Carbohydrates, Protein, Lipids, and Dna

Carbohydrates have two major functions in plants and animals: they serve as fuel and building material. Plants store most of their energy in the form of starch. Starch is a polysaccharide (consisting of several conjoined sugars). Synthesizing starch enables the plant to stockpile surplus glucose, and since glucose is a major cellular fuel, starch represents stored energy. The sugar can later be withdrawn by hydrolysis, which breaks the bonds between monomers. Animals, in turn, have enzymes that can hydrolyze plant starch, making glucose available as a nutrient for cells. Cellulose, on the other hand, is used in plant cells for structural support. For example, wood is a strong building material that is rich in cellulose. Like starch, cellulose is a polymer of glucose, but cellulose is composed of beta glucose (as opposed to alpha glucose in starch). Beta glucose forms chains with extra hydrogen bonds on top of each other, making extra-tough microfibrils that cannot be digested by most organisms.

Proteins accomplish many different tasks, including structural support, storage, transport, movement, and defense against foreign substances. One noteworthy type of protein, called an enzyme, acts as a catalyst, speeding up chemical reactions in the cell. All proteins are composed of the same set of 20 amino acids, and each amino acid differs from the other only by the "R group" attached to it. The polymers of those amino acids are called polypeptides, and the order of amino acids is vital to the function of the protein. For example, if just one amino acid is substituted for another in the primary structure of hemoglobin, the protein that carries oxygen in a red blood cell, it can seriously hinder the blood cell's ability to carry oxygen, and the cell's circular shape becomes deformed into a sickle shape, which can clog blood vessels.

Lipids include fats, and the main function of fat is energy storage in animals. Fats are even richer in energy than carbohydrates. A gram of fat stores more than twice as much energy as a gram of starch. Animals must carry their energy stores with them, so fats are beneficial for storing energy without being weighed down by water. In addition, they also cushion vital organs such as kidneys. Phospholipids (two fatty acids connected to a glycerol) are a major component of cell membranes; they from a bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward.

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