The Cell - Structure, Functions, and Organization

Introduction to the Cell

  • Fundamental Definition: The cell is the basic structural as well as functional unit of all living organisms.
  • Structural Analogy: Just as a building is constructed from individual bricks, the bodies of all plants and animals are composed of cells.
  • Ubiquity Across Life forms: Cells form the structural basis of all living entities, ranging from microscopic unicellular organisms like bacteria and Amoeba to large multicellular organisms like elephants, whales, and gigantic trees.
  • Essential Biological Functions: Cells independently perform fundamental life processes required for organism survival:
    • Nutrition
    • Respiration
    • Growth
    • Development
    • Reproduction

History and Discovery of the Cell

  • Timeline of Discovery: Cells remained undiscovered for a long period because the vast majority are microscopic and invisible to the unaided eye. Their discovery depended on the development of optical instruments in the seventeenth century.
  • Robert Hooke (1665):
    • First scientist to observe and document cells in 16651665
    • Examined thin slices of cork (derived from tree bark) using a self-designed microscope.
    • Observed a structural pattern resembling a honeycomb, composed of distinct compartments.
    • Coined the term cell from the Latin word cella, meaning "a little room".
    • These compartments were later identified as dead cells bounded by rigid cell walls.
  • Dynamic Nature of Human Skin Cells:
    • The outermost layer of human skin consists entirely of dead cells.
    • This layer is periodically shed and continuously replaced by newer cells.
    • An individual human loses approximately 4 kg4\,kg of skin cells every year.

Variation in Cell Number, Shape, and Size

  • Classification by Cell Number:
    • Unicellular Organisms: Organisms consisting of a single cell that independently performs all life functions (e.g., Amoeba, Euglena, Paramoecium, Chlamydomonas).
    • Multicellular Organisms: Organisms composed of millions to trillions of cells working in a coordinated manner (e.g., earthworms, human beings, birds, trees).
    • Organism Size Relationship: The overall physical size difference between organisms (e.g., Amoeba vs. earthworm) is primarily due to the total number of constituent cells rather than individual cell size.
  • Variation in Cell Shape:
    • Cell shapes vary significantly between different species and between different organs within the same organism.
    • Common Shapes: Oval, spherical, cuboidal, fibre-like, polygonal, disc-shaped, or irregular.
    • Functional Correlation: A cell's shape is directly adapted to its specific location and biological function within a tissue:
      • Nerve Cells (Neurons): Possess long, elongated fibre-like structures specialized for transmitting nerve impulses to organs across distant body regions.
      • Red Blood Cells: Possess a characteristic biconcave disc shape optimized for moving through capillaries and transporting gases.
      • Amoeba: Exhibits an irregular, continuously changing shape for locomotion and engulfing food.
      • Other Documented Cell Shapes: Spherical liver cells, globular fat cells, spindle-shaped smooth muscle cells, and unicellular green algae like Chlamydomonas.
  • Variation in Cell Size:
    • Smallest Known Cell: PPLO (Pleuro pneumonia-like organism), also termed Mycoplasma, measuring approximately 0.1 μm0.1\,\mu m in diameter (1 μm=10−6 m=10−3 mm1\,\mu m = 10^{-6}\,m = 10^{-3}\,mm).
    • Largest Known Cell: Ostrich egg, measuring approximately 170 mm170\,mm in diameter.
    • Hen's Egg: Represents a single large cell visible to the unaided eye, measuring approximately 60 mm60\,mm.
    • Red Blood Cell Lifespan: Human red blood cells have an average lifespan of about 120 days120\,days.
  • Approximate Sizes of Selected Plant and Animal Cells:
    • Amoeba: 1000 μm1000\,\mu m
    • Hen's egg: 60 mm60\,mm
    • Ostrich egg: 170 mm170\,mm
    • Green alga (Chara): 10 cm10\,cm
  • Approximate Sizes of Specific Human Body Cells:
    • Red blood cell: 9 μm9\,\mu m
    • Liver cell: 20 μm20\,\mu m
    • Human ovum: 0.1 mm0.1\,mm (or 100 μm100\,\mu m)
    • Nerve cell: Approximately 1 m1\,m

Laboratory Procedures for Cell Observation

  • Preparation and Microscopic Observation of Animal Cells (Human Cheek Cells):
    1. Gently scratch the inner lining of the cheek using a clean toothpick to obtain frothy material.
    2. Rub the collected material onto the center of a clean glass slide.
    3. Add a drop of methylene blue stain to stain cellular structures.
    4. Allow the stain to react for 1 minute1\,minute.
    5. Cover gently with a cover slip and observe under a microscope.
    6. Observed Features: Isolated or clustered polygonal cells showing a distinct outer cell membrane, cytoplasm, and a darkly stained central nucleus.
  • Preparation and Microscopic Observation of Plant Cells (Onion Peel):
    1. Place a drop of clean water in the center of a glass slide.
    2. Place a small, neatly cut piece of onion peel (the thin membrane surrounding fleshy scale leaves) onto the water drop.
    3. Add 1 to 2 drops of saffranin stain.
    4. Allow to stain for 1 minute1\,minute.
    5. Place a cover slip on top and observe under the microscope.
    6. Observed Features: Cells neatly arranged in linear rows, bound by distinct rigid cell walls surrounding a cytoplasm with a dark, centrally located nucleus.
  • Observation of Human Blood Cells:
    1. Prepare a fresh blood mount or use a prepared permanent slide.
    2. Observed Features: Disc-shaped red blood cells exhibiting a characteristic red pigmentation, alongside distinct, irregularly shaped (Amoeba-like) white blood cells distributed between them.
  • Anatomical Structure of a Hen's Egg:
    • Breaking the outer calcareous shell reveals two main internal parts surrounding the embryo:
      • Yolk: The central yellow spherical mass.
      • Albumen: The transparent, white, jelly-like fluid surrounding the central yolk.
      • Biological Function: Yolk and albumen serve as stored reserve food material within the cell cytoplasm.
      • Labeled Structural Parts: Outer shell, Air sac, Cell membrane, Albumen, Yolk.

Structural Components of the Cell

  • Protoplasm:
    • The living substance of the cell, bounded externally by the cell membrane.
    • Composed of two main components: the cytoplasm and the nucleus.
  • Cell Membrane (Plasma Membrane):
    • A thin, living outer boundary that encloses the inner gel-like protoplasm in all living cells.
    • Selectively Permeable Nature: Controls the passage of substances into and out of the cell, allowing only specific materials to cross based on cellular requirements.
  • Cell Wall:
    • An additional rigid outer boundary found outside the plasma membrane exclusively in plant cells, fungi, and bacteria.
    • Key Functions: Provides structural rigidity, mechanical support, protection against environmental fluctuations (temperature, wind, moisture), and maintains a fixed shape and size.
  • Cytoplasm:
    • The portion of protoplasm located inner to the plasma membrane and outer to the nuclear membrane (kytos=hollow\text{kytos} = \text{hollow}, plasma=liquid\text{plasma} = \text{liquid}).
    • Functions as the primary ground substance for all metabolic and cellular activities.
    • Composed of carbohydrates, proteins, fats, minerals, vitamins, and a large percentage of water.
  • Nucleus:
    • The central control center of cellular activities, usually located in the center of the cell (can be peripheral in mature plant cells).
    • Enclosed by a double-layered nuclear membrane.
    • Nucleoplasm: The specialized protoplasm contained inside the nuclear membrane.
    • Chromatin: A delicate, thread-like interconnected network of genetic material present in non-dividing cells.
    • Chromosomes: Thick, rod-like structures formed when chromatin condenses during cell division.
    • Genes: Functional units of inheritance carried on chromosomes responsible for transferring genetic traits from parents to offspring.
  • Somatic Cell Chromosome Numbers Across Species:
    • Man: 4646
    • Dog: 7878
    • Pigeon: 8080
    • Yeast: 3232
    • Wheat: 4242

Cell Organelles and Cellular Extensions

  • Plastids:
    • Large organelles characteristic of plant cells; contain specialized pigments.
    • Chloroplasts: Green-colored plastids containing chlorophyll; manufacture food via photosynthesis.
    • Chromoplasts: Pigmented plastids (containing colors other than green); impart bright colors to flowers, fruits, and vegetables.
    • Leucoplasts: Colorless plastids; function as storage centers for nutrients such as starch, proteins, and oils.
  • Mitochondria:
    • Rod-shaped or spherical membrane-bound organelles.
    • Responsible for cellular respiration and energy production (ATP generation).
    • Known as the "powerhouse of the cell". Present in elevated quantities in metabolically active cells.
  • Endoplasmic Reticulum (ER):
    • An extensive network of interconnected membranes providing structural channels for internal transport of materials.
    • Rough ER: Surface studded with ribosomes; plays an essential role in protein synthesis.
    • Smooth ER: Free of ribosomes; functions in the synthesis of lipids/fats.
  • Golgi Complex:
    • Composed of flattened, sac-like structures stacked directly above one another.
    • Involved in modifying, processing, sorting, and packaging cellular products.
  • Vacuole:
    • Fluid-filled membrane-bound storage spaces appearing empty in the cytoplasm.
    • Plant cells contain a single, massive central vacuole storing excess water, nutrients, and cellular waste.
    • Animal cells contain small, temporary vacuoles or lack them entirely.
    • Specialized Functional Vacuoles: Single-celled Amoeba forms food vacuoles to retain and digest captured food particles; Paramoecium contains contractile vacuoles.
  • Ribosomes:
    • Tiny granular structures present freely in cytoplasm or attached to the Rough Endoplasmic Reticulum.
    • Serve as the primary sites for cellular protein synthesis.
  • Cilia and Flagella:
    • Specialized locomotory membrane extensions projecting from the cell surface; aid in locomotion and food collection.
    • Cilia: Numerous short, hair-like projections covering the entire cell surface (e.g., Paramoecium).
    • Flagella: A long, whip-like single extension (e.g., Euglena).
    • Structural features of Euglena: Flagellum, Cytoplasm, Chloroplast, Pellicle, Nucleus.
    • Structural features of Paramoecium: Cilia, Contractile vacuole, Macro-nucleus, Micro-nucleus, Food vacuole, Cytoplasm.

Levels of Biological Organisation

  • Unicellular Organisation:
    • A single cell independently carries out all vital life processes including food capture, digestion, respiration, excretion, growth, and reproduction (e.g., Amoeba).
  • Multicellular Structural Hierarchy:
    • Cell: The basic structural and functional unit.
    • Tissue: A specialized group of similar cells working together to perform a specific activity (e.g., nervous tissue).
    • Organ: A structure composed of different tissues working together to perform a specific function (e.g., kidney).
    • Organ System: A group of organs operating in coordination to execute complex biological functions (e.g., digestive system).
    • Organism: A complete living entity integrated by multiple organ systems.
  • Major Organ Systems in the Human Body:
    • Digestive System
    • Respiratory System
    • Circulatory System
    • Excretory System
    • Skeletal System
    • Muscular System
    • Nervous System
    • Reproductive System
    • Endocrine System
    • Integumentary System

Comparison Between Plant and Animal Cells

  • Key Differences Overview:
    • Shape: Plant cells generally possess a rigid, fixed, definite shape due to the outer cell wall. Animal cells lack a cell wall and are bounded only by a flexible plasma membrane, allowing significant shape variation.
    • Plastids: Plant cells contain plastids (chloroplasts, chromoplasts, leucoplasts); plastids are completely absent in animal cells.
    • Vacuoles: Plant cells possess a single, large central vacuole; animal cells either lack vacuoles or possess small, scattered vacuoles.
  • Summary Table of Cellular Differences:
    • Shape: Plant Cell = Fixed shape; Animal Cell = Irregular / Not fixed shape.
    • Cell Wall: Plant Cell = Present; Animal Cell = Absent.
    • Plastids: Plant Cell = Present; Animal Cell = Absent.
    • Vacuoles: Plant Cell = One large central vacuole present; Animal Cell = Vacuoles are either absent or present only as small vacuoles.