers of ATP in the aerobic cells. ATP, the energy currency, generated in mitochondria is exported to all parts of the cell to provide energy for the cellular work.

The mitochondrial matrix contains a circular double-stranded DNA (mtDNA), RNA and ribosomes. Thus, the mitochondria are equipped with an independent protein synthesizing machinery. It is estimated that about 10% of the mitochondrial proteins are produced in the mitochondria.

The structure and functions of mitochondria closely resemble prokaryotic cells. It is hypothesized that mitochondria have evolved from aerobic bacteria. Further, it is believed that during evolution, the aerobic bacteria developed a symbiotic relationship with primordial anaerobic eukaryotic cells that ultimately led to the arrival of aerobic eukaryotes.

Endoplasmic reticulum:

The network of membrane enclosed spaces that extends throughout the cytoplasm constitutes endoplasmic reticulum (ER). Some of these thread­like structures extend from the nuclear pores to the plasma membrane.

A large portion of the ER is studded with ribosomes to give a granular appearance which is referred to as rough endoplasmic reticulum. Ribosomes are the factories of protein biosynthesis. During the process of cell fractionation, rough ER is disrupted to form small vesicles known as microsomes. It may be noted that microsomes as such do not occur in the cell.

The smooth endoplasmic reticulum does not contain ribosomes. It is involved in the synthesis of lipids (triacylglycerol’s, phospholipids, and sterols) and metabolism of drugs, besides supplying Ca²⁺ for the cellular functions.

Golgi apparatus:

Eukaryotic cells contain a unique cluster of membrane vesicles known as dictyosomes which, in turn, constitute Golgi apparatus (or Golgi complex). The newly synthesized proteins are handed over to the Golgi apparatus which catalyse the addition of carbohydrates, lipids or sulfate moieties to the proteins. These chemical modifications are necessary for the transport of proteins across the plasma membrane.

Certain proteins and enzymes are enclosed in membrane vesicles of Golgi apparatus and secreted from the cell after the appropriate signals. The digestive enzymes of pancreas are produced in this fashion. Golgi apparatus are also involved in the membrane synthesis, particularly for the formation of intracellular organelles (e.g. peroxisomes, lysosomes).

Lysosomes:

Lysosomes are spherical vesicles enveloped by a single membrane. Lysosomes are regarded as the digestive tract of the cell, since they are actively involved in digestion of cellular substances— namely proteins, lipids, carbohydrates and nucleic acids. Lysosomal enzymes are categorized as hydrolases.

These include the following enzymes (with substrate in brackets):

i. α-Glucosidase (glycogen)

ii. Cathepsins (proteins)

iii. Lipases (lipids)

iv. Ribonucleases (RNA)

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 lysosomal enzymes are responsible for maintaining the cellular compounds in a dynamic state, by their degradation and recycling.

The degraded products leave the lysosomes, usually by diffusion, for reutilization by the cell. Sometimes, however, certain residual products, rich in lipids and proteins, collectively known as lipofuscin accumulate in the cell. Lipofuscin is the age pigment or wear and tear pigment which has been implicated in ageing process.

The digestive enzymes of cellular compounds are confined to the lysosomes in the best interest of the cell. Escape of these enzymes into cytosol will destroy the functional macromolecules of the cell and result in many complications. The occurrence of several diseases (e.g. arthritis, muscle diseases, allergic disorders) has been partly attributed to the release of lysosomal enzymes.

Peroxisomes:

Peroxisomes, also known as micro bodies, are single membrane cellular organelles. They are spherical or oval in shape and contain the enzyme catalase. 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 (>C₁₈) and synthesis of plasminogen’s and glycolipids. Plants contain glyoxysomes, a specialized type of peroxisomes, which are involved in the glyoxylate pathway.

Cytosol and cytoskeleton:

The cellular matrix is collectively referred to as cytosol. Cytosol is basically a compartment containing several enzymes, metabolites and salts in an aqueous gel like medium. More recent studies however, indicate that the cytoplasm actually contains a complex network of protein filaments, spread throughout, that constitutes cytoskeleton. The cytoplasmic filaments are of three types- microtubules, actin filaments and intermediate filaments. The filaments which are polymers of proteins are responsible for the structure, shape and organization of the cell.

Plant Cell:

A diagrammatic representation of a typical plant cell is depicted in Fig. 62.2. The cell wall, chloroplasts and vacuoles are the most important and distinguishing components of plant cells when compared to animal cells.

In the Table 62.2, a comparison between plant and animal cells is given. The salient features of plant cell organelles are briefly described.

Cell wall:

The plant cell wall is usually rigid, non-living and permeable component, surrounding the plasma membrane. Cell walls are of two types — primary and secondary.

The primary cell wall is the one that is formed during the course of cell division. It is mainly composed of cellulose, and is flexible in nature.

The secondary cell wall is rigid and more complex in nature. Chemically, it is made up of more cellulose, and high content of lignin. Lignin is the major component of wood. The secondary cell wall is inextensible and determines the final shape and size of the plant cell. Besides cellulose and lignin, hemicelluloses, pectins, and extensins (oligosaccharides) are also present in the cell wall.

Chloroplasts:

The chloroplasts are found only in the plant cells, and are the sites of photosynthesis. The general term plastids is often used to collectively represent chloroplasts (green plastids containing chlorophylls), chromoplasts (yellow to reddish colour plastids containing carotenoids) and leucoplasts (colourless plastids).

Chloroplasts have a double membrane system. Internally, chloroplasts contain a system of flattened membranous discs called thylakoids. Piles of thylakoids are located in the central region called stroma. Chlorophyll, the sunlight-capturing green pigment is present in the thylakoids.

Vacuoles:

Plant cells have membrane-bound, liquid filled vesicles called vacuoles, which may occupy as much as 90% of the cells total volume. The vacuoles may contain wide range of dissolved molecules such as salts, sugars, pigments and toxic wastes. The pigments of vacuoles contribute to the colours of flowers and leaves. The physical support of plant tissues comes from the high internal pressure of water maintained within the vacuoles.