Cells & Cellular Organization — Science

  1. Grade 7 — Cell Theory & Discovering Cells
  2. Grade 7 — Cell Structures & Organelles
  3. Grade 7 — Plant vs. Animal Cells
  4. Grade 7 — Levels of Organization: Cell to Organism
  5. Grade 10 — Cell Theory & the Discovery of Cells

    ↑ Deepens: cell theory revisited with the tools of high-school biology.

  6. Grade 10 — Prokaryotic & Eukaryotic Cells & Their Organelles
  7. Grade 10 — The Cell Membrane
  8. Grade 10 — Cellular Transport
  9. Grade 10 — Homeostasis & Feedback

Apply it in a new setting

Problem 1

A cell is shaped like a cube. Compare a small cube 2 units on each edge with a larger cube 4 units on each edge. For the LARGER (4-unit) cube, what is its surface-area-to-volume ratio? (Surface area of a cube = 6 × edge²; volume = edge³.)
Hints

Work out the surface area and the volume of the 4-unit cube separately, then divide.

Surface area = 6 × 4² ; volume = 4³.

Ratio = surface area ÷ volume.

Answer and explanation

1.5

Surface area = 6 × 4² = 6 × 16 = 96 square units.

Volume = 4³ = 64 cubic units.

Ratio = 96 ÷ 64 = 1.5 (down from 3 for the 2-unit cube — as a cell grows it has relatively less surface to exchange materials, which is why cells stay small or divide).

Problem 2

A plant cell is placed in a beaker of distilled (pure) water. Predict what happens and why.
  1. Water enters the cell by osmosis; the cell swells and becomes firm (turgid), but the rigid cell wall keeps it from bursting.
  2. Water leaves the cell and it shrivels, because water always moves out of a cell into pure water.
  3. Water rushes in and the cell bursts, exactly as an animal cell would in the same beaker.
  4. Nothing happens, because distilled water and the cell's cytoplasm are isotonic.
Hints

Compare the solute concentration inside the cell with the pure water outside — which way does water move by osmosis?

Water moves toward the MORE concentrated (more solute) solution. Where is that here?

Then ask what a plant cell has that an animal cell does not, that stops it bursting.

Answer and explanation

Water enters the cell by osmosis; the cell swells and becomes firm (turgid), but the rigid cell wall keeps it from bursting.

The cytoplasm holds solutes while distilled water has essentially none, so the cell is hypertonic to the water and water moves IN by osmosis.

The cell swells, but the rigid cell wall resists further expansion, making the cell turgid instead of bursting (an animal cell, lacking a wall, would burst).

Water in, wall prevents bursting, therefore A.

Problem 3

A cell is moving glucose INTO itself, from a region of low glucose outside to high glucose inside. A poison then shuts down the cell's ATP production. Predict what happens to this glucose movement, and why.
  1. It continues at the same rate, because glucose always diffuses freely across membranes.
  2. It speeds up, because losing ATP makes the membrane more permeable to glucose.
  3. It stops, because moving glucose from low to high concentration is active transport, which requires ATP.
  4. It reverses and glucose leaves, because diffusion now pushes it outward.
Hints

Notice the direction: glucose is going from LOW to HIGH concentration. Is that with or against the gradient?

Movement against a concentration gradient cannot happen by passive diffusion — what does it require?

If that energy source is cut off, what must happen to the uphill movement?

Answer and explanation

It stops, because moving glucose from low to high concentration is active transport, which requires ATP.

Glucose is moving against its concentration gradient (low → high), which is active transport.

Active transport is powered by ATP; with ATP production shut down, the pump has no energy.

So the uphill movement stops, therefore C.

Problem 4

A gland cell is specialized to manufacture and export large amounts of a protein enzyme. Which combination of organelles would you expect this cell to contain in unusually large numbers?
  1. Chloroplasts and a large central vacuole.
  2. Ribosomes, rough endoplasmic reticulum, and Golgi apparatus.
  3. Lysosomes and a cell wall.
  4. Smooth endoplasmic reticulum only.
Hints

Trace the pathway a protein takes from being built to being shipped out of the cell.

Which organelle builds proteins? Which one modifies and packages them for export?

Pick the set that covers make → modify → package of proteins.

Answer and explanation

Ribosomes, rough endoplasmic reticulum, and Golgi apparatus.

Proteins are built on ribosomes, many studding the rough ER; the rough ER folds them and the Golgi modifies and packages them for secretion.

Chloroplasts (choice A) do photosynthesis and smooth ER (choice D) mainly makes lipids — neither fits a protein-exporting cell.

The protein-secretion pathway needs ribosomes, rough ER, and Golgi, therefore B.

Problem 5

A biologist describes a newly found single-celled organism: it has NO nucleus and no membrane-bound organelles, but it does have a cell wall and one circular loop of DNA floating in the cytoplasm. What is the best conclusion?
  1. It is a eukaryote, because having a cell wall is a eukaryotic feature.
  2. It must be a plant cell, since only plant cells have cell walls.
  3. It cannot be a living cell, because every living cell must contain a nucleus.
  4. It is a prokaryote; its single circular DNA sits in the cytoplasm (a nucleoid region) rather than in a nucleus.
Hints

The most decisive clue is what the cell LACKS: a nucleus and membrane-bound organelles.

Cell walls are not unique to eukaryotes or plants — bacteria have them too.

Which cell type keeps its DNA loose in the cytoplasm instead of inside a nucleus?

Answer and explanation

It is a prokaryote; its single circular DNA sits in the cytoplasm (a nucleoid region) rather than in a nucleus.

No nucleus and no membrane-bound organelles are the defining features of prokaryotic cells.

Cell walls occur in bacteria, plants, and fungi, so a wall alone proves neither eukaryote nor plant; and a cell need not have a nucleus at all.

A wall-bearing cell with circular DNA and no nucleus is a prokaryote, therefore D.

Problem 6

Cell theory states that all living things are made of one or more cells, and that all cells arise from pre-existing cells. Which NEW observation would most directly CHALLENGE cell theory?
  1. Discovering a previously unknown organelle inside a familiar cell.
  2. Finding a living organism that carries out all life processes but is not made of cells at all.
  3. Finding a single cell far larger than any measured before.
  4. Watching one cell divide to form two new cells.
Hints

A challenge is an observation that would CONTRADICT one of the theory's claims, not just add a detail.

Re-read the two claims: everything living is cellular, and cells come only from other cells.

Which option would violate one of those two claims?

Answer and explanation

Finding a living organism that carries out all life processes but is not made of cells at all.

A non-cellular living organism would directly violate the claim that all living things are made of cells.

A new organelle, a giant cell, and cell division are all consistent with (or even support) cell theory.

Only the non-cellular organism contradicts it, therefore B.

Problem 7

After a sugary snack, your blood glucose rises. The pancreas releases insulin, body cells take up glucose, and blood glucose falls back to its normal level. This regulation is best described as:
  1. positive feedback, because the response happens in reaction to the rise in glucose.
  2. an acquired trait, because it depends on what food you happened to eat.
  3. negative feedback, because the response reverses the original change and returns glucose toward a set point.
  4. simple diffusion, because glucose just spreads out evenly on its own.
Hints

Ask whether the response amplifies the change or cancels it out.

Negative feedback opposes a change and restores a set point; positive feedback pushes it further.

Here glucose went up and the response brought it back DOWN — which is that?

Answer and explanation

negative feedback, because the response reverses the original change and returns glucose toward a set point.

The rise in glucose triggers insulin, which lowers glucose again — the response opposes and cancels the original change.

Opposing a change to restore a set point is the definition of negative feedback (positive feedback would drive glucose even higher).

This is negative feedback, therefore C.

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