How do animals store glucose?
Animals store glucose in the form of glycogen. Glycogen is stored in the liver and skeletal muscle cells.
Glucose gets converted into Glycogen during the process called Glycogenesis. During Glycogenesis the glucose molecules are added to chains of Glycogen for storage in the body depending on the future demand for glucose availability and ATP energy requirement.
Glycogen is mostly available in the liver cells as compared to that of the skeletal muscle cells.
Glucose is a monosaccharide while Glycogen is a polysaccharide. Meaning that glucose is a simple sugar molecule whereas, glycogen is a polymer made of many glucose molecules joined together.
Both glucose and glycogen are carbohydrates, and the difference is that glucose is a monomer while glycogen is a polymer.
The glucose along with amino acids and fats is the main energy source of the animal body and this is also what that fuels the brain.
When the body doesn’t need to use the excess glucose for energy, it stores it in the liver and muscles.
The animal body also has a biochemical mechanism to store that glucose in the form of glycogen as a future reservoir of energy.
Muscle glycogen is converted into glucose by muscle cells and liver glycogen gets converted to glucose for use throughout the body including the Central Nervous System (CNF).
Do animals make their own glucose?
The glucose is the most important and abundant source of instant energy in any biological body.
In animals that glucose is stored as glycogen, while in plants that glucose is stored in the form of starch as a carbohydrate reserve.
Yes, animals can make their own glucose but in order to do so it needs to feed on lactose, sucrose, maltose, starch like carbohydrates containing food.
But, if the animal body doesn’t get any kind of carbohydrate then it can’t produce glucose of its own.
Now, if the animal body gets glucose directly from the food then it’s all okay. But, if the body takes the food rich in other disaccharides or polysaccharides then the body will synthesize glucose of its own from the disaccharides and polysaccharides.
So, the animal body needs to get carbohydrates from somewhere. So, it gets carbohydrates in the form of glucose or other disaccharides or polysaccharides by eating food like fish, meat, bread, rice, potatoes, vegetables, fruit, sugar, yogurt, and milk.
So, the main source of glucose is the food that the animal eats. In general, 300 g of glucose is produced per day by the conversion of food.
If the animals have eaten food with disaccharides like fructose and galactose, then these molecules will travel to the liver, where they can be converted to glucose.
The breakdown of glucose-containing polysaccharides like starch, cellulose, etc. can also happen in the animal’s mouth during chewing of the food by means of amylase which is contained in saliva, and also by means of maltase, lactase, and sucrase enzymes secreted in the small intestine.
Now, it is also to be noted that, glucose in very small amounts can also be synthesized from other metabolites in the body’s cells. So, the body can also make glucose of its own due to the metabolites of other biochemical reactions going inside the complex body.
Just for instance, if you see, glucose may also be produced from non-carbohydrate precursors, such as pyruvate, amino acids, and glycerol.
What actually is Glycogen in animals?
Glycogen is a polysaccharide carbohydrate reserve in animals that acts as future reserve of energy. It is often referred to as the animal starch.
It is present in high concentration in the liver, followed by the muscles, brain, etc. The liver cells and skeletal muscle cells are the sites of glycogen storage while the brain uses glucose from glycogen as fuel.
The structure of glycogen is similar to that of amylopectin with more number of branches. Both glycogen and amylopectin are the polymers of glucose.
Glucose is actually the repeating units in Glycogen joined together by α(1→4) glycosidic bonds, and α(1→6) glycosidic bonds at branching points.
Glucose is a simple sugar with the molecular formula C6H12O6. While Glycogen is a complex and long sugar polymer of glucose with molecular formula (C6H10O5)n where ‘n’ is the number of glucose molecules in the glycogen polymer.
Glycogen is a polymer of glucose with up to 120,000 glucose residues or in a minimum of 2,000-60,000 glucose residues per one molecule of glycogen.
The Glycogen is composed of units of glucose linked by α(1→4) with branches occurring α(1→6) approximately after every 8 to 12 residues.
Glucose is an osmotic molecule and can have profound effects on osmotic pressure in high concentrations possibly leading to cell damage or cell death if stored in the cell without being modified.
Glycogen is also a non-osmotic molecule, so it can also be used as a solution to storing glucose in the cell without disrupting osmotic pressure.
How & Where do animals get their glucose?
Animals get their glucose from the carbohydrate rich food that they do eat like meat, fish, leaves, vegetables, rice, grass, etc.
If the animals directly consume food like fish, meat, rice, etc. then they will directly get the glucose they will need.
If the animals consume food like fish, meat, sugar cane, green leaves, potato, sweet root vegetables such as beetroot and carrots, etc. then they will get disaccharides, polysaccharides from the food which will be broken down in their body to derive the monosaccharide glucose from it.
So, in very simple words carbohydrate containing food is the one and only direct or indirect source of glucose.
The food that includes meat, fish, bread, rice, pasta, potatoes, vegetables, fruit, sugar, yogurt, and milk are the top-most sources of carbohydrate for both humans and animals alike.
After eating the food, the animal bodies are able to change 100% of the carbohydrate into glucose for energy needs.
So, you will often see that after eating food the blood sugar levels can be changed or affected quickly, within an hour or two after eating.
So, How the glucose is taken? The saliva contains amylase that can break down amylose, starch, and amylopectin into smaller chains of glucose, called dextrins and maltose.
The stomach and small intestines absorb the glucose by the enzymes amylase, maltase, lactase, and sucrase, and which get absorbed into the bloodstream.
Now, the glucose that enters the bloodstream from the breakdown of food can either enter into the cell for its utilization or can be converted to glycogen by glycogenesis in the liver and skeletal muscle cells.
The glucose can be intaken directly in the body cells from the blood stream to produce ATP energy following a mechanism of facilitated diffusion through a family of hexose transporters embedded in the cell membrane by the action of insulin hormone.
For example: In the skeletal muscle cells, a kind of hexose transporter embedded in the cell membrane is GLUT4 to intake glucose.
If excess glucose is to be converted than those glucose molecules will go through the process of gluconeogenesis majorly in the liver. Gluconeogenesis in very small amounts can also take place in the skeletal muscles, kidney, brain, and also in the heart muscles.
How do animals utilize the stored glucose to get energy?
The stored glucose in the animal body is actually the glycogen and nothing else. The glucose (or glycogen) stored in the animal body leads to the production of energy for the body’s cells by glycolysis.
In simple words, Glycolysis is defined as a sequence of reactions converting glucose (or glycogen) to pyruvate or lactate with the production of ATP as energy for fulfilling the body’s energy requirements and for carrying on the body’s metabolic activities.
Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism.
This process of glycolysis consists of both of an energy-requiring phase followed by an energy-releasing phase.
This glycolysis process takes place inside the each and every cell of the body. And, all of the enzymes for carrying on glycolysis are present inside the cytoplasm of every cell.
In fact, Glycolysis is a major pathway for ATP energy synthesis in cells lacking mitochondria. For example in RBC cells, eye cornea, lens, etc.
Remembered, we said glycolysis leads to converting glucose (or glycogen) to pyruvate or lactate with ATP production. So, here Pyruvate is the result of aerobic respiration whereas, Lactate is the result of anaerobic respiration.
So, Glycolysis occurs both in the absence of oxygen (anaerobically) or in the presence of oxygen (aerobically).
Lactate is the end product under anaerobic conditions. In the aerobic condition, Pyruvate is formed which is then oxidized into CO2 and H2O as end products.
Why do animal cells store glycogen and not glucose?
The first reason why animal cells store glycogen and not glucose is that if glucose molecules remain present in the cell then it will eventually start taking up water inside the cell through a process called osmosis.
This can cause the cell to burst and rupture if excess water gets inside the cell. So, glycogen being a very large polymer doesn’t intake water by osmosis.
The glucose is present in very little quantity in the blood and is well soluble in water. If the glucose is present in higher quantity in the blood then it will suck the water out of cells into the blood stream thus, causing problem for the cell and altering the normal body functioning.
So, the excess glucose is stored as glycogen inside the cells as macromolecular particles that are insoluble in water.
So, in very simple words, glucose cannot be stored in the cells of the animal body due to the high osmotic pressure which glucose exerts inside the cell, causing the entry of water in the cell by endosmosis.
This is the reason that the glucose is converted to glycogen in the liver for storage because glycogen is osmotically more stable than glucose.
Also another reason is that the controlled breakdown of glycogen and release of glucose increase the amount of glucose that is available between meals. Hence, glycogen serves as a buffer to maintain blood-glucose levels.