Unit 2 - Cells: Lipids, Membranes, Cell Structure, and Communication

Study Guide - Smart Notes

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Lipids are a diverse group of hydrophobic molecules, including fats (triglycerides), phospholipids, and steroids.
Phospholipids are the primary lipid component of cell membranes, while cholesterol (a steroid) is also present in animal cell membranes.
Fats serve mainly as energy storage, while waxes and glycolipids have structural or signaling roles.
Example: Phosphatidylcholine is a common phospholipid in eukaryotic membranes.

The major lipid in cell membranes is the phospholipid, which is amphipathic (having both hydrophilic and hydrophobic regions).
This property allows phospholipids to form bilayers, creating a selective barrier between the cell and its environment.
Cholesterol modulates membrane fluidity and stability in animal cells.

The plasma membrane is organized as a phospholipid bilayer with embedded proteins.
Integral (transmembrane) proteins span the membrane and often function as channels or transporters.
Peripheral proteins are attached to the membrane surface and may play roles in signaling or structural support.
The chemistry of amino acid side chains (hydrophobic vs. hydrophilic) determines how proteins associate with the membrane.

Passive transport does not require energy and moves substances down their concentration gradient (e.g., diffusion, facilitated diffusion, osmosis).
Active transport requires energy (usually from ATP) to move substances against their concentration gradient.
Example: The sodium-potassium pump is an active transporter in animal cells.

Selective permeability means the membrane allows some substances to cross more easily than others.
Factors influencing permeability include lipid composition, presence of transport proteins, temperature, and molecule size/charge.
Small, nonpolar molecules (e.g., O2, CO2) cross easily; ions and large polar molecules require transport proteins.

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, cell remains stable.

Solution Type	Relative Solute Concentration	Effect on Animal Cell
Hypertonic	Higher outside	Shrinks (crenation)
Hypotonic	Lower outside	Swells/bursts (lysis)
Isotonic	Equal	No change

Prokaryotic cells (e.g., bacteria, archaea) lack a nucleus and membrane-bound organelles.
Eukaryotic cells (e.g., plants, animals, fungi, protists) have a nucleus and various organelles.
Prokaryotes are generally smaller and structurally simpler.

Nucleus: Contains genetic material (DNA), site of transcription.
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: Digestive organelles containing hydrolytic enzymes.
Chloroplasts: (in plants/algae) Site of photosynthesis.
Organelle composition influences cell function (e.g., muscle cells have many mitochondria).

Proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.
Evidence: Double membranes, their own DNA, and ribosomes similar to bacteria.

Proteins are directed to specific cellular locations by signal sequences in their amino acid chains.
Proteins destined for the nucleus have a nuclear localization signal (NLS).
Proteins for secretion or the endomembrane system enter the ER during translation and are trafficked via vesicles.

Microfilaments (actin filaments): Support cell shape, involved in movement.
Intermediate filaments: Provide mechanical strength.
Microtubules: Involved in cell division, intracellular transport, and structure (e.g., cilia, flagella).

Animal ECM: Composed mainly of proteins (collagen, elastin) and polysaccharides (proteoglycans).
Plant ECM (cell wall): Composed of cellulose, hemicellulose, and pectin.
ECM provides structural support and mediates cell signaling.

Animal cells: Connected by tight junctions, desmosomes, and gap junctions (allowing direct communication).
Plant cells: Connected by plasmodesmata (channels through cell walls for cytoplasmic exchange).

Lipid-soluble hormones (e.g., steroid hormones) can cross the plasma membrane and bind to intracellular receptors.
Lipid-insoluble hormones (e.g., peptide hormones) bind to cell surface receptors, triggering signal transduction pathways.
Both types regulate gene expression and cellular responses, but via different mechanisms.

Signal transduction is the process by which a cell converts an extracellular signal into a functional response.
Major mechanisms include:
- G protein-coupled receptors (GPCRs): Activate intracellular G proteins, which trigger downstream signaling cascades.
- Receptor tyrosine kinases (RTKs): Dimerize and autophosphorylate, activating signaling proteins inside the cell.
- Second messengers (e.g., cAMP, Ca2+): Amplify and propagate the signal within the cell.
Activation often involves ligand binding, receptor conformational change, and phosphorylation events.