Cell Structure and Functions

Cell Structure and Functions – Complete CBSE Class 8 Science Notes

Introduction to Cell Structure and Functions

Cells are the fundamental units of life. Every living organism, from the smallest bacterium to the largest whale, is made up of cells. Understanding cell structure and functions is essential to grasping how life operates at the microscopic level.

This comprehensive guide covers the CBSE Class 8 Science chapter on Cell Structure and Functions, including the discovery of cells, types of cells, cell organelles, and their specific roles. Whether you're preparing for exams or simply curious about biology, this resource will help you master the topic.

What is a Cell?

A cell is the smallest structural and functional unit of life. It is a self-contained unit capable of performing all the basic functions necessary for life, such as:

  • Nutrition
  • Respiration
  • Growth
  • Reproduction
  • Excretion

Cells can exist independently (as in unicellular organisms like Amoeba) or work together in groups (as in multicellular organisms like humans and plants).

Terminology

Term Definition
Cytology Study of the structure of cells
Cell Biology Study of the structure and functions of cells
Protoplasm Living substance of the cell, including cytoplasm and nucleus

Historical Background of Cell Discovery

Robert Hooke (1665)

  • First to observe and name "cells"
  • Examined thin slices of cork under a microscope
  • Saw honeycomb-like compartments, which he called "cells"
  • Published findings in his book Micrographia
  • Actually observed dead cell walls, not living cells

Anton van Leeuwenhoek (1674)

  • First to observe living cells and microorganisms
  • Discovered bacteria, protozoa, spermatozoa, and red blood cells
  • Called them "animalcules"

Robert Brown (1831)

  • Discovered the nucleus in orchid root cells

Schleiden and Schwann (1838–1839)

  • Proposed the Cell Theory:
    1. All living organisms are made of cells
    2. Cell is the basic structural and functional unit of life
    3. New cells arise from pre-existing cells (later added by Rudolf Virchow, 1855)

Rudolf Virchow (1855)

  • Modified cell theory with the principle: "Omnis cellula e cellula" (every cell arises from a pre-existing cell)

Classification of Cells

Based on Complexity

1. Prokaryotic Cells

  • No true nucleus (nucleoid region without nuclear membrane)
  • Lack membrane-bound organelles
  • Single chromosome
  • Examples: Bacteria, Blue-green algae
  • Size: 1–10 µm

2. Eukaryotic Cells

  • Well-defined nucleus with nuclear membrane
  • Membrane-bound organelles present (mitochondria, ER, Golgi apparatus, etc.)
  • Multiple chromosomes
  • Examples: Plants, animals, fungi
  • Size: 50–100 µm
Feature Prokaryotic Cell Eukaryotic Cell
Nucleus No nuclear membrane Nuclear membrane present
Size Smaller (1–10 µm) Larger (50–100 µm)
Organelles Absent Present
Chromosome Single Multiple
Examples Bacteria, cyanobacteria Animals, plants, fungi

Based on Number of Cells

Unicellular Organisms

  • Made of a single cell
  • Perform all life functions independently
  • Examples: Amoeba, Paramecium, Euglena, bacteria

Multicellular Organisms

  • Made of many cells
  • Cells are specialized for specific functions
  • Examples: Humans, animals, plants
  • A fully grown human has about 100 trillion cells (10¹⁴)

Cell Size and Shape

Cell Size

  • Most cells are microscopic and measured in microns (µm) or micrometers (1 µm = 1 millionth of a meter)
  • Size varies widely:
Cell Type Size
Ostrich egg (largest animal cell) 170 mm × 130 mm
Nerve cells (longest in humans) Over 1 meter
Mycoplasma (smallest cell) 0.1 × 0.5 µm
Red blood cells (smallest in humans) 7–9 µm

Cell Shape

  • Shape depends on the function of the cell
  • Examples:
    • Round/spherical: Chlamydomonas
    • Irregular: Amoeba
    • Slipper-shaped: Paramecium
    • Elongated: Nerve cells, muscle cells
    • Flat: Cheek cells
    • Kidney-shaped: Guard cells of stomata

Parts of a Cell

Every cell has three main components:

  1. Cell Membrane (Plasma Membrane)
  2. Cytoplasm
  3. Nucleus

1. Cell Membrane (Plasma Membrane)

  • Thin, flexible outer covering of the cell
  • Made of proteins and phospholipids
  • Selectively permeable: allows only certain substances to pass through

Functions:

  • Protects the cell
  • Provides shape
  • Regulates entry and exit of substances

2. Cell Wall (in Plant Cells Only)

  • Rigid outer layer outside the plasma membrane
  • Made of cellulose (in plants), chitin (in fungi)
  • Absent in animal cells

Functions:

  • Provides shape and support (plants lack a skeleton)
  • Protects against environmental stress (temperature, wind, moisture)

3. Cytoplasm

  • Jelly-like substance between the cell membrane and nucleus
  • Contains water, salts, proteins, and cell organelles
  • Site of most metabolic activities

Functions:

  • Holds cell organelles in place
  • Site of cellular activities

4. Nucleus

  • Control center of the cell
  • Usually spherical and located in the center
  • Enclosed by nuclear membrane (porous, allows material exchange)

Components of Nucleus:

Part Description
Nuclear membrane Double-layered, porous membrane
Nucleoplasm Jelly-like fluid inside nucleus
Nucleolus Dense, spherical body; synthesizes ribosomes
Chromosomes Thread-like structures made of DNA and proteins; carry genes

Functions:

  • Controls all cellular activities
  • Stores and transmits genetic information (heredity)
  • Synthesizes ribosomes (via nucleolus)

Cell Organelles and Their Functions

1. Endoplasmic Reticulum (ER)

  • Network of membranes throughout the cytoplasm
  • Discovered by K.R. Porter (1945)
  • Continuous from nuclear membrane to plasma membrane

Types:

  • Rough ER (RER): Studded with ribosomes; synthesizes proteins
  • Smooth ER (SER): No ribosomes; synthesizes lipids, steroids, and detoxifies drugs

Functions:

  • Transport of materials
  • Protein and lipid synthesis
  • Provides structural support

2. Golgi Apparatus (Golgi Complex)

  • Discovered by Camillo Golgi
  • Composed of flattened membrane-bound sacs called cisternae
  • Located near the nucleus

Functions:

  • Packaging and secretion of proteins and lipids
  • Formation of lysosomes
  • Synthesis of cell wall materials (in plants)
  • Formation of acrosome in sperm cells

3. Ribosomes

  • Smallest organelles in the cell
  • Discovered by George Palade
  • Composed of RNA and proteins
  • Found free in cytoplasm or attached to ER

Function:

  • Protein synthesis (translation of mRNA)

4. Mitochondria (Power House of the Cell)

  • Rod-shaped or sausage-shaped organelles
  • Have double membrane (outer and inner)
  • Inner membrane folded into cristae (increase surface area)
  • Contain their own DNA and ribosomes

Function:

  • Cellular respiration: Produces ATP (energy currency of the cell)
  • Site of Krebs cycle and electron transport chain

5. Lysosomes (Suicidal Bags)

  • Discovered by Christian de Duve (1955)
  • Small, spherical organelles
  • Contain digestive enzymes (e.g., acid phosphatase)

Functions:

  • Intracellular digestion
  • Break down worn-out organelles (autophagy)
  • Digest foreign particles (heterophagy)
  • Cell death (autolysis)

6. Plastids (Only in Plant Cells)

  • Colored organelles found only in plant cells
  • Contain their own DNA and ribosomes

Types:

Type Pigment Function
Chloroplasts Chlorophyll (green) Photosynthesis
Chromoplasts Carotenoids (red, yellow, orange) Provide color to flowers and fruits
Leucoplasts None (colorless) Storage of starch, oils, proteins

7. Vacuoles

  • Membrane-bound sacs filled with cell sap
  • Large and central in plant cells
  • Small and numerous in animal cells

Functions:

  • Storage of water, nutrients, and waste
  • Maintain turgor pressure in plant cells
  • Food vacuoles in Amoeba

8. Centrosome and Centrioles (Only in Animal Cells)

  • Centrosome: Region near nucleus containing two centrioles
  • Centrioles: Cylindrical structures; help in cell division

Comparison: Plant Cell vs Animal Cell

Feature Plant Cell Animal Cell
Cell Wall Present (cellulose) Absent
Chloroplasts Present Absent
Vacuoles Large, central (1–3) Small, numerous
Centrioles Absent Present
Shape Fixed (due to cell wall) Variable
Nucleus Position Peripheral (pushed by vacuole) Central
Plasmodesmata Present Absent

How Organelles Work Together

Endoplasmic Reticulum and Golgi Apparatus

  1. Rough ER synthesizes proteins
  2. Proteins are packaged into transport vesicles
  3. Vesicles move to Golgi apparatus
  4. Golgi modifies, sorts, and packages proteins
  5. Vesicles bud off from Golgi and transport proteins to their destination (cell membrane, lysosomes, or secretion)

This is called the endomembrane system.

Functions of Mitochondria and Chloroplasts Explained

Mitochondria

  • Convert glucose into ATP through cellular respiration
  • Krebs cycle occurs in the matrix
  • Electron transport chain occurs on cristae
  • Called "powerhouse" because they produce energy for all cellular activities

Chloroplasts

  • Perform photosynthesis (convert light energy into chemical energy)
  • Contain chlorophyll in thylakoid membranes (grana)
  • Produce glucose and oxygen
  • Found only in plant cells

Both are semi-autonomous organelles (have their own DNA and ribosomes).

Role of the Cytoskeleton During Cell Division

The cytoskeleton is a network of protein filaments (microtubules, microfilaments, intermediate filaments) that provides:

  • Structural support
  • Cell shape
  • Movement of organelles

During cell division:

  • Microtubules form the spindle apparatus
  • Spindle fibers attach to chromosomes and pull them apart
  • Centrioles organize spindle formation (in animal cells)
  • Cytokinesis (division of cytoplasm) is aided by contractile ring of microfilaments

Examples of Organelle Defects and Related Diseases

Organelle Defect Disease/Condition
Mitochondria Mutations in mitochondrial DNA Mitochondrial myopathies, diabetes, neurodegenerative diseases
Lysosomes Enzyme deficiency Lysosomal storage diseases (e.g., Tay-Sachs disease)
Peroxisomes Enzyme defects Zellweger syndrome
Endoplasmic Reticulum Protein misfolding Cystic fibrosis, Alzheimer's disease
Golgi Apparatus Trafficking defects Congenital disorders of glycosylation

Formulas and Concepts (Quick Reference Table)

Concept Formula/Expression Explanation
Cell size 1 micron (µm) = 10⁻⁶ meters Unit of measurement for cells
Cell number in humans ~10¹⁴ cells Total cells in a fully grown human
Prokaryote size 1–10 µm Smaller than eukaryotes
Eukaryote size 50–100 µm Larger, more complex
Chromosome pattern 9 + 2 arrangement Microtubules in cilia and flagella
ATP production Cellular respiration in mitochondria Converts glucose → ATP

Conclusion

Understanding cell structure and functions is fundamental to biology. Cells are the building blocks of life, and every organism, from bacteria to humans, relies on the coordinated functioning of cellular components.

This guide has covered:

  • The discovery and history of cell biology
  • Types of cells (prokaryotic vs eukaryotic, unicellular vs multicellular)
  • Detailed structure and functions of cell organelles
  • Differences between plant and animal cells
  • Real-world applications and diseases related to organelle dysfunction

Master this chapter to build a strong foundation for advanced biology topics in higher classes.

Additional Resources