Explain the properties of water (polarity, cohesion, solvent ability, heat capacity) and the bonding properties of carbon that make it the backbone of biological molecules (MA STE HS-LS1-6 supporting).

What this topic is asking

The Massachusetts STE framework (HS-LS1-6) asks you to explain how carbon, hydrogen, and oxygen from sugar molecules combine to form the large carbon-based molecules of life. Behind that standard sit two foundations the MCAS expects you to understand: the special properties of water, the medium in which all cell chemistry happens, and the bonding properties of carbon, the element that forms the backbone of every biological molecule. These are short topics, but they explain why life is built the way it is, and the test uses them to check that you can reason from a molecule's structure to its behavior.

Why water is the solvent of life

Because water is polar, water molecules attract one another and attract other charged or polar substances. This single fact explains most of water's biological importance:

• Solvent ability. Polar water surrounds and separates the particles of polar and charged substances (salts, sugars, many proteins), dissolving them. Since most molecules of metabolism dissolve in water, water is where the reactions of life take place.
• Cohesion and adhesion. Water molecules stick to one another (cohesion) and to other surfaces (adhesion). Cohesion lets water be pulled up a plant in continuous columns and gives water surface tension.
• High heat capacity. Water absorbs a lot of heat for a small rise in temperature, so it resists sudden temperature change. This helps organisms and cells keep a stable internal temperature, which supports homeostasis.

Most of a cell is water, so these properties are not background detail; they are why cells can function at all.

Why carbon is the backbone of life

Biological molecules are built mostly from carbon, hydrogen, and oxygen, with nitrogen, phosphorus, and sulfur in some classes. Carbon sits at the center because of how it bonds.

A carbon atom has four outer electrons, so it can form four covalent bonds with other atoms. This lets carbon:

• build long chains (the backbone of fatty acids and many polymers),
• form branches, giving complex shapes,
• close into rings (as in glucose and the bases of DNA),
• and bond to many different elements, including other carbon atoms.

Because of this versatility, a small set of elements can be arranged into an almost endless variety of stable molecules. That is why the four classes of biological molecule, despite doing very different jobs, are all carbon-based. The framework standard HS-LS1-6 makes this concrete: cells take the carbon, hydrogen, and oxygen in sugars and rearrange them to build amino acids, fats, and other large molecules.

How water and carbon connect to the rest of biology

These properties are the foundation under every later topic. Water's role as a solvent and as a reactant or product appears in photosynthesis and respiration. Its polarity is the reason the phospholipid bilayer forms a barrier. Carbon's bonding ability is the reason the four classes of biological molecule can exist at all.

Try this

Q1. State two properties of water that come from its polarity. [2]

• Cue. Any two of: it is a good solvent (dissolves polar and charged substances), it shows cohesion (molecules stick together), it has a high heat capacity (resists temperature change).

Q2. Explain why carbon can form such a large variety of molecules. [2]

• Cue. Each carbon atom forms four covalent bonds, so it can build chains, branches, and rings and bond to many different atoms, giving great variety.