BIOLOGY FORM 4 NOTES CHAPTER 3 : MOVEMENT OF SUBSTANCES ARCOSS THE PLASMA MEMBRANE

CHAPTER 3:

MOVEMENT OF SUBSTANCES ARCOSS THE PLASMA MEMBRANE

3.1 MOVEMENT ARCOSS THE PLASMA MEMBRANE

3.1.1     The need for the movement of substances across the plasma membrane:

            (a) to obtain nutrients and gases

            (b) to excrete metabolic waste

            (c) to maintain a suitable PH and ionic concentration within the cell for enzyme activity.

           

3.1.2       The structure of the plasma membrane

1.     It is called Fluid-mosaic model, introduced by Singer and Nicolson in 1972

2.     Plasma membranes are composed mainly of phospholipids and proteins.

3.     The phospholipids molecules arrange themselves in a layer of two molecules thick which is called  phospholipids bilayer.

4.     Each phospholipid molecule has a polar head that gives it the hydrophilic property (like water) and a pair of non-polar tails gives it the hydrophobic property (dislike water)

5.     The phospholipids bilayer acts as barrier which isolates the two sides of the membranes.

6.     Phospholipids bilayer also contains cholesterol that helps to stabilize and strengthen the plasma membrane, making it more flexible but less permeable to water soluble substances.

7.     Transport proteins regulate the movement of the water soluble molecules and ions through the plasma membrane.

8.     There are two types of Transport proteins, ie channel proteins and carrier proteins.

9.     The channel proteins have pore to allow particular molecules or ions to across the plasma membrane; the carrier proteins act as carrier to take up specific molecules on one side of the plasma membrane and release them on the other side.

10.  Why the plasma membrane model is called the Fluid-mosaic model?

(a)   The phospholipids bilayer, proteins and other components are not rigid or static, this causes the membrane to have a ‘fluid’ characteristic

(b)   The protein molecules float about in the phospholipids bilayer to form a mosaic pattern.

3.1.3       The permeability of the plasma membrane

1.     The plasma membrane is semi-permeable or selectively permeable. This means that some substances can move across the membrane freely while others cannot.

2.     Two factors that determine whether a molecule can pass through the plasma membrane or not: the size and the polarity of the molecules.

3.     Lipid-soluble molecules such as fatty acids and glycerol can pass through phospholipids bilayer freely.

4.     Non-polar molecules such as oxygen and carbon dioxide can move through the phospholipids bilayer with ease.

5.     Small water-soluble molecules and ions pass through phospholipids bilayer by pore proteins.

6.     Large water-soluble molecules such as glucose and amino cannot pass through phospholipids bilayer but carry by carrier proteins.

3.1.4       The movement of substances across the plasma membrane: Passive Transport

1.     SIMPLE DIFFUSION

(a)   Definition: The movement of molecules or ions from a region of higher concentration to a region of lower concentration, thus going down a concentration gradient until an equilibrium is achieved.

(b)   Factors affecting the rate of diffusion:

(i)     Surface area between the two region:

--- the larger the surface area, the higher the rate of diffusion

(ii)    Distances over which diffusion occurs:

--- the shorter the distance over which diffusion occurs, the higher the rate of the diffusion across it.

(iii)   Concentration gradient:

--- the greater the difference in concentration between two regions, the higher the rate of diffusion.

(iv)  Size and nature of the particles:

--- smaller particles diffuses faster than larger particles.

(v)   Temperature:

--- at higher temperatures, the particles have more kinetic energy to diffuse in higher rate.

(c)   Example: Gaseous exchange in the alveoli and blood capillaries.

2.     OSMOSIS

(a)   Definition: The movement of water molecules from a region of low solute concentration to a region of high solute concentration (going down the concentration gradient of water) through semi-permeable membrane.

(b)   Osmosis is the diffusion of water only and NOT the substances that dissolved in water.

(c)   Example: Absorption of water by root hairs in a plant

SIMILARITIES IN SIMPLE DIFFUSION AND OSMOSIS

DIFFERENCES BETWEEN SIMPLE DIFFUSION AND OSMOSIS
SIMPLE DIFFUSION	OSMOSIS

1.     FACILITATED DIFUSSION

(a)   Definition: The movement of hdydrophilic molecules or ions across the plasma membrane with the help of transport proteins.

(b)   Channel proteins provide a functional pore in membrane for the diffusion of ions.

(c)   Carrier proteins pick up amino acids, glucose or small protein on one side and release them on the other side. The relationship between the carrier protein and the transport molecules is specific. For example, glucose molecules can only combine with carrier proteins which are specific for glucose. This is because the carrier proteins have binding sides that can combine reversibly with specific molecules only.

(d)   Example: Absorption of digested food in the villus

(e)   The mechanism of facilitated diffusion with the aid of a carrier protein.

3.1.2       The movement of substances across the plasma membrane: Active Transport

1.     Definition: The movement of molecules or ions from a region of lower concentration to a region of higher concentration (against the concentration gradient) across the plasma membrane, with the use of cellular energy.

2.     Active transport requires the use of carrier proteins and cellular energy to transport molecules against the concentration gradient.

3.     Active transport can only take place in living organism.

4.     Example: Intake of mineral salts and ions by root hairs of a plant.

The mechanism of active transport for sodium ions.

SIMILARITIES IN PASSIVE TRANSPORT AND ACTIVE TRANSPORT

DIFFERENCES BETWEEN PASSIVE TRANSPORT AND ACTIVE TRANSPORT
PASSIVE TRANSPORT	ACTIVE TRANSPORT

3.1          MOVEMENT OF SUBSTANCES ACROSS THE PLASMA MEMBRANE IN EVERYDAY LIFE.

3.2.1       Hypotonic, Isotonic and Hypertonic

1.     When a cell is surrounded by an external solution that is more dilute than the cytoplasm fluid or vacuole, the external solution is said to be hypotonic to the cell. A cell in hypotonic solution will gain water by osmosis.

2.     When a cell is surrounded by an external solution that is more concentrate than the cytoplasm fluid or vacuole, the external solution is said to be hypertonic to the cell. A cell in hypertonic solution will lose water by osmosis.

3.     When a cell is surrounded by an external solution that has the same concentration as the cytoplasm fluid or vacuole, the external solution is said to be isotonic to the cell. A cell in isotonic solution will not gain or lose water by osmosis.

3.2.2     The effects and application of osmosis in everyday life

1.     Wilting in plants

(a)   Excessive use of fertilizers

-       Excessive fertilizers which dissolve in the soil water make the soil water more concentrate than and hypertonic to the cell sap pf plant roots.

-       Water diffuses from the cell sap into the soil by osmosis.

-       The plasmolysed cells become flaccid and cause the plant to wilt.

-       The plant will eventually die if water is not supply immediately.

(b)   Shortage of water in soil

-       As the soil dries up, the remaining soil water becomes more concentrated and hypertonic to the cell sap pf plant roots.

-       The plant loses water by osmosis, causing it to wilt.

2.     Food preservation

-       Food can be preserved by using salt, vinegar and sugar.

-       These preservatives make the surrounding solution hypertonic to content of the food

-       The hypertonic solution causes water to leave the food by osmosis and the preservatives to enter the cell sap.

-       The dehydrated condition of the food prevents the growth of bacteria and fungi which can spoil the food.

3.2          APPRECIATING THE MOVEMENT OF SUBSTANCES ACROSS THE PLASMA MEMBRANE

1.     The movement of substances across the plasma membrane in a control manner is important for the survival of the cell, because the plasma membrane :

(a)   act as ‘gatekeeper’, regulating what goes in and out of the cell;

(b)   act as barrier between the contents of the cell and the surrounding environment.

2.     How to maintain the proper functioning of plasma membrane?

(a)   we need to take good care of our food and water intake

(b)   drinking sufficient water to hydrate body cells as regulate the osmotic pressure of the blood.