Cells, Transport & Cellular Energy — Chapter Practice, Grade 7

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

Problem 1

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 2

Why is cork a limited model of a living cell?
Answer and explanation

Much of the living content is absent; the observed compartments mainly show remaining cell walls.

Problem 3

What other information is needed to infer mass from cell count?
Answer and explanation

Cell sizes and composition matter. More cells do not supply a complete mass measurement by themselves.

Problem 4

Why can a bacterium be an organism without organs?
Answer and explanation

One cell can carry out the required interacting life processes; organs are arrangements in some multicellular organisms.

Problem 5

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 6

A newly observed unicellular organism has no tissue. Does that contradict cell theory?
Answer and explanation

No. Unicellular organisms consist of one cell that carries out life processes. Tissues are not required in every organism.

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 7

Which fits cell theory?
  1. Every cell has chloroplasts
  2. All organisms contain many cells
  3. New cells arise from existing cells
  4. Every living cell has an enclosed nucleus
Answer and explanation

New cells arise from existing cells

Cell theory includes cells arising from existing cells; the other statements are not universal.

Problem 8

Starting with two cells, each divides once and then each resulting cell divides once. How many cells result?
Answer and explanation

8 cells

Two becomes four, then eight, assuming every stipulated division occurs.

Problem 9

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 10

Does a nucleus itself assemble all proteins?
Answer and explanation

No. Ribosomes build proteins using cellular instructions; the nucleus stores much genetic information in a typical eukaryotic cell.

Problem 11

Why is “mitochondria create energy” misleading?
Answer and explanation

They help transfer energy from food into usable forms; energy is not created.

Problem 12

Why is labeling organelles not a complete explanation of cell function?
Answer and explanation

An explanation also connects each structure with its role and interactions in the functioning system.

Problem 13

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 14

A bacterium has DNA and ribosomes but no nucleus. Can it still make proteins?
Answer and explanation

Yes. Ribosomes can use cellular instructions without an enclosed nucleus; the arrangement differs from the typical eukaryotic-cell 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 15

Which structure builds proteins?
  1. Cell wall
  2. Ribosome
  3. Scale bar
  4. Whole organ system
Answer and explanation

Ribosome

Ribosomes assemble proteins using cellular instructions.

Problem 16

Why do plant cells need usable energy even though they photosynthesize?
Answer and explanation

Building, transporting, repairing, and maintaining cellular conditions require energy transfers. Photosynthesis stores energy in food; respiration releases energy into usable forms.

Problem 17

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 18

What provides selective exchange in a plant cell?
Answer and explanation

The membrane, not simply the rigid cell wall. The wall contributes support and protection.

Problem 19

Does absence of chloroplasts prove a root cell is an animal cell?
Answer and explanation

No. Many plant cells are not photosynthetic; other structures and tissue context help identify them.

Problem 20

Why can a wall alone not identify a plant?
Answer and explanation

Other organisms, including many fungi and bacteria, also have walls. Additional features and context are needed.

Problem 21

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 22

A bulb cell has a wall and nucleus but no visible chloroplasts. Must the observation be wrong?
Answer and explanation

No. Many plant tissues lack chloroplasts, and visible detail also depends on the instrument. Use tissue context and additional features.

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 23

Which statement is accurate?
  1. Every root cell photosynthesizes
  2. Plant cells can respire
  3. Fungi are animals because they lack chloroplasts
  4. Walls replace membranes
Answer and explanation

Plant cells can respire

Plant cells release usable energy through respiration; a wall does not replace selective membrane exchange.

Problem 24

Why might a leaf cell and root cell have different visible structures?
Answer and explanation

Their functions differ. Many leaf cells photosynthesize and contain chloroplasts; many root cells obtain and transport materials without photosynthesis.

Problem 25

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 26

Does diffusion equilibrium mean all particles stop?
Answer and explanation

No. Motion continues; opposing movements can balance so there is no net concentration change.

Problem 27

What must be known before predicting osmosis?
Answer and explanation

Which materials can cross, their concentrations, and relevant pressure or other conditions; a membrane is not universally permeable.

Problem 28

Is movement of a dissolved particle always osmosis?
Answer and explanation

No. Osmosis describes net water movement across a selective membrane; other particles and processes require their own explanation.

Problem 29

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 30

A membrane is freely permeable to the solute in a second model. Can the previous nonpenetrating-solute prediction be copied unchanged?
Answer and explanation

No. Solute redistribution changes the relevant conditions. Determine permeability and concentrations before predicting water movement.

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 31

What moves in the simple osmosis model?
  1. Only cell walls
  2. Water across the selective membrane
  3. Every solute equally
  4. Whole organ systems
Answer and explanation

Water across the selective membrane

Osmosis concerns net water movement across a selectively permeable membrane.

Problem 32

A model compartment has 12 solute units in 3 volume units. Find concentration.
Answer and explanation

4 solute units/volume unit

12/3=4 in the stated model units.

Problem 33

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.

Problem 34

Why does a root without chloroplasts still need sugar?
Answer and explanation

It uses organic material and releases usable energy through cellular processes such as respiration.

Problem 35

Does falling carbon dioxide around a lit plant prove it stopped respiring?
Answer and explanation

No. Photosynthetic uptake can exceed respiration while both continue; the measurement is a net change.

Problem 36

Why is “energy cycles just like carbon” misleading?
Answer and explanation

Carbon atoms can be reused in cycles, while usable energy flows through processes and ultimately dissipates as thermal energy.

Problem 37

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 38

A student says plants use respiration only at night. Correct the claim.
Answer and explanation

Plants respire in light and darkness. Photosynthesis can occur in suitable light at the same time, so a net gas change must account for both processes.

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 39

Which organism can both photosynthesize and respire?
  1. A typical green plant
  2. Only an animal
  3. A rock
  4. A scale bar
Answer and explanation

A typical green plant

Green plants can perform both; the processes have different roles.

Problem 40

A gas reading changes from 300 to 260 units. Find the signed change.
Answer and explanation

-40 units

Final minus initial: 260−300=−40 units.

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