Water & Solutions — The Chemistry of Life

Water (H₂O) is so common that we take it for granted — but chemically, it is deeply unusual. Almost every property that makes life possible depends on water's strange behaviour. Your body is about 60% water by mass. Blood is ~92% water. Every chemical reaction in every cell of your body happens in a watery environment. Water isn't just a background liquid — it is an active participant in almost all biochemistry. What makes water so special? Its molecular structure. The two hydrogen atoms bond to oxygen at an angle of 104.5°. Because oxygen pulls electrons much more strongly than hydrogen, the oxygen end carries a slight negative charge (δ–) and the hydrogen ends carry slight positive charges (δ+). Water is a polar molecule. This polarity allows water molecules to hydrogen-bond extensively with each other and with other polar molecules — and this explains almost every unusual property water has.

A solution is a mixture where one substance (the solute) is dissolved in another (the solvent). In body fluids, water is always the solvent. What dissolves in water and what doesn't: - Hydrophilic ("water-loving") substances dissolve in water. They are ionic (like NaCl) or polar (like glucose, amino acids). Polar water molecules surround them, pulling them apart. - Hydrophobic ("water-fearing") substances do NOT dissolve in water. Non-polar molecules like fats and oils cannot form hydrogen bonds with water — they clump together instead. Concentration measures how much solute is dissolved in a given volume of solution. In medicine, this matters enormously — a drug's dose is based on achieving the right concentration in the patient's blood or target tissue. Too little: no effect. Too much: toxic. (More on this in the Concentration lesson.) Molarity (mol/L): The standard scientific measure — moles of solute per litre of solution. mg/dL: Common in clinical blood tests (e.g. blood glucose: normal is 70–100 mg/dL). mmol/L: Used for electrolytes (e.g. normal serum sodium: 135–145 mmol/L).

Osmosis is the movement of water across a semi-permeable membrane from an area of lower solute concentration to higher solute concentration. Water moves to dilute the more concentrated side. Think of it this way: water "wants" to equalise concentrations on both sides of a membrane. Since the solute can't cross, the water does the moving. Osmotic pressure is the pressure needed to stop osmosis. Solutions with a higher solute concentration have a higher osmotic pressure — they pull water toward them. Clinical importance — IV fluids: - Isotonic saline (0.9% NaCl) has the same solute concentration as blood plasma. Water doesn't move into or out of red blood cells. Used for fluid replacement. - Hypotonic solutions (more dilute than blood) — water enters cells → cells swell → risk of lysis. Dangerous if given inappropriately. - Hypertonic solutions (more concentrated than blood) — water leaves cells → cells shrink. Hypertonic saline can treat brain swelling (oedema) by drawing water out of swollen brain cells. Dehydration: When you're dehydrated, blood plasma becomes more concentrated. Your kidneys retain water, and osmoreceptors in the hypothalamus signal thirst. The body is trying to restore osmotic balance.

Diffusion is the net movement of particles from an area of high concentration to low concentration — down their concentration gradient. No energy is required; it's driven by random molecular motion. Diffusion is fundamental to life: - Oxygen diffuses from air sacs (alveoli, high O₂) into blood (lower O₂) in the lungs - Carbon dioxide diffuses from blood (high CO₂) into alveoli (lower CO₂) to be exhaled - Glucose diffuses from blood into cells when blood glucose rises after a meal - Drug absorption — many oral drugs diffuse across the gut wall down their concentration gradient Fick's Law (simplified): The rate of diffusion increases with: - Larger concentration difference (steeper gradient) - Larger surface area - Thinner membrane - Higher temperature This is why the lungs have 300 million tiny alveoli (enormous surface area) with capillary walls only one cell thick — maximising gas exchange by every principle of Fick's Law. Active transport moves substances AGAINST their concentration gradient — requiring energy (ATP). Example: the sodium-potassium pump moves Na⁺ out of cells and K⁺ in, maintaining the electrical gradient essential for nerve and muscle function.