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Chemistry · Grades 10–12

Membranes and amphipathic molecules

Relate molecular regions to behavior in water.

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Learn the idea

Amphipathic

A molecule can contain both water-interacting and water-avoiding regions.

Bilayer

Phospholipids can organize with hydrophilic heads facing water and hydrophobic tails shielded inside.

Permeability

Size, polarity, charge and transport proteins influence membrane passage.

Context

A membrane is dynamic, with proteins and other components; it is not simply a solid wall.

Worked examples

What does amphipathic mean here?

Different regions interact differently with water.

Having both hydrophilic and hydrophobic regions.

Where are phospholipid tails mainly found in a bilayer?

The hydrophobic regions are grouped away from bulk water.

Shielded in the bilayer interior.

Check your understanding

  1. Why do many ions need membrane transport proteins?

    1. Their charge makes passage through the hydrophobic interior unfavorable.
    2. Ions are larger than whole cells.
    3. Ions cannot move in water.

  2. Why is the membrane not accurately described as completely impermeable?

  3. A model has two leaflets containing 40 phospholipids each. How many phospholipids are represented?

Answers and marking notes

Answers and marking notes
  1. Their charge makes passage through the hydrophobic interior unfavorable.

    Charged particles face an energetic barrier in the lipid interior.

  2. Some substances cross directly and others through regulated proteins.

    Selective permeability is more accurate.

  3. 80

    2 × 40 = 80.

Further reading and review references

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Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.