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.

BIOLOGY FORM 4 NOTES CHAPTER 4 : CHEMICAL COMPOSITION OF THE CELL

CHAPTER 4: CHEMICAL COMPOSITION OF THE CELL

4.1 CHEMICAL COMPOSITION OF THE CELL

1. All living and non living things are made of substances called elements.

2. The essential elements in human body:

Major bioelements (96%)	Other bioelements (4%)	Trace elements (0.01%)
Oxygen (65%) Carbon (18.5%) Hydrogen (9.5%) Nitrogen (3.3%)	Calcium Potassium Phosphorus Sulphur Spdium Chloride Magnesium Ferum (Iron)	Zinc Manganese Cobalt Copper Iodine Boron Chromium Molybdenum Selenium Fluorine

1. Common elements combined with each other to form various chemical compounds in the cell.

2. The chemical compounds can be divided into 2 types:

(a)   organic compounds (contain carbon)

- Carbohydrates, lipids, protein and nucleic acids

(b)   inorganic compounds (do not contain carbon)

- Water

3. The importance of chemical compounds in Cells:

Chemical compounds	Importance
Carbohydrates
Lipids
Chemical compounds	Importance
Proteins
Nucleic acids
Water

4.2 CARBOHYDRATES

1.     Carbohydrates contain carbon, hydrogen and oxygen.

4.3 PROTEINS

1. Proteins are made up of carbon, hydrogen, oxygen and nitrogen. Some proteins contain sulphur and phosphorus.

2. All proteins are made up of monomers called amino acids.

3. Two amino acids can combine to form a dipeptide by condensation. Conversely a dipeptide can be broken down into amino acids through hydrolysis.

	Amino acid + amino acid                                   dipeptide + water

 

4. Long chains of amino acids are called polypeptides. Polypeptides can be broken down through hydrolysis reaction into amino acids by digestive enzymes.

Polypeptides + water                               dipeptides or amino acids

5. Protein structures:

(a)   Primary structure

(b)   Secondary structure

(c)   Tertiary structure

(d)   Quarternary structure

1. Types of amino acids ( 20 types)

Essential amino acids (9)	Non-essential amino acids (11)
Cannot be synthesized by the body. Can only be obtained from the diet.	Can be synthesized by the body. Derived from other amino acids.

Animal proteins	Plant proteins
Contain all the essential amino acids. First class proteins.	Do not contain all the essential amino acids. Second class proteins.

4.4   LIPIDS

1.     Lipids are made up of carbon, hydrogen and oxygen. Some lipids contain nitrogen and phosphorus.

2.     Lipids are insoluble in water but soluble in other lipids and organic solvents such as alcohol and ether.

3.     Types of lipids:

(a)   Fats and oils

·         Fats and oils are triglycerides

·         A triglyceride is formed by the condensation of one molecule of glycerol and three molecules of fatty acids.

·         A triglyceride is can be broken into fatty acids and glycerol through hydrolysis.

Glycerol         3 molecules of fatty acids                                    triglyceride

·         Two types of fats: saturated fats and unsaturated fats. Comparison between saturated fats and unsaturated fats.

SIMILARITIES

Both are triglycerides and contain fatty acids.

Saturated fats	Unsaturated fats
Contains saturated fatty acids.	Contains unsaturated fatty acids.
Do not have any double bonds between the carbon atoms.	Have a least one double bond between the carbon atoms.
Contains maximum number of hydrogen.	Contains less than maximum number of hydrogen. It still can take in hydrogen.
Solid at room temperature.	Liquid at room temperature
Has a high melting point.	Has a low melting point
Increases the cholesterol level in the blood.	Decreases the cholesterol level in the blood.
Example: Animal fats such as butter.	Example: Vegetable oil such as corn oil.

(a)   Waxes

·         Found on the cuticles of the epidermis of leaves, fruits and seeds of some plants.

·         Waterproof: preventing entry and evaporation of water.

·         Sebum contains wax that soften our skin.

(b)   Phospholipids

·         Important components in the formation of plasma membrane.

(c)   Steroids

·         Complex organic compounds include cholesterol and hormone (testosterone, oestrogen and progesterone)

4.5     ENZYMES

4.5.1  The Role of Enzymes in Organisms

1.     Metabolism:

(a)   Anabolism: the metabolic reactions that build complex molecules, for example, photosynthesis

(b)   Catabolism: the metabolic reactions that break down complex molecules, for example digestion

2.     Definition of enzyme: Enzyme is organic catalyst, usually a protein which speeds up biochemical reactions in living cells.

4.5.2   The General Characteristics of Enzymes

        

1.     Enzymes are proteins which are synthesized by living organisms.

2.     In enzymatic reactions, enzymes bind to their substrates and convert them to products.

3.     Enzymes speed up the rates of chemical reactions but remain unchanged at the end of the reactions. Enzymes are not destroyed by the reactions they catalyzed.

4.     Enzymes are highly specific.

-       Enzymes have specific sites called active sites to bind with specific substrates.

-       Each enzyme can usually catalyse only one kind of substrates.

5.     Enzymes are needed in small quantities

-       Enzymes are not used up at the end of a reaction

-       The same enzyme molecule can process a large quantity of substrate molecules

6.     Most enzyme-catalysed reactions are reversible.

-       Enzyme can catalyse the reaction in either direction.

7.     The activity of an enzyme can be slowed down or completely stopped by inhibitors.

      -     Examples of inhibitors: heavy metal such as lead and mercury

8.     Many enzymes require helper molecules, called cofactors, to function

-       Inorganic cofactors: ferum and copper

-       Organic cofactors / coenzymes: water-soluble vitamins

4.5.3   Naming of Enzymes

1.     An enzyme is named according to the name of the substrate it catalysed.

2.     Most enzymes have a name derived by adding the suffix –ase at the end of the name of their substrates.

Substrates	Enzymes
Sucrose
Maltose
Lactose
Lipid

1.     However, there are other enzymes that were named before a systematic way of naming the enzymes was formed. For example, pepsin, trypsin and rennin

4.5.2   The Sites of Enzyme Synthesis.

1.     Ribosomes are the sites of enzymes synthesis

2.     The information for the synthesis of enzymes is carried by the DNA

3.     The different sequences of bases in the DNA are codes to make different proteins

4.     During the process, messenger RNA is formed to translate the codes into a sequence of amino acids. These amino acids are bonded together to form specific enzymes according to the DNA’s codes.

4.5.3   Intracellular and Extracellular Enzymes

1.     Intracellular enzymes: Enzymes which are synthesized and retained in the cell for the use of the cell itself.

Extracellular enzymes: Enzymes which are synthesized in the cell but secreted from a cell to work externally.

4.5.2   The Mechanism of Enzyme Action

        

1.     The way an enzyme binds to its substrate can be explained by the “lock and key hypothesis”.

2.     The substrate molecule is represented by the ‘key’ while the enzyme molecule is represented by the ‘lock’

4.5.2   Factors Affecting Enzyme Activity

1.     The factors which affect enzyme activity include:

(a)   temperature

(b)   pH

(c)   substrate concentration

(d)   enzyme concentration

2.     The effects of temperature on the activity of enzyme

(a)   At low temperature, an enzyme-catalysed reaction takes places slowly. This is because the substrate molecules are moving at a relatively slow rate.

(b)   An increase in temperature leads to an increase in the rate of enzyme-catalysed reaction. This is because as the temperature increase, the movement of substrate molecules increases, thus collisions between the substrate and enzyme molecules occur more frequent.

(c)   For every 10⁰C rise in temperature, the rate of reaction is doubled. However, this is only true up to the optimum temperature.

(d)   The optimum temperature is the temperature at which an enzyme catalysed a reaction at the maximum rate.(Example: for human and most animals, optimum temperature for enzyme-catalysed reaction is 37⁰C)

(e)   Beyond the optimum temperature, any increase in temperature causes the rate of reaction to decrease sharply until it stops completely at 60⁰C.This is because the bond that hold enzyme molecules together begin to break at high temperatures, and eventually destroy their active sites. This means that substrates can no longer fit into the active sites of the enzymes. The enzymes are said to be denatured at very high temperature. This is called denaturation.

(f)    Denaturation is irreversible, hence, the body needs to maintain its temperature at 37⁰C for the optimal functioning of enzymes.

  

§  At low temperature, an enzyme-catalysed reaction takes places slowly

§  As the surrounding temperature increases, the rate of reaction is increased until it reaches the optimum temperature.

§  The rate of reaction is at the maximum at the optimum temperature.

§  Beyond the optimum temperature, the rate of reaction decreases due to the denaturation of enzymes.

3.     The effects of pH on the activity of enzyme

(a)   Each enzyme can only function optimally at a particular pH.

(b)   The optimum pH is the pH at which the rate of reaction is at the maximum.

(c)   A change in pH can alter the charges on the active sites of an enzyme and the surface of a substrate. This can reduce the ability of both molecules to bind each other.

(d)   The effects of pH on enzymes are normally reversible. When the pH in the environment reverts to the optimum level for the enzymes, the ionic charges on the active sites are restored. Thus, the enzymes resume their normal function.

4.     The effects of substrate concentration the activity of enzyme

(a)   The rate of enzyme-catalysed reaction increases when the substrate concentration increases until the reaction reaches maximum rate.

(b)   Beyong the maximum rate, the active sites of enzyme molecules are fully occupied by substrate concentration further has no effect on the rate of reaction. The rate of reaction becomes constant.

(c)   This means there is an excess of substrate molecules.

(d)   The concentration of enzymes becomes a limiting factor.

5.     The effects of enzyme concentration on the activity of enzyme

(a)   The rate of enzyme-catalysed reaction increases when the concentration of the enzyme increases until a maximum rate is achieved.

(b)   Beyong the maximum rate of reaction, the concentration of substrate becomes a limiting factor.

(c)   When the enzyme concentration is doubled, the rate of reaction will be doubled as long as the substrates are present in excess concentration.

4.5.3   `The Uses of Enzyme

Application	Enzymes	Uses
1. Food Processing (a) Dairy	Lipase
	Lactase
	Renin
(b) Brewing	Zymase
(c) Baking	Amylase
(d) Meat	Protease
(e) Fish	Protease
(f)  Starch	Amylase
(g) Cereal grain	Cellulase
(h) Seaweed	Cellulase
2. Biological Detergents	Protease
	Amylase
	Lipase
3. Textile Industry	Amylase
4. Paper Industry	Ligninase
5. Leather Industry	Protease / Lipase
6. Medical /   Phamaceutical Industry	Pancreatic trypsin
	Microbial trypsin

                            The Uses of Enzyme

Application	Enzymes	Uses
1.     Food processing (a) Dairy	Lipase	Ripening of cheese
	Lactase	Hydrolyses lactose to glucose and galactose in the making of ice cream
	Renin	Solidifies milk proteins
(b) Brewing	Zymase	Converts sugars into ethanol
(c) Baking	Amylase	Convert starch flours into sugar in the making of bread and dough
(d) Meat	Protease	Tenderise meat
(e) Fish	Protease	Removes the skin of fish
(f)  Starch	Amylase	Change starch to sugar in the making of syrup
(g) Cereal grain	Cellulase	Breaks down cellulose and removes the seed coat from cereal grain
(h) Seaweed	Cellulase	Extracts agar from seaweed
2. Biological Detergents	Protease	Acts on stains containing proteins, for examples, blood and saliva
	Amylase	Removes stains containing starch, for examples sauces, ice cream and gravy
	Lipase	Removing oil and grease
3. Textile Industry	Amylase	Removes the starch that is used as stiffeners of fabric
4. Paper Industry	Ligninase	Removes lignin from pulp
5. Leather Industry	Protease / Trypsin	Removes hairs from animal hides
6. Medical /   Phamaceutical Industry	Pancreatic trypsin	Treats inflammation
	Microbial trypsin	Dissolves blood clots