Cells, Transport & Cellular Energy — Problem set, Grade 7

Five focused units, a three-session investigation, chapter review, and chapter quiz.

Review the key ideas

Cell Theory & Discovering Cells

Cells are the smallest units of life. Some organisms consist of one cell; others contain many cells with specialized functions. Cell theory states that organisms are made of cells, cells are basic units of structure and function, and new cells come from existing cells. These statements connect observations across very different organisms rather than defining one animal or plant as the model for all life.

Cell Structures & Organelles

A cell is an interacting system with boundaries, internal material, genetic information, and structures that perform tasks. The membrane surrounds the cell and helps regulate exchanges. Cytoplasm contains material and structures inside the membrane. An organelle is a specialized structure within a cell; many familiar organelles occur in eukaryotes, while prokaryotes lack a membrane-enclosed nucleus.

Plant, Animal & Other Cell Types

Plant and animal cells are eukaryotic and share many structures, including a membrane, cytoplasm, ribosomes, mitochondria, and typically a nucleus. Comparing their structures explains some different functions, but a general diagram should not be mistaken for every cell in every tissue. Specialized cells can differ from the typical models.

Membrane Transport, Diffusion & Osmosis

Cells exchange materials across selective membranes. Diffusion is the net movement of particles from a region of higher concentration toward lower concentration through their random motion. Particles do not intentionally seek an even arrangement, and individual particles can move either way. The net pattern depends on concentrations and conditions.

Photosynthesis & Cellular Respiration

Photosynthesis stores energy by using light to support the production of sugar from carbon dioxide and water. In the plant model, chloroplasts perform this process and oxygen is released. Light is an energy input, not matter that becomes the carbon atoms in sugar. Carbon enters mainly from carbon dioxide, while water and other resources also matter.

Problem 1

Illustrative record: a living cell divides into two cells; each later divides once. A student claims the resulting four cells must be four times the original mass. Use cell theory to explain the number change and evaluate the mass claim.
Answer and explanation

The sequence fits new cells arising from existing cells and gives four cells after the two division rounds. The mass claim is unsupported: cells can differ in size, and growth requires material uptake. Cell count alone does not establish four times the starting mass. Additional mass or size observations would be needed.

Count one to two to four cells.

Connect new cells with previous cells dividing.

Distinguish cell production from total mass.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 2

Does cell theory require every organism to have many cells?
Answer and explanation

No. A single cell can be an organism; many organisms contain multiple cells.

Problem 3

Illustrative cell record: the membrane is damaged and substances leak outward; ribosomes remain visible. A draft says visible ribosomes prove the whole cell functions normally. Explain the membrane’s role, evaluate the draft, and identify a useful further observation.
Answer and explanation

The membrane helps maintain regulated exchanges and internal conditions. Leakage suggests disruption even though ribosomes remain visible. Presence of one structure does not prove normal system function. Measurements of exchange, internal conditions, and cell activity would help assess the extent of damage.

Locate the exchange boundary.

Connect uncontrolled leakage with disrupted internal conditions.

Separate visible presence from measured function.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 4

Why would a completely sealed boundary be a poor cell model?
Answer and explanation

It would prevent needed exchanges such as uptake of materials and removal of wastes.

Problem 5

Illustrative records: A has wall, enclosed nucleus, and chloroplasts; B has wall and DNA without an enclosed nucleus; C has membrane and enclosed nucleus without a wall. Identify categories supported by the combination of features and qualify the conclusions.
Answer and explanation

A fits the supplied photosynthetic plant model; B fits a prokaryotic cell; C fits a typical animal model. These are supported model comparisons, not a complete identification of species. Other organisms and specialized cells can share selected features, and microscope resolution and sample context matter.

A’s combination fits a photosynthetic plant-cell model.

B’s DNA location fits a prokaryotic model.

C fits a typical animal-cell model but needs sample context.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 6

What shared energy process occurs in both plant and animal cells?
Answer and explanation

Both can carry out cellular respiration; plant photosynthesis does not replace it.

Problem 7

Illustrative model: equal-pressure compartments A and B start with equal volumes; A has 5 nonpenetrating solute particles per volume unit and B has 10. Water can cross; solute cannot. Predict initial net water movement, identify what crosses, and explain why equilibrium need not mean no motion.
Answer and explanation

Initial net water movement tends from A toward B under these simplified conditions. Water crosses; the stipulated solute does not. This can change concentrations and pressures, limiting later flow. At equilibrium water molecules can still move both ways even when there is no net change. The conclusion depends on the stated permeability and pressure assumptions.

Compare nonpenetrating solute concentrations.

Apply the stipulated water permeability.

Separate net movement from individual crossings.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 8

Why can particle count alone fail to show concentration?
Answer and explanation

Concentration depends on both amount and volume; counts in unequal volumes are not directly comparable.

Problem 9

Illustrative sealed-chamber data for a short study: carbon dioxide reading begins at 400 units, falls to 350 under light, and rises to 370 during a later dark interval. Readings share one calibrated unit; plant condition and temperature are otherwise stated comparable. Calculate both changes and explain them through net processes without claiming either process was observed directly.
Answer and explanation

The light interval shows a 50-unit decrease, consistent with net photosynthetic uptake exceeding respiratory release. The dark interval shows a 20-unit increase, consistent with respiration without the same light-driven uptake. These net gas measurements do not separately measure each process; leaks, other organisms, and instrument conditions would need checking in an actual investigation.

Light change: 350−400=−50 units.

Dark change: 370−350=+20 units.

Compare photosynthetic uptake and respiratory release under the model.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 10

Where does much of a growing plant’s carbon come from?
Answer and explanation

Carbon dioxide taken from the surroundings supplies carbon used in organic molecules; light supplies energy rather than carbon matter.

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