Essential elements are indispensable to living organisms, including macroelements and trace elements. The bio functions of non-essential elements are still unclear. Long-term intake may pose health risks. Common harmful elements include lead and mercury.

Water in organisms ranges from 60% to 90%. Free water or unbound water can flow freely in living organisms. Bound water is trapped in the vicinity of organic matter. They can be converted to each other. The bound water increases resistance to harsh conditions.

Why is water important to life? Hydrogen bonds are formed between water molecules. It gives water high specific heat capacity, very strong surface tension, and ice is lighter than water.

Monosaccharides are the units that make up sugars. Common ones are: glucose, fructose and galactose. We introduce their structure and biological functions, physical and chemical properties.

Disaccharides consist of two monosaccharide molecules. We describe the physical, chemical, biological properties of three common disaccharides: sucrose, maltose and lactose. We also introduce their structure and bio functions. The dangers of consuming too much sucrose are also described.

Starch is a common polysaccharide found in plants. They are divided into amylopectin and amylose. We describe their structure, physical and chemical properties in detail.

Synthesis and degradation of liver and skeletal muscle glycogen: small muscle glycogen (β particle) is mobilized fast, while large liver glycogen (α particle) for storing energy and stable blood sugar is mobilized slowly. We also cover the lactic acid cycle connecting these two glycogens.

Fats or Triglyceride are classified into saturated and unsaturated fats. Fat has more carbon and hydrogen and stores more energy than sugar. It also keeps you warm, moderates shock and promotes the absorption of fat-soluble vitamins.

Differences in the structure of double bonds lead to cis fats and trans fats. Trans fats: Hydrogenation and refining of vegetable oils, frying and baking, food irradiation.

There are two types of trans fatty acids: artificial or industrial trans fatty acid, and natural or ruminant trans fatty acid. The position of their double bonds is different. The natural trans fats don't damage health, but artificial trans fats hazard health: heart disease, diabetes, cognitive impairment and obesity.

Phospholipids are common lipids divided into glycerophospholipids and sphingomyelin. We introduce their structure, physical, chemical properties, and biological functions. The hydrophilic head and hydrophobic tail form the phospholipid bilayer of the cell membrane.

Structure, physical and chemical properties of cholesterol and sterols. They not only make cell membranes more stable, but also increase fluidity. Other functions are also introduced: hormones, vitamin D, antioxidant.

This article describes the physical, chemical properties, physiological functions, and applications of natural bio-waxes in the food, cosmetic and pharmaceutical industries. It ends with a description of several common biowaxes: beeswax, Brazilian carnauba wax and spermaceti.

Proteins are amino acids polymers make up about 50% organic content in lives. Proteins have unique conformation and perform a specific function: motion, structure, catalysis, signal recognition, defense, transportation, energy Source.

Structure, physical, chemical properties of proteinogenic amino acids: melting point, solubility, optical activity, amphoteric compound. They are classified into hydrophilic, hydrophobic, neutral, basic, and acidic amino acids based on the R group.

Peptide bonds have partial properties of double bonds. Physical, chemical properties of polypeptide are determined by hydrophilic or hydrophobic R groups. They fold into complex structures. Oligopeptide promotes nutrient absorption. Some bio peptide resist bacteria.

Peptide bonds and disulfide bonds are covalent bonds that determine primary structure of proteins. Secondary structures include α-Helix, β-Sheet, β-Turn, β-Bulge and Random Coil. Tertiary and quaternary structures are maintained by salt bonds, hydrogen bonds and hydrophobic interactions.

There're 6 interactions to maintain 4 orders of protein structure. Peptide bonds, disulfide bond are covalent. Salt bridges, hydrogen bonds, hydrophobic interactions, and van der Waals forces are all secondary chemical bonds.

The chemical bonds that maintain the three-dimensional structure of proteins are relatively weak. They will break in the presence of high temperatures, ions and organic solvents, thus changing the protein shape. Loss of protein structure results in loss of function.

Cooking, chewing, stomach and small intestine are all necessary steps for protein digestion. Proteins are hydrolyzed into polypeptides. The polypeptide is decomposed into oligopeptide and amino acids for absorption. Peptide can also be absorbed directly. Rabbit starvation is also discussed by us.

The human body decomposes sugars, fats, and proteins to obtain energy. Sugar served as the main energy source, while fat is used to store energy. When there is a shortage of fat and sugar, protein serves as the body's energy source. They are broken down to produce energy in the following order: glucose, glycogen, fat and protein.