1. Metabolism, cell respiration and photosynthesis
Topic 8
Metabolism, cell respiration and photosynthesis

2. 8.1 Metabolism
Metabolism is the sum of all the chemical reactions that occur within organisms.
Metabolic pathways consist of chains and cycles of enzyme catalyzed reactions.
Metabolism = Anabolism + Catabolism
Anabolism uses energy to build larger, more complex molecules.
Catabolism breaks down large, complex molecules and releases energy.

3. Reactions that take in energy (anabolic) are said to be endergonic and the products of the reaction contain more energy than the reactants. Ex: photosynthesis
Reactions that release energy (catabolic) are said to be exergonic and the products of the reaction have less energy than the reactants.
Ex: Cellular respiration

4. Metabolic Pathways Glucose enzyme Pyruvate Acetyl CoA
Substrate A Substrate B Substrate C
Each arrow represents a specific enzyme that changes one substrate into the next.
Original research described enzyme substrate interaction as lock and key
Induced-fit enzyme action says the shape of the active site on the enzyme changes when the substrate fits into it. R groups
enzyme

5. Activation Energy
The energy needed to destabilize the bonds of the substrate.
Enzymes lower the activation energy required do they cause the reaction to happen faster.

6. Mechanism of Enzyme Action
Substrate contacts active site of enzyme
Enzyme changes shape
Temporary complex called enzyme-substrate complex forms.
Activation energy is lowered and substrate is altered.
Transformed substrate (product) is released from the active site.
Enzyme is unchanged E + S ES E + P

7. Inhibition Certain molecules can alter activity of an Enzyme.
Competitive Inhibition
A competitive inhibitor, that resembles the substrate, competes for the active site. Rate of the reaction decreases because the active sites are being blocked by the inhibitor.
Can be reversed by adding more substrate.
Ex: Sulfanilamide competes with PABA so folic acid can’t be made in bacteria

8. Inhibition
Non-competitive inhibition (allosteric) doesn’t compete for the active site.
It interacts with another site on the enzyme called the allosteric site. When it binds with the allosteric site, it changes the shape of the enzyme’s active site so it can’t interact with the substrate.

9. End-product inhibition
When the end product of a metabolic pathway reaches a sufficient level, the product itself can act as an inhibitor so that you don’t make more than is needed.
Usually inhibits the first enzyme in the pathway.
Allosteric inhibition, and a form of negative feedback.

10. Competitive inhibitors must be kept at a high level so they can compete. If you increase the substrate, the reaction will increase.
Non-competitive inhibitors continue to keep reaction slow even when you increase the substrate.

11. 8.2 Cell Respiration
Cell respiration involves oxidation and reduction of electron carriers.
NAD+ is reduced to NADH
NADH is oxidized to NAD+
There are catabolic pathways and anabolic pathways

12. Oxidation Reduction Comparison
Oxidation Reduction Loss of electrons Gains electrons Gain of oxygen loses oxygen Results in many C-O bonds C-H bonds Results in a compound with Higher energy lower potential energy Oil RIG oxidation is loss of electrons reduction is gain

13. Occur together. One molecules loss is another molecules gain.
Glucose is oxidized, O2 is reduced
CO2 and H2O have less energy.
Electrons carry energy so things that are reduced gain energy, oxidized loses energy

14. Overview of Respiration
In 2.8 we discussed 3 aspects
Glycolysis, anaerobic respiration (fermentation), aerobic respiration.
Glycolysis in cytoplasm, makes 2 ATP and 2 pyruvate.
If no oxygen available (Anaerobic), pyruvate turned into lactate or CO2 and ethanol. (fermentation)

15. If oxygen is available (aerobic), pyruvate enters the mitochondria and makes a large number of ATPs, CO2 and water.
We will now learn more about glycolysis, and about the link reaction, the Krebs cycle and oxidative phosphorylation

16. Glycolysis Glycolysis needs no oxygen. No organelles are involved.
Occurs in prokaryotes and eukaryotes.
Glucose is split into pyruvate in many steps, we will look at three main stages.
Phosphorylation of glucose, splitting of the glucose into two g-3-p molecules, oxidation of the g-3-p molecules.

17. Phosphorylation of glucose
C-C-C-C-C-C
6 carbon glucose
2 ATP
2 ADP
P-C-C-C-C-C-C- P
Fructose-1, 6-biphosphate

18. Splitting (Lysis) of the 6 carbon molecule
P-C-C-C C-C-C-P
Two Glyceraldehyde-3-phosphates (G3P)

19. Oxidation of G3P 2 P-C-C-C-P 2 C-C-C pyruvate 2 C-C-C-P G3P 2 NAD+ 2 P
2 NADH
2 P-C-C-C-P
4 ADP
4 ATP
C-C-C
pyruvate

20. Summary of Glycolysis Two ATPs used to start
Four ATPs produced for net gain of 2ATPs
Two NADHs produced
Two pyruvates produced
Pathway involves substrate-level phosphorylation, lysis, oxidation and ATP formation
Occurs in the cytoplasm
Controlled by enzymes end product inhibition

21. Mitochondria
The remainder of aerobic respiration takes place in the mitochondria in the presence of oxygen.
Remember, mitochondria have a double membrane, the inside area is called the matrix and the space between the two membranes is called the intermembrane space.

22. The Link Reaction
Pyruvate enter the mitochondrial matrix via active transport.
Each pyruvate loses a CO2 (decarboxylation).
Each pyruvate is oxidized (NAD+ to NADH)
Each pyruvate receives coenzyme A
We now have 2 Acetyl CoA
No ATP is produced

23. Krebs Cycle
If ATP levels are high, Acetyl CoA can be converted into a lipid for storage.
If ATP levels are low, the Acetyl CoA enters the Krebs cycle, also in the matrix.
Step 1 Acetyl CoA combines with a 4 carbon molecule called oxaloacetate. =6 carbon citrate, CoA leaves.
Step 2 Citrate is oxidized (NAD+ to NADH) and decarboxylated into a 5 carbon molecule.

24. Step 3 The 5 carbon molecule is oxidized (NAD+ to NADH) and decarboxylated into a 4 carbon molecule.
Step 4 The 4 carbon molecule undergoes various changes making several products.
Another NAD+ to NADH, FAD is reduced to FADH2
ADP is reduced to ATP
At the end, the 4 carbon molecule becomes oxaloacetate. Back where you started.

26. Remember, the Krebs cycle will run twice for each glucose you began with.
Taking into account the two acetyl CoA molecules go through the cycle, to total is:
2 ATPs
6 NADHs
2 FADH2s
4 CO2s

27. Two CO2s were released from the link reaction and four more from the Krebs cycle. This accounts for all 6 of the carbons in the initial glucose molecule. Glucose has been completely catabolized and its original energy is now carried by NADH, FADH2 and ATP.
So far we have only made 4 ATPs by a process called substrate-level phosphorylation.
Only 34 more to go

28. Electron Transport Chain
We will make the remaining ATPs by a process called oxidative phosphorylation.
This occurs on the inner membrane and cristae.
Embedded in the inner membrane are molecules easily reduced and oxidized.
These molecules are going to receive and pass along electrons

29. Each molecule has a different level of attraction for electrons due to different electronegativity.
Most of these molecules are proteins with haem (heme) groups called cytochromes.
One of them is not a protein and is called coenzyme Q
In this chain, electrons are passed from one carrier to another because the receiving molecule has a greater attraction for the electron than the previous one.

30. As the electrons are being passed along, small amounts of energy are released.
The source for the electrons are the NADHs and FADH2s produced in previous steps.
FADH2 electrons enter the chain at a lower level than NADH electrons so each FADH2 will create 2 ATPs while each NADH will create 3.
At the end of the chain, the low energy level electrons combine with oxygen.

31. Oxygen is the final electron acceptor and very electronegative.
After 2 electrons combine with oxygen, they attract 2 hydrogen ions H+ and water is formed.
The energy lost as the electrons are passed along the chain will be used to make the ATP.

32. Chemiosmosis
Involves the movement of hydrogen ions, H+, from the matrix into the inner membrane space.
Imbedded in the inner membrane is an enzyme called ATP synthase, and this is where the ATP will be created.
The Hydrogen ions that are being moved into the inner membrane space are building up and creating a concentration gradient.
They move back into the matrix by going through the ATP synthase and their energy is used to attach a P onto an ADP to make ATP
This is called oxidative phosphorylation because the energy being used is coming from oxidation.

33. Summary of ATP Production
The energy flow can be given as follows:
Glucose NADH or FADH2 Electron transport chain Chemiosmosis ATP
Process ATP
Glycolysis 2
Krebs Cycle 2
Elect chain/ Chem 32

34. Often, not all 36 ATPs that are possible are actually created, usually its closer to 30.
Why? Some H+ manage to get back into the matrix without going through the ATP synthase.
Some energy is used to move the pyruvate into the mitochondria. (remember, it was active transport).
The 30 ATPs represent about 30% of the energy for the glucose, the rest is lost as heat.