Movement of Substances Across The Plasma Membrane

Substance that move into the cells :

1. Glucose       : Production of energy through cellular respiration
2. Amino Acids   : Production of proteins such as enzymes ,antibodies and muscles
3. Fats          : Production of energy and synthesis of cellular structure
4. Vitamins &
5. minerals      : Coenzymes which are required for cell metabolism
6. Water         : Reaction medium for most of the biochemical reactions
7. Oxygen        : Production of energy through cellular respiration

Substances that move out from the cells :

Carbon dioxide     - Waste product of respiration
Oxygen             - Waste product of photosynthesis
Nitrogenous wastes - From the breakdown of excessive amino acids
                   - E.g.urea, uric acid, ammonia
Secretions         - Secreted by particular cells
                   - E.g. extracellular enzymes, hormones, neurotransmitter

Structure of plasma membrane 

1. Plasma membrane is made up of mainly phospholipid bilayer and protein molecules.

2. Each phospholipid molecule consists of two parts:
     a) Hydrophilic head (water loving) which made up of phosphate group.
     b) Hydrophobic tail (water hating) which made up of fatty acids.


3. The fluid mosaic model proposed By Singer and Nicholson stated that the     phospholipids, proteins and other components of the membrane form a dynamic and fluid structure.

4. Plasma membrane is semi-permeable because only certain substances can move across the plasma membrane.

5. The phospholipid bilayer is only permeable to
     a) Small, fat soluble molecules such as fatty acids, glycerol, steroids and vitamins A,D,E,K.
     b) Small uncharged molecules such as oxygen, carbon dioxide and water.

6. the proteins embedded in the phospholipid bilayer consisted of
     a) Carrier protein
     b) Pore protein/channel protein

7. Slightly bigger polar molecules such as glucose and amino acids cannot move across the phospholipid bilayer.These substances pas through the plasma membrane with the help of the carrier proteins.

8. Small charged mineral ions such as Na+ and K+ ions cannot move across the phospholipid bilayer. These substances pass through the plasma membrane with the help of the pore proteins.

Various Types of Substances Moving Across The Plama Membrane

Movement of substances across the Plasma Membrane

              

Simple Diffusion (Passive Transport)

1. Diffusion is the net movement of substances from the higher concentration to the lower concentration region until a dynamic equilibrium is achieved.

2. The examples of simple diffusion

   a) Gaseous exchange in unicellular organisms such as Amoeba sp.

   b) Gaseous exchange between alveoli and blood capillaries during respiration.

   c) Gaseous exchange through the stomata of leaves .

   d) Small fat-soluble molecules such as fatty acids, glycerol, vitamin A,D,E,K 

      diffuse across the plasma membrane.

             

Osmosis (Passive Transport)

1. Osmosis is the diffusion of water molecules across a semi-permeable membrane until a dynamic equilibrium is achieved.

2. The example of osmosis

   a) The uptake of water from soil by the root hairs.

          

The short video below shows that how osmosis process works.

 

Facilitated Diffusion (Passive Transport)

1. Facilitated diffusion is the diffusion of certain substances through the cell 

   membrane with the help of carrier protein or pore protein.

2. In the facilitated diffusion, the substances will be transported down the gradient concentration until equilibrium is reached.

3. The examples of facilitated transport

   a) The absorption of glucose and amino acids from the ileum into the villus 

      through carrier protein.

   b) The absorption of mineral ions from the ileum into the villus through pore 

      protein.

 

Absorption of glucose molecules into the villus through carrier protein (facilitated diffusion)

Active Transport

1. Active transport is the movement of substances across the plasma membrane   against concentration gradient with the use of ATP energy and carrier proteins called pumps.
        
  

2. The phosphate bond in ATP molecules is broken down to release ADP, P, and
   energy.

3. Active transport can be slowed down or inhibited by respiratory poisons such
   as cyanide or a lack of oxygen.

4. The examples of active transport

   a) Absorption of mineral ions such as potassium ion, phosphate and nitrate by
      root hair.
   b) Sodium-potassium pump ( in nerve and animal cells ) that pump sodium ions
      out of the cell and potassium ions into the cell.
   c) Uptake of the sodium ion and glucose from the lumen of the tubules of
      kidney nephron.
   d) Accumulation of the iodine ions by algae, Nitella sp. Despite the
      concentration of iodine int he cell being higher than that of the sea.

Transport of the sodium ion out and potassium ion into a cell through a carrier protein (active transport)

                

Comparison between passive transport and active transport

              

Video below shows the processes of passive and active transport

 

Hypotonic, Hypertonic And Isotonic Solutions

There are 3 types of solutions in accordance with their solute dissolved in water:

   a) Hypotonic solution contains a lower concentration of solute relative to
      another solution.
   b) Hypertonic solution contains a higher concentration of solute relative to
      another solution.
   c) Isotonic solution contains the same concentration of solute as another
      solution.

Effect of hypotonic,hypertonic and isotonic solutions in plant cells

 

Hypotonic solution

• Water diffuses into the large central vacuole by osmosis.
• This causes the vacuole to expand and push against the cell membrane.
• The cell membrane pushes against the cell wall.
• The cell is turgid.
• The turgidity within the plant cell is important to support and maintain the shape of the cell.
• The plant cell does not burst because the cell wall is rigid and strong enough to resist the turgor pressure.

              

 

Hypertonic solution 

• Water diffuses out of the large central vacuole by osmosis.
• The vacuole and the cytoplasm shrink.
• The plasma membrane pulls away from the cell wall.
• The cell becomes flaccid.
• This process is called plasmolysis.
• The flaccidity of plant cells causes wilting of the plant.
• If the plasmolysed cell placed in a hypotonic solution, water moves in by osmosis and the cell becomes turgid again. This process is called deplasmolysis.

          

 

Isotonic solution

• Water diffuses into and out of the cell at equal rates.
• There is no net movement of water across the plasma membrane.
• The cell retains its normal shape.

                

Effect of hypotonic,hypertonic and isotonic solutions in animal cells

Hypotonic solution

• Water enters the cell by osmosis.
• The red blood cells swells up and eventually burst because the plasma membrane is too thin to contain the water.
• The bursting of red blood cells is known as haemolysis.

        

Hypertonic solution

• Water moves out of the red blood cells by osmosis.
• This causes the red blood cell to shrink or shrivel.
• The red blood cells have undergone crenation.

          

Isotonic solution

• Water diffuses into and out of the cell at equal rates.
• The red blood cells maintained their shape.
• The concentration of a red blood cell is the same as the concentration as 0.85% sodium chloride solution.

       






      

Video below shows the experiment carry out to determine the effects of the egg membrane in the hypotonic and hypertonic solutions.

The Effect And Application Of Osmosis In Daily Life

Wilting in Plants

1. Excessive use of fertilisers can cause plants to wilt.
2. Fertilisers will dissolve in the water in the soil and cause the soil water to hypertonic to the cell sap of the plant roots.
3. Water diffuses from the cell sap into the soil by osmosis.
4. The plant cells lose water to the surroundings, causing the cell to be flaccid and plasmolysed.
5. This causes the plant to wilt as there is no support for the plant.

Diagram below shows the wilting of plant due to excessive used of fertilisers.

                      

Preservation of food

1. Food such as fruits, vegetables and fish can be preserved to increase its shelf life by using salt, sugar and vinegar.
2. The food is put in salt/sugar/vinegar solution which is hypertonic to the food cells.
3. Water diffuses from the food cell into the salt/sugar/vinegar solution by osmosis.
4. The lack of water in the food cells prevents the growth of bacteria and fungi, thus preventing spoilage of food.

Diagram below shows the preservation of food