Glucose as a Fuel Molecule #1

Question 1

Describe glucose as a fuel molecule (the overall process)

Answer 1

• hydrolysis of carbohydrates to give glucose
• absorption of glucose (getting glucose across membranes into the body)
• cells that use glucose as a fuel molecule
• glycolysis
• the product of glycolysis in aerobic and anaerobic conditions

Question 1

describe glucose as a molecule

Answer 1

• simple sugar/monosacchride
• hexose (6 carbons)
• drawn in different conformations (need to be able to recognise, but not draw them) - ring, open, chair

Question 2

describe carbohydrates (the source of glucose)

Answer 2

• large molecules/polysaccharides
• starch from plants
• polymers of up to 1 million glucose units joined by glycosidic bonds
  - amylose: a linear polymer of glucose units (glycosidic bonds between monomers)
  - amylopectin: a branched polymer glucose units (glycosidic bonds between monomers)
• glycogen from meat (liver, muscle) and storage
  - similar branched structure to amylopectin
• disaccharides such as sucrose, lactose and maltose
• two sugars joined together by a glycosidic bond

Question 3

describe the digestion of carbohydrates

Answer 3

• the enzyme amylase hydrolyses glycosidic bonds n large polysaccharides to give smaller oligosaccharides continues until disaccharides are produced
• disaccharides are cleaved into monosaccharides by specific enzymes (depending on what two monosaccharides are bonded together)
• End result: glucose (and other monosaccharides) in the GI tract

Question 4

describe glucose absorption and its transport across membranes

Answer 4

• sugars are highly polar (water soluble) and cannot simply diffuse across cell membranes
• require specific transporter proteins anchored in the membrane

Two types of transport:
- active transport: moves molecules against concentration gradient, requires energy input
- for glucose: SGLT (sodium-glucose linked transporter; SGLT1 in absorption from GI)
- facilitative transport: molecules move down a concentration gradient, does not require energy input
- for glucose: GLUT (glucose transporter: GLUT2 in absorption from the GI)

Question 5

describe the actual pathway of glucose absorbtion and its transport across membranes

Answer 5

SGLT: symport of glucose and Na+ into the epithelial cells
- glucose up concentration gradient (active transport so needs to be coupled to Na+ so it doesn’t use ATP)
- Na+ down concentration gradient
Na+ concentration increases in the cell

Na+/K+ ATPase:
- Na+ moved up concentration gradient into interstitial fluid (uses ATP)
- ATP hydrolysis
- K+ moved into epithelial cells
- K+ returned to interstitial fluid down concentration gradient

GLUT: glucose moves down concentration gradient into interstitial fluid then into blood (capillary)
- storage/glycogen
- fuel molecule

Question 6

describe glycolysis in different organisms

Answer 6

• oxidised in glycolysis in all organisms (animals, fungi, plants, bacteria)
• glycolysis is usually cytoplasm in eukaryotes (other pathways mitochondrial)
• In mammals, all cells use glucose in glycolysis. some cells in mammals reply on/preferentially use glucose

Question 7

describe why glucose is the essential fuel for red blood cells

Answer 7

red blood cells do not have mitochondria, so do not have other pathways
- as they develop, they tend to get rid of most of their cellular contents (including the mitochondrea)

Question 8

why is glucose the preferred fuel molecule for the brain?

Answer 8

brain has high energy requirement: human brain requires around 120g of glucose per day
brain cells have mitochondria

Hypotheses (we currently don’t know the actual reason!):
1. Supply (traditional view textbooks - but has been proven to be incorrect)
- glucose easily crosses the blood-brain barrier but fats do not
2. It is safer
- using glucose provides a quick source of ATP without risk of damage

Question 9

describe why glucose is the preferential fuel molecule in the eye

Answer 9

blood vessels (bringing oxygen) and mitochondria (using oxygen) would refract light in the optical path (lens, cornea) to retina
- instead we use glucose which doesn’t use oxygen and does its role in the cytoplasm (which is clear)

Question 10

why is glucose the fuel molecule for muscle?

Answer 10

white muscle tends to use glucose because it can get energy out of glucose quickly (white muscle used for quick, fast, powerful movements)
red muscle uses fats because it needs ATP at a much slower rate