Cells: The Living Units – Structure and Function

Cells: The Living Units

Cell Theory and Cell

The cell is the fundamental structural and functional unit of life. Understanding cell theory is essential for studying anatomy and physiology.

• Cell Theory: States that all living things are composed of cells, and all cells arise from pre-existing cells. This theory was proposed by Schwann and Schleiden.

• Organismal Activity: Depends on the individual and collective activity of cells.

• Biochemical Activities: Are dictated by subcellular structure.

• Cell Count: The human body contains about 75 trillion cells.

• Cell Size: Measured in micrometers (μm), where 1 μm = 1/1000 mm.

• Human Egg Cell: Diameter of about 140 μm, barely visible without a microscope.

• Red Blood Cell (RBC): Diameter of about 7.5 μm.

• Muscle Cells: Range between 20 to 500 μm long.

• Cell Diversity: Cells have different sizes and shapes, reflecting their specialized functions.

A Composite Cell

There is no single 'typical' cell, as cells vary greatly. However, most cells share three basic parts:

• Cell Membrane (Plasma Membrane): The outer boundary of the cell.

• Nucleus: The control center containing genetic material.

• Cytoplasm: The intracellular fluid containing organelles.

Structure of a Generalized Cell

Plasma Membrane

The plasma membrane, also called the plasmalemma, is a flexible, elastic barrier that separates the cell's internal environment from the extracellular fluid.

• Maintains cell integrity and controls the movement of substances in and out of the cell.

• It is selectively permeable (allows some substances to pass while blocking others).

• Can receive stimuli and respond via signal transduction.

Plasma Membrane Structure

• Composed mainly of lipids, proteins, and carbohydrates (Fluid Mosaic Model).

• Lipid Bilayer: Forms the basic structure; consists of phospholipids with hydrophobic tails and hydrophilic heads.

• Contains cholesterol molecules for rigidity and cell shape.

• Proteins in the membrane serve various functions:

  • Receptor Proteins: Sites where specific molecules (ligands) can bind, initiating cellular responses.

  • Integral Proteins: Channel proteins that help substances (e.g., ions, molecules) move through the membrane. Some control the movement of Na+ and K+ ions, important in nerve signaling.

  • Cell Marker Proteins: Expressed by cells for identification; also known as cell antigens.

Example: Oxygen (O2) and carbon dioxide (CO2) can diffuse through the lipid bilayer, while ions require channel proteins.

Cell Membrane Junctions

Cells connect to each other via specialized junctions, known as intercellular junctions. These junctions help maintain tissue integrity and facilitate communication.

• Tight Junctions: Membranes are fused, preventing passage of substances between cells (e.g., lining of the gastrointestinal tract, blood vessels).

• Desmosome Junctions: Membranes are occasionally fused, providing spot welds for mechanical strength (e.g., skin cell lining).

• Gap Junctions: Cells are connected by tubular channels, allowing cytoplasmic exchange (e.g., cardiac and smooth muscle cells).

Cytoplasm

The cytoplasm is the cellular material between the plasma membrane and the nucleus. It consists of cytosol (fluid), organelles, and inclusions.

• Contains cytosol (liquid), cellular organelles, and protein rods and tubules (cytoskeleton).

Major Organelles

1. Endoplasmic Reticulum (ER):

   • Complex organelle of membranous sacs, canals, and vesicles.

   • Transports molecules within the cell.

   • Associated with protein synthesis.

   • Rough ER: Studded with ribosomes; synthesizes proteins.

   • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

2. Ribosomes:

   • Can be free in cytoplasm or bound to ER.

   • Composed of protein and RNA.

   • Sites of protein synthesis.

3. Golgi Apparatus (Golgi Body):

   • Consists of flattened sacs called cisternae.

   • Packages and delivers proteins within the cell.

Additional info: Other important organelles (not detailed in the provided notes) include mitochondria (energy production), lysosomes (digestion), and peroxisomes (detoxification).