Human digestion system

State definition of ingestion

State definition of ingestion

Consumption of / taking food into gut

Define digestion

Mechanical and chemical breakdown of large insoluble molecules; carbohydrates, fats and proteins into smaller soluble products

Define absorption

The process by which the soluble products of digestion are transported across the gut wall into the blood and the lymph

Define egestion

Undigested food is eliminated from the gut through the anus (cellulose or fibre)

Define assimilation

Using digested food molecules in the body eg) to make new proteins or use in respiration

State and define the two types of metabolic reactions?

anabolic = make things (assimilation), Catabolic = break down things (digestion)

What are accessory organs?

Not part of the alimentary canal but aid in digestion

State accessory organs examples

salivary glands, Liver, Gall bladder. Bile duct, Pancreas, Appendix

State the 4 layers of the gut in order (from outside to inside)

lumen, Mucosa, Submucosa, Circulary muscles, Longitudinal muscles, Serosa

State function of circular and longitudinal muscles

peristalsis, Circular: contract to narrow diameter of gut, Longitudinal: contract to shorten gut

What type of tissue is the mucosa

epithelium

what type of tissue is submucosa?

connective tissue

Describe function of submucosa

layer of connective tissue, Contains blood and lymph vessels, That carry away products of digestion

Describe function of mucosa

Epithelium tissue lines lumen, Secretes mucus, May be specialised to secrete enzymes for absorption

State functions of mucus in gut?

lubricant, Protection

Define bolus of food

A mouthful f food that you swallow

Describe peristalsis

circular muscles behind bolus of food contract and forces bolus down, longitudinal muscles relax to let it through, Occurs along the whole of the gut

Where does mechanical digestion happen?

mouth: chewing with teeth, Stomach: due to churning action of the stomach walls and the peristaltic action of the muscular layers

State a difference between mechanical and chemical digestion?

Mechanical digestion increases surface area but doesn’t break down molecules

Define chemical digestion?

The hydrolysis of larger molecules into smaller, more soluble ones by the action of enzymes

Where is saliva secreted from?

3 pairs of salivary glands in the mouth

what does saliva contain?

amalayse: begins hydrolysis of starch to maltose, Carbonate and hydrogen carbonate ions: to maintain slightly alkaline pH, Mucus: lubricates the food

Why is there no amalayse in the stomach?

optimum pH 7-8.3 (slightly alkaline), Denatures in acidic stomach environment

What feature of the stomach keeps food inside?

esophageal sphincter (at top), Pyloric sphincter (at bottom)

State the layers of the stomach wall? (Outside to inside)

gastric pits, Glastric glands (mucosa), Blood vessels (Submucosa), Oblique muscles, Circular msucles, Longitudinal msucles, Serosa (connective tissue)

Describe function of oblique layer?

Enables churning which further increases surface area of the food

What is the function of ruggae?

Allows the stomach to expand after a meal

State the cells in the gastric pits?

chief cells, Ozyntic/ parietal cells, Goblet cells

Describe function of chief cells?

Secretes pepsinogen

What is pepsinogen?

An inactive form of pepsin

Describe function of parietal cells?

secrete HCL, Lowers pH to about 2, Kills bacteria on the food & converts pepsinogen to pepsin & provides optimum pH for pepsin

State function of goblet cells?

secretes mucus, Protects the mucosa from digestive enzymes and lubricates food

State the types of protein digesting enzymes?

endopeptidase, Exopeptidase

describe function of endopeptidase? + give example

catalyses the hydrolysis of peptide bonds within the polypeptide chain, Pepsin/trypsin

Describe function of exopeptidase?

Catalyses hydrolysis of peptide bonds at the ends of short polypeptides

Does endopeptidase or exopeptidase act first? And why?

endopeptidase, So exopeptidase will have more ends to act on

What is pepsinogen? + explain role?

inactive form of pepsin, To prevent auto-digestion of stomach wall

What is the length and pH of the small intestine (approx)

6.5m, PH 7.8

Describe duodenum?

short - 25cm, Secretions from Brunner’s gland, gall bladder and pancreas enter here, Digestion of all major food groups occurs here, Some absorption occurs

Describe role of ileum?

Absorption of all molecules is completed, Absorption of small soluble products occurs

Describe neutralisation of stomach acid

the pyloric sphincter relaces and allows the passage of the chyme into the duodenum little at a time. Is mixed with the alkaline secretions from Brunner’s gland which is high in hydrogen carbonate ions. Bile, made in the liver and stored in the gall bladder is also alkaline. Secretions from the pancreas are also alkaline

Define chyme

Partially digestion food

Describe the role of bile

bile is a complex mixture; the contents include bile salts and bile pigmentsLipids are non polar and form large globules in the aqueous chyme. The bile salts are amphoteric (have polar and non-polar parts). They emulsify the lipid globule breaking into small droplets and preventing it recoalescing. Provides a much larger SA for digestion

Explain what type of gland is the pancreas is?

an endocrine gland: secretes hormones into the blood which circulate affecting cells that have receptors. An exocrine gland: secretions delivered via ducts to the site of an action

Describe the contents of pancreatic juice?

contains endopeptidases including trypsinogen

How is trypsinogen converted to trypsin and where?

converted by the enterokinase enzyme, found on external surface of epithelial cells lining duodenum

what are the roles of amylase and lipase in the small intestine?

amylase: continues the hydrolysis of starch to maltose. lipase: catalyses the hydrolysis of triglycerides to monoglycerides and fatty acids

state the importance of bile in the small intestine?

needed to emulsify the lipids so that the lipase can digest the lipids in the time that the food is in the gut

what are the epithelial cells lining the gut called?

enterocytes

where are enterocytes found?

they form a single layer on the outer surface of mucosa layer

what is the ‘brush border’?

the membrane of the enterocyte adjacent to the lumen is folded into microvilli to form the ‘brush border’

why do the cells on the microvilli need to be replaced?

Cells at the tips of the microvilli are constantly being sloughed off (lost)

What replaces the cells on the microvilli which are constantly being sloughed off (lost)?

By cells originally formed in the crypts

Describe the role of the membranes of the microvilli in digestion?

contain enzymes: exopeptidase and dipeptidase

what is the function of dipeptidase?

digest dipeptides formed by exopeptidase enzymes into amino acids

what happens to disaccharides in the small intestines?

actively transported into the enterocyte, then digested by disaccharide enzymes in cell membrane

state adaptations for absorption in small intestine?

large SA, short diffusion distance, blood capillaries, muscle fibres, many mitochondria, lacteal

Explain the large SA of the small intestine?

due to folds of the intestinal wall, villi and microvilli

Explain the short diffusion distance of the small intestine?

due to the single layer of epithelial cells

Explain the importance of blood capillaries of the small intestine?

to carry away the products of protein & carbohydrate digestion

Explain the importance of muscle fibres of the small intestine?

in each villus contract to move the villus and mixes the gut contents → increased contact between food & the enzymes of microvilli membranes

Explain the importance of many mitochondria of the small intestine?

present in epithelial cells for active transport

explain importance of lacteal in small intestine?

in each villus to carry away absorbed lipids

state and explain adaptations for absorption in small intestine/villus?

A very large surface area due to: Folds of the intestinal wall, Villi, Microvilli. A short distance due to the single layer of epithelial cells. dense capillary network to carry away the products of protein and carbohydrate digestion. Lacteal to carry away absorbed lipids. goblet cells secrete mucus: lubricates and protects against pathogenic invasion. muscle walls: peristalsis

Describe and explain the absorption of monosacharides?

Glucose moves into the enterocyte by co-transport with sodium ions. Sodium ions are actively transported out of the enterocyte to maintain concentration gradient; they enter the capillary by facilitated diffusion. Glucose leaves the enterocyte by facilitated diffusion and enters the capillary dissolving in the blood plasma. Some glucose also may be actively absorbed from the lumen to ensure all glucose is absorbed

Describe and explain the absorption of amino acids?

Glucose moves into the enterocyte by co-transport with sodium ions. Sodium ions are actively transported out of the enterocyte to maintain concentration gradient; they enter the capillary by facilitated diffusion. Glucose leaves the enterocyte by facilitated diffusion and enters the capillary dissolving in the blood plasma. Some glucose also may be actively absorbed from the lumen to ensure all glucose is absorbed

describe and explain absorption of minerals?

Minerals are absorbed by facilitated diffusion and water soluble vitamins by diffusion

describe and explain the absorption of water in small intestine?

Water follows the absorption of solutes by osmosis

what happens to absorbed nutrients in the capillaries?

The capillaries drain into the hepatic portal vein so absorbed nutrients are carried first to the liver

Describe and explain the absorption of lipids?

Fatty acids and glycerol diffuse into the enterocytes and into the lacteals. Fat soluble vitamins and cholesterol are also absorbed into the lacteal

where are absorbed nutrients in lacteal transported to?

The lacteal drains into the lymphatic system which eventually empties into the subclavian vein near the heart

State the parts of the large intestine

caecum, appendix, Colon (ascending, transverse and descending), Rectum

What substances pass into the colon?

undigested food, Mucus, Dead cells, Bacteria

What is absorbed in the large intestine and what is produced?

water, Vitamin K, Folic acid is produced by gut microbiomes

Describe the adaptations of the large intestine?

folds & villi - increase SA for absorption, Glands secrete mucus, Undigested food forms semi-solid faeces - stimulates muscle contractions

What is faeces made up of?

cellulose, cholesterol, bile, mucus, mucosa cells(250gof cells lost each day), bacteria, water

describe what happens to glucose after absorption?

glucose is absorbed (by diffusion and active transport) across the epithelium of villi into the capillaries, then to the venules and then travel via the hepatic portal vein to the liver. Glucose is absorbed from the blood by cells, for energy release in respiration, any excess stored as glycogen or converted to fat

describe what happens to amino acids after absorption

Absorbed (by diffusion and active transport) across the epithelium of villi into the capillaries, then to the venules and then travel via the hepatic portal vein to the liver. Amino acids are absorbed for protein synthesis; excess cannot be stored so are deaminated - the removed amino groups (NH2) are converted to urea and the deaminated remainder is converted to carbohydrate and stored

describe what happens to fatty acids and glycerol after absorption?

fatty acids and glycerol are passed into the lacteal (blind ended lymph vessel), then through the lymphatic system to the blood stream opening at the thoracic duct. Lipids are used for membranes and hormones, the excess is stored as fat