8.1 Cellular Respiration
8.1.1 State that oxidation involves the loss of electrons from an element, whereas reduction involves a gain of electrons; and that oxidation frequently involves gaining oxygen or losing hydrogen, whereas reduction frequently involves losing oxygen or gaining hydrogen.
● Oxidation is the loss of electrons from an element; or gaining of oxygen; or the loss of hydrogen.
● Reduction is the gain of electrons in an element; or the loss of oxygen; or the gain of hydrogen.
8.1.2 Outline the process of glycolysis, including phosphorylation, lysis, oxidation and ATP formation.
In the cytoplasm, one hexose sugar is converted into two three-carbon atom compounds (pyruvate) with a net gain of two ATP and two NADH + H+.
● In glycolysis, a hexose sugar (usually glucose) is converted to form two pyruvate molecules. It occurs within the cytoplasm and is an anaerobic process that does not require oxygen gas.
● The first step isl phosphorylation. Two ATP are invested to reduce glucose into a hexose biphosphate.
● Then lysis occurs, splitting the hexose biphosphate into two triose phosphate molecules.
● Both of these molecules are then oxidized by the electron carrier, NAD+. NAD+ is reduced to NADH and a H+ ion is released. Energy released in this process is used to bind two inorganic phosphates to each of the 3C sugars.
● ATP formation then occurs as the two phosphate ions are removed from each 3C sugar and added to ADP to form ATP. The result is the formation of four ATP molecules and two pyruvates.
● Therefore there is a net gain of two ATP molecules.
● ATP is formed by substrate level phosphorylation.
● Glycolysis is regulated through end-product inhibition.
8.1.3 Draw and label a diagram showing the structure of a mitochondrion as seen in electron micrographs.
● outer membrane; intermembrane space / outer compartment;inner membrane;matrix;cristae; ribosome; naked / circular DNA; ATP synthase;
8.1.4 Explain aerobic respiration, including the link reaction, the Krebs cycle, the role of NADH + H+, the electron transport chain and the role of oxygen.
● Pyruvate enters the matrix of the mitochondria and undergoes the link reaction and oxidative decarboxylation, where three things occur:
○ Pyruvate is oxidized by NAD+, and NAD+ is reduced, forming NADH.
○ Pyruvate undergoes decarboxylation, and CO2 is removed.
○ Coenzyme A (CoA) binds to the acetyl group, forming acetyl CoA.
● Acetyl CoA enters the Krebs cycle and the following reactions occur (draw a diagram):
○ C2 + C4 → C6
■[Acetyl CoA (2C) and oxaloacetate (4C) combine to form a 6C molecule.]
○ C6 → C5 + CO2 released
■[The 6C molecule releases a carbon as CO2, resulting in a 5C molecule. It also becomes oxidized by NAD+, which releases a hydrogen ion and results in reduced
NADH.]
NADH.]
○ C5 → C4 + CO2 released
■[The 5C molecule releases a carbon as CO2, resulting in a 4C molecule. This molecule is also oxidized again, resulting in NADH. ATP formation occurs using one phosphate molecule from the substrate. Subsequently, the 4C molecule undergoes two more oxidations, reducing NAD+ to NADH and FAD to FADH2. Oxaloacetate is now regenerated.]
● The substrate in the Krebs cycle is oxidized by hydrogen-carrying coenzymes, NAD+ and FAD. These become reduced to NADH and FADH2, and are later used in the electron transport chain.
○ 1 ATP is directly produced in the Krebs cycle by substrate level phosphorylation.
● The total yield per turn of the Krebs cycle is: two CO2, 3 NADH + H+, one FADH2. 1 ATP (directly produced).
8.1.5 Explain oxidative phosphorylation in terms of chemiosmosis.
● The electron transport chain is a series of embedded membrane proteins called electron carriers, located in the inner membrane of the mitochondrion.
● NADH and FADH2 supply electrons to the electron carriers, and the energy from electrons is used to pump H+ ions from the matrix and into the intermembrane space of the mitochondrion.
● The inner mitochondrial membrane is impermeable to protons, so as result an electrochemical proton gradient is formed and there is a store of potential energy in the intermembrane space.
● H+ ions flow back into the matrix through a channel called ATP synthase, which is embedded into the inner membrane and also acts as an enzyme. The energy from the movement of protons through ATP synthase causes it to rotate and catalyzes the phosphorylation of ADP (ADP + Pi → ATP) to form ATP.
● Oxygen is the terminal electron acceptor, which binds with H+ to form water.
○ 1/2O2 + 2H+ → H2O
● Since ATP production relies on energy released by oxidation, the process is called oxidative phosphorylation.
● Since ATP production relies on a proton gradient, the process is also called chemiosmosis.
8.1.6 Explain the relationship between the structure of the mitochondrion and its function.
Limit this to cristae forming a large surface area for the electron transport chain, the small space between inner and outer membranes for accumulation of protons, and the fluid matrix containing enzymes of the Krebs cycle.
● There are many folds in the cristae - this increases the surface area available for oxidative phosphorylation in the electron transport chain.
● The double membrane of the mitochondria provides a very small inter-membrane space - the small area allows efficient accumulation of protons.
● The fluid matrix of the mitochondria - contains the enzymes of the Krebs cycle.
● The inner membrane of the mitochondria is impermeable to protons - which alllows H+ ions to accumulate and establish a proton gradient in the inter-membrane space, which drives ATP production.
8.2 Photosynthesis
8.2.1 Draw and label a diagram showing the structure of a chloroplast as seen in electron micrographs.
8.2.2 State that photosynthesis consists of light-dependent and light-independent reactions.
These should not be called “light” and “dark” reactions.
8.2.3 Explain the light-dependent reactions.
Include the photoactivation of photosystem II, photolysis of water, electron transport, cyclic and non-cyclic photophosphorylation, photoactivation of photosystem I, and reduction of NADP+.
● Light energy is funneled to the reaction centre of photosystem II. It becomes photooxidized/photoactivated becase the electrons of chlorophyll are raised to a higher energy level.
● Chlorophyll releases the high-energy electrons and they are passed along a chain of electron carriers.
● The electrons lost by PSII are replaced by electrons yielded from the photolysis of water. O2 is releases as a waste product, and H+ remains within the thylakoid lumen.
● In the chain of electron carriers in the thylakoid membrane, the energy from electron flow causes protons in the form of protons to be pumped into the thylakoid membrane.
○ The proton gradient drives the production of ATP by ATP synthase in the thylakoid membrane. The process is called photophosphorylation.
● Electrons pass to photosystem I at end of carrier chain. They become re-excited and emitted by photosystem I after being photoactivated/photooxidized by light energy.
● Non-cyclic electron flow:
○ Electrons from PSI are carried to the electron acceptor, NADPH+, which becomes reduced to NADPH + H+.
● Cyclic electron flow:
○ Excited electrons flow from PSI to the electrons of PSI and then back to PSI, continuing in a cyclic manner. Cyclic photophosphorylation uses photosystem I electron carriers and ATP synase only.
8.2.4 Explain photophosphorylation in terms of chemiosmosis.
● Chemiosmosis is synthesis of ATP coupled to electron transport and proton movement;
● Photophosphorylation is the production of ATP with energy from light;
● A proton gradient is established within the thylakoid lumen in the following ways:
○ Light energy causes the photolysis of water. This provides replacement electrons for those lost by excited chlorophyll in PSII. This also contributes to the gradient by leaving an H+ in the thylakoid lumen.
○ The movement of electrons along the chain of electron carriers on the thylakoid lumen causes the pumping of protons across the thylakoid membrane.
● Protons move down the concentration gradient and into the stroma by flowing through ATP synthase.
● Energy from the passive movement of protons causes the head of ATP synthase to rotate and this drives ATP formation.
8.2.5 Explain the light-independent reactions.
Include the roles of ribulose bisphosphate (RuBP) carboxylase, reduction of glycerate 3-phosphate (GP) to triose phosphate (TP), NADPH + H+, ATP, regeneration of RuBP, and subsequent synthesis of more complex carbohydrates.
● The Calvin cycle occurs in the stroma of the chloroplast. The products of the light-dependent reactions, ATP and NADPH, are used in the light-independent reactions.
● Carbon dioxide from the atmosphere enters the chloroplast and is fixed using ribolose bisphosphate. The reaction is catalyzed with the enzyme rubisco (ribulose bisphosphate carboxylase).
● This forms a 6-carbon compound that is unstable and breaks into two molecules of glycerate 3-phosphate.
● The glycerate 3-phosphate molecules undergo reduction with NADPH and are acted on by ATP. As a result they are converted to trios phosphates.
● Some of the trios phosphates (5/6) are used to regenerate RuBP. Five trios phosphates convert to three RuBP molecules.
● Some of the trios phosphates (1/6) is further metabolized to form glucose or starch both within or outside the chloroplast.
8.2.6 Explain the relationship between the structure of the chloroplast and its function.
Limit this to the large surface area of thylakoids for light absorption, the small space inside thylakoids for accumulation of protons, and the fluid stroma for the enzymes of the Calvin cycle.
● There is an extensive membrane surface area on the thylakoids – this allows for greater surface area for absorption of light for photosynthesis.
● The lumen within thylakoids is small – this allows for faster accumulation of protons to create a concentration gradient.
● The fluid stroma – houses the enzymes of the Calvin cycle.
8.2.7 Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants.
A separate spectrum for each pigment (chlorophyll a, chlorophyll b, and so on) is not required.
● Action spectra show the percentage use of different wavelengths of light during photosynthesis.
● Absorption spectra show the percentage of absorption of different wavelengths of light.
● The action and absorption spectra of most plants show strong similarities:
○ The greatest absorption is in the violet-blue range.
○ There is also high absorption in the red range.
○ The least absorption is in the yellow-green range. Most of the light is reflected.
● There are few differences:
○ An action spectrum may show absorption of pigments like blue-green or yellow-green; however an action spectrum will not show photosynthesis for those wavelengths. This is due to the ability of accessory pigments (xanthophylls and carotene) to absorb wavelengths that chlorophyll cannot.
8.2.8 Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide.
The limiting factor determines the rate of photosynthesis, because when it is changed the rate of photosynthesis changes.
Light
● Rate of photosynthesis increases as light intensity increases
○ At high light intensities, more photons are absorbed to make ATP and NADPH + H+, and subsequently drive the Calvin cycle.
○ At low light intensities there is a shortage of products of the light dependent reactions, and this limits the Calvin cycle because glycerate 3-phosphate cannot be reduced.
● Photosynthetic rate reaches plateau at high light levels because photosystems are absorbing photons at a maximum rate, and some other factor is limiting.
CO2:
● Rate of photosynthesis increases as carbon dioxide concentration increases.
○ At high concentrations, more CO2 is reacted with ribulose biphosphate (RuBP) and catalyzed by ribulose bisphosphate carboxylase (rubisco) to make triose phosphate.
○ At low concentrations CO2 becomes the limiting reactant of the Calvin cycle.
● Photosynthetic rate reaches plateau at high CO2 levels because rubisco is fixing carbon at a maximum rate.
Temperature:
● Rate of photosynthesis increases with increase in temperature up to an optimal level.
○ At higher temperatures there is greater kinetic energy creates more collisions between enzymes and substrates.
○ At low temperatures, the enzymes of the Calvin cycle function at slower rates and NADPH accumulates.
● Very high temperatures reduce the rate of photosynthesis because high temperature denatures enzymes by breaking intermolecular forces and altering the active sites of the enzymes. This prevents substrates from binding with the enzymes.
If asked to explain “aerobic respiration”:
| Explain the process of aerobic respiration including oxidative phosphorylation. glucose converted to pyruvate (two molecules); by glycolysis; pyruvate enters the mitochondria; pyruvate converted to acetyl CoA / ethyl CoA; by oxidative decarboxylation / NADH and CO2 formed; fatty acids / lipids converted to acetyl CoA; acetyl groups enter the Krebs cycle (accept acetyl CoA); FAD / NAD+ accepts hydrogen (from respiratory substrates) to form NADH / FADH 2; FADH2 / NADH donates electrons / hydrogen to electron transport chain (reject 2 donates H+); electrons release energy as they pass along the chain; oxygen final electron acceptor; production of water; builds up proton gradient / protons pumped across inner membrane; protons flow into matrix of mitochondria through ATPase; ATP produced; produces 36 / 38 ATP (per glucose); | Explain the process of aerobic cellular respiration. glucose is broken down to pyruvate in the cytoplasm; with a small yield of ATP/net yield of 2 ATP; and NADH + H+ /NADH; aerobic respiration in the presence of oxygen; pyruvate converted to acetyl CoA; acetyl CoA enters Krebs cycle; Krebs cycle yields a small amount of ATP/one ATP per cycle; and 2 FADH/FADHH+ + / NADH/ NADHH+ + / reduced compounds / electron collecting molecules; these molecules pass electrons to electron transport chain; oxygen is final electron acceptor/water produced; electron transport chain linked to creation of an electrochemical gradient; electrochemical gradient/chemiosmosis powers creation of ATP; through ATPase; |
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