6.1 digestion and absorption Flashcards
1
Q
briefly outline the digestive system
A
- mouth: chewing (mechanical digestion); saliva moistens food to make a bolus for swallowing; salivary amylase behinds chemical digestion of starch
- 5: food enters the pharynx past the epiglottis (now closed) and move down to the stomach via peristalsis
- oesophagus: wave of muscle contractions (peristalsis) pushes the bolus into the stomach
- 5: sphincter muscles regulate the movement of food into and also out of the stomach (esophageal and pyloric sphincters)
- stomach: muscular contractions continues mechanical digestion; acid kills bacteria; pepsin begins digestion of proteins
- 5: food enters through pyloric sphincter into the small intestines
- duodenum (small intestine): bile from the liver and gall bladder neutralises acid and emulsifies fats; pancreatic amylase and lipase digest carbohydrates and fats; trypsin digests polypeptides to amino acids
- jejunum
- ileum (small intestine): lower half of small intestine absorbs nutrients into the blood via the villi
- large intestine: water is reclaimed and returned to the blood, leaving semi-solid faeces which is stored in the rectum
- egestion: faeces (containing undigested food, dead cells and other waste) is forced out of the anus
2
Q
how does food move along the gut?
A
- peristalsis is the main way that food is pushed along the alimentary canal, with the aid of antagonistic muscles
- circular muscles contract behind food mass and constrict/narrow the intestines, reducing the cross-sectional diameter of the intestines
- contraction of longitudinal muscles ahead the bolus of food moves the food forward
- longitudinal muscles work as antagonistic muscles to the circular muscles and opposes the effects of the circular muscles
- when the longitudinal muscles contract in front of food, it opens up the lumen of the intestines and allows space for the food to move forwards (contraction of circular muscles is what forces food mass forward)
3
Q
how are macromolecules digested in humans?
A
- enzymes digest most macromolecules in food into monomers in the small intestine
- many of the molecules of nutrients that humans consume are large molecules, or even in the form of polymers (macro-molecules)
- for the nutrients to be useful in the human body, they must be absorbed and these molecules are usually too large to be absorbed, and/or are relatively insoluble
- hence need to be broken down into smaller molecules, to facilitate their absorption through the cell membranes on the intestinal villi
- carried out with the help of digestive enzymes secreted into the digestive tract by exocrine glands/organs
- 1 of the main types of chemical reactions catalysed by digestion enzymes would be hydrolysis reactions
4
Q
what is the pancreas’s purpose in digestion?
A
- an organ found adjacent to the stomach
- secretes enzymes into the lumen of the small intestine
- besides secreting hormones as part of the endocrine system, it can also act as an exocrine gland and secrete enzymes and other products as part of pancreatic juice into the small intestines
- pancreatic juice contains enzymes like amylases, lipases and proteases, as well an alkaline solution rich in bicarbonates
- secretes amylase, lipase and endopeptidase (hydrolyse internal peptide bonds)
- pancreatic juice is released from the pancreas, via the pancreatic duct, to the upper portion of the duodenum
5
Q
what is the enzymatic digestion in small intestine?
A
CARBOHYDRATES
- polysaccharides -> [pancreatic amylase] maltose (and other disaccharides) -> [maltase, sucrase, lactase, etc] monosaccharides
PROTEINS
- polypeptides -> [trypsin, chymotrypsin] -> smaller polypeptides -> [various peptidases] amino acids
NUCLEIC ACIDS
- dna and rna -> [nucleases] nucleotides -> [other enzymes] nitrogenous bases, sugars, phosphates
FATS
- fat globules -> [bile salts] fat droplets (emulsified) -> [lipase] fatty acids and glycerol
6
Q
what is the main adaptation of the intestines to increase rate of absorption?
A
- folding of the epithelium into large circular folds, and throughout the surface of the folds there are finger-like projections called villi (singular: villus)
- on surface of epithelial cells of the intestinal villi, there are also microvilli present the increases the surface area even further
- villi increase the surface area of epithelium over which absorption is carried out
- main way which molecules are absorbed from the intestinal lumen is via diffusion and other related modes of transport (active transport)
- hence, surface area of the intestines (epithelium) directly affects the rate of absorption
7
Q
what other structures facilitate the absorption of nutrients in the intestinal villi?
A
- increasing the surface area via microvilli
- rich supply of capillaries that run close to the epithelium, to maintain a concentration gradient for diffusion, as well as providing a short path for diffusion
- lymphatic vessels found in the villi known as the lacteal, and are mainly involved in absorption and transport of lipids
- villi absorb monomers formed by digestion as well as mineral ions and vitamins
8
Q
what are the 3 steps of villi absorption?
A
- digestion: break down of complex molecules into monomers with the aid of enzymes
- absorption: movement of molecules through the intestinal epithelial cells into the blood vessels or lacteal
- assimilation: after transport to the target tissues, the molecules are taken into cells and used in metabolism
- along with vitamins and minerals, all products of digestion are absorbed by the intestinal villi
9
Q
what is the method of transport for absorption of lipids?
A
- simple diffusion
- non-polar and therefore can pass freely through hydrophobic core of the plasma membrane into the epithelial cells (down the concentration gradient )
10
Q
what is the method of transport for absorption of fructose and vitamins?
A
- facilitated diffusion
- water-soluble (hydrophilic) molecules use channel proteins to pass phospholipid bilayer and enter the epithelial cells (down the concentration gradient)
11
Q
what is the method of transport for absorption of glucose, amino acids and mineral ions?
A
- active transport
- protein pumps use ATP to move molecules against the concentration gradient into the epithelial cells
12
Q
what is the method of transport for absorption of antibodies from breast milk?
A
- endocytosis (pinocytosis)
- plasma membrane folds inward to form vesicles to absorb larger molecules without digesting them
13
Q
what is starch? (the structure etc)
A
- polysaccharide that is made up of repeated monomers of glucose, and is made up of both amylose and amylopectin molecules/chains
- in straight chains of amylose, the glucose molecules are bound together mainly via 1-4 glycosidic bonds
- in amylopectin, there are the usual 1-4 glycosidic bonds, as well as 1- 6 glycosidic bonds
14
Q
how does starch digestion occur?
A
- amylases are able to break the 1-4 glycosidic bonds via hydrolysis, but such amylases are not capable of breaking the 1-6 glycosidic bonds
- pullulanase and limit dextrinases are examples of enzymes that can break the 1-6 glycosidic bonds
- amylases break down starch chains into maltose molecules, and hence maltase is needed to cleave the maltose molecules into two glucose molecules
15
Q
how does glucose digestion occur?
A
- after digestion of starch, glucose molecules are produced
- glucose transported into epithelial cells mainly via active transport, where energy is expended to ensure as many molecules of glucose are absorbed regardless of concentration gradient
- in active transport of glucose into the epithelial cells, glucose is co-transported along with sodium ions (good to know but nvm)
- primary active transport: 2 K in 3 Na out ; secondary active transport: glucose and Na in
- glucose moves via facilitated diffusion towards the capillaries
- diffuses into the blood stream across the capillary walls
- blood flows from the capillaries in the villi into venules, which then converges and drain into the hepatic portal vein to enter the liver
- depending on the blood glucose levels, the liver will convert excess glucose into glycogen to be stored within the liver
