Vitamin amino acids. Most vitamins are not single molecules, but groups of related molecules called vitamers. For example, vitamin E consists of four tocopherols and four tocotrienols. The thirteen vitamins required by human metabolism are: vitamin A (retinols and carotenoids), vitamin B (thiamine), vitamin B (riboflavin), vitamin B (niacin), vitamin B (pantothenic acid), vitamin B (pyridoxine), vitamin B (biotin), vitamin B (folic acid or folate), vitamin B (cobalamins), vitamin C (ascorbic acid), vitamin D (calciferols), vitamin E (tocopherols and tocotrienols), and vitamin K (quinones). Vitamins have diverse biochemical functions. Some forms of vitamin A function as regulators of cell and tissue
B2 First the Reading and Use of English papers have been combined into a single paper. B2 First is available in two versions: B2 First and B2 First for Schools both have the same exam format (e.g. number of papers, number of questions, time allowance), both support learners to develop real-life communication skills, and both versions lead to the same certificate. The exams use different topics and content: Both versions of the exam (B2 First and B2 First for Schools) are made up of four papers, which cover all the key language skills (Reading and Use of Language, Writing, Listening, and Speaking).