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

Cell Structure and Function

Cell Structure, Membranes, and Organelles

Understanding the relationship between cell structure and function is fundamental in biology. Cells are the basic units of life, and their internal organization determines their capabilities.

• Cell Structure: Refers to the physical components of a cell, including the plasma membrane, cytoplasm, and organelles.

• Cell Membrane: A selectively permeable barrier composed mainly of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It regulates the movement of substances in and out of the cell.

• Cell Wall: Found in plants, fungi, and some prokaryotes, providing structural support and protection.

• Eukaryotic Cell Organelles: Specialized structures within eukaryotic cells, such as the nucleus, mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, and cytoskeleton.

• Prokaryotic vs. Eukaryotic Cells: Prokaryotes lack membrane-bound organelles and a nucleus, while eukaryotes possess both.

• Specialized Cell Types: Cells such as macrophages or lymphocytes have unique structures and functions, often reflected in their abundance of specific organelles.

• Emergent Properties: When individual components of a system interact, new properties emerge that are not present in the individual parts (e.g., a functional organelle or cell).

Example: The mitochondrion is known as the "powerhouse" of the cell because it generates ATP through cellular respiration, a function that emerges from the coordinated activity of its proteins and membranes.

Cell Junctions and Communication

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

• Cell Junctions: Structures that connect cells to one another, including tight junctions, desmosomes, and gap junctions in animal cells, and plasmodesmata in plant cells.

• Function: These junctions facilitate communication, adhesion, and the passage of materials between cells.

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

Membrane Structure and Function

Fluid Mosaic Model and Membrane Dynamics

The plasma membrane is described by the fluid mosaic model, which highlights its dynamic and heterogeneous nature.

• Fluid Mosaic Model: The membrane is a fluid structure with a "mosaic" of various proteins embedded in or attached to a double layer of phospholipids.

• Membrane Fluidity: Influenced by lipid composition (saturated vs. unsaturated fatty acids), cholesterol content, and temperature.

• Asymmetry: The two leaflets of the bilayer have different lipid and protein compositions, contributing to membrane function.

Example: Cholesterol acts as a "fluidity buffer," preventing membranes from becoming too rigid or too fluid under temperature changes.

Transport Across Membranes

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

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

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (usually ATP).

• Bulk Transport: Endocytosis and exocytosis allow large molecules or particles to enter or exit the cell.

• Electrochemical Gradients: Membranes can generate gradients of ions, which are used for processes like nerve impulse transmission and ATP synthesis.

Example: The sodium-potassium pump ($Na^+/K^+$ ATPase) actively transports $3 Na^+$ ions out and $2 K^+$ ions into the cell, maintaining membrane potential.

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane, and tonicity describes the effect of a solution on cell volume.

• Isotonic Solution: No net movement of water; cell volume remains constant.

• Hypotonic Solution: Water enters the cell; cell may swell or burst.

• Hypertonic Solution: Water leaves the cell; cell shrinks.

Example: Plant cells become turgid in hypotonic solutions due to water influx, which is essential for structural support.

Cell Communication and Signal Transduction

Types of Cell Signaling

Cells communicate using chemical signals that can be classified based on the distance they travel and the type of target cell.

• Direct Contact: Via cell junctions or cell surface molecules.

• Paracrine Signaling: Local signaling to nearby cells.

• Endocrine Signaling: Hormones travel through the bloodstream to distant targets.

• Synaptic Signaling: Specialized form of signaling in neurons.

Example: Neurotransmitters released at synapses transmit signals between nerve cells.

Signal Transduction Pathways

Signal transduction involves converting an external signal into a functional response inside the cell.

• Reception: Signal molecule binds to a receptor protein.

• Transduction: Signal is relayed and amplified by intracellular signaling molecules (second messengers such as cAMP, Ca2+).

• Response: Activation of cellular processes, such as gene expression or metabolic changes.

Example: The binding of epinephrine to its receptor activates a cascade involving cAMP, leading to the breakdown of glycogen in liver cells.

Regulation of Signal Transduction

• Amplification: One signal molecule can activate many downstream molecules.

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

• Termination: Signals are terminated by degradation of signaling molecules or deactivation of receptors.

Example: Phosphodiesterase breaks down cAMP, terminating the signal.

Metabolism: Energy Transfer and Transformation

Thermodynamics in Biology

Biological systems obey the laws of thermodynamics, which govern energy transfer and transformation.

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

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

• Free Energy ($G$): The portion of a system's energy that can perform work. Changes in free energy ($\Delta G$) determine whether a reaction is spontaneous.

Equation:

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

• $\Delta G$ = change in free energy

• $\Delta H$ = change in enthalpy (total energy)

• $T$ = temperature in Kelvin

• $\Delta S$ = change in entropy

Exergonic Reaction: Releases energy ($\Delta G < 0$), spontaneous. Endergonic Reaction: Requires energy input ($\Delta G > 0$), non-spontaneous.

ATP and Energy Coupling

ATP (adenosine triphosphate) is the primary energy currency of the cell, coupling exergonic and endergonic reactions.

• ATP Hydrolysis: $ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

• The energy released is used to drive cellular work, such as muscle contraction and active transport.

Example: The sodium-potassium pump uses ATP hydrolysis to transport ions against their gradients.

Enzyme Structure and Function

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

• Active Site: The region of the enzyme where substrate binds and reaction occurs.

• Induced Fit: The enzyme changes shape to better fit the substrate upon binding.

• Cofactors and Coenzymes: Non-protein helpers required for enzyme activity.

Enzyme Kinetics: The rate of enzyme-catalyzed reactions can be affected by substrate concentration, temperature, pH, and inhibitors.

• Competitive Inhibition: Inhibitor competes with substrate for the active site.

• Noncompetitive Inhibition: Inhibitor binds elsewhere, changing enzyme shape and function.

Allosteric Regulation: Enzyme activity is regulated by molecules binding to sites other than the active site, which can activate or inhibit function.

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

Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Cell Size	Small (1-10 μm)	Larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Table: Types of Cell Junctions

Junction Type	Location	Function
Tight Junction	Animal cells	Prevents leakage of extracellular fluid
Desmosome	Animal cells	Anchors cells together
Gap Junction	Animal cells	Allows passage of ions and small molecules
Plasmodesmata	Plant cells	Channels for transport and communication

Additional info:

• Some explanations and examples were expanded for clarity and completeness.

• Tables were constructed to summarize key comparisons and classifications relevant to the study guide topics.