Homeostasis & Water

Homeostasis

• The ability of the body to keep its internal environment stable.

• Thousands of chemical reactions occur in a precise rhythm, even when the environment changes.

→ Ex: constant body temperature, heart rate, O₂ levels, water levels, etc.

Importance of Water

• Vital for human beings - makes up ~ 60% of our mass.

• Most important characteristic: polar molecule.

• Electrons are shared unequally between the oxygen atom and the hydrogen atoms.

• Oxygen is more “electron-greedy”

  → Draw electrons closer to the O-side of the molecule than the H-side.

  • O-side gains a slightly negative charge

  • H-end gains a slightly positive charge

  → O-end attracts H-end through a hydrogen bond, which pulls molecules together.

  • These bonds alter how molecules behave.

• Polar → Has several characteristics

High molecular cohesion

• Cohesion: Molecules stick to themselves.

  → Creates surface tension.

• Surface tension: a property allowing liquids to resist external force.

• Insects can walk on water, and this is why water “beads up”.

H-bonding between water molecules

• Solid ice is less dense than liquid water!

• Water expands when it freezes and becomes less dense when frozen.

• Thus, lakes and rivers don’t freeze from the bottom up but instead have a layer of insulating ice on the top.

Water is a lubricant (= move easily)

H₂O molecules cohere under pressure in joints.

Water has polar adhesion to other polar molecules

• Adhesion: water molecules stick to other substances.

• Can, therefore, spread easily.

• H₂O molecules move through tiny spaces by adhering to polar surfaces and by cohering to each other.

  • causes “capillary action”: the ability of a liquid to flow upward in a narrow space without the help of external forces like gravity.

    → Ex: up plant stems and roots

Water has a high specific heat capacity

• Specific heat capacity = the amount of energy needed to raise the temperature of something

  → Ex: Water takes more energy to boil as compared to oil, because H-bonds between water molecules have to be broken.

• Waters warms (or cools) slowly → Allows for a stable body temperature.

• Many reactions occur at a certain rate depending on temperature.

Water is an excellent solvent

• Allows chemicals to move within organisms.

• Polar molecules help attract positive and negative ions and polar molecules.

• Very common on Earth.