Metabolism and Energy Storage Systems

Flashcards covering energy storage systems, glycogen metabolism, fatty acid synthesis, glucose homeostasis, and muscle energetics during exercise.

Why does the human body need energy storage systems?

Because we cannot eat constantly and still need $ATP$ between meals.

What are the two major forms of energy storage in humans?

Glycogen (short-term) and fat/TAGs (long-term).

What is the body’s major long-term energy storage molecule?

Triacylglycerols (TAGs).

Where are TAGs mainly stored in the human body?

In adipocytes (fat cells).

What makes fat an effective long-term energy storage molecule?

It is extremely energy dense and stored without water.

Approximately how much energy does fat provide per gram?

About $38\,kJ/g$.

How much energy do carbohydrates and proteins provide per gram?

About $17\,kJ/g$ each.

Why does fat store more energy per gram than glycogen?

Fat is stored in an anhydrous form without water.

Compared to hydrated glycogen, how much more energy does $1\,g$ of fat store?

More than $6$ times as much energy.

What is the primary function of white adipose tissue?

Energy storage.

What is the primary function of brown adipose tissue?

Heat production.

How does brown fat generate heat?

By uncoupling the electron transport chain from $ATP$ synthesis.

What are the two components of a Triacylglycerol (TAG)?

Glycerol and fatty acids.

Where does fatty acid synthesis primarily occur?

In the cytosol of liver cells.

What is the starting molecule for fatty acid synthesis?

Acetyl-CoA.

Into what molecule is acetyl-CoA converted during the synthesis of fatty acids?

Malonyl-CoA.

How many carbons are contained in an acetyl-CoA molecule?

$2$ carbons.

How many carbons are contained in a malonyl-CoA molecule?

$3$ carbons.

Which enzyme is responsible for converting acetyl-CoA into malonyl-CoA?

Acetyl-CoA carboxylase (ACC).

Does the conversion of acetyl-CoA to malonyl-CoA require energy?

Yes, it requires $ATP$.

What is the primary reducing molecule used in fatty acid synthesis?

NADPH.

What is the role of fatty acid synthase?

It builds long-chain fatty acids.

Why are fats unable to travel freely in the blood?

Because fats are hydrophobic and do not dissolve well in water.

What molecule transports TAGs from the liver to adipose tissue?

Very low-density lipoproteins (VLDLs).

What does the acronym VLDL stand for?

Very low-density lipoprotein.

What are the four components of a VLDL?

TAGs, cholesterol, phospholipids, and apoproteins.

What is the definition of glycogen?

A highly branched polymer of glucose.

Approximately how many glucose units are found in one glycogen molecule?

Around $50,000$ glucose units.

Why is glycogen designed with a highly branched structure?

To allow rapid glucose release from many branch ends.

What is the primary purpose of glycogen stored in the liver?

Global energy storage and maintaining blood glucose.

What is the primary purpose of muscle glycogen?

Local energy storage for muscle activity.

What is the term for the process of glycogen synthesis and storage?

Glycogenesis.

What is the term for the process of glycogen breakdown?

Glycogenolysis.

Which enzyme converts glucose into glucose-6-phosphate?

Hexokinase or glucokinase.

Which enzyme is responsible for converting glucose-6-phosphate to glucose-1-phosphate?

Phosphoglucomutase.

Which enzyme adds glucose units onto a glycogen chain?

Glycogen synthase.

Which enzyme is responsible for creating glycogen branches?

Glycogen branching enzyme.

What hormone stimulates the synthesis of glycogen?

Insulin.

Why does insulin stimulate the synthesis of glycogen?

Because excess glucose after eating should be stored.

Which enzyme is responsible for the breakdown of glycogen?

Glycogen phosphorylase.

Which enzyme removes glycogen branches during the degradation process?

Glycogen debranching enzyme.

Besides glycogen phosphorylase and debranching enzyme, what other enzyme is involved in glycogen degradation?

Acid alpha-glucosidase.

Which hormone stimulates the breakdown of glycogen?

Glucagon.

Why does glucagon stimulate the breakdown of glycogen?

To raise blood glucose during fasting.

Which molecule signals low cellular energy to stimulate glycogen breakdown?

AMP.

Which ion stimulates the breakdown of glycogen during muscle contraction?

$Ca^{2+}$.

Which molecule inhibits glycogen breakdown when energy is abundant?

$ATP$.

Which glucose metabolite acts as an inhibitor for glycogen breakdown?

Glucose-6-phosphate.

Theoretically, how much $ATP$ can one glucose molecule produce according to Metabolism Lecture 4?

About $38\,ATP$.

Which hormone is released by adipocytes to signal fullness?

Leptin.

In brown adipose tissue, what protein provides a shortcut for hydrogen flow to release energy as heat?

UCP1 protein.

What are apolipoproteins and what is their function?

Specialized proteins on the surface of particles that act as structural "glue" and chemical keys for cell recognition.

What are the two major potential fates of Glucose-6-phosphate (G6P)?

It can be broken down through glycolysis to make pyruvate or used to make glycogen.

What is the specific function of glycogen synthase?

It is the enzyme that builds glycogen chains.

How do glycogen synthase regulators ($ATP$, $AMP$, $Ca^{2+}$, and $G6-P$) influence glycogen storage?

High energy ($ATP$) and high $G6P$ stimulate building glycogen; low energy ($AMP$) and muscle contraction ($Ca^{2+}$) stimulate breakdown.

What is the fate of glucose transported by GLUT1?

It is immediately used for $ATP$ (basal survival energy).

What are the functions of GLUT5 in the intestine and sperm?

In the intestine, it absorbs fructose into the blood; in sperm, it uses fructose for $ATP$.

What are the roles of GLUT2 in the liver and beta-cells?

In the liver, it stores glucose as glycogen or uses it for $ATP$; in beta-cells, it senses glucose to trigger insulin.

What is the destination and purpose of glucose transported via GLUT3?

It goes to the brain for $ATP$ production only (no storage).

What is the role of GLUT4 in the muscles?

It transports glucose to the muscle for $ATP$ and glycogen storage.

In the context of integrated metabolism, what is the definition of coordinated metabolic activity?

The coordination of organs and hormones to maintain blood glucose homeostasis.

How is blood glucose homeostasis defined?

Keeping blood glucose levels within a safe range.

What are the dangers associated with low blood glucose?

It can cause brain dysfunction and coma.

What are the dangers associated with chronically high blood glucose?

It damages tissues and contributes to diabetes complications.

Which organ is considered the major consumer of fuel in the body?

The brain.

Which organ is considered the major provider of fuel for the body?

The liver.

Why are red blood cells limited only to glycolysis for energy production?

They have no mitochondria, so they cannot perform the Krebs cycle, ETC, or beta oxidation.

Which three tissue/cell types mainly use ketone bodies during prolonged fasting?

Brain, muscle, and heart cells.

Which glucose transporter is described by the phrase "the door is always open"?

GLUT1.

What is the primary function of GLUT1?

Basal glucose uptake into cells.

Where is GLUT1 primarily found in the body?

Most cells, red blood cells, and the blood-brain barrier.

Which glucose transporter is specifically designated as a glucose sensor?

GLUT2.

In which cells is GLUT2 mainly located?

Liver cells and pancreatic beta cells.

Why is GLUT2 referred to as a glucose sensor?

Because the more glucose there is in the blood, the more glucose enters the cell.

Which glucose transporter is categorized as insulin-dependent?

GLUT4.

Where is GLUT4 mainly found?

Skeletal muscle, adipose tissue, and heart muscle.

What is the status of GLUT4 inside the cell when insulin is absent?

GLUT4 mostly stays inside the cell.

What occurs to GLUT4 when insulin is present?

GLUT4 moves to the cell membrane and allows glucose into the cell.

What does the "lock and key" analogy represent regarding GLUT4?

Insulin is the "key" that triggers GLUT4 to move to the membrane and "open the door" for glucose entry.

Which transporter is responsible for the movement of fructose?

GLUT5.

By what title is insulin known in regards to metabolic states?

The fed-state/storage hormone.

By what title is glucagon known in regards to metabolic states?

The fasting/release hormone.

What generally happens to blood glucose levels immediately after eating?

Blood glucose rises.

Which specific pancreatic cells are responsible for releasing insulin?

Beta cells.

What signal does insulin send to the body?

"We have energy available — store it."

What signal does glucagon send to the body?

"We need energy — release stored fuels."

What are the major metabolic effects of insulin?

Increased glucose uptake, glycogen synthesis, and fat storage; decreased fat breakdown and glucose production.

What are the major metabolic effects of glucagon?

Increased glycogen breakdown, lipolysis, and gluconeogenesis.

Define glycogenolysis in the context of glucose homeostasis.

The breakdown of glycogen to release glucose.

What is the definition of lipolysis?

The breakdown of triglycerides into fatty acids and glycerol.

What is the definition of gluconeogenesis?

The production of NEW glucose from non-carbohydrate molecules.

What three molecules can be used as substrates for gluconeogenesis?

Lactate, glucogenic amino acids, and glycerol.

Why can fatty acids not be utilized to create new glucose?

Fatty acids become acetyl-CoA, which cannot be converted back into glucose.

What metabolic state is indicated by a HIGH insulin to glucagon ratio?

The fed state.

Which processes dominate metabolic activity during the fed state?

Glycogen synthesis, fat synthesis, protein synthesis, and glucose uptake.

What metabolic state is indicated by a LOW insulin to glucagon ratio?

The fasted state.

Which processes dominate metabolic activity during the fasted state?

Glycogen breakdown, gluconeogenesis, lipolysis, and ketogenesis.

In the first step of insulin release, how does glucose enter the beta cell?

Through the GLUT2 transporter.

In step 2 of insulin release, what happens after glucose enters the beta cell?

It undergoes glycolysis, the Krebs cycle, and ETC, causing $ATP$ levels to rise.

In step 3 of insulin release, what is the effect of increased $ATP$?

It closes $ATP$-sensitive potassium ($K^{+}$) channels.