Cell - Notes, Biology
When we observe the natural world, we see objects that are living and others that are not. The principal feature that distinguishes living organisms from non-living objects is the presence of cells. A cell is the smallest structural and functional unit of life. Some organisms are unicellular (single-celled) and can live independently as a single cell; others are multicellular and are composed of many specialised cells working together.
What is a Cell?
A cell is a bounded, organised unit capable of performing all vital functions of life such as metabolism, growth, reproduction and response to stimuli. Without a complete cell organisation, independent life is not possible. Thus, the cell is the fundamental building block and operational unit of all living beings.
Early milestones in the study of cells:
- Anton van Leeuwenhoek was the first to observe and describe living cells (micro-organisms) using simple microscopes.
- Robert Brown discovered and described the nucleus in plant cells (around 1831).
- Advances in microscopy, especially the electron microscope, later revealed the fine internal (ultrastructural) organisation of cells and their organelles.
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Cell Theory
The modern cell theory emerged in the 19th century from observations by botanists and zoologists and later contributions that explained how new cells arise.
- In 1838, Matthias Schleiden, a German botanist, concluded that plants are made of cells.
- In 1839, Theodor Schwann, a German physiologist, extended similar observations to animals and proposed that animal tissues are also composed of cells. He described the presence of a thin outer layer (later recognised as the plasma membrane) in animal cells and noted that plant cells have an additional rigid cell wall.
- In 1855, Rudolf Virchow provided the missing explanation for cell origin by stating "Omnis cellula e cellula" - every cell arises from a pre-existing cell by cell division.
These contributions established the core postulates of cell theory:
All living organisms are composed of cells and their products.
All cells arise from pre-existing cells.
An Overview of Cells
- Onion cells are a common example of plant cells and show a distinct outer cell wall and an inner plasma membrane.
- Human cheek (buccal) cells are typical animal cells with a plasma membrane surrounding cytoplasm and a prominent nucleus containing DNA-chromosomes.
- Eukaryotic cells possess a membrane-bound nucleus and membrane-bound organelles (for example, endoplasmic reticulum, Golgi apparatus, mitochondria, plastids, lysosomes, vacuoles).
- Prokaryotic cells lack a true nucleus and membrane-bound organelles; their genetic material is not enclosed within a nuclear membrane.
- Ribosomes are non-membrane-bound organelles present in both prokaryotes and eukaryotes; they synthesise proteins and are found free in the cytoplasm or associated with membranes (for example, rough endoplasmic reticulum).
- Some organelles are present only in particular groups: the centrosome is prominent in animal cells and aids cell division; chloroplasts are found in plants and certain algae and carry out photosynthesis.
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Size and Shape of Cells
Cells vary greatly in size and shape according to their function.
- Smallest cells: Mycoplasmas are among the smallest cells (~0.3 µm).
- Bacteria: Many bacteria have sizes in the range of 3-5 µm, though sizes vary widely across species.
- Largest single cell (isolated): The ostrich egg is the largest single cell that can be seen with the naked eye.
- Human red blood cells (RBCs): approximately 7.0 µm in diameter.
- Nerve cells (neurons): may be extremely long (extensions called axons) but with very narrow diameters.
- Shapes: Cells may be disc-like, polygonal, columnar, cuboidal, thread-like or irregular; shape often reflects specialised function.
Types of Cells
Prokaryotic Cells
Definition: Prokaryotic cells are generally unicellular organisms that lack a membrane-bound nucleus and membrane-bound organelles. Organisms in the domains Bacteria and Archaea are prokaryotes.
Key features:
- Genetic material present as a single, circular DNA molecule located in a nucleoid region (no nuclear membrane).
- Ribosomes are present (70S), but membrane-bound organelles are absent.
- Many prokaryotes possess a complex cell envelope and specialised surface structures for movement or attachment.
Eukaryotic Cells
Definition: Eukaryotic cells have a well-defined, membrane-bound nucleus containing linear chromosomes and numerous membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi apparatus.
Key features:
- Possess 80S ribosomes (in cytoplasm) and numerous membrane-bound organelles.
- Generally larger and structurally more complex than prokaryotic cells.
Comparison: Prokaryotic Cell vs Eukaryotic Cell
| Prokaryotic Cell | Eukaryotic Cell |
|---|---|
| Simple and primitive in organisation. | Developed and comparatively complex in organisation. |
| Nucleus not well organised; no nuclear membrane or nucleolus. | Well-organised nucleus with nuclear membrane and nucleolus. |
| No membrane-bound organelles (except ribosomal granules). | Many membrane-bound organelles present. |
| Chromosomes not well defined during cell division. | Chromosomes formed in the nucleus during cell division. |
| DNA generally present as a single circular thread in the nucleoid region. | DNA is organised into chromosomes in the nucleus. |
Comparison: Plant Cell vs Animal Cell
| Plant Cell | Animal Cell |
|---|---|
| Rigid cell wall present outside the plasma membrane. | Cell wall absent; only plasma membrane present. |
| Usually larger in size. | Generally smaller in size. |
| Shape fixed; cannot change shape easily. | Often capable of changing shape. |
| Plastids (for example, chloroplasts) are present. | Plastids usually absent. |
| Chlorophyll-containing chloroplasts present in photosynthetic cells. | Chlorophyll absent. |
| Large central vacuole present in mature cells. | Many small vacuoles or vesicles present. |
| Nucleus often pushed to the periphery by the large vacuole. | Nucleus usually near the centre. |
| Generally fewer mitochondria. | Generally more numerous mitochondria. |
| Plant mitochondria often have tubular cristae. | Animal mitochondria often have plate-like cristae. |
| Do not burst in hypotonic solutions due to cell wall. | May burst (lyse) in hypotonic solutions unless regulatory mechanisms exist. |
Cell Envelope and its Modifications (in Prokaryotes)
Many prokaryotes, especially bacteria, have a multilayered cell envelope made of several tightly associated layers that together protect and support the cell. Important components include:
- Glycocalyx: A variable outermost layer that may be a loose slime layer or a well-defined capsule. It aids adhesion, protection and can be involved in evading host defences.
- Cell wall: Provides shape and mechanical strength. In bacteria, it commonly contains peptidoglycan.
- Plasma membrane: The innermost boundary that is selectively permeable and participates in transport and energy transduction.
- Mesosomes: Infoldings or extensions of the plasma membrane seen under some preparations; implicated in roles such as cell wall formation, DNA processes and respiration (note: some mesosome observations are artefacts of preparation, but they are historically described structures).
Bacteria are classified as Gram-positive or Gram-negative according to their cell envelope structure and their reaction to the Gram staining method:
- Gram-positive bacteria: Thick peptidoglycan layer retains crystal violet stain and cells appear purple after Gram staining.
- Gram-negative bacteria: Thin peptidoglycan layer plus an outer membrane; they do not retain the crystal violet stain and appear pink/red after counterstaining.
Gram-positive & Gram-negativeOther surface structures and modifications of bacterial cells:
- Chromatophores: Pigment-containing structures in some photosynthetic bacteria.
- Motility: Bacteria may be motile or non-motile.
- Flagella: Long filamentous appendages for locomotion; composed of three principal parts: filament, hook and basal body.
- Pili (singular: pilus): Tubular protein structures involved in attachment and genetic exchange (conjugation).
- Fimbriae: Short, bristle-like fibres that aid attachment to surfaces and host cells.
Ribosomes and Inclusion Bodies
Ribosomes: Sites of protein synthesis.
- Prokaryotic ribosomes are 70S (formed from 50S and 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S).
- Ribosomes may be free in the cytoplasm or attached to the rough endoplasmic reticulum; multiple ribosomes attached to a single mRNA form a polysome.
- Ribosomes are composed of ribosomal RNA (rRNA) and proteins and catalyse polypeptide chain formation.
Inclusion bodies: Non-membrane-bound storage granules in the cytoplasm containing reserve materials such as glycogen, polyphosphate (volutin), or pigment granules.
Gas vacuoles: Found in some aquatic photosynthetic bacteria; they help control buoyancy.
Cell Wall (Plant Cells and Others)
The cell wall is a rigid protective layer outside the plasma membrane in plants, fungi and many prokaryotes. It was first described by Robert Hooke (1665) in the cork. Composition varies by group:
- Plant cell walls: mainly cellulose (a polysaccharide of glucose), plus hemicellulose, pectin and structural proteins.
- Fungal cell walls: mainly chitin (a polymer of N-acetylglucosamine).
- Bacterial cell walls: commonly contain peptidoglycan (murein); composition and layering differ between Gram-positive and Gram-negative bacteria.
Components of Cell WallComponents of the Plant Cell Wall
- Cellulose: Main load-bearing polysaccharide forming microfibrils.
- Hemicellulose: Matrix polysaccharides that bind cellulose microfibrils together.
- Pectin: A group of heterogeneous polysaccharides abundant in the middle lamella; acts as a cementing substance between adjacent cells.
- Primary wall and secondary wall: Growing cells have a flexible primary wall; some cells develop an inner, thicker secondary wall after growth stops.
- Middle lamella: Rich in pectins (for example, calcium pectate) and glues neighbouring cell walls together.
- Plasmodesmata: Microscopic channels traversing the cell wall and middle lamella that connect the cytoplasm of adjacent plant cells, allowing transport and communication.
Cell Membrane (Plasma Membrane)
The plasma membrane is the selectively permeable membrane that surrounds the living contents of a cell, separating the cytoplasm from the external environment. It is thin, flexible and capable of regeneration.
Cell MembraneThe most accepted structural model is the fluid mosaic model (Singer & Nicolson): a lipid bilayer (phospholipids) in which proteins are embedded and can move laterally, giving the membrane fluidity and a mosaic of components.
- Membrane locations: Plasma membrane in all cells; internal biomembranes form the boundaries of organelles (for example, nuclear envelope, ER, Golgi, mitochondria).
- Functions: Selective permeability and transport, compartmentalisation, signal transduction, cell recognition and adhesion, endocytosis and exocytosis.
- Transport mechanisms: Passive transport (simple diffusion, facilitated diffusion, osmosis), and active transport (energy-dependent pumps such as the Na+/K+ ATPase that move ions against their concentration gradients).
What are Cell Organelles?
Cell organelles are specialised structures within the cytoplasm that carry out distinct functions. Organelles may be non-membrane-bound or membrane-bound, and together they coordinate the activities necessary for cell survival, growth and reproduction.
- Organelles without membranes: cell wall (external in plants), ribosomes, components of the cytoskeleton.
- Single membrane-bound organelles: vacuoles, lysosomes, Golgi apparatus, endoplasmic reticulum.
- Double membrane-bound organelles: nucleus, mitochondria, chloroplasts.
List of Cell Organelles and Their Functions
1. Plasma Membrane
- The selectively permeable membrane composed of a lipid bilayer and embedded proteins.
- Controls movement of substances into and out of the cell, provides mechanical support and participates in cell signalling.
- The fluid mosaic model describes its organisation: proteins float in a fluid lipid bilayer.
2. Cytoplasm
The cytoplasm is the jelly-like material between the plasma membrane and nucleus that contains organelles, cytosol, dissolved solutes and enzymes.
- Site of many metabolic reactions and pathways; organelles are embedded in it.
3. Nucleus
The nucleus is a double-membraned organelle present in eukaryotes. It contains the cell's genetic material (DNA) organised as chromosomes and acts as the control centre for cellular activities.
- Nuclear envelope: double membrane with nuclear pores that regulate transport between nucleus and cytoplasm.
- Chromosomes: thread-like DNA-protein complexes carrying genes; genes are functional units of heredity.
- Nucleolus: dense region within the nucleus where ribosomal RNA (rRNA) and ribosomal subunits are assembled.
4. Endoplasmic Reticulum (ER)
The ER is an extensive network of membrane-bound tubules and cisternae that functions in synthesis and transport.
- Rough ER (RER): Studded with ribosomes; site of protein synthesis and initial protein folding/modification.
- Smooth ER (SER): Lacks ribosomes; involved in lipid and steroid synthesis, detoxification and calcium storage.
5. Mitochondria
- Mitochondria are double membrane-bound organelles often called the powerhouses of the cell because they generate ATP by aerobic respiration.
- They have an outer membrane and a folded inner membrane (cristae) that increases surface area for oxidative phosphorylation; the internal matrix contains enzymes, circular DNA and 70S-type ribosomes in many eukaryotes.
- Mitochondrial DNA is typically inherited maternally in many organisms.
6. Plastids
Plastids are double-membrane organelles found in plants and algae; they are involved in synthesis and storage of food and pigments.
- Chloroplasts: Contain chlorophyll and other pigments; site of photosynthesis. Internal thylakoid membranes form grana and stroma; chloroplasts contain circular DNA and 70S ribosomes.
- Chromoplasts: Pigment-containing plastids (carotenoids) that give yellow, orange or red colours to fruits and flowers.
- Leucoplasts: Colourless plastids specialised for storage: amyloplasts (starch), aleuroplasts (protein), elaioplasts (lipids).
7. Ribosomes
- Non-membrane-bound organelles composed of rRNA and proteins; they synthesise polypeptides according to mRNA templates.
- Ribosomes are described by their sedimentation coefficients: 70S in prokaryotes (50S+30S) and 80S in eukaryotes (60S+40S).
8. Golgi Apparatus
The Golgi apparatus (Golgi complex) consists of flattened membrane-bound cisternae and functions in modification, sorting and packaging of proteins and lipids for secretion or delivery to other organelles.
- Receives vesicles from the ER, modifies macromolecules (for example, glycosylation), and dispatches them in secretory vesicles.
9. Microbodies
- Small, membrane-bound vesicular organelles containing enzymes (for example, peroxisomes) involved in metabolic reactions such as fatty acid oxidation and detoxification.
10. Cytoskeleton
The cytoskeleton is a dynamic network of protein filaments (microfilaments, intermediate filaments and microtubules) that provides cell shape, mechanical resistance, intracellular transport, and is essential during cell division and motility.
- Microtubules and microfilaments reorganise rapidly as the cell requires to move organelles or change shape.
11. Cilia and Flagella
- Cilia: Numerous, short hair-like projections that move fluids over cell surfaces or help locomotion in single-celled organisms.
- Flagella: Longer, usually fewer (one or two) structures for propulsion; prokaryotic and eukaryotic flagella differ structurally and in mechanism.
- The core axoneme of eukaryotic cilia/flagella has a "9+2" arrangement: nine peripheral microtubule doublets and two central microtubules, with associated radial spokes and dynein arms. Basal bodies anchor them at the cell surface.
12. Centrosome and Centrioles
- The centrosome is a microtubule-organising centre in many animal cells and contains a pair of centrioles at right angles to each other.
- Each centriole is a cylindrical structure made of nine triplets of microtubules arranged in a circle; centrioles help form basal bodies for cilia/flagella and participate indirectly in spindle formation during mitosis.
13. Vacuoles
- Vacuoles are membrane-bound storage compartments containing water, nutrients, pigments or waste products.
- Plant cells typically have a large central vacuole that maintains turgor pressure, stores metabolites and contributes to cell growth; animal cells have smaller, more numerous vacuoles or vesicles.
A Brief Summary on Cell Organelles
Cells are diverse in form and function but share common organisational principles: a boundary plasma membrane, internal cytoplasm with organelles, and genetic material that directs cell activities. Prokaryotic and eukaryotic cells differ mainly in the presence of a membrane-bound nucleus and organelles in eukaryotes. Organelles such as mitochondria, chloroplasts, nucleus, endoplasmic reticulum and Golgi apparatus perform specialised tasks that together sustain life at the cellular level.
FAQs on Cell - Notes, Biology
| 1. What is a cell and why is it called the basic unit of life? |  |
| 2. What's the difference between prokaryotic and eukaryotic cells in simple terms? | |
| 3. How do mitochondria and chloroplasts work differently in plant and animal cells? | |
| 4. What do all the different cell organelles actually do inside the cell? | |
| 5. Why do plant cells have cell walls while animal cells don't? | |
