DIGESTION
1. Ingestion & Mechanical Digestion (Mouth)
Ingestion is the process of taking food into the body. The mouth is the first site
where digestion starts, both mechanically and chemically.
● Mechanical Digestion: The teeth and tongue work together to physically
break down food into smaller pieces (mastication). This increases the
surface area for enzymes to work on.
● Chemical Digestion: Salivary glands secrete saliva, which contains the
enzyme amylase. Salivary amylase starts the breakdown of starches
(carbohydrates) into simpler sugars like maltose. It begins the chemical
digestion of carbohydrates.
Enzyme Involved:
○ Salivary Amylase: Breaks down starch (a polysaccharide) into
maltose (a disaccharide).
2. Swallowing & Passage through the Esophagus
Once the food is sufficiently broken down in the mouth, it forms a bolus, which is
swallowed. The bolus travels down the esophagus through a process called
peristalsis (rhythmic contraction of muscles).
No digestion occurs in the esophagus itself, but the food moves toward the
stomach.
3. Stomach - Protein Digestion
When food reaches the stomach, the environment changes drastically. The
stomach is highly acidic (pH ~2), and the primary role of the stomach is to further
break down food mechanically and chemically.
● Mechanical Digestion: The stomach churns the food (via smooth muscle
contractions), mixing it with gastric juices. This turns the food into a
semi-liquid substance called chyme.
● Chemical Digestion: Gastric glands in the stomach secrete gastric juice,
which contains hydrochloric acid (HCl) and the enzyme pepsinogen
(which is converted into pepsin).
○ Pepsin is the active enzyme that begins the breakdown of proteins
into smaller peptides (polypeptides). Pepsin works best in acidic
conditions.
● Enzyme Involved:
○ Pepsin: Breaks down proteins into smaller peptides.
● Other Components:
○ Hydrochloric acid (HCl): Lowers the pH of the stomach, creating an
acidic environment optimal for enzyme activity and also killing
harmful microbes.
○ Mucus: Protects the stomach lining from being digested by the acid
and pepsin.
4. Small Intestine - Further Digestion & Absorption
After food leaves the stomach, it enters the small intestine, where most digestion
and absorption occur. The small intestine is divided into three parts: the
duodenum, the jejunum, and the ileum.
Duodenum (First Part of the Small Intestine) - Major Digestive Activity
Here, the acidic chyme from the stomach is neutralized and mixed with bile and
pancreatic juices. This is where the final stages of digestion of carbohydrates,
proteins, and lipids occur.
● Bile: Produced by the liver and stored in the gallbladder, bile is released
into the duodenum. Bile does not contain enzymes but is essential for the
emulsification of fats, which means it breaks large fat globules into
smaller droplets, increasing the surface area for digestion by enzymes.
● Pancreatic Juice: The pancreas releases a mixture of enzymes and
bicarbonate ions into the duodenum. Bicarbonate neutralizes the stomach
acid, raising the pH to around 7-8, which is ideal for enzyme activity.
Enzymes Involved:
○ Pancreatic Amylase: Continues the breakdown of carbohydrates
(starch) into maltose, similar to salivary amylase.
○ Proteases (Trypsin & Chymotrypsin): These enzymes further
break down proteins into smaller peptides and amino acids.
○ Lipase: Breaks down fats into fatty acids and glycerol
(monoglycerides).
○ Nucleases: Break down nucleic acids (DNA and RNA) into their
component nucleotides.
Jejunum and Ileum (Absorption of Nutrients)
The jejunum and ileum are primarily responsible for absorbing the nutrients that
were digested in the duodenum. The walls of the small intestine are lined with villi
and microvilli, tiny finger-like projections that greatly increase the surface area for
absorption.
Nutrients such as amino acids, simple sugars (e.g., glucose), fatty acids,
vitamins, and minerals are absorbed into the bloodstream or lymphatic system
for transport to the rest of the body.
5. Large Intestine - Water Reabsorption & Formation of Feces
After nutrients are absorbed in the small intestine, any undigested food moves
into the large intestine, where water and electrolytes (like sodium) are
reabsorbed.
● Mechanical Digestion: The large intestine does not have enzymes to
digest food. Instead, it uses peristalsis to move waste material toward the
rectum.
● Fermentation: The large intestine contains a large population of bacteria
that help ferment undigested carbohydrates, producing gases (like
methane) and some short-chain fatty acids that can be absorbed and
utilized by the body.
● Feces Formation: The remaining material, now solid, is stored in the
rectum and eliminated through the anus as feces.
Key Enzymes Involved in Digestion:
1. Salivary Amylase (Mouth): Breaks down starch into maltose.
2. Pepsin (Stomach): Breaks down proteins into peptides.
3. Pancreatic Amylase (Small Intestine): Continues carbohydrate digestion,
converting starch into maltose.
4. Trypsin & Chymotrypsin (Small Intestine): Breaks down proteins into
smaller peptides.
5. Lipase (Small Intestine): Breaks down fats into fatty acids and glycerol.
6. Nucleases (Small Intestine): Breaks down nucleic acids (DNA/RNA) into
nucleotides.
Overview of Respiration
Respiration involves the breakdown of glucose and other nutrients to produce ATP, which cells
use for energy. There are two primary types of respiration:
1. Aerobic Respiration (with oxygen)
2. Anaerobic Respiration (without oxygen)
Aerobic respiration is more efficient and occurs in the presence of oxygen, while anaerobic
respiration occurs when oxygen is scarce.
We’ll focus primarily on aerobic respiration, which is the process in eukaryotic cells and
provides the most energy. This involves four main stages:
1. Glycolysis
2. Pyruvate Decarboxylation (Link Reaction)
3. Citric Acid Cycle (Krebs Cycle)
4. Electron Transport Chain & Oxidative Phosphorylation
1. Glycolysis (Occurs in the Cytoplasm)
Glycolysis is the first step in both aerobic and anaerobic respiration. It breaks down one
molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon
compound), producing a small amount of ATP and NADH in the process.
Key Reactions:
● Glucose (6-carbon) → 2 Pyruvate (3-carbon)
● ATP Production: In the initial phase, energy is used to phosphorylate glucose and its
intermediates. In the later steps, ATP is produced.
● NADH Production: NAD⁺ is reduced to NADH, which will be used later in the electron
transport chain.
Enzymes Involved:
1. Hexokinase: Adds a phosphate group to glucose, forming glucose-6-phosphate.
2. Phosphofructokinase (PFK): Converts fructose-6-phosphate into
fructose-1,6-bisphosphate, an important regulatory step in glycolysis.
3. Aldolase: Splits the 6-carbon sugar into two 3-carbon molecules.
4. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH): Catalyzes the conversion of
glyceraldehyde-3-phosphate (G3P) to 1,3-bisphosphoglycerate while reducing NAD⁺
to NADH.
5. Pyruvate Kinase: Catalyzes the final step where phosphoenolpyruvate (PEP) is
converted into pyruvate, producing ATP in the process.
2. Pyruvate Decarboxylation (Link Reaction) (Occurs in the Mitochondrial
Matrix)
Once glycolysis produces pyruvate in the cytoplasm, pyruvate enters the mitochondrial matrix
(in eukaryotic cells) where it undergoes decarboxylation to form acetyl-CoA, which can then
enter the citric acid cycle.
Key Reactions:
● Pyruvate (3-carbon) → Acetyl-CoA (2-carbon) + CO₂ (carbon dioxide is released)
● NAD⁺ is reduced to NADH, which will later contribute to the electron transport chain.
Enzyme Involved:
1. Pyruvate Dehydrogenase Complex (PDC): This multi-enzyme complex catalyzes the
decarboxylation of pyruvate and its conversion into acetyl-CoA.
3. Citric Acid Cycle (Krebs Cycle) (Occurs in the Mitochondrial Matrix)
The Citric Acid Cycle (also known as the Krebs Cycle or TCA Cycle) is a series of
enzyme-catalyzed reactions that take place in the mitochondria. This cycle is responsible for
further breaking down the acetyl-CoA produced in the previous step and extracting high-energy
electrons for the electron transport chain.
Key Reactions:
● Acetyl-CoA (2-carbon) enters the cycle and combines with oxaloacetate (4-carbon) to
form citrate (6-carbon).
● Through a series of reactions, citrate is progressively oxidized and decarboxylated,
releasing carbon dioxide (CO₂) and reducing NAD⁺ to NADH and FAD to FADH₂.
● ATP (or GTP) is also generated.
For each acetyl-CoA, the cycle produces:
● 3 NADH
● 1 FADH₂
● 1 ATP (or GTP)
● 2 CO₂ (released as waste)
Enzymes Involved:
1. Citrate Synthase: Catalyzes the reaction between acetyl-CoA and oxaloacetate to form
citrate.
2. Aconitase: Converts citrate to isocitrate.
3. Isocitrate Dehydrogenase: Converts isocitrate to alpha-ketoglutarate, reducing NAD⁺ to
NADH and releasing CO₂.
4. Alpha-Ketoglutarate Dehydrogenase: Converts alpha-ketoglutarate to succinyl-CoA,
producing NADH and CO₂.
5. Succinate Dehydrogenase: Converts succinate to fumarate, producing FADH₂.
6. Malate Dehydrogenase: Converts malate to oxaloacetate, producing NADH.
4. Electron Transport Chain & Oxidative Phosphorylation (Occurs in the
Inner Mitochondrial Membrane)
The final step of aerobic respiration is the Electron Transport Chain (ETC) and Oxidative
Phosphorylation, which takes place in the inner mitochondrial membrane. The goal here is to
use the high-energy electrons carried by NADH and FADH₂ to produce a large amount of ATP.
Key Reactions:
● Electron Transport Chain: NADH and FADH₂ donate their electrons to protein
complexes (I, II, III, and IV) in the inner mitochondrial membrane. As electrons move
through these complexes, protons (H⁺) are pumped across the membrane, creating a
proton gradient.
● ATP Synthase: This proton gradient drives ATP synthesis by ATP synthase, which uses
the energy from protons moving back across the membrane to convert ADP to ATP.
● Oxygen is the final electron acceptor and combines with electrons and protons to form
water (H₂O).
Enzymes and Components Involved:
1. Complex I (NADH Dehydrogenase): Transfers electrons from NADH to ubiquinone (Q),
pumping protons across the membrane.
2. Complex II (Succinate Dehydrogenase): Transfers electrons from FADH₂ to
ubiquinone.
3. Complex III (Cytochrome bc₁ Complex): Transfers electrons from ubiquinol to
cytochrome c, continuing the proton pumping.
4. Complex IV (Cytochrome c Oxidase): Transfers electrons to oxygen (final electron
acceptor), forming water and pumping protons.
5. ATP Synthase: Uses the proton gradient to synthesize ATP from ADP and inorganic
phosphate (Pi).
ATP Yield from Aerobic Respiration:
● Glycolysis: 2 ATP (net), 2 NADH
● Pyruvate Decarboxylation: 2 NADH (since 2 pyruvate are produced from 1 glucose)
● Citric Acid Cycle: 6 NADH, 2 FADH₂, 2 ATP
● Electron Transport Chain: The NADH and FADH₂ produced in glycolysis, pyruvate
decarboxylation, and the citric acid cycle will be used to generate about 34 ATP.
Total ATP from one molecule of glucose (aerobic respiration) is approximately 38 ATP (the
exact number can vary depending on the system used for counting ATP production).
Anaerobic Respiration:
When oxygen is not available, cells can perform anaerobic respiration, which is less efficient
but still generates some ATP. In this process, glycolysis produces pyruvate, but since oxygen is
lacking, pyruvate is converted into lactic acid (in animals) or ethanol and CO₂ (in yeast),
regenerating NAD⁺ so glycolysis can continue.
