Carbohydrates: Monosaccharides and Disaccharides

Carbohydrates Overview

• Carbohydrates contain carbon, hydrogen, and oxygen.
• The ratio of hydrogen to oxygen is approximately 2:1.
• Carbohydrates include sugars, starches, cellulose, chitin, and glycogen.
• They provide energy, store energy, and provide structural support.

Monosaccharides

• Defined as simple sugars.
• Commonly contain 3, 5, or 6 carbon atoms.
• Examples of 6-carbon sugars (hexoses) include glucose, fructose, and galactose.
• Most living organisms use it for energy.
• Glucose, fructose, and galactose are composed of hydrogen, carbon, and oxygen.
• The formula for glucose (and all hexose sugars) is C6H12O6.

Disaccharides

• Formed when two monosaccharide molecules condense
• This forms one large molecule with two rings joined
• This process releases water (dehydration synthesis).
• Sucrose (cane sugar) is a common disaccharide formed from the condensation of glucose and fructose, with the formula C12H22O11.
  • Plants use sucrose for energy transport and storage and can be extracted from sugar cane and maple trees.
• Lactose is formed from the condensation of glucose and galactose.
  • It is the principal energy source in milk.
  • Lactase enzyme breaks it down in the gut, producing glucose and galactose.
  • Lactose intolerance occurs when people lack sufficient lactase.
  • Galactosemia is a genetic disorder where galactose accumulates to dangerous levels.

Polysaccharides

• Formed when many monosaccharides link together, creating large, complex structures that vary in length, the sugars used, side chains, and branching patterns.
• Examples include starches, glycogen, cellulose, and chitin.
• Starches are the most common storage chemical in plants, usually a mixture of branched and unbranched polysaccharides.
  • Easily broken down by hydrolysis to release glucose (energy). Branched starches are hydrolyzed more quickly than unbranched starches.
• Glycogen is a major storage carbohydrate in animals, stored in the liver and muscles, and represents approximately a day's worth of glucose storage.
• Cellulose is a structural polysaccharide providing strength and rigidity to plant cell walls, with many molecules lying side-by-side, forming fibers that mat together in the cell walls.

Proteins Overview

• Proteins are essential components of cells needed for energy
• Proteins facilitate chemical reactions as enzymes
• Proteins are responsible for movement across cell membranes as carrier molecules
• Proteins are involved in antibody production
• Proteins make up muscle fibres, haemoglobin, collagen, elastin, hormones, and hair.

Amino Acids

• Elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), and sometimes Sulfur (S) (depending on the side chain).
• Consists of a central carbon (α-carbon), an amino group (-NH2​), a carboxylic acid group (-COOH), and a side chain (R-group) which varies.
• The R groups can vary; examples include Glycine (R = H), Cysteine (R = CH2​-SH), Alanine (R = CH3​), and Tryptophan (complex ring structure).
• There are about 20 amino acids found in nature.
• Amino acids can bond with one another to make long chains of amino acids, giving a huge range of possible polypeptides.
• The bond between amino acids is called a peptide bond/linkage.
• Amino acid sequence of polypeptides is coded for by genes. these are synthesized on ribosomes
• The links between amino acids (peptide bonds) are formed in condensation reactions, creating polypeptides.
• A protein may consist of a single polypeptide or more than one polypeptide linked together.

Denaturation

• Denaturation is the process where a protein loses its native shape due to disruption of the intramolecular bonds (secondary and tertiary structures), leading to the loss of its biological function.
• Heat causes vibration within proteins, breaking intramolecular bonds and changing the conformation reversibly
• Proteins have an optimal pH at which their conformation is normal but changes in pH can break intramolecular bonds, leading to denaturation.

Common Proteins

• Keratin is a helical protein with three polypeptide chains intertwined, forming bundles of fibers stacked together to form hair, and is the principal constituent of hair and a fibrous protein.
• Collagen gives strength to tissues (tendons, ligaments, bone, and skin), has the same tensile strength as steel, reinforces structures like steel reinforces concrete, and is a fibrous protein.
• Antibodies are made specifically to attack viruses and other foreign material, consist of two heavy chains and two light chains, and are globular proteins.
• Haemoglobin transports oxygen in the red blood cells, is a globular protein, and contains haem groups which bind to oxygen.
• Myoglobin stores oxygen in the tissues (especially the muscles), is related to haemoglobin, and is a globular protein.
• Protein functions include:
  • ENZYMES with Rubisco used in photosynthesis
  • SIGNALLING with Insulin being a hormone made of amino acids
  • RECOGNITION with Immunoglobulins as antibodies
  • TRANSPORT with Albumin in blood
  • STRUCTURE with Collagen for spider silk
  • OTHER with Rhodopsin as a pigment in the rod cells of eyes.

Lipids Overview

• Lipids are fats and oils.
• Lipids have many vital uses in the body including:
  • Energy storage
  • Insulation against the cold in animals
  • Protection of organs against physical damage in animals
  • Energy storage in seeds (e.g., peanuts) in plants
  • Waterproofing (waxes) to keep water in in plants.
• Examples:
  • Fat under the skin of a seal protects it from the cold.
  • Fat around the kidneys protects them.
  • Wax on the surface of leaves repels water and reduces water loss.
  • Lipids on a duck's feathers repel water.

Triglycerides

• Mainly used as energy stores in animals and plants.
• Consist of one molecule of glycerol and three fatty acids, with fatty acids that can vary in length and in bond types.
• Glycerol is a simple polyol compound that is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic.
• Fatty acids are a carboxylic acid consisting of a hydrocarbon chain and a terminal carboxyl group, especially any of those occurring as esters in fats and oils.

Formation of Triglycerides

• Glycerol's reaction with three fatty acids through esterification forms ester bonds, releasing three water molecules in the process
• Glycerol + 3 x Fatty Acid → Triglyceride + 3 x H20
• comparison:
  • Lipids have about twice the energy per gram compared to carbohydrates.
  • Lipids are lighter for the same energy storage, are insoluble (no osmosis problems in cells), and have slower digestion and energy release, while carbohydrates are heavier, soluble (easily transported), and have more easily digested with energy released more rapidly.

Phospholipids

• Consist of:
  • a polar head containing a negatively charged phosphate group (hydrophilic - water-loving), and
  • a non-polar tail is hydrophobic (water-hating)
• When placed in water, phospholipids arrange themselves so that the water-loving end faces out, giving them the unique property of forming ultrathin membranes and bilayers.
• Cell membranes are composed primarily of phospholipid bilayers.

Cholesterol

• It is a complex fat with a polar end, normally made by the liver as needed.
• Fills holes in the bilipid layer of all membranes and is essential for healthy animal cells (plants and fungi use other sterols).
• Cholesterol is a steroid-based structure (C27H46O) with a polar (-OH) group at one end and a nonpolar hydrocarbon chain.
• Cholesterol molecules are inserted into the phospholipid bilayer of cell membranes, modulating membrane fluidity by making the membrane less fluid at high temperatures and preventing it from solidifying at low temperatures.
• Cholesterol helps maintain the integrity and stability of the cell membrane.
• Cholesterol is synthesized in the liver through a complex series of enzymatic reactions regulated by feedback mechanisms to maintain appropriate cholesterol levels in the body.
• Cholesterol-related heart disease arises in people who inherit a tendency to make large amounts of cholesterol, leading to atherosclerosis, which narrows the arteries and causes heart attacks and strokes by blocking blood vessels due to the buildup of fats, cholesterol, and other substances in and on the artery walls, forming plaques that can narrow arteries and blocking blood flow.

Steroid Hormones

• Steroid hormones are a type of lipid
• Steroid hormones are made from cholesterol
• Examples are testosterone, progesterone, and oestrogen.

Waxes

• Waterproofing.
• Waxes consist of long fatty acid esters.
• Cannot be digested by most animals.

Glossary

• Lipids: A class of organic compounds that are fatty acids or their derivatives and are insoluble in water but soluble in organic solvents.
• Fat: A natural oily or greasy substance occurring in animal bodies, especially when deposited as a layer under the skin or around certain organs.
• Oil: A viscous liquid derived from petroleum, especially for use as a fuel or lubricant.
• Wax: A substance of plant, animal, or synthetic origin that is solid at room temperature, pliable when warm, and typically lustrous, translucent, and waterproof.
• Triglyceride: An ester formed from glycerol and three fatty acid groups; triglycerides are the main constituents of natural fats and oils, and high concentrations in the blood indicate an elevated risk of stroke.
• Phospholipid: A lipid containing a phosphate group in its molecule.
• Cell membrane: The semipermeable membrane surrounding the cytoplasm of a cell.
• Cholesterol: A compound of the sterol type found in most body tissues, including the blood and the nerves. Cholesterol and its derivatives are important constituents of cell membranes and precursors of other steroid hormones, but a high proportion of cholesterol in the blood is associated with an increased risk of atherosclerosis.