Physiology and Cell Structure: Foundations for Biology

Course Foundations and Logistics

Course Structure and Assessment

This section outlines the organization of the course, including instructors, assessment methods, and recommended study strategies.

• Instructors: Dr. Bethany Waddington (weeks 6–10), John (weeks 1–5).

• Assessments: Two exams (100 pts each), eight weekly quizzes (20 pts each), and ungraded Dynamic Study Modules.

• Textbook: Campbell Biology (latest edition) is required for quizzes and study modules.

• Active Learning: Strategies include note-taking, highlighting, drawing diagrams, and teaching concepts to others.

Introduction to Physiology

Definition and Scope

Physiology is the study of how living organisms function, focusing on the mechanisms that maintain internal stability (homeostasis) in response to environmental changes.

• Structure–Function Relationship: The form of biological structures is closely linked to their function.

• Organ Systems: Includes nervous, circulatory, skeletal, muscular, reproductive, and endocrine systems.

Historical Context

• Hippocrates (≈400 BC): Early ideas on internal stability.

• Claude Bernard (late 1800s): Introduced the concept of the "internal milieu," a precursor to homeostasis.

Homeostasis

Concept and Examples

Homeostasis refers to the maintenance of a relatively constant internal environment despite external fluctuations.

• Examples: Regulation of body temperature and blood oxygen concentration.

• Importance: Essential for survival; loss of homeostasis can quickly lead to physiological failure (e.g., lack of oxygen is fatal within minutes).

Cell Theory and Cellular Organization

Cell Theory Fundamentals

The Cell Theory is a foundational concept in biology, established in the 19th century.

• All living organisms are composed of one or more cells.

• The cell is the smallest unit that exhibits all characteristics of life.

• All cells arise from pre-existing cells.

Historical Figures: Robert Hooke (first observed cells, 1665), Matthias Schleiden & Theodor Schwann (formalized cell theory).

Why Cells Are Small: Surface Area-to-Volume Ratio (SA:V)

Cell size is limited by the surface area-to-volume ratio (SA:V), which affects the efficiency of nutrient and waste exchange.

• Surface Area (SA): Total area exposed to the environment.

• Volume (V): Space occupied by the cell.

• As a cell grows, SA increases with the square of the radius, while V increases with the cube of the radius.

• Thus, SA:V ratio decreases as cell size increases, limiting diffusion rates and driving multicellularity.

Formulas for a cube with side length a:

• Surface area: $SA = 6a^2$

• Volume: $V = a^3$

• SA:V ratio: $\frac{SA}{V} = \frac{6}{a}$

Example Calculations:

• For a = 1: $SA = 6$, $V = 1$, $SA:V = 6:1$

• For a = 5: $SA = 150$, $V = 125$, $SA:V = 1.2:1$

Biological Implication: High SA:V enables efficient exchange; low SA:V limits cell size.

Examples of Cell Size

• Amoeba: ~2–3 µm

• Human cheek epithelial cell: ~10–15 µm

• Blue whale: ~30 m (composed of billions of cells)

Scale of Cellular Organization

• The human body contains approximately 35 trillion cells.

• Cellular diversity allows for specialization (e.g., muscle, nerve, kidney cells) and the development of complex physiological systems.

Microscopy Techniques

Overview of Microscopy

Microscopy is essential for studying cells and their structures. Different techniques provide varying levels of detail.

Technique	Principle	Typical Magnification	What It Shows
Light Microscopy	Visible light passes through specimen; lenses focus image	Up to ~1,000×	Whole cells, tissue structure
Transmission Electron Microscopy (TEM)	Electron beam transmitted through thin specimen	10,000–1,000,000×	Internal ultrastructure (e.g., organelles)
Scanning Electron Microscopy (SEM)	Electron beam scans surface; reflected electrons detected	10,000–500,000×	3-D surface topology

Examples: Light micrographs show red blood cells; TEM reveals membrane layers; SEM displays 3-D surface features.

Cell Structure: Prokaryotes vs. Eukaryotes

Universal Organelles

All cells share certain structures:

• Plasma membrane (phospholipid bilayer)

• Ribosomes (protein synthesis)

• DNA (genetic material)

• Cytoplasm (gel-like interior)

Comparison Table: Prokaryotes vs. Eukaryotes

Feature	Prokaryotes (Bacteria & Archaea)	Eukaryotes
Nucleus	No true nucleus; DNA in nucleoid region	Membrane-bound nucleus
Organelles	No membrane-bound organelles	Numerous membrane-bound organelles (mitochondria, ER, Golgi, etc.)
Cell Wall	Usually present (peptidoglycan in bacteria, pseudo-peptidoglycan in archaea)	Present in plants/fungi (cellulose, chitin); absent in animal cells
Size	0.1–10 µm (most)	10–100 µm; some cells (e.g., neurons) >1 m
Genetic Layout	Single circular chromosome; often plasmids	Multiple linear chromosomes
Motility Structures	Flagella (simple) or fimbriae; not universal	Cilia, flagella (complex), cytoskeletal-based movement

Additional info: Mitochondria are absent in Bacteria and Archaea; they originated from an endosymbiotic event after the divergence of prokaryotes.

Plasma Membrane Structure

Phospholipid Bilayer and Components

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins and carbohydrates.

• Phosphate head (hydrophilic): Faces outward, interacts with water.

• Fatty-acid tails (hydrophobic): Face inward, form the membrane's core.

• Proteins: Integral (span the bilayer) and peripheral (attached to one side); involved in transport and signaling.

• Carbohydrates: Glycoproteins and glycolipids; important for cell recognition and signaling.

Function: Controls the movement of substances (O₂, CO₂, glucose, ions) into and out of the cell.

Endomembrane System

Components and Functions

The endomembrane system is a network of organelles that synthesize, modify, package, and transport macromolecules.

1. Nuclear envelope: Double membrane with pores; continuous with rough ER.

2. Rough Endoplasmic Reticulum (RER): Ribosome-studded; site of protein synthesis and modification.

3. Smooth Endoplasmic Reticulum (SER): Synthesizes lipids, sterols, and some carbohydrates; detoxifies chemicals.

4. Golgi apparatus: Modifies, sorts, and packages proteins and lipids into vesicles.

5. Transport vesicles: Shuttle cargo between organelles and to the plasma membrane.

6. Lysosomes: Contain hydrolytic enzymes for degrading macromolecules and recycling components.

Flow of Materials: Proteins are synthesized in the RER, transported to the Golgi for modification, and then sent to their destinations via vesicles.

Mitochondria and Cellular Energy

Structure and Function

Mitochondria are double-membrane organelles responsible for generating ATP through oxidative phosphorylation.

• Outer membrane: Smooth, allows passage of small molecules.

• Inner membrane: Highly folded into cristae, increasing surface area for the electron transport chain.

Key Reaction (Cellular Respiration):

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \longrightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{~38 ATP}$

• Fuel: Carbohydrates (glucose), fats, and (less commonly) proteins.

• By-product: CO₂ (exhaled).

• ATP: Provides energy for cellular processes such as protein activity, membrane transport, and muscle contraction.

• Cellular Variation: Cells with high energy demands (e.g., cardiac muscle) contain many mitochondria.

Summary Table: Key Organelles and Their Functions

Organelle	Main Function
Nucleus	Stores genetic material; controls cell activities
Rough ER	Protein synthesis and modification
Smooth ER	Lipid synthesis; detoxification
Golgi apparatus	Modification, sorting, and packaging of macromolecules
Lysosome	Digestion and recycling of cellular materials
Mitochondrion	ATP production via cellular respiration

Course Expectations and Study Strategies

• Active participation in lectures and discussions is expected.

• Regular review of textbook chapters and completion of quizzes reinforce learning.

• Utilize dynamic study modules for self-paced practice.

• Apply active learning strategies: note-taking, highlighting, drawing, and teaching concepts to others.