Comprehensive Study Guide on the Water and Carbon Cycles

Elemental Composition and the Importance of Water

• Oxygen is the most abundant element in the human body, a status primarily attributed to its presence in water ($H_2O$). Water is a compound of critical importance to all living organisms.

• The percentage of total body weight accounted for by water is significant across different life forms:

  • Up to $60\%$ of the weight of the human body is water.

  • Up to $90\%$ of plant weight is composed of water.

• The elemental makeup of the human body by total body weight is distributed as follows:

  • Oxygen: $65.0\%$

  • Carbon: $18.5\%$

  • Hydrogen: $9.5\%$

  • Nitrogen: $3.2\%$

  • Calcium: $1.5\%$

  • Phosphorus: $1.0\%$

  • Potassium: $0.4\%$

  • Sodium: $0.2\%$

  • Chlorine: $0.2\%$

  • Magnesium: $0.1\%$

  • Sulfur: $0.04\%$

• Four primary elements—Oxygen, Carbon, Hydrogen, and Nitrogen—constitute $96.2\%$ of the total human body weight.

• Carbon is categorized as the "backbone" of the macromolecules that comprise living organisms. This is illustrated by the molecular structure of glucose ($C_6H_{12}O_6$), which relies on a carbon-based framework.

Cellular Macromolecules and Their Functions

• Cellular macromolecules are categorized into four main types, each with specific elemental compositions and physiological roles based on the dry mass of a cell:

• Proteins

  • Primary Elements: Carbon, Hydrogen, Oxygen, Nitrogen ($C-H-O-N$).

  • Average Percent of Dry Cell Mass: $50\%$

  • Function: Proteins, specifically enzymes, are responsible for carrying out almost all chemical reactions within cells.

• Nucleic Acids

  • Primary Elements: Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus ($C-H-O-N-P$).

  • Average Percent of Dry Cell Mass: $18\%$

  • Function: Deoxyribonucleic acid (DNA) serves as the storage for the genetic code of life.

• Carbohydrates

  • Primary Elements: Carbon, Hydrogen, Oxygen ($C-H-O$).

  • Average Percent of Dry Cell Mass: $15\%$

  • Function: Cellulose provides structural support in plants. Glucose ($C_6H_{12}O_6$) acts as a readily accessible source of cellular energy for both plants and animals.

• Lipids

  • Primary Elements: Carbon, Hydrogen, Oxygen, Phosphorus ($C-H-O-P$).

  • Average Percent of Dry Cell Mass: $10\%$

  • Function: Fats serve as an accessible fuel source. Phospholipids are essential components of cellular membranes.

The Water Cycle: Key Transformations and Processes

• The movement of water through ecosystems is tracked through several specific processes, primarily following the transformation of oxygen atoms:

• Evaporation

  • Mechanism: Liquid water ($H_2O$) from oceans, lakes, and rivers is released into the atmosphere as water vapor (gas).

  • Impact: This process is responsible for $90\%$ of the water vapor found in the atmosphere.

• Transpiration

  • Mechanism: Liquid water within plants is released into the atmosphere as vapor, primarily through plant leaves.

  • Impact: This process accounts for the remaining $10\%$ of atmospheric water vapor.

• Condensation

  • Mechanism: As water vapor rises into the air, cooler temperatures cause it to transform into liquid droplets and ice crystals, which aggregate to form clouds.

• Precipitation

  • Mechanism: Air currents move clouds globally. Cloud particles collide and grow, eventually falling to the surface as liquid (rain) or solid (snow/ice) precipitation.

• Infiltration

  • Mechanism: Liquid or frozen precipitation seeps into the soil.

  • Outcome: This water may be absorbed by plant roots or percolate into deeper ground layers to join the groundwater system, which serves as a source of drinking water for humans.

• Surface Runoff

  • Mechanism: Precipitation that does not infiltrate the soil flows over the Earth's surface toward oceans, lakes, and rivers.

  • Nutrient Transport: As water travels, it picks up essential nutrients from the soil, such as nitrogen ($N$) and phosphates ($P$).

The Carbon Cycle: Transformations and Storage

• Carbon cycles through various forms, moving between the atmosphere, living organisms, and the Earth's crust:

• Photosynthesis

  • Reactants: Carbon dioxide ($CO_2$) and water ($H_2O$).

  • Products: Glucose ($C_6H_{12}O_6$) and oxygen ($O_2$).

  • Function: This is the primary process for energy production in plant cells.

• Cellular Respiration

  • Reactants: Glucose ($C_6H_{12}O_6$) and oxygen ($O_2$).

  • Products: Carbon dioxide ($CO_2$), water ($H_2O$), and adenosine triphosphate (ATP).

  • Function: Used by aerobic organisms (such as humans and other mammals) to generate cellular energy.

• Assimilation

  • Mechanism: Organisms break down consumed glucose and other materials to serve as building blocks for their own macromolecules.

• Deposition

  • Mechanism: Organic matter from deceased organisms decays, releasing carbon into the ground.

  • Long-term Storage: Over millions of years, large accumulations of this organic material can transform into fossil fuels (coal, oil, and natural gas).

• Sedimentation

  • Mechanism: Atmospheric $CO_2$ dissolves in the ocean and is converted into calcium carbonate ($CaCO_3$) by aquatic organisms like corals and oysters for shell production.

  • Geological Formation: When these organisms die, their remains settle into ocean sediments, eventually forming limestone.

• Combustion

  • Mechanism: The burning of fossil fuels or organic matter (such as forests) releases $CO_2$ gas back into the atmosphere.

Interactions Between the Water and Carbon Cycles

• The cycles are interconnected through atmospheric and chemical processes:

  • Carbonic Acid Formation: As precipitation falls, $CO_2$ gas dissolves in rainwater. This chemical reaction produces a weak carbonic acid ($H_2CO_3$), which contributes to the chemical weathering of rocks.

  • Nutrient Cycling: The water cycle facilitates the movement of carbon-based organic matter and other nutrients (nitrogen and phosphorus) through runoff and infiltration, linking hydrological movement to biological availability.

Questions & Discussion

1. Explain why over 96% of the human body is made up of only four elements. Be sure to use evidence, including data from the figure and table, to support your answers.

• The four elements—Oxygen ($65.0\%$), Carbon ($18.5\%$), Hydrogen ($9.5\%$), and Nitrogen ($3.2\%$)—total $96.2\%$ of body weight because they are the primary components of water and cellular macromolecules. Oxygen and hydrogen predominate due to water making up $60\%$ of body weight. Furthermore, all four major macromolecules (Proteins, Nucleic Acids, Carbohydrates, and Lipids) rely on these elements: Proteins and Nucleic Acids utilize all four ($C-H-O-N$), while Carbohydrates and Lipids utilize three ($C-H-O$). Given that proteins alone make up $50\%$ of the dry mass of a cell, the concentration of these specific elements is necessary for structural and functional biological processes.

2. What three elements are found in all macromolecules?

• Carbon ($C$), Hydrogen ($H$), and Oxygen ($O$) are present in all four types of macromolecules (Proteins, Nucleic Acids, Carbohydrates, and Lipids).

3. Phosphorus represents only 1% of human body weight. However, it is a building block of very important macromolecules. Describe some of the cellular functions that would not be possible without phosphorus.

• Without phosphorus, the storage of genetic information would be impossible because phosphorus is a required element in Nucleic Acids (DNA). Additionally, cellular boundaries could not be maintained as Lipids (specifically phospholipids) require phosphorus to form cellular membranes. Energy storage and transfer within the cell would also fail because phosphorus is a key component of ATP produced during cellular respiration.**