🥗 Nutrition and Absorption
Learn digestion and absorption of carbohydrates, proteins and fats for CUET Agriculture. Intestinal villi, active transport and assimilation.
Complete Summary of Digestion
The following table traces the step-by-step breakdown of each major nutrient as it passes through the digestive system. Notice how digestion is a sequential process — each region builds on the work of the previous one, and the final products are always the simplest absorbable molecules.
| Nutrient | Mouth | Stomach | Duodenum (Pancreatic) | Small Intestine (Brush border) | Final Product |
|---|---|---|---|---|---|
| Starch | Salivary amylase → Maltose | — | Pancreatic amylase → Maltose | Maltase → Glucose | Glucose |
| Proteins | — | Pepsin → Peptides | Trypsin, Chymotrypsin → Peptides | Aminopeptidase, Dipeptidase → Amino acids | Amino acids |
| Fats | — | Gastric lipase (minor) | Bile salts emulsify; Pancreatic lipase → Fatty acids + Glycerol | Intestinal lipase | Fatty acids + Glycerol |
| Sucrose | — | — | — | Sucrase → Glucose + Fructose | Monosaccharides |
| Lactose | — | — | — | Lactase → Glucose + Galactose | Monosaccharides |
| Nucleic acids | — | — | Nucleases → Nucleotides | Nucleotidase, Nucleosidase → Bases + Sugars | Bases + Sugars |
TIP
Starch digestion begins in the mouth (salivary amylase) but is paused in the stomach (acid inactivates amylase) and resumes in the duodenum (pancreatic amylase). Protein digestion is the opposite — it begins in the stomach with pepsin and continues in the intestine.
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Complete Summary of Digestion
The following table traces the step-by-step breakdown of each major nutrient as it passes through the digestive system. Notice how digestion is a sequential process — each region builds on the work of the previous one, and the final products are always the simplest absorbable molecules.
| Nutrient | Mouth | Stomach | Duodenum (Pancreatic) | Small Intestine (Brush border) | Final Product |
|---|---|---|---|---|---|
| Starch | Salivary amylase → Maltose | — | Pancreatic amylase → Maltose | Maltase → Glucose | Glucose |
| Proteins | — | Pepsin → Peptides | Trypsin, Chymotrypsin → Peptides | Aminopeptidase, Dipeptidase → Amino acids | Amino acids |
| Fats | — | Gastric lipase (minor) | Bile salts emulsify; Pancreatic lipase → Fatty acids + Glycerol | Intestinal lipase | Fatty acids + Glycerol |
| Sucrose | — | — | — | Sucrase → Glucose + Fructose | Monosaccharides |
| Lactose | — | — | — | Lactase → Glucose + Galactose | Monosaccharides |
| Nucleic acids | — | — | Nucleases → Nucleotides | Nucleotidase, Nucleosidase → Bases + Sugars | Bases + Sugars |
TIP
Starch digestion begins in the mouth (salivary amylase) but is paused in the stomach (acid inactivates amylase) and resumes in the duodenum (pancreatic amylase). Protein digestion is the opposite — it begins in the stomach with pepsin and continues in the intestine.
Absorption
Absorption is the process by which the final products of digestion pass through the intestinal mucosa into the blood or lymph. Different nutrients use different transport mechanisms and enter different circulatory pathways. The small intestine (especially the jejunum) is the primary absorption site due to its enormous surface area (~250 m²).
| Nutrient | Mechanism | Site | Route |
|---|---|---|---|
| Glucose, amino acids | Active transport (Na⁺-coupled cotransport) | Small intestine (jejunum mainly) | Blood capillaries → portal vein → liver |
| Fructose | Facilitated diffusion (GLUT5 transporter) | Small intestine | Blood capillaries → liver |
| Fatty acids, glycerol | Diffusion into enterocytes; re-esterified to triglycerides → packaged into chylomicrons | Small intestine | Lacteals → lymph → thoracic duct → blood |
| Water | Osmosis (follows solute movement) | Small and large intestine | Blood |
| Electrolytes (Na⁺, K⁺, Ca²⁺, Fe²⁺) | Active transport / facilitated diffusion | Small intestine | Blood |
| Vitamin B₁₂ | Active transport (requires intrinsic factor from parietal cells) | Terminal ileum specifically | Blood |
| Fat-soluble vitamins (A, D, E, K) | Absorbed with fats as part of micelles | Small intestine | Lacteals → lymph |
| Water-soluble vitamins (B, C) | Diffusion / active transport | Small intestine | Blood |
IMPORTANT
Fats take a unique absorption route. Unlike water-soluble nutrients that enter blood capillaries directly, fatty acids are reassembled into triglycerides inside enterocytes, packaged into chylomicrons (lipoprotein particles), and released into lacteals (lymphatic vessels). From there, they travel through the lymphatic system to the thoracic duct and finally enter the bloodstream at the left subclavian vein — completely bypassing the liver initially.
Why does Vitamin B₁₂ deficiency cause pernicious anaemia?
Vitamin B₁₂ absorption is uniquely dependent on **intrinsic factor**, a glycoprotein secreted by the parietal cells of the stomach. If parietal cells are damaged (as in autoimmune gastritis) or the stomach is surgically removed, intrinsic factor is absent, and B₁₂ cannot be absorbed in the terminal ileum — even if the diet contains adequate B₁₂. This leads to **pernicious anaemia**, characterized by abnormally large, immature red blood cells (megaloblasts) and neurological symptoms.Large Intestine
The large intestine is the final segment of the alimentary canal, measuring approximately 1.5 m in length with a diameter of about 6 cm. Although no enzymatic digestion occurs here, the large intestine performs several vital functions related to water recovery, vitamin production, and waste elimination.
Parts (in sequence): Caecum (with vermiform appendix) → Ascending colon → Transverse colon → Descending colon → Sigmoid colon → Rectum → Anal canal → Anus
Functions of the Large Intestine:
- Water absorption — recovers ~90% of remaining water from the undigested residue, converting liquid chyme into semi-solid feces
- Electrolyte absorption — actively absorbs Na⁺ and Cl⁻ ions
- Bacterial synthesis of vitamins — the resident gut flora (especially E. coli and Bacteroides) synthesize vitamin K, biotin, and folic acid, which are absorbed here. This is a crucial source of vitamin K for the body.
- Feces formation and storage — compacts undigested residue, dead bacteria, mucus, and shed epithelial cells into feces
- Mucus secretion — the large intestine secretes mucus (but no enzymes) to lubricate feces and bind them together for easier passage
The defecation reflex is triggered when feces distend the rectum. This initiates mass peristalsis and relaxation of the internal anal sphincter (smooth muscle, involuntary). The external anal sphincter (skeletal muscle, voluntary) can be consciously controlled, allowing defecation to be delayed when socially inappropriate.
What is the role of the vermiform appendix?
The **vermiform appendix** is a narrow, finger-like projection attached to the caecum. Long considered vestigial, recent research suggests it serves as a **reservoir for beneficial gut bacteria**, helping to repopulate the intestine after diarrhoeal illness. It also contains lymphoid tissue, contributing to mucosal immunity. However, if it becomes blocked and inflamed, it causes **appendicitis** — a surgical emergency.GI Hormones
The gastrointestinal tract is the largest endocrine organ in the body — it secretes numerous hormones that coordinate digestive secretions, motility, and appetite. These hormones act in a finely tuned sequence that matches digestive secretions to the type of food present.
| Hormone | Secreted By | Stimulus | Action |
|---|---|---|---|
| Gastrin | G-cells (pyloric stomach) | Protein in stomach, vagal stimulation | Stimulates HCl secretion and gastric motility — ramps up stomach activity when food arrives |
| Secretin | S-cells (duodenal mucosa) | Acidic chyme entering duodenum | Stimulates pancreatic bicarbonate secretion to neutralize acid; inhibits gastric acid — protects the duodenum |
| Cholecystokinin (CCK) | I-cells (duodenal mucosa) | Fats and amino acids in duodenum | Stimulates pancreatic enzyme secretion; causes gallbladder contraction (releasing bile); inhibits gastric emptying — ensures fats are properly emulsified before more chyme arrives |
| GIP (Gastric Inhibitory Peptide) | K-cells (duodenal/jejunal mucosa) | Glucose, fats in duodenum | Stimulates insulin release from pancreas; inhibits gastric acid and motility — links gut activity to blood sugar regulation |
| Enterogastrone | Duodenal mucosa | Fats in duodenum | Inhibits gastric secretion and motility — slows stomach emptying when fatty food is detected (fat takes longest to digest) |
| VIP (Vasoactive Intestinal Peptide) | Neurons in gut wall | Various | Causes vasodilation in gut blood vessels; stimulates intestinal secretion; inhibits acid production |
NOTE
Secretin holds a special place in biology history — it was the first hormone ever discovered (by Bayliss and Starling in 1902). Its discovery proved that chemical messengers, not just nerves, could coordinate body functions.
Disorders of the Digestive System
| Disorder | Features |
|---|---|
| Jaundice | Yellowing of skin and eyes due to excess bilirubin in the blood. Three types: obstructive (bile duct blocked by gallstone or tumour), hepatic (liver damage from hepatitis or cirrhosis), hemolytic (excessive RBC breakdown producing too much bilirubin) |
| Vomiting | Forceful expulsion of stomach contents through the mouth; controlled by the vomiting center in the medulla oblongata. Prolonged vomiting can cause dehydration and metabolic alkalosis (loss of HCl). |
| Diarrhoea | Frequent loose or watery stools; caused by infection, toxins, or motility disorders. Major danger is dehydration and electrolyte loss — treated with ORS (oral rehydration solution). |
| Constipation | Infrequent, hard stools that are difficult to pass; caused by low fibre intake, dehydration, reduced colonic motility, or medication side effects |
| Indigestion / Dyspepsia | Discomfort in upper abdomen; commonly caused by overeating, spicy food, anxiety, or NSAID use |
| Peptic ulcer | Erosion of the mucosal lining in the stomach (gastric ulcer) or duodenum (duodenal ulcer); most commonly caused by Helicobacter pylori infection, which damages the protective mucus layer |
| GERD | Gastro-oesophageal reflux disease; stomach acid refluxes into the oesophagus due to a weak cardiac sphincter, causing heartburn and potential oesophageal damage |
| Appendicitis | Inflammation of the vermiform appendix, usually due to blockage; presents with pain in the lower right abdomen; a surgical emergency if untreated (risk of rupture and peritonitis) |
| Gallstones (Cholelithiasis) | Crystallized deposits of cholesterol or bile pigments in the gallbladder; can block the bile duct, causing severe pain (biliary colic) and obstructive jaundice |
| Cirrhosis | Chronic, irreversible liver damage with extensive fibrosis (scar tissue replacing functional liver cells); commonly caused by chronic alcohol abuse or viral hepatitis (B, C) |
| Hiatal hernia | Part of the stomach pushes upward through the diaphragm into the thoracic cavity; can worsen GERD symptoms |
Protein-Energy Malnutrition (PEM)
PEM is a serious nutritional deficiency that primarily affects young children in developing regions. The two major forms represent a spectrum of malnutrition:
| Condition | Deficiency | Age | Features |
|---|---|---|---|
| Marasmus | Total calorie deficiency (both protein and energy) | <1 year typically | Severe wasting of muscles and fat, extremely thin limbs, "old man face" appearance due to loss of buccal fat pads, child remains alert despite extreme emaciation |
| Kwashiorkor | Primarily protein deficiency (calories may be adequate from starchy foods) | 1–5 years | Characteristic oedema (especially distended belly from ascites), moon face, skin lesions (flaky paint dermatosis), fatty liver (proteins needed for lipoprotein synthesis are lacking), apathy and irritability |
TIP
"Kwashiorkor" comes from a Ghanaian word meaning "the disease the first child gets when the second child is born" — referring to when the older child is weaned onto a protein-poor, starchy diet.
Vitamins
Vitamins are organic compounds required in small amounts for normal metabolism. They cannot be synthesized in sufficient quantities by the body and must be obtained from the diet. They are classified into two groups based on solubility.
Fat-Soluble Vitamins (A, D, E, K)
These vitamins dissolve in fats and are absorbed along with dietary fats in the small intestine via micelles and lacteals. They can be stored in the liver and adipose tissue, so deficiency develops slowly but toxicity (hypervitaminosis) is also possible with excessive intake.
| Vitamin | Chemical Name | Sources | Deficiency Disease |
|---|---|---|---|
| A | Retinol | Carrot, liver, fish oil, dairy, yellow-orange vegetables | Night blindness (nyctalopia), xerophthalmia (dry eyes), keratomalacia (corneal damage) |
| D | Calciferol | Sunlight (UV converts skin cholesterol to vitamin D), fish oil, egg yolk | Rickets (children — soft, bowed bones), osteomalacia (adults — soft bones) |
| E | Tocopherol | Vegetable oils, nuts, wheat germ, seeds | Sterility (reproductive failure), haemolytic anemia in infants |
| K | Phylloquinone | Green leafy vegetables, gut bacteria synthesize it | Bleeding disorders — impaired blood clotting (vitamin K is essential for synthesis of prothrombin and factors VII, IX, X) |
Water-Soluble Vitamins (B-complex, C)
These vitamins dissolve in water and are not stored in significant amounts — excess is excreted in urine. Therefore, they need to be consumed regularly. The B-complex vitamins primarily function as coenzymes in metabolic reactions.
| Vitamin | Chemical Name | Deficiency Disease |
|---|---|---|
| B₁ | Thiamine | Beriberi — two forms: wet beriberi (cardiac — heart failure, oedema) and dry beriberi (neurological — peripheral neuropathy, muscle wasting) |
| B₂ | Riboflavin | Cheilosis (cracked, inflamed lips), glossitis (swollen tongue) |
| B₃ | Niacin | Pellagra — remembered by the 3 D's: Dermatitis, Diarrhoea, Dementia (a 4th D — Death — if untreated) |
| B₅ | Pantothenic acid | Burning feet syndrome (rare, as B₅ is found in almost all foods — "pantothenic" means "from everywhere") |
| B₆ | Pyridoxine | Peripheral neuropathy, sideroblastic anaemia |
| B₇ | Biotin | Dermatitis, hair loss (rare, as gut bacteria also produce biotin) |
| B₉ | Folic acid | Megaloblastic anaemia (large, immature RBCs); neural tube defects in fetus (spina bifida, anencephaly) — hence folic acid supplementation is critical before and during early pregnancy |
| B₁₂ | Cyanocobalamin | Pernicious anaemia (requires intrinsic factor for absorption in terminal ileum); also causes neurological damage. B₁₂ is found only in animal products — strict vegans are at risk. |
| C | Ascorbic acid | Scurvy — bleeding gums, loose teeth, poor wound healing, weakened blood vessels (vitamin C is essential for collagen synthesis) |
Minerals
Minerals are inorganic elements essential for numerous physiological processes. Unlike vitamins, they cannot be destroyed by heat or cooking but can be lost by leaching into cooking water.
| Mineral | Function | Deficiency |
|---|---|---|
| Calcium (Ca) | Bones, teeth, blood clotting, muscle contraction, nerve signaling | Rickets (children), osteoporosis (elderly — brittle bones), tetany (muscle spasms from low Ca²⁺) |
| Phosphorus (P) | Bones, teeth, ATP (energy currency), nucleic acids, phospholipid membranes | Weakness, bone pain (rare as phosphorus is abundant in diet) |
| Iron (Fe) | Component of hemoglobin (O₂ transport), myoglobin (muscle O₂ storage), cytochromes (electron transport chain) | Iron-deficiency anaemia — the most common nutritional deficiency worldwide |
| Iodine (I) | Required for thyroid hormones (T₃ and T₄) that regulate metabolism | Goitre (enlarged thyroid), cretinism (children — stunted growth, intellectual disability), myxoedema (adults) |
| Sodium (Na) | Fluid balance, nerve impulse conduction, muscle contraction | Hyponatremia (muscle cramps, confusion, seizures in severe cases) |
| Potassium (K) | Nerve impulse, muscle contraction (especially cardiac), fluid balance | Hypokalemia (weakness, cardiac arrhythmia — potentially fatal) |
| Zinc (Zn) | Enzyme cofactor (over 300 enzymes), wound healing, immune function, taste sensation | Growth retardation, delayed wound healing, impaired immunity |
| Fluorine (F) | Strengthens tooth enamel by forming fluoroapatite (more resistant to acid) | Dental caries (tooth decay); excess → fluorosis (mottled teeth, skeletal abnormalities) |
Ruminant Digestive System
Ruminants (cattle, sheep, goat, buffalo) are herbivores with a highly specialized digestive system. Since mammals lack the enzyme cellulase needed to digest cellulose (the main component of plant cell walls), ruminants rely on symbiotic microorganisms housed in a 4-chambered stomach:
| Chamber | Function |
|---|---|
| Rumen | The largest chamber — essentially a massive fermentation vat. Symbiotic bacteria and protozoa break down cellulose and produce volatile fatty acids (VFAs): acetic acid, propionic acid, and butyric acid. These VFAs are absorbed directly through the rumen wall and serve as the primary energy source for the animal. |
| Reticulum | Has a distinctive "honeycomb" pattern on its inner surface. Works closely with the rumen to sort food particles. Smaller, well-digested particles pass forward; larger particles are regurgitated back to the mouth for further chewing — this is the process of rumination (cud chewing). |
| Omasum | Known as the "many plies" or "book stomach" due to its leaf-like internal folds. Absorbs water and VFAs; reduces particle size mechanically before passing food to the abomasum. |
| Abomasum | The "true stomach" — functionally equivalent to the monogastric (single-chambered) stomach. Contains gastric glands that secrete HCl and pepsin for enzymatic protein digestion. |
- Rumination cycle: food → rumen (initial fermentation) → regurgitation to mouth → re-chewing (cud chewing) → re-swallowing → rumen → reticulum → omasum → abomasum
- Rumen microbes can digest cellulose using cellulase — an enzyme completely absent in mammalian cells
- Rumen microbes also synthesize B vitamins and vitamin K, making ruminants largely independent of dietary sources for these vitamins
NOTE
Ruminants produce significant amounts of methane (CH₄) as a byproduct of microbial fermentation in the rumen. A single cow can produce 70–120 kg of methane per year, making livestock a notable contributor to greenhouse gas emissions.
Beginner's Box — Practice Questions
Set 1: Digestive Anatomy
-
Total number of permanent teeth in adult human: Answer: 32
-
The hardest substance in the human body is: Answer: Enamel
-
Which salivary gland secretes primarily serous (watery) secretion? Answer: Parotid gland
-
The enzyme in saliva that digests starch is: Answer: Salivary amylase (ptyalin)
-
The longest part of the alimentary canal is: Answer: Small intestine (~6 m)
Set 2: Stomach and Enzymes
-
Parietal cells secrete: Answer: HCl and intrinsic factor
-
Pepsinogen is activated to pepsin by: Answer: HCl
-
The pH of gastric juice is approximately: Answer: 1.5–3.5
-
Enterokinase activates which enzyme? Answer: Trypsinogen → Trypsin
-
Bile is produced by which organ? Answer: Liver (hepatocytes)
Set 3: Absorption and Nutrition
-
Fatty acids are absorbed into: Answer: Lacteals (lymphatic vessels)
-
Vitamin B₁₂ is absorbed in which part of the intestine? Answer: Terminal ileum
-
Deficiency of niacin (B₃) causes: Answer: Pellagra
-
Kwashiorkor is caused primarily by deficiency of: Answer: Protein
-
The "true stomach" of a ruminant is: Answer: Abomasum
Set 4: Hormones and Disorders
-
Secretin stimulates secretion of: Answer: Pancreatic bicarbonate
-
CCK stimulates which two actions? Answer: Pancreatic enzyme secretion and gallbladder contraction
-
Peptic ulcers are commonly caused by: Answer: Helicobacter pylori
-
Which bacteria synthesize vitamin K in the large intestine? Answer: Gut flora (intestinal bacteria, e.g., E. coli)
-
The dental formula for milk teeth is: Answer: I 2/2, C 1/1, M 2/2 = 20 teeth
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Complete Digestion Summary | Mouth: salivary amylase (starch → maltose, pH 6.8) Stomach: pepsin (proteins → peptides, pH 1.5–2.5), gastric lipase (minor fat digestion), rennin (milk curdling in infants) Duodenum: bile salts emulsify fats; pancreatic enzymes (trypsin, chymotrypsin, lipase, amylase) Ileum: intestinal juice completes digestion (maltase, sucrase, lactase, peptidases) |
| Absorption — Small Intestine | Primary site of absorption (~90% of nutrients) Monosaccharides & amino acids: absorbed into blood capillaries of villi via active transport and facilitated diffusion Fatty acids & glycerol: absorbed into lacteals (lymph vessels) as chylomicrons Vitamins: fat-soluble (A, D, E, K) absorbed with fats; water-soluble (B, C) absorbed directly Iron: absorbed in duodenum Vitamin B₁₂: absorbed in ileum (requires intrinsic factor from stomach) |
| Absorption Mechanisms | Active transport: against concentration gradient (glucose, amino acids, Na⁺) Passive diffusion: water, small lipids Facilitated diffusion: fructose Pinocytosis: large molecules |
| Large Intestine | ~1.5 m long; regions: caecum (with vermiform appendix), ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal Functions: water absorption, electrolyte absorption, vitamin K and B₁₂ synthesis by gut bacteria, faeces formation |
| GI Hormones | Gastrin (from G-cells of stomach): stimulates HCl and pepsinogen secretion Secretin (from duodenum): stimulates pancreatic bicarbonate secretion (neutralises acid) CCK / Cholecystokinin (from duodenum): stimulates pancreatic enzyme secretion + gallbladder contraction GIP / Gastric Inhibitory Peptide: inhibits gastric secretion, stimulates insulin release Enterogastrone: slows gastric emptying when fats enter duodenum |
| GI Disorders | Jaundice: excess bilirubin in blood → yellow skin/eyes Vomiting: controlled by vomiting centre in medulla oblongata Diarrhoea: frequent loose stools, abnormal watery stool frequency Constipation: infrequent bowel movements, hard dry stools Indigestion (Dyspepsia): pain/discomfort in upper abdomen Peptic ulcers: commonly caused by Helicobacter pylori |
| PEM — Marasmus | Deficiency of both protein and calories Age: usually <1 year Features: extreme wasting, no oedema, shrunken limbs, "old man" appearance, body weight <60% of normal |
| PEM — Kwashiorkor | Deficiency of protein (adequate calories) Age: usually 1–5 years Features: oedema (swollen belly, feet), moon face, flaky paint dermatosis, fatty liver, muscle wasting under oedema |
| Fat-Soluble Vitamins | Vitamin A (Retinol): night vision, epithelial health; deficiency → xerophthalmia, night blindness; source: carrots, liver Vitamin D (Calciferol): calcium absorption; deficiency → rickets (children), osteomalacia (adults); source: sunlight, fish oil Vitamin E (Tocopherol): antioxidant, fertility; deficiency → sterility; source: vegetable oils, nuts Vitamin K (Phylloquinone): blood clotting (prothrombin synthesis); deficiency → bleeding disorders; source: green leafy vegetables, gut bacteria |
| Water-Soluble Vitamins | Vitamin B₁ (Thiamine): deficiency → beriberi Vitamin B₂ (Riboflavin): deficiency → cheilosis (cracked lips) Vitamin B₃ (Niacin): deficiency → pellagra (3 D's: dermatitis, diarrhoea, dementia) Vitamin B₅ (Pantothenic acid): coenzyme A component Vitamin B₆ (Pyridoxine): amino acid metabolism Vitamin B₇ (Biotin): deficiency → dermatitis, hair loss Vitamin B₉ (Folic acid): DNA synthesis; deficiency → megaloblastic anaemia, neural tube defects Vitamin B₁₂ (Cobalamin): RBC formation; deficiency → pernicious anaemia; requires intrinsic factor Vitamin C (Ascorbic acid): collagen synthesis; deficiency → scurvy |
| Important Minerals | Calcium: bones, teeth, clotting, muscle contraction; deficiency → osteoporosis Iron: haemoglobin; deficiency → iron-deficiency anaemia Iodine: thyroid hormones; deficiency → goitre, cretinism Sodium/Potassium: nerve impulse, fluid balance Fluorine: enamel hardening; excess → fluorosis Zinc: enzyme cofactor, immunity, wound healing |
| Ruminant Digestive System | 4 stomach chambers: Rumen (largest, microbial fermentation) → Reticulum (honeycomb, sorts food, regurgitation for cud chewing) → Omasum (absorbs water) → Abomasum ("true stomach", has gastric glands, enzymatic digestion) Cellulose digested by symbiotic bacteria and protozoa in rumen |
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