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Cell Biology Organelles and Membrane Transport Cheat Sheet
A printable reference covering organelles, protein targeting, vesicle trafficking, membrane transport, diffusion, osmosis, and membrane potential for college.
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Cell biology connects organelle structure, membrane organization, and transport processes to explain how eukaryotic cells maintain internal order. This cheat sheet helps college biology students review how proteins reach the correct compartments, how molecules cross membranes, and how ion gradients create electrical signals. These ideas are essential for understanding metabolism, secretion, cell signaling, nerve function, and disease mechanisms. The core concepts include the endomembrane system, protein targeting signals, vesicle budding and fusion, passive and active transport, and electrochemical gradients. Important relationships include Fick's law for diffusion, osmotic water movement, the Nernst equation, and the Goldman equation. A strong grasp of these topics helps students predict whether a solute will move, which membrane protein is required, and how ion movement affects membrane voltage.
Key Facts
- The nucleus stores genomic DNA, and proteins with a nuclear localization signal enter through nuclear pores using importins and Ran-GTP.
- Secreted, lysosomal, plasma membrane, and many organelle membrane proteins usually enter the rough endoplasmic reticulum through an ER signal peptide and the SRP pathway.
- Vesicle trafficking depends on coat proteins for budding, Rab proteins for targeting, and SNARE proteins for membrane fusion.
- Simple diffusion moves small nonpolar molecules down their concentration gradient without ATP, and the net flux can be described by J = -D(dC/dx).
- Facilitated diffusion moves solutes down their electrochemical gradient through channels or carriers and does not directly require ATP.
- Primary active transport uses ATP directly, such as the Na+/K+ ATPase, which moves 3 Na+ out and 2 K+ in per ATP hydrolyzed.
- Secondary active transport uses the downhill movement of one solute to drive the uphill movement of another, as in Na+-glucose symport.
- For one ion at equilibrium, the Nernst equation is Eion = (RT/zF) ln([ion]outside/[ion]inside), and at 37 degrees Celsius it is often approximated as Eion = 61.5 mV/z log([out]/[in]).
Vocabulary
- Signal sequence
- A short amino acid sequence that directs a newly made protein to a specific cellular location or membrane system.
- Endomembrane system
- The connected network of the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, endosomes, vesicles, and plasma membrane.
- Facilitated diffusion
- Passive movement of a solute down its gradient through a specific membrane channel or carrier protein.
- Electrochemical gradient
- The combined effect of a concentration difference and an electrical voltage difference across a membrane.
- Membrane potential
- The voltage difference across a membrane caused mainly by unequal ion distributions and selective membrane permeability.
- Osmosis
- The net movement of water across a selectively permeable membrane toward the side with higher effective solute concentration.
Common Mistakes to Avoid
- Confusing passive transport with transport through any protein is wrong because channels and carriers can be passive or active depending on whether solutes move down or against their electrochemical gradients.
- Assuming all membrane proteins are made on free ribosomes is wrong because many secreted and membrane proteins enter the rough ER during translation through the SRP pathway.
- Treating concentration gradient and electrochemical gradient as the same thing is wrong because charged ions are affected by both concentration differences and membrane voltage.
- Forgetting the ion charge in the Nernst equation is wrong because z changes both the sign and size of the equilibrium potential.
- Saying water always moves toward higher total solute is incomplete because only nonpenetrating or effectively impermeable solutes create sustained osmotic water movement.
Practice Questions
- 1 A cell has [K+]inside = 140 mM and [K+]outside = 5 mM. Using Eion = 61.5 mV/z log([out]/[in]) at 37 degrees Celsius, estimate the K+ equilibrium potential.
- 2 A transporter moves 2 Na+ ions into a cell down their gradient while moving 1 glucose molecule into the cell against its gradient. Is this primary active transport, secondary active transport, or facilitated diffusion?
- 3 A solute has a membrane permeability of 2 x 10^-5 cm/s, outside concentration of 10 mM, and inside concentration of 2 mM. Using flux proportional to P(Coutside - Cinside), what is the proportional net driving difference and direction of movement?
- 4 A protein lacks an ER signal peptide but has a nuclear localization signal. Explain where it is most likely synthesized and how it reaches its destination.