Complete Biology notes on Cell its structure and functions for BPSC and Other Competitive Exams in 2025

Complete Biology notes on Cell its structure and functions for BPSC and Other Competitive Exams in 2025

General Science- 14 (Biology)

Cell its Structure and Functions

The cell is the fundamental structural and functional unit of all known living organisms. It is the smallest unit of life that can replicate independently.

1. Introduction to the Cell

• Discovery: The term “cell” was first coined by Robert Hooke in 1665, who observed honeycomb-like structures in a thin slice of cork under his self-made microscope.
• Cell Theory: Proposed by Matthias Schleiden (1838) and Theodor Schwann (1839), and later elaborated by Rudolf Virchow (1855). Its main postulates are:
  1. All living organisms are composed of one or more cells.
  2. The cell is the basic structural and functional unit of life.
  3. All cells arise from pre-existing cells (Omnis cellula e cellula).
• Organisms based on Cell Number:

• Unicellular Organisms: Composed of a single cell (e.g., Amoeba, Bacteria, Yeast). All life functions are carried out by this single cell.
• Multicellular Organisms: Composed of many cells (e.g., Plants, Animals, Fungi). Cells are specialized to perform different functions, leading to division of labor.

2. Types of Cells

Living organisms primarily consist of two major types of cells: Prokaryotic and Eukaryotic.

A. Prokaryotic Cells:

• Characteristics:
  • Simpler and generally smaller in size.
  • Lack a well-defined or true nucleus; genetic material (DNA) is located in a region called the nucleoid, not enclosed by a nuclear membrane.
  • Lack membrane-bound organelles (e.g., mitochondria, ER, Golgi apparatus, lysosomes are absent).
  • Contain ribosomes (smaller 70S type).
  • Have a cell wall (except Mycoplasma).
  • Genetic material is typically a single, circular chromosome.
• Examples: Bacteria, Cyanobacteria (blue-green algae), Archaea.

B. Eukaryotic Cells:

• Characteristics:
  • More complex and generally larger in size.
  • Possess a true nucleus enclosed by a double nuclear membrane, containing the genetic material.
  • Contain various membrane-bound organelles that perform specialized functions (e.g., mitochondria, ER, Golgi apparatus, lysosomes, vacuoles, chloroplasts).
  • Contain ribosomes (larger 80S type).
  • Genetic material is organized into multiple linear chromosomes.
• Examples: Plant cells, Animal cells, Fungi, Protists.

3. Basic Cell Structure (Common Components)

All cells, whether prokaryotic or eukaryotic, share certain basic components:

1. Cell Membrane (Plasma Membrane):
   • Structure: A thin, flexible, selectively permeable outer boundary of the cell, composed primarily of a lipid bilayer (phospholipids) with embedded proteins.
   • Function:
     • Regulates the passage of substances into and out of the cell (selective permeability).
     • Maintains cell integrity and shape.
     • Involved in cell recognition and communication.

2. Cytoplasm:
   • Structure: The jelly-like substance that fills the cell, enclosed by the cell membrane. It includes the cytosol (the fluid portion) and the various organelles suspended within it.
   • Function:
     • Site of most cellular metabolic reactions (e.g., glycolysis).
     • Holds organelles in place.

3. Nucleus (in Eukaryotic Cells):
   • Structure: The largest and most prominent organelle in eukaryotic cells, typically spherical or oval. It is enclosed by a double membrane called the nuclear envelope, which has nuclear pores. Inside the nucleus are nucleoplasm (nuclear sap), chromatin (complex of DNA and proteins forming chromosomes), and a dense structure called the nucleolus.
   • Function:
     • Contains the cell’s genetic material (DNA) in the form of chromosomes.
     • Controls and regulates the growth, metabolism, and reproduction of the cell.
     • The nucleolus is involved in the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomes.

4. Cell Organelles (Structure & Function)

Organelles are specialized sub-cellular structures that perform specific functions necessary for the cell’s survival.

1. Mitochondria:
   • Structure: Double-membraned, rod-shaped organelles. The inner membrane is highly folded into structures called cristae, increasing surface area.
   • Function: Often called the “Powerhouse of the Cell.” It is the primary site of cellular respiration, where glucose is broken down to produce Adenosine Triphosphate (ATP), the energy currency of the cell.

2. Endoplasmic Reticulum (ER):
   • Structure: A network of interconnected membranes forming sacs and tubules extending throughout the cytoplasm. It can be of two types:
     • Rough Endoplasmic Reticulum (RER): Has ribosomes attached to its surface, giving it a rough appearance.
     • Smooth Endoplasmic Reticulum (SER): Lacks ribosomes, appearing smooth.
   • Function:
     • RER: Involved in the synthesis, folding, modification, and transport of proteins (especially those destined for secretion or insertion into membranes).
     • SER: Involved in lipid and steroid synthesis, detoxification of drugs and poisons, and storage of calcium ions.

3. Ribosomes:
   • Structure: Small, granular structures composed of ribosomal RNA (rRNA) and proteins. They are not membrane-bound.
   • Function: Site of protein synthesis (translation). Found either free in the cytoplasm or attached to the RER.

4. Golgi Apparatus (Golgi Complex/Body):
   • Structure: Consists of flattened, membrane-bound sacs called cisternae, stacked together.
   • Function: Modifies, sorts, and packages proteins and lipids synthesized in the ER for secretion or delivery to other organelles. Also involved in the formation of lysosomes.

5. Lysosomes:
   • Structure: Small, single-membrane bound vesicles containing powerful digestive enzymes.
   • Function: “Suicidal bags of the cell” or “waste disposal system.” They break down waste materials, cellular debris, foreign particles, and worn-out cell organelles. They also play a role in programmed cell death (apoptosis).

6. Vacuoles:
   • Structure: Membrane-bound sacs primarily used for storage. Their size and number vary significantly between cell types.
   • Function: Store water, nutrients, waste products, and provide turgor pressure in plant cells.
   • In Plant Cells: Typically a single, large central vacuole that occupies up to 90% of the cell volume.
   • In Animal Cells: Usually small and numerous, or sometimes absent.

7. Plastids (Found primarily in Plant Cells and some Protists):
   • Structure: Double-membraned organelles.
   • Types & Function:
     • Chloroplasts: Contain the green pigment chlorophyll. They are the primary site of photosynthesis, where light energy is converted into chemical energy (sugars).
     • Chromoplasts: Contain pigments (e.g., carotenoids) that give color to fruits and flowers.
     • Leucoplasts: Colorless plastids involved in the storage of starch, oils, or proteins.

8. Cell Wall (Found in Plant Cells, Fungi, Bacteria, Algae – NOT in Animal Cells):
   • Structure: A rigid, protective outer layer present outside the cell membrane. In plants, it is primarily made of cellulose. In fungi, it’s chitin; in bacteria, peptidoglycan.
   • Function: Provides structural support and shape to the cell, protects the cell from mechanical stress and osmotic lysis (prevents bursting from water intake).

9. Centrioles (Found in Animal Cells and some lower plants – NOT in higher Plant Cells):
   • Structure: Small, cylindrical structures usually found in pairs, located near the nucleus in an area called the centrosome.
   • Function: Involved in cell division by forming spindle fibers that help separate chromosomes during mitosis and meiosis. Also form the basal bodies of cilia and flagella.

5. Differences Between Plant and Animal Cells

6. Cell Division (Brief Overview)

Cells reproduce by dividing into two or more daughter cells. This process is crucial for growth, repair, and reproduction of organisms.

• Mitosis:
  • Purpose: Cell division for growth, repair, and asexual reproduction.
  • Outcome: A single parent cell divides into two genetically identical diploid (2n) daughter cells.
  • Location: Occurs in somatic (body) cells.

• Meiosis:
  • Purpose: Cell division for sexual reproduction (to form gametes: sperm and egg cells).
  • Outcome: A single parent cell divides into four genetically distinct haploid (n) daughter cells.
  • Location: Occurs in germline cells (reproductive organs).

Practice Questions

1. Who first coined the term “cell” after observing cork slices?
   • A) Anton van Leeuwenhoek
   • B) Matthias Schleiden
   • C) Theodor Schwann
   • D) Robert Hooke
   • Rationale: Robert Hooke observed cell structures in cork and named them ‘cells’.
2. Which of the following statements is a postulate of the Cell Theory?
   • A) Cells can spontaneously generate.
   • B) Only animals are made of cells.
   • C) All cells arise from pre-existing cells.
   • D) Cells are the largest units of life.
   • Rationale: The principle ‘Omnis cellula e cellula’ states that cells originate from pre-existing cells.
3. A key distinguishing feature of prokaryotic cells compared to eukaryotic cells is the absence of a:
   • A) Cell membrane
   • B) True nucleus
   • C) Cytoplasm
   • D) Ribosomes
   • Rationale: Prokaryotic cells lack a membrane-bound nucleus, a defining characteristic that differentiates them from eukaryotic cells.

4. The cell membrane is selectively permeable, meaning it:
   • A) Allows all substances to pass freely.
   • B) Blocks all substances from entering.
   • C) Regulates the passage of specific substances.
   • D) Only allows water to pass through.
   • Rationale: Selective permeability is the ability to control which substances can enter or leave the cell.
5. Which part of the eukaryotic cell contains the cell’s genetic material and controls cellular activities?
   • A) Cytoplasm
   • B) Mitochondria
   • C) Nucleus
   • D) Cell wall
   • Rationale: The nucleus houses the DNA and acts as the control center of the eukaryotic cell.
6. The jelly-like substance filling the cell and surrounding the organelles is known as the:
   • A) Nucleoplasm
   • B) Vacuole
   • C) Cytoplasm
   • D) Cell sap
   • Rationale: Cytoplasm refers to all the contents within the cell membrane, excluding the nucleus.

7. Which cell organelle is known as the “Powerhouse of the Cell” responsible for ATP production?
   • A) Ribosome
   • B) Mitochondria
   • C) Lysosome
   • D) Golgi apparatus
   • Rationale: Mitochondria are the sites of cellular respiration, generating ATP.
8. The primary function of ribosomes is:
   • A) Lipid synthesis
   • B) Energy production
   • C) Protein synthesis
   • D) Detoxification
   • Rationale: Ribosomes are the cellular machinery for translating genetic information into proteins.
9. Which organelle contains digestive enzymes and is often called the “suicidal bag” of the cell?
   • A) Vacuole
   • B) Endoplasmic Reticulum
   • C) Lysosome
   • D) Chloroplast
   • Rationale: Lysosomes contain hydrolytic enzymes for breaking down waste and cellular debris.
10. The site of photosynthesis in plant cells is the:
    • A) Mitochondria
    • B) Nucleus
    • C) Chloroplast
    • D) Vacuole

Rationale: Chloroplasts contain chlorophyll and are where photosynthesis occurs

11. Which of the following cell organelles is present in plant cells but absent in animal cells?
    • A) Mitochondria
    • B) Cell membrane
    • C) Nucleus
    • D) Cell wall
    • Rationale: The cell wall is a distinguishing feature of plant cells, providing rigid support.
12. The process of cell division that results in two genetically identical daughter cells is known as:
    • A) Meiosis
    • B) Binary fission
    • C) Mitosis
    • D) Fertilization
    • Rationale: Mitosis produces two diploid daughter cells that are exact copies of the parent cell, essential for growth and repair.