Unit 1: Part 2 Notes: Cell Membrane & Enzymes
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Cell membrane is a phospholipid bilayer with embedded proteins and carbohydrate chains. Diagram features a protein channel and receptors on the outside.
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Function: separates cell components from environment, acts as a gatekeeper, and helps maintain homeostasis (stable internal balance).
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Structure: double-layered phospholipid bilayer; proteins embedded in the bilayer (channels and pumps); glycoproteins on the surface identify cells.
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Phospholipid heads are polar and hydrophilic; attracted to water (hydrophilic).
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Lipid tails are nonpolar and hydrophobic; face away from water.
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Phospholipids arrange into two layers facing each other due to water on both sides: the phospholipid bilayer.
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Transport/transmembrane proteins allow material movement; glycoproteins enable cell recognition and binding.
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Fluid Mosaic Model: plasma membrane is a dynamic mosaic of phospholipids, proteins (integral and transmembrane), cholesterol, and carbohydrates.
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Biomolecule functions in membrane:
Phospholipids: provide flexibility and a barrier
Cholesterol: embedded; reduces flexibility and permeability
Proteins: form channels and pumps
Carbohydrates: markers for cell recognition
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Permeability terms:
Permeable: allows passage
Impermeable: does not allow passage
Semi-permeable/Selectively Permeable: allows only certain substances
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Solution terms:
Solute: substance being dissolved
Solvent: dissolving medium
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Concentration: amount of a substance in an area; Concentation gradient: difference between areas.
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Types of transport:
Passive: no energy; moves with the gradient (high to low)
Active: requires energy; moves against the gradient (low to high)
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Passive transport: no energy; three types: Diffusion, Osmosis, Facilitated Diffusion.
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Diffusion: movement of particles from high concentration to low concentration along the gradient.
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Example: dye diffusing in water; perfume spreading in air.
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Gas exchange between lungs and bloodstream is driven by diffusion of CO₂ and O₂ across membranes.
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In alveoli, CO₂ diffuses out of blood and O₂ diffuses in; equilibrium approaches as gases move.
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Solution: solute dissolved in solvent (e.g., sugar in water).
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Osmosis: diffusion of water through a semipermeable membrane; water moves, solutes cannot pass freely; membrane is permeable to water but not to certain solutes.
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Water molecule can pass through the semipermeable cell membrane.
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Osmosis terms:
Hypertonic: high solutes, low solvent
Hypotonic: low solutes, high solvent
Isotonic: equal solute/solvent
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Water moves toward higher solute concentration due to the concentration gradient.
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Osmosis effects on cells:
Hypotonic: water enters → cell swells or bursts
Isotonic: cell remains normal
Hypertonic: water leaves → cell shrinks
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Red blood cell and plant cell responses: cytolysis (cell bursts) in hypotonic solutions and plasmolysis (cell shrivels) in hypertonic solutions.
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Further examples: water moves into or out of cells; in salt water, cells lose water.
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Facilitated diffusion: diffusion with help from membrane proteins (channels or carriers); no energy required; larger openings for molecules like glucose.
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Example: glucose moves from outside to inside via a carrier/channel protein without energy input: glucose (high outside) → inside.
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Visual example of carrier-mediated transport across the plasma membrane.
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Carrier proteins span the membrane and assist specific molecules to cross.
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Active Transport: requires energy (ATP); moves substances from low to high (against gradient); includes Endocytosis and Exocytosis.
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Pumps use ATP to move ions/molecules against the gradient (low to high).
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Example A: plants uptake of minerals by active transport; outside soil concentrations are often low; pumps move Ca²⁺ into cells using ATP.
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Energy use in active transport enables transport against gradients across membranes.
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Example B: CO₂ is pumped from cells into surrounding blood to be carried to lungs; requires energy to move against gradient when necessary.
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Endocytosis/Exocytosis: mechanisms for very large molecules.
Endocytosis: into cell
Exocytosis: out of cell
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Pinocytosis: liquid uptake; vesicles form.
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Exocytosis: vesicles fuse with the cell membrane to release contents outside.
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Exocytosis steps: vesicle moves to surface and fuses with membrane, releasing contents.
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Sodium-Potassium Pump: moves in and out; essential for neurons and muscle cells.\$2K^{+}\$ in, $\$3Na^{+}\$ out
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Flow chart: Homeostasis is maintained by the cell membrane; Passive Transport (no energy) includes Diffusion, Osmosis, Facilitated Diffusion; Active Transport (requires energy) includes Endocytosis and Exocytosis.
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Chemical Reactions
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Chemical Reactions: Reactant(s) enter; products are produced.
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Dehydration Synthesis: builds polymers by removing water; Monomer1 + Monomer2 → Polymer +
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Hydrolysis: breaks bonds by adding water; Polymer + → Monomer1 + Monomer2
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Hydrolysis and Dehydration Synthesis are fundamental to polymer cycling in biology.
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Enzymes: proteins that act as catalysts by reducing the activation energy of a chemical reaction.
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Activation Energy: energy required for a reaction to occur; enzymes speed up reactions by lowering this barrier.
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Why enzymes matter: without them, many life-sustaining processes would be too slow; enzymes increase reaction rates.
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How to recognize enzymes: many end with -ase (e.g., polymerase, ligase, helicase).
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Substrate and Active Site: substrate binds to the enzyme at the active site.
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Enzyme-Substrate complex: specificity; lock-and-key model; structure determines function.
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Enzyme efficiency: optimal temperature and pH maximize activity; graph shows activity vs temperature/pH; enzymes have narrow optima.
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Denaturation: if temperature or pH deviates too far from optimum, enzyme loses shape and function.
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Amoeba Sisters: Enzymes overview resource.