Download LECTURE OUTLINE (Chapter 7) I. Energy Currency and Energy

LECTURE OUTLINE (CHAPTER 7)
I. Energy Currency and Energy Carrier Molecules (Sections 7.1 and 7.2)
A. Energy harvesting—Everest’s deadly problem: having only 30 percent of the oxygen
available in the atmosphere at sea level. How many people know why we need to breathe
oxygen? To extract energy from food (covered in this lecture). Does all energy harvesting
require oxygen?
B. Relevant questions to be answered:
1. How do cells convert food into energy? Do they convert fats differently from
carbohydrates? Do you get fat by eating fat or calories?
2. Why do we need to breathe? How does exercise affect metabolism?
3. What about energy supplements? What does it mean to have a fast or slow
metabolism?
C. ATP is the most important energy storage molecule: Figure 7.1.
1. Potential energy from food breakdown is used to drive the endergonic synthesis of
ATP (like recharging a battery).
2. The charged ATP has energy that can be released at any time (by breaking off the third
phosphate to do a variety of actions).
D. Electrons from food carry energy to make ATP.
1. Electrons from glucose run downhill. Transferred by carriers, the electron drop powers
uphill synthesis of ATP.
2. Electron transfers to molecules—redox reactions occur side by side:
a. One molecule is oxidized—loses electrons.
b. Another molecule is reduced—gains those electrons (reduces charge).
3. Intermediate electron carriers serve to shuttle electrons through these reactions,
transferring energy as they go: Figure 7.3.
a. NAD+ (empty city cab) in redox reaction is an oxidizing agent (removes electrons,
causing a substance to be oxidized); it accepts a hydrogen atom and one electron
becomes NADH (full cab).
b. NADH can carry electrons (proceed down energy hill) to another acceptor, thus
being regenerated (empty cab).
c. NAD+ is made by cells from the vitamin niacin.
4. Enzymes coordinate all these transfers by bringing together the glucose derivatives
with energy carrier molecules.
II. Cellular Respiration (Sections 7.3 through 7.6)
A. Overview of the three stages:
1. General reaction—C6H12O6 + 6O2 + ADP  6CO2 + 6H2O + ATP
2. Energy coupling—downhill breakdown of glucose releases electrons, carried along and
used to transfer energy to drive uphill synthesis of ATP.
3. Overview and importance of each of the three stages: Figure 7.4.
a. Glycolysis—for eukaryotes, this is the first stage. It begins breakdown of glucose,
yielding little energy, but it does transfer electrons to NAD+. On the plus side, it
doesn’t require oxygen and occurs in the cytoplasm, and some prokaryotes and
single-celled eukaryotes have long used it as the sole source of energy.
b. Krebs cycle and electron transport chain—evolved later, but generate larger
quantities of energy; only problems are that they occur only in mitochondria (only
eukaryotes) and that the electron transport chain requires oxygen.
B. Glycolysis: means “sugar splitting”: Figure 7.5.
1. Steps in the process:
a. Sugar in bloodstream enters cytoplasm, where this breakdown begins. Enzymes
catalyze each reaction in metabolic pathway (first is hexokinase, which adds a
phosphate from ATP, a process called phosphorylation).
b. Although breaking sugar apart generates energy, it requires some activation energy
(another ATP is used to attach another phosphate: –2 ATP total).
c. Rearrangement eventually leads to splitting the molecule in half, from one 6-carbon
sugar into two 3-carbon sugars (pyruvic acid is the end product).
d. Oxidation by 2 NAD+ transfers electrons and leads to the attachment of a highenergy phosphate to each sugar. Enough energy is generated by the eventual release
of these four total phosphates in the next two steps to attach them to ADP to make
ATP (+ 4 ATP).
2. Ledger:
a. Plus side—very fast reactions cut glucose in half, generating small amount of energy
(net 2 ATP), and electrons (2 NADH), but no oxygen was required.
b. Minus side—what is the next redox reaction in which electrons can be transferred to
empty the cab (NADH) for more passengers (NAD+)? Not much ATP made for all
the work.
3. Essay: When Energy Harvesting Ends at Glycolysis, Beer Can Be the Result
a. Bacteria and certain eukaryotes can use only glycolysis. Problem—how can they
recycle the NAD+?
b. Solution—alcoholic fermentation. Occurs in yeast in the absence of oxygen (e.g., to
produce bread and wine); must regenerate NAD+, so they dump electrons from
NADH onto the acetaldehyde (converted from pyruvic acid and spewing off CO2),
reducing it to ethanol, but regenerating the NAD+.
c. Solution—lactate fermentation. Occurs in animals in the absence of oxygen (muscle
fatigue); pyruvate accepts electrons from NADH and regenerates NAD+ but is
converted into lactic acid (muscle burn).
4. Essay: Energy and Exercise—huge quantities of ATP are required to contract skeletal
muscle, but ATP is generated in different ways depending on circumstances.
a. First burst of activity (6 seconds): cells have stockpile of ATP and phosphocreatine:
essay figure 2.
b. Look at essay figure 1—what is greatest source of energy at 30 seconds? Aerobic or
anaerobic? What about at 10 minutes? Why? Answer: although aerobic respiration is
slower, it is much more productive. The body moves to aerobic respiration as soon
as it can, using anaerobic respiration only under extreme exertion when oxygen
drops: essay figure 2.
C. Krebs cycle sugar derivatives are oxidized to yield electrons in the interior of the inner
membrane of mitochondria: Figure 7.6.
1. Each of the two pyruvic acids travels into the mitochondria, where they combine with
coenzyme A to make acetyl CoA, one NADH and CO2 (breathe out): Figure 7.7.
2. Steps in the process: Figure 7.8.
a. Acetyl CoA combines with oxaloacetic acid to make citric acid and continues around
through a series of reactions that finally yield oxaloacetic acid again (cycle).
b. During the cycle of reactions, as the acetyl CoA is transformed, it is being oxidized
by electron carrier molecules NAD+ and FAD.
c. Also, ATP and CO2 are produced.
3. Ledger—6 NADH, 2 FADH2, and 2 ATP; acetyl CoA completely broken down into
CO2.
4. Majority of electrons for next stage (electron transport chain).
D. Electron transport chain (ETC)—series of molecules in the mitochondrial inner
membrane that are the destination of the electrons carried by NADH and FAHD2.
1. Steps in the process: Figure 7.9.
a. NADH arrives, and it bumps the ETC’s first carrier, which accepts the electrons,
then passes them on along the chain (like a hot potato).
b. Movement of electrons at each transfer releases enough energy to power the
movement of H+ ions from the inner compartment into the outer compartment (like
the heat of a hot potato dissipating as it is passed). The ions are being pumped
against their concentration gradient (uphill).
c. Hydrogen ions are allowed to flow downhill through an enzyme in the membrane
called ATP synthase, like a waterwheel spinning; as the ions pass, energy is used to
transfer phosphate onto ADP to make ATP.
2. Greatest amount of ATP is made in this stage (32 ATP per glucose).
3. At the end of the ETC, which carrier accepts the electron?
1/2 O2 + 2 electrons + 2 H+ = H2O
III. Other Foods, Other Respiratory Pathways (Section 7.7)
A. Fats, proteins, and other sugars can also enter pathway to be converted to energy, but not
in exactly the same way: Figure 7.10.
B. Food eaten in excess of caloric demands can also be converted from amino acids, fatty
acids, and sugars into proteins, fats, and carbohydrates for structure or storage (98 percent
of energy reserves of animals are fats).
C. Example: fats.
1. Triglyceride is converted to fatty acids and glycerol.
2. Glycerol is converted to glyceraldehyde phosphate (glycolysis intermediary)
downstream.
3. Fatty acids can be broken apart and used to make acetyl CoA.
KEY TERMS
ATP synthase
cellular respiration
citric acid cycle
electron carrier
glycolysis
oxidation
Krebs cycle
redox reaction
nicotinamide adenine dinucleotide (NAD)
electron transport chain (ETC)
reduction
LECTURE OUTLINE (CHAPTER 8)
I. Introduction: Photosynthesis and Energy (Section 8.1)
A. Relevance.
1. Try to name something you eat that isn’t from a plant or from an animal that ate a
plant.
2. All food comes from plants. Molecules of our bodies are made from food we eat, but
plants make their own food from sunlight.
a. Food is used for creating macromolecules from monomers such as glucose and
amino acids.
b. More important, food is used in respiration to generate cellular energy, ATP. Plants
are the nearly universal source of energy for all living things.
c. Plants take energy-poor reactants (water and carbon dioxide) and use solar energy to
drive the uphill reaction of trapping those reactants in complex, ordered bonds of
glucose.
3. Oxygen needed for respiration is produced as a by-product of photosynthesis.
II. Light Energy Drives Photosynthesis (Section 8.2)
A. Nature of light.
1. Energy rays have different wavelengths in a spectrum, from gamma rays to radio
waves, only a portion of which is visible light: Figure 8.2.
2. Explain what it means to see a plant as red or green in terms of absorption and
reflection and why a black car is hotter on a sunny day than a white car. (Black absorbs
all light and reflects none; white absorbs little and reflects almost all.) Explain what it
means to absorb light by a pigment.
3. Photosynthesis is driven by only part of the visible spectrum (blue and red); plant
pigments in the green plant reflect green and absorb blue and red.
B. Tour of a leaf, where plants absorb light: Figure 8.3.
1. Blade.
2. Leaf section, epidermis, stomata, mesophyll.
3. Chloroplasts, inner and outer membranes.
4. Grana and stroma.
5. Thylakoid membrane and compartment.
6. Pigments.
C. Photosynthesis occurs in two essential phases.
1. Light-dependent phase: “photo” of photosynthesis.
a. Power of sunlight excites electrons in pigment molecules.
b. Excited electrons are carried down transport chain of redox reactions like those in
mitochondria.
c. Energy is used to make a gradient of H+ ions to drive synthesis of ATP, and
electrons may be transferred by a carrier molecule such as NAD+, NADP+.
d. Pigment electrons are replaced by electrons stripped from water, making O2 gas.
2. Light-independent phase.
a. ATP and NADPH are not good permanent storage molecules, so the plants convert
energy into several bonds in a glucose molecule.
b. Electrons from carriers are brought together with CO2 and H2O to make this glucose.
D. Photosystems are the working units that absorb solar energy.
1. Aggregates of hundreds of pigment molecules serve as antennae to absorb solar energy.
2. Reaction center of aggregate contains pair of chlorophyll molecules with electrons that
absorb the energy and jump to electron carrier molecules: Figure 8.4.
E. Energy transfer is possible using redox reactions.
1. One substance loses electrons (oxidized) while another gains electrons (reduced).
2. Electrons move down the energy hill, losing energy as they go (analogy of people
passing a hot potato, warming each hand as it drops, giving off some heat as it goes.
The last person to get the potato gets some heat and food as well). The final recipient
of the electron in this case is NADP+: Figure 8.5.
III. Light Reactions (Sections 8.3 and 8.4)
A. Follow the pathway: resources for this chapter include Web Animation 8.1 Properties of
Light.
1. Photosystem II absorbs solar energy.
2. Electron jumps to the primary electron acceptor.
3. Chlorophyll is left without an electron, making it an oxidizing agent that grabs an
electron from water, splitting it into H+ ions and O2.
4. Ejected electron falls back down the energy hill through a series of electron transfer
molecules and a series of redox reactions until it reaches photosystem I (another
reaction center also receiving solar energy).
5. Again, energized electrons from photosystem I are transferred back down the energy
hill, until they are received by NADP+, an electron carrier that ferries electrons to the
second stage, the light-independent stage of photosynthesis.
6. Travel took place from thylakoid to stroma: Figure 8.6.
B. Importance of the light-dependent phase.
1. Oxygen is formed.
2. Energized electrons are being transferred, not just giving off heat of fluorescing, and
are ferried in NADPH.
3. ATP is formed, which is used to power the second stage, the light-independent
reactions.
IV. The Calvin Cycle or Light-Independent Reactions (Section 8.5)
A. The Calvin (C3) cycle—the “synthesis” of photosynthesis, making food, trapping CO2:
Figure 8.7.
1. Enzyme called rubisco brings together CO2 and sugar, carbon fixation—three lowenergy molecules of CO2 from the atmosphere are combined with three 5-carbon
sugars (RuBP).
2. Six-carbon product is unstable and splits into two 3-carbon products (3-PGA).
3. ATP places a phosphate group on each 3-PGA; NADPH donates a pair of electrons,
yielding a high-energy food, G3P.
4. Only one G3P exits the cycle; the other five are used to regenerate the starting material,
RuBP.
5. Review both stages together: Figure 8.8.
V. Photorespiration and Plant Adaptation (Section 8.6)
A. Glitch in the system—photorespiration.
1. Rubisco often combines O2 instead of CO2 with RuBP, unproductively.
2. Occurs with one O2 for every three CO2.
3. Undercuts food production in crops that use C3 cycle.
4. Especially troublesome in hot weather because of water evaporation. Plant closes
stomata in leaves to prevent evaporation, but as water is kept in, CO2 is kept out. As
the light-dependent reactions continue, O2 builds up, combining with RuBP
unproductively.
B. C4 plants (warm climate adaptation): Figure 8.9.
1. Examples: grasses, corn, sugarcane, and sorghum.
2. Use a different enzyme located in bundle-sheath cells: Figure 8.10.
3. Costs ATP to shuttle CO2 to bundle-sheath cells; in sunny climates this is not an issue,
because with abundant sunlight, ATP is plentiful.
4. In northern climates, C4 plants are not as well adapted.
VI. CAM Plants (Section 8.7)—Another Adaptation That Saves Water in Hot Climates
A. Examples: cactus, pineapple, mint, and orchid.
B. Close stomata during the day; open them at night
C. Start C4 metabolism at night by fixing CO2 but wait for day to use abundant ATP to
finish.
D. Comparison of three strategies: Table 8.1.
KEY TERMS
C4 photosynthesis
Calvin cycle
CAM photosynthesis
chlorophyll a
chloroplast
fixation
photorespiration
photosynthesis
photosystem
reaction center
rubisco
stomata
stroma
thylakoid