Cellular Function

Modern Cell Theory

The modern cell theory is foundational to understanding anatomy and physiology. It explains the role of cells as the basic unit of life and the building blocks of all living organisms.

• The cell is the smallest unit of life. All physiological processes of an organism ultimately depend on the activities of its cells.

• All living organisms are composed of one or more cells. This includes both unicellular and multicellular organisms.

• An organism's structure and function are due to the collective activities of its cells. The specialized functions of tissues and organs arise from the properties and interactions of their constituent cells.

Cell Size and Surface Area

Why Are Cells Small?

Most cells are between 10–30 μm in diameter. The small size of cells is crucial for efficient exchange of materials and cellular function.

• Surface Area to Volume Ratio: As a cell increases in diameter, its volume increases faster than its surface area. This limits the size a cell can attain while still efficiently exchanging materials with its environment.

• Mathematical Relationship: If the diameter (D) of a cell doubles, the surface area increases by a factor of 4 (), but the volume increases by a factor of 8 ().

• Advantages of Many Small Cells: Having trillions of small cells allows for specialization, redundancy, and efficient transport of substances.

Types of Cells in the Human Body

Germ Cells vs. Somatic Cells

• Germ Cells: Reproductive cells, including sperm and ova (egg cells).

• Somatic Cells: All other cells in the body that are not involved in reproduction.

Key Cellular Terms

• Intracellular Fluid (ICF) or Cytosol: The fluid within the cell.

• Extracellular Fluid (ECF) or Interstitial Fluid: The fluid outside the cell, surrounding tissue cells.

• Cytoplasm: The cellular contents between the nucleus and the plasma membrane, including:

  • Cytosol (fluid component)

  • Organelles (specialized structures)

  • Inclusions (stored substances)

Extracellular Materials

Substances contributing to body mass but located outside of cells.

• Extracellular Fluid (ECF): Includes interstitial fluid, plasma, and other body fluids.

• Cellular Secretions: Substances produced and released by cells (e.g., mucus, enzymes).

• Extracellular Matrix: A network of proteins and polysaccharides outside cells, providing structural and biochemical support.

Basic Cell Structure

• Nucleus: Controls cellular activities and contains genetic material (DNA).

• Cytoplasm: Highly compartmentalized, containing the endomembrane system and organelles ("little organs").

• Plasma Membrane: The outer boundary of the cell, separating the internal environment from the external environment.

Note: The concept of an "average" cell is a simplification; real cells show significant variation and specialization.

Plasma Membrane

Functions

• Physical Boundary: Separates intracellular fluid (ICF) from extracellular fluid (ECF) and contains organelles.

• Transport: Regulates movement of substances into and out of the cell via selective permeability.

• Cell Identification: Uses the glycocalyx for cell recognition.

• Cell Signaling: Membrane proteins act as receptors for communication and signal transduction.

Structure

• Plasma Membrane Lipids: Phospholipids and cholesterol.

• Plasma Membrane Proteins: Integral and peripheral proteins with various functions.

Plasma Membrane Lipids

• Phospholipid Bilayer: Composed of amphiphilic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Fluid Mosaic Model: Describes the flexible, dynamic nature of the membrane, where phospholipids and proteins can move laterally.

• Unsaturated Fatty Acids: Kinked tails provide spacing and increase membrane fluidity.

• Cholesterol: Stabilizes and stiffens the membrane, modulating its fluidity and permeability.

Phospholipid Structure

• Amphiphilic Nature: Each phospholipid has a polar (hydrophilic) head and two nonpolar (hydrophobic) tails.

• Arrangement: In the bilayer, hydrophilic heads face outward toward water, and hydrophobic tails face inward, away from water.

Example: The plasma membrane of all animal cells is primarily composed of a phospholipid bilayer with embedded proteins and cholesterol, allowing selective transport and communication with the environment.

Additional info: Later sections would cover membrane proteins, transport mechanisms, and further details on organelles and cell function.