Cells: The Living Units

Cell Basics

The cell is the fundamental unit of life, responsible for carrying out essential biological functions. The cell theory states that all living things are composed of cells, cells are the smallest unit of life, and new cells arise from pre-existing cells through processes such as mitosis and meiosis.

• Definition: The smallest unit of life capable of independent existence.

• Functions: Contains DNA, synthesizes proteins, produces ATP for energy.

• Cell Theory:

  • All living organisms are made of cells.

  • Cells are the basic unit of structure and function.

  • Cells arise from pre-existing cells.

• Three Major Regions:

  • Plasma Membrane: Acts as a barrier and regulates selective transport.

  • Cytoplasm: Contains fluid (cytosol) and organelles.

  • Nucleus: Control center, stores genetic material (DNA).

Extracellular Materials

Materials outside the cell play important roles in communication, support, and transport of substances.

• Extracellular Fluid: Includes interstitial fluid, plasma, and cerebrospinal fluid (CSF); provides nutrients and removes wastes.

• Secretions: Such as saliva, gastric fluids, and lubricants, facilitate various physiological processes.

• Extracellular Matrix: A network of proteins and carbohydrates that acts as a "glue" to hold cells together.

Cell Structure and Function

Plasma Membrane

The plasma membrane is a dynamic structure that separates the cell from its environment and regulates the movement of substances.

• Structure: Composed of a lipid bilayer and proteins, described by the "fluid mosaic model."

• Phospholipids: Have hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• Cholesterol: Stabilizes the membrane and maintains fluidity.

• Proteins: Serve as transporters, receptors, enzymes, structural components, and recognition molecules.

• Carbohydrates: Form the glycocalyx, a "sugar coat" that acts as identification tags for cell recognition.

• Cell Junctions:

  • Tight Junctions: Create waterproof seals between cells.

  • Desmosomes: Provide anchoring, similar to Velcro.

  • Gap Junctions: Allow communication between cells via channels.

Transport Across the Plasma Membrane

Cells regulate the movement of substances through passive and active transport mechanisms.

• Passive Transport (No ATP Required):

  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2, fats) down their concentration gradient.

  • Facilitated Diffusion: Movement of larger or charged molecules (e.g., glucose, ions) via protein channels or carriers.

  • Osmosis: Diffusion of water through aquaporins.

  • Tonicity:

    • Isotonic: Equal solute concentration; cell size remains unchanged.

    • Hypertonic: Higher solute concentration outside; cell shrinks.

    • Hypotonic: Lower solute concentration outside; cell swells.

• Active Transport (Requires ATP):

  • Primary Active Transport: Direct use of ATP to move substances (e.g., Na+/K+ pump).

  • Secondary Active Transport: Uses energy from primary transport to move other substances (co-transport, symport, antiport).

  • Vesicular Transport:

    • Endocytosis: Uptake of substances into the cell.

      • Phagocytosis: "Cell eating" of large particles.

      • Pinocytosis: "Cell drinking" of fluids.

      • Receptor-mediated Endocytosis: Specific uptake via receptors.

    • Exocytosis: Release of substances from the cell (secretion, waste removal).

Example: The Na+/K+ pump maintains cellular ion gradients essential for nerve impulse transmission.

Key Formula:

Where \Delta C is the concentration gradient driving diffusion.

Cytoplasm and Organelles

Cytoplasm

The cytoplasm is the region between the plasma membrane and nucleus, containing cytosol, inclusions, and organelles.

• Cytosol: Fluid containing dissolved solutes.

• Inclusions: Stored substances such as glycogen and melanin.

• Organelles:

  • Mitochondria: Site of ATP production; contains its own DNA.

  • Ribosomes: Protein synthesis; free in cytosol or bound to endoplasmic reticulum.

  • Rough Endoplasmic Reticulum (ER): Protein production and processing.

  • Smooth ER: Lipid synthesis, detoxification, calcium storage.

  • Golgi Apparatus: Modifies and packages proteins and lipids for transport.

  • Lysosomes: Digestion and waste removal.

  • Peroxisomes: Detoxification and neutralization of free radicals.

  • Cytoskeleton: Provides structural support and facilitates movement; composed of microfilaments, intermediate filaments, and microtubules.

  • Centrosome/Centrioles: Organize spindle fibers during cell division.

Cell Extensions

Cells may possess extensions that aid in movement or increase surface area.

• Cilia: Move substances across cell surfaces (e.g., respiratory tract).

• Flagella: Propel cells (e.g., sperm).

• Microvilli: Increase surface area for absorption (e.g., intestine, kidney).

Comparison Table:

Nucleus

Nuclear Structure and Function

The nucleus is the control center of the cell, housing genetic material and coordinating cellular activities.

• Nuclear Envelope: Double membrane with pores for transport of molecules.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

Example: The nucleolus assembles ribosomal subunits, which are essential for protein synthesis.

Cytoskeletal Elements

Structure and Function

The cytoskeleton provides structural support, maintains cell shape, and enables movement.

• Microfilaments: Composed of actin; involved in cell movement and shape changes.

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Hollow tubes; facilitate intracellular transport and cell division.

Additional info: Microtubules form the core of cilia and flagella, enabling their movement.