Cell The Unit of Life Biology Notes - Cell Theory, Prokaryotic and Eukaryotic Cells, Cell Organelles, Nucleus and Chromosomes

• A cell is the fundamental, structural, and functional unit of all living organisms.
• Robert Hooke: Discovered cell.
• Anton Von Leeuwenhoek: First observed and described a live cell.
• The invention of the compound and electron microscopes revealed all the structural details of the cell.

Cell Theory

• Matthias Schleiden (1838): Observed that all plants are composed of different kinds of cells.
• Theodore Schwann (1839): Found that cells have a thin outer layer (plasma membrane). He also found that plant cells have a cell wall. He proposed a hypothesis that animals and plants are composed of cells and products of cells.
• Schleiden and Schwann formulated the cell theory.
• 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.
• Cell theory states that:
  • All living organisms are composed of cells and products of cells.
  • All cells arise from pre-existing cells.

An Overview of Cell

• All cells contain:
  • Cytoplasm: A semi-fluid matrix where cellular activities and chemical reactions occur. This keeps the cell in a ‘living state’.
  • Ribosomes: Non-membrane bound organelles seen in cytoplasm, chloroplasts, mitochondria, and on rough ER.
• Cells differ in size, shape, and activities.
  • Smallest cells: Mycoplasmas (0.3 µm in length).
  • Largest isolated single cell: Egg of ostrich.
  • Longest cells: E.g., Nerve cell.
  • Size of bacteria: 3 to 5 µm (Typical: 1 to 2 µm).
  • Human RBCs are about 7.0 µm in diameter.
• Based on the functions, shape of cells may be disc-like, polygonal, columnar, cuboid, thread-like, or irregular.
• Cells are of two types: Prokaryotic cells and Eukaryotic cells.

Prokaryotic Cells

• They have no membrane-bound nucleus and organelles.
• They include bacteria, blue-green algae, mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).
• They are generally smaller and multiply more rapidly than eukaryotic cells.
• They vary in shape and size. E.g., Bacteria have four basic shapes: Bacillus, Coccus, Vibrio, and Spirillum.

Cell Organelles in Prokaryotic Cells

1. Cell Envelope

It is a chemically complex protective covering made of three tightly bound layers.

• Glycocalyx: Outer layer. Its composition and thickness vary in different bacteria. It may be a slime layer (loose sheath) or capsule (thick and tough).
• Cell Wall: Middle layer. Seen in all prokaryotes except mycoplasma. It gives shape to the cell and provides structural support to prevent the bacterium from bursting or collapsing.
• Plasma Membrane: Inner layer. It is semi-permeable and interacts with the outside. Structurally similar to that of eukaryotes.

Based on the types of cell envelopes and response to Gram staining, bacteria are two types:

• Gram Positive: They take up and retain the Gram stain.
• Gram Negative: They do not retain the Gram stain.

2. Mesosomes and Chromatophores (Membranous Structures)

Mesosome: Formed by infoldings of the plasma membrane. It includes vesicles, tubules, and lamellae.

Functions: Mesosomes help in:

• Cell wall formation.
• DNA (chromosome) replication.
• Distribution of chromosomes to daughter cells.
• Respiration and secretion processes.
• Increasing the surface area of the plasma membrane and enzymatic content.

Chromatophores: Pigment-containing membranous infoldings in some prokaryotes (e.g., cyanobacteria).

3. Nucleoid

• Formed of non-membranous (naked) circular genomic DNA (single chromosome/genetic material) and protein.
• Many bacteria have small circular DNA (plasmid) outside the genomic DNA. It gives unique phenotypic characters (e.g., resistance to antibiotics) to bacteria.

4. Flagella

• Thin filamentous extensions from the cell wall of motile bacteria. Their number and arrangement vary in different bacteria.
• Bacterial flagellum has three parts: filament, hook, and basal body.
• The filament is the longest portion and extends from the cell surface to the outside.

5. Pili and Fimbriae

• Surface structures that have no role in motility.
• Pili (sing. Pilus): Elongated tubular structures made of a special protein (pilin).
• Fimbriae: Small bristle-like fibers sprouting out of the cell. In some bacteria, they help attach the bacteria to rocks in streams and to host tissues.

6. Ribosomes

• Associated with the plasma membrane of prokaryotes.
• About 15 nm by 20 nm in size.
• Made of two subunits: 50S and 30S (Svedberg’s unit). Together, they form 70S prokaryotic ribosomes. (S = sedimentation coefficient; a measure of density and size).
• Function: Ribosomes are the site of translation (protein synthesis). Several ribosomes may attach to a single mRNA to form a chain called polyribosomes (polysome). Ribosomes translate the mRNA into proteins.

7. Inclusion Bodies

• Non-membranous, stored reserve material seen freely in the cytoplasm of prokaryotic cells.
• E.g., phosphate granules, cyanophycean granules, glycogen granules, gas vacuoles, etc.
• Gas vacuoles: Found in blue-green and purple and green photosynthetic bacteria.

• Eukaryotic Cells have a well-organized, membrane-bound nucleus and organelles.
• Presence of membranes gives clear compartmentalization of cytoplasm.
• Their genetic material is organized into chromosomes.
• They have complex locomotory and cytoskeletal structures.

Cell Organelles in Eukaryotic Cells

1. Cell Membrane

• Chemical studies on human RBCs show that the cell membrane is composed of a lipid bilayer, protein, and carbohydrate.
• Lipids (mainly phosphoglycerides) have an outer polar head and inner hydrophobic tails, protecting the non-polar tail of saturated hydrocarbons from the aqueous environment.
• The ratio of protein and lipid varies in different cells. E.g., in human RBCs, the membrane has 52% protein and 40% lipids.
• Based on ease of extraction, membrane proteins are two types:
  • Integral proteins: Partially or totally buried in the membrane.
  • Peripheral proteins: Lie on the surface of the membrane.
• Fluid mosaic model of cell membrane: Proposed by Singer & Nicolson (1972). The quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity.

Functions:

• Transport of molecules. The membrane is selectively permeable to some molecules present on either side of it.
• Due to its fluid nature, the plasma membrane can help in cell growth, formation of intercellular junctions, secretion, endocytosis, and cell division.

Types of Transport:

• Passive transport: Movement of molecules across the membrane along the concentration gradient (from higher to lower concentration) without energy expenditure. It is two types:
  1. Simple diffusion: Movement of neutral solutes across the membrane.
  2. Osmosis: Movement of water by diffusion across the membrane.
• Polar molecules cannot pass through the non-polar lipid bilayer, so they require membrane carrier proteins for transport.
• Active transport: Movement of molecules against the concentration gradient (from lower to higher concentration) with energy expenditure (ATP is utilized). E.g., Na+/K+ pump.

2. Cell Wall

• A non-living rigid structure found outer to the plasma membrane of fungi and plants.
• Cell wall of algae is made of cellulose, galactans, mannans, and minerals like CaCO3. In other plants, it consists of cellulose, hemicellulose, pectins, and proteins.
• The cell wall of a young plant cell (primary wall) is capable of growth. It gradually diminishes as the cell matures, and the secondary wall forms on the inner side (towards the membrane).
• The middle lamella is a layer containing calcium pectate, which glues neighboring cells together. Cell wall and middle lamellae may be traversed by plasmodesmata, connecting the cytoplasm of neighboring cells.

Functions:

• Gives shape to the cell.
• Protects the cell from mechanical damage and infection.
• Helps in cell-to-cell interaction.
• Acts as a barrier to undesirable macromolecules.

3. Endomembrane System

• A group of membranous organelles with coordinated functions.
• Includes endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles.

Endoplasmic Reticulum (ER)

• A network of tiny tubular structures scattered in the cytoplasm.
• Divides the intracellular space into two compartments: luminal (inside ER) and extra-luminal (cytoplasm).
• Endoplasmic reticulum is two types:
  1. Rough endoplasmic reticulum (RER): Bears ribosomes on its surface. Frequently observed in cells actively involved in protein synthesis and secretion. Extends to the outer membrane of the nucleus.
  2. Smooth endoplasmic reticulum (SER): Lacks ribosomes. Major site for lipid synthesis. In animal cells, steroidal hormones are synthesized in SER.

Golgi Apparatus

• Densely stained reticular structures near the nucleus.
• First observed by Camillo Golgi (1898).
• Consists of flat, disc-shaped sacs (cisternae) of 0.5–1.0 µm diameter, stacked parallelly.
• Cisternae are concentrically arranged with a convex cis (forming) face and a concave trans (maturing) face. Cis and trans faces are totally different but interconnected.

Functions:

• Secretes materials to intra-cellular targets or outside the cell.
• Materials to be packaged as vesicles from the ER fuse with the cis face and move towards the trans face, explaining the close association with the endoplasmic reticulum.
• Proteins synthesized by ribosomes on the ER are modified in the cisternae before release from the trans face.
• Formation of glycoproteins and glycolipids.

Lysosomes

• Membrane-bound vesicular structures formed by packaging in the Golgi apparatus.
• Contain almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases), active at acidic pH, which digest carbohydrates, proteins, lipids, and nucleic acids.

Vacuoles

• Membrane-bound spaces in the cytoplasm containing water, sap, excretory products, and other materials not useful for the cell.
• Bound by a single membrane called tonoplast.
• In plant cells, vacuoles can occupy up to 90% of the cell volume.
• In plants, the tonoplast facilitates transport of ions and other materials against concentration gradients into the vacuole, resulting in higher concentrations in the vacuole than in the cytoplasm.
• In Amoeba, the contractile vacuole aids in excretion.
• In many cells (e.g., protists), food vacuoles form by engulfing food particles.

Eukaryotic Cells

4. Mitochondria

• Mitochondria are clearly visible only when stained.
• Number, shape, and size vary depending on the cell’s physiological activity.
• Sausage-shaped or cylindrical, with a diameter of 0.2–1.0 µm (average 0.5 µm) and length 1.0–4.1 µm.
• A mitochondrion is a double membrane-bound structure with an outer membrane and an inner membrane, dividing the lumen into two compartments: the outer compartment and the inner compartment (matrix).
• The inner membrane forms infoldings (cristae) towards the matrix, increasing surface area.
• Both membranes have specific enzymes associated with mitochondrial function.
• The matrix contains circular DNA, a few RNA molecules, 70S ribosomes, and components for protein synthesis.
• Mitochondria divide by fission.
• Function: Sites of aerobic respiration, producing energy in the form of ATP. Hence, called the ‘power houses’ of the cell.

5. Plastids

• Found in all plant cells and euglenoides.
• Large-sized and easily observable under a microscope.
• Contain specific pigments.
• Based on pigment type, plastids are three types:
  1. Chloroplasts: Contain chlorophyll and carotenoid pigments, trapping light energy for photosynthesis.
  2. Chromoplasts: Contain fat-soluble carotenoid pigments (e.g., carotene, xanthophylls), giving yellow, orange, or red color.
  3. Leucoplasts: Colorless plastids of varied shapes and sizes with stored nutrients, including:
     • Amyloplasts: Store starch (e.g., potato).
     • Elaioplasts: Store oils and fats.
     • Aleuroplasts: Store proteins.

Chloroplasts:

• Double membrane-bound organelles, mainly in mesophyll cells of leaves.
• Lens-shaped, oval, spherical, discoid, or ribbon-like, with length 5–10 µm and width 2–4 µm.
• Number varies from 1 (e.g., Chlamydomonas) to 20–40 per cell in mesophyll.
• Inner membrane is less permeable.
• The space within the inner membrane is called stroma, containing flattened membranous sacs called thylakoids.
• Thylakoid membranes enclose a lumen.
• Chlorophyll pigments are present in thylakoids.
• Thylakoids are stacked into grana or connected by intergranal thylakoids.
• Flat membranous tubules, stroma lamellae, connect thylakoids of different grana.
• Stroma contains double-stranded circular DNA, ribosomes, and enzymes for synthesizing carbohydrates and proteins.
• Chloroplast ribosomes are 70S, smaller than cytoplasmic 80S ribosomes.

6. Ribosomes

• Non-membranous granular structures composed of RNA and proteins.
• First observed by George Palade (1953).
• Eukaryotic ribosomes have two subunits: 60S (large) and 40S (small), forming 80S.

7. Cytoskeleton

• A network of filamentous proteinaceous structures in the cytoplasm.
• Provides mechanical support, motility, and maintains cell shape.

8. Cilia and Flagella

• Hair-like outgrowths of the cell membrane.
• Cilia: Small structures that work like oars, causing movement of the cell or surrounding fluid.
• Flagella: Longer, responsible for cell movement. Prokaryotic and eukaryotic flagella are structurally different.
• Cilia and flagella are covered with plasma membrane. Their core (axoneme) has microtubules in a 9+2 array: nine pairs of radially arranged peripheral microtubule doublets and a central pair.
• The central tubules are connected by bridges and enclosed by a central sheath, linked to one tubule of each peripheral doublet by a radial spoke. Peripheral doublets are interconnected by linkers.
• Emerge from basal bodies, centriole-like structures.

9. Centrosome and Centrioles

• Centrosome contains two non-membrane-bound cylindrical structures called centrioles.
• Surrounded by pericentriolar materials.
• Centrioles lie perpendicular to each other, made of nine evenly spaced peripheral fibrils of tubulin, each a triplet. Adjacent triplets are linked.
• The central part, called the hub, is proteinaceous and connected to peripheral triplets by radial spokes.
• Centrioles form the basal body of cilia or flagella and spindle fibers for the spindle apparatus during cell division in animal cells.

10. Nucleus

• First described by Robert Brown (1831).
• Stainable material named chromatin by Flemming.
• Typically, one nucleus per cell, but some cells have multiple (e.g., certain fungi), and some lack a nucleus (e.g., mammalian RBCs, sieve tube cells).
• The interphase nucleus contains:
  • Nuclear envelope: Double-layered membrane with a perinuclear space (10–50 nm). Acts as a barrier between nucleus and cytoplasm. The outer membrane is continuous with ER and bears ribosomes. Has nuclear pores for RNA and protein movement.
  • Nuclear matrix (nucleoplasm): Fluid within the nucleus.
  • Chromatin: Nucleoprotein fibers containing DNA, histones, non-histone proteins, and RNA. Condenses into chromosomes during cell division.
  • Nucleolus: Non-membranous spherical body, continuous with nucleoplasm, site of ribosomal RNA synthesis.

Chromosomes

• A human cell has 2 m of DNA distributed among 46 chromosomes (23 pairs).
• Each chromosome has a centromere (primary constriction) with disc-shaped kinetochores on its sides.
• Based on centromere position, chromosomes are four types:
  • Metacentric: Middle centromere, forming two equal arms.
  • Sub-metacentric: Centromere nearer one end, forming one shorter and one longer arm.
  • Acrocentric: Centromere close to one end, forming one very short and one very long arm.
  • Telocentric: Terminal centromere.
• Some chromosomes have a non-staining secondary constriction at a constant location, called a satellite.

11. Microbodies

• Membrane-bound minute vesicles containing various enzymes.
• Present in both plant and animal cells.

Differences Between Plant and Animal Cells

Comparison Between Prokaryotic and Eukaryotic Cells