The Cell and Membrane Transport: Structure, Function, and Dynamics

Chapter 4: The Cell

Overview of Cellular Structure and Function

Cells are the fundamental units of life, forming the basis for all living organisms. Despite their diversity, all cells share common features and reflect evolutionary ancestry. Multicellular organisms consist of specialized cells that work together, while unicellular organisms perform all life functions within a single cell.

• Cells maintain homeostasis and respond to environmental changes.

• All organisms are made of cells, which are the simplest units of life.

• Cells share a common ancestry and have evolved diverse forms and functions.

4.1: Microscopy and the Study of Cells

• History of Cell Discovery: Cells were first observed by Robert Hooke in 1665 using a light microscope. The development of microscopy enabled the discovery of cell structure and function.

• Microscope Terminology:

  • Magnification: The ratio of an object's image size to its real size.

  • Resolution: The measure of image clarity; the minimum distance two points can be separated and still be distinguished.

  • Contrast: The difference in brightness between parts of the sample.

• Size Ranges:

  • Plant and animal cells: 10–100 μm

  • Bacteria: 1–10 μm

  • Viruses: 20–300 nm

• Types of Microscopes:

  • Light Microscopes (LM): Use visible light to illuminate specimens; suitable for living cells and general structure.

  • Electron Microscopes (EM): Use electron beams for much higher resolution; Transmission EM (TEM) for internal structures, Scanning EM (SEM) for surface details.

• Microscope Parts: Include ocular lens, objective lenses, stage, light source, coarse and fine focus knobs, and condenser.

4.2: Prokaryotic vs. Eukaryotic Cells and Compartmentalization

• Prokaryotes: Lack membrane-bound organelles; DNA is in the nucleoid region. Examples: Bacteria, Archaea.

• Eukaryotes: Have membrane-bound organelles, including a nucleus. Examples: Plants, animals, fungi, protists.

• Compartmentalization:

  • Pros: Increases efficiency by separating incompatible reactions, allows specialization.

  • Cons: Requires energy for transport and communication between compartments.

• Animal vs. Plant Cells:

  • Plant cells have cell walls, chloroplasts, and large central vacuoles; animal cells do not.

• Cell Size and Surface Area:Volume Ratio: Cells are small to maximize surface area relative to volume, facilitating efficient exchange of materials. Equation: $\text{Surface Area} = 6s^2$ (for a cube), $\text{Volume} = s^3$

4.3: The Nucleus and Ribosomes

• Nucleus: (Eukaryotes only) Contains genetic material (DNA); surrounded by a double membrane (nuclear envelope) with pores for transport.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

• Chromosomes: DNA molecules with associated proteins; carry genetic information.

• Chromatin: DNA and protein complex; condenses to form chromosomes during cell division.

• Ribosomes: Sites of protein synthesis; can be free in cytosol or bound to the endoplasmic reticulum (ER).

4.4: The Endomembrane System

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies drugs, stores calcium ions.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport.

• Lysosomes: Contain hydrolytic enzymes for intracellular digestion (mainly in animal cells).

• Vesicles/Vacuoles: Membrane-bound sacs for storage and transport; large central vacuole in plant cells for storage and maintaining turgor pressure.

• Plasma Membrane: Selectively permeable boundary of the cell.

• Endomembrane System Includes: Nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, vacuoles, and plasma membrane.

4.5: Mitochondria, Chloroplasts, and Peroxisomes

• Endosymbiotic Theory: Mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

  • Evidence: Double membranes, own DNA and ribosomes, reproduce independently within the cell.

• Mitochondria: (All eukaryotes) Site of cellular respiration; converts glucose and oxygen into ATP.

• Chloroplasts: (Plants and algae) Site of photosynthesis; converts solar energy into chemical energy (glucose).

• Peroxisomes: Break down fatty acids and detoxify harmful substances; produce hydrogen peroxide as a byproduct.

4.6: The Cytoskeleton

• Cytoskeleton: Network of protein fibers (microtubules, microfilaments, intermediate filaments) that provides structural support, cell shape, and movement.

• Microtubules: Hollow rods made of tubulin; involved in cell shape, organelle movement, and chromosome separation during cell division.

• Motor Proteins: Move along cytoskeletal tracks to transport vesicles and organelles.

• Centriole: (Animal cells) Organizes microtubules during cell division; found in centrosome.

• Cilia and Flagella: Structures for cell movement; cilia are short and numerous, flagella are longer and usually singular.

4.7: Extracellular Components and Cell Connections

• Cell Wall: (Plants, fungi, some protists, and prokaryotes) Rigid structure outside the plasma membrane; provides support and protection.

• Extracellular Matrix (ECM): (Animals) Network of glycoproteins and other molecules outside the cell; supports and anchors cells, facilitates communication.

4.8: Integration of Cellular Components

• Organelle Interactions: Example: The rough ER synthesizes proteins, which are then modified and sorted by the Golgi apparatus before being transported in vesicles.

• Structure-Function Relationships: Example: The double membrane of mitochondria creates compartments for efficient ATP production; the extensive surface area of the inner membrane (cristae) supports more ATP-generating enzymes.

Chapter 5: Cell Transport

Overview of Membrane Structure and Function

The plasma membrane is a selectively permeable boundary that regulates the movement of substances into and out of the cell. Its structure is conserved across all domains of life but is specialized for different environments and cell types.

5.1: Membrane Structure – The Fluid Mosaic Model

• Phospholipids: Composed of a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails. In water, they form a bilayer with heads facing outward and tails inward.

• Saturated vs. Unsaturated Phospholipids:

  • Saturated: No double bonds; pack tightly; less fluid at low temperatures.

  • Unsaturated: One or more double bonds; prevent tight packing; maintain fluidity at lower temperatures.

• Fluid Mosaic Model: Describes the membrane as a mosaic of proteins floating in or on a fluid lipid bilayer.

• Cholesterol: Steroid molecule interspersed in the membrane; stabilizes membrane fluidity across temperature changes.

• Membrane Proteins:

  • Integral Proteins: Span the membrane; involved in transport and cell signaling.

  • Peripheral Proteins: Loosely attached to the membrane surface; involved in signaling and maintaining cell shape.

  • Only integral proteins can function as transport proteins.

• Glycolipids and Glycoproteins: (Advanced) Carbohydrate chains attached to lipids or proteins; involved in cell recognition and signaling.

5.2: Selective Permeability of Membranes

• Selective Permeability: The membrane allows some substances to cross more easily than others due to its structure.

• Transport of Molecules:

  • Small, nonpolar molecules (e.g., O2, CO2): Pass easily through the lipid bilayer.

  • Small, polar molecules (e.g., H2O): Pass through slowly or via protein channels (aquaporins).

  • Ions (e.g., Na+): Require specific transport proteins due to charge.

  • Large molecules (e.g., glucose): Require carrier proteins or bulk transport mechanisms.

• Examples:

  • Small: O2

  • Medium: Glucose

  • Large: Proteins

  • Nonpolar: O2

  • Polar: H2O

  • Ionic: Na+

5.3: Passive Transport

• Diffusion: Movement of molecules from high to low concentration due to random motion; does not require energy.

• Concentration Gradient: Difference in concentration across a membrane; substances move down their gradient.

• Passive vs. Active Transport:

  • Passive: No energy required; moves substances down their gradient.

  • Active: Requires energy (usually ATP); moves substances against their gradient.

• Types of Passive Transport:

  • Simple Diffusion: Direct movement through the lipid bilayer (e.g., O2).

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

  • Facilitated Diffusion: Movement via channel or carrier proteins (e.g., glucose transporters).

• Transport Proteins:

  • Channel Proteins: Provide corridors for specific molecules or ions.

  • Carrier Proteins: Change shape to move substances across the membrane.

  • Pumps: Use energy to move substances against their gradient (active transport).

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

  • Hypotonic: Lower solute concentration outside; water enters cell.

  • Hypertonic: Higher solute concentration outside; water leaves cell.

  • Isotonic: Equal solute concentrations; no net water movement.

  • Non-woody plants prefer hypotonic environments (turgor pressure); animal cells prefer isotonic environments.

5.4: Active Transport

• Active Transport: Uses energy (usually ATP) to move solutes against their concentration gradients via protein pumps.

• ATP: Transfers a phosphate group to the pump protein, causing a conformational change that moves the solute.

5.5: Bulk Transport

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

• Endocytosis: Cell takes in materials by forming vesicles from the plasma membrane.

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

  • Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

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

Table: Comparison of Prokaryotic and Eukaryotic Cells

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

Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions via channels
Osmosis	No	Down water potential gradient	Water via aquaporins
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Bulk Transport	Yes (ATP)	In or out	Exocytosis, endocytosis

Additional info: Some explanations and examples were expanded for clarity and completeness, following standard introductory biology textbooks.