Cell: The Basic Unit of Life

A cell is the fundamental structural and functional unit of all living organisms. Cells can exist as single independent units (unicellular organisms) or as part of a larger multicellular structure.

1. Types of Cells

A. Prokaryotic Cells

• Found in bacteria and archaea
• Lack a true nucleus (DNA is in a nucleoid region)
• No membrane-bound organelles
• Smaller and simpler in structure
• Example: Escherichia coli (E. coli)

B. Eukaryotic Cells

• Found in plants, animals, fungi, and protists
• Have a true nucleus enclosed by a nuclear membrane
• Contain membrane-bound organelles (mitochondria, ER, Golgi apparatus)
• More complex and larger in size
• Example: Human skin cells, plant leaf cells

2. Cell Structure and Organelles

A. Plasma Membrane

• Composed of a phospholipid bilayer with proteins
• Regulates the movement of substances in and out of the cell
• Maintains homeostasis

B. Cytoplasm

• Jelly-like fluid that fills the cell
• Contains organelles, enzymes, and molecules necessary for cellular function

C. Nucleus (Eukaryotic cells only)

• Contains DNA and controls cellular activities
• Enclosed by the nuclear membrane
• Contains a nucleolus, where ribosomes are synthesized

D. Mitochondria

• Known as the "powerhouse of the cell"
• Site of cellular respiration (ATP production)

E. Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; involved in protein synthesis
• Smooth ER: No ribosomes; involved in lipid synthesis and detoxification

F. Ribosomes

• Sites of protein synthesis
• Found floating in the cytoplasm or attached to the rough ER

G. Golgi Apparatus

• Modifies, packages, and transports proteins and lipids

H. Lysosomes (Animal Cells)

• Contain digestive enzymes to break down waste and foreign substances

I. Vacuoles

• Large central vacuole in plant cells (stores water, nutrients)
• Small vacuoles in animal cells

J. Chloroplasts (Plant Cells Only)

• Contain chlorophyll for photosynthesis

K. Cytoskeleton

• Provides structure and shape to the cell
• Helps in cell movement and division

3. Cell Division

A. Mitosis (For growth and repair)

• Produces two identical daughter cells
• Occurs in somatic (body) cells

B. Meiosis (For reproduction)

• Produces four genetically different cells
• Occurs in gametes (sperm and egg cells)

4. Cell Transport Mechanisms

• Passive Transport (No energy required)
• Diffusion – Movement of molecules from high to low concentration
• Osmosis – Movement of water through a membrane
• Facilitated diffusion – Transport through protein channels
• Active Transport (Requires energy – ATP)
• Endocytosis – Engulfing substances into the cell
• Exocytosis – Releasing substances out of the cell
• Sodium-Potassium Pump – Moves ions against the concentration gradient

5. Differences Between Plant and Animal Cells

Cells are the building blocks of life. Understanding their structure, function, and processes helps us explore fields like genetics, microbiology, and medicine.