A1.1: water
In the Water unit students are introduced to the structure and function of water, as the medium of life. Water has many useful properties, and so it is ubiquitous in life on earth. The useful properties of water arise from its structure.
The unit is planned for:
The unit is planned for:
- 2 hrs SL
- 1 hr HL
Guiding questions
- What physical and chemical properties of water make it essential for life?
- What are the challenges and opportunities of water as a habitat?
Understanding:
A1.1.1—Water as the medium for life
- Define what is meant by "water as the medium for life"
- Explain why water is essential for biochemical reactions.
- Origin of life in water:
- Early Earth had abundant liquid water, providing the medium where organic molecules could interact.
- Water’s ability to dissolve various substances made it an ideal environment for chemical reactions leading to life.
- Medium for biological processes:
- Cellular processes such as nutrient transport, waste removal, and enzyme activity occur in aqueous solutions.
- Example:
- Cytoplasm is primarily water, enabling efficient biochemical reactions.
- In photosynthesis, water provides electrons during the light-dependent reactions.
- Significance:
- Without water, the transport of gases, ions, and nutrients would not occur efficiently, halting life processes.
A1.1.2—Hydrogen bonds as a consequence of the polar covalent bonds within water molecules
- Draw two water molecules, including partial charges and hydrogen bonds.
- Explain the importance of hydrogen bonds in maintaining the properties of water.
- State why water is a polar molecule.
image from http://www.whatischemistry.unina.it/
Polarity of water molecules:
- Oxygen's high electronegativity attracts shared electrons more strongly than hydrogen, creating a partial negative charge (δ⁻) on oxygen and a partial positive charge (δ⁺) on hydrogen.
- Weak bonds form between the δ⁺ hydrogen of one molecule and the δ⁻ oxygen of another, resulting in cohesion and unique physical properties.
- Water’s high boiling point compared to similar molecules like H₂S is due to extensive hydrogen bonding.
- Hydrogen bonding affects water's thermal properties, surface tension, and solvent abilities, which are critical for life.
A1.1.3—Cohesion of water molecules due to hydrogen bonding and consequences for organisms
- Define cohesion and provide an example of its importance in plants.
- Explain how surface tension supports life in aquatic habitats.
- Describe the role of cohesion in xylem transport.
Cohesion is the attraction between water molecules due to hydrogen bonding, which creates a network of interconnected molecules.
Consequences for organisms:
Consequences for organisms:
- Transport in xylem: Cohesion allows water to be pulled up tall plants as a continuous column under tension.
- Surface tension: Water forms a “skin” on its surface, enabling light organisms like water striders to move across without sinking.
- Dewdrops remain spherical because of cohesion.
- Redwood trees rely on cohesion to transport water from roots to leaves, overcoming gravitational forces.
A1.1.4—Adhesion of water to materials that are polar or charged and impacts for organisms
- Define adhesion and describe one example of its biological importance.
- Explain how adhesion and cohesion work together in capillary action.
- Compare the roles of adhesion and cohesion in water transport in plants.
https://bio.libretexts.org/
Adhesion refers to water’s attraction to polar or charged surfaces, enabling it to stick to other materials.
Impacts for organisms:
Capillary action:
Impacts for organisms:
Capillary action:
- Water climbs up narrow spaces in soil or xylem due to adhesion and cohesion working together.
- Adhesion to soil particles helps water reach plant roots.
- Adhesion allows water to stick to the walls of xylem vessels, assisting its upward movement in plants.
- In soil, adhesion helps distribute water to root surfaces.
A1.1.5—Solvent properties of water linked to its role as a medium for metabolism and for transport in
plants and animals
plants and animals
- Define the term "universal solvent" and explain why water is described this way.
- Outline the role of water as a solvent in plants and animals.
- Explain how water’s solvent properties support enzymatic reactions.
Universal solvent:
- Water’s polarity allows it to dissolve ionic compounds (e.g., NaCl) and polar molecules (e.g., glucose).
- Many biochemical reactions occur in aqueous solutions, where water acts as a solvent.
- Example: Water dissolves salts, sugars, and gases like CO₂ and O₂ for transport in blood and plants.
- Dissolved nutrients and ions are transported efficiently in plants and animals.
- Example: Xylem transports dissolved minerals in plants; blood plasma transports glucose in humans.
- Molecules like lipids do not dissolve in water but play essential roles, such as forming cell membranes.
A1.1.6—Physical properties of water and the consequences for animals in aquatic habitats
- Compare the thermal properties of water and air. (3 marks)
Explain the significance of water’s high specific heat capacity for aquatic life. (3 marks)
Describe how buoyancy affects aquatic organisms.
Buoyancy:
- Water provides support to aquatic organisms, reducing energy expenditure.
- Water’s moderate viscosity allows streamlined movement of aquatic organisms.
- High specific heat capacity stabilizes aquatic environments by absorbing heat without significant temperature changes.
- Example: Marine ecosystems remain relatively stable even during temperature fluctuations.
- Water is denser and more thermally conductive than air, affecting insulation strategies in aquatic animals.
- Example: Blubber in ringed seals (Pusa hispida) provides insulation in water.
Note: When referring to an organism in an examination, either the common name or the scientific
name is acceptable.
name is acceptable.
image from tes.com
hl only
A1.1.7—Extraplanetary origin of water on Earth and reasons for its retention
- Outline the hypotheses for the origin of water on Earth.
- Explain how Earth’s position in the solar system contributed to water retention.
- Discuss the role of asteroid impacts in delivering water to Earth.
Hypotheses for Water's Origin:
- Asteroids and Comets:
- During Earth’s early formation, water-rich asteroids and icy comets bombarded the planet.
- Evidence: Analysis of isotopic ratios (deuterium to hydrogen, D/H) in Earth’s water and water from asteroids shows similarities, suggesting a common source.
- Volcanic Outgassing:
- Early volcanic activity released water vapor from Earth's interior, which condensed to form oceans.
- This hypothesis is supported by studies of volcanic gases today, which still release water vapor.
- Solar Nebula:
- Water molecules may have formed in the protoplanetary disk around the young Sun and were incorporated into Earth’s building blocks.
Retention of Water:
Examples for Context:
- Earth’s size and gravity:
- Unlike smaller planets like Mars or Mercury, Earth’s gravity was strong enough to prevent water molecules from escaping into space.
- Magnetic field:
- Protected the planet’s atmosphere, including water vapor, from being stripped away by solar winds.
- Distance from the Sun:
- Earth is within the "Goldilocks zone," where temperatures allowed water to exist in liquid form without boiling off or freezing completely.
Examples for Context:
- The D/H ratio of water in meteorites from the asteroid belt closely matches Earth’s water.
- Venus, despite being similar in size to Earth, lost most of its water due to its proximity to the Sun and the lack of a magnetic field.
A1.1.8—Relationship between the search for extraterrestrial life and the presence of water
- Define the "Goldilocks Zone" and its importance in the search for extraterrestrial life.
- Explain why the presence of liquid water is a critical factor in evaluating a planet’s habitability.
- Discuss the potential for life on Europa or Enceladus based on their water evidence.
Goldilocks Zone:
- A region around a star where conditions are "just right" for liquid water to exist, crucial for life as we know it.
- Too close: Water boils away due to high temperatures (e.g., Mercury, Venus).
- Too far: Water freezes into ice (e.g., outer planets like Jupiter and beyond).
- Example: Earth’s average distance from the Sun (1 AU) places it in the habitable zone.
Water as a Key to Life:
- Unique Properties:
- Solvent for biochemical reactions.
- Medium for nutrient transport in cells.
- Stability of liquid water allows long-term chemical evolution.
- Extraterrestrial Water:
- Evidence of water increases the potential for extraterrestrial life:
- Mars: Ancient riverbeds and subsurface ice suggest liquid water existed in the past.
- Europa (moon of Jupiter): Subsurface oceans under an icy crust could harbor microbial life.
- Enceladus (moon of Saturn): Geysers ejecting water vapor suggest the presence of liquid water beneath its surface.
- Missions like NASA’s Perseverance Rover (Mars) and ESA’s JUICE Mission (Jupiter’s moons) aim to detect water or signs of life.
- Techniques:
- Spectroscopy: Identifies water molecules in a planet’s atmosphere.
- Probing subsurface ice and liquid layers for microbial signatures.
- The presence of liquid water is a strong indicator of a planet's or moon's habitability.
- Understanding Earth’s water history helps scientists narrow down the search for life elsewhere.
- Examples for Context:
- Mars: Findings from rovers like Curiosity and Perseverance suggest ancient river valleys and deltas, indicating the planet once had liquid water on its surface.
- Europa: Evidence of a global subsurface ocean beneath a thick layer of ice raises the possibility of hydrothermal vents, similar to those on Earth where life thrives.
Key Terms:
|
polarity
thermal metabolic reaction coolant vaporization |
covalent
polar ionic ion cation |
anion
hydrogen bond polymer carbon hydrogen |
condensation
hydrolysis hydrophobic hydrophilic adhesion |
density
capillary action solvent surface tension |
Classroom Material:
Powerpoint by Luke Templer
| A1.1 Water Presentation.pptx |
Correct use of terminology is a key skill in Biology. It is essential to use key terms correctly when communicating your understanding, particularly in assessments. Use the quizlet flashcards or other tools such as learn, scatter, space race, speller and test to help you master the vocabulary.
Useful Links:
Water
Animated Bonds
Hydrogen Bonds and Water
Structure of Water
The Chemistry of Water
Phases of Water
Water properties and behavior
Salt Dissolving in Water
Water balloons in space
Water properties and behavior
Simple H-bonds from John Gianni
H-bonds in water, from Northland College
Properties of water, from Northland College
Molecule polarity simulation from PhET Labs (allow Java to run)
Solubility in water simulation from PhET Labs (allow Java to run)
PhET Labs: Salts and Solubility and also Dissolving Sugars and Salts
Water
Animated Bonds
Hydrogen Bonds and Water
Structure of Water
The Chemistry of Water
Phases of Water
Water properties and behavior
Salt Dissolving in Water
Water balloons in space
Water properties and behavior
Simple H-bonds from John Gianni
H-bonds in water, from Northland College
Properties of water, from Northland College
Molecule polarity simulation from PhET Labs (allow Java to run)
Solubility in water simulation from PhET Labs (allow Java to run)
PhET Labs: Salts and Solubility and also Dissolving Sugars and Salts
In the News:
International-mindedness:
- There are challenges for the increasing human population in sharing water resources equitably for drinking and irrigation, electricity generation and a range of industrial and domestic processes.
TOK
- Claims about the “memory of water” have been categorized as pseudoscientific. What are the criteria that can be used to distinguish scientific claims from pseudoscientific claims?
Video Clips
Hank teaches us why water is one of the most fascinating and important substances in the universe.
In this video Paul Andersen explains how the polarity of water makes life on the planet possible. Oxygen is highly electronegative and pulls the electrons closely creating a partial negative charge. The polarity of water (and the corresponding hydrogen bonds) create cohesion, adhesion, capillary action, high specific heat, and a universally good solvent.
Water is essentially everywhere in our world, and the average human is composed of between 55 and 60% water. So what role does water play in our bodies, and how much do we actually need to drink to stay healthy? Mia Nacamulli details the health benefits of hydration
How does water behave in zero gravity? Find out at the international space station (this is really good):
