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Human Body

WAtEr, mInErAls, And FluId comPArtmEnts When biologists are asked what sort of life we have on Earth, the reply used to be carbon based, but now it is water based. Waters unique properties allow all the biochemical and electrical transformations in life to take place, and it is the main constituent of your body. Body fluids come in three major compartments blood, extracellular fluid ECF, the salty soup between cells, and intracellular fluid ICF, the viscous cytosol inside cells. Nearly half of your blood volume is cells, mainly red cells, and a percent or two white cells and platelets. The fluid portion of blood, plasma, is quite similar to extracellular fluid, but very large proteins like albumin stay in the blood, contributing to its osmotic ability to keep water within capillaries. About 30 litres of fluid seep out of capillaries per day, and all but 3 litres drained as lymph seeps back in, carrying cell products. Fluid ratios vary with body type, gender, and age, but an adult has about in litres BLOOD 5, ECF 9, ICF 29. Most blood is pooled as a slow flowing reservoir in veins, between carrying everything else around. Brain and kidneys take a fifth of nonlung flow each, and the busiest current organs take most of the rest, especially muscles if youre running, gut and liver after a meal, or skin, if youre hot. In terms of dissolved molecules rather than ions, ICF composition varies hugely, depending on cell type and phase of activities. Plasma carries all circulating substances, and ECF mediates between ICF plasma. Charged ions are kept at stable levels in fluid compartments, because life processes operate within narrow local electrical and acidbase windows. An ion is surrounded by a multi layered morphing aquahedron i. of water molecules, defining its sphere of influence. Potassium and sodium ions both carry a single positive charge, but though K is heavier, it is less electrodense, so has a smaller, less influential aquahedron. This makes it the tame cation of choice for intracellular use, and for stabilising the membrane voltage by seeping through small cation channels. Sodium performs the faster, more electrodense processes like rushing through voltagegated channels, and is ejected from cells by NaKATP pumps, 3 Na out to 2 K in per ATP molecule. The salty sea of ECF has chloride Cl as its main anion, while phosphates and proteins carry most ICF negative charges. MAJOR ION CONCENTRATIONS mmolL1 Plasma ECF ICF Sodium 144 146 11 Potassium 4 4 148 Calcium 4 3 0.13 Magnesium 2 2 35 Chloride 104 118 4 Bicarbonate 25 27 13 Phosphate 2 2 40100 Protein anions 20 1 52 Others 5 6 60 Of other minerals, iron mainly for hemoglobin is more abundant in men, whereas copper is higher in women. Rarer elements Cu, Mg, Mn, Zn, Cr, Se, etc are mainly enzyme components, for instance molybdenum, the rarest element you need, which is in xanthine oxidase. A cobalt sits in the middle of vitamin B 12 ii., a coenzyme, like most vitamins. Sulphur appears mainly in proteins, playing a special role in forming their 3D shape. Iodine is used in thyroxine synthesis, being superadded to iodinecontaining peptides. A fourth fluid category is special extracellular secretions, which includes the cerebrospinal fluid the brain floats in, the eyes aqueous and vitreous humours, and the ears endolymph and perilymph, and also any fluid or mucus matrix cells deposit around them, for lubrication, insulation, protection, and as an extended molecular territory for cellular activities. About ninetenths of your water comes in what you eat and drink, but, incredibly, the other tenth is made by you as fresh new water molecules, a little during protein synthesis, but most of it during mitochondrial ATP synthesis. i. ii. 54 molEculAr nutrItIon WHAT YOU HAVE TO EAT DRINK is water minerals, lipids fats, carbohydrates sugars, proteins, and vitamins. The complex molecules in your food have already been synthesised by the combined efforts of bacteria, fungi, plants, and animals if you eat meat. The energy source for the synthesis of complex organic molecules is the Sun. Your body can interconvert most substances, but some molecules the body cannot synthesise are essential nutrients, mainly certain aminoacids, lipids and vitamins. Everything else can be made from other molecules. Catabolism is the breakdown of complex molecules into simple ones that are used to supply energy or to build other compounds. Anabolism is the synthesis of complex compounds. These processes require energy input, which in most non mitochondrial pathways comes as ATP a.. The Krebs cycle is the multistep circular pathway by which mitochondria produce ATP. The brain has to use glucose b. to fuel this process, and other cells prefer to, as it can be split via a simple elevenstep process into two pyruvates eg c. which enter the Krebs cycle two steps later. Most of the pathways of sugar, fat, and protein catabolism converge around here. Short term lack of oxygen diverts pyruvate to lactate d. in an emergency pathway. Dietary sugars can be simple like glucose, or the fruit sugar fructose e., disaccharides like sucrose b. e. or lactose f., the milk sugar which is the main nonplant carbohydrate youre likely to ingest, or complex sugar polymers starches. Bean sugars are initially digested by gas producing bacteria, hence the flatulence. Sugars can be stored as glycogen, a glucose polymer, synthesised by liver and skeletal muscle cells as a rapid access reserve, but 99 of energy storage is as fat, mainly triglycerides e.g. g., the destination of all excess sugars, proteins, and lipids. The two essential dietary fats are linoleic and linolenic acid h. found in all plant cells. Other dietary fats are cholesterol essential in cell walls and for making steroid hormones, bile salts vitamin D, and phospholipids like lecithin, the main constituents of cell walls. Derivatives of the 20carbon arachidonic acid, made from linolenic acid, are also used as paracrine messengers between cells, both within tissues, and in moveable operations like damage repair and immunication. Plants make all aminoacids j., and we can synthesise 10 of the basic 20, the rest being essential aminoacids like tryptophan k.. Peptide bonds join aminoacids to make proteins of all sizes, from dipeptides two amino acids to huge muscle proteins like titin, a chain of 27,000. If you have to use proteins for energy, you remove their nitrogen containing parts, creating ammonia, which is also made at many other metabolic stages. Birds excrete excess nitrogen as urates in semisolid guano, but mammals excrete ammonia dissolved as urea l., which is the main reason you have to drink so much water. By the time you eat your food, the hard work of synthesis has already been done. Our metabolic processes have evolved with the organisms we eat, and use the same substances in different pathways, so a diet of good water, fruit, nuts, roots, vegetables and some extra a. adenosinetriphosphate b. c. d. e. f. g. h. j. R radical k. l. 55
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