Definition
Autotrophic nutrition is the ability to make one's own food from simple inorganic substances (as plants do via photosynthesis). Heterotrophic nutrition means obtaining food ready-made by consuming other organisms — humans and other animals cannot make their own food, so they rely entirely on heterotrophic nutrition.
The five steps of nutrition in animals, in order: ingestion (taking food in), digestion (breaking it down), absorption (taking digested nutrients into the body), assimilation (using those nutrients in cells), and egestion (removing undigested waste).
Definition
Digestion is the breakdown of large, complex food molecules (polymers, like starch, proteins, and fats) into smaller, simpler molecules (monomers, like glucose, amino acids, and fatty acids) — this is necessary because large polymers are too big to pass through cell membranes and enter the bloodstream, while their smaller monomer building blocks can be absorbed.
Definition
The oral cavity begins digestion mechanically (chewing, which increases surface area for enzymes) and chemically (salivary glands release saliva containing amylase, which starts breaking down starches into simpler sugars).
As food is chewed and mixed with saliva, it forms a soft, rounded mass called a bolus, ready to be swallowed.
Swallowing pushes the bolus from the mouth into the oesophagus; peristalsis is the coordinated, wavelike contraction of muscles that then pushes the bolus (and later, food generally) along the length of the digestive tract.
During swallowing, a flap called the epiglottis closes over the opening of the trachea (windpipe), preventing food from entering the lungs and directing it into the oesophagus instead.
Definition
The oesophagus is the part of the digestive system where no chemical digestion takes place — it serves only as a muscular tube that transports the bolus from mouth to stomach via peristalsis.
Definition
The stomach churns food (mechanical digestion) and continues chemical digestion, producing gastric juice containing HCl (hydrochloric acid) and the enzyme pepsinogen.
Functions of HCl in gastric juice: it kills most bacteria ingested with food, provides the strongly acidic environment () that pepsin needs to work effectively, and converts inactive pepsinogen into active pepsin.
Pepsinogen is converted into active pepsin by the action of HCl (and by pepsin itself, once some is formed, in a self-sustaining process) — pepsin then begins the digestion of proteins into shorter peptide chains.
The cardiac sphincter (at the top of the stomach, where the oesophagus enters) controls food entering the stomach and normally prevents stomach contents from flowing back up; the pyloric sphincter (at the bottom, leading to the small intestine) controls the release of stomach contents into the duodenum in controlled amounts.
The soft, soup-like mixture formed after food is churned and partly digested in the stomach is called chyme.
Remember
A malfunctioning cardiac sphincter can lead to acid reflux, because if it doesn't close properly, acidic stomach contents can flow back up into the oesophagus, whose lining (unlike the stomach's) isn't well-protected against strong acid, causing irritation and a burning sensation (heartburn).
Definition
The liver produces bile, which is stored in the gallbladder and released via the common bile duct into the duodenum. Bile doesn't contain digestive enzymes — instead, it emulsifies large fat droplets into many smaller droplets (a process that greatly increases their combined surface area), making it easier for lipase enzymes to act on the fat effectively.
Bile needs to be stored in the gallbladder before release because it's produced continuously by the liver, but only needed intermittently, specifically when fatty food actually enters the duodenum — storing it lets a concentrated amount be released exactly when needed, rather than being wasted or under-supplied.
Four other functions of the liver besides bile production: detoxification of harmful substances (like alcohol and drugs), storage of glucose as glycogen (and its release when blood sugar is low), production of blood plasma proteins, and breakdown of old red blood cells.
The pancreas produces digestive enzymes and releases them into the duodenum (via the pancreatic duct) — these include enzymes that digest carbohydrates, proteins (like trypsin), and fats.
Remember
Someone with liver disease has difficulty digesting fats because a damaged liver can't produce enough bile — without sufficient bile to emulsify fat, lipase enzymes have far less surface area to work on, making fat digestion and absorption much less efficient.
Definition
The three portions of the small intestine, in order: duodenum, jejunum, and ileum.
Carbohydrates, lipids, and proteins are all digested in the small intestine (their digestion, begun in the mouth/stomach, is completed here), aided by enzymes from the pancreas and the intestine's own lining, plus bile from the liver/gallbladder for fats.
Trypsin (a protein-digesting enzyme from the pancreas) is found and active in the small intestine, where it continues breaking down proteins/peptides that pepsin started digesting in the stomach.
Amylase enzymes (from saliva and the pancreas) break down starches into simpler sugars.
The small intestine has many folds, and its lining is covered in tiny finger-like projections called villi (each covered in even smaller microvilli) — all of this dramatically increases the internal surface area available for absorbing digested nutrients into the blood, which is essential since absorption efficiency depends directly on how much surface area is in contact with digested food.
Definition
Structure of a villus: it has a thin, single layer of cells (short diffusion distance for nutrients), a rich network of blood capillaries (to carry away absorbed sugars and amino acids), and a lacteal (a small lymph vessel) at its core (to carry away absorbed fats).
Definition
Absorption in the small intestine: digested sugars (like glucose) and amino acids pass into the blood capillaries within the villi and are carried directly to the liver via the bloodstream. Digested fat products, however, are absorbed differently — they enter the lacteals (lymph vessels) within the villi rather than blood capillaries directly, travelling through the lymphatic system before eventually entering the bloodstream, since fat digestion products first need to be reassembled into a form that can travel this way.
Definition
The three parts of the large intestine: caecum, colon, and rectum.
In the large intestine, water and some remaining salts are absorbed from the indigestible remains of food, which is gradually compacted into solid faeces for elimination.
It's important to maintain a healthy balance of gut bacteria because beneficial bacteria help digest certain substances the body can't break down alone, produce some vitamins, and help keep harmful bacteria in check — an imbalance can lead to digestive problems and make the gut more vulnerable to infection.
The appendix is a small pouch attached near the junction of the small and large intestine; while its exact function in humans is debated (possibly a minor role in gut immunity or maintaining beneficial gut bacteria), if it becomes infected (appendicitis) it can become dangerously inflamed and, if untreated, can rupture, causing a serious, potentially life-threatening infection that requires urgent surgical removal.
Definition
Ulcers are painful sores that form in the lining of the digestive tract, most often caused by infection with the bacterium H. pylori (or overuse of certain pain-relief drugs), which damages the protective mucus lining and lets stomach acid irritate the tissue beneath. Symptoms include burning stomach pain, bloating, and nausea.
Based on their location, ulcers in the stomach, duodenum, and oesophagus are called gastric ulcers, duodenal ulcers, and oesophageal ulcers respectively.
Diarrhoea (frequent, loose/watery stools) is usually caused by infection (bacterial, viral, or parasitic) or food intolerance, often via contaminated food or water. It's prevented mainly through good hygiene, safe drinking water, and proper food handling and cooking.
Constipation involves infrequent, difficult bowel movements with hard stools. Signs/symptoms include straining, hard/dry stool, and a feeling of incomplete emptying. Common causes include low fibre intake, insufficient water intake, and lack of physical activity. Treatment/prevention centres on increasing dietary fibre, drinking more water, and regular exercise.
Quick Recap
- ●Humans: heterotrophic nutrition; five steps: ingestion digestion absorption assimilation egestion
- ●Mouth: chewing salivary amylase bolus; swallowing epiglottis (blocks trachea) peristalsis oesophagus (no chemical digestion) stomach
- ●Stomach: HCl (kills bacteria, activates pepsin, gives acidic pH) pepsin (protein digestion) chyme; cardiac/pyloric sphincters control flow
- ●Liver: bile (emulsifies fat, stored in gallbladder) detox/glycogen storage/plasma proteins/RBC breakdown. Pancreas: digestive enzymes (incl. trypsin) into duodenum
- ●Small intestine (duodenum, jejunum, ileum): completes digestion; villi/microvilli maximize absorption surface area; sugars/amino acids blood capillaries, fats lacteals (lymph)
- ●Large intestine (caecum, colon, rectum): absorbs water/salts, forms faeces; gut bacteria balance matters; appendix — infection is dangerous
- ●Disorders: ulcers (H. pylori, acid damage), diarrhoea (infection/contamination), constipation (low fibre/water/activity)
