Biology: Cell Structure and Organelles Quiz

The mitochondrion runs the Krebs cycle and oxidative phosphorylation, which produce most of a cell's ATP. The Golgi apparatus modifies and packages proteins; it does not make energy.

The nucleolus is a dense region of the nucleus that transcribes rRNA and assembles ribosome subunits. The finished subunits leave through nuclear pores and become the ribosomes you see in the cytoplasm.

Plant cells have chloroplasts (plus a cell wall and usually a large central vacuole). A common mistake is thinking plants have chloroplasts instead of mitochondria: plant cells have both, because they also do cellular respiration.

Prokaryotes (bacteria and archaea) keep their DNA in a nucleoid region with no surrounding membrane. They still have DNA, a plasma membrane and ribosomes (smaller 70S ribosomes).

Smooth ER has no ribosomes; it makes lipids and steroids, stores calcium and detoxifies drugs (liver cells have lots of it). Making secreted proteins is the job of rough ER, whose ribosomes give it the rough look.

Lysosomes contain hydrolytic enzymes that digest old organelles, macromolecules and material taken in by endocytosis. Lysosomal storage diseases such as Tay-Sachs happen when one of these enzymes is missing.

Phospholipids arrange tails-in, heads-out to form the bilayer. Cholesterol sits between them to adjust fluidity, and proteins float in it like tiles in a mosaic. Cellulose is part of the plant cell wall, not the membrane.

Hypertonic means the outside has a higher solute concentration, so water moves out by osmosis and the cell shrivels. Swelling and bursting (lysis) happen in a hypotonic solution, the opposite case.

Primary active transport such as the Na+/K+ pump hydrolyzes ATP to move ions against their concentration gradients. Facilitated diffusion uses a protein but no energy, because molecules move down their gradient.

The pump exports 3 sodium ions and imports 2 potassium ions per ATP. Because more positive charge leaves than enters, it helps keep the inside of the cell negative (the resting membrane potential).

Surface area = . Volume = . Ratio = 24 / 8 = 3 : 1. A 4 µm cube gives 96 / 64 = 1.5 : 1, which shows why bigger cells have relatively less membrane for exchange and why cells stay small.

DNA is copied during the S (synthesis) phase of interphase. G1 is growth before replication and G2 is preparation for mitosis; by prophase the chromosomes have already been copied and are condensing.

In anaphase the cohesin holding sister chromatids together is cut and spindle fibers pull them apart. In metaphase they are still joined and lined up at the cell's equator.

Mitosis produces two genetically identical diploid cells, each with the same 46 chromosomes as the parent. Halving to 23 happens only in meiosis, when gametes are made.

Homologous chromosomes pair up (synapsis) and swap segments in prophase I. By meiosis II the homologs are already in separate cells, so there is no homolog to cross over with.

Mitochondria resemble free-living bacteria: circular DNA, small ribosomes, two membranes, and they divide by a fission-like process. Prokaryotes do not have mitochondria, which is part of why the idea is that an ancestral cell engulfed one.

Secreted proteins are made by ribosomes on the rough ER, travel in vesicles to the Golgi for modification and sorting, then leave in secretory vesicles that fuse with the plasma membrane (exocytosis).

Spindle fibers are microtubules that grow from the centrosomes and attach to kinetochores. Actin microfilaments form the cleavage furrow during cytokinesis in animal cells, which is a different step.

False. Viruses carry DNA or RNA but have no cytoplasm, ribosomes or metabolism, and they can reproduce only inside a host cell. Cell theory does not count them as cells.