Unit 2 - Cells: Structure, Function, and Communication

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Lipids are a diverse group of hydrophobic molecules essential for cell structure and function.

Phospholipids: Major component of cell membranes, consisting of a hydrophilic head and two hydrophobic fatty acid tails.
Triglycerides: Storage form of energy, composed of glycerol and three fatty acids.
Steroids: Lipids with a characteristic four-ring structure; cholesterol is a key example in animal membranes.
Glycolipids: Lipids with attached carbohydrate groups, important for cell recognition.

Example: Cholesterol modulates membrane fluidity in animal cells.

Phospholipids form a bilayer, creating a semi-permeable barrier between the cell and its environment.
The amphipathic nature (hydrophilic head, hydrophobic tails) allows for membrane formation in aqueous environments.

The fluid mosaic model describes the membrane as a dynamic structure with proteins embedded in or attached to a phospholipid bilayer.
Integral proteins span the membrane, while peripheral proteins are attached to the membrane surface.
Proteins may associate with the membrane via hydrophobic interactions, lipid anchors, or electrostatic interactions.

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).

Example: The sodium-potassium pump is an example of active transport.

The membrane allows some substances to cross more easily than others.
Factors affecting permeability include lipid composition, presence of transport proteins, and size/charge of molecules.

Hypertonic solution: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink.
Hypotonic solution: Lower solute concentration outside; water enters the cell, causing it to swell or burst.
Isotonic solution: Equal solute concentration; no net water movement.

Example: Red blood cells placed in a hypotonic solution will swell and may lyse.

Prokaryotic cells: Lack a nucleus and membrane-bound organelles; DNA is in the nucleoid region (e.g., bacteria, archaea).
Eukaryotic cells: Have a nucleus and membrane-bound organelles (e.g., animals, plants, fungi, protists).

Nucleus: Stores genetic material and coordinates cell activities.
Mitochondria: Site of cellular respiration and ATP production.
Endoplasmic reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.
Golgi apparatus: Modifies, sorts, and packages proteins and lipids.
Lysosomes: Contain digestive enzymes for waste breakdown.
Chloroplasts (plants): Site of photosynthesis.

Additional info: Organelle abundance and specialization can influence cell function (e.g., muscle cells have many mitochondria).

Proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.
Evidence includes double membranes, their own DNA, and similarities to prokaryotes.

Proteins are synthesized in the cytosol or on the rough ER, then sorted to their destinations (e.g., lysosomes, plasma membrane) via vesicles.
Signal sequences direct proteins to specific organelles.

Microfilaments (actin filaments): Involved in cell shape, movement, and muscle contraction.
Intermediate filaments: Provide structural support and maintain cell shape.
Microtubules: Involved in cell division, intracellular transport, and cilia/flagella movement.

Animal ECM: Composed of proteins (e.g., collagen, elastin) and polysaccharides; provides structural support and mediates cell signaling.
Plant cell wall: Made of cellulose, hemicellulose, and pectin; provides rigidity and protection.

Animal cells: Connected by tight junctions, desmosomes, and gap junctions (allowing direct communication).
Plant cells: Connected by plasmodesmata, channels that allow exchange of molecules.

Lipid-soluble hormones (e.g., steroid hormones) pass through the membrane and bind to intracellular receptors.
Lipid-insoluble hormones (e.g., peptide hormones) bind to cell surface receptors, triggering signal transduction pathways.

Signal transduction involves converting an extracellular signal into a cellular response.
Common mechanisms include:
- G protein-coupled receptors (GPCRs): Activate intracellular G proteins, leading to second messenger production.
- Receptor tyrosine kinases (RTKs): Dimerize and autophosphorylate upon ligand binding, activating downstream pathways.
- Second messengers: Small molecules (e.g., cAMP, Ca2+) that amplify the signal within the cell.
Activation often leads to changes in gene expression or cellular activity.

Type of Cell Junction	Location	Function
Tight Junction	Animal cells	Prevents leakage of extracellular fluid
Desmosome	Animal cells	Anchors cells together
Gap Junction	Animal cells	Allows direct communication between cells
Plasmodesmata	Plant cells	Channels for molecular exchange