Chapter 3: The Cell – Structure and Function in Human Anatomy & Physiology

Basic Processes of Cells

Introduction to Cellular Functions

Cells are the fundamental units of life, performing essential processes to maintain homeostasis and support the organism. These processes include metabolism, substance transport, communication, and reproduction.

• Cell Metabolism: The sum of all chemical reactions within a cell that are necessary for life. This includes:

  • Anabolic Reactions: Synthesize complex molecules from simpler ones (e.g., protein synthesis).

  • Catabolic Reactions: Break down complex molecules into simpler ones (e.g., cellular respiration).

  • Oxidation-Reduction (Redox) Reactions: Involve the transfer of electrons, crucial for energy production.

• Substance Transport: Movement of compounds into, out of, or within the cell. This includes nutrient uptake, waste removal, and intracellular trafficking.

• Communication: Cells interact with their environment and other cells via chemical signals and membrane receptors.

• Cell Reproduction: Most cells divide to produce new cells, essential for growth, repair, and maintenance.

Overview of Cell Structure

Major Components of Animal Cells

Animal cells share three primary structural components, each with specialized functions:

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

• Cytoplasm: The region between the plasma membrane and the nucleus, containing:

  • Cytosol: Also called intracellular fluid (ICF); a watery gel with proteins, dissolved solutes, and various forms of RNA. It is the site of many metabolic reactions and may contain storage bodies called inclusions.

  • Organelles: Specialized structures ("molecular machines") that perform and compartmentalize specific cellular functions (e.g., mitochondria, endoplasmic reticulum).

  • Cytoskeleton: A network of protein filaments that supports the cell, maintains its shape, holds organelles in place, and provides tracks for intracellular transport.

• Nucleus: The control center of the cell, surrounded by a nuclear envelope. It contains most of the cell's DNA and is the site for RNA synthesis. DNA and RNA direct cellular activities by coding for and regulating protein synthesis.

Functions of the Plasma Membrane

The plasma membrane is a dynamic structure with several critical roles:

• Physical Isolation: Acts as a barrier, separating the cell from its surroundings.

• Structural Support: Maintains cell shape and integrity.

• Communication: Contains proteins that allow the cell to communicate with other cells and the environment.

• Regulation of Transport: Controls the entry and exit of substances, maintaining internal conditions.

• Cell Identification: Surface molecules help identify the cell to other cells and the immune system.

The plasma membrane also defines two main fluid compartments:

• Intracellular Space: The area within the cell, containing intracellular fluid (cytosol).

• Extracellular Space: The area outside the cell, containing extracellular fluid (ECF).

Cell Size and Diversity

Cells vary greatly in size, shape, and function. This diversity allows for specialization, enabling different cell types to perform unique roles within the body (e.g., muscle cells, nerve cells, blood cells).

• Example: Red blood cells are small and biconcave to maximize surface area for gas exchange, while neurons have long extensions for transmitting signals.