Human Physiology: 6.1 Digestion and Absorption

Question 1

Define Digestion

Answer 1

Digestion is a process in which relatively large, insoluble molecules in food (such as starch and proteins) are broken down into smaller, soluble molecules that can be absorbed into the bloodstream and delivered to cells in the body

Question 2

There are two major groups of organs which comprise the human digestive system

Answer 2

The alimentary canal consists of organs through which food actually passes (oesophagus, stomach, small & large intestine)
The accessory organs produce substances that aid in digestion but do not actually transfer food (salivary glands, pancreas, liver, gall bladder)

Question 3

What is peristalsis and its purpose?

Answer 3

Peristalsis is the principal mechanism of movement in the oesophagus, although it also occurs in both the stomach and gut. Peristalsis is series of muscle contractions in the walls of the oesophagus (or stomach/gut) that pass like a wave along the alimentary canal

Food is moved unidirectionally along the alimentary canal in a caudal direction (mouth to anus)
These contractions are controlled unconsciously by the autonomic nervous system

Question 4

How does peristalsis occur?

Answer 4

Peristalsis is controlled by circular and longitudinal muscles
These muscles are smooth muscle (not striated)
Circular muscles contract to reduce the diameter of the lumen of the oesophagus or small intestine
This prevents the food moving backwards towards the mouth
Longitudinal muscles contract to reduce the length of that section in the oesophagus or the small intestine
This forces the food forwards through the alimentary canal

Question 5

What is mechanical digestion?

Answer 5

In mechanical digestion, food is physically broken down into smaller fragments via the acts of chewing (mouth), churning (stomach) and segmentation (small intestine)

Question 6

Describe the mechanical digestion of segmentation in the intestines

Answer 6

Segmentation involves the contraction and relaxation of non-adjacent segments of circular smooth muscle in the intestines
Segmentation contractions move chyme in both directions, allowing for a greater mixing of food with digestive juices containing enzymes
While segmentation helps to physically digest food particles, its bidirectional propulsion of chyme can slow overall movement allowing time for digestion

Question 7

Outline the mechanical digestion of food by chewing (mouth)

Answer 7

Food is initially broken down in the mouth by the grinding action of teeth (chewing or mastication)
The tongue pushes the food towards the back of the throat, where it travels down the esophagus as a bolus
The epiglottis prevents the bolus from entering the trachea, while the uvula prevents the bolus from entering the nasal cavity

Question 8

Outline the mechanical digestion of food by Churning (Stomach)

Answer 8

The stomach lining contains muscles which physically squeeze and mix the food with strong digestive juices (‘churning’)
Food is digested within the stomach for several hours and is turned into a creamy paste called chyme
Eventually the chyme enters the small intestine (duodenum) where absorption will occur

Question 9

What is the role of mucus?

Answer 9

Mucus is produced to continually lubricate the food mass and reduce friction

Question 10

What is chemical digestion?

Answer 10

n chemical digestion, food is broken down by the action of chemical agents (such as enzymes, acids and bile)

Question 11

Outline the role of stomach acids in chemical digestion in the stomach

Answer 11

The stomach contains gastric glands which release digestive acids to create a low pH environment (pH ~2)
The acidic environment functions to denature proteins and other macromolecules, aiding in their overall digestion
The stomach epithelium contains a mucous membrane which prevents the acids from damaging the gastric lining
The pancreas releases alkaline compounds (e.g. bicarbonate ions), which neutralise the acids as they enter the intestine

Question 12

Describe the role of bile in chemical digestion (where it is produced, what it does, how it helps)

Answer 12

The liver produces a fluid called bile which is stored and concentrated within the gall bladder prior to release into the intestine
Bile contains bile salts which interact with fat globules and divide them into smaller droplets (emulsification)
The emulsification of fats increases the total surface area available for enzyme activity (lipase)

Question 13

Describe the role of enzymes in chemical digestion

Answer 13

Enzymes are biological catalysts which speed up the rate of a chemical reaction (i.e. digestion) by lowering activation energy
Enzymes allow digestive processes to therefore occur at body temperatures and at sufficient speeds for survival requirements
Enzymes are specific for a substrate and so can allow digestion of certain molecules to occur independently in distinct locations

Question 14

Where are digestive enzyme produced mostly and what determines the type of enzyme secreted

Answer 14

Digestive enzymes are secreted predominantly by the pancreas, although other organs also contribute (salivary gland, stomach)

The type of enzyme secreted and location of secretion depends on the specific macromolecule required for hydrolysis

Question 15

How are enzymes synthesised in pancreas?

Answer 15

Enzymes are synthesised on the ribosomes of the rough endoplasmic reticulum. They are then processed within the Golgi apparatus before being secreted by exocytosis into the lumen of the small intestine

Question 16

What stimulates Secretion of pancreatic enzymes?

Answer 16

Secretion of pancreatic enzymes is stimulated by the release of hormones into the stomach and intestines in response to ingestion of food
This is an automatic response of the autonomic nervous system

Question 17

What enzymes are found in pancreatic juice?

Answer 17

The enzymes found in pancreatic juice include:
Amylase - for the partial digestion of starch into maltose
Lipase - for digestion of triglycerides into fatty acids and glycerol/monoglycerides
Phospholipase - for digestion of phospholipids into fatty acids, glycerol and phosphate
Protease - for the partial digestion of proteins and polypeptides into shorter peptides

Question 18

Where are digestive enzymes produced in pancreas? (Structure of pancreas)

Answer 18

Digestive enzymes are produced in specialised gland cells which are known as acinar cells
These cells are located in clusters around the ends of tubes called ducts
Ducts join together to form larger ducts and eventually, one pancreatic duct
This is where the pancreatic juices, containing enzymes, are secreted into the duodenum of the small intestine

Question 19

What enzymes are made by the small intenstine?

Answer 19

As well as those enzymes found in pancreatic juices, enzymes are also produced in the walls of the small intestine
These enzymes break the products of pancreatic enzyme digestion down into monomers, e.g.
Nucleases break down nucleic acids
Lactase digests lactose
Sucrase digests sucrose
Maltase digests maltose
Dipeptidase digests dipeptides

Question 20

What are some distinctive features of enzymes produced from the small intestine?

Answer 20

Some enzymes are secreted from the epithelial cells into the intestinal lumen with partially digested food
Other enzymes e.g. maltase, are immobilised and are attached to the membrane of the epithelial cells where they digest substrate molecules as the food is forced through the small intestine
These enzymes are examples of integral proteins

Question 21

What are endopeptidases and exopeptidases?

Answer 21

Pancreatic juice contains endopeptidases and exopeptidases
Endopeptidases hydrolyse peptide bonds within polypeptide chains to produce dipeptides
Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains to produce dipeptides

Question 22

Describe digestion of protein (from stomach to small intestine)

Answer 22

Protein digestion begins in the stomach with the release of proteases that function optimally in an acidic pH (e.g. pepsin = pH 2)
Smaller polypeptide chains enter the small intestine where they are broken down by endopeptidases released by the pancreas
These endopeptidases work optimally in neutral environments (pH ~ 7) as the pancreas neutralises the acids in the intestine

Lastly, there are dipeptidase enzymes found within the cell surface membrane of the epithelial cells in the small intestine. These enzymes hydrolyse dipeptides into amino acids which are released into the cytoplasm of the cell

Question 23

Outline digestion of lipids

Answer 23

The digestion of lipids takes place solely in the lumen of the small intestine
Lipase enzymes break down lipids to glycerol and fatty acids
Lipids can also be broken down into monoglycerides and fatty acids
Lipase enzymes are produced in the pancreas and secreted into the small intestine

Question 24

Describe starch digestion to disaccharides and glucose monomers

Answer 24

The digestion of starch is initiated by salivary amylase in the mouth and continued by pancreatic amylase in the intestines
Amylase hydrolyses the 1,4 glycosidic bonds found in both amylose and amylopectin
Amylase digests amylose into maltose subunits (disaccharide) and digests amylopectin into branched chains called dextrins (Amylase is unable to digest the 1,6 glycosidic bonds)

The next stage of starch digestion involves enzymes immobilised in the membranes of the microvilli e.g.
Maltase - a disaccharidase which hydrolyses maltose into glucose
Dextrinase - digests the 1,6 glycosidic bonds found in dextrins

Question 25

State the function of the two intestines in human

Answer 25

The human intestines function to absorb the products of digestion and have specialised structures to fulfil this function

The small intestine absorbs usable food substances (i.e. nutrients – monosaccharides, amino acids, fatty acids, vitamins, etc.)
The large intestine absorbs water and dissolved minerals (i.e. ions) from the indigestible food residues

Question 26

Describe the structure of small intestine

Answer 26

The small intestine is composed of four main tissue layers, which are (from outside to centre):

Serosa – a protective outer covering composed of a layer of cells reinforced by fibrous connective tissue
Muscle layer – outer layer of longitudinal muscle (peristalsis) and inner layer of circular muscle (segmentation)
Submucosa – composed of connective tissue separating the muscle layer from the innermost mucosa
Mucosa – a highly folded inner layer which absorbs material through its surface epithelium from the intestinal lumen

Question 27

Describe features of the villi Mnemonic: MR SLIM

Answer 27

The inner epithelial lining of the intestine is highly folded into finger-like projections called villi (singular: villus)

Many villi will protrude into the intestinal lumen, greatly increasing the available surface area for material absorption
Intestinal villi contain several key features which facilitate the absorption of digestive products (monomers, ions and vitamins):
Mr Slim

Microvilli – Ruffling of epithelial membrane further increases surface area
Rich blood supply – Dense capillary network rapidly transports absorbed products
Single layer epithelium – Minimises diffusion distance between lumen and blood
Lacteals – Absorbs lipids from the intestine into the lymphatic system
Intestinal glands – Exocrine pits (crypts of Lieberkuhn) release digestive juices
Membrane proteins – Facilitates transport of digested materials into epithelial cells

Question 28

What is absorption?

Answer 28

During absorption, digested food monomers must pass from the lumen into the epithelial lining of the small intestine, and then transported to the liver or cells via the bloodstream

Question 29

How are amino acids absorbed?

Answer 29

Specific amino acid co-transport proteins (a type of carrier protein) are found within the cell-surface membrane of the epithelial cells lining the ileum

They transport amino acids only when there are sodium ions present

For every sodium ion that is transported into the cell, an amino acid is also transported in
This occurs via facilitated diffusion, which requires the movement of molecules down their concentration gradient

Amino acids diffuse across the epithelial cell and then pass into the capillaries via facilitated diffusion

The concentration gradient of sodium ions from the lumen of the ileum into the epithelial cell is maintained by the active transport of sodium ions out of the cell and into the blood via a sodium-potassium pump at the capillary end of the cell

Question 30

How is glucose transported?

Answer 30

Glucose is polar so cannot pass into the blood by diffusion
The glucose carrier proteins in the cell-surface membrane of the small intestine work in a similar way to the amino acid carrier proteins
Sodium ions and glucose molecules are co-transported into the epithelial cells via facilitated diffusion
This is a passive process but depends on the concentration gradient of sodium ions from the lumen of the ileum into the epithelial cell
The gradient is maintained by the active transport of sodium ions out of the cell and into the blood via a sodium-potassium pump at the capillary end of the cell
The glucose molecules diffuse across the epithelial cell and enter the capillary by facilitated diffusion through a glucose channel protein