General Biology Study Guide: Cell Structure, Membrane Function, Metabolism, and Cell Communication

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

Cells are classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotic cells lack a nucleus and membrane-bound organelles. Their genetic material is found in the nucleoid region.

• Eukaryotic cells possess a true nucleus and various membrane-bound organelles.

• Key differences: Eukaryotes have compartmentalization, larger size, and more complex internal structures.

Nucleus and Its Components

The nucleus is the control center of the cell, storing genetic information and coordinating cellular activities.

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

• Chromatin: DNA-protein complex that condenses to form chromosomes during cell division.

• Nuclear envelope: Double membrane that separates the nucleus from the cytoplasm.

Ribosomes

Ribosomes are molecular machines responsible for protein synthesis.

• Free ribosomes: Located in the cytosol; produce proteins for use within the cell.

• Bound ribosomes: Attached to the endoplasmic reticulum; synthesize proteins for secretion or membrane insertion.

Endomembrane System

The endomembrane system is a network of membranes involved in transport and processing of cellular materials.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Smooth ER: Lipid synthesis, detoxification, and calcium storage.

• Rough ER: Studded with ribosomes; involved in protein synthesis and modification.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for delivery.

• Cis face: Receiving side, closest to the ER.

• Trans face: Shipping side, releases vesicles to other destinations.

Lysosomes

Lysosomes are organelles containing hydrolytic enzymes for intracellular digestion.

• Function: Breakdown of macromolecules, recycling of cellular components.

Vacuoles

Vacuoles are membrane-bound sacs for storage and waste disposal, especially prominent in plant cells.

• Central vacuole: Maintains cell turgor, stores nutrients and waste products.

Mitochondria and Chloroplasts

These organelles are responsible for energy conversion in cells.

• Mitochondria: Site of cellular respiration; double membrane structure with inner membrane forming cristae.

• Chloroplasts: Site of photosynthesis in plants; contains thylakoid membranes and stroma.

Peroxisomes

Peroxisomes carry out oxidation reactions, breaking down fatty acids and detoxifying harmful substances.

Cytoskeleton

The cytoskeleton provides structural support and facilitates cell movement.

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

• Microfilaments: Actin filaments; support cell shape and enable movement.

• Intermediate filaments: Provide mechanical strength.

Cell Wall and Extracellular Matrix

Plant cells have a rigid cell wall for protection and support; animal cells have an extracellular matrix for structural integrity and signaling.

Intercellular Junctions

Junctions connect cells and facilitate communication.

• Plant cells: Plasmodesmata

• Animal cells: Tight junctions, desmosomes, gap junctions

Membrane Structure and Function

Phospholipids and Membrane Fluidity

Cell membranes are composed of a phospholipid bilayer with embedded proteins.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Fluidity: Influenced by saturation of hydrocarbon tails and presence of cholesterol.

• Cholesterol: Modulates membrane fluidity at different temperatures.

Membrane Proteins

Proteins embedded in the membrane perform various functions.

• Integral proteins: Span the membrane.

• Peripheral proteins: Attached to the surface.

• Functions: Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to cytoskeleton and ECM.

Membrane Carbohydrates

Carbohydrates attached to proteins or lipids are involved in cell-cell recognition.

Origin of Plasma Membrane

The plasma membrane originates from the endoplasmic reticulum and Golgi apparatus through vesicle trafficking.

Transport Across Membranes

Cells regulate the movement of substances across membranes.

• Diffusion: Passive movement of molecules from high to low concentration.

• Facilitated diffusion: Passive transport via membrane proteins.

• Active transport: Movement against concentration gradient using energy (usually ATP).

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

Osmotic Conditions

Condition	Description
Hypotonic	Lower solute concentration outside cell; water enters cell
Hypertonic	Higher solute concentration outside cell; water leaves cell
Isotonic	Equal solute concentration; no net water movement

Membrane Potential and Electrochemical Gradients

Membrane potential is the voltage difference across a membrane, created by ion gradients.

• Electrochemical gradient: Combination of chemical and electrical forces driving ion movement.

Transport Proteins

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

• Carrier proteins: Change shape to shuttle substances across membrane.

• Co-transport: Coupled transport of two substances via the same protein.

Bulk Transport

• Exocytosis: Export of large molecules via vesicles.

• Endocytosis: Import of large molecules; includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Metabolism and Enzyme Function

Introduction to Metabolism

Metabolism encompasses all chemical reactions in a cell, divided into catabolic and anabolic pathways.

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones.

Kinetic and Potential Energy

Energy exists in two main forms:

• Kinetic energy: Energy of motion.

• Potential energy: Stored energy due to position or structure.

Thermodynamics

• First Law: Energy cannot be created or destroyed, only transformed.

• Second Law: Every energy transfer increases the entropy (disorder) of the universe.

Free Energy ($G$)

Free energy is the portion of a system's energy available to do work.

• Equation: $G = H - TS$ where $H$ = enthalpy, $T$ = temperature, $S$ = entropy

Exergonic vs. Endergonic Reactions

Type	Change in Free Energy ($riangle G$)	Energy Flow
Exergonic	$riangle G < 0$	Releases energy
Endergonic	$riangle G > 0$	Requires energy input

Energy Coupling and ATP

Cells couple exergonic and endergonic reactions using ATP as an energy shuttle.

• ATP (Adenosine Triphosphate): Main energy currency; transfers energy via phosphorylation.

• Phosphorylation: Addition of a phosphate group to a molecule, making it more reactive.

Enzymes

Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Substrate: The reactant an enzyme acts upon.

• Active site: Region on enzyme where substrate binds.

• Induced fit model: Enzyme changes shape to fit substrate more snugly.

Factors Affecting Enzyme Activity

• Temperature: Affects kinetic energy and enzyme stability.

• pH: Alters enzyme structure and function.

• Cofactors: Non-protein helpers (e.g., metal ions, vitamins).

• Enzyme inhibitors: Molecules that reduce enzyme activity; can be competitive (bind active site) or noncompetitive (bind elsewhere).

• Allosteric regulation: Regulation by binding at sites other than the active site, affecting enzyme activity.

Cell Communication

Signal-Transduction Pathways

Cells communicate using signal-transduction pathways, converting signals into cellular responses.

• Stages: Reception, Transduction, Response

Types of Signaling

• Local signaling: Paracrine and synaptic signaling

• Long-distance signaling: Endocrine (hormonal) signaling

Membrane Receptor Proteins

• G-protein linked receptors: Activate intracellular G-proteins

• Receptor tyrosine kinases: Trigger phosphorylation cascades

• Ion channel receptors: Allow ions to pass in response to signal

Reception

Reception involves the binding of a signaling molecule (ligand) to a receptor.

Transduction

Transduction is the relay and amplification of the signal inside the cell, often involving phosphorylation.

• Phosphorylation: Addition of phosphate groups to proteins, activating or deactivating them.

• Second messengers: Small molecules (e.g., cAMP, Ca2+) that propagate the signal.

Response

The final stage leads to a specific cellular activity, such as gene expression or metabolic change.

• Amplification: One signal can trigger many responses.

• Specificity: Different cells respond differently to the same signal.

• Efficiency: Scaffold proteins organize components for rapid signaling.

• Termination: Signals are turned off to reset the pathway.