Cell Structure and Function

• All organisms share similarities in structure and function, and their bodies are made of cells.
• The cell is the basic structural and functional unit of all living organisms.
• The study of cells includes their variety in shape, size, structure, and functions across different organisms.

Discovery of the Cell

• Cells are the basic structural unit of living beings.
• Cells were undiscovered for a long time due to their small size.
• The discovery of the cell and its features occurred in the seventeenth century with the advent of optical instruments.
• Robert Hooke observed thin slices of cork in 1665 using a self-designed microscope.
• Hooke observed honey-comb like structures with compartments, which he called "cells" (Latin for "a little room").
• The compartments Hooke observed were dead cells bounded by a cell wall.
• Living organisms are made of cells with similar basic structures, but they differ in number, shape, and size.
• "Did you know?": The outermost layer of skin consists of dead cells, which are shed and replaced. A person may lose about four kilograms of skin cells every year.

Variation in Cell Number, Shape and Size

• Cell Number:
  • Organisms can be unicellular or multicellular based on number of cells.
  • Amoeba is a unicellular organism, while an earthworm is a multicellular organism.
• Cell Shape:
  • Cell shapes vary across organisms and within the same organism.
  • Cell shapes can be oval, spherical, cuboidal, fiber-like, or polygonal.
  • Cell shape is related to its location and function in the tissue (e.g., nerve cells have a long fiber-like structure to transmit nerve impulses).
• Cell Size:
  • Cell sizes vary considerably.
  • Mycoplasma (PPLO) is the smallest cell (0.1 micron in diameter).
  • Ostrich egg is the largest cell (170 mm in diameter).
• Hen's egg is a single cell, large enough to be seen with the naked eye.
• Table 1 provides approximate sizes of plant and animal cells, including Amoeba (1000 μm), hen's egg (60 mm), ostrich egg (170 mm), and green alga Chara (10 cm).
• Table 2 provides approximate sizes of human body cells, including red blood cell (9 μm), liver cell (20 μm), human ovum (0.1 mm or 100 μm), and nerve cell (about 1 m).
• "Did you know?": The life span of a red blood cell is about 120 days.

Activities

• Observing animal cells: prepare a temporary mount of cheek cells.
  • Take a clean toothpick and gently scratch the inner side of your cheek.
  • Rub the material on a clean glass slide, add methylene blue, let it stain, put a coverslip, and observe under a microscope.
• Observing plant cells: make a slide of onion peel.
  • Put a drop of water on a glass slide, place a small piece of onion peel, add saffranin, stain, put a coverslip, and observe under the microscope.
  • Onion peel cells are arranged in rows, with a dark structure (nucleus) in the center of each cell.
• Observing blood cells: request a prepared slide.
  • Red blood cells have a characteristic red color and disc shape. Also, a few Amoeba-like white blood cells are present.

Parts of a Cell

• Cells consist of protoplasm (cytoplasm and nucleus) surrounded by a cell membrane.
• Cytoplasm contains cell organelles, which perform specific functions.
• Cell Membrane:
  • All living cells are bound by a plasma membrane (cell membrane).
  • It surrounds the protoplasm and controls the entry and exit of substances.
  • Plant, fungi, and bacteria cells have an additional outer covering called the cell wall.
• Cell Wall:
  • Plant cells have a cell wall for protection, rigidity, definite shape, size, and support.
• Cytoplasm:
  • Cytoplasm is the portion of protoplasm inside the cell membrane but outside the nuclear membrane (kytos = hollow, plasma = liquid).
  • It's a ground substance for cell activities, made of carbohydrates, proteins, fats, minerals, vitamins, and water.
• Nucleus:
  • Nucleus is the most important part of the cell and generally lies in the center.
  • It controls cell activities.
  • Nucleoplasm is the protoplasm of the nucleus.
• Chromatin is a thread-like network in the nucleoplasm.
  • During cell division, chromatin condenses to form chromosomes, responsible for inherited characters (genes).
  • Many small living structures, called cell organelles, are present in the cell.
• Cell Organelles:
  • Plastids are large organelles characteristic of plant cells containing pigments.
    • Chloroplasts are green plastids responsible for photosynthesis.
    • Chromoplasts impart colors (other than green) to the plant parts.
    • Leucoplasts are colorless plastids that store starch, proteins, and oils.
  • Mitochondria are rod-shaped or spherical structures responsible for cellular respiration and energy generation i.e. the powerhouse of a cell.
  • Endoplasmic Reticulum (ER) is a network for transport of materials.
    • Rough ER has ribosomes for protein synthesis.
    • Smooth ER lacks ribosomes and helps in the synthesis of fats.
  • Golgi Complex are sac-like structures involved in processing and packaging of materials.
  • Vacuole appears as an empty space to store excess water and waste.
  • Amoeba has food vacuoles for digestion.
  • Ribosomes are found in the cytoplasm and rough ER, that helps in protein synthesis.
  • Cilia and flagella are small extensions for locomotion and movement of food.
  • Euglena has a single flagellum, Paramoecium have numerous cilia.

Levels of Organization in an Organism

• Single cells perform all necessary functions in unicellular organisms like Amoeba i.e. capturing and digesting food, respiration, excretion and reproduction.
• Multicellular organisms: cells specialize to perform specific functions.
• A group of cells forms a tissue (i.e., nervous tissue), which performs a specialized function.
• Multiple tissues form an organ (i.e., kidney), which performs a specific function.
• Several organs working together form an organ system (i.e., digestive system).
• All cells have common features, but they can appear different in different parts of an organism, such as blood cells versus liver cells.

Comparison between Plant and Animal Cells

• Plant cells have a definite shape due to the rigid cell wall, while animal cells have a cell membrane, that provide them flexibility.
• Plant cells have plastids, which are absent in animal cells.
• Plant cells generally have large vacuoles, while animal cells either lack vacuoles or have very small vacuoles.

Difference between a Plant and an Animal Cell

• Components/Characters:
  • Shape: Fixed vs Irregular/Not fixed
  • Cell Wall: Present vs Absent
  • Plastids: Present vs Absent
  • Vacuoles: One large vacuole is present vs Vacuoles are either absent, or are present only as small vacuoles