Cell Structure and Membrane Function: Study Notes for General Biology

TEMA 1: La célula (Cell Structure and Function)

Introduction

The cell is the fundamental unit of life. Understanding its structure and function is essential for all biological sciences. This topic covers the tools used to study cells, the differences between cell types, and the main organelles and their roles.

Microscopy and Cell Fractionation

• Microscopy: Techniques such as light microscopy and electron microscopy allow visualization of cell structures at different resolutions.

• Cell Fractionation: Laboratory process used to separate cellular components for detailed study.

• Example: Differential centrifugation separates organelles based on size and density.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles. Example: Escherichia coli.

• Eukaryotic cells: Have a nucleus and various organelles. Example: Animal and plant cells.

• Key differences: DNA location, organelle presence, cell size.

Animal vs. Plant Eukaryotic Cells

• Animal cells: Lack cell walls, have centrioles, and lysosomes.

• Plant cells: Have cell walls, chloroplasts, and large central vacuoles.

• Comparison Table:

Feature	Animal Cell	Plant Cell
Cell Wall	No	Yes
Chloroplasts	No	Yes
Vacuole	Small/absent	Large central
Lysosomes	Yes	Rare
Centrioles	Yes	No

Cell Organelles: Structure and Function

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

• Ribosomes: Sites of protein synthesis.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplasts: Site of photosynthesis in plant cells.

• Lysosomes: Contain digestive enzymes; break down waste.

• Vacuoles: Storage and structural support, especially in plant cells.

• Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments; provides shape and movement.

Functional Relationships: Nucleus and Ribosomes

• The nucleus contains DNA, which is transcribed into RNA.

• Ribosomes translate RNA into proteins.

• These organelles work together to regulate gene expression and protein synthesis.

Endomembrane System

• Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, and vesicles.

• Responsible for synthesis, modification, and transport of cellular products.

• Example: Secretory proteins are synthesized in the rough ER, processed in the Golgi, and transported via vesicles.

Cytoskeleton and Cell Motility

• Microtubules: Hollow tubes; involved in cell shape, transport, and division.

• Microfilaments: Thin fibers; involved in cell movement and muscle contraction.

• Intermediate filaments: Provide structural support.

• Motility structures: Cilia, flagella, and pseudopodia.

Cellular Compartments and Extracellular Structures

• Cell wall: Provides protection and support in plants, fungi, and some protists.

• Extracellular matrix (ECM): Network of proteins and carbohydrates outside animal cells; provides structural support and signaling.

• Cell junctions: Tight junctions, desmosomes, and gap junctions facilitate communication and adhesion between cells.

Vocabulary

• Microscopio de luz (Light microscope)

• Transmisión (Transmission)

• Centriolos (Centrioles)

• Mitocondria (Mitochondria)

• Cloroplastos (Chloroplasts)

• Ribosomas (Ribosomes)

• Retículo endoplásmico (Endoplasmic reticulum)

• Vacuola (Vacuole)

• Lisosoma (Lysosome)

• Flagelos, Granas (Flagella, Grana)

• Uniones celulares (Cell junctions)

• Membrana nuclear (Nuclear membrane)

• Vesículas (Vesicles)

• Genoma (Genome)

• Colágeno (Collagen)

• Proteoglucano (Proteoglycan)

TEMA 2: Estructura y función de las membranas biológicas (Structure and Function of Biological Membranes)

Introduction

Biological membranes are essential for compartmentalization, protection, and regulation of cellular processes. This topic explores membrane structure, models, transport mechanisms, and related vocabulary.

Importance of the Cell Membrane

• Defines cell boundaries and maintains homeostasis.

• Controls the movement of substances in and out of the cell.

• Facilitates communication and signaling.

Fluid Mosaic Model

• Describes the membrane as a dynamic structure composed of a phospholipid bilayer with embedded proteins.

• Proteins and lipids can move laterally within the layer, providing fluidity.

• Components: Phospholipids, cholesterol, integral and peripheral proteins, carbohydrates.

• Diagram: Shows proteins interspersed in a sea of lipids.

Membrane Components and Functions

• Phospholipids: Form the bilayer; hydrophilic heads and hydrophobic tails.

• Cholesterol: Modulates fluidity and stability.

• Proteins: Transport, signaling, enzymatic activity.

• Carbohydrates: Cell recognition and adhesion.

Selective Permeability

• Membranes allow some substances to pass while blocking others.

• Small nonpolar molecules (e.g., O2, CO2) pass easily; large or charged molecules require transport proteins.

Transport Mechanisms

• Passive transport: No energy required; includes diffusion, osmosis, and facilitated diffusion.

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

• Example equations:

$J = -D \frac{dC}{dx}$ (Fick's law of diffusion)

$\text{Rate} = k[\text{Substrate}]$ (Facilitated diffusion rate)

$\text{Na}^+/\text{K}^+$ pump: 3 Na+ out, 2 K+ in per ATP hydrolyzed$

Osmosis and Tonicity

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

• Tonicity: Relative concentration of solutes; affects cell volume.

• Types: Isotonic (no net water movement), hypotonic (cell swells), hypertonic (cell shrinks).

Specialized Transport Proteins

• Aquaporins: Channel proteins that facilitate rapid water movement.

• Ion channels: Allow specific ions to pass through the membrane.

• Pumps: Use energy to move substances against gradients.

Bulk Transport: Endocytosis and Exocytosis

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

• Endocytosis: Cell engulfs external substances via vesicle formation.

• Types of endocytosis: Phagocytosis (solid particles), pinocytosis (liquids), receptor-mediated endocytosis (specific molecules).

Comparing Environments: Hypotonic, Isotonic, Hypertonic

Environment	Effect on Animal Cell	Effect on Plant Cell
Isotonic	No net change	Flaccid
Hypotonic	Swells, may burst	Turgid (normal)
Hypertonic	Shrinks (crenates)	Plasmolyzed

Vocabulary

• Difusión facilitada (Facilitated diffusion)

• Osmosis

• Gradiente (Gradient)

• Modelo-mosaico fluido (Fluid mosaic model)

• Proteínas integrales y periféricas (Integral and peripheral proteins)

• Canales (Channels)

• Colesterol (Cholesterol)

• Exocitosis (Exocytosis)

• Endocitosis (Endocytosis)

• Fagocitosis (Phagocytosis)

• Pinocitosis (Pinocytosis)

• Receptores (Receptors)

• Glucoproteínas (Glycoproteins)

• Permeabilidad selectiva (Selective permeability)

• Aquaporinas (Aquaporins)

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