Topic 8 - Metabolism, Cell Respiration, and Photosynthesis Flashcards
1
Q
anabolism
A
- type of metabolic reaction
- synthesizes complex molecules from its constituents
- endergonic (requires absorption of energy)
e. g. photosynthesis
2
Q
catabolism
A
- type of metabolic reaction
- breaks down complex molecules into its constituents
- exergonic (releases energy)
e. g. cell respiration
3
Q
induced fit enzyme model
A
- similar to lock and key model
- except it proposes that the substrate causes a conformational change in the enzyme
- this is due to changes in R-groups of amino acids at the enzyme’s active site
4
Q
mechanism of enzyme action
A
- Surface of substrate contacts enzyme’s active site
- Enzyme changes shape to accommodate substrate
- Temporary complex (enzyme-substrate complex) forms
- Activation energy is lowered and the substrate is altered by the rearrangement of atoms
- The product is released from the active site
- The (unchanged) enzyme is free to combine with other substrates
5
Q
competitive inhibition
A
- the inhibitor only binds to the active site
- so it has to “compete” with the substrate to bind
- it should have a similar structure to the substrate
- can be reversible or irreversible
- if reversible, it can be minimized by increasing substrate conc
6
Q
non-competitive inhibition
A
AKA allosteric inhibition
- the inhibitor binds away from the active site
- the site it binds to is the “allosteric site”
- the binding will cause a change to the active site that prevents substrates from binding to it
- can be reversible or irreversible
7
Q
end-product inhibition
A
- metabolic reactions occur in an assembly-line process
- each step is catalyzed by a different, specific enzyme
- when the end product reaches a sufficient concentration, the assembly line is shut down via end-product inhibition
8
Q
how end-product inhibition works
A
- at sufficient concentrations, the end product acts as a non-competitive inhibitor to the enzyme at the first step of the assembly line
- when the concentration is low, less inhibition occurs and thus there is more enzyme activity
- thus the inhibition of the first pathway prevents the build-up of intermediates in the cell
9
Q
oxidation
A
- lose electrons
- gain oxygen
- lose hydrogen
- results in many C-O bonds
- results in product with lower potential energy
10
Q
reduction
A
- gain electrons
- lose oxygen
- gain hydrogen
- results in many C-H bonds
- results in product with higher potential energy
11
Q
glycolysis
A
- first step of all forms of cell respiration
- occurs in cytosol of a cell
- does not use oxygen!
- uses a hexose (usually glucose)
12
Q
most commonly used hydrogen carrier (oxidant)
A
NAD (nicotinamide adenine dinucleotide)
- as an oxidant, it undergoes reduction
NAD + 2H → reduced NAD
reduced NAD usually takes the form of NADH + H+
13
Q
phosphorylation
A
- reaction in which a phosphate group (PO 4 3-) is added to an organic molecule
- usually transferred from ATP
- creates a less stable molecule which is therefore more likely to react
- can turn a slow endothermic rxn into a fast exothermic rxn
14
Q
process of glycolysis
A
- 2 ATP molecules used to begin glycolysis
- phosphorylation occurs to form fructose-1,6-biphosphate
- 2 ATP molecules used to begin glycolysis
- phosphorylated hexose splits into two 3-carbon sugars triose phosphate (G3P) in a process called “lysis” (literally ‘splitting of molecules)
- once the two G3P molecules are formed, they are oxidized with NAD to remove 2 H from each G3P
- reduced NAD+ (NADH) forms
- released energy used to add another phosphate to the remaining 3-carbon compound
- results in a compound with 2 phosphate groups
- enzymes remove phosphate groups so that they can be added to ADP to produce ATP
- once the two G3P molecules are formed, they are oxidized with NAD to remove 2 H from each G3P
15
Q
results of glycolysis
A
- 4 ATP (as 2 ATP were used up, there’s a net gain of 2 ATP)
- 2 NADP
- 2 pyruvate (ionized form of pyruvic acid)
16
Q
what happens to pyruvate in anaerobic conditions
A
- no oxygen
- so oxidation of pyruvate cannot occur
- products: ethanol + lactate (in humans) or CO2 (in yeast)
17
Q
what happens to pyruvate in aerobic conditions
A
- more energy can be obtained from oxidation of pyruvate
- obviously this only happens in aerobic respiration
- link reaction and Krebs cycle proceed
- products: CO2 and H2O
18
Q
link reaction
A
- follows glycolysis
- occurs before krebs cycle
19
Q
decarboxylation
A
removal of CO2
20
Q
oxidative decarboxylation
A
simultaneous oxidation and decarboxylation
- in link reaction it’s removal of H and CO2 simultaneously
21
Q
link reaction: process
A
- pyruvate enters matrix of mitochondria via active transport
- enzymes in matrix catalyse oxidative decarboxylation with NAD+ and coenzyme A (CoA)
byproducts: reduced NAD (NADH), CO2
main product: Acetyl CoA
- if ATP levels are high already, acetyl CoA can be synthesized into a lipid for storage
22
Q
krebs cycle
A
AKA tricarboxylic acid cycle
- series of reactions
- begins and ends with the same substance (thus ‘cycle’)
- doesn’t directly require oxygen but does require products from electron transport chain
- occurs in matrix of mitochondria
- requires Acetyl CoA (2C)
- starting compound is oxaloacetate (4C)
- each glucose molecule = 2 pyruvates = 2 Acetyl CoAs
- so for each glucose molecule = 2 krebs cycles
- actually hexose is not a requirement, other lipids/carbohydrates can be substituted
- acetyl CoA can be produced from most carbohydrates and fats
23
Q
krebs cycle: process of 1 cycle
A
- Acetyl CoA (2C) + oxaloacetate (4C) → citrate (6C)
byproduct: CoA - citrate (6C) undergoes oxidative decarboxylation with NAD to form 5C compound
byproducts: NADH, CO2 - 5C compound undergoes oxidative decarboxylation with NAD to form 4C compound
byproducts: reduced NAD, CO2 - 4C compound undergoes various changes to form oxaloacetate
- coenzyme FAD reduced to form FADH2
- NAD+ reduced to form NADH
- ADP reduced (1 phosphate group added) to form ATP
byproducts: NADH, FADH2, ATP
CYCLE REPEATS!
24
Q
products of each krebs cycle
A
- 1 ATP
- 3 NADH
- 2 FADH2
- 4 CO2 (waste)
25
Q
usefulness of NADH and FADH2
A
- used as e- sources in electron transport chain
- FADH2 enters the electron transport chain at a lower free energy level than NADH
- so FADH2 produces 2 ATPs while NADH produces 3 ATPs
- at the very end of the chain, the de-energized e-s combine with available oxygen
26
Q
electron transport chain
A
- series of e- carriers in the inner membrane of the mitochondrion (including cristae)
- most of the ATPs from aerobic respiration are produced here
- only stage of cellular respiration where oxygen is needed
- releases small amounts of energy whenever an e- is transported
- e-s step down in potential energy as they are passed from one carrier to another
- coenzymes NADH and FADH2 (byproducts in krebs cycle) are used as e- sources
27
Q
nature of e- carriers in electron transport chain
A
- mostly proteins
- packed close together to reduce amount of energy lost in exchanges
- easily reduced/oxidized
- pass the e-s from one to another using an energy gradient
- some carriers act as proton pumps and pump H+ from the matrix to the intermembrane space
- each carrier has a slightly different electronegativity
- 1 of the carriers is not a protein but CoQ (coenzyme Q)
- oxygen is the final acceptor as it’s highly electronegative
28
Q
why is the energy released from each exchange in the electron transport chain low?
A
- low energy is easily harnessed
- this ensures that the energy is maximised for use (phosphorylation of ADP)
- if it was too high, the cell could be damaged
29
Q
role of oxygen in electron transport chain
A
- oxygen is the last e- acceptor (terminal electron acceptor)
- it’s the last bc it’s highly electronegative
- this occurs in the matrix (on the surface of the inner membrane)
- at the same time, oxygen accepts H+ ions to form water
- the use of protons contributes to the proton gradient across the inner mitochondrial membrane
- only stage where oxygen is used in cell respiration
30
Q
what if oxygen isn’t available?
A
- e- flow stops and reduced NAD can’t be converted back
- NAD supplies in mitochondrion run out so the link reaction and Krebs cycle stops
- the only part of cell respiration that can continue is glycolysis
31
Q
chemiosmosis
A
- generation of ATP using energy released by movement of protons across a membrane
- the electron transport chain proteins act as proton pumps that pump H+ out of the matrix and into the intermembraneous space of the mitochondrion
- they diffuse back via ATP synthase and the energy released upon diffusion is harnessed to produce ATP
32
Q
ATP synthase
A
- enzyme located in the inner mitochondrial membrane
- allows protons to diffuse back across the membrane to the matrix
- energy released by the protons upon diffusion is harnessed by ATP synthase to produce ATP
33
Q
features of a mitochondrion
A
- outer mitochondrial membrane
- matrix
- cristae
- inner mitochondrial membrane
- intermembraneous space
34
Q
mitochondrial intermembraneous space
A
H+ (proton) reservoir
35
Q
cristae
A
- tubular regions surrounded by membranes
- increases SA for oxidative phosphorylation
36
Q
matrix
A
- internal cytosol-like area
- contains enzymes for Krebs cycle and link reaction
37
Q
plastid
A
- group of closely related organelles that occur in photosynthetic eukaryotic cells
- all plastids developed from a common proplastid
38
Q
types of plastids and their functions
A
- chloroplasts, which are green and involved in photosynthesis
- leucoplasts, which are white or ‘clear’ and function as energy storehouses
- chromoplasts, which are brightly coloured and synthesize and store large amounts of orange, red, or yellow pigments
39
Q
photosystems
A
- large groups of pigment molecules
- they work together to harvest light energy
40
Q
components of photosystems
A
- accessory pigments
its reaction centre contains:
- a pair of chlorophyll a molecules
- a protein matrix
- a primary electron acceptor.
41
Q
differences between types of photosystems
A
- Photosystem I absorbs light more efficiently at 700 nm (so it’s called P700)
- Photosystem II absorbs light more efficiently at 680 nm (so it’s called P680)
42
Q
nature of pigments
A
- absorb certain wavelengths of light
- as certain wavelengths cause excitement of an e- in the pigment molecule
43
Q
how pigments in photosystems work together
A
- when an e- of a pigment is excited, it’s passed from pigment to pigment
- until it reaches a special chlorophyll molecule at the reaction centre of the photosystem
- the chlorophyll passes pairs of excited e-s away to e- acceptors in the thylakoid membrane
44
Q
where does the light-dependent reaction occur?
A
in the thylakoids/grana of the chloroplast
45
Q
photophosphorylation
A
production of ATP in chloroplasts
46
Q
chemiosmosis
A
- generation of ATP using energy released by movement of protons across a membrane
- facilitated by some e- carriers acting as proton pumps in the electron transport chain
47
Q
process of light-dependent reaction
A
- photon of light absorbed by pigment in photosystem II is transferred until it reaches the reaction centre
- chlorophyll e-s in p680 (photosystem II) get excited and are passed to a chain of carriers
- water is photolysed by an enzyme
- produces e-s, H+ ions, and an oxygen atom
- e-s are supplied one by one to chlorophyll a molecules in P680
- water is photolysed by an enzyme
- (process similar to electron transport chain)
- at one stage enough energy is released to pump protons from stroma across the thylakoid membrane
- this contributes to a proton gradient
- ATP synthase allows protons to diffuse back to the stroma
- uses energy released to bring about phosphorylation of ADP to produce ATP (chemiosmosis) - P700 undergoes a similar process in first 2 steps
- de-energized e-s from P680 fills the void left by the newly energized electron
- the energized e- is then passed down a second electron transport chain, now involving the carrier ferredoxin
- de-energized e-s from P680 fills the void left by the newly energized electron
- NADP reductase catalyses transfer of the e- from ferredoxin to NADP+
- 2 e-s used to reduce NADP+ to NADPH
48
Q
final products of light-dependent reaction in photosynthesis
A
- NADPH
- ATP
49
Q
light-independent reaction in photosynthesis
A
- occurs in the stroma
- the ATP and NADPH made with the light-dependent reaction are needed for the light-independent reaction
- involves the Calvin cycle
50
Q
Calvin cycle process
A
- Ribulose bisphosphate (RuBP, a 5C compound) binds to incoming CO2 (carbon fixation, catalysed by rubisco)
- product: 6 unstable 6C compounds - Being unstable, the 6C compound breaks down
- product: 12 (3C) glycerate 3-phosphate (GP) molecules - 12 ATP and 12 NADPH reduce GP
- products: 12 (3C) triose phosphates (TP)
- byproducts: 12 ADP, 12 reduced NADP, 12 H+ - TP then undergoes one of 2 pathways:
- continue in the cycle to reproduce the originating compound of the cycle, RuBP
- leave the cycle to become sugar phosphates that may become more complex carbohydrates (this is also where the ‘glucose’ comes from in the summarized photosynthetic reaction) - Normally 10 of the 12 TPs are left and ATP is used to regain RuBP from those TP molecules
- product: 6 RuBP
- byproducts: 6 ADP, 6 H+
