General Biology Study Guide: Cell Structure, Membranes, and Metabolism the main one

Cell Structure and Function

Plasma Membrane (Castle = cell, castle walls = cell membrane, guards = selective barrier that decides what comes in and out)

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

• Structure: Composed 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.

Ribosomes (Factory. Blueprint = mRNA, Worker = ribosomes, Deliverers = tRNA)

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.

Nucleus (Brain. Skull is the double membrane, DNA are the memories, nucleolus is like a special part of the brain that creates communication tools which are the ribosomes

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 and ribosome assembly begins.

Endomembrane System

The endomembrane system is a network of membranes involved in protein and lipid synthesis, modification, and transport.

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

• Key Organelles:

  • Rough ER: Studded with ribosomes; site of protein synthesis and modification.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

  • 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 large 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) increasing surface area for ATP production.

• Compartments: Matrix (site of Krebs cycle) and intermembrane space.

Chloroplasts

Chloroplasts are organelles found in plant cells responsible for photosynthesis.

• Structure: Double membrane, internal thylakoid membranes (site of light-dependent reactions), and stroma (site of Calvin cycle).

Cytoskeleton

The cytoskeleton provides structural support, facilitates cell movement, and organizes organelles.

• Components:

  • Microfilaments (actin filaments): Involved in cell movement and shape.

  • Microtubules: Hollow tubes that maintain cell shape and serve as tracks for organelle movement.

  • Intermediate Filaments: Provide mechanical strength.

Extracellular Matrix (ECM) and Cell Wall

The extracellular matrix (in animal cells) and cell wall (in plant cells) provide structural support and mediate cell signaling.

• ECM: Network of proteins and carbohydrates outside animal cells.

• Cell Wall: Rigid structure made of cellulose in plants.

Intracellular Junctions

Intracellular junctions connect cells and facilitate communication.

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

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, fluid structure with proteins embedded in or attached to the bilayer.

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

Membrane Fluidity

• Fluidity is influenced by temperature and the composition of fatty acids (saturated vs. unsaturated).

• Cholesterol acts as a buffer, maintaining fluidity across temperature changes.

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 maintaining cell shape.

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, requiring energy (usually ATP).

• Bulk Transport: Includes endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) and exocytosis.

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

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

• Na+/glucose transporter: Example of secondary active transport (uses the 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.

• Maintained by the unequal distribution of ions (Na+, K+, Cl-) across the membrane.

• Na+/K+ ATPase and leak channels are key contributors.

Action Potentials

• Rapid changes in membrane potential that transmit signals along neurons.

• Involve voltage-gated Na+ and K+ channels.

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.

• Endergonic Reactions: Require energy input; non-spontaneous.

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

ATP: The Energy Currency

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

• Energy is released by hydrolysis of the terminal phosphate group.

• ATP drives endergonic reactions via phosphorylation.

Enzymes

Enzyme Structure and Function

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

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

• Induced Fit Model: Enzyme changes shape to better fit the substrate upon binding.

Enzyme Activity

• Affected by temperature, pH, substrate concentration, and presence of inhibitors or activators.

• Enzyme activity typically increases with temperature up to an optimum, then decreases due to denaturation.

• pH affects the ionization of amino acids at the active site.

Reaction Progress and Energy Diagrams

• Enzyme-catalyzed reactions have lower activation energy compared to uncatalyzed reactions.

• Exergonic reactions: Products have lower free energy than reactants.

• Endergonic reactions: Products have higher free energy than reactants.

Enzyme Regulation

• Enzymes can be regulated by inhibitors (competitive and noncompetitive) and activators.

• Allosteric regulation involves binding of molecules at sites other than the active site, changing enzyme activity.

Example Table: Types of Cytoskeletal Filaments

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

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard General Biology curriculum.