Cell Structures and Their Functions: Study Notes for Anatomy & Physiology

Chapter 3: Cell Structures and Their Functions

Introduction

This chapter explores the fundamental structures of human cells and their diverse functions. Understanding cell anatomy and physiology is essential for grasping how tissues, organs, and systems operate in the human body.

Functions of the Human Cell

Overview of Cellular Functions

• Metabolism and Energy Use: Cells perform metabolic reactions to generate energy, which is used for activities such as muscle contraction and heat production.

• Synthesis of Molecules: Cells synthesize essential molecules including proteins, nucleic acids, and lipids for growth, repair, and regulation.

• Communication: Cells communicate with each other, for example, nervous tissue interacts with muscle tissue to coordinate movement.

• Reproduction: Cells replicate their genetic information to produce new cells, ensuring growth and tissue repair.

Structure of the Human Cell

Plasma Membrane

The plasma membrane is the outer boundary of the cell, separating extracellular substances from intracellular components. It acts as a selective barrier for movement into and out of the cell and plays a role in cell communication.

• Selective Permeability: The membrane allows certain substances to pass while restricting others.

• Communication: Membrane proteins facilitate signaling between cells.

Additional info: The plasma membrane is composed primarily of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

Movement Through the Cell Membrane

Transport Mechanisms

Cells must exchange materials with their environment to maintain homeostasis. This is achieved through two main transport systems:

• Passive Membrane Transport: Does not require energy; substances move down their concentration gradient.

• Active Membrane Transport: Requires energy (ATP); substances move against their concentration gradient.

Passive Transport

• Diffusion: Movement of solutes from an area of higher concentration to lower concentration.

• Facilitated Diffusion: Movement of molecules across the membrane via protein channels or carriers; important for non-lipid soluble molecules.

• Osmosis: Diffusion of water across a selectively permeable membrane from a region of higher water concentration to lower concentration.

Example: If compartment A contains 100% distilled water and compartment B contains 97% water and 3% solute, water will move from A to B.

Tonicity

Tonicity describes the effect of a solution on cell volume:

• Isotonic Solution: No net movement of water; cell volume remains unchanged.

• Hypotonic Solution: Water enters the cell; cell swells.

• Hypertonic Solution: Water leaves the cell; cell shrinks.

Type of Solution	Solute %	Water %	Effect on Cell
Isotonic	10%	90%	No net movement
Hypotonic	5%	95%	Cell swells
Hypertonic	15%	85%	Cell shrinks

Active Transport

• Active Transport: Uses membrane proteins and ATP to move substances against their concentration gradient.

• Sodium-Potassium Pump: Transports Na+ out of cells and K+ into cells, maintaining electrochemical gradients.

• Endocytosis: Cell engulfs substances into vesicles.

• Exocytosis: Vesicles fuse with the membrane to release contents outside the cell.

Equation for Active Transport:

$\text{Rate} = k[\text{Carrier Protein}][\text{ATP}]$

Cell Organelles and Their Functions

Major Organelles

• Nucleus: Contains genetic material (DNA); surrounded by the nuclear envelope; controls cell activities.

• Ribosomes: Sites of protein synthesis; can be free or attached to the endoplasmic reticulum.

• Endoplasmic Reticulum (ER): Network for protein and lipid synthesis and transport.

  • Rough ER: Studded with ribosomes; synthesizes proteins.

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

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

• Mitochondria: "Powerhouse" of the cell; site of cellular respiration and ATP production.

• Lysosomes: Contain digestive enzymes; break down waste and foreign materials.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

Cytoskeleton

• Microfilaments, Intermediate Filaments, Microtubules: Provide structural support, shape, and facilitate movement within the cell.

Cell Membrane Extensions

• Cilia: Move mucus and particles out of the respiratory tract.

• Microvilli: Increase surface area for absorption in the intestine and kidneys.

• Flagella: Propel sperm cells; only one per cell in humans.

Genetic Material and Protein Synthesis

DNA and Chromosomes

• Chromosomes: Human cells have 23 pairs; composed of DNA and proteins.

• DNA: Contains genetic code for protein synthesis; double helix structure.

• Nucleic Acids: DNA and RNA; RNA is synthesized from DNA and is involved in protein production.

Protein Synthesis

• Transcription: DNA is used to make messenger RNA (mRNA) in the nucleus.

• Translation: mRNA travels to ribosomes, where its codons are translated into a chain of amino acids, forming a protein.

Equation for Protein Synthesis:

$\text{DNA} \xrightarrow{\text{Transcription}} \text{mRNA} \xrightarrow{\text{Translation}} \text{Protein}$

Cell Cycle

Phases of the Cell Cycle

• Interphase: Cell carries out normal metabolic functions and replicates DNA in preparation for division.

• Mitosis: Division of the nucleus to produce two identical daughter cells.

• Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

Additional info: The cell cycle is tightly regulated to ensure proper growth, development, and tissue repair.