storage mechanisms and control in carbohydrate metabolism

Question 1

what happens when we consume a meal high in carbohydrates

Answer 1

body converts the excess glucose into glycogen for storage
primarily in the liver and muscles

glycogen serves as a highly efficient energy reserve, capable of being mobilized rapidly when the body needs glucose

Question 2

it is a branched polymer of glucose

Answer 2

glycogen

Question 3

what is the average chain length of the branches in glucose

Answer 3

13 glucose residues

it is important for its ability to
- store [ long-lasting source of glucose for activities ranging from short bursts of exercise to extended fasting]
- deliver energy quickly [enables rapid glycogenolysis (breakdown of glycogen into glucose)]

= maintaining homeostasis

Question 4

how does starch and glycogen store glucose for energy in plants and animals

Answer 4

Starch (Used by plants):
Amylose: A straight chain of glucose molecules (simple and unbranched).
Amylopectin: A chain with fewer branches compared to glycogen.
Plants store glucose as starch in seeds and roots to use as energy later.

Glycogen (Used by animals):
Highly branched structure: This allows glycogen to pack tightly and be broken down quickly when energy is needed.
Animals store glycogen in their liver (to regulate blood sugar) and muscles (for quick energy during movement).

The Difference:
Branching: Glycogen is much more branched than starch (amylopectin). This is what makes glycogen faster to access when the body needs energy.
Who uses it?
Starch = plants’ energy storage.
Glycogen = animals’ energy storage.
In simple terms: glycogen is like a “fast-access energy bank” for animals, while starch is a slower but steady energy source for plants.

Question 5

briefly explain the glycogen breakdown
- in the liver
- in the muscle

Answer 5

LIVER
* The release of glycogen stored in the liver is triggered by low levels of glucose in blood.
* Liver glycogen breaks down to glucose-6 phosphate, which is hydrolyzed to give glucose which then released into the bloodstream

MUSCLE
* When it needs energy, it converts from glycogen to glucose-6-phosphate
* It uses for muscle movement instead of converting to glucose

Question 6

what are the 2 steps in Glycogenolysis

Answer 6

[1] Phosphorolysis: Formation of Glucose-1-P
- from glycogen to G1P
- enzyme: glycogen phosphorylase (this breaks the a(1-4) linkages) + debranching enzyme —> degrade (1-6) linkages
- phosphorolysis (glucose + P = G1P)

• NO ATP IS HYDROLYZED
• hence, glycogen produce net of 3 ATP instead of 2 bcs the conversion of glucose to G6P is skipped as that takes 1 ATP
  = save, more efficient energy production

[2] Isomerization: Formation of Glucose-6-P
- from G1P to G6P
- enzyme: PGM, phosphoglucomutase
- isomerization (transfer P from 1 to 6. why? so that it can enter the glycolysis)

Question 7

function of debranching enzyme

Answer 7

1. branch transfer
   - take a small branch of 3 glucose molecules and moves to the end of another main branch
2. bond breaking
   - breaks a(1-6) bond at the branch point

Question 8

what are the 2 types of glycosidic bond

Answer 8

a(1-4) bonds:
form long linear chains of glucose units

a(1-6) bonds:
create branch points where side chains connect to the main chains

Question 9

what is the net gain of ATP molecules when when glycogen rather than glucose is the starting
material for glycolysis

Answer 9

3 rather than 2 because

because the hexokinase step, which consumes ATP is removed

Question 10

briefly explain the glycogen production

Answer 10

Energy is Needed: Making glycogen requires energy, which comes from a molecule called UTP (similar to ATP).

Stage 1 – Preparing the Building Block:

G1P reacts with UTP to form another molecule called UDP-glucose with a by-product of
pyrophosphate (PPi)
enzyme: UDP-glucose pyrophosphorylase.

step 2:

UDP-glucose (UDPG) added to the growing glycogen chain
enzyme: glycogen synthase
it connects the glucose from UDPG to the chain.
creates a new bond called an α(1 → 4) glycosidic bond, which links glucose molecules in a straight chain.

*Glycogen synthase can only add glucose to an existing chain. It can’t start a new chain from scratch.

another enzyme: branching enzyme
takes a small segment (7 glucose units) from the end of the growing glycogen chain.
moves this segment to a different part of the chain and creates a branch point by forming a new bond called an α(1 → 6) glycosidic linkage.

In Short:
Glycogen synthase adds glucose units one by one in a straight line using α(1 → 4) bonds.
The branching enzyme creates branches by forming α(1 → 6) bonds, making the glycogen structure more efficient for energy storage and release.

Question 11

what is the energy release in glycogen production

Answer 11

G1P reacts with UTP = UDPG + PPi
- energy change (ΔG°’) is close to zero because this reaction can easily go forward or backward.

PPi breaks down into 2 phosphate molecules (2Pi) with the help of water
- releases energy (ΔG°’ = -30.5 kJ/mol or -7.3 kcal/mol). it ensures the reaction moves forward by removing PPi

Overall Charge:
Glucose-1-phosphate + UTP → UDPG + 2Pi
ΔG°’ = -30.5 kJ/mol (-7.3 kcal/mol).

The energy released makes the reaction irreversible, which helps the body efficiently prepare UDP-glucose for glycogen synthesis without reversing the process.

Question 12

what are the forms of glycogen phosphorylase

Answer 12

Phosphorylase a (active): This form is phosphorylated (a phosphate group is added), making it active and able to break down glycogen.

Phosphorylase b (inactive): This form is dephosphorylated (no phosphate group), so it’s less active.

Question 13

Phosphorylation (Activation):
Dephosphorylation (Deactivation):

Answer 13

Phosphorylation (Activation):
An enzyme called phosphorylase kinase adds a phosphate group to glycogen phosphorylase (converts it to the active “a” form).
This process uses energy from ATP.

Dephosphorylation (Deactivation):
Another enzyme, phosphoprotein phosphatase, removes the phosphate group (converts it back to the inactive “b” form).

Question 14

R and T States

Answer 14

R form (Relaxed and Active):
When glycogen phosphorylase is active, it’s in the R form, ready to break down glycogen.
Molecules like AMP (a signal of low energy) promote this state.

T form (Tense and Inactive):
When inactive, it’s in the T form. Molecules like ATP or glucose-6-phosphate (G6P) (indicators of enough energy) stabilize this state.

Question 15

what controls the Glycogen phosphorylase

Answer 15

Allosteric control: Molecules like AMP (activates) or ATP/G6P (inhibits) regulate its activity.

Covalent modification: Phosphorylation (activation) and dephosphorylation (deactivation) control its form.

This regulation ensures glycogen is only broken down when the body needs energy.

_______________________________________________________

Covalent Modification:
Enzymes involved in glycogen breakdown or synthesis can be activated or deactivated by adding or removing phosphate groups. This is influenced by hormones like glucagon or epinephrine.

Allosteric Control:
Molecules like AMP, ATP, glucose, and glucose-6-phosphate act as signals to tell the enzymes when to start or stop glycogen metabolism. For example:
AMP signals low energy and promotes glycogen breakdown.
ATP or glucose-6-phosphate signals enough energy and slows glycogen breakdown.

Question 16

what is the allosteric control of glycogen phosphorylase in the liver and muscles

Answer 16

1. In the Liver
   Key Player: Glucose
   Glucose acts as an inhibitor for the active form of glycogen phosphorylase (phosphorylase a).
   When glucose binds to the enzyme, it makes the enzyme shift to the inactive T state.
   This helps stop glycogen breakdown because the liver senses there’s already enough glucose in the blood.

This explanation focuses on allosteric control (how molecules regulate enzyme activity) of glycogen phosphorylase in the liver and muscles.

1. In the Liver
   Key Player: Glucose
   Glucose acts as an inhibitor for the active form of glycogen phosphorylase (phosphorylase a).
   When glucose binds to the enzyme, it makes the enzyme shift to the inactive T state.
   This helps stop glycogen breakdown because the liver senses there’s already enough glucose in the blood.
2. In the Muscles
   Key Players: ATP, AMP, and G6P
   These molecules regulate glycogen phosphorylase activity based on the muscle’s energy needs:

Low ATP, High AMP (low energy):
High AMP signals low energy in the muscle.
AMP activates glycogen phosphorylase b by shifting it to the active R form to break down glycogen and provide energy.

High ATP, Low AMP (high energy):
High ATP means the muscle has enough energy.
ATP stabilizes the inactive T form, reducing glycogen breakdown.

High Glucose-6-phosphate (G6P):
High G6P indicates plenty of stored energy.
G6P also stabilizes the inactive T form, further preventing glycogen breakdown.

Question 17

Opposite Behavior to Glycogen Phosphorylase

Answer 17

Glycogen phosphorylase (breaks down glycogen): Active when phosphorylated.

Glycogen synthase (builds glycogen): Active when unphosphorylated.
- When phosphorylated, glycogen synthase becomes inactive, stopping glycogen production.

Question 18

How Phosphorylation Happens

Answer 18

Hormonal Signals (Glucagon or Epinephrine):
- These hormones signal the body that more energy is needed (e.g., during fasting or stress).
- The signal triggers an enzyme called cAMP-dependent protein kinase, which adds phosphate groups to glycogen synthase.
- This phosphorylation inactivates glycogen synthase, halting glycogen synthesis.

At the same time:
- The hormonal signal activates glycogen phosphorylase (to break down glycogen).
- It inactivates glycogen synthase (to stop making glycogen).

This ensures the body doesn’t simultaneously break down and build glycogen, optimizing energy use.

In Simple Terms
Glycogen synthase builds glycogen but is turned off by phosphorylation when hormones like glucagon or epinephrine signal the need for energy.
This allows the body to prioritize breaking down glycogen for energy instead of storing it.

Question 19

What is Glycogen?

Answer 19

store glucose for later use

Question 20

Why is Glycogen Important?

Answer 20

When your body needs energy (e.g., during exercise or fasting), glycogen is broken down to release glucose.

When you don’t need energy (e.g., after eating), glucose is stored as glycogen for future use.

Question 21

Process by which pyruvate is converted to glucose

Answer 21

Gluconeogenesis

Question 22

what are the 3 irreversible steps of gluconeogenesis

Answer 22

1. Production of pyruvate and ATP from phosphoenolpyruvate
2. Production of fructose-1,6-bisphosphate from fructose-6-phosphate
3. Production of glucose-6 phosphate from glucose

Question 23

Conversion of Pyruvate to Phosphoenolpyruvate

Answer 23

[Step 1] Pyruvate is carboxylated to oxaloacetate
* Requires energy, which is available from the hydrolysis of ATP
* Catalyzed by pyruvate carboxylase, which is activated by acetyl-CoA
* Reaction requires biotin, which is a CO2 carrier, and Mg2+
- biotin is a carrier f carbon dioxide, it has a specific site for covalent attachment of CO2

[Step 2] Conversion of oxaloacetate to phosphoenolpyruvate
* Catalyzed by phosphoenolpyruvate carboxykinase (PEPCK)
* Involves the hydrolysis of GTP

Question 24

what are the Fates of the Oxaloacetate Formed in the Mitochondria

Answer 24

1. Pathway 1: Stay and Turn into PEP
   Oxaloacetate can be converted directly into phosphoenolpyruvate (PEP) by an enzyme.
   PEP is then transported out of the mitochondria to continue building glucose in the cytosol (outside the mitochondria).
2. Pathway 2: Turn into Malate and Travel
   If oxaloacetate can’t leave the mitochondria directly, it gets converted into malate using an enzyme called mitochondrial malate dehydrogenase. This step also uses NADH (a helper molecule that carries energy).
   Malate acts as a “sneaky traveler” and exits the mitochondria.
   Once malate is outside in the cytosol, it gets converted back into oxaloacetate by cytosolic malate dehydrogenase. This step regenerates NADH, which is needed for other steps in gluconeogenesis.

Why These Options?
The direct PEP route is faster but might not always be possible.
The malate route helps balance NADH levels outside the mitochondria, which is necessary for gluconeogenesis to continue smoothly.

Question 25

F-1,6BP to FGP

Answer 25

-hydrolyze
- enzyme: fructose-1,6-bisphosphatase + NADH

Enzyme Regulation:

Low Energy (Low ATP, High AMP):
When the cell is running low on energy, there’s more AMP and less ATP.
AMP inhibits fructose-1,6-bisphosphatase, so gluconeogenesis slows down.
Instead, the body favors glycolysis (breaking down glucose to produce energy).

High Energy (High ATP, Low AMP):
When the cell has plenty of energy, there’s more ATP and less AMP.
ATP stimulates fructose-1,6-bisphosphatase, encouraging gluconeogenesis to store energy by building glucose.

Question 26

what is the final step of gluconeogenesis

Answer 26

Key Reaction: Glucose-6-phosphate → Glucose
In this step, glucose-6-phosphate (a sugar with a phosphate attached) is broken down to produce:
- Glucose (usable sugar for the body).
- A free phosphate ion.

This reaction releases energy, which helps make it happen efficiently.

It is catalyzed by an enzyme called glucose-6-phosphatase, which removes the phosphate group.

*It happens in the endoplasmic reticulum (a specialized structure inside the cell). After glucose is formed, it is transported out of the endoplasmic reticulum and can then be released into the bloodstream.

Question 27

briefly explain the reciprocal regulation in glucose metabolism

Answer 27

reciprocal regulation to make sure glycolysis (breaking down glucose) and gluconeogenesis (making glucose) don’t happen at full speed at the same time, as this would waste energy

1. Gluconeogenesis:
   Converts fructose-1,6-bisphosphate to fructose-6-phosphate.
   This is done by the enzyme fructose-1,6-bisphosphatase.
   Inhibited by:
   Fructose-2,6-bisphosphate (signals glycolysis is needed).
   AMP (low energy levels mean glucose needs to be broken down, not made).
2. Glycolysis:
   Converts fructose-6-phosphate to fructose-1,6-bisphosphate.
   This is done by the enzyme phosphofructokinase (PFK).
   Activated by:
   Fructose-2,6-bisphosphate and AMP (signal low energy, so glucose breakdown is needed).
   Inhibited by:
   ATP (indicates energy is plentiful).
   Citrate (signals that the cell already has enough building blocks and energy).

Question 28

How Reciprocal Regulation Works:

Answer 28

If energy is low:
AMP and fructose-2,6-bisphosphate activate PFK for glycolysis and inhibit fructose-1,6-bisphosphatase, so glucose is broken down for energy.

If energy is high:
ATP and citrate inhibit PFK, and gluconeogenesis is favored to store glucose for later use.

Question 29

What F2,6BP Does

Answer 29

Stimulates Glycolysis:
It activates phosphofructokinase-1 (PFK-1), the key enzyme in glycolysis, encouraging the breakdown of glucose.

Inhibits Gluconeogenesis:
It blocks fructose-1,6-bisphosphatase (FBPase-1), a key enzyme in gluconeogenesis, slowing down the production of glucose.

Question 30

How F2,6BP Levels Are Controlled

Answer 30

Made by PFK-2:
The enzyme phosphofructokinase-2 (PFK-2) makes F2,6BP from fructose-6-phosphate. This increases F2,6BP levels, favoring glycolysis.

Broken Down by FBPase-2:
The enzyme fructose-2,6-bisphosphatase-2 (FBPase-2) breaks F2,6BP back into fructose-6-phosphate, lowering its levels and favoring gluconeogenesis.

When F2,6BP is High or Low:
High F2,6BP:

Stimulates glycolysis (to generate energy).
Happens when energy is needed.
Low F2,6BP:

Stimulates gluconeogenesis (to make glucose).
Happens when the body needs to maintain blood sugar, like during fasting.

Question 31

how does Phosphorylation Controls the Balance

Answer 31

Phosphorylation = FBPase-2 on, PFK-2 off → Stimulates gluconeogenesis (makes glucose).

Dephosphorylation = PFK-2 on, FBPase-2 off → Stimulates glycolysis (breaks glucose for energy).

In Simple Terms:
Think of this protein as a switchable enzyme tool:

When it has a phosphate group, it breaks down F2,6BP, turning off glycolysis and turning on gluconeogenesis.
When the phosphate is removed, it builds F2,6BP, turning on glycolysis and turning off gluconeogenesis.

Question 32

Glycogen Synthase:
Glycogen Phosphorylase:

Answer 32

Glycogen Synthase:
Builds glycogen (stores glucose for later use).

Glycogen Phosphorylase:
Breaks down glycogen (releases glucose for energy).

These enzymes are regulated in opposite ways to ensure that glycogen is either built or broken down, but not both at the same time.

Question 33

what hormones control the glycogen metabolism through enzymatic cascades

Answer 33

Hormones like insulin and glucagon

1. Breakdown of Glycogen (Triggered by Glucagon or Epinephrine):
   - Hormones activate an enzyme cascade starting with adenylate cyclase, which produces cAMP.
   - cAMP activates protein kinase enzymes, which phosphorylate (activate) glycogen phosphorylase to break down glycogen.
2. Synthesis of Glycogen (Triggered by Insulin):
   - Insulin triggers a cascade that activates glycogen synthase kinase.
   - This phosphorylates glycogen synthase D (inactive form).
   - Another enzyme, phosphoprotein phosphatase, removes the phosphate, converting glycogen synthase to its active form (glycogen synthase I), which builds glycogen.

In Simple Terms:
Glycogen breakdown (energy release): Activated by glucagon or epinephrine when blood sugar is low or energy is needed.
Glycogen synthesis (energy storage): Activated by insulin when blood sugar is high and energy needs are low.

Question 34

Insulin

Answer 34

helps store energy

When is it secreted?
- Insulin is released when blood glucose levels are high (like after eating).

What does it do?
- It signals cells to take in glucose (through a -transporter called GLUT4).
- It triggers a protein kinase cascade inside the cell that activates glycogen synthase, the enzyme that builds glycogen (stores glucose).
- This helps lower blood glucose levels by turning excess glucose into stored glycogen.

Question 35

Glucagon & Epinephrine

Answer 35

helps release energy

When are they secreted?
- These hormones are released when blood glucose levels are low (like between meals or during exercise).

What do they do?
- They bind to receptors on cells and trigger a protein kinase cascade that activates glycogen phosphorylase, the enzyme that breaks down glycogen (releases glucose).
- They also inhibit glycogen synthase, which stops the creation of more glycogen.
- This ensures that glucose is released from glycogen to raise blood glucose levels when energy is needed.

In Simple Terms:
Insulin: After eating, insulin helps cells take in glucose and store it as glycogen, lowering blood glucose.
Glucagon and Epinephrine: When blood sugar is low, these hormones signal the body to break down glycogen and release glucose into the blood for energy.

These hormones use signal cascades (a series of events inside the cell) to amplify their effects and make sure glucose levels are properly regulated!

Question 36

where does these following hormone secretes
INSULIN:
GLUCAGON:
EPINEPHRINE:

Answer 36

INSULIN: beta cells (increase BG lvl)
GLUCAGON: alpha cells (decrease BG lvl)
EPINEPHRINE: alpha cells (BG lvl)

Question 37

DOMS

Answer 37

delayed onsight muscle soreness

Question 38

Control process in which opposing reactions are catalyzed by different enzymes

Answer 38

Substrate Cycling

-Opposing reactions can be independently regulated and can have different rates
- Hydrolysis of ATP is the energetic price paid for independent control of the opposing reactions

[How Substrate Cycling Works:

Opposing Reactions:
In substrate cycling, there are two reactions that essentially do the opposite of each other. For example:
- One reaction converts fructose-6-phosphate (a simple sugar molecule) to fructose-1,6-bisphosphate (another sugar molecule).
- The reverse reaction takes fructose-1,6-bisphosphate and converts it back to fructose-6-phosphate.

Energy Cost:
- These reactions are energy-intensive, meaning they require ATP (energy) to go in one direction and release energy when they go in the opposite direction.
- The net effect of this cycling is a cost of ATP and water as part of the process, but it allows the body to regulate the reactions independently and control their rates separately.]

Question 39

Example: Fructose-6-Phosphate Conversion

Answer 39

First Reaction (Energy Input):

Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP
This requires energy from ATP and is exergonic (releases energy). The change in energy is -25.9 kJ/mol (or -6.2 kcal/mol).

Second Reaction (Energy Release):

Fructose-1,6-bisphosphate + H2O → Fructose-6-phosphate + Pi (inorganic phosphate)
This releases energy and is also exergonic. The change in energy is -8.6 kJ/mol (or -2.1 kcal/mol).

Question 40

Why Use Substrate Cycling?

Answer 40

Independent Regulation:
These two opposing reactions can be independently controlled, allowing the body to adjust the rates at which glucose is made or broken down based on its energy needs.

ATP Hydrolysis:
The cost of this process is that ATP is consumed during one part of the cycle and released during the reverse part, which is the energetic “price” for having this independent control.

In Simple Terms:
Substrate cycling is like a two-way street where glucose molecules can be moved in and out of storage, and the body can fine-tune how fast each process happens, depending on whether more glucose is needed or stored. The catch is that it costs some ATP to keep this balance going!

Question 41

Cori Cycle
1. in the muscle
2. in the bloodstream
3. in the liver
4. back to the muscles

Answer 41

• explains how different parts of the body work together to maintain energy levels when muscles are working hard, especially when there isn’t enough oxygen available (like during intense exercise).

1. In the muscles:
   When the muscles are doing a lot of work (especially during intense activity), they may not get enough oxygen. This is called oxygen debt. As a result, the muscles perform glycolysis, which breaks down glucose into pyruvate, but since there’s not enough oxygen, pyruvate is converted into lactate (also known as lactic acid).
2. In the bloodstream: The lactate produced in the muscles is released into the bloodstream and transported to the liver.
3. In the liver: The liver uses a process called gluconeogenesis to convert lactate back into glucose. This process first converts lactate into pyruvate (which can be used to make glucose).
4. Back to the muscles: The new glucose produced in the liver is sent back to the muscles through the bloodstream, where it can be used again for energy.

Question 42

why is the Cori Cycle important

Answer 42

helps the body keep producing energy, even when muscles are working hard and oxygen is low, by recycling lactate into glucose. This cycle is especially important during high-intensity exercise, where muscles need energy quickly but can’t rely on oxygen alone.

Question 43

Pentose Phosphate Pathway

Answer 43

helps the body produce two important things:

1. Five-carbon sugars (like ribose), which are used to make DNA and RNA.
2. NADPH, a molecule that helps with biosynthesis (building new molecules like fatty acids and cholesterol) and also protects cells from damage.

Question 44

briefly explain the 2 main types of reactions of the Pentose Phosphate Pathway

Answer 44

Oxidative Reactions (First Part)

1. Glucose-6-phosphate to 6-phosphogluconate:
   - Enzyme: Glucose-6-phosphate dehydrogenase
   - What happens: Glucose-6-phosphate (a form of glucose) gets oxidized (loses electrons), which produces NADPH (a molecule that helps in building things like fatty acids and protecting cells) and transforms glucose-6-phosphate into 6-phosphogluconate.
2. 6-Phosphogluconate to Ribulose-5-phosphate:
   - Enzyme: 6-phosphogluconate dehydrogenase
   - What happens: 6-phosphogluconate loses a carbon dioxide (CO2) molecule, and the remaining structure gets converted into ribulose-5-phosphate, which is a five-carbon sugar important for making DNA and RNA.

Nonoxidative Reactions (Second Part)

In the second part of the pathway, ribulose-5-phosphate is rearranged to form different sugars that can be used in other processes, like glycolysis (energy production).

1. Isomerization Reactions:
   - First Reaction: Ribulose-5-phosphate gets converted into xylulose-5-phosphate. This is done by an enzyme called phosphopentose-3-epimerase, which changes the arrangement of atoms in the sugar.
   - Second Reaction: Ribulose-5-phosphate is converted into ribose-5-phosphate, an important sugar for making nucleotides (DNA/RNA), by phosphopentose isomerase.
2. Group-Transfer Reactions:
   - These reactions rearrange the sugars and link the pentose phosphate pathway with glycolysis (the process that breaks down glucose for energy).
   - Transketolase transfers a two-carbon unit, and transaldolase transfers a three-carbon unit, helping to rearrange sugars like xylulose-5-phosphate and ribose-5-phosphate.
   - The result is the formation of fructose-6-phosphate and glyceraldehyde-3-phosphate, which can enter glycolysis to produce energy.
3. Final Step:
   - Transketolase helps xylulose-5-phosphate react with erythrose-4-phosphate to produce fructose-6-phosphate and glyceraldehyde-3-phosphate, which are also important for energy production in glycolysis.

In Simple Terms:
The Pentose Phosphate Pathway helps make key molecules for cell function, like NADPH (for building things) and ribose (for DNA/RNA). It also rearranges sugars to feed into energy production pathways (like glycolysis).

Question 45

Pentose Phosphate Pathway: Regulation

Answer 45

regulated based on the cell’s needs, and it can adjust which products it makes depending on what’s needed more—NADPH (used in building and protection) or ribose-5-phosphate (used for making DNA and RNA).

1. If the cell needs more NADPH than ribose-5-phosphate (for processes like protecting cells or building molecules), the pathway will make more NADPH, even if it means not making as much ribose-5-phosphate.
2. If the cell needs more ribose-5-phosphate (for DNA/RNA building) than NADPH, the pathway can switch gears. The non-oxidative reactions of the pathway can run in reverse to produce more ribose-5-phosphate.

This flexibility in the pathway helps the cell meet its changing needs for different molecules. The enzymes that carry out these steps (like transketolase and transaldolase) are reversible, so they can shift the direction of the reactions as needed.