General Biology Study Guide: Cell Structure, Membranes, and Metabolism

Cell Structure and Function

Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell, controlling the movement of substances in and out.

• Structure: Composed mainly of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Function: Maintains homeostasis, facilitates communication, and provides structural support.

• Organization: The fluid mosaic model describes the dynamic arrangement of lipids and proteins.

Ribosomes

Ribosomes are molecular machines responsible for protein synthesis.

• Free Ribosomes: Located in the cytosol; synthesize proteins for use within the cell.

• Bound Ribosomes: Attached to the rough endoplasmic reticulum (ER); synthesize proteins for secretion or membrane insertion.

Nucleus

The nucleus is the control center of the cell, containing genetic material (DNA).

• Structure: Surrounded by a double membrane (nuclear envelope) with nuclear pores.

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

Endomembrane System

The endomembrane system includes organelles involved in synthesis, modification, and transport of cellular materials.

• Pathway of Protein Synthesis: DNA → mRNA (nucleus) → ribosome (rough ER) → vesicle → Golgi apparatus → vesicle → plasma membrane (for secretion).

• Rough ER: Studded with ribosomes; synthesizes and modifies proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

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

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes.

• Function: Breakdown of macromolecules, cellular debris, and foreign substances.

• Processes: Phagocytosis (engulfing particles) and autophagy (recycling cell components).

Mitochondria

Mitochondria are the powerhouses of the cell, generating ATP through cellular respiration.

• Structure: Double membrane with inner folds (cristae) and a matrix.

• Compartments: Outer membrane, intermembrane space, inner membrane, and matrix.

• Function: Site of aerobic respiration and energy (ATP) production.

Chloroplasts

Chloroplasts are organelles found in plant cells responsible for photosynthesis.

• Structure: Double membrane, stroma, and internal thylakoid membranes (stacked as grana).

• Function: Convert light energy into chemical energy (glucose).

Cytoskeleton

The cytoskeleton provides structural support, facilitates movement, and organizes cellular components.

• Microfilaments (actin filaments): Thin fibers involved in cell movement and shape.

• Microtubules: Hollow tubes that maintain cell shape, enable intracellular transport, and form spindle fibers during cell division.

• Intermediate Filaments: Provide mechanical strength.

Cell Walls and Extracellular Matrix

Cell walls (in plants) and the extracellular matrix (in animals) provide structural support and mediate cell interactions.

• Plant Cell Wall: Composed mainly of cellulose.

• Extracellular Matrix (ECM): Network of proteins and carbohydrates outside animal cells.

Intercellular Junctions

Intercellular junctions connect adjacent cells and facilitate communication.

• Plasmodesmata: Channels between plant cells.

• Tight Junctions: Seal cells together in animal tissues.

• Desmosomes: Anchor cells together.

• Gap Junctions: Allow passage of ions and small molecules between animal cells.

Cell Membranes & Membrane Transport

Phospholipids and Membrane Structure

Cell membranes are primarily composed of phospholipids, which are amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Phospholipid Bilayer: Forms the basic structure of biological membranes.

• Fluid Mosaic Model: Describes the membrane as a dynamic, fluid structure with proteins embedded or attached.

Membrane Proteins

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

• Peripheral Proteins: Loosely attached to the membrane surface; involved in signaling and structural support.

Membrane Transport Mechanisms

Cells regulate the movement of substances across membranes through various transport mechanisms.

• Passive Transport: Movement down a concentration gradient without energy input (e.g., diffusion, osmosis, facilitated diffusion).

• Active Transport: Movement against a concentration gradient, requiring energy (ATP).

• Bulk Transport: Endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis for large molecules.

Osmosis and Tonicity

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

• Tonicity: The effect of a solution on cell volume (hypotonic, isotonic, hypertonic).

Active Transport Examples

• Na+/K+ ATPase: Pumps 3 Na+ out and 2 K+ into the cell, maintaining electrochemical gradients.

• Na+-glucose transporter: Example of secondary active transport (uses Na+ gradient to transport glucose).

Passive Transport in Action: Neurons

Resting Membrane Potential

The resting membrane potential is the voltage difference across the neuronal membrane when the neuron is not transmitting a signal.

• Typical Value: About -70 mV (inside negative relative to outside).

• Ion Distribution: More Na+ outside, more K+ inside.

Action Potentials

• Depolarization: Na+ channels open, Na+ enters the cell.

• Repolarization: K+ channels open, K+ leaves the cell.

• Na+/K+ ATPase: Restores ion gradients after an action potential.

Intro to Metabolism

Thermodynamics in Biology

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

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

Free Energy and Chemical Reactions

• Free Energy (G): The portion of a system's energy that can perform work.

• Exergonic Reactions: Release energy; spontaneous ($\Delta G < 0$).

• Endergonic Reactions: Require energy input; non-spontaneous ($\Delta G > 0$).

ATP: The Energy Currency

• ATP (Adenosine Triphosphate): Stores and transfers energy for cellular processes.

• Hydrolysis of ATP: Releases energy by breaking a phosphate bond ($\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$).

Enzymes

Enzyme Function and Kinetics

• Enzymes: Biological catalysts that speed up chemical reactions by lowering activation energy.

• Active Site: Region where substrate binds and reaction occurs.

• Enzyme-Substrate Complex: Temporary association during catalysis.

• Factors Affecting Activity: Temperature, pH, substrate concentration, and inhibitors.

Reaction Progress and Energy Diagrams

• Activation Energy (Ea): The energy required to start a reaction.

• Exergonic vs. Endergonic: Exergonic reactions release energy; endergonic reactions absorb energy.

Enzyme Regulation

• Allosteric Regulation: Binding of molecules at sites other than the active site to regulate activity.

• Feedback Inhibition: End product of a pathway inhibits an earlier step.

Summary Table: Types of Cytoskeletal Filaments

Filament Type	Main Protein	Function
Microfilaments	Actin	Cell movement, shape, muscle contraction
Microtubules	Tubulin	Cell shape, intracellular transport, chromosome movement
Intermediate Filaments	Various (e.g., keratin)	Mechanical strength

Key Equations

• Free Energy Change: $\Delta G = \Delta H - T\Delta S$

• ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$