Photosynthesis is a broadly used term to refer to the process through which the energy of sunlight is used for the synthesis of carbohydrates from CO2 and water. The "Calvin Cycle" is the name given to the sequence of carbon fixation reactions that involved the incorporation of CO2 to produce particular organic compounds. We divided the Clavin Cycle into three stages, to help us understand the entire process. Which statement below reflects what happens in the first step of the first stage of the Calvin Cycle?
CO2 is attached to a 5C intermediate (ribose 1,5 bisphosphate) to make a 6C intermediate
What happens in the second step of the first stage of the Calvin Cycle?
The 6C intermediate is cleaved into two, 3C molecules of 3-phosphoglycerate
what happens in the first step of the second stage of the Clavin Cycle
3-phosphoglycerate is converted to 1,3 bisphosphoglycerate using ATP
The second stage of the Calvin Cycle results in the production of 3C compounds known as triose phosphate. What is the fate of a significant portion of these compounds?
They are converted to glucose
They are used for the synthesis of sucrose and starch
They are oxidized in the glycolysis pathway
all depending on cellular requirements
What happens in the third stage of the Calvin Cycle?
Most of the triose phosphates are converted back to ribulose 1,5-bisphosphate
Fructose 2,6-bisphosphate is a compound with similar structure to the glycolysis/gluconeogenesis substrate fructose 1,6-bisphosphate. However, its role is as an allosteric modulator that catalyze a key step in glycolysis/gluconeogenesis, and subsequently sucrose, glycogen biosynthesis. What effect does it have?
It increases the rate of glycolysis and decreases the rate of gluconeogenesis
[Frucotse 2,6 BP] is itself regulated by a number of factors, which in green plants are controlled by the day/night cycle. What happens to the [Fructose 2,6 BP] in these species during the night?
The [Fructose 2,6 BP] increases
In the first bypass step of gluconeogenesis:
Pyruvate is converted to phosphoenoylpyruvate
OAA is produced as an intermediate
Both ATP and HCO3- (CO2) are required
The second bypass step of gluconeogenesis:
converts fructose 1,6 bisphosphate to fructose 6-phosphate
is an exergonic reaction
is catalyzed by fructose 1,6-bisphosphatase
The third bypass step of gluconeogenesis:
Converts glucose 6-phosphate to glucose
is an exergonic step
Which reaction is the key regulator step for the synthesis of fatty acids:
acetyl-CoA + HCO3- + ATP --> malonyl-CoA + ADP
which enzyme catalyzes the key regulatory step for the synthesis of fatty acids:
Even though we refer to NH4+ as the biologically "available" form of nitrogen, the real 'pool' of nitrogen for the synthesis of amino acids, nucleotides and other nitrogen-containing compounds is provided by:
The key regulatory enzyme that fixes NH4+ into the molecule(s) that serve as the 'pool' of nitrogen for biosynthesis reactions is:
Which molecules have an inhibitory effect on this enzyme (glutamine synthetase)
What happens to cholesterol after it has been synthesized?
it is transported to tisses for use in cell membranes as a "fluidity" regulator
it is transported to specific tissues for conversion into steroid hormones
it is converted to bile salts and acids that aid the digestion of dietary lipids.
Approximately how many ATP and NADP does it take to "turn" the Calvin Cycle once, resulting in the production of one triose phosphate (glyceraldehyde 3-phosphate)?
9 ATP, 6 NADPH
The class of enzymes which create unsaturated fatty acids (introduce double bonds) are known as:
mixed function oxidases
The enzyme(s) that catalyzes the nitrogen fixation process require ATP. In this situation, ATP hydrolysis helps promote the fixation process by:
binding to, and causing a conformational change in the enzyme(s), turning it into a powerful reductant
Steroid hormones, derived from cholesterol, have roles in:
the regulation of secondary sex characteristics and reproduction cycles
Photorespiration refers to:
the wasteful consumption of O2 and production of CO2 by plants
The chloroplast triose phosphate antiporter exports triose phosphate, such as dihydroxyacetone phosphate (DHAP), from the chloroplast in exchange for something. What is it?
Prior to the "condensation" reaction, how are the substrates for fatty acid synthesis joined to the functional groups on Fatty Acid Synthase?
Through a Thioester bond
What is the purpose of gluconeogenesis?
to convert compounds such as pyruvate to glucose
"C4" plants, which include many species of tropical origin (e.g. maize) have a molecular strategy to avoid the metabolic losses associated with photorespiration. Briefly stated, these include:
fixing CO2 into Oxaloacetate/malate, using an enzyme with little affinity for O2
transporting the "fixed" CO2 to tissues below the surface, where O2 is not as prevalent
maintaining the normal "C3" CO2 fixing "Calvin Cycle" enzymes in tissues where compe... O2 is not as prevalent
Glycolysis and Gluconeogenesis
Have 7 of 10 reactions in common
occur in most, if not all, known species
are responsive to hormonal regulation
Many of the intermediate steps of cholesterol synthesis involve structures that belong to which family of compounds?
Which of the following correctly describes the substrates involved in triacylglycerol (TAG) biosynthesis?`
(3) fatty acyl-CoA + glycerol-phosphate --> --> TAG
Which of the following compounds can serve as precursors for the synthesis of glucose in animals?* double check answer
pyruvate & oxaloacetate
Cholesterol, as well as other lipids, are exported from their point of synthesis to the other tissues in lipid/protein complexes known as lipoproteins. What role(s) does the protein component of these complexes have?
activation of extracellular lipases, which converts TAG to fatty acids
interaction with specific cellular receptors, to target the lipids to specific tissues (adipose, muscle, etc.)
interaction with proteins on other lipoprotein complexes
What role does citrate play in lipid biosynthesis?
it transports the carbon to be used in fatty acid biosynthesis
it allosterically regulates the primary regulatory enzyme for fatty acid biosynthesis
some pathways, such as glycolysis and gluconeogenesis, catalyze similar sets of reactions but proceed in opposite metabolic "directions." What characteristics distinguish these similar, yet opposing pathways?
The pathways will often, but not always, take place in different cellular compartments (e.g. cytosol vs. mitochondria)
the different pathways will be regulated in a reciprocal manner (i.e if one is active, the other will be inactive)
Each pathway will contain at least one highly exergonic reaction, which is catalyzed by a unique enzyme
The process of nitrogen fixation is catalyzed by bacteria which possess:
the nitrogenase complex
The biological nitrogen cycle involves the fixation of atmospheric nitrogen into biologically "available" forms such as NH4+. Which equation correctly repressents this conversion by various bacteria (do not be overly concerned about stoichiometry)?
N2 + 16 ATP + 8 electrons + 10H+ <--> 2 NH4+ + 16 ADP + 16 Pi + H2
The substrate for the addition of 2C Units by fatty acid synthase during each round of fatty acid elongation is:
Where does the NADPH for fatty acid synthesis in mammals come from?
pentose phosphate pathway
Where do the reactions of "simple" (C16) fatty acid biosynthesis occur?
Cholesterol is synthesized from
How in general, do anabolic pathways differ from catabolic pathways
anabolic pathways generally consume both ATP and NADPH
The net result of the Calvin Cycle is
That 3 CO2's and one Pi are converted to a triose phosphate, using several ATP and NADPH
Why are highly density lipoprotein particles (HDL's) considered to be the "good" cholesterol
They contain a relatively low proportion of cholesterol, at least initially
They remove excess cholesterol from tissues and other lipoprotein particles
They transport excess cholesterol back to the liver for storage or conversion to other products
Rubisco is a key enzyme of the Calvin Cycle. It is activated indirectly through the action of light. how is this accomplished?
light-induced proton pumping (photophosphorylation increases the pH of the Stroma. (?) High pH creates the environment necessary for the activation of the enzyme.
_________ drives the synthesis of the 5-, 10-,15-, and 30-carbon intermediates of cholesterol synthesis is:
The production of pyrophosphate (PPi) as one of the products, which is very quickly converted to 2 Pi's
The ATP necessary for reactions in the Calvin Cycle and for glucose, sucrose, and starch synthesis in photosynthetic organisms come primarily from:
Which molecule supplies the carbon for the synthesis of fatty acids?
What is the primary regulatory step in the synthesis of cholesterol?
the conversion of HMG-CoA (6C) to Mevalonate (6C)
Rubisco is a key enzyme in the Calvin Cycle. What reaction does it Catalyze?
The incorporation of CO2 into an organic molecule
The enzyme(s) that catalyze nitrogen fixation are very sensitive to oxygen, which comprises approximately 20% of the atmosphere. How have N-fixing microorganisms adapted to overcome this problem?
They colonize plant root tissues, which provide them with leghemoglobin, an oxygen rich protein
They "burn off" oxygen by reducing it to H2O in their own electron transport process.