Cell Structure, Function, and Membrane Transport: Study Guide for Exam 2 (Chapters 4-5)

Cell Structure and Function

Discovery of Cells and Cell Theory

The study of cells is fundamental to understanding biology. The cell theory is a cornerstone of modern biology, describing the properties and significance of cells.

• Discovery of Cells: Cells were first observed by Robert Hooke in 1665 using a microscope. Later, Anton van Leeuwenhoek observed living cells.

• Three Tenets of the Cell Theory:

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

  2. The cell is the basic unit of structure and organization in organisms.

  3. All cells arise from pre-existing cells.

• Surface Area to Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting the size a cell can attain. This ratio is crucial for efficient exchange of materials.

Types of Microscopes

Microscopes are essential tools for studying cells. There are several types, each with unique applications.

• Light Microscopes: Use visible light to observe living or stained cells.

• Electron Microscopes: Use beams of electrons for much higher resolution, allowing observation of ultrastructure.

• Structural Similarities of All Cells: All cells have a plasma membrane, cytoplasm, DNA, and ribosomes.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their internal structure.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. Examples: Bacteria and Archaea.

• Eukaryotic Cells: Have a nucleus and membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

• Main Components of Bacterial Cells: Cell wall, plasma membrane, cytoplasm, ribosomes, and genetic material (DNA).

• Domains: Archaea and Bacteria are both prokaryotic but differ in membrane lipids, cell wall composition, and genetic machinery.

Eukaryotic Cell Structure and Organelles

Overview of Eukaryotic Organelles

Eukaryotic cells contain specialized structures called organelles, each with distinct functions.

• Nucleus: Contains genetic material (DNA) and controls cell activities.

• Endomembrane System: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles. Responsible for synthesis, modification, and transport of proteins and lipids.

• Endosymbiotic Theory: Explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Mitochondria and Chloroplasts: Both have their own DNA and double membranes, supporting the endosymbiotic theory.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) that provide structural support, cell movement, and intracellular transport.

• Types of Cell Junctions: Structures that connect cells to one another, including tight junctions, desmosomes, and gap junctions.

Cell Membrane Structure and Function

Structure of the Cell Membrane

The cell membrane, or plasma membrane, is a selectively permeable barrier that surrounds the cell.

• Purpose: Regulates the movement of substances in and out of the cell, provides protection and support.

• Fluid Mosaic Model: Describes the membrane as a fluid combination of phospholipids, cholesterol, and proteins.

• Components:

  • Phospholipid Bilayer: Forms the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

  • Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

  • Cholesterol: Stabilizes membrane fluidity.

Membrane Transport Mechanisms

Cells transport substances across their membranes using various mechanisms, classified as passive or active transport.

• Passive Transport: Movement of substances down their concentration gradient without energy input.

  • Diffusion: Movement of molecules from high to low concentration.

  • Facilitated Diffusion: Movement via transport proteins.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (ATP).

  • Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump).

  • Secondary Active Transport: Uses energy from the movement of another substance down its gradient.

• Bulk Transport: Movement of large particles via vesicles.

  • Endocytosis: Uptake of materials into the cell (includes phagocytosis and receptor-mediated endocytosis).

  • Exocytosis: Release of materials from the cell.

Osmosis and Tonicity

Osmosis is critical for maintaining cell volume and function.

• Osmosis: Movement of water from an area of low solute concentration to high solute concentration.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

  • Isotonic: No net water movement; cell remains the same size.

  • Hypertonic: Water leaves the cell; cell shrinks (crenation in animal cells).

  • Hypotonic: Water enters the cell; cell swells and may burst (lysis in animal cells).

Transport Proteins and Mechanisms

Transport proteins facilitate the movement of substances across the membrane.

• Carrier Proteins: Bind and transport specific molecules.

• Channel Proteins: Form pores for specific ions or molecules.

• Types of Carrier Proteins:

  • Symporters: Move two substances in the same direction.

  • Antiporters: Move two substances in opposite directions.

  • Uniporters: Move one substance at a time.

• Examples: Sodium/Potassium pump (Na+/K+ ATPase), Glucose/Sodium pump.

Endocytosis and Exocytosis

Cells use vesicles to transport large molecules or particles.

• Endocytosis: Process of taking in materials by engulfing them in a vesicle.

  • Phagocytosis: "Cell eating"; uptake of large particles.

  • Pinocytosis: "Cell drinking"; uptake of fluids and dissolved substances.

  • Receptor-mediated Endocytosis: Uptake of specific molecules via receptor proteins.

• Exocytosis: Release of substances from the cell via vesicles fusing with the plasma membrane.

Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Cell Size	Small (1-10 μm)	Larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose via carrier protein
Osmosis	No	High to Low (water)	Water movement
Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Bulk Transport	Yes (ATP)	Varies	Endocytosis, Exocytosis

Key Equations

• Surface Area of a Sphere: $A = 4\pi r^2$

• Volume of a Sphere: $V = \frac{4}{3}\pi r^3$

• Osmotic Pressure (van't Hoff equation): $\Pi = iMRT$ where $\Pi$ = osmotic pressure, $i$ = ionization constant, $M$ = molarity, $R$ = gas constant, $T$ = temperature in Kelvin

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard General Biology curriculum.