The cell membrane: a selective barrier

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

1. Read this first

   Use the table

   Lab card (model data), not to scale. A lab-made bilayer had 20 particles of each kind placed on one side. The card gives the count that reached the far side. Version 1: the bilayer alone. Version 2: the bilayer with carrier proteins built for glucose.

   Particle	Version 1 (bilayer alone)	Version 2 (with glucose carrier proteins)
   carbon dioxide molecules	17	17
   potassium ions	1	1
   glucose molecules	1	14

   The test was read about only: a teacher ran it, and no one handled the lab-made bilayer.

2. 1.

   Luz reads a lab card for a plain bilayer and a bilayer with glucose carriers. Which explanation fits the potassium ions row?

   • About the same, because the membrane is a solid wall in both versions
   • About the same, because ions cross the plain bilayer as easily as gases
   • About the same, because carrier proteins help gases, not sugars or ions
   • About the same, because the carriers fit glucose, not potassium ions
3. 2.

   Water molecules are small, yet Esme’s lab card shows far more crossed once aquaporins were added. Which explanation fits?

   • Aquaporins are channel proteins that let water pass at a very high rate
   • Aquaporins turned the bilayer into a thinner solid wall for water
   • Water crosses a bilayer freely, so the proteins made no difference
   • Aquaporins pushed water across the bilayer by using up some of the cell’s ATP energy
4. 3.

   Jun wonders why sodium ions did not cross in version 2, even though channel proteins were added. Which explanation fits?

   • Sodium ions cross a plain bilayer alone, so they need no channel
   • The channels stopped working in version 2, so no ions could pass through them
   • Channels are specific, so these fit potassium ions and not sodium ions
   • Proteins rest on the surface, so they cannot help any ion cross
5. 4.

   Channel proteins were added, yet Pia’s card shows the carbon dioxide count did not change. Which explanation fits?

   • Carbon dioxide has no charge and slips through the lipid middle, so it needs no protein
   • Carbon dioxide is charged, so the new proteins repelled it
   • Any particle crosses a bilayer freely, so proteins make no difference
   • Carbon dioxide crossed through the new proteins in both versions

6. Use this

   Membrane drawing of an animal cell (model, not to scale). A strip of phospholipids with watery fluid outside the cell above and watery fluid inside the cell below. The round heads touch the fluids and the thin tails meet in the middle. Letter K points to a protein that reaches all the way across the strip. Small arrows beside the strip show that the phospholipids and the protein move a little relative to one another.

7. 5.

   Kira’s notes say the cell membrane is a solid wall that never changes. Which correction fixes the notes?

   • It is a solid wall, but its proteins rest on the surface of the bilayer
   • It is a solid wall, but its tails face the fluid
   • It is fluid, so any molecule passes through it freely
   • It is a fluid mosaic: its lipids and proteins float and move
8. 6.

   Membrane drawing (model, not to scale). No proteins are drawn. Milo’s notes say oxygen cannot cross a bilayer unless a channel protein is present. Which correction fixes the notes?

   • Oxygen needs a carrier protein, not a channel protein, to cross
   • Oxygen cannot cross, but sodium ions slip through the lipid tails of the bilayer alone
   • Oxygen crosses because the membrane is open to any molecule
   • Oxygen has no charge, so it slips through the lipid middle with no protein
9. 7.

   “model, not to scale” Tala’s notes say sodium ions slip between the tails, so channel protein K is just extra. Which correction fixes the notes?

   • Sodium ions are charged and repelled by the tails, so they need a protein
   • Sodium ions need a protein because the membrane is one solid wall around the cell
   • Sodium ions need no protein, but glucose molecules do
   • Sodium ions meet the tails first, because tails face the fluid

10. Use this

    Membrane drawing (model, not to scale). A bilayer of phospholipids with watery fluid outside the cell above and watery fluid inside the cell below. Letter M points to a protein that reaches all the way across the bilayer, with a tunnel shaped to fit chloride ions. Outside the cell: 4 oxygen molecules and 4 chloride ions.

11. 8.

    Zee must pick the best claim, with its reason, about oxygen molecules and chloride ions near this bilayer.

    • Oxygen crosses alone and chloride needs a protein, because chloride has a charge
    • Neither crosses alone, because the cell controls entry with proteins
    • Chloride crosses alone and oxygen needs a protein, because ions are small
    • Both cross alone, because the bilayer is open to any particle
12. 9.

    Ari reads that this animal cell lives in cold conditions. Which claim about cholesterol, with its reason, fits the drawing?

    • Cholesterol lets ions cross in the cold, because it forms a tunnel
    • Cholesterol makes the membrane a solid wall in the cold, because it fills the gaps
    • Cholesterol helps the membrane stay fluid in the cold, because it lies alongside the phospholipids
    • Cholesterol marks the cell in the cold, because it carries sugar chains
13. 10.

    Ren compares claims about protein K, which lets calcium ions cross. Which claim, with its reason, fits the drawing?

    • K is an integral protein, because it spans the bilayer from fluid to fluid
    • K is a peripheral protein, because proteins stay on the surface of the membrane
    • K is a piece of a solid wall, because the membrane cannot change
    • K is unnecessary, because calcium ions slip between the tails