Water as a Chemical Component of the Cell

Water as the main chemical component

Content standard 4.1 opens the study of chemical components of the cell with water, which makes up the largest proportion of both cell and body mass. Its molecular structure, a slightly negative oxygen and slightly positive hydrogens, makes it a polar molecule, which is the basis for nearly all of its important properties.

The proportion of water differs between body parts: blood plasma is almost entirely water, most soft tissues contain a majority of water, while bone and fat store far less. This difference reflects how metabolically active and how well hydrated a tissue needs to be.

Water is not only a passive medium either, it is a direct reactant in the hydrolysis reactions that digest carbohydrates, proteins and lipids, and a product of the condensation reactions that build these same molecules, as well as a reactant in photosynthesis and a product of aerobic respiration.

Properties of water and their biological importance

Roles of water inside a living organism

• Acts as a medium in which metabolic reactions, such as enzyme reactions, take place.
• Acts as a transport medium, for example, plasma in blood and cell sap in the vacuole.
• Provides support through turgor pressure in plant cells, keeping non-woody parts upright.
• Regulates body and leaf temperature through sweating and transpiration.
• Acts as a reactant in the hydrolysis reactions that break down carbohydrates, proteins and lipids during digestion, and is released as a product of the condensation reactions that build these molecules.
• Acts as a lubricant, for example, in the fluid that reduces friction between bones at a joint and in the film of moisture that keeps mucous membranes and the surface of the eye moist.

How it is examined

Exam questions commonly ask you to state a property of water and link it to a named biological function, for example explaining why water's high specific heat capacity helps stabilise body temperature, or why cohesion between water molecules allows continuous water transport in the xylem. You may also need to explain why water is described as the medium for metabolic reactions.

You may also be given a list of properties of water and asked to match each one to its correct biological function, or to compare the water content of two named tissues and explain the difference in terms of metabolic activity. Questions on transpiration or osmosis often expect you to state the specific property of water, cohesion, adhesion or its role as a solvent, responsible for the observation described.

Worked exam-style question

Question. The table below compares the temperature rise of two liquids, water and cooking oil, after being heated at the same rate for five minutes. Water: temperature rose from 28°C to 33°C.

Cooking oil: temperature rose from 28°C to 45°C. (a) Name the property of water shown by this result.

(b) Explain, using this property, how water helps to keep the internal environment of a cell stable. (c) State one other property of water and explain how it allows water to be pulled up a tall tree in the xylem.

Model answer. (a) High specific heat capacity. (b) Because water needs to absorb a large amount of heat energy before its temperature rises even a small amount, it resists sudden changes in temperature; this keeps the internal environment of the cell relatively stable so that enzymes are not denatured by a sudden rise in temperature and metabolic reactions continue at a steady rate.

(c) Cohesion, water molecules are attracted to one another through hydrogen bonding, so they stay linked together as a continuous, unbroken column inside the narrow xylem vessels; this cohesion, together with adhesion to the walls of the xylem, allows the whole column of water to be pulled upward as water is lost by transpiration from the leaves.

Practice question

Try this. A doctor advises a patient who is sweating heavily during a fever to drink plenty of water. (a) Name the property of water that allows sweating to cool the body.

(b) Explain, in terms of this property, why sweating lowers body temperature. (c) Suggest one reason why the patient is advised to replace the water lost through sweating.

Exam tip

Key terms

• Carbohydrate, a molecule built partly through condensation reactions that release water.
• Protein, a molecule built from amino acids joined by peptide bonds through condensation.
• Lipid, an energy-storage molecule formed from glycerol and fatty acids by condensation.
• DNA, the genetic molecule whose structure depends on hydrogen bonding, the same bonding responsible for water's solvent action, cohesion and high specific heat capacity.
• Osmosis, the movement of water itself across a partially permeable membrane, from a region of higher to lower water potential.

Source:SRC-DSKP-EN

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