Digestion Flashcards
Starch
Starch is a naturally insoluble polymer made from lots of glucose molecules; the molecules are too big to be absorbed across the bloodstream
Test for glucose
Benedict’s Reagent turns from blue to yellow, to brown, eventually forming a brick-red precipitate
Monosaccharides
Simple saccharide (sugar) molecules, such as alpha-glucose, galactose and fructose. These hexose sugars have a six-sided “ring” shape. General formula CnH2nOn
Gylcosidic bond
A chemical bond between two monosaccharides, created by a condensation reaction. A [1, 4] glycosidic bond gives a straight molecules; a [1, 6] glycosidic bond gives a “kinked” or branched molecule. The [1, 4] refers to the carbon atoms between which the bond is formed
Disaccharides
Two monosaccharide molecules joined by a glycosidic bond. Lactose (a component of mammalian milk) is made up of α-glucose and galactose; maltose (formed from starch and broken down into glucose during seed germination) is made up of α-glucose and α-glucose; sucrose (a sugar transported around plants in the phloem) is made up of α-glucose and fructose. General formula CnH2n-2On-1
Hydrolysis reaction
A reaction in which a water molecule is added to a glycosidic bond, causing a disaccharide or polysaccharide molecule to break down into monosaccharides; carried out by hydrolytic enzymes (e.g. during digestion)
Condensation reaction
A reaction in which a water molecule is lost from a pair of monosaccharides, creating a glycosidic bond. Condensation polymerisation causes many disaccharides to join into one polysaccharide
Polysaccharides
Sugar molecules are made into a polymer by a condensation polymerisation reaction
Test for disaccharides
At first, Benedict’s reagent tests negative; heat the sugar with acid to break the glycosidic bonds; neutralise the solution then Benedict’s reagent will test positive
Test for starch
Iodine turns blue-black in the presence of starch
Starch
Starch is a storage carbohydrate found in plants that is made up of many α-glucose molecules joined by glycosidic bonds. Starch provides a store of energy. It is an unreactive macromolecule that is insoluble, so it exerts little osmotic effect on the cell, and cannot diffuse out of the cell
Amylopectin
Amylopectin is a part of the starch molecule that is branched (it has 1,6 glycosidic bonds). The branched structure allows the rapid mobilisation of glucose units (by hydrolysis) for use in respiration
Amylopectin
Amylopectin is a part of the starch molecule that is branched (it has 1,6 glycosidic bonds). The branched structure allows the rapid mobilisation of glucose units (by hydrolysis) for use in respiration
Lactose intolerance
Lactose intolerance occurs due to the absence of lactase enzyme. Lactose cannot be digested and so remains in the gut and the large intestine, where there wouldn’t normally by any sugar. This has two consequences: a lower water potential in the lumen, causing water to be absorbed into the faeces by osmosis, which results in explosive diarrhoea; bacteria in the colon also break down the lactose anaerobically, producing acids and gases which cause flatulence
Sodium-potasium pump
Sodium-potassium adenosine tri-phosphase is an active transport protein present in all animal cell membranes. The protein pumps sodium ions out of the epithelial cell, and potassium ions into the cell, gaining energy from the hydrolysis of ATP. The ions are transported against their concentration gradients, resulting in a large build-up of sodium ions in the lumen of the gut
Facilitated diffusion protein
Sodium-glucoseco-transporter protein is a facilitated diffusion protein found in the epithelial membrane of cells in the ileum (small intestine). The protein has two binding sites, for glucose and sodium ions, and both molecules must be carried together down their concentration gradients
Absorption of glucose
Glucose and other monosaccharides are absorbed from the gut by coupled active transport. The sugars are transported by a facilitated diffusion protein coupled to an active transport protein, and sodium ions are pumped into the ileum to maintain a high concentration gradient. The glucose diffuses from the epithelial cell into the tissue fluid by facilitated diffusion, then into the gaps in the capillaries
Cholera
Cholera is an infectious disease caused by Vibrio cholerae, a rod-shaped prokaryotic cell with a single flagellum. Symptoms of cholera include stomach cramps, vomiting, fever and severe diarrhoea, causing severe water loss (20 litres per day) and eventually death. It adheres to the intestinal epithelium and secretes cholera toxin
Cholera toxin
Cholera toxin is secreted by the cholera bacterium, and activates a chloride ion channel in the cell membrane. Chloride ions diffuse out of the cells into the lumen, lowering the water potential of the gut, causing water to be lost from cells by osmosis
Oral rehydration therapy
ORT is a simple and cheap treatment for diarrhoea, consisting of an oral rehydration solution of glucose and salt. Sodium ions and glucose in the lumen bind to the sodium-glucose co-transporter protein, and are carried into the cell. This lowers the water potential inside the epithelial cells, causing diffusion of water and rehydration
