The Cell Membrane — Practice · Grade 10 Science

Chapter: Cells, Membranes & Exchange

What this practice covers

By the end of The Cell Membrane you should be able to:

10 questions, multiple choice and written answer, each with a worked explanation.

Vocabulary used: phospholipid, hydrophilic, hydrophobic, phospholipid bilayer, fluid mosaic model, selective permeability, membrane protein.

Problem 1

In a phospholipid, the phosphate head is ___ and the fatty-acid tails are ___.
  1. hydrophilic; hydrophobic
  2. hydrophobic; hydrophilic
  3. both hydrophilic
  4. both hydrophobic
Answer and explanation

hydrophilic; hydrophobic

The phosphate head is water-loving (hydrophilic) and the fatty-acid tails are water-fearing (hydrophobic) — the split personality that lets a bilayer form.

Problem 2

In the fluid mosaic model, what does the word 'mosaic' describe?
  1. The rigid, crystal-like arrangement of phospholipids
  2. The colorful appearance of cells under a microscope
  3. The variety of proteins embedded throughout the membrane like tiles
  4. The way water freezes on the membrane
Answer and explanation

The variety of proteins embedded throughout the membrane like tiles

'Mosaic' refers to the patchwork of many different proteins set into the fluid phospholipid sheet.

Problem 3

Which molecule can pass most easily through the phospholipid bilayer WITHOUT a protein?
  1. A sodium ion (charged)
  2. Glucose (large, polar)
  3. A protein
  4. Oxygen (small, nonpolar)
Answer and explanation

Oxygen (small, nonpolar)

Small nonpolar molecules like oxygen dissolve straight through the hydrophobic core; charged or large molecules need transport proteins.

Problem 4

Explain what 'selective permeability' means and why it is essential for a cell.
Answer and explanation

Selective permeability means the membrane lets some substances cross while blocking or controlling others. It is essential because it lets the cell keep its internal chemistry different from the outside — taking in what it needs, removing wastes, and keeping harmful things out — which is the basis of homeostasis.

Problem 5

A hormone that cannot cross the membrane still changes a cell's behavior. Which membrane component makes this possible?
  1. A receptor protein that detects the signal and relays it inward
  2. The hydrophobic tails
  3. Cholesterol
  4. The nucleus
Answer and explanation

A receptor protein that detects the signal and relays it inward

Receptor proteins act as antennae: they bind the signal on the outside and trigger a response inside without the signal itself entering the cell.

Problem 6

Why does oxygen often cross a lipid bilayer more readily than a sodium ion? Explain the reasoning in your own words.
Answer and explanation

Different chemical properties cause different permeabilities.

Oxygen is small and nonpolar.

A charged ion interacts poorly with the hydrophobic interior and generally needs a protein pathway.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

Why can a channel make diffusion selective? Show the reasoning, not just the conclusion.
Hints

A membrane is neither an open sieve nor an absolute wall.

Answer and explanation

Its structure permits certain particles and not others; movement can still occur down an electrochemical gradient.

Problem 8

Does adding a membrane transport protein necessarily make transport active?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. Channels and carriers can mediate passive transport; active transport requires an energy-coupled mechanism.

Problem 9

A membrane can allow one substance while limiting anotherA membrane can allow one substance while limiting another+Small nonpolar soluteIon: chargedPermeability depends on the substance and the available pathways.
Original qualitative permeability comparison. Arrow length is not a numerical permeability coefficient, and omitted channels could change actual ion transport.
Interpret the supplied visual. Can an ion cross simply because it is smaller than a protein?
Answer and explanation

Size alone is insufficient; its charge makes passage through the lipid interior unfavorable, and suitable transport proteins may be required.

Selective permeability means passage depends on a substance and the membrane’s pathways. Small nonpolar molecules can often enter the lipid interior more readily than ions, whose charge and surrounding interactions create a barrier. Some water crosses lipid membranes, and aquaporin proteins greatly increase water permeability in many cells. A molecule being small does not automatically guarantee rapid passage. Channels and carriers have selectivity and regulation, so identify the relevant substance, concentration/electrical conditions and available protein before predicting transport.

Size alone is insufficient; its charge makes passage through the lipid interior unfavorable, and suitable transport proteins may be required.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 10

A small charged solute crosses a cell membrane only after a matching channel opens. Explain why size alone and the phrase “fluid membrane” do not predict its passage.
Answer and explanation

Charge and lipid-interior interactions limit the direct route; the selective channel changes the available pathway. Fluidity concerns component motion, not unrestricted permeability to every substance.

Charge and lipid-interior interactions limit the direct route; the selective channel changes the available pathway. Fluidity concerns component motion, not unrestricted permeability to every substance.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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