Polysaccharides are complex carbohydrates made of simpler sugars like glucose. They're the essential energy sources of plant and animal bodies. Their systems and interactions are unseen but vital to health and ecology.
Polysaccharides in plants and animals are made of long chains of monosaccharides, or simple sugars, mainly glucose. Starch, glycogen, and cellulose are the most abundant polysaccharides.
Their functions include energy storage, structural support and cellular communication. Starch, glycogen and cellulose yield mainly glucose when broken down, commonly used as fuel for plant and animal bodies.
Omnipresent in nature, polysaccharides join by glycosidic bonds, formed by removal of a water molecule. They can be homopolymers, made of one type of monosaccharide, or heteropolymers, made from different types.
Polysaccharides come from the process of photosynthesis. Green plants absorb sunlight, carbon dioxide and water to create glucose. This simple sugar is then transformed to polysaccharides.
In plants, starch serves as the main polysaccharide synthesized in chloroplasts, or sac-like organelles. For example, potato tubers contain 70% starch, and rice up to 90%.
Cellulose is formed in the cell walls to add rigidity. About 33% of the dry weight of plant biomass is cellulose.
In animals, glycogen is the primary storage form of glucose. Stored in liver and muscles, it represents about 1% of body weight in an average human.
Starch
Starch is the main storage component in plants.
Description: Found in granules, starch is a polymer of glucose, which is linked in a specific manner that allows for easy breakdown.
Unique Traits: Starch is a mix of two types of glucose polymers, amylose (linear chains) and amylopectin (branched chains). This structure facilitates effective storage of energy to be used later by the plant.
Where it's Found: Starch is primarily found in plant storage organs, such as seeds like corn, rice, wheat; tubers such as potatoes and cassava; and plant stems. While tubers are called roots, they're actually part of the stem.
Function: Plants use starch as a readily available energy reserve. Animals obtain energy from starch by consuming plant matter. Starch is a reserve of energy, primarily located in seeds and tubers.
For instance, a single medium-sized potato contains about 37 grams of starch. When plants need energy, enzymes break down starch into glucose to fuel activities. Starch is a major part of carbohydrate intake.
Glycogen: The Animal Starch
Glycogen is the storage form of glucose in animals and fungi, similar to the way starch works in plants.
Description: Glycogen is the primary storage polysaccharide in animals, often referred to as "animal starch." Like starch, it's a polymer of glucose.
Unique Traits: Glycogen is a highly branched structure, which allows for faster mobilization of glucose when energy is needed.
Where it's Found: Glycogen is primarily stored in the liver and muscle tissues of animals. In humans makes up to 1% of body weight. The liver stores about 100 g of glycogen. Skeletal muscles store about 400 g.
Function: It's a short-term energy source, quickly broken down to release glucose for cellular fuel. During manual work, exercise or running for the bus, muscles can use stored glycogen to generate energy rapidly.
Cellulose
Cellulose, the most abundant organic polymer on Earth, makes up the structural component of plant cell walls.
Description: Cellulose is the structural polysaccharide of plants. It's a linear polymer of glucose, but glucose molecules link in different ways than in starch or glycogen.
Unique Traits: The unique linkage between the glucose molecules in cellulose forms strong hydrogen bonds within and between cellulose chains.
Cellulose forms microfibrils to give plants rigidity and strength. The structure of cotton fiber, made primarily of cellulose, is an example of this property.
Where it's Found: Cellulose is the main component of plant cell walls. It's found in plant materials and is a source of dietary fiber for humans. Bodies can't break down cellulose, but it aids digestion and relieves constipation.
Function: Cellulose maintains structural integrity of plants, helping them resist external pressures and remain robust. Cotton, paper, and wood are all primarily made of cellulose.
Homopolymers: A Common Thread
These three crucial polysaccharides - starch, glycogen, and cellulose - are classified as homopolymers. Each is made up of only one type of monosaccharide, glucose.
Despite being made of the same building block, unique structures and bonding arrangements lead to distinct characteristics and functions.
Hydrolysis: Breaking the Bonds
Hydrolysis breaks down polysaccharides into monosaccharides. This reaction involves adding a water molecule, which cleaves the glycosidic bonds among the monosaccharides.
Hydrolysis is catalyzed by specific enzymes, each designed to break down a particular type of polysaccharide. For instance, amylase breaks down starch, while cellulase breaks down cellulose.
For humans, most of the focus is on starch, some glycogen and little or no breakdown of cellulose. In the human body, enzymes like amylase, lactase, and cellulase facilitate hydrolysis of polysaccharides during digestion.
Starch is broken down into glucose by amylase, an enzyme and protein that speeds up chemical reactions in the body. Amylase helps digest carbohydrates. Most is made by the pancreas and salivary glands.
Facts about Polysaccharides
Medicine: Polysaccharides are important to medicine. Examples include use of heparin as an anticoagulant and hyaluronic acid in skin care products. Hyaluronic acid is a natural substance in fluids in the eyes and joints. It's a cushion and lubricant in the joints and other tissues.
Digestive Health: While starch and glycogen provide essential energy, cellulose significantly benefits digestive health by facilitating GI tract mobility and promoting regular bowel movements.
Biofuel Production: Cellulose can be transformed into biofuels. This offers a renewable energy source to help reduce dependence on fossil fuels. However the money remains on oil. Biodiesel costs are currently 70% to 130% higher than petrol and diesel on the wholesale market.
Glycogen Storage Disease: Certain genetic conditions hinder the proper breakdown of glycogen, leading to metabolic disorders known as glycogen storage diseases. These affect muscle and liver function.
Plant Fibers: Cotton, hemp and flax (linen), primarily composed of cellulose, are natural fibers used for textiles.
Polysaccharides are essential biomolecules. They power life and provide structural support. Starch, glycogen, and cellulose are examples of their versatility and importance as glucose-based homopolymers.
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