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

Cell Structure and Function

Cell Structure, Membranes, and Organelles

Cells are the basic units of life, and their structure is closely related to their function. Understanding the organization of cell membranes and organelles is essential for studying cellular processes.

• Cell Structure: Cells are composed of a plasma membrane, cytoplasm, and various organelles. The plasma membrane acts as a selective barrier, controlling the movement of substances in and out of the cell.

• Membrane Composition: The cell membrane consists of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates, which contribute to its fluidity and functionality.

• Eukaryotic Cell Organelles: Eukaryotic cells contain membrane-bound organelles such as the nucleus, mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and peroxisomes. Each organelle has a specific function, such as energy production (mitochondria) or protein synthesis (ribosomes).

• Specialized Cell Types: Cells can be specialized for particular functions, such as macrophages for immune defense or neurons for signal transmission.

• Compartmentalization: Organelles create distinct environments within the cell, allowing for specialized metabolic processes.

Example: The mitochondrion is known as the "powerhouse" of the cell because it generates ATP through cellular respiration.

Cell Junctions and Communication

Cells interact with each other and their environment through specialized junctions and signaling mechanisms.

• Cell Junctions: Structures such as tight junctions, gap junctions, and desmosomes connect cells and facilitate communication and transport.

• Animal vs. Plant Cell Junctions: Animal cells have tight and gap junctions, while plant cells have plasmodesmata for intercellular communication.

Example: Gap junctions allow ions and small molecules to pass directly between neighboring animal cells, enabling rapid communication.

Cell Membranes: Structure and Properties

Selective Permeability and Fluidity

Cell membranes are selectively permeable barriers that regulate the movement of substances.

• Phospholipid Bilayer: The membrane's structure is based on amphipathic phospholipids, with hydrophilic heads and hydrophobic tails.

• Fluid Mosaic Model: Membranes are dynamic, with proteins and lipids able to move laterally within the bilayer.

• Cholesterol: Cholesterol modulates membrane fluidity and stability.

• Temperature Effects: Changes in temperature can affect membrane fluidity; higher temperatures increase fluidity, while lower temperatures decrease it.

Example: The presence of unsaturated fatty acids in phospholipids increases membrane fluidity.

Transport Across Membranes

Cells transport molecules across membranes using various mechanisms.

• Passive Transport: Includes simple diffusion, facilitated diffusion, and osmosis. No energy is required.

• Active Transport: Requires energy (usually ATP) to move substances against their concentration gradient.

• Electrochemical Gradients: Membranes can generate gradients that drive transport, such as the sodium-potassium pump.

Example: The Na+/K+ pump maintains electrochemical gradients essential for nerve impulse transmission.

Key Transport Equations

• Diffusion Rate: $J = -D \frac{dC}{dx}$

• Nernst Equation (for membrane potential): $E = \frac{RT}{zF} \ln \frac{[\text{ion outside}]}{[\text{ion inside}]}$

Cell Communication and Signal Transduction

Cell Signaling Mechanisms

Cells communicate using chemical signals that can be lipid-soluble or water-soluble. Signal transduction pathways convert these signals into cellular responses.

• Receptors: Proteins that bind signaling molecules (ligands) and initiate cellular responses.

• Signal Transduction: Involves a cascade of events, often including second messengers such as cAMP or Ca2+.

• Second Messengers: Small molecules that amplify and propagate the signal within the cell.

• Gene Expression: Signals can lead to changes in gene expression, affecting cell function.

Example: The binding of a hormone to its receptor can activate a G-protein, leading to the production of cAMP and activation of downstream enzymes.

Signal Transduction Pathway Example

• Steps:

  1. Ligand binds to receptor.

  2. Receptor activates G-protein.

  3. G-protein activates adenylate cyclase.

  4. Adenylate cyclase converts ATP to cAMP.

  5. cAMP activates protein kinase A.

  6. Protein kinase A phosphorylates target proteins.

Metabolism: Energy Transfer and Transformation

Thermodynamics in Biology

Energy transfer and transformation are fundamental to all biological processes. The laws of thermodynamics govern how energy is used and conserved in living systems.

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

• Second Law of Thermodynamics: Entropy (disorder) tends to increase in isolated systems.

• Spontaneous vs. Non-Spontaneous Reactions: Spontaneous reactions occur without input of energy; non-spontaneous reactions require energy.

• Gibbs Free Energy: Determines whether a reaction is spontaneous.

  • $\Delta G = \Delta H - T\Delta S$

  • Negative $\Delta G$ indicates a spontaneous (exergonic) reaction.

Example: Cellular respiration is an exergonic process that releases energy by breaking down glucose.

Enzyme Function and Regulation

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy. Their activity is regulated by various mechanisms.

• Enzyme-Substrate Interaction: Enzymes bind substrates at their active sites, facilitating the reaction.

• Activation Energy: The energy required to initiate a reaction; enzymes lower this barrier.

• Enzyme Kinetics: The study of reaction rates and how they are affected by substrate concentration and inhibitors.

• Allosteric Regulation: Enzymes can be regulated by molecules that bind at sites other than the active site, changing their activity.

• Feedback Inhibition: The end product of a pathway inhibits an earlier step, preventing overproduction.

Example: ATP acts as an allosteric inhibitor of phosphofructokinase in glycolysis, regulating energy production.

Enzyme Kinetics Equations

• Michaelis-Menten Equation: $v = \frac{V_{max}[S]}{K_m + [S]}$

HTML Table: Comparison of Cell Junctions

Junction Type	Location	Function
Tight Junction	Animal cells	Prevents leakage of extracellular fluid
Gap Junction	Animal cells	Allows passage of ions and small molecules
Desmosome	Animal cells	Anchors cells together
Plasmodesmata	Plant cells	Connects cytoplasm of adjacent cells

HTML Table: Types of Membrane Transport

Transport Type	Energy Required	Example
Simple Diffusion	No	O2 and CO2 across membrane
Facilitated Diffusion	No	Glucose via GLUT transporters
Active Transport	Yes (ATP)	Na+/K+ pump
Co-transport (Symport/Antiport)	Indirect (uses gradient)	Sodium-glucose symporter

Additional info:

• Some content was inferred and expanded for clarity and completeness, such as the detailed explanations of cell junctions, membrane transport, and enzyme regulation.

• Scientific terms and processes were defined and contextualized to ensure the notes are self-contained and suitable for exam preparation.