Explain the properties of water that make it essential to life and describe ATP as the cell's energy currency (North Carolina Standard Course of Study, Biology, LS.Bio.3).

What this topic is asking

North Carolina LS.Bio.3 is about energy use in living things, and it rests on two chemical foundations: water, the medium in which life happens, and ATP, the molecule that carries usable energy. For the Biology EOC you need to know the properties of water that make it essential (polarity, cohesion, its role as a solvent, and temperature stability) and how ATP stores and releases energy as the cell's "energy currency." These ideas underpin photosynthesis, respiration, and homeostasis.

Why water is essential

Polarity explains water's most important properties for life:

• Universal solvent. Because water is polar, it surrounds and separates charged and polar substances, dissolving them. Most of the chemistry of life happens in solution, and blood, cytoplasm, and sap all rely on water as the solvent that carries dissolved nutrients, gases, and wastes.
• Cohesion and adhesion. Water molecules stick to each other (cohesion) and to other surfaces (adhesion). Cohesion lets water be pulled up a plant in continuous columns and produces surface tension.
• Temperature stability. Water absorbs a large amount of heat before its temperature rises, and releases it slowly, so it resists temperature change. This keeps the inside of cells and large bodies of water at stable temperatures, protecting organisms from rapid swings.

Because so much of biology happens in water, the EOC treats these properties as background knowledge for transport, homeostasis, and the reactions of photosynthesis and respiration.

ATP: the energy currency

Calling ATP the "energy currency" captures the idea that it is the form of energy a cell can actually spend: just as money is the currency you exchange for goods, ATP is the currency a cell exchanges for movement, active transport, building molecules, and other work. Glucose stores a lot of energy, but the cell must first convert that energy into ATP (through respiration) before it can use it for these tasks.

How the foundations connect

These two foundations link the whole bioenergetics module. Photosynthesis captures light energy and stores it as glucose; cellular respiration releases the energy from glucose and packages it as ATP; and water is both a reactant and a product in these processes and the medium that carries everything. When you study photosynthesis and respiration, watch for where water appears in the equations and where ATP is made or used.

Try this

Q1. State two properties of water that make it important to living things, and link each to its polarity. [2]

• Cue. Any two of: solvent (polarity dissolves substances), cohesion (polar molecules attract each other), temperature stability (hydrogen bonds absorb heat).

Q2. Describe how ATP releases energy and how it is recharged. [2]

• Cue. ATP releases energy by losing a phosphate to become ADP; it is recharged by adding a phosphate to ADP using energy from respiration.