Download Digestion and Metabolism

Metabolic Pathways
Digestion – the process of converting large molecules to small ones that can be absorbed by cells.
Digestion of carbohydrates:
- The digestion starts in mouth:
Salivary α-amylase starts breaking down starches  dextrins, maltose
- Continues in stomach for ~1 hr.
- In the small intestine:
α-amylase from pancreas breaks down the starch and dextrins  maltose
other enzymes hydrolyze the branches (α-1,6-glycosidic bonds)
then maltose, sucrose and lactose are hydrolyzed by maltase, sucrase and lactase.
- Monosaccharides pass through the intestinal walls into the bloodstream and are transported to cells.
- In the liver, fructose and galactose are isomerized into glucose.
Digestion of triacylglycerols (lipids):
(lots of energy is stored in our bodies as fat.)
- The breakdown of the triacylglycerols does not occur in the stomach, but starts once the partially digested
food has entered the small intestine.
- In the small intestine bile salts from the gall bladder emulsify fats by forming micelles (bile salts are
similar to soaps with a nonpolar part and a charged part.).
- Pancreatic lipases hydrolyze some of the triglycerides to form monoglycerides, diglycerides, fatty acids
and glycerol. These are absorbed into the intestinal lining, then re-attached to form TAG’s. Finally, they are
coated with proteins to form lipoproteins (known as chylomicrons) that can travel through the bloodstream.
- In cells, TAG’s are hydrolyzed to get the glycerol and the fatty acids.
Digestion of proteins:
Proteins are mostly not used for energy. (Usually, only 10% of our energy needs are satisfied by
metabolism of amino acids. But, if fasting or starvation occurs, you can get the energy from breaking down
of proteins, which in turn breaks down essential tissues.) They are broken down to amino acids, which are
then used for building new proteins.
- Digestion starts in the stomach. The stomach acid (pH 2) denatures proteins and activates pepsin, which
starts hydrolyzing the peptide bonds.
- In the small intestine (pH 8), trypsin and chymotrypsin continue hydrolysis.
- The amino acids are then absorbed through the intestinal walls into the bloodstream and travel to the cells.
Our proteins are constantly being degraded and new ones are formed.
A. Reyes - free to copy for educational purposes
1
Chem 30B
Metabolism – Is the complete set of reactions that occur in living cells.
1. Catabolic reactions – break down molecules and produce energy.
2. Anabolic reactions – use energy to build larger molecules or to do work in the cell (like contracting muscle).
Spontaneous and non spontaneous reactions:
A spontaneous reaction (or exergonic rection) is "a chemical change that occurs without the addition of energy";
it can also be described as a "reaction that will occur without any energy input from the surroundings or a reaction
that will occur on its own."
Ex.
ATP +
Adenosine
triphosphate
H2 O

ADP +
Adenosine
diphosphate
Pi
+
7.3 kcal/mol (or 31 kJ/mol)
Inorganic phosphate: HPO4-2 or H2PO4at pH 7 there’s equal amounts.
For the above reaction the ΔG = -7.3kcal/mol (or 31 kJ/mol). The negative sign indicates that the system
(the reaction in this case) lose energy, giving it to the surroundings.
A non spontaneous reaction (also called an unfavorable reaction or an endergonic reaction) is "a chemical
change that does not occur without the addition of energy". In other words, it needs to be “pushed”.
Ex.
Glucose
+
Pi

Glucose-6-P
+
H2 O
For this case, the ΔG = +3.3kcal/mol. The positive sign indicates that the system (the reaction in this case)
gained energy (increased its energy by 3.3 kcal/mol), and that energy comes from the surroundings (it was the
“push” for the reaction to happen.).
Endergonic reactions (non spontaneous) can be pushed by coupling them to another reaction which is strongly
exergonic (spontaneous), through a shared intermediate. This is often how biological reactions proceed. For
example, if we take the two reactions above we get:
ATP
+
+
Glucose
Glucose
H2 O
+
+

ADP
+
Pi
ΔG = +7.3 kcal/mol
Pi  Glucose-6-P + H2O
ATP  Glucose-6-P + ADP
ΔG = –3.3 kcal/mol
ΔG = –4.0 kcal/mol
Again, the negative sign indicates that the process is spontaneous overall. (The hydrolysis or ATP provides
the energy for the formation of glucose-6-phosphate)
We can think of ATP as the energy “currency” of the cell. In the cell, energy can be “stored” by forming ATP from
ADP. Then the ATP can be used later, when needed.
A. Reyes - free to copy for educational purposes
2
Chem 30B