ENERGY METABOLISM
Contents
EXPLAIN THE CONCEPT OF FOOD AS A FUEL FOR THE PRODUCTION OF ENERGY IN THE BODY, AND HOW ENERGY
REQUIREMENTS VARY DURING REST, EXERCISE AND STARVATION. ......................................................................... 1
DESCRIBE THE PATHWAYS INVOLVED IN ENERGY METABOLISM: GLYCOLYSIS, GLUCONEOGENESIS, BETAOXIDATION, AMINO ACID BREAKDOWN, TCA CYCLE AND ELECTRON TRANSPORT CHAIN. FOR EACH, INCLUDE THE
CELLULAR LOCATION, THE MAJOR ORGANS IN WHICH EACH PATHWAY IS ACTIVE AND THE EFFECT OF
STARVATION ON FLUX OF SUBSTRATES THROUGH THE PATHWAY............................................................................ 2
OUTLINE HOW CHEMICAL ENERGY RELEASED FROM THE OXIDATION OF FOOD MOLECULES IS USED TO DRIVE ATP
SYNTHESIS AND DESCRIBE THE ROLE OF THE ELECTRON TRANSPORT CHAIN AS AN INTERMEDIATE IN THIS
PROCESS. ................................................................................................................................................................... 4
REFERENCES.............................................................................................................................................................. 5
EXPLAIN THE CONCEPT OF FOOD AS A FUEL FOR THE PRODUCTION OF ENERGY IN THE BODY, AND HOW ENERGY
REQUIREMENTS VARY DURING REST, EXERCISE AND STARVATION.



Food is a source of fuel for the body as it can be oxidised to produce energy in the body
It is a source of “building blocks” for the production of new cells and tissue
It is a source of “chemicals” that are necessary to conduct reactions within the body
ATP is the energy unit of the body
Proteins, lipids and carbohydrates that make up the major part of the food that we eat must be broken down into
smaller molecules before cells can use them as energy
At rest (The well-fed/ absorptive state)





Immediately after a meal, the blood glucose level rises and stimulates the release of insulin
Major target for insulin are liver, muscle and adipose tissue
Insulin promotes glycogen synthesis in liver and muscle
After glycogen stores are filled, the liver converts excess glucose to fatty acids and triglycerides
Brain tissue and red blood cells are insulin independent. They derive their energy from oxidising glucose to
CO2 and water in both well-fed and normal fasting states. Only in prolonged fasting does this change.
Starvation (prolonged)





Levels of glucagon markedly elevated
Lipolysis is rapid; resulting in excess acetyl CoA that is used for ketone synthesis
Levels of both lipids and ketones are thus increased in the blood
Muscles use fatty acids as the major fuel and brain adapts to using ketones for some of its energy
The shift from glucose to ketones as the major fuel diminishes the amount of protein that must be degraded to
Jess Q – Week 9 – A matter of opinion
Page 1
support gluconeogenesis.
Exercise
 The primary fuel used to support muscle contraction depends on the magnitude and duration of exercise as
well as the major fibres involved
 Skeletal muscle has stores of both glycogen and some triglycerides
 During moderately high, continuous exercise, oxidation of glucose and fatty acids are both important, but after
1 to 3 hours of continuous exercise at this level, muscle glycogen stores become depleted and the intensity of
exercise declines to a rate that can be supported by oxidation of fatty acids
 During low-intensity exercise, fat oxidation predominates as the energy source with some contribution by
glucose
DESCRIBE THE PATHWAYS INVOLVED IN ENERGY METABOLISM: GLYCOLYSIS, GLUCONEOGENESIS, BETAOXIDATION, AMINO ACID BREAKDOWN, TCA CYCLE AND ELECTRON TRANSPORT CHAIN. FOR EACH, INCLUDE
THE CELLULAR LOCATION, THE MAJOR ORGANS IN WHICH EACH PATHWAY IS ACTIVE AND THE EFFECT OF
STARVATION ON FLUX OF SUBSTRATES THROUGH THE PATHWAY.
Jess Q – Week 9 – A matter of opinion
Page 2
This diagram is a very basic version of the human body’s metabolic pathway. The different processes are further
discussed below
Glycolysis
- Metabolic pathway that turns glucose into pyruvate
- It involves 10 definitive steps and 10 intermediate compounds
- Occurs in cytosol of cell
- If cell has mitochondria and oxygen, glycolysis is aerobic. If mitochondria or oxygen is lacking, glycolysis may
occur anaerobically (eg: exercising skeletal muscle)
Gluconeogenesis
- Metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate
and pyruvate
- The process of gluconeogenesis helps keep blood glucose levels within critical limits
- Occurs primarily in the liver (cytoplasm and mitochondria)
- The pathways are promoted by glucagon and adrenaline and inhibited by insulin. In fasting, glycogen reserves
drop dramatically in the first 12 hours, during which time gluconeogenesis increases. After 24 hours, it represents
the sole source of glucose.
Beta Oxidation
- A process in which fatty acids molecules are broken down, through a series of intermediate steps, into acetyl
coenzyme A. Acetyl coenzyme A then enters the TCA cycle along with metabolites of carbohydrates and proteins
- Pathway is a repetition of four steps. Each four-step cycle releases one acetyl CoA and reduces NAD and FAD
(producing NADH and FADH2).
- In muscle and adipose tissue (in the mitochondria), Acetyl CoA enters the TCA; in the liver ATP may be used for
gluconeogenesis and the acetyl CoA stimulates gluconeogenesis
- In a fasting state, the liver produces more acetyl CoA from β oxidation than is used in the citric acid cycle. Much
of this acetyl CoA is used to synthesise ketone bodies
Amino Acid breakdown
- Proteins are made up of amino acids. Thus, to use proteins as a source of fuel, they must be broken back down into
amino acids
- Amino acids released from proteins usually lose their amino group through transamination or deamination
- Body protein is catabolised primarily in muscle and in liver (in the mitochondria)
- During prolonged fasting or starvation, protein may be used as an energy source
TCA cycle
- Occurs in the mitochondria
- Primary function is oxidation of acetyl Co A to carbon dioxide
- Activity of the TCA is necessary irrespective of the fed or fasting state
- Start and end products of the TCA
Electron Transport Chain (ETC)
Jess Q – Week 9 – A matter of opinion
Page 3
-
Releases a large amount of energy
Occurs in mitochondria
Continues in next LO
OUTLINE HOW CHEMICAL ENERGY RELEASED FROM THE OXIDATION OF FOOD MOLECULES IS USED TO DRIVE
ATP SYNTHESIS AND DESCRIBE THE ROLE OF THE ELECTRON TRANSPORT CHAIN AS AN INTERMEDIATE IN THIS
PROCESS.
 Energy is extracted from food via oxidation
 This occurs in four stages
Stage I
-
Foods that have been eaten are broken down into their primary components
I.e.: Carbohydrates into glucose, Proteins into amino acids and Fats into fatty acids
These components are absorbed
This occurs in the GIT
Stage II
- Carbohydrates, proteins and fatty acids are further degraded by different pathways
- Carbohydrates are broken down by glycolysis, amino acids by amino acid degradation and fatty acids by β
oxidation
- All the pathways use a common metabolic intermediate acetyl CoA
- Most of the energy at this step is conserved in the chemical bonds of acetyl CoA
- A small portion is conserved by reducing NAD to NADH and FAD to FADH2
Stage III
- The citric acid (or Krebs or TCA) cycle oxidises CoA to CO2.
- The energy released in this process is primarily conserved by reducing NAD to NADH and FAD to FADH2
Stage IV
- This stage is oxidative phosphorylation
- The energy of NADH and FADH2 is released via the electron transport chain (ETC)
- It is used by an ATP synthase to produce ATP
- It requires O2
Jess Q – Week 9 – A matter of opinion
Page 4
Specific to the ETC
- Electrons are transported down a chain coupled to the transport of protons across the inner membrane of the
mitochondria
- This occurs at three specific proton pumping sites  in doing so, creating an electrochemical gradient
- Any energy not trapped as ATP is released as heat
REFERENCES
Hansen, B. & Jorde, L. (N.D). USMLE Step 1 Lecture Notes. USA: Kaplan medical.
Sherwood, L. (2004). Human Physiology: From Cells to Systems (5th Edition). Australia: Thompson.
Lecture notes: Liver as integrator; Fasting and Exercise; Diet and nutrition
Jess Q – Week 9 – A matter of opinion
Page 5