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

The Cell

Introduction to Cells

Cells are the fundamental units of life in all plants and animals. They are the smallest structures capable of performing all vital physiological functions, and each cell maintains homeostasis at the cellular level.

• Definition: A cell is the basic structural and functional unit of living organisms.

• Homeostasis: Each cell regulates its internal environment to maintain stable conditions necessary for survival.

• Example: Muscle cells contract to produce movement, while nerve cells transmit electrical signals.

Cellular Components

Extracellular Fluid and Cytoplasm

Cells are surrounded by a watery medium called extracellular fluid (also known as interstitial fluid). The plasma membrane separates the cell's internal environment (cytoplasm) from the extracellular fluid.

• Cytoplasm: The material inside the cell, excluding the nucleus.

• Cytosol: The liquid portion of the cytoplasm, also called intracellular fluid.

• Organelles: Specialized structures within the cytoplasm that perform specific functions.

Plasma Membrane

Functions of the Plasma Membrane

The plasma membrane is a dynamic structure that serves several essential functions for the cell:

• Physical Isolation: Acts as a barrier, separating intracellular fluid from extracellular fluid.

• Regulation of Exchange: Controls the entry of ions and nutrients and the exit of wastes and cellular products.

• Environmental Monitoring: Senses changes in extracellular and intracellular fluid composition, such as sodium (Na+) and potassium (K+) levels.

• Structural Support: Anchors cells and tissues, providing mechanical stability.

Plasma Membrane Lipids

The plasma membrane is primarily composed of a double layer of phospholipids, which creates a selective barrier for the cell.

• Phospholipid Bilayer: Consists of hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) fatty-acid tails facing inward.

• Barrier Function: Prevents free passage of ions and water-soluble compounds, maintaining distinct internal and external environments.

• Cholesterol: Embedded within the membrane, cholesterol helps maintain membrane integrity and fluidity, and plays a role in cell signaling.

Semipermeable Nature of the Cell Membrane

The cell membrane is semipermeable, meaning it selectively allows certain substances to pass while restricting others.

• Selective Permeability: The membrane determines what enters and exits the cell, based on size, charge, and solubility.

• Phospholipid Bilayer: The arrangement of hydrophilic heads and hydrophobic tails creates a barrier to most polar molecules and ions.

Diagram Description

The provided diagram illustrates the structure of a typical animal cell, labeling key organelles such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosome, peroxisome, and plasma membrane. It also shows the phospholipid bilayer with hydrophilic heads and hydrophobic tails, emphasizing the semipermeable nature of the membrane.

Key Terms and Definitions

• Cell: The smallest unit of life capable of independent function.

• Extracellular Fluid: Fluid outside the cell, providing nutrients and removing wastes.

• Plasma Membrane: The cell's outer boundary, controlling interactions with the environment.

• Cytoplasm: The internal environment of the cell, containing cytosol and organelles.

• Phospholipid Bilayer: The fundamental structure of the plasma membrane.

• Semipermeable: Allows selective passage of substances.

• Cholesterol: A lipid that stabilizes membrane structure.

Summary Table: Plasma Membrane Components

Component	Structure	Function
Phospholipid Bilayer	Double layer of phospholipids with hydrophilic heads and hydrophobic tails	Forms the basic barrier; controls permeability
Cholesterol	Interspersed within the bilayer	Stabilizes membrane; affects fluidity
Proteins	Integral and peripheral proteins	Transport, signaling, structural support

Additional info:

• Integral proteins span the membrane and function as channels or transporters for polar substances.

• Peripheral proteins are attached to the inner or outer surface and act as enzymes, receptors, or markers.

• The plasma membrane's selective permeability is essential for maintaining cellular homeostasis and communication.