Spm Bio Revision Note

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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

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.


Passive Transport - Simple Diffusion

What is passive transport?

Passive transport is the movement of substances across the cell membrane without the use of energy by the cell.

During passive transport, substances move down their concentration gradient, hence no energy is required.

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.

Passive Transport - Osmosis

What is Osmosis?

Osmosis is the diffusion of a water through a semi-permeable membrane, from a solution of low solute concentration to a solution with high solute concentration.

It is a physical process in which a solvent moves, without input of energy, across a semi-permeable membrane separating two solutions of different concentrations.

Important Points:
• It is the diffusion of water (normally) through a semi-permeable membrane. • It is from a dilute solution to a more concentrated solution.

Passive Transport - Facilitated Diffusion

What is Facilitated Diffusion?

Facilitated diffusion is the passive transport of substances across the plasma membrane with the help of transport proteins such as the channel protein and the carrier protein.

Substances Pass through the Plasma Membrane through Facilitated Diffusion

Particles undergo facilitated diffusion are the particles that cannot diffuse through the phospholipid bilayer such as 1. Large particles such as glucose, amino acids, proteins and nucleic acids 2. Some ions such as the sodium ions and chloride ions

2 Types of Transport Protein

Facilitated diffusion occurs through 2 types of transport protein, namely 1. Channel Protein
2. Carrier Protein


Mechanism of Facilitated Diffusion

Click on the links below to see how facilitated diffusion happen through the plama membrane of the cell.

Concentration Gradient

Facilitated diffusion happens down a concentration gradient.

Similarities between Simple Diffusion and Facilitated Diffusion

1. Down the concentration gradient (From high concentration to low concentration) 2. No energy is required

Differences between Simple Diffusion and Facilitated Diffusion


Active Transport

What is Active Transport?

Active transport is the movement of substances across the plasma membrane of cells against the concentration...
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