Metabolism Flashcards
Without exaggeration, you could spend your entire life studying metabolism. We know you'd prefer not to do that, so we've created these 120 cards to dig deep into topics like glycolysis and the Krebs cycle when necessary while not obsessing over irrelevant details.
Define:
metabolism
It collectively refers to the biological processes that occur within cells.
Specifically, these processes either generate energy through the breakdown of molecules or use energy to build molecules.
Metabolism is also known as “cellular respiration.”
Define:
catabolism
It is the biological breakdown of molecules into smaller units.
Catabolic processes are accompanied by the generation of energy.
The opposite of this class of metabolic reactions is anabolism.
Define:
anabolism
It is the creation of larger biomolecules from smaller units.
Anabolic processes require energy input.
The opposite of this class of metabolic reactions is catabolism.
What broad distinction separates aerobic and anaerobic respiration?
- Aerobic respiration requires oxygen
- Anaerobic respiration occurs in the absence of oxygen.
As part of metabolism, both processes involve the breakdown of biological molecules and the eventual release of energy.
What is the chemical formula of glucose?
C6H12O6
Broadly, glucose is a carbohydrate; specifically, it is a monosaccharide.
The majority of glucose molecules enter the cell via which transport method?
facilitated diffusion
This occurs with the assistance of a family of transport proteins, GLUT 1-4.
This process is promoted by high plasma glucose levels, which creates a concentration gradient that drives glucose into the cells. It is also promoted by the activity of insulin, which increases the number of GLUT 4 transporters on the membranes of certain cell types.
In eukaryotes, which metabolic process occurs in the cytosol regardless of the presence or absence of oxygen?
Glycolysis occurs anaerobically in the cytosol.
Glycolysis is a biochemical process that forms pyruvate from the breakdown of glucose. The pathway produces 2 NADH and a net total of 2 ATP per glucose molecule.
Fill in the blank.
In prokaryotes, glycolysis occurs in the _______.
cytosol
Glycolysis occurs in the cytoplasm for all cells, whether prokaryotic or eukaryotic. (Even if you didn’t know this, you should absolutely know that prokaryotic cells lack membrane-bound organelles, meaning that metabolic processes generally occur in the cytoplasm.)
Describe the net reaction of glycolysis.
C6H12O6 + 2 NAD+ + 2 Pi + 2 ADP → 2 pyruvate + 2 ATP + 2 NADH + 2 H2O + 2 H+
While glycolysis only produces a net of two ATP molecules, it generates a total of four. The other two molecules of ATP are used as reactants in early glycolytic steps.
Fill in the blank.
Glucose contains ________ carbons, while pyruvate contains ________ carbons.
six, three
As such, complete glycolysis of a single glucose molecules produces two pyruvate molecules for use in later metabolic processes.
In the net reaction of glycolysis, for every pyruvate molecule that is produced, how many ADP molecules are consumed?
one
You should know the net reaction of glycolysis inside and out! When one glucose molecule undergoes glycolysis, it consumes a net of two ADP molecules and produces two pyruvate molecules. Thus, the ratio of pyruvate to ADP is 1:1.
If six glucose molecules underwent complete glycolysis, how many molecules of NADH would be produced?
12
Per the net reaction of glycolysis, for every glucose molecule consumed, two molecules of NADH are produced. Thus, complete glycolysis of six glucose molecules would produce 12 NADH.
In the complete glycolysis of a single glucose molecule, how many carbon atoms are lost as carbon dioxide?
zero
Glycolysis does not produce carbon dioxide (CO2)! Instead, all of the carbon atoms in glucose end up in pyruvate.
In contrast, other metabolic processes do yield CO2, most notably the Krebs cycle.
Glucose enters the cells through a family of GLUT transporters. What process ensures that glucose does not exit by a similar mechanism?
In the first step of glycolysis, glucose is converted to glucose 6-phosphate (G6P). Since GLUT transporters only facilitate the movement of glucose, G6P cannot utilize them to leave the cell.
Additionally, since G6P is very negative, it is even more incapable than glucose of exiting directly through the hydrophobic cell membrane.
In the first step of glycolysis, glucose is converted to glucose 6-phosphate (G6P). What enzyme catalyzes this reaction?
hexokinase
Remember, kinases are a class of enzymes that phosphorylate other molecules. You can use the name “hexokinase” to predict this molecule’s role: it is a kinase that phosphorylates hexoses.
Which reaction is the rate-limiting step of glycolysis?
Step 3 involves the phosphorylation of fructose 6-phosphate to form fructose 1,6-bisphosphate.
This reaction is catalyzed by the enzyme phosphofructokinase-1 (PFK-1). For this reason, PFK-1 is sometimes called the rate-limiting enzyme of glycolysis.
Step 2 of glycolysis involves the conversion of glucose 6-phosphate to fructose 6-phosphate. The enzyme that catalyzes this reaction falls into which of the six main classes of enzymes?
isomerase
You should know that glucose and fructose are isomers, so glucose 6-phosphate and fructose 6-phosphate are isomers as well! Isomers are interconverted by isomerase enzymes. Specifically, the enzyme that catalyzes this reaction is phosphoglucose isomerase.
The enzyme that catalyzes step 10 of glycolysis falls into which of the six main classes of enzymes?
transferase
Specifically, this enzyme is pyruvate kinase (a kinase is a type of transferase). Pyruvate kinase catalyzes the conversion of phosphoenolpyruvate into pyruvate, a reaction that also produces ATP from ADP.
In which step of glycolysis does the six-carbon substrate split into two three-carbon molecules, and which enzyme catalyzes this step?
Step 4 of glycolysis, which is catalyzed by aldolase.
From this point onward, you should imagine twice the substrate molecules at play (since one glucose substrate has been transformed into two 3-carbon substrates).
Name the two 3-carbon molecules that are produced in step 4 of glycolysis.
- Glyceraldehyde 3-phosphate
- Dihydroxyacetone phosphate
The production of these molecules is catalyzed by the enzyme aldolase.
By step number (for example, “step 1”), which steps of glycolysis are irreversible?
Steps 1, 3, and 10
This means that these steps are so highly exergonic that they cannot effectively be reversed. The remaining seven steps of glycolysis are reversible.
Which glycolytic enzymes catalyze irreversible steps?
Hint: There are three of them.
- Hexokinase (or glucokinase)
- Phosphofructokinase-1 (PFK-1)
- Pyruvate kinase
Hexokinase and glucokinase catalyze step 1 of glycolysis. PFK-1 catalyzes step 3, and pyruvate kinase catalyzes the final step (step 10).
True or false.
The seven reversible steps of glycolysis are reversible because they are highly endergonic.
False
While irreversible steps are irreversible because they are highly exergonic, reversible steps are not highly endergonic (nonspontaneous). Instead, they tend to have ΔG values that are close to 0.
Which step(s) of glycolysis require(s) the input of ATP?
Steps 1 and 3
Predictably, both of these steps involve the phosphorylation of the substrate. In step 1, glucose is converted into glucose 6-phosphate, while in step 3, fructose 6-phosphate is converted into fructose 1,6-bisphosphate.