ALTERNATIVE METABOLIC PATHWAYS SBI4U Biology

Glucose Metabolism Lack of Oxygen Lack of Glucose Too much Glucose Cells rely primarily on the Glycolytic Pathway for most ATP. Carbs other than glucose need minimal modification before entering this pathway. Alternate pathways deal with either: Lack of Oxygen Lack of Glucose Too much Glucose

Lack of Oxygen Lactic Acid Fermentation Low blood O2 = cell metabolism ‘backs up’ Pyruvate  Lactate Regenerates NAD+

Lack of Oxygen Alcoholic Fermentation Yeast is an anaerobe Pyruvate  Ethanol Very simple!

GLYCOGENOLYSIS Low blood Glucose = Glycogen broken down to glucose, for delivery to cells. Daily Glycogen depletion/repletion cycle

GLYCOGENOLYSIS Involves several organs & tissues Pancreas: Glucagon, Insulin Liver: stores Glycogen *Adipose cells: store body fat.

Gluconeogenesis Gluco = sugar Neo = new Genesis = formation Making glucose or its intermediates, often from non-carb sources.

Amino Acids & Energy: glucose-Alanine Cycle Amino Acid Alanine can be converted to glucose & vice versa. Useful when proteins are plentiful but carbs in short supply.

Amino Acids & Energy Various glucogenic amino acids can be metabolised to produce glucose in times of need.

Amino Acids & Energy Glucogenic *Act as Both: Phe, Trp, Tyr, Ile Ketogenic: Leucine, Lysine Ketogenic amino acids enter the regular pathway of metabolism indirectly, but still help yield ATP energy.

Amino Acids & Energy Proteins yield less energy than lipids or carbs They are metabolised as a last resort in starvation

Amino Acids & Energy Starvation is a more complex metabolic process than people imagine….

Lipid Metabolism Lipolysis: Glycerol enters glycolysis as PGAL Fatty Acids enter Krebs as Acetyl CoA Along the way, NADH & FADH2 are generated.

Lipids & Carbs: Cori Cycle Uses energy derived from Fatty Acid metabolism drives this type of gluconeogenesis Helps body clear lactate in a useful way.

Too much of a good thing! After feeding, surplus protein & carbohydrate are converted via Krebs cycle intermediates into fats. When fasting protein amino acids are converted into carbohydrates via the Krebs cycle & gluconeogenesis to maintain our blood glucose levels. Fats provide energy and are mostly oxidized to CO2 and water

Lipogenesis from Glucose So the fate of glucose after a meal is: Fast Energy 2. Stored as Glycogen 3. Stored as Fat

Summary The body is resourceful, but frugal It finds ways of getting energy when fuel is low It directs molecules to their best possible use: energy, storage, or structure. Catabolic & Anabolic processes are constant