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 primarily of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

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

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

• Example: Transport proteins in the membrane allow glucose uptake into cells.

Ribosomes

Ribosomes are molecular machines responsible for protein synthesis.

• Free Ribosomes: Float in the cytoplasm and synthesize proteins for use within the cell.

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

• Example: Insulin is produced by ribosomes bound to the rough ER in pancreatic cells.

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: Region within the nucleus where ribosomal RNA is synthesized.

• Function: Stores genetic information and coordinates cell activities like growth and reproduction.

Endomembrane System

The endomembrane system is a group of organelles involved in the synthesis, modification, and transport of cellular materials.

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

• Pathway: Newly synthesized proteins move from the rough ER to the Golgi apparatus, then to vesicles for secretion or delivery to other organelles.

• Example: Digestive enzymes are produced in the rough ER, processed in the Golgi, and sent to lysosomes.

Endoplasmic Reticulum (ER)

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

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

Golgi Apparatus

• Structure: Stacks of flattened membranous sacs (cisternae).

• Function: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Example: Glycosylation (addition of sugars to proteins) occurs in the Golgi.

Lysosomes

• Function: Contain hydrolytic enzymes for intracellular digestion.

• Processes: Involved in phagocytosis (engulfing particles) and autophagy (recycling cell components).

Mitochondria

• Structure: Double-membraned organelle with inner folds called cristae.

• Function: Site of cellular respiration and ATP production.

• Compartments: Matrix (contains enzymes for Krebs cycle) and intermembrane space.

Chloroplasts

• Structure: Double-membraned organelle with internal thylakoid membranes.

• Function: Site of photosynthesis in plant cells.

• Compartments: Stroma (fluid), thylakoid space (inside thylakoids).

Cytoskeleton

• Components: Microfilaments (actin), intermediate filaments, and microtubules.

• Function: Provides structural support, enables cell movement, and organizes organelles.

• Comparison Table:

Component	Protein	Function
Microfilaments	Actin	Cell shape, movement
Intermediate Filaments	Various	Structural stability
Microtubules	Tubulin	Organelle movement, cell division

Cell Walls and Extracellular Matrix

• Plant Cell Wall: Composed of cellulose; provides rigidity and protection.

• Extracellular Matrix (ECM): Network of proteins and carbohydrates outside animal cells; provides support and regulates cell behavior.

Intercellular Junctions

• Types: Tight junctions, desmosomes, gap junctions (animals); plasmodesmata (plants).

• Function: Facilitate communication and adhesion between 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.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins floating in or on the fluid lipid bilayer.

• Lipid Bilayer: Two layers of phospholipids with tails facing inward and heads facing outward.

• Environmental Effects: Membrane fluidity is influenced by temperature and lipid composition (e.g., unsaturated vs. saturated fatty acids).

Membrane Proteins

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

• Peripheral Proteins: Attached to the membrane surface; involved in cell signaling and structure.

• Comparison: Integral proteins are embedded within the bilayer, while peripheral proteins are loosely attached to the surface.

Transport Across Membranes

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

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

• Bulk Transport: Endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis move large particles or volumes.

• Example: Sodium-potassium pump ($Na^+/K^+$-ATPase) actively transports $Na^+$ out and $K^+$ into animal cells.

Osmosis and Tonicity

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

• Tonicity: The ability of a solution to cause a cell to gain or lose water (hypotonic, isotonic, hypertonic solutions).

Transport Proteins

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

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

Passive Transport in Action: Neurons

Resting Membrane Potential

• Definition: The voltage difference across the plasma membrane when a neuron is not transmitting a signal.

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

• Ion Distribution: More $Na^+$ outside and more $K^+$ inside the neuron.

Action Potentials

• Action Potential: Rapid change in membrane potential that travels along the neuron.

• Phases: Depolarization (influx of $Na^+$), repolarization (efflux of $K^+$), and return to resting potential.

• Role of $Na^+/K^+$-ATPase: Restores ion gradients after action potentials.

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.

• Change in Free Energy ($\Delta G$): Determines whether a reaction is spontaneous.

$\Delta G = \Delta H - T\Delta S$

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

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

ATP and Energy Coupling

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

• Hydrolysis of ATP: Releases energy to drive endergonic reactions.

• Phosphorylation: Transfer of a phosphate group to another molecule, often activating it.

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

Enzymes

Enzyme Structure and Function

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

• Active Site: Region on the enzyme where the substrate binds.

• Induced Fit Model: Enzyme changes shape slightly to fit the substrate more closely.

Enzyme Activity

• Factors Affecting Activity: Temperature, pH, substrate concentration, and presence of inhibitors or activators.

• Reaction Progress Diagram: Shows the energy changes during a reaction with and without an enzyme.

• Effect of pH and Temperature: Each enzyme has an optimal pH and temperature for activity.

Enzyme Regulation

• Allosteric Regulation: Binding of molecules at sites other than the active site can increase or decrease enzyme activity.

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

Additional info: Some content was inferred and expanded for completeness and clarity based on standard General Biology curriculum topics.