3.1.1 Passive Transport – Simple Diffusion

What is passive transport?

  1. Passive transport is the movement of substances across the cell membrane without the use of energy by the cell.
  2. During passive transport, substances move down their concentration gradient, hence no energy is required.
  3. Passive transports can happen through three different channels, namely
    1. lipid bilayer
    2. pore protein
    3. carrier protein

What is diffusion?

Diffusion is the movement of particles from a high density region to a low density region. No energy is needed and no membrane involves in diffusion.

The Dynamic Equilibrium

Diffusion will continue until the concentration in all region is the same. When this happen, we say it has reached the dynamic equilibrium.

Factors Affecting the Rate of Diffusion (How fast diffusion happens)

Particles that Move Through the Plasma Membrane Through Diffusion

  1. Substances soluble in fat: fatty acid, glycerol, some vitamins (A,D,E,K)
  2. Neutral particles: water, oxygen, carbon dioxide,

Example of Diffusion

Between alveoli and blood capillaries in the lung during gases exchange.

3.1 The Plasma Membrane

The Plasma Membrane

The plasma membrane is a semi-permeable lipid bilayer found in all cells that controls water and certain substances in and out of the cell.

Function of the Plasma Membrane

  1. Protects the cell.
  2. Separates the intracellular components from the extracellular environment.
  3. Controls what enters and exits the cell

Necessities for the Movement of Substances across the Plasma Membrane

  1. To transport nutrients into the cell.
  2. For gases exchange
  3. To excrete metabolic waste.
  4. To maintain the pH value and ionic concentration of the cell.

Substances In and Out through the Membrane

Structure of the Plasma Membrane

Fluid Mosaic Model

Permeability of the Phospholipids Bilayer

The permeability of the phospholipids bilayer is determined by:
  1. the size
  2. the charge and
  3. the polarity
of the substances pass through it.