Cell Structure and Function

Overview & Fundamental Concept

Definitive AISSEE Class 9 General Science guide for Cell Structure and Function (Topic 3). Features pointwise cytology facts, organelle comparison matrices, plant vs animal cell tables, memory mnemonics, and 10 exam-standard MCQs.

• Discovery of Cell: Discovered by English scientist Robert Hooke in 1665. He observed thin slices of dead tree cork under a crude compound microscope and noticed box-like compartments resembling honeycombs. He coined the term 'cell' (Latin: cella = small room).
• First Living Cell: Observed by Anton van Leeuwenhoek in 1674 (pond water microbes, protozoa, and bacteria) using an improved single-lens microscope.
• Discovery of Nucleus: Discovered by Scottish botanist Robert Brown in 1831 in orchid root cells.
• Protoplasm Coined: The living substance inside a cell was named Protoplasm by J. E. Purkinje in 1839. Protoplasm = Cytoplasm + Nucleus (the physical basis of life).
• The Classic Cell Theory: Formulated jointly by German botanist Matthias Schleiden (1838) and German zoologist Theodor Schwann (1839):
  • All living plants and animals are composed of cells.
  • The cell is the fundamental structural and functional unit of all living organisms.
• Cell Theory Expansion: Expanded by Rudolf Virchow in 1855 with the Latin aphorism 'Omnis cellula-e-cellula' — all living cells arise exclusively from pre-existing living cells through cell division.
• Cell Dimensions & Extremes:
  • Smallest Cell: Mycoplasma (PPLO — Pleuro-Pneumonia Like Organism), measuring only 0.1 to 0.5 micrometres (μm).
  • Largest Single Cell: The unfertilised egg of an ostrich, measuring approximately 170 mm × 130 mm.
  • Longest Human Cell: The Nerve cell (Neuron), which can measure up to 1 metre in length.
  • Smallest Human Cell: Human sperm cell (head ~5 μm) / Red Blood Corpuscle (RBC ~7 μm diameter).
• Cell Shape Specialisation: Shapes correlate directly with functions:
  • Amoeba and White Blood Cells (WBCs): Irregular and shape-changing (pseudopodia for movement/phagocytosis).
  • Red Blood Cells (RBCs): Biconcave circular discs (optimizes surface area for oxygen transport).
  • Muscle Cells: Spindle-shaped, elongated, and pointed at both ends (for contraction).
  • Nerve Cells: Long, branched with an axon and dendrites (for rapid electrical impulse transmission).

• Prokaryotic Cells (Primitive Nucleus): (Greek: pro = primitive/before; karyon = nucleus).
  • Cells that lack a true membrane-bound nucleus.
  • Genetic material (DNA) lies freely in the cytoplasm in an undefined region called the Nucleoid.
  • Membrane-bound organelles (Mitochondria, Endoplasmic Reticulum, Golgi apparatus, Lysosomes, Plastids) are strictly absent.
  • Ribosomes are smaller (70S type).
  • Single, circular chromosome.
  • Examples: Bacteria, Blue-Green Algae (Cyanobacteria), and Mycoplasma.
• Eukaryotic Cells (True Nucleus): (Greek: eu = true/well; karyon = nucleus).
  • Cells that have a well-organized nucleus enclosed by a double nuclear membrane.
  • Genetic material is organized into linear chromosomes made of DNA and histone proteins.
  • Possess membrane-bound organelles performing compartmentalized functions (Mitochondria, Golgi body, ER, Lysosomes).
  • Ribosomes are larger (80S type in cytoplasm).
  • Examples: All plant cells, animal cells, fungi, and protozoa (Amoeba, Paramecium).

• 1. Plasma Membrane (Cell Membrane): A thin, delicate, elastic, selectively permeable outer boundary composed of lipids and proteins. Regulates entry and exit of substances by diffusion and osmosis.
• 2. Cell Wall: A rigid, non-living, fully permeable outer protective layer present exclusively in plant cells, fungi, and bacteria (absent in all animal cells). In plants, it is composed of cellulose and provides structural rigidity, mechanical strength, and prevents bursting in hypotonic solutions.
• 3. Cytoplasm: A jelly-like, semi-fluid living matrix occupying the space between the plasma membrane and the nuclear envelope. Holds all cell organelles and is the site of vital metabolic chemical reactions.
• 4. Nucleus ('Control Center / Brain of Cell'): Largest organelle, bounded by a porous double nuclear membrane.
  • Nucleolus: Dense spherical body that synthesizes ribosomes (RNA).
  • Chromatin & Chromosomes: Thread-like chromatin condenses into rod-shaped Chromosomes during cell division. Chromosomes carry Genes, the hereditary units that transfer genetic characters from parents to offspring. Normal human cell contains 46 chromosomes (23 pairs).
• 5. Mitochondria ('Powerhouse of the Cell'): Double-membraned organelle with folded inner membrane (cristae). Generates energy in the form of ATP (Adenosine Triphosphate) — the universal energy currency — via cellular respiration. Contains its own circular DNA and 70S ribosomes.
• 6. Plastids: Present strictly in plant cells. Three major types:
  • Chloroplasts: Contain green pigment chlorophyll; sites of photosynthesis ('Kitchen of the Cell').
  • Chromoplasts: Contain carotenoid pigments (yellow, orange, red); impart attractive colors to flowers and ripe fruits to attract pollinators.
  • Leucoplasts: Colorless plastids; store starch (amyloplast), oils (elaioplast), and proteins (aleuroplast) in roots, tubers, and seeds.
• 7. Vacuoles ('Storage Sacks'): Plant cells have a single, permanent, gigantic central vacuole occupying up to 90% of cell volume, bounded by a membrane called the Tonoplast, providing turgidity. Animal cells have multiple small, temporary vacuoles.
• 8. Endoplasmic Reticulum (ER): Network of membranous tubules. Rough ER (RER): Studded with ribosomes; synthesizes proteins. Smooth ER (SER): Free of ribosomes; synthesizes lipids/fats and detoxifies poisons.
• 9. Golgi Apparatus: Discovered by Camillo Golgi. Stacks of flattened cisternae; packages, modifies, and secretes cellular proteins and enzymes.
• 10. Lysosomes ('Suicide Bags / Scavengers of Cell'): Membranous sacs packed with powerful hydrolytic digestive enzymes. When a cell gets damaged or infected, lysosomes burst and their enzymes digest their own cell.

• Plant and animal cells share common eukaryotic features (plasma membrane, nucleus, mitochondria, cytoplasm, Golgi, ER), but exhibit critical taxonomic differences.
• Shape & Rigidity: Plant cells have a fixed, regular, rigid polygonal/rectangular shape maintained by the cellulose cell wall. Animal cells have a flexible, irregular, often spherical or oval shape.
• Cell Wall: Present in plant cells; totally absent in animal cells.
• Plastids: Present in plant cells (chloroplasts, chromoplasts, leucoplasts); absent in animal cells (animals cannot synthesize their own food).
• Vacuole Size & Position: Plant cell has one massive central vacuole (tonoplast-bound), which pushes the nucleus toward the peripheral edge. Animal cell has multiple tiny, dispersed vacuoles; nucleus stays near the center.
• Centrosome & Centrioles: Present in animal cells (organize spindle fibres during cell division); absent in higher plant cells.