Digestion

Question 1

Starch

Answer 1

Starch is a naturally insoluble polymer made from lots of glucose molecules; the molecules are too big to be absorbed across the bloodstream

Question 2

Test for glucose

Answer 2

Benedict’s Reagent turns from blue to yellow, to brown, eventually forming a brick-red precipitate

Question 3

Monosaccharides

Answer 3

Simple saccharide (sugar) molecules, such as alpha-glucose, galactose and fructose. These hexose sugars have a six-sided “ring” shape. General formula CnH2nOn

Question 4

Gylcosidic bond

Answer 4

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

Question 5

Disaccharides

Answer 5

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

Question 6

Hydrolysis reaction

Answer 6

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)

Question 7

Condensation reaction

Answer 7

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

Question 8

Polysaccharides

Answer 8

Sugar molecules are made into a polymer by a condensation polymerisation reaction

Question 9

Test for disaccharides

Answer 9

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

Question 10

Test for starch

Answer 10

Iodine turns blue-black in the presence of starch

Question 11

Starch

Answer 11

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

Question 12

Amylopectin

Answer 12

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

Question 13

Amylopectin

Answer 13

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

Question 14

Lactose intolerance

Answer 14

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

Question 15

Sodium-potasium pump

Answer 15

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

Question 16

Facilitated diffusion protein

Answer 16

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

Question 17

Absorption of glucose

Answer 17

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

Question 18

Cholera

Answer 18

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

Question 19

Cholera toxin

Answer 19

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

Question 20

Oral rehydration therapy

Answer 20

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