Chapter 10 (Cell Biology)
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What two things must organelles receive during growth and duplication?
New lipids for membrane expansion and new proteins (membrane + soluble) to fill and operate the organelle.
How do proteins “know” where to go inside the cell?
They contain sorting signals—short amino-acid sequences acting like an address label that directs them to the right organelle.
What do free polyribosomes make?
Proteins that stay in the cytosol or go to nucleus, mitochondria, or chloroplasts — not membrane or secretory proteins.
What do membrane-bound polyribosomes make?
Membrane, endomembrane-system, and secreted proteins — they evolved with internal membranes.
Where does synthesis of almost all proteins begin?
On cytosolic ribosomes, before sorting signals direct them elsewhere.
What happens to proteins without a sorting signal?
They stay in the cytosol.
Give an example of a sorting signal sequence.
The KDEL sequence (Lys-Asp-Glu-Leu) at the C-terminus retains proteins in the ER.
What are the three main mechanisms of protein transport into organelles?
I – Through nuclear pores (fully folded proteins). II – Via protein translocators across membranes (unfolded). III – By transport vesicles between endomembrane compartments.
How does the nuclear envelope get its lipids and proteins?
Through lateral (backfilling) movement along the continuous ER membrane monolayers.
What makes the nucleus a “protected compartment”?
It stores DNA and uses selective nuclear pores to tightly control what enters or leaves.
What kinds of proteins must enter the nucleus?
Soluble proteins such as DNA/RNA-binding proteins and polymerases that interact with DNA.
About how many proteins make up the NPC?
More than 30 different proteins.
What is the function of FG repeats in nuclear pore proteins?
They create a flexible mesh that blocks large molecules but allows small ones to pass.
What are the protruding structures on each side of a nuclear pore?
Protein fibrils, which form a basket-like structure on the nuclear side.
What size proteins can diffuse freely through pores?
Small hydrophilic molecules and proteins up to ~20–40 kDa.
What is a nuclear localization signal (NLS)?
A short amino-acid sequence that serves as a “ticket” + address for nuclear import.
What recognizes the NLS on a cargo protein?
A nuclear import receptor (NIR), also called an importin.
How does the NIR guide the protein through the pore?
It binds to cytosolic fibrils, then hops along FG repeats inside the pore until reaching the nucleus.
How does the cargo protein detach from its importin?
Inside the nucleus, Ran-GTP binds the importin, causing cargo release.
How does the importin return to the cytosol?
The importin–Ran-GTP complex exits; then Ran hydrolyzes GTP → GDP with RanGAP, freeing importin for reuse.
What drives nuclear import energetically?
The Ran-GTP gradient — high GTP in nucleus, low in cytosol = active transport.
Do proteins stay folded during nuclear transport?
Yes, they pass through pores fully folded, unlike mitochondrial/chloroplast import.
How fast can nuclear transport occur?
Up to ~1000 translocations per second per pore.