Cell Structure, Function, and Signaling: General Biology Study Guide

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Diffusion and osmosis are fundamental processes for the movement of molecules across cell membranes, crucial for maintaining cellular homeostasis.

Diffusion: Movement of molecules from areas of high concentration to areas of low concentration, driven by a concentration gradient.
Osmosis: A specific type of diffusion involving water molecules across a selectively permeable membrane. Water moves from areas of lower solute concentration to higher solute concentration.
Example: Plant cells placed in pure water will swell due to water entering by osmosis; in salty water, they may shrink as water leaves the cell.

Additional info: Osmosis is vital for processes such as nutrient absorption and waste removal in cells.

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

Composition: Phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.
Functions:
- Maintains homeostasis by regulating entry and exit of molecules.
- Facilitates communication and signaling between cells.
- Provides structural support and protection.
Example: Transport proteins in the membrane allow glucose to enter cells for energy production.

Organelles are specialized structures within cells that perform distinct functions necessary for cellular survival and activity.

Nucleus: Contains genetic material (DNA); site of transcription and replication.
Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.
Mitochondria: Site of cellular respiration; produces ATP (energy currency of the cell).
Chloroplasts: Found in plant cells; site of photosynthesis.
Cell Wall: Provides rigidity and protection in plant, fungal, and bacterial cells (not found in animal cells).

Additional info: Eukaryotic cells have membrane-bound organelles, while prokaryotic cells do not.

Cells use various mechanisms to transport molecules across membranes, maintaining internal balance and responding to environmental changes.

Passive Transport: Includes diffusion and osmosis; does not require energy.
Facilitated Diffusion: Movement of molecules via transport proteins; still passive.
Active Transport: Requires energy (usually ATP) to move substances against their concentration gradient.
Example: Sodium-potassium pump in animal cells maintains electrochemical gradients.

Equation:

Additional info: Active transport is essential for nerve impulse transmission and muscle contraction.

Cells are connected and supported by specialized structures that facilitate communication and maintain tissue integrity.

Cell Junctions:
- Tight Junctions: Prevent leakage of materials between cells.
- Desmosomes: Anchor cells together, providing mechanical strength.
- Gap Junctions: Allow direct communication between adjacent cells.
Extracellular Matrix (ECM): Network of proteins and carbohydrates outside cells; provides structural support and regulates cell behavior.
Example: Collagen fibers in the ECM give skin its strength and elasticity.

Viruses and bacteria are both microscopic agents, but they differ fundamentally in structure and function.

Bacteria: Living prokaryotic cells; can reproduce independently; some are beneficial, others pathogenic.
Viruses: Non-living infectious particles; require host cells to reproduce; consist of genetic material (DNA or RNA) surrounded by a protein coat.
Example: Escherichia coli is a common bacterium found in the human gut; influenza virus causes the flu.

Viruses infect host cells and hijack their machinery to replicate, often causing disease.

Retroviruses: Use reverse transcriptase to convert RNA into DNA inside host cells.
Antibiotic Resistance: Bacteria can evolve resistance to antibiotics, posing challenges for treatment.
Plasmids: Small DNA molecules in bacteria that can carry antibiotic resistance genes.

Additional info: Plasmids are used in biotechnology for gene cloning and genetic engineering.

Cell signaling involves the transmission of signals from the environment or other cells to elicit specific cellular responses.

Signal Molecules: Hormones, neurotransmitters, and other molecules that bind to receptors on target cells.
Receptors: Proteins on the cell surface or inside the cell that recognize and bind signal molecules.
Signal Transduction: The process by which a signal is converted into a cellular response, often involving a cascade of molecular events.
Second Messengers: Small molecules (e.g., cAMP) that amplify the signal within the cell.
Example: Insulin signaling regulates glucose uptake in cells.

Equation:

Additional info: Quorum sensing in bacteria allows populations to coordinate behavior based on cell density.

Feature	Prokaryotic Cells	Eukaryotic Cells
Size	Smaller	Larger
Nucleus	Absent	Present
Organelles	None (except ribosomes)	Membrane-bound organelles
Cell Wall	Present (peptidoglycan)	Present in plants/fungi (cellulose/chitin)
Reproduction	Binary fission	Mitosis/meiosis
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists