Cell Structure and Function: The Living Units in Human Anatomy & Physiology

Cells: The Smallest Living Units

Cell Theory

The cell is the fundamental structural and functional unit of life. The overall function of an organism depends on the individual and combined activities of its cells. The relationship between structure and function is complementary, meaning the shape and internal organization of a cell determine its biochemical activities.

• Cell: The smallest unit capable of independent life.

• Structure and Function: The shape and subcellular structures of a cell dictate its specific functions.

• Cell Origin: Cells arise only from preexisting cells (principle of biogenesis).

Cell Diversity

Human bodies contain over 250 different types of cells, each specialized for particular functions. These differences are reflected in their size, shape, and internal components.

• Examples of Cell Types:

  • Erythrocytes: Red blood cells, transport oxygen.

  • Fibroblasts: Connective tissue cells, produce fibers.

  • Muscle Cells: Skeletal and smooth muscle cells, enable movement.

  • Fat Cells: Store nutrients.

  • Macrophages: Fight disease.

  • Nerve Cells: Gather information and control body functions.

  • Sperm Cells: Reproduction.

• Functional Diversity: Cell specialization allows for complex tissue and organ functions.

Generalized Cell Structure

Basic Parts of Human Cells

Despite their diversity, all human cells share three basic structural components:

1. Plasma Membrane: Flexible outer boundary that separates the cell from its environment.

2. Cytoplasm: Intracellular fluid containing organelles.

3. Nucleus: DNA-containing control center that regulates cell activities.

Example: The generalized cell diagram includes structures such as the nucleus, nucleolus, nuclear envelope, plasma membrane, cytoplasm, mitochondria, lysosome, centrosome, Golgi apparatus, rough endoplasmic reticulum, ribosomes, cytoskeletal elements, microtubules, intermediate filaments, peroxisome, and secretory vesicles.

Extracellular Materials

Substances Found Outside Cells

Extracellular materials are substances located outside the cell and play important roles in tissue function and communication.

• Extracellular Fluids:

  • Interstitial Fluid: Bathes and surrounds cells.

  • Blood Plasma: Fluid component of blood.

  • Cerebrospinal Fluid: Surrounds nervous system organs.

• Cellular Secretions: Includes substances like saliva and mucus.

• Extracellular Matrix: Network of proteins and polysaccharides that acts as a glue to hold cells together.

Plasma Membrane

Function and Structure

The plasma membrane acts as an active barrier, separating intracellular fluid (ICF) from extracellular fluid (ECF). It controls what enters and leaves the cell, maintaining homeostasis.

• Also Known As: Cell membrane.

• Dynamic Role: Regulates transport, communication, and cell recognition.

Structure of Plasma Membrane

The plasma membrane is primarily composed of lipids and proteins, forming a flexible bilayer. Its structure is often described as a fluid mosaic due to the dynamic arrangement of its components.

• Membrane Lipids:

  • Lipid Bilayer: Two layers of phospholipids with hydrophilic (water-loving) heads facing outward and hydrophobic (water-hating) tails facing inward.

  • Phospholipids: Make up about 75% of the membrane; provide barrier and fluidity.

  • Glycolipids: Lipids with attached sugar groups, present on the outer surface.

  • Cholesterol: About 20% of membrane lipids; stabilizes membrane structure.

• Membrane Proteins:

  • Integral Proteins: Embedded within the membrane; often span the bilayer (transmembrane); function as channels, carriers, enzymes, or receptors.

  • Peripheral Proteins: Loosely attached to the membrane surface; function as enzymes, motor proteins, or in cell-to-cell connections.

• Glycocalyx: Surface sugars attached to lipids (glycolipids) or proteins (glycoproteins); serve as biological markers for cell recognition.

• Cell Junctions: Specialized structures that help hold cells together and facilitate communication.

Example: The fluid mosaic model illustrates the dynamic and complex nature of the plasma membrane, with proteins and lipids moving within the bilayer.

*Additional info: The plasma membrane's selective permeability is essential for maintaining the internal environment of the cell and for communication with other cells.*