3.1 Movement of Substances Across The Plasma Membrane (continue)

The permeability of the plasma membrane

Keypoints :

• The plasma membrane acts as a partially permeable membrane and selectively controls the movement of substances in and out of a cell.
• Factors that determine whether a molecule can pass through the plasma membrane are the size and polarity of the molecule.

Passive Transport

1. Simple diffusion
2. Facilitated diffusion
3. Osmosis
   

Simple diffusion

• simple diffusion is the random movement of particles (molecules) from a region of high concentration to a region of low concentration.
• this process will continue until a dynamic equilibrium reached.
• example - diffusion of oxygen from the alveoli into the adjacent blood capillaries.
  

The process of diffusion

Simple diffusion through plasma membrane

Facilitated diffusion

• facilitated diffusion is the passive movement of molecules or ions down a concentration gradient.
• the process involves special pore proteins (channel proteins) or specific carrier proteins in the plasma membrane.
• the particular molecules combine with the specific active site of the carrier protein. The carrier protein changes its shape to allow the molecules to pass through to the other side of the plasma membrane. After assisting the movement of molecules to the other side of the plasma membrane, it is free to bind with other molecules.
• example- the reabsorption of glucose and amino acid from the villi into the blood capillaries.
  

Facilitated diffusion through a pore protein in the plasma membrane

Facilitated diffusion through a carrier protein in the plasma membrane

Osmosis

• is the random movement of water molecules from a region of high concentration of water molecules to a region of low concentration of water molecules through a partially permeable membrane.
• Example - the reabsorption of water molecules from the nephrons into the blood capillaries.
  

Important!

Q - What is the difference between simple diffusion & facilitated diffusion?

A - Both describes the tendency of molecules to spread out randomly from the site of high concentration to low concentration (with concentration gradient) except facilitated diffusion requires the help of carrier protein to facilitate the movement of the larger molecules.

Q - What are the similarities between simple diffusion & facilitated diffusion?
A : both are following the concentration gradient and they do not require energy to achieve movement of substances

3.1 Movement of Substances Across The Plasma Membrane

Learning Objective:

Analysing the movement of substances across the plasma membrane

Learning Outcomes :
At the end, a student is able to:

• state the substances required by living cells,
• state the substances that have to be eliminated from cells,
• explain the necessity for movement of substances across the plasma membrane,
• describe the structure of the plasma membrane

Substances Transported Into (Enter) The Cells :

1. Oxygen
2. Digested food substances :
   

• Glucose
• Amino acid
• Glycerol
• Fatty acid
  

Substances Eliminated From (Leave) the Cells :

1. Waste products from metabolic processes :
   

• Carbon dioxide
• Urea
• Lactic acid
• Excess water
  

Necessity for Movement of Substances Across The Plasma Membrane

1. A cell is surrounded by a plasma membrane that separates it from the outer environment.
2. For cellular activities to be carried out, the cell has to move substances into and out of the cell.
3. Movement of substances into or out of a cell is important to :

• provide nutrients for metabolism and growth
• supply oxygen for respiration
• regulate solute concentration and suitable pH for maintaining a stable internal environment for optimal enzymatic activities
• maintain an ion concentration gradient required for nerve and muscle cell activities
• secrete useful substances, for example, digestive enzymes and hormones
• eliminate toxic waste products such as urea and carbon dioxide

The Structure of the Plasma Membrane

1. The fluid-mosaic model proposes that the plasma membrane consists of protein molecules scattered in a mosaic pattern on a fluid bilayer of phospholipids molecules.
2. The phospholipid bilayer have different solubility properties of the two ends of phospholipid molecules.
   
3. There are various types of protein which are either partially or fully embedded in the membrane. Pore proteins forms a channel (allow small molecules, either polar or non-polar, to pass through freely) whereas carrier protein acts as a carrier (attach to specific glucose molecules, before transporting the molecules across the plasma membrane).
4. Cholesterol molecules help to stabilise the structure of the plasma membrane.
5. The phospholipid bilayer, proteins and other components are not rigid or static but form a dynamic and flexible structure.