Cells

Introduction to Cells

• Definition: The cell is the basic structural and functional unit of life.
• Cell Theory:
  1. All living organisms are composed of cells.
  2. The cell is the basic unit of structure and function in living organisms.
  3. All cells arise from pre-existing cells (Rudolf Virchow, 1855).
• Discoveries:
  • Robert Hooke (1665): First observed cells in cork tissue.
  • Anton van Leeuwenhoek (1674): Observed living cells under a microscope.

Types of Cells

1. Prokaryotic Cells:

   • Characteristics:
     • Lack a nucleus; genetic material (DNA) is present in the nucleoid region.
     • No membrane-bound organelles.
     • Generally smaller (1–10 µm).
   • Examples: Bacteria, Archaea.

2. Eukaryotic Cells:

   • Characteristics:
     • Have a true nucleus surrounded by a nuclear membrane.
     • Contain membrane-bound organelles (e.g., mitochondria, Golgi apparatus).
     • Larger in size (10–100 µm).
   • Examples: Animal cells, Plant cells, Fungi.

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Organelles	Absent (except ribosomes)	Present
Size	Smaller (1–10 µm)	Larger (10–100 µm)
Cell Division	Binary fission	Mitosis or meiosis
Examples	Bacteria, Archaea	Animals, Plants, Fungi

Structure of a Cell

1. Plasma Membrane:

   • Thin, flexible outer covering of the cell.
   • Composed of a lipid bilayer with embedded proteins.
   • Functions:
     • Regulates the entry and exit of substances (selectively permeable).
     • Provides shape and protection.

2. Cytoplasm:

   • Jelly-like substance inside the cell.
   • Contains all organelles and is the site of metabolic activities.

3. Nucleus:

   • Control center of the cell.
   • Contains genetic material (DNA) arranged in chromosomes.
   • Functions:
     • Regulates cell activities.
     • Facilitates cell division.

4. Cell Wall (in plants, fungi, and some bacteria):

   • Rigid, protective layer outside the plasma membrane.
   • Composed of cellulose (plants) or chitin (fungi).
   • Provides structure and support.

Specialized Structures in Cells

1. Ribosomes:

   • Small, spherical structures involved in protein synthesis.
   • Present in both prokaryotic and eukaryotic cells.

2. Flagella and Cilia:

   • Flagella: Long, whip-like structures used for movement (e.g., in sperm cells).
   • Cilia: Short, hair-like structures used for movement or creating currents (e.g., in the respiratory tract).

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Organelles in Eukaryotic Cells

1. Mitochondria:

   • Known as the "powerhouse of the cell."
   • Structure:
     • Double membrane: Outer membrane is smooth; inner membrane has folds (cristae).
     • Contains its own DNA and ribosomes.
   • Function: Site of cellular respiration; generates ATP (energy currency of the cell).

2. Endoplasmic Reticulum (ER):

   • A network of membranous tubules and sacs.
   • Types:
     • Rough ER (RER): Studded with ribosomes; involved in protein synthesis.
     • Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis, detoxification, and storage of calcium ions.
   • Function: Transportation of substances within the cell.

3. Golgi Apparatus:

   • Composed of flattened membrane-bound sacs (cisternae).
   • Function: Modifies, sorts, and packages proteins and lipids for secretion or internal use.

4. Lysosomes:

   • Small vesicles containing digestive enzymes.
   • Function:
     • Breakdown of cellular waste and debris (intracellular digestion).
     • Autophagy: Removal of damaged organelles.
     • Autolysis: Self-destruction of cells.

5. Peroxisomes:

   • Contain enzymes like catalase and oxidase.
   • Function: Breakdown of fatty acids and detoxification of harmful substances.

6. Chloroplasts (in plant cells):

   • Site of photosynthesis.
   • Structure:
     • Double membrane.
     • Contains chlorophyll (green pigment) and its own DNA.
   • Function: Converts sunlight into chemical energy (glucose).

7. Vacuoles:

   • Membrane-bound sacs for storage.
   • Large central vacuole in plant cells provides structural support and stores water, nutrients, and waste.

Organelle	Structure	Function
Mitochondria	Double membrane, cristae	ATP production
ER (Rough & Smooth)	Membranous network	Protein and lipid synthesis
Golgi Apparatus	Flattened sacs (cisternae)	Packaging and secretion
Lysosomes	Vesicles with enzymes	Intracellular digestion
Chloroplasts	Double membrane, chlorophyll	Photosynthesis

Other Important Cellular Components

1. Cytoskeleton:

   • A network of protein filaments (microfilaments, microtubules, intermediate filaments).
   • Function: Provides shape, support, and facilitates movement of organelles.

2. Centrioles:

   • Found in animal cells.
   • Composed of microtubules arranged in a 9+2 pattern.
   • Function: Help in cell division (formation of spindle fibers).

3. Plasmodesmata (in plant cells):

   • Channels that connect neighboring plant cells.
   • Function: Facilitates transport and communication.

4. Extracellular Matrix (ECM):

   • Present in animal cells.
   • Composed of proteins and polysaccharides.
   • Function: Provides structural support and facilitates communication between cells.

Differences Between Plant and Animal Cells

Feature	Plant Cell	Animal Cell
Cell Wall	Present (made of cellulose)	Absent
Chloroplasts	Present	Absent
Vacuole	Large central vacuole	Small and numerous
Centrioles	Absent	Present
Shape	Fixed, rectangular	Flexible, round

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Cell Division

Cell division is the process by which cells reproduce to ensure growth, repair, and reproduction.

1. Types of Cell Division:

   • Mitosis:

     • Definition: A type of cell division where a single cell divides into two identical daughter cells.
     • Purpose: Growth and repair in multicellular organisms; asexual reproduction in unicellular organisms.
     • Stages:
       1. Prophase: Chromosomes condense, spindle fibers form, and the nuclear envelope breaks down.
       2. Metaphase: Chromosomes align at the cell's equatorial plane.
       3. Anaphase: Sister chromatids are pulled apart to opposite poles.
       4. Telophase: Nuclear membranes reform, and chromosomes decondense.
       5. Cytokinesis: Division of the cytoplasm to form two separate cells.

   • Meiosis:

     • Definition: A type of cell division that reduces the chromosome number by half, producing four haploid cells.
     • Purpose: Formation of gametes (sperm and egg cells) in sexual reproduction.
     • Stages: Two rounds of division – Meiosis I and Meiosis II.
       • Meiosis I: Homologous chromosomes separate.
       • Meiosis II: Sister chromatids separate.
     • Results in genetic variation due to crossing over and independent assortment.

Feature	Mitosis	Meiosis
Number of Divisions	One	Two
Number of Cells Formed	Two diploid cells	Four haploid cells
Genetic Variation	Absent	Present
Purpose	Growth and repair	Gamete formation

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Transport Across Cell Membranes

1. Passive Transport:

   • Does not require energy.
   • Movement occurs along the concentration gradient (high to low).
   • Types:
     • Diffusion: Movement of molecules (e.g., oxygen, carbon dioxide).
     • Osmosis: Movement of water across a semi-permeable membrane.
     • Facilitated Diffusion: Uses protein channels or carriers for transport.

2. Active Transport:

   • Requires energy (ATP).
   • Movement occurs against the concentration gradient (low to high).
   • Example: Sodium-potassium pump in nerve cells.

Transport Type	Energy Required	Direction	Example
Passive Transport	No	High to Low	Diffusion, Osmosis
Active Transport	Yes	Low to High	Sodium-Potassium Pump

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Specialized Cell Types

1. Red Blood Cells (RBCs):

   • Specialized for oxygen transport.
   • Lack nucleus to maximize space for hemoglobin.

2. Nerve Cells (Neurons):

   • Long and branched to transmit electrical signals.

3. Muscle Cells:

   • Contain actin and myosin filaments for contraction.

4. Guard Cells (in plants):

   • Control the opening and closing of stomata for gas exchange.

5. Sperm and Egg Cells:

   • Involved in sexual reproduction.

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Importance of Cells

1. Foundation of Life:
   • All organisms are composed of cells, which perform vital life processes.
2. Disease Understanding:
   • Study of cells aids in understanding diseases like cancer.
3. Applications in Biotechnology:
   • Genetic engineering and stem cell research are based on cellular processes.

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