Cell: The Unit of Life - Comprehensive Notes
Biology as the Study of Living Organisms
Biology studies living organisms, initially focusing on their diverse forms and appearances.
The cell theory highlights the unity among diverse life forms, emphasizing cellular organization.
Cell structure and growth via division are detailed in this unit.
The integrity of cellular organization is essential for living processes.
Physico-chemical approaches, using cell-free systems, are employed to study physiological and behavioral processes at a molecular level.
Analyzing living tissues for elements and compounds helps describe processes in molecular terms.
Understanding the functions of compounds within cells reveals the molecular basis of physiological processes like digestion, excretion, memory, defense, and recognition.
This approach also explains abnormal processes in diseased conditions.
‘Reductionist Biology’ applies physics and chemistry concepts to understand biology.
Biomolecules are briefly described in Chapter 9.
G.N. Ramachandran
G.N. Ramachandran was a prominent figure in protein structure and founder of the ‘Madras school’ of conformational analysis of biopolymers.
In 1954, he discovered the triple helical structure of collagen, published in Nature.
His analysis of allowed protein conformations using the ‘Ramachandran plot’ is a major contribution to structural biology.
Born on October 8, 1922, near Cochin, India; his father was a mathematics professor.
Graduated top of his class in B.Sc. (Honors) Physics at the University of Madras in 1942.
Received a Ph.D. from Cambridge University in 1949.
Inspired by Linus Pauling’s work on α-helix and β-sheet structures, he focused on solving the structure of collagen.
Passed away on April 7, 2001, at the age of 78.
Cell: The Basic Unit of Life
Living organisms are distinguished from non-living things by the presence of the cell.
All organisms are composed of cells; unicellular organisms consist of a single cell, while multicellular organisms consist of many cells.
Unicellular organisms can exist independently and perform all essential life functions.
A complete cell structure is necessary for independent living.
The cell is the fundamental structural and functional unit of all living organisms.
Antonie Von Leeuwenhoek first observed and described a live cell.
Robert Brown later discovered the nucleus.
The invention of the microscope and its subsequent improvement to the electron microscope revealed detailed cell structures.
Cell Theory
In 1838, Matthias Schleiden, a German botanist, noted that plants consist of different kinds of cells forming plant tissues.
In 1839, Theodore Schwann, a German zoologist, reported that animal cells have a thin outer layer, now known as the ‘plasma membrane’.
Schwann concluded that animal and plant bodies are composed of cells and cell products.
Schleiden and Schwann jointly formulated the cell theory.
Rudolf Virchow (1855) explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellula).
The modern cell theory states:
All living organisms are composed of cells and cell products.
All cells arise from pre-existing cells.
Overview of the Cell
Onion cells have a distinct cell wall as their outer boundary and a cell membrane inside it.
Human cheek cells have an outer cell membrane.
Inside each cell is a dense, membrane-bound nucleus containing chromosomes and DNA.
Eukaryotic cells have membrane-bound nuclei, while prokaryotic cells do not.
A semi-fluid matrix called cytoplasm occupies the cell volume in both cell types.
The cytoplasm is the main site for cellular activities and chemical reactions that maintain the cell's living state.
Eukaryotic cells contain membrane-bound organelles like the endoplasmic reticulum (ER), Golgi complex, lysosomes, mitochondria, microbodies, and vacuoles.
Prokaryotic cells lack these membrane-bound organelles.
Ribosomes, which are non-membrane bound organelles, are found in all cells.
Ribosomes are present in the cytoplasm, chloroplasts (in plants), mitochondria, and on rough ER.
Animal cells have centrosomes, which are non-membrane bound organelles that help in cell division.
Cells vary in size, shape, and activities.
Mycoplasmas, the smallest cells, are about 0.3 \mu m in length.
Bacteria are 3 to 5 \mu m.
The largest single cell is the ostrich egg.
Human red blood cells are about 7.0 \mu m in diameter.
Nerve cells are among the longest cells.
Cells can be disc-like, polygonal, columnar, cuboid, thread-like, or irregular, depending on their function.
Prokaryotic Cells
Prokaryotic cells include bacteria, blue-green algae, mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).
They are generally smaller and multiply faster than eukaryotic cells.
They vary in shape and size.
The four basic shapes of bacteria are:
Bacillus (rod-like)
Coccus (spherical)
Vibrio (comma-shaped)
Spirillum (spiral)
Prokaryotic cell organization is similar across different shapes and functions.
Most prokaryotes have a cell wall around the cell membrane, except for mycoplasma.
The cytoplasm is a semi-fluid matrix.
There is no well-defined nucleus; the genetic material is naked and not enclosed by a nuclear membrane.
Many bacteria have small circular DNA called plasmids, in addition to the genomic DNA (single chromosome/circular DNA).
Plasmids provide unique characteristics, like antibiotic resistance.
Plasmids are used to monitor bacterial transformation with foreign DNA.
Prokaryotic cells lack membrane-bound organelles, except for ribosomes.
Inclusions are unique to prokaryotes.
Mesosomes, formed by infoldings of the cell membrane, are characteristic of prokaryotes.
Cell Envelope and Modifications
Most prokaryotic cells, especially bacteria, have a chemically complex cell envelope.
The cell envelope has three layers:
Outermost glycocalyx
Cell wall
Plasma membrane
Each layer has a distinct function, but together they act as a single protective unit.
Bacteria are classified as Gram positive or Gram negative based on their cell envelopes and response to Gram staining.
Glycocalyx varies in composition and thickness:
In some bacteria, it is a loose sheath called the slime layer.
In others, it is thick and tough, called the capsule.
The cell wall determines the cell shape and provides structural support, preventing bursting or collapsing.
The plasma membrane is selectively permeable and interacts with the external environment, similar in structure to eukaryotes.
Mesosomes, extensions of the plasma membrane, are in the form of vesicles, tubules, and lamellae.
Mesosomes aid in cell wall formation, DNA replication, distribution to daughter cells, respiration, secretion processes, increasing surface area, and enzymatic content.
Cyanobacteria have membranous extensions called chromatophores containing pigments.
Bacterial cells may be motile or non-motile. Motile bacteria have flagella.
Bacterial flagellum has three parts:
Filament (longest portion, extending from cell surface)
Hook
Basal body
Pili and fimbriae are surface structures that do not aid in motility.
Pili are elongated tubular structures made of a special protein.
Fimbriae are small bristle-like fibers.
Fimbriae help bacteria attach to rocks in streams and host tissues.
Ribosomes and Inclusion Bodies in Prokaryotes
Ribosomes are associated with the plasma membrane.
They measure about 15 nm by 20 nm and consist of two subunits: 50S and 30S, forming 70S prokaryotic ribosomes.
Ribosomes are the site of protein synthesis.
Multiple ribosomes attached to a single mRNA form a chain called polyribosomes or polysome.
Polysomes translate mRNA into proteins.
Inclusion bodies store reserve materials in the cytoplasm.
They are not bound by membranes and include:
Phosphate granules
Cyanophycean granules
Glycogen granules
Gas vacuoles are found in blue-green, purple, and green photosynthetic bacteria.
Eukaryotic Cells
Eukaryotes include protists, plants, animals, and fungi.
Eukaryotic cells feature extensive compartmentalization of cytoplasm via membrane-bound organelles.
They possess an organized nucleus with a nuclear envelope.
They have complex locomotory and cytoskeletal structures.
Their genetic material is organized into chromosomes.
Plant and animal cells differ:
Plant cells have cell walls, plastids, and a large central vacuole.
Animal cells have centrioles.
Cell Membrane
Detailed structure was studied after the advent of the electron microscope in the 1950s.
Chemical studies of red blood cells (RBCs) helped deduce the plasma membrane structure.
The cell membrane is mainly composed of lipids and proteins.
Lipids are phospholipids arranged in a bilayer, with polar heads facing outward and hydrophobic tails facing inward to protect them from the aqueous environment.
The membrane also contains cholesterol.
Biochemical investigations revealed the presence of proteins and carbohydrates.
The ratio of protein to lipid varies across different cell types.
In human erythrocytes, the membrane is approximately 52% protein and 40% lipid.
Membrane proteins are classified as integral or peripheral based on ease of extraction.
Peripheral proteins lie on the membrane surface.
Integral proteins are partially or totally buried in the membrane.
Fluid Mosaic Model
Singer and Nicolson (1972) proposed the fluid mosaic model.
The quasi-fluid nature of the lipid allows lateral movement of proteins within the bilayer, defining membrane fluidity.
Membrane fluidity is important for cell growth, intercellular junctions, secretion, endocytosis, and cell division.
Membrane Transport
The plasma membrane facilitates the transport of molecules across it.
The membrane is selectively permeable.
Passive transport allows molecules to move across without energy.
Neutral solutes move via simple diffusion along the concentration gradient (high to low).
Water moves via osmosis (diffusion of water from high to low concentration).
Polar molecules require carrier proteins for transport across the nonpolar lipid bilayer.
Active transport moves ions or molecules against their concentration gradient (low to high), requiring energy (ATP), e.g., Na^+/K^+ pump.
Cell Wall
The non-living, rigid cell wall is an outer covering of the plasma membrane in fungi and plants.
The cell wall provides shape, protects against mechanical damage and infection, facilitates cell-to-cell interaction, and acts as a barrier to undesirable macromolecules.
Algae cell walls are made of cellulose, galactans, mannans, and minerals like calcium carbonate.
Plant cell walls consist of cellulose, hemicellulose, pectins, and proteins.
The primary wall of a young plant cell can grow, but this diminishes as the cell matures and forms a secondary wall on the inner side.
The middle lamella, composed of calcium pectate, glues neighboring cells together.
Plasmodesmata traverse the cell wall and middle lamellae, connecting the cytoplasm of adjacent cells.
Endomembrane System
The endomembrane system includes the endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles.
These organelles coordinate their functions.
Mitochondria, chloroplasts, and peroxisomes are not part of the endomembrane system because their functions are not coordinated with the other components.
Endoplasmic Reticulum (ER)
Electron microscopy reveals a network of tiny tubular structures called the endoplasmic reticulum (ER) scattered in the cytoplasm.
The ER divides the intracellular space into luminal (inside ER) and extra luminal (cytoplasm) compartments.
Rough endoplasmic reticulum (RER) has ribosomes attached to its outer surface; smooth endoplasmic reticulum (SER) does not.
RER is abundant in cells involved in protein synthesis and secretion and is continuous with the outer nuclear membrane.
SER is the major site for lipid synthesis; it synthesizes lipid-like steroidal hormones in animal cells.
Golgi Apparatus
Camillo Golgi (1898) first observed densely stained reticular structures near the nucleus, later named Golgi bodies.
They consist of flat, disc-shaped sacs or cisternae of 0.5 \mu m to 1.0 \mu m diameter stacked parallel to each other.
A Golgi complex contains multiple cisternae arranged concentrically near the nucleus with distinct faces:
Convex cis face (forming face)
Concave trans face (maturing face)
The cis and trans faces are different but interconnected.
The Golgi apparatus primarily packages materials for intracellular delivery or secretion.
Vesicles from the ER fuse with the cis face and move toward the trans face.
Many proteins synthesized by ribosomes on the ER are modified in the Golgi cisternae before release from the trans face.
The Golgi apparatus is a key site for the formation of glycoproteins and glycolipids.
Lysosomes
Lysosomes are membrane-bound vesicular structures formed by packaging in the Golgi apparatus.
They are rich in hydrolytic enzymes (hydrolases like lipases, proteases, carbohydrases) that optimally function at acidic pH.
These enzymes digest carbohydrates, proteins, lipids, and nucleic acids.
Vacuoles
The vacuole is a membrane-bound space in the cytoplasm containing water, sap, excretory products, and other non-useful materials.
It is bound by a single membrane called the tonoplast.
In plant cells, vacuoles can occupy up to 90% of the cell volume.
The tonoplast facilitates the transport of ions and other materials against concentration gradients into the vacuole, resulting in higher concentrations than in the cytoplasm.
In Amoeba, the contractile vacuole regulates osmoregulation and excretion.
In many cells, food vacuoles are formed by engulfing food particles.
Mitochondria
Mitochondria are not easily visible without specific staining.
The number of mitochondria per cell varies with cellular activity.
They are typically sausage-shaped or cylindrical, with a diameter of 0.2-1.0 \mu m (average 0.5 \mu m) and length of 1.0-4.1 \mu m.
Each mitochondrion is a double membrane-bound structure with two aqueous compartments: an outer compartment and an inner compartment.
The inner compartment is filled with a dense homogeneous substance called the matrix.
The outer membrane forms the boundary of the organelle.
The inner membrane has infoldings called cristae that increase the surface area.
The two membranes have specific enzymes associated with mitochondrial function.
Mitochondria are the sites of aerobic respiration.
They produce cellular energy in the form of ATP and are called ‘power houses’ of the cell.
The matrix contains a single circular DNA molecule, RNA molecules, 70S ribosomes, and components for protein synthesis.
Mitochondria divide by fission.
Plastids
Plastids are found in all plant cells and in euglenoids.
They are easily observed under a microscope due to their large size.
They contain specific pigments that impart colors to plants.
Plastids are classified into chloroplasts, chromoplasts, and leucoplasts based on the type of pigments.
Chloroplasts contain chlorophyll and carotenoid pigments for trapping light energy in photosynthesis.
Chromoplasts contain fat-soluble carotenoid pigments like carotene and xanthophylls, giving yellow, orange, or red colors.
Leucoplasts are colorless plastids that store nutrients:
Amyloplasts store carbohydrates (starch), e.g., potato.
Elaioplasts store oils and fats.
Aleuroplasts store proteins.
Chloroplasts are mainly found in mesophyll cells of leaves.
They are lens-shaped, oval, spherical, discoid, or ribbon-like, with a length of 5-10 \mu m and width of 2-4 \mu m.
Their number varies from 1 per cell in Chlamydomonas to 20-40 per cell in mesophyll.
Chloroplasts are double membrane-bound, with a less permeable inner membrane.
The space limited by the inner membrane is called the stroma.
The stroma contains organized flattened membranous sacs called thylakoids, arranged in stacks called grana.
Stroma lamellae are flat membranous tubules connecting thylakoids of different grana.
The thylakoid membrane encloses a space called the lumen.
The stroma contains enzymes for carbohydrate and protein synthesis, small circular DNA molecules, and ribosomes.
Chlorophyll pigments are present in the thylakoids.
Chloroplast ribosomes are smaller (70S) than cytoplasmic ribosomes (80S).
Ribosomes
Ribosomes are granular structures first observed under the electron microscope by George Palade (1953).
They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by a membrane.
Eukaryotic ribosomes are 80S, while prokaryotic ribosomes are 70S.
Each ribosome has two subunits: a larger and a smaller subunit.
80S ribosomes have 60S and 40S subunits, while 70S ribosomes have 50S and 30S subunits.
‘S’ (Svedberg’s Unit) is the sedimentation coefficient, measuring density and size.
Cytoskeleton
The cytoskeleton is a network of filamentous proteinaceous structures including microtubules, microfilaments, and intermediate filaments in the cytoplasm.
It provides mechanical support, motility, and maintains cell shape.
Cilia and Flagella
Cilia (singular: cilium) and flagella (singular: flagellum) are hair-like outgrowths of the cell membrane.
Cilia are small structures that move like oars, causing movement of the cell or surrounding fluid.
Flagella are longer and responsible for cell movement.
Prokaryotic bacteria also have flagella, but they differ structurally from eukaryotic flagella.
Electron microscopy shows cilia and flagella are covered with plasma membrane.
Their core, called the axoneme, has microtubules running parallel to the long axis.
The axoneme typically has nine doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules (9+2 array).
Central tubules are connected by bridges and enclosed by a central sheath, connected to one tubule of each peripheral doublet by a radial spoke.
There are nine radial spokes.
Peripheral doublets are interconnected by linkers.
Cilia and flagella emerge from centriole-like structures called basal bodies.
Centrosome and Centrioles
The centrosome contains two cylindrical structures called centrioles, surrounded by amorphous pericentriolar materials.
Centrioles are perpendicular to each other and have a cartwheel-like organization.
They consist of nine evenly spaced peripheral fibrils of tubulin protein, each a triplet.
Adjacent triplets are linked.
The central part of the proximal region is proteinaceous, called the hub, and connected to tubules by radial spokes.
Centrioles form the basal body of cilia or flagella and spindle fibers, giving rise to the spindle apparatus during cell division in animal cells.
Nucleus
The nucleus was first described as a cell organelle by Robert Brown in 1831.
Flemming named the nuclear material stained by basic dyes as chromatin.
The interphase nucleus (non-dividing cell) contains nucleoprotein fibers called chromatin, nuclear matrix, and nucleoli.
The nuclear envelope consists of two parallel membranes separated by a space (10 to 50 nm) called the perinuclear space, forming a barrier between nuclear and cytoplasmic materials.
The outer membrane is continuous with the endoplasmic reticulum and bears ribosomes.
Nuclear pores, formed by fusion of the two membranes, interrupt the nuclear envelope, allowing movement of RNA and protein molecules between the nucleus and cytoplasm.
Typically, there is one nucleus per cell, but variations occur.
Some mature cells lack a nucleus, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants.
The nuclear matrix or nucleoplasm contains the nucleolus and chromatin.
Nucleoli are spherical structures within the nucleoplasm, continuous with it, and not membrane-bound.
They are sites for active ribosomal RNA synthesis.
Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.
Chromosomes
During cell division, structured chromosomes replace the loose chromatin network of the interphase nucleus.
Chromatin contains DNA, basic proteins called histones, non-histone proteins, and RNA.
A single human cell has approximately two meters of DNA distributed among 46 chromosomes (23 pairs).
Every chromosome (visible only in dividing cells) has a primary constriction or centromere with disc-shaped kinetochores on its sides.
The centromere holds two chromatids of a chromosome.
Chromosomes are classified into four types based on the position of the centromere:
Metacentric: middle centromere forming two equal arms.
Sub-metacentric: centromere slightly away from the middle, resulting in one shorter and one longer arm.
Acrocentric: centromere close to the end, forming one extremely short and one very long arm.
Telocentric: terminal centromere.
Some chromosomes have non-staining secondary constrictions at a constant location, giving the appearance of a small fragment called the satellite.
Microbodies
Microbodies are membrane-bound minute vesicles containing various enzymes, present in both plant and animal cells.
Biology studies living organisms, focusing on their diverse forms and appearances.
The cell theory highlights the unity among diverse life forms, emphasizing cellular organization.
The integrity of cellular organization is essential for living processes.
Physico-chemical approaches, using cell-free systems, are employed to study physiological and behavioral processes at a molecular level.
Understanding the functions of compounds within cells reveals the molecular basis of physiological processes like digestion, excretion, memory, defense, and recognition.
All organisms are composed of cells; unicellular organisms consist of a single cell, while multicellular organisms consist of many cells.
Unicellular organisms can exist independently and perform all essential life functions.
A complete cell structure is necessary for independent living.
The cell is the fundamental structural and functional unit of all living organisms.
All living organisms are composed of cells and cell products.
All cells arise from pre-existing cells.
Eukaryotic cells have membrane-bound nuclei, while prokaryotic cells do not.
The cytoplasm is the main site for cellular activities and chemical reactions that maintain the cell's living state.
Eukaryotic cells contain membrane-bound organelles like the endoplasmic reticulum (ER), Golgi complex, lysosomes, mitochondria, microbodies, and vacuoles.
Prokaryotic cells lack these membrane-bound organelles.
Ribosomes, which are non-membrane bound organelles, are found in all cells.
Prokaryotic cells include bacteria, blue-green algae, mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).
They are generally smaller and multiply faster than eukaryotic cells.
Most prokaryotes have a cell wall around the cell membrane, except for mycoplasma.
There is no well-defined nucleus; the genetic material is naked and not enclosed by a nuclear membrane.
Plasmids provide unique characteristics, like antibiotic resistance.
Prokaryotic cells lack membrane-bound organelles, except for ribosomes.
Mesosomes, formed by infoldings of the cell membrane, are characteristic of prokaryotes.
Bacteria are classified as Gram positive or Gram negative based on their cell envelopes and response to Gram staining.
Mesosomes aid in cell wall formation, DNA replication, distribution to daughter cells, respiration, secretion processes, increasing surface area, and enzymatic content.
Eukaryotes include protists, plants, animals, and fungi.
Eukaryotic cells feature extensive compartmentalization of cytoplasm via membrane-bound organelles.
They possess an organized nucleus with a nuclear envelope.
Their genetic material is organized into chromosomes.
The cell membrane is mainly composed of lipids and proteins.
Lipids are phospholipids arranged in a bilayer.
Singer and Nicolson (1972) proposed the fluid mosaic model.
The quasi-fluid nature of the lipid allows lateral movement of proteins within the bilayer, defining membrane fluidity.
The plasma membrane facilitates the transport of molecules across it.
The membrane is selectively permeable.
Passive transport allows molecules to move across without energy.
Active transport moves ions or molecules against their concentration gradient (low to high), requiring energy (ATP).
The non-living, rigid cell wall is an outer covering of the plasma membrane in fungi and plants.
The endomembrane system includes the endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles.
RER is abundant in cells involved in protein synthesis and secretion.
SER is the major site for lipid synthesis.
The Golgi apparatus primarily packages materials for intracellular delivery or secretion.
Lysosomes are membrane-bound vesicular structures formed by packaging in the Golgi apparatus and are rich in hydrolytic enzymes.
Mitochondria produce cellular energy in the form of ATP and are called ‘power houses’ of the cell.
Plastids are classified into chloroplasts, chromoplasts, and leucoplasts based on the type of pigments.
Eukaryotic ribosomes are 80S, while prokaryotic ribosomes are 70S.
The cytoskeleton provides mechanical support, motility, and maintains cell shape.
Cilia and flagella are hair-like outgrowths of the cell membrane used for movement.
The centrosome contains two cylindrical structures called centrioles, important for cell division.
The interphase nucleus (non-dividing cell) contains nucleoprotein fibers called chromatin, nuclear matrix, and nucleoli.
During cell division, structured chromosomes replace the loose chromatin network of the interphase nucleus.
Chromatin contains DNA, basic proteins called histones, non-histone proteins, and RNA.