AP Biology: Water & Chemical Elements

The structure of a biological molecule dictates its properties. Differences in structure can include:
- The components from which a molecule is constructed.
- The sequence in which these components appear.
Within water molecules, electrons are shared unequally between the oxygen and hydrogen atoms.
- The oxygen atom acquires a partial negative charge (\delta^{-}).
- The hydrogen atoms acquire a partial positive charge (\delta^{+}).
This unequal distribution of charge renders water a polar molecule.
The polar nature of water facilitates the formation of weak attractive forces known as hydrogen bonds.

Water's polarity and the resulting hydrogen bonds endow it with properties crucial for living systems.
Key properties essential for the survival of organisms include:
- Cohesion
- Adhesion
- High specific heat capacity
- High heat of vaporization
- Low density when frozen
- Excellent solvent capabilities

Definition: Attractive forces between molecules of the same type.
In water, cohesion means that water molecules are attracted to each other.
Importance in living organisms:
- Plant Transport: Water molecules are drawn upwards in the xylem tissue, following other water molecules due to cohesive forces.
  - This mechanism ensures a continuous water supply to plants and enables the transport of dissolved mineral ions.
- Surface Tension: Cohesive forces between water molecules at the surface and those below create surface tension.
  - Surface tension permits small organisms, such as pond skaters (insects), to walk on the surface of ponds and lakes.

Definition: Attractive forces between molecules of a different type.
In water, adhesion means that water molecules are attracted to other types of molecules.
Importance in living organisms:
- Plant Transport: Adhesion between water molecules and the walls of the xylem vessels draws water upwards via capillary action.
- The upward movement of water in plant xylem is a combined result of both cohesion and adhesion.

Property	Description	Importance for Living Organisms
High Specific Heat Capacity	Hydrogen bonds restrict the movement of water molecules, necessitating a significant amount of energy to increase water's temperature.	Water exhibits thermal stability, helping organisms maintain a constant internal temperature.
High Heat of Vaporization	A large amount of energy is required to overcome hydrogen bonds and convert water from its liquid state to a gaseous state.	Processes like sweating in animals and transpiration in plants facilitate cooling through evaporative heat loss.
Low Density When Frozen	When water freezes, hydrogen bonds form an open, crystalline lattice structure, causing ice to be less dense than liquid water, allowing it to float.	Floating ice provides a habitat for certain polar animals and acts as an insulating layer over bodies of water, preventing them from freezing solid and protecting aquatic life below.
Excellent Solvent	Polar water molecules are strongly attracted to charged particles in a solution, enabling ionic compounds (e.g., Sodium Chloride, NaCl) and other polar substances to dissolve readily.	Water serves as a crucial transport medium for substances within organisms (e.g., blood plasma) and provides the aqueous environment necessary for most biochemical reactions to occur.

The tissues of living organisms are composed of numerous macromolecules, including:
- Proteins: Found in structures like cell membranes.
- Nucleic Acids: Such as DNA.
- Carbohydrates: Used both as energy storage compounds and as structural components (e.g., cellulose for building plant cells).

Organisms acquire the necessary raw materials to construct these macromolecules by continuously exchanging matter with their external environment.
This exchange involves fundamental biological processes such as:
- Eating (ingestion)
- Breathing (respiration)
- Excreting (waste removal)

The continuous exchange of materials is vital for organisms to:
- Grow
- Reproduce
- Maintain their complex organization
Examples of essential atoms exchanged with the environment and their roles in building macromolecules include:
- Carbon (C): A fundamental component used to build:
  - Carbohydrates
  - Lipids
  - Proteins
  - Nucleic acids
- Nitrogen (N): Essential for constructing:
  - Proteins (amino acids)
  - Nucleic acids (nitrogenous bases)
- Phosphorus (P): Required for the synthesis of:
  - Nucleic acids (phosphate backbone)
  - Some lipids (e.g., phospholipids in cell membranes)