Unit 2: Cell Structure and Function
Inside the cell: the organelles, membranes, and structures every living cell depends on to survive.
9 topics · 29 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Cell Theory and the Discovery of Cells
- How it worksHow did Schleiden, Schwann, and Virchow's separate observations combine into one cell theory?
- Common mistakeWhy isn't cell theory just a guess since nobody has watched every organism under a microscope?
- Going furtherWhat happens to cell theory's third claim when a new cell forms through division?
Your turn1 of 2
Robert Hooke first described cork's tiny box-like compartments in 1665, but a full theory that ALL organisms are made of cells didn't emerge until the 1830s. Why did that gap of over 150 years exist?Prokaryotic vs Eukaryotic Cells
- How it worksHow does a eukaryotic cell wall off its DNA inside a nucleus while a prokaryote leaves its DNA loose?
- Going furtherWhat changes about a cell's size limit once it evolves membrane-bound organelles?
- Common mistakeWhy isn't a prokaryotic cell just a simpler, incomplete version of a eukaryotic cell?
Your turn1 of 2
A bacterium has no nucleus, no mitochondria, and none of the membrane-bound organelles a human cell has, yet it survives and reproduces just fine. Why doesn't lacking those compartments cripple it?The Nucleus and Genetic Control
- How it worksHow do pores in the nuclear envelope let RNA out while keeping DNA safely inside?
- How it worksWalk me through how the nucleolus assembles the parts of a ribosome before they leave the nucleus.
- Common mistakeWhy doesn't the nucleus disappear completely once a cell finishes dividing?
Your turn1 of 2
A cell's DNA carries instructions that must be copied and read constantly, yet eukaryotic cells wall that DNA off inside a nucleus rather than leaving it loose in the cytoplasm. What does that separation actually protect?Ribosomes, the ER, and the Golgi Apparatus
- How it worksHow does a protein travel from the rough ER through the Golgi apparatus before leaving the cell?
- How it worksHow does the Golgi apparatus tag a finished protein with the address of where it needs to go?
- Common mistakeWhy isn't the smooth ER just a plain version of the rough ER with the ribosomes missing?
- Going furtherWhat happens to protein export once the Golgi apparatus stops working?
Your turn1 of 2
The smooth ER lacks the ribosomes that stud the rough ER's surface, and a student concludes the smooth ER is just an unfinished or incomplete version of the rough ER. What's wrong with that conclusion?Mitochondria and Chloroplasts
- How it worksHow does a mitochondrion's folded inner membrane give it more room to make ATP?
- Common mistakeWhy isn't a chloroplast just a mitochondrion that also happens to be green?
- Ties togetherHow do the reactions inside a chloroplast and inside a mitochondrion end up as opposite halves of one cycle?
Your turn1 of 2
Folding a mitochondrion's inner membrane into cristae increases its surface area by a factor of 5 compared to a smooth, unfolded membrane of area 29 square micrometers. What is the folded surface area?Lysosomes and Vacuoles
- How it worksHow does a lysosome break down worn-out organelles without digesting the whole cell?
- Going furtherWhat happens to a plant cell's shape when its central vacuole loses water?
- Common mistakeWhy doesn't a lysosome's own membrane get digested by the enzymes it carries?
Your turn1 of 2
A lysosome is packed with enzymes powerful enough to break down proteins, lipids, and worn-out organelles. Why doesn't that same lysosome digest itself along with everything it processes?The Cytoskeleton, Cilia, and Flagella
- How it worksHow do microtubules act like tracks that motor proteins walk organelles along?
- How it worksWalk me through how a cilium's beating whip pattern moves fluid past a cell's surface.
- Common mistakeWhy isn't the cytoskeleton a fixed, permanent scaffold like the beams inside a building?
Your turn1 of 2
A cell membrane alone is a flexible, fluid sheet with no rigidity of its own, yet a cell still holds a distinct shape instead of collapsing into a shapeless puddle. What actually keeps the cell's shape stable?Cell Size and Surface Area to Volume Ratio
- How it worksHow does a cell's surface area fall behind its volume as the cell gets larger?
- Worked exampleFind the surface area to volume ratio of a cube-shaped cell that is 2 micrometers on each side.
- Going furtherWhat happens to a cell's ability to get oxygen in and waste out once its volume outgrows its surface?
- Common mistakeWhy doesn't a bigger cell just grow a bigger nucleus to keep up with all the extra cytoplasm?
Your turn1 of 2
Find the surface area to volume ratio of a cube-shaped cell that is 6 micrometers on each side.Comparing Plant and Animal Cells
- How it worksHow does a rigid cell wall outside the membrane give a plant cell its fixed shape?
- Common mistakeWhy isn't a cell wall the same structure as a cell membrane?
- Ties togetherHow do chloroplasts and a central vacuole change what a plant cell can do that an animal cell can't?
Your turn1 of 2
A plant cell holds a rigid, boxy shape even when placed in pure water, while an animal cell stays round and can flex or squeeze through tight spaces. What structural difference actually explains this contrast?