2027 · High School Specialised Subject · Biochemistry

2.5 The Fitness of the Aqueous Environment

Thermal buffering, density anomaly, cohesion and availability

Why water makes life possible: high heat capacity and heat of vaporisation, the density anomaly that lets lakes stratify safely, cohesion and long-distance transport, osmotic balance, water activity, dissolved oxygen and salinity.

10 SECTIONS17 FIGURESEnglish edition

01

The conditions life requires from water

A liquid range that spans biologically relevant temperatures
Ability to dissolve ions and polar molecules while excluding nonpolar ones
Sufficient availability and stability over geological time

Figure 2-5-1The conditions life requires from water — reference figure

Table 2.5-1Water properties linked to living phenomena

Table 2.5-4Water-related terms and what to watch for

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01

The conditions life requires from water

A liquid range that spans biologically relevant temperatures
Ability to dissolve ions and polar molecules while excluding nonpolar ones
Sufficient availability and stability over geological time

Figure 2-5-1The conditions life requires from water — reference figure

02

High heat capacity and heat of vaporisation buffer organisms and environments

Water absorbs large amounts of heat with a small temperature change
Evaporative cooling removes heat efficiently through sweating and transpiration
Large water bodies moderate regional climate

Figure 2-5-2The conditions life requires from water — reference figure

Table 2.5-2Representative values used in heat calculations

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02

High heat capacity and heat of vaporisation buffer organisms and environments

Water absorbs large amounts of heat with a small temperature change
Evaporative cooling removes heat efficiently through sweating and transpiration
Large water bodies moderate regional climate

Figure 2-5-2The conditions life requires from water — reference figure

03

The density anomaly protects aquatic life in winter

Ice is less dense than liquid water and floats
The 4 °C maximum density means the bottom of a lake stays liquid
Stratification slows heat loss and allows survival below the ice

Figure 2-5-3High heat capacity and heat of vaporisation buffer organisms and environments — reference figure

04

Cohesion and adhesion support long-distance transport

Hydrogen bonds create high surface tension and tensile strength
Cohesion allows water columns to be pulled through xylem under tension
Capillary action combines adhesion to walls with cohesion within the column

Figure 2-5-4High heat capacity and heat of vaporisation buffer organisms and environments — reference figure

05

Cells balance water across membranes

Osmosis moves water toward the higher solute concentration
Animal cells regulate volume by pumping ions; plant cells rely on the cell wall and turgor
Aquaporins increase water permeability without passing ions

Figure 2-5-5The density anomaly protects aquatic life in winter — reference figure

Table 2.5-3Components of water potential

06

Water availability is measured as water activity

Water activity reflects the fraction of water available for reactions
High solute concentrations lower water activity and limit microbial growth
Preservation by salting or sugaring works through reduced water activity

Figure 2-5-6The density anomaly protects aquatic life in winter — reference figure

07

Temperature and dissolved oxygen constrain aquatic life

Warm water holds less oxygen than cold water
Thermal stratification can isolate a hypoxic layer
Metabolic demand rises with temperature while supply falls

Figure 2-5-7Cohesion and adhesion support long-distance transport — reference figure

Table 2.5-5Approximate oxygen saturation in fresh water at sea-level pressure

08

Salinity defines both the advantages and limits of water

Osmotic stress requires osmoregulation in marine and freshwater organisms
Halophiles accumulate compatible solutes to balance external salt
Salinity shapes species distribution in estuaries and coastal waters

Figure 2-5-8Cells balance water across membranes — reference figure

09

Is liquid water a sufficient condition for life?

Liquid water appears necessary for known life, but necessity is not sufficiency
Energy sources, essential elements and stable conditions are also required
The question motivates the search for habitable environments beyond Earth

Figure 2-5-9Cells balance water across membranes — reference figure

10

Unit synthesis: from molecules to ecosystems

Molecular properties scale up to cellular, organismal and ecological consequences
The same hydrogen bond explains protein folding, transpiration and lake stratification
Reading a property at one level and predicting another is the recurring method of biochemistry

Figure 2-5-10Water availability is measured as water activity — reference figure

Table 2.5-6How unit 2 connects

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10

Unit synthesis: from molecules to ecosystems

Molecular properties scale up to cellular, organismal and ecological consequences
The same hydrogen bond explains protein folding, transpiration and lake stratification
Reading a property at one level and predicting another is the recurring method of biochemistry

Figure 2-5-10Water availability is measured as water activity — reference figure

References

**Table 2.5-1**Water properties linked to living phenomena
Property	Molecular basis	Biological meaning	Limit or caution
High specific heat	Energy exchange in the hydrogen-bond network	Damping of temperature change	Does not prevent extreme heat input
High heat of vaporisation	Energy needed for the liquid–gas transition	Sweating and transpirational cooling	Involves water loss
Cohesion and adhesion	Intermolecular forces and surface interaction	Transport and surface tension	Tall trees need transpirational pull
Low density of the solid	Open crystal structure of ice	Buoyancy of ice; surface freezing	Salinity and mixing alter freezing
Polarity and solvent behaviour	Hydration and ion dispersal	Transport of reactants, ions and metabolites	Excess water causes osmotic stress

**Table 2.5-2**Representative values used in heat calculations
Quantity	Representative value	Condition
Specific heat of water	≈4.18 J/(g·K)	Liquid water near room temperature
Latent heat of vaporisation	≈2.4 kJ/g	Approximate value near room temperature
Energy example	192 kJ	Ideal case: 80 g of water fully evaporated

**Table 2.5-3**Components of water potential
Term	Meaning	Typical sign
Ψs	Solute potential: lowered by the presence of solute	Usually zero or negative
Ψp	Pressure potential: turgor positive, tension may be negative	Positive or negative
Ψg	Gravitational potential with height	Depends on the reference point
Ψm	Interaction of water with walls and soil particles	Usually zero or negative

**Table 2.5-4**Water-related terms and what to watch for
Term	Meaning	Caution in class
Water content	Mass fraction of water in a material	Different from water activity
Water activity aw	State variable defined from the vapour-pressure ratio	Range 0–1; depends on temperature
Osmotic stress	Impaired function from differences in water movement	Consider solute permeability
Compatible solute	Accumulated solute that preserves protein function	Not every species uses the same strategy

**Table 2.5-5**Approximate oxygen saturation in fresh water at sea-level pressure
Temperature	Representative saturated DO (mg/L)
0 °C	14.6
5 °C	12.8
10 °C	11.3
15 °C	10.1
20 °C	9.1
25 °C	8.3
30 °C	7.6

**Table 2.5-6**How unit 2 connects
Phenomenon	Core concept	Linked section
Body and environmental temperature	Specific heat, heat of vaporisation, heat balance	2.1 Hydrogen bonding
Water movement across cells	Osmosis, water potential, aquaporins	2.1 Colligative properties
Solute and enzyme function	Hydration, pH, water activity	2.2–2.3 Acid–base and buffering
Water as a participant in reactions	Hydrolysis, water oxidation	2.4 Water as a reactant
Maintenance of aquatic ecosystems	Density, dissolved oxygen, stratification, salinity	2.5 Fitness of the aqueous environment

The conditions life requires from water — reference figure — Figure 2-5-1The conditions life requires from water — reference figure

The conditions life requires from water — reference figure — Figure 2-5-1The conditions life requires from water — reference figure

The conditions life requires from water — reference figure — Figure 2-5-2The conditions life requires from water — reference figure

The conditions life requires from water — reference figure — Figure 2-5-2The conditions life requires from water — reference figure

High heat capacity and heat of vaporisation buffer organisms and environments — reference figure — Figure 2-5-3High heat capacity and heat of vaporisation buffer organisms and environments — reference figure

High heat capacity and heat of vaporisation buffer organisms and environments — reference figure — Figure 2-5-4High heat capacity and heat of vaporisation buffer organisms and environments — reference figure

The density anomaly protects aquatic life in winter — reference figure — Figure 2-5-5The density anomaly protects aquatic life in winter — reference figure

The density anomaly protects aquatic life in winter — reference figure — Figure 2-5-6The density anomaly protects aquatic life in winter — reference figure

Cohesion and adhesion support long-distance transport — reference figure — Figure 2-5-7Cohesion and adhesion support long-distance transport — reference figure

Cells balance water across membranes — reference figure — Figure 2-5-8Cells balance water across membranes — reference figure

Cells balance water across membranes — reference figure — Figure 2-5-9Cells balance water across membranes — reference figure

Water availability is measured as water activity — reference figure — Figure 2-5-10Water availability is measured as water activity — reference figure

Water availability is measured as water activity — reference figure — Figure 2-5-10Water availability is measured as water activity — reference figure