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Transport Across Cell Membranes. Getting things in and out of cells Part One. The cell membrane described as “semi-permeable”. It allows some substances to pass through freely, but not others. Which types of molecules can pass through the membrane?. Easily passes through:

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transport across cell membranes

Transport Across Cell Membranes

Getting things in and out of cells

Part One


The cell membrane described as “semi-permeable”.

  • It allows some substances to pass through freely, but not others.
which types of molecules can pass through the membrane
Which types of molecules can pass through the membrane?

Easily passes through:

  • Small nonpolar, or polar, molecules
    • e.g. O2, CO2, H2O
  • Large, nonpolar molecules

Can’t pass through:

  • Large, polar molecules
    • e.g., hydrophilic proteins; glucose
  • Charged molecules (ions)

The ECF (extracellular fluid) contains many substances.

  • The cell needs a way to transport these substances in, and to control which ones come in or leave.
four types of transport
Four types of transport
  • Simple diffusion
  • Facilitated diffusion
  • Osmosis
  • Active transport



1 simple diffusion
1. Simple diffusion
  • Particles of solute are constantly in motion in a solution.
  • Diffusion is the movement of molecules from an area of high concentration to an area of lower concentration.
  • If a difference in particle density (concentration) exists between regions, we say a concentration gradient exists.
2 facilitated diffusion
2. Facilitated diffusion
  • via ion channels/pores, OR
  • via carrier protein
  • “Facilitated” – Diffusion that is assisted
  • Charged, and polar molecules
      • e.g. ions, glucose
2a ion channel pore
2a. Ion channel/pore
  • Ions are small, but are charged
  • Ion channel – Integral protein through which ions can pass, along their concentration gradients.

Some channels are not open all the time, and have "gates" that can be opened or closed, depending on cellular conditions.

      • Important when we look at the nervous system
2b carrier protein
2b. Carrier protein
  • Another integral protein.
  • Molecule still diffuses through it, but mechanism is different.

Molecule binds to the carrier protein

  • Binding  Conformational change
  • Molecule is presented onto the opposite side of the membrane. The protein’s affinity for the molecule is reduced.
  • Molecule unbinds, and allows protein to return to original conformation.


Glucose transporter

3 osmosis
3. Osmosis
  • Osmosis = Diffusion of water.
  • Note: Water is very polar, but is still somehow able to diffuse across the membrane.

Tonicity is a relative concept. Can only use it when comparing solute concentrations.

  • Hypertonic – Higher [solute]
  • Isotonic – SAME [solute]
  • Hypotonic – Lower [solute]