1. Chapter 25 Metabolism and Nutrition
The food we eat is our only source of energy for performing biological work.
There are three major metabolic destinations for the principle nutrients. They will be used for energy for active processes, synthesized into structural or functional molecules, or synthesized as fat or glycogen for later use as energy.

2. Glucose Catabolism

3. Coenzymes
Two coenzymes are commonly used by living cells to carry hydrogen atoms: nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD).
An important point to remember about oxidation-reduction reactions is that oxidation is usually an energy-releasing reaction.

4. Mechanisms of ATP Generation
ADP + P = ATP
Phosphorylation is
bond attaching 3rd phosphate group contains stored energy
Mechanisms of phosphorylation
within animals
substrate-level phosphorylation in cytosol
oxidative phosphorylation in mitochondria
in chlorophyll-containing plants or bacteria
photophosphorylation.

5. Phosphorylation in Animal Cells
In cytoplasm (1)
In mitochondria (2, 3 & 4)

6. Carbohydrate Review
In GI tract
polysaccharides broken down into simple sugars
absorption of simple sugars (glucose, fructose & galactose)
In liver
fructose & galactose transformed into glucose
storage of glycogen (also in muscle)
In body cells --functions of glucose
oxidized to produce energy
conversion into something else
storage energy as triglyceride in fat

7. Fate of Glucose
Glucose can be used to form amino acids, which then can be incorporated into proteins.
Excess glucose can be stored by the liver and skeletal muscles as glycogen, a process called glycogenesis.
If glycogen storage areas are filled up, liver cells and fat cells can convert glucose to glycerol and fatty acids that can be used for synthesis of triglycerides (neutral fats) in the process of lipogenesis.

8. Glucose Movement into Cells
In GI tract and kidney tubules
Na+/glucose symporters
Most other cells
GluT facilitated diffusion transporters
insulin increases the insertion of GluT transporters in the membrane of most cells
in liver & brain, always lots of GluT transporters
Glucose 6-phosphate forms immediately inside cell (requires ATP) thus, glucose is “hidden” when it is in the cell.
Concentration gradient remains favorable for more glucose to enter.

9. Glucose Catabolism

10. Glucose Oxidation Cellular respiration 4 steps are involved
glucose + O2 produces H2O + energy + CO2
Anaerobic respiration
called glycolysis (1)
formation of acetyl CoA (2) is transitional step to Krebs cycle
Aerobic respiration
Krebs cycle (3) and electron transport chain (4)

11. Glycolysis
Glycolysis refers to the breakdown of the six-carbon molecule, glucose, into two three-carbon molecules of pyruvic acid.
10 step process occurring in cell cytosol
use two ATP molecules, but produce four, a net gain of two (Figure 25.3).

12. Glycolysis of Glucose & Fate of Pyruvic Acid
Breakdown of six-carbon glucose molecule into 2 three-carbon molecules of pyruvic acid
Pyruvic acid is converted to acetylCoA, which enters the Kreb’s Cycle.
The Kreb’s Cycle will require NAD+
NAD+ will be reduced to the high-energy intermediate NADH.

13. Glycolysis of Glucose & Fate of Pyruvic Acid
When O2 falls short in a cell
pyruvic acid is reduced to lactic acid
coupled to oxidation of NADH to NAD+
NAD+ is then available for further glycolysis
lactic acid rapidly diffuses out of cell to blood
liver cells remove lactic acid from blood & convert it back to pyruvic acid

14. Pyruvic Acid
The fate of pyruvic acid depends on the availability of O2.

15. Formation of Acetyl Coenzyme A
Pyruvic acid enters the mitochondria with help of transporter protein
Decarboxylation
pyruvate dehydrogenase converts 3 carbon pyruvic acid to 2 carbon fragment acetyl group plus CO2.

16. Formation of Acetyl Coenzyme A
2 carbon fragment (acetyl group) is attached to Coenzyme A to form Acetyl coenzyme A, which enter Krebs cycle
coenzyme A is derived from pantothenic acid (B vitamin).

17. Krebs Cycle
The Krebs cycle is also called the citric acid cycle, or the tricarboxylic acid (TCA) cycle. It is a series of biochemical reactions that occur in the matrix of mitochondria (Figure 25.6).

18. Krebs Cycle

19. Krebs Cycle
The large amount of chemical potential energy stored in intermediate substances derived from pyruvic acid is released step by step.
The Krebs cycle involves decarboxylations and oxidations and reductions of various organic acids.
For every two molecules of acetyl CoA that enter the Krebs cycle, 6 NADH, 6 H+, and 2 FADH2 are produced by oxidation-reduction reactions, and two molecules of ATP are generated by substrate-level phosphorylation (Figure 25.6).
The energy originally in glucose and then pyruvic acid is primarily in the reduced coenzymes NADH + H+ and FADH2.

20. Krebs Cycle (Citric Acid Cycle)
The oxidation-reduction & decarboxylation reactions occur in matrix of mitochondria.
acetyl CoA (2C) enters at top & combines with a 4C compound
2 decarboxylation reactions peel 2 carbons off again when CO2 is formed

21. Potential energy (of chemical bonds) is released step by step to reduce the coenzymes (NAD+NADH & FADFADH2) that store the energy
Review:
Glucose 2 acetyl CoA molecules
each Acetyl CoA molecule that enters the Krebs cycle produces
2 molecules of C02
3 molecules of NADH + H+
one molecule of ATP
one molecule of FADH2
Krebs Cycle

22. Electron Transport Chain
The electron transport chain involves a sequence of electron carrier molecules on the inner mitochondrial membrane, capable of a series of oxidation-reduction reactions.
As electrons are passed through the chain, there is a stepwise release of energy from the electrons for the generation of ATP.
In aerobic cellular respiration, the last electron receptor of the chain is molecular oxygen (O2). This final oxidation is irreversible.
The process involves a series of oxidation-reduction reactions in which the energy in NADH + H+ and FADH2 is liberated and transferred to ATP for storage.

23. Electron Transport Chain
Pumping of hydrogen is linked to the movement of electrons passage along the electron transport chain.
It is called chemiosmosis (Figure 25.8.)
Note location.

24. Chemiosmosis
H+ ions are pumped from matrix into space between inner & outer membrane
High concentration of H+ is maintained outside of inner membrane
ATP synthesis occurs as H+ diffuses through a special H+ channels in the inner membrane

25. Electron Transport Chain

26. Steps in Electron Transport
Carriers of electron transport chain are clustered into 3 complexes that each act as a proton pump (expelling H+)
Mobile shuttles (CoQ and Cyt c) pass electrons between complexes.
The last complex passes its electrons (2H+) to oxygen to form a water molecule (H2O)

27. Proton Motive Force & Chemiosmosis
Buildup of H+ outside the inner membrane creates + charge
The potential energy of the electrochemical gradient is called the proton motive force.
ATP synthase enzymes within H+ channels use the proton motive force to synthesize ATP from ADP and P

28. Summary of Aerobic Cellular Respiration
The complete oxidation of glucose can be represented as follows:
C6H12O6 + 6O2 => 36 or 38ATP + 6CO2 +6H2O
During aerobic respiration, 36 or 38 ATPs can be generated from one molecule of glucose.
Two of those ATPs come from substrate-level phosphorylation in glycolysis.
Two come from substrate-level phosphorylation in the Krebs cycle.

29. Review Table 25.1 summarizes the ATP yield during aerobic respiration.
Figure 25.8 summarizes the sites of the principal events of the various stages of cellular respiration.

30. Glycogenesis & Glycogenolysis
glucose storage as glycogen
4 steps to glycogen formation in liver or skeletal muscle
stimulated by insulin
Glycogenolysis
glucose release

31. Glycogenesis & Glycogenolysis
glucose storage as glycogen
Glycogenolysis
glucose release
not a simple reversal of steps
Phosphorylase enzyme is activated by glucagon (pancreas) & epinephrine (adrenal gland)
Glucose-6-phosphatase enzyme is only in hepatocytes so muscle can not release glucose into the serum.

32. Gluconeogenesis
Gluconeogenesis is the conversion of protein or fat molecules into glucose (Figure 25.12).

33. Gluconeogenesis
Glycerol (from fats) may be converted to glyceraldehyde-3-phosphate and some amino acids may be converted to pyruvic acid. Both of these compounds may enter the Krebs cycle to provide energy.
Gluconeogenesis is stimulated by cortisol, thyroid hormone, epinephrine, glucagon, and human growth hormone.