A1.1 Bio

Water is

the medium in which most biological processes occur.

A medium is

the substance or environment in which something happens.

metabolism

Turning food into energy for growth, repair and to eliminate waste

Why is water necessary?

1. Origin of the first cells

2. Metabolic reactions

3. Cell membranes

types of metabolism

Anabolism

Catabolism

Anabolism

Synthesis of complex molecules from simpler molecules

Catabolism

Breakdown of complex to simpler molecules

1. Origin of the first cells

• The first cells developed in an aquatic environment.

• Water allowed naturally occurring biological molecules to interact.

• These interactions helped molecules grow, combine and copy themselves.

2. Metabolic reactions

• Cells contain a watery fluid called the cytoplasm.

• Substrates (reactants) and enzymes are usually dissolved in this fluid.

• This allows them to interact so metabolic reactions can occur.

Cell membranes

• Cell membranes function in an aqueous environment.

• They separate the inside of a cell from its surroundings.

• Water allows the movement and interaction of substances around cells.

Oxygen and hydrogen are connected through

Covalent bond

Covalent bond

Atoms share electrons. non metal to non metal

a strong chemical bond where two atoms share one or more pairs of valence electrons to achieve a stable electronic configuration

Ionic bond

formed through the electrostatic attraction between oppositely charged ions created by the complete transfer of electrons from a metal to a non-metal

Electronegativity

the ability of an atom's nucleus to attract shared electrons in a covalent bond.

2 factors of electronegativity

Factor	Effect
Number of protons	More protons generally increase attraction to electrons.
Number of electron shells	More shells increase distance and shielding, reducing attraction to shared electrons.

Oxygen has a stronger attraction to shared electrons than hydrogen.

Therefore:

1. Oxygen pulls the shared electrons closer to itself.

2. The electrons spend more time near oxygen.

3. Oxygen develops a partial negative charge.

4. Hydrogen develops a partial positive charge.

This creates a polar covalent bond.

Polarity

an uneven distribution of electrical charge.

dipole

An electric dipole is a pair of equal and opposite electric charges separated by a small distance.

Polar covalent bond

Within one water molecule

Shares electrons

Relatively strong

Shown as a solid line

Hydrogen bonds

Between water molecules

Attraction between partial charges

Relatively weak

Shown as a dashed line

Cohesion

The attraction between molecules of the same substance.

Water has strong cohesion because

hydrogen bonds attract neighbouring water molecules. thats why a very light object like a paper clip can stay on surface of water.

• Hydrogen bonds form between water molecules.

• These attractions pull water molecules together.

Surface tension

the resistance of a liquid surface to being increased or broken.

ex. like a paper clip on water or water striders

Why does it surface tension happen?

1. Water molecules at the surface are attracted to neighbouring water molecules.

2. These hydrogen bonds create a cohesive force.

3. This makes the surface resist disruption.

Important application: Detergents and pollutants can

reduce surface tension, making it harder for water striders to remain on the surface.

vascular tissue

Xylem

is vascular tissue in plants that transports water and dissolved mineral ions from the roots towards the leaves.

How water is transported to leaves? also known as cohesion-tension mechanism

• Water evaporates from the moist surfaces of cells inside the leaves.

• Water vapour diffuses out through the stomata. This is called transpiration.

• As water evaporates, it creates tension (negative pressure) in the remaining water.

• This tension pulls neighbouring water molecules upwards through the xylem.

• Cohesion between water molecules maintains a continuous column of water.

• Water is therefore pulled from the roots towards the leaves, against gravity.

What do hydrogen bonds do in plant?

Hydrogen bonds cause cohesion, and cohesion allows water to form a continuous column that can be pulled upwards through the xylem.

Without cohesion, the water column would be much more likely to break.

Transpiration

Loss of water vapour from aerial parts of plants, mainly through stomata

Tension

Pulling force created by water loss

Continuous water column

Connected column of water molecules in xylem

Adhesion

The attraction between molecules of different substances.

How adhesion work?

• Water has partial positive and negative charges.

• Polar or charged surfaces have electrical charges.

• Opposite charges attract.

• Therefore, water molecules are attracted to these surfaces.

Capillary action

is the movement of water through narrow spaces or tubes due to cohesion and adhesion.

How does Capillary Action work

• Water molecules are attracted to the polar surface of a narrow tube. This is adhesion.

• Other water molecules are attracted to the water molecules already touching the surface. This is cohesion.

• Together, these forces allow water to move upwards through narrow spaces.

• Water can move against gravity.

Capillary action occurs when

adhesion to the surface is sufficiently strong relative to cohesion.

2 examples of capillary action

Example 1: Soil

• Water adheres to soil particles.

• Cohesion helps neighbouring water molecules move with it.

• Water moves through tiny spaces in the soil.

• This helps maintain moisture around plant roots.

Example 2: Plant cell walls

• Plant cell walls contain cellulose fibres.

• Water adheres to the hydrophilic cellulose.

• Capillary action draws water into spaces between the fibres.

• This helps replenish water lost through evaporation from leaf cells.

A meniscus

is the curved surface of a liquid in a container.

Water forms a concave meniscus in glass because

adhesion between water and glass is stronger than cohesion between water molecules at the surface.

Solvent

A substance that dissolves another substance.

Hydrophilic substances

are attracted to water and are generally soluble in water.

They are usually:

• Polar.

• Ionic.

• Charged.

examples of hydrophilic

Substance	Why it dissolves
Glucose	Has polar regions
Sucrose	Has many polar hydroxyl groups
Sodium chloride	Contains oppositely charged ions
Amino acids	Have charged or polar groups

Salt contains

• Sodium ions (Na⁺).

• Chloride ions (Cl⁻).

• The partially negative oxygen faces the positive sodium ions.

• The partially positive hydrogen faces the negative chloride ions.

• Water molecules surround and separate the ions.

• The ions become dispersed throughout the solution.

Hydrophobic substances

not attracted to water and are generally insoluble in water.

They are usually non-polar.

Examples:

• Oils.

• Fats.

• Lipids.

Cell membranes

• Cell membranes contain phospholipids.

• Their hydrophobic regions face inwards, away from the watery environments.

• This creates a barrier between the inside and outside of the cell.

Aquaporins

Aquaporins are membrane proteins that allow water to move efficiently(rapidly) across cell membranes.

• Their hydrophobic exterior interacts with the hydrophobic interior of the membrane.

• Their hydrophilic interior forms a channel through which water can pass.

This is a perfect example of how both hydrophilic and hydrophobic properties are important for cell function.

Phloem

• Transports dissolved organic substances.

• Mainly transports sucrose.

• Also transports amino acids and other metabolites.

Catabolic

Catabolic reactions: Water can act as a reactant in hydrolysis, helping break chemical bonds.

Anabolic reactions

Water can be released as a product in condensation reactions, when molecules join together.

Extraplanetary(outside earth) origin of water

The hypothesis:

1. Water-rich asteroids collided with the early Earth.

2. These collisions delivered water to Earth.

3. Over many collisions, significant quantities of water accumulated.

4. As Earth cooled, the water could condense into liquid form.

Why was Earth able to retain water?

Gravity

• Earth has sufficient gravitational force because of its mass.

• Gravity attracts and retains water on Earth's surface.

• This prevents water from easily escaping into space.

2. Moderate temperatures

• As Earth cooled, water vapour could condense into liquid water.

• Earth's temperatures allow liquid water to exist.

• Water does not continuously boil away or remain permanently frozen.

Earth retained water because its gravitational force prevented water from easily escaping into space, while moderate temperatures allowed water vapour to condense and remain as liquid water.

Why do scientists search for water when looking for life?

Because all known life depends on water as a medium for biological chemical reactions.

The Goldilocks zone (habitable zone)

the region around a star where temperatures may allow liquid water to exist on a planet's surface.

• Too close to the star = potentially too hot, causing water to evaporate.

• Too far from the star = potentially too cold, causing water to freeze.

• Just the right distance = temperatures may allow liquid water.

Being in the Goldilocks zone does NOT guarantee that a planet contains life.

Other factors also matter, including atmospheric conditions and whether liquid water is actually present.