Chapter-8 Cell: The Unit of Life

Cell Theory and Cell as the Basic Unit of Life

All living organisms are composed of cells. Some are made of a single cell (unicellular) and others of many cells (multicellular). The cell is the fundamental, structural, and functional unit of all living organisms.

Definition: Cell theory states that all living organisms are composed of cells and products of cells, and all cells arise from pre-existing cells.

The formulation of cell theory was a collaborative effort. Matthias Schleiden (1838) examined plants and stated that all plants are composed of different kinds of tissues which form the cells. Theodor Schwann (1839) studied animal and plant cells and reported that cells have a thin outer layer today known as the plasma membrane. He also concluded that the presence of a cell wall is a unique character of plant cells. Schleiden and Schwann together formulated the cell theory. However, this theory did not explain how new cells were formed. Rudolf Virchow (1855) first explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the cell theory to give it a final shape.

Key Points of Modern Cell Theory

• All living organisms are made up of cells and their products.
• All cells come from pre-existing cells.
• Metabolic reactions take place inside cells.
• Cells contain hereditary information (DNA) passed from cell to cell during division.

Exam-Oriented Note: Remember the contributions of Schleiden, Schwann, and Virchow clearly for objective questions. Do not confuse who discovered plant cell walls versus plasma membranes.

Structure of Prokaryotic and Eukaryotic Cells

Based on the organization of their nucleus and membranes, cells are broadly classified into two types: prokaryotic and eukaryotic.

Prokaryotic Cells

Prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms). They are generally smaller and multiply much faster than eukaryotic cells. They lack a membrane-bound nucleus and true membrane-bound organelle systems.

• Genetic Material: The genomic DNA is naked, not enveloped by a nuclear membrane. It typically consists of a single circular DNA molecule known as the nucleoid.
• Plasmids: Many bacteria have small circular DNA outside the genomic DNA, called plasmids. These confer unique phenotypic characters, such as antibiotic resistance.
• Inclusion Bodies: Reserve material in prokaryotic cells is stored in the cytoplasm in the form of inclusion bodies (e.g., phosphate granules, cyanophycean granules, glycogen granules).

Eukaryotic Cells

Eukaryotic cells include all protists, plants, animals, and fungi. They possess an organized nucleus with a nuclear envelope and distinct membrane-bound cytoplasmic organelles, allowing for compartmentalization of metabolic functions.

Cell Envelope: Cell Membrane and Cell Wall

Most prokaryotic cells, particularly bacterial cells, have a chemically complex cell envelope. The cell envelope consists of a tightly bound three-layered structure: the outermost glycocalyx, followed by the cell wall and then the plasma membrane.

Cell Membrane (Plasma Membrane)

The plasma membrane is selectively permeable to some molecules present on either side of it. In eukaryotes, it is composed of a lipid bilayer with proteins embedded within it.

Practical Application: The fluid mosaic model describes the structure of the plasma membrane, where lipids facilitate the fluid nature, allowing lateral movement of proteins. This fluidity is important for functions like cell growth, formation of intercellular junctions, secretion, and endocytosis.

Cell Wall

The non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of fungi and plants. Plant cell walls are primarily made of cellulose, hemicellulose, pectins, and proteins, while algal cell walls are made of cellulose, galactans, mannans, and minerals like calcium carbonate.

• Middle Lamella: The cell wall of a young plant cell, the primary wall, is capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on the inner side (towards membrane). The middle lamella is a layer mainly composed of calcium pectate which holds or glues the different neighbouring cells together.
• Plasmodesmata: The cell wall and middle lamella may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells.

Endomembrane System

While each of the membranous organelles is distinct in terms of its structure and function, many of these are considered together as an endomembrane system because their functions are coordinated. The endomembrane system includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vacuoles.

Endoplasmic Reticulum (ER)

Electron microscopic studies of eukaryotic cells reveal a network or reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum. ER divides the intracellular space into two compartments: luminal (inside ER) and extra-luminal (cytoplasm) compartments.

• Rough Endoplasmic Reticulum (RER): Bears ribosomes on their surface. RER is actively involved in protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus.
• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes and is the major site for synthesis of lipid-like steroidal hormones in animal cells.

Golgi Bodies (Golgi Apparatus)

Camillo Golgi (1898) first observed densely stained reticular structures near the nucleus. These are called Golgi bodies. They consist of many flat, disc-shaped sacs or cisternae of 0.5 micrometer to 1.0 micrometer diameter.

• Cisternae are stacked parallel to each other.
• Convex cis (forming) face and concave trans (maturing) face.
• Function: Principal function is the packaging of materials, to be delivered either to the intra-cellular targets or secreted outside the cell. Important site of formation of glycoproteins and glycolipids.

Lysosomes

Lysosomal vesicles are membrane-bound vesicular structures formed by the process of packaging in the Golgi apparatus. Isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases - lipases, proteases, carbohydrases) optimally active at the acidic pH. These enzymes are capable of digesting carbohydrates, proteins, lipids, and nucleic acids.

Vacuoles

The vacuole is the membrane-bound space found in the cytoplasm of plant cells. It contains water, sap, excretory product, and other materials not useful for the cell. The vacuole is bound by a single membrane called tonoplast.

• In plant cells, vacuoles can occupy up to 90 percent of the volume of the cell.
• In plants, the tonoplast facilitates the transport of a number of ions and other materials concentration against concentration gradients into the vacuole, hence their concentration is significantly higher in the vacuole than in the cytoplasm.
• In Amoeba, the contractile vacuole is important for excretion and osmoregulation. In many cells, as in protists, food vacuoles are formed by engulfing food particles.

Other Cellular Organelles

Mitochondria

Mitochondria (singular: mitochondrion), unless specifically stained, are not easily visible under the microscope. The number of mitochondria per cell is variable depending on the physiological activity of the cells.

• Ultrastructure: Each mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments, namely the outer compartment and the inner compartment.
• Matrix: The inner compartment is filled with a dense homogeneous substance called the matrix. The inner membrane forms a number of infoldings called cristae towards the matrix, which increase the surface area.
• Function: Mitochondria are the sites of aerobic respiration. They produce cellular energy in the form of ATP, hence they are called the power houses of the cell. The matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S), and the components required for the synthesis of proteins.

Ribosomes

Ribosomes are granular structures first observed under the electron microscope as dense particles by George Palade in 1953. They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane.

• Eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S. (Here 'S' stands for sedimentation coefficient or Svedberg's unit; it indirectly measures density and size).
• 70S ribosomes are also found in mitochondria and chloroplasts of eukaryotic cells.

Plastids

Plastids are found in all plant cells and in euglenoids. They are easily observed under the microscope as they are large. They bear some specific pigments, thus imparting specific colours to the plants.

• Chloroplasts: Contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.
• Chromoplasts: Fat-soluble carotenoid pigments like carotene, xanthophylls, and others are present in the chromoplast. This gives the part of the plant a yellow, orange, or red colour.
• Leucoplasts: Colourless plastids of varied shapes and sizes with stored nutrients: Amyloplasts store carbohydrates (starch), elaioplasts store oils and fats, and aleuroplasts store proteins.
• Ultrastructure of Chloroplast: Double membrane-bound. The inner space limited by the inner membrane of the chloroplast is called the stroma. A number of organised flattened membranous sacs called thylakoids are present in the stroma. Thylakoids are arranged in stacks like piles of coins called grana. Flat membranous tubules called the stromal lamella connect the thylakoids of the different grana.

Microbodies

Many membrane bound minute vesicles called microbodies that contain various enzymes are present in both plant and animal cells.

Cytoskeleton

An elaborate network of filamentous proteinaceous structures consisting of microtubules, microfilaments, and intermediate filaments present in the cytoplasm is collectively referred to as the cytoskeleton.

• Function: Involved in many functions such as mechanical support, motility, maintenance of the shape of the cell.

Cilia and Flagella

Cilia (singular: cilium) and flagella (singular: flagellum) are hair-like outgrowths of the cell membrane. Cilia are small structures which work like oars, causing the movement of either the cells or the surrounding fluid. Flagella are comparatively longer and responsible for cell movement.

• The electron microscopic study of a cilium or the flagellum shows that they are covered with plasma membrane. Their core called the axoneme possesses a number of microtubules running parallel to the long axis.
• The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. This is known as the 9+2 array arrangement.

Centrosome and Centrioles

Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel.

• They are made up of nine evenly spaced peripheral fibrils of tubulin protein.
• Each of the peripheral fibril is a triplet. The adjacent triplets are also linked.
• The central part of the proximal region of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein.
• Function: The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

Nucleus and its Ultrastructure

The nucleus as a cell organelle was first described by Robert Brown as early as 1831. Later the material of the nucleus stained by basic dyes was given the name chromatin by Flemming.

Definition: The interphase nucleus (nucleus of a cell when it is not dividing) has highly elaborated nucleoprotein fibres called chromatin, nuclear matrix, and one or more spherical bodies called nucleoli.

• Nuclear Envelope: The nuclear envelope consists of two parallel membranes with a space between (10 to 50 nm) called the perinuclear space. It forms a barrier between the materials present inside the nucleus and that of the cytoplasm. The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.
• Nuclear Pores: At a number of places the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes. These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between nucleus and cytoplasm.
• Nucleoplasm: The nuclear matrix or the nucleoplasm contains chromatin and one or more spherical bodies called nucleoli. The nucleoli are spherical structures present in the nucleoplasm. It is not a membrane-bound structure and is a site for active ribosomal RNA synthesis.
• Chromatin: Interphase nucleus has a loose and indistinct network of nucleoprotein fibres called chromatin. During different stages of cell division, cells show structured chromosomes in place of the nucleus. Chromatin contains DNA and basic proteins called histones, non-histone proteins, and RNA.
• Chromosomes: A single human cell has approximately two metre long thread of DNA distributed among its 46 (23 pairs) chromosomes. Every chromosome (visible only in dividing cells) essentially has a primary constriction or the centromere on the sides of which disc-shaped structures called kinetochores are present.

Classification of Chromosomes based on Centromere Position

• Satellite: Few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite.