HOW CELLS RELEASE STORED ENERGY

HOW CELLS RELEASE STORED

ENERGY

Chapter 7

Energy-releasing Pathways

Sun producers consumers (heterotrophs) Producers harvest the sun’s energy to make glucose Consumers eat the producers (and other consumers) to

obtain energy (glucose) Consumers break down glucose to convert its energy to

ATP using aerobic (with O2) or anaerobic (without O2) respiration

The chemical equation for aerobic respiration shows its relationship to photosynthesis

C6H12O6 + 6O2 6CO2 + 6H2O + ATP See the small figure on page 120 showing the relationship

between photosynthesis and aerobic respiration


Both aerobic and anaerobic respiration start with glycolysis in the cytoplasm Produces a small amount of ATP

Aerobic respiration continues in the mitochondria with the Kreb’s cycle and electron transfer phosphorylation (This is the same as electron transfer chain. The book uses phosphorylation, I will refer to it as the electron transfer chain.)

Produces a large amount of ATP

Fig. 7-2b, p.108

Fig. 7-3, p.109

Questions

What are heterotrophs? Which type of respiration occurs in the presence of

oxygen? What metabolic pathway is used by both aerobic

and anaerobic respiration? Where does glycolysis occur? Where does the Krebs cycle occur? Which type of respiration produces the most ATP?

Glycolysis

Glycolysis is a metabolic pathway that occurs in the cytoplasm

Glucose is broken down through a series of intermediates to two pyruvate molecules Study unit 7.3 and figure 7.5 The following slide highlights some important steps of

the pathway

Glycolysis

1. Two ATP molecules are used to energize the rearrangement of glucose into two 3-carbon molecules called PGAL For glycolysis you have to spend some energy to earn some

energy

2. Both PGAL molecules are rearranged through several intermediates Electrons are stripped from PGAL producing NADH ATP is produced when phosphate groups are transferred to

ADP (phosphorylation)

3. The final product is two pyruvate molecules

Glycolysis

Results per glucose molecule 2 ATP total

Used 2 ATP Made 4 ATP

2 NADH molecules Co-enzymes carrying electrons and H+ Will be shuttled to the electron transport chain

2 pyruvate molecules

Questions

What starting molecule is broken down during glycolysis?

How much energy is required to get glycolysis started? After the input of energy what 3-carbon intermediate is

formed? What is the final product of glycolysis? How much ATP is generated (gross and net)? How much NADH is generated?

Krebs Cycle

The Krebs cycle is a cyclic metabolic pathway that occurs in the mitochondria

Pyruvate is rearranged to form Acetyl-CoA which then enters the Krebs cycle where it is broken down in a series of steps to CO2

Study unit 7.4 and figure 7.7 The following slide highlights some important

steps of the pathway

Krebs Cycle

1. Pyruvate enters the mitochondrion (remember there are two pyruvate molecules for every glucose molecule)

2. Pyruvate is changed to Acetyl-CoA by the following reactions Electrons and H+ are stripped to make NADH CO2 is released

Co-enzyme A is attached

3. Acetyl-CoA combines with oxaloacetate to form citrate

4. Several rearrangements and intermediates result in Two more CO2 molecules are released

Three more NADH (and one FADH2, which is another electron carrying co-enzyme) form by stripping electrons and H+ from the intermediates

One molecule of ATP is formed

5. Ultimately oxaloacetate is reformed to start the cycle again

Fig. 7-6a, p.113

Krebs Cycle

Results per glucose molecule (remember there are two pyruvates produced per molecule of glucose)

Six CO2 molecules released Accounts for all six carbons found in glucose C6H12O6

Eight NADH plus two FADH2 molecules Co-enzymes carrying electrons and H+ Will be shuttled to the electron transport chain

Two ATP molecules

Questions

Where does the Krebs cycle occur? Prior to starting the Krebs cycle pyruvate is changed to

what molecule? Acetyl-CoA is bound to ______ to form _____. During the Krebs cycle electrons and hydrogen atoms are

stripped and carried by what two co-enzymes? How many ATP molecules are formed (per one glucose)? How many NADH molecules are formed (per one

glucose)?

Electron Transfer Chain(aka: electron transfer phosphorylation)

Electron transfer is an energy producing process that occurs over the inner mitochondrial membrane Study unit 7.5 and figure 7.8 The following slides highlight some important steps of

the transfers


1. NADH and FADH2 carry the electrons and H+ to the electron transfer chain

2. As the electrons move through the chain they release small amounts of energy allowing the transfer chain to shuttle H+ over the membrane

Fig. 7-7b, p.114


3. An H+ gradient forms in the outer mitochondrial compartment (in between the two membranes)

4. The resulting gradient propels H+ across the mitochondrial membrane through ATP synthases

5. The flow has enough force to cause the synthases to attach phosphate to ADP, forming ATP The process is called chemiosmosis or H+ electrochemical gradient

Fig. 7-7b, p.114


6. Once the electrons have moved through the electron transfer chain, they are accepted by O2 which is the terminal electron acceptor

7. O2 combines the electrons and H+ to form water O2 + H+ + electrons H2O

Fig. 7-7b, p.114


Results per glucose molecule 32 molecules of ATP! Depending on the needs of the cell the amounts can

fluctuate Shifting concentrations of reactant, intermediates and

products Shuttling mechanisms for moving NADH may use some ATP

Questions

Where is the electron transfer chain? What carries the electrons to the chain? What happens when energy is released by electrons

moving through the electron transfer chain? The build up of a H+ concentration gradient provides the

force to cause what important event? For aerobic respiration what is the terminal electron

acceptor? How many molecules of ATP can be formed by the

electron transfer chain (per glucose molecule)?


Total possible for one molecule of glucose (cells don’t always harvest this much as they may use the intermediates in other processes)

Glycolysis = 2 ATP Kreb’s = 2 ATP Electron transfer = 32 ATP Total = 36 ATP

See figure 7.9 for a summary of aerobic respiration

Fig. 7-8, p.115

Energy-releasing Pathways Anaerobic respiration uses glycolysis, but due to the lack of oxygen

Kreb’s and electron transfer are not used Instead of oxygen being the terminal electron acceptor, an organic

substance is used for NADH to donate the electrons (NADH must be recycled to NAD+ to be used in glycolysis again) Alcoholic fermentation (fig 7.10)

Pyruvate is converted to acetaldehyde which accepts electrons from NADH producing ethanol and CO2

Used to produce yeast breads and alcoholic beverages Lactate fermentation (figure 7.11)

Pyruvate accepts electrons from NADH producing lactate Muscle cells use this pathway when they are not receiving enough O2

Anaerobic pathways are referred to as fermentation pathways

Fig. 7-9b, p.116

Fig. 7-10a, p.117

Fig. 7-10b, p.117

Fig. 7-10c, p.117

Fig. 7-9c, p.116

Fig. 7-11, p.117

Alternative Energy Sources

Glucose is a type of carbohydrate Carbohydrates are an important part of the diet to

provide energy Proteins and lipids (fats) can also be used for

energy The molecules are broken down to form PGAL or Krebs

cycle intermediates They can then be used to produce ATP See figure 7.12

Fig. 7-12, p.119

Questions

Anaerobic respiration can also be called _____. During anaerobic respiration what type of molecule

becomes the electron accepter for NADH? What pathway is used by yeast in bread making? What pathway can be used by some muscle fibers? What other molecules can be used for energy? What are alternate energy sources generally broken

down to?

Summary

Aerobic respiration Glycolysis Kreb’s Electron transfer chain

Anaerobic respiration/fermentation Ethanol Lactic acid

Alternate energy sources

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HOW CELLS RELEASE STORED ENERGY