Grade 8 Chapter 10: Cell and Cell Organelles

Introduction

The cell is the fundamental structural and functional unit of all living organisms. Just as bricks are the basic units of a building, cells are the basic units of life. From the smallest bacteria to the largest whale, all living things are composed of cells. This chapter will take us on a microscopic journey to explore the discovery of the cell, its different types, the distinctions between plant and animal cells, and the intricate structures within a cell known as cell organelles, each performing vital functions to sustain life.

The Cell: Basic Unit of Life

• Discovery of the Cell: In 1665, Robert Hooke observed thin slices of cork under his self-designed microscope. He noticed box-like compartments, which he called 'cells' (from the Latin word 'cella' meaning a small room).
• Cell Theory:
  • All living organisms are composed of cells.
  • The cell is the basic structural and functional unit of life.
  • All cells arise from pre-existing cells.

Types of Cells

Based on the presence or absence of a well-defined nucleus and membrane-bound organelles, cells are classified into two main types:

1. Prokaryotic Cells

• These are primitive and simpler cells.
• Lack a true nucleus (genetic material is not enclosed by a nuclear membrane).
• Lack membrane-bound organelles (like mitochondria, endoplasmic reticulum, Golgi complex).
• Generally smaller in size (1-10 µm).
• Example: Bacteria, Blue-green algae (Cyanobacteria).

2. Eukaryotic Cells

• These are advanced and complex cells.
• Possess a true nucleus (genetic material enclosed by a nuclear membrane).
• Contain various membrane-bound organelles.
• Generally larger in size (10-100 µm).
• Example: Plant cells, Animal cells, Fungi, Protists.

Plant Cell vs. Animal Cell

Both plant and animal cells are eukaryotic, but they have some distinct differences in their structure.

Cell Organelles

Cell organelles are specialized structures within the cytoplasm of eukaryotic cells that perform specific functions necessary for the cell's survival.

1. Cell Wall

• Location: Outermost rigid layer found only in plant cells, fungi, and bacteria. Absent in animal cells.
• Composition: In plants, primarily made of cellulose.
• Function: Provides structural support, protection, and maintains the shape of the cell. Prevents the cell from bursting when it takes in too much water.

2. Cell Membrane (Plasma Membrane)

• Location: Present in both plant and animal cells. It is the outermost boundary in animal cells and lies just inside the cell wall in plant cells.
• Composition: Made of lipids and proteins.
• Function: Regulates the movement of substances into and out of the cell (selectively permeable membrane). Provides protection and maintains cell shape in animal cells.

3. Cytoplasm

• Location: The jelly-like substance filling the cell, enclosed by the cell membrane. It contains various cell organelles.
• Composition: Mostly water, salts, organic molecules, and various organelles.
• Function: Site of most cellular activities and metabolic reactions. Holds all the cell organelles.

4. Nucleus

• Location: Generally a large, spherical organelle usually located in the center of animal cells and towards the periphery in mature plant cells (due to large central vacuole).
• Structure: Enclosed by a double-layered nuclear membrane, contains chromatin (DNA and proteins), and a nucleolus.
• Function: Controls all cellular activities. Contains the genetic material (DNA) and is responsible for heredity, cell growth, and reproduction. Often called the 'control center' of the cell.

5. Endoplasmic Reticulum (ER)

• Location: A network of membrane-bound tubules and sacs (cisternae) extending from the nuclear membrane throughout the cytoplasm.
• Types:
  • Rough Endoplasmic Reticulum (RER): Has ribosomes attached to its surface, giving it a rough appearance. Involved in protein synthesis and modification.
  • Smooth Endoplasmic Reticulum (SER): Lacks ribosomes. Involved in lipid synthesis, detoxification of drugs and poisons, and storage of calcium ions.
• Function: Serves as a transport system within the cell. RER synthesizes and transports proteins. SER synthesizes lipids and detoxifies substances.

6. Golgi Complex (Golgi Apparatus/Golgi Body)

• Location: A stack of flattened, membrane-bound sacs called cisternae, usually located near the nucleus.
• Function: Processes, modifies, sorts, and packages proteins and lipids synthesized in the ER. It acts as the 'post office' of the cell, preparing materials for secretion or delivery to other organelles. Forms lysosomes.

7. Mitochondria

• Location: Rod-shaped or oval-shaped organelles found in the cytoplasm of eukaryotic cells.
• Structure: Double-membraned organelle; the inner membrane is folded into cristae to increase surface area.
• Function: Site of cellular respiration, where glucose is broken down to release energy in the form of ATP (Adenosine Triphosphate). Often called the 'powerhouse' of the cell.

8. Plastids

• Location: Present only in plant cells and some algae. Absent in animal cells.
• Types:
  • Chloroplasts: Contain green pigment chlorophyll. Site of photosynthesis.
  • Chromoplasts: Contain pigments other than chlorophyll, giving color to flowers and fruits (e.g., red, yellow, orange).
  • Leucoplasts: Colorless plastids involved in the storage of food (starch, oil, protein).
• Function: Chloroplasts perform photosynthesis (food production). Chromoplasts provide color. Leucoplasts store food.

9. Vacuoles

• Location: Membrane-bound sacs found in the cytoplasm.
• Size: In plant cells, typically a single large central vacuole (occupying up to 90% of cell volume). In animal cells, they are small and numerous or sometimes absent.
• Function: Stores water, nutrients, waste products, and other substances. In plant cells, it provides turgidity and rigidity to the cell. In animal cells, they are involved in storage and waste removal.

10. Lysosomes

• Location: Small, spherical, membrane-bound sacs found in the cytoplasm.
• Function: Contain digestive enzymes that break down waste materials and cellular debris. They also digest worn-out cell organelles and foreign invaders like bacteria. Often called 'suicidal bags' of the cell because they can digest the entire cell if their membrane bursts.

11. Centrosome

• Location: Found only in animal cells, usually near the nucleus. Absent in plant cells.
• Structure: Consists of two centrioles arranged perpendicularly.
• Function: Plays a crucial role in cell division by forming spindle fibers that help in the separation of chromosomes.

Summary

• The cell is the basic structural and functional unit of life, discovered by Robert Hooke.
• Prokaryotic cells lack a true nucleus and membrane-bound organelles (e.g., bacteria).
• Eukaryotic cells have a true nucleus and membrane-bound organelles (e.g., plant and animal cells).
• Plant cells have a cell wall, chloroplasts, and a large central vacuole, which are absent in animal cells. Animal cells have a centrosome, which is absent in most plant cells.
• Key cell organelles and their functions:
  • Cell Wall: Support and protection (plants, fungi, bacteria).
  • Cell Membrane: Regulates substance movement (all cells).
  • Cytoplasm: Jelly-like substance, site of reactions.
  • Nucleus: Controls cell activities, contains DNA.
  • Endoplasmic Reticulum: Network for transport; RER (protein synthesis), SER (lipid synthesis, detoxification).
  • Golgi Complex: Processes, packages, and sorts proteins/lipids.
  • Mitochondria: 'Powerhouse,' produces ATP through cellular respiration.
  • Plastids: Chloroplasts (photosynthesis in plants), Chromoplasts (color), Leucoplasts (food storage).
  • Vacuoles: Storage, turgidity (large in plants).
  • Lysosomes: 'Suicidal bags,' contain digestive enzymes.
  • Centrosome: Helps in cell division (animal cells).

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 10: Cell and Cell Organelles.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.