Unit Notes

Digestion

Digestion is the process of mechanical and chemical breakdown of nutrients into smaller components in order to be absorbed into the body (blood stream)

Nutrients

Carbohydrates include monosaccharides (fructose and glucose) that combine to form disaccharides (sucrose and maltose) and polysaccharides (starch and glycogen). They consist of the elements carbon, hydrogen and oxygen.

Lipids include glycerides (made from fatty acids and glycerol) and steroids. They consist of the elements carbon, hydrogen and oxygen.

NOTE: although carbohydrates and lipids consist of the same elements, the ratios and molecular structures differ predictably.

Proteins are one or more peptides which consist of a sequence of amino acids. They consist of the elements carbon, hydrogen oxygen and nitrogen (and sulfur in many cases).

Metabolism

Metabolism is the web of all enzyme-catalyzed reactions in a cell or organism.

Metabolism is separated into two categories.

• Anabolism​ is​ the synthesis of complex molecules from simpler molecules. This includes formation of macromolecules from monomers by condensation reactions.

Condensation reactions are chemical reactions in which reactants are joined together to form a product with water as a by product.

Consider:

Carbohydrates: Monosaccharide + Monosaccharide --> Disaccharide + Water

Example: Glucose + Glucose --> Maltose + Water

Lipids: 3 Fatty Acids + Glycerol --> Triglyceride + Water

Proteins: Amino Acid + Amino Acid --> Dipeptide + Water

• Catabolism is the breakdown of complex molecules into simpler molecules. This includes breakdown of macromolecules into monomers by hydrolysis reactions.

Hydrolysis reactions are chemical reactions in which reactants are separated using water to form products.

Consider:

Carbohydrates: Disaccharide + Water --> Monosaccharide + Monosaccharide

Example: Maltose + Water --> Glucose + Glucose

Lipids: Triglyceride + Water --> 3 Fatty Acids + Glycerol

Proteins: Dipeptide + Water​ --> Amino Acid + Amino Acid

Collision Theory

Collision Theory states that a chemical reaction will occur if 3 criteria are met:

1. Reactants (including enzymes and substrates) collide

2. with the correct orientation (i.e., in the "right" place)

3. with enough energy (E ≥ Ea)

Ea = activation energy = the minimum amount of energy required for a chemical reaction to occur)

It is helpful to recall Kinetic Theory (it is usually applied to gasses but generally applies to all matter) whose main principles are:

• All matter is made of particles

• All particles are in constant motion

• The motion is random but linear

• Collisions are elastic​

Matter – anything that has mass and volume

Reaction rate

Reaction rate is the change in a quantity per unit time.

• In terms of reactants, it is the rate at which they disappear. This is a negative change.

• In terms of products, it is the rate at which they appear. This is a positive change.

Equation

• The equation is change in amount ÷ change in time.

• Amount may be volume (of a gas), mass, temperature (change is a consequence of the reaction), height, length and so forth.

Graph

Amount - time graph

• Average rate is the change in amount / time (the slope)

• Instantaneous rate is the slope of a tangent at a given point

As the reaction progresses, the concentration of reactants decreases. This leads to lower frequency of collision with the correct orientation and sufficient energy. Thus, rate decreases as reactions progress - highest at the beginning and lowest at the end.

Rate - factor graph

• Rate is read from the y-axis.

Enzyme

Definition

An enzyme is a biological (protein) catalyst.

• ​A catalyst increases reaction rate but is not used up (it is recycled).

• An enzyme is biological which means that it is found in living things. It is a protein.

Digestive enzymes

Enzyme specificity

an enzyme binds to and reacts with specific substrates according to the lock-n-key model. This explains substrate-enzyme specificity.

• The enzyme is the lock.

• The active site is the keyhole where the reaction takes place in the enzyme.

The substrate must fit the active site – the shape of the substrate is complementary to the shape of the active site.

• The substrate is the key that fits into the active site which is the keyhole.

The substrate is the reactant.

NOTE: the lock-n-key model is actually outdated and a revised model, the induced fit model, is used

Enzyme catalysis

an enzyme increases reaction rate but is not permanently changed in the reaction

• An enzyme reacts with a substrate through an alternate reaction pathway

• The activation energy of alternate reaction pathway is less than that of the reaction pathway without the enzyme

Factors

Factors that affect reaction rate in enzymes are:

Temperature

changing temperature → changes the average kinetic energy of the particles

→ changes the speed of the particles → changes the frequency of collisions → changes the probability of successful collisions → changes reaction rate

AND

→ changes the number of particles with E ≥ Ea → changes the frequency of successful collisions → changes reaction rate

UP UNTIL THE ENZYME DENATURES – the active site changes shape so that the substrate no longer binds to it and rate declines. There is an OPTIMUM temperature.

Concentration

changing concentration → changes the number of particles within a specified space → changes the frequency of collisions → changes the probability of successful collisions → changes reaction rate

UP UNTIL ALL ACTIVE SITES ARE USED ON THE ENZYME SUCH THAT ANY ADDITIONAL CHANGE HAS NOT EFFECT – rate reaches a maximum.

Surface area

changing surface area → changes the number of particles exposed → changes the frequency of collisions → changes the probability of successful collisions → changes reaction rate

UP UNTIL ALL ACTIVE SITES ARE USED ON THE ENZYME SUCH THAT ANY ADDITIONAL CHANGE HAS NOT EFFECT – rate reaches a maximum.

pH

changing pH → modifies the interaction of the substrate with the active site

UP UNTIL THE ENZYME DENATURES – the active site changes shape so that the substrate no longer binds to it and rate declines. There is an OPTIMUM pH.​