SPM Biology Form 4 Notes – Terminology and Concepts: Movement of Substances Across the Plasma Membrane (Part I)

by BerryBerryTeacher

in Berry Reference (Notes)

Berry Berry Easy is back with SPM Form 4 Biology notes for Movement of substances across the plasma membrane“. The plasma membrane is a membrane of biological nature which forms the barrier between the interior of a cell against external environment. However, do not just think of it as a layer which do not have any function. Typically, movement of substances to and from cells to outer environment is controlled (due to its permeability) by the plasma membrane. The key concept here for this topic is to understand the selective-permeability nature of a plasma membrane. The rest of the topic are rather easy and intuitive. Do note that plasma membrane is also known as cell membrane and plasmalemma.

SPM Biology Form 4 – Terminology and Concepts: Movement of Substances Across the Plasma Membrane (Part 1)

Permeability of the fruit skin

Permeability of the fruit skin

1. Uniqueness of Plasma Membrane (also known as cell membrane):

  • it is a semi-permeable cell membrane
  • it allows water and certain substances to move in and out of the cell.

2. Importance of Plasma Membrane:

  • - cells obtain nutrients and gases
  • cells excrete metabolic wastes
  • cells can maintain pH for enzyme activity
  • cells can maintain ionic concentration of the cells for enzyme activity
  • control the types and the amount of substances
  • allow useful substance (hormones/enzymes) to secrete from cells
  • protect cells
  • a boundary between the inside and outside of cell.

3. Structure of the basic unit of plasma membrane

  • Phospholipid molecule:
    ‘Head’ – hydrophilic: a polar phosphate molecule (philic~loves water / attracted to water)
    ‘Tail’ – hydrophobic: two non-polar fatty acids (phobic~hates water / repelled to water)
  • Formation:
    heads pointing outwards
    tails pointing inwards
    (Bilayer phospolipid)

Fluid Mosaic Model (Protein embedded in the bilayer)

Carrier protein

  • carrier for some molecules (glucose, amino acids, proteins and nucleic acids)
  • controls the movement of ions and particles (Na+, Ca2+ and K+)
  • Glycoprotein


  • combination of lipids and polysaccharides

4. Permeability

Permeable (allow to pass through)

  • small non-polar molecules (vitamins A, D, E, K, fatty acids, glycerol and steroids)

Impermeable (not allow to pass through but with help of carrier protein and cellular energy, it is allow to pass through)

  • large polar molecules (glucose, amino acids, mucleic acids and polysaccharides)
  • charged ions (H+, Na+, K+, Cl- and Ca2+)

Substances that are allowed to move in the cell:

  • CO2
  • O2
  • excess H2O
  • waste: nitrogenous

Substances that are allowed to move out of the cell:

  • CO2
  • O2
  • amino acids
  • ionic salts
  • glucose

This is the end of Part 1 of this topic. Do check out Part 2 which covers passive and active transport.

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February 18, 2012
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