Cycles
• Life is composed of cycles - cycles of energy, cycles of materials, cycles of information
(reproduction)
• Life is maintained by a constant flow of chemical energy
• Energy means motion - kinetic energy is energy of motion
> Light
> Heat
> Electricity
> Moving objects
• Energy can also be stored - potential energy
> Water behind a dam
> Batteries
> Gasoline
> Firewood
> Stretched rubber bands
• Gasoline has potential energy, stored in the chemical bonds between its component atoms
• Burn it, releases kinetic energy - heat, light, motion - that was stored in the molecules
• Some released energy powers engine, much escapes as heat energy, radiates away
• First Law of Thermodynamics - energy can neither be created nor destroyed, but it can be
changed from one form into another
• Energy enters ecosystems as solar energy, (kinetic) transformed by photosynthesis into
chemical energy (potential - chemical bonds in glucose)
• Energy transformation never 100% efficient
• Much energy lost as heat, radiates away
• Conversion of auto gas (chemical energy) to moving car parts (mechanical energy) is about
20-30% efficient
• Cellular metabolism about 50% efficient
• Second Law of Thermodynamics - universe tends to become more disordered - over time,
energy becomes scattered or dispersed
• Entropy - universal tendency towards disorder
• Life is a highly ordered state, seems to defy entropy
• Eventually the Second Law predicts less energy locked up in stable bonds in molecules and
more scattered and dispersed
• Heat death of the universe predicted by cosmologists
• Sun pays the entropic price for all of us
• Sun is exploding, consuming itself, generates tremendous amount of scattered energy
• Earth intercepts a tiny piece of the energy released by the sun’s thermonuclear explosions -
enough to cause local reversals in entropy
• Life is a local reversal of entropy
• Photosynthesis converts solar energy into chemical energy
• How efficient is photosynthesis??
• Only 2% efficiency
• Photosynthesis combines carbon dioxide, water, sunlight, to form glucose (simple sugar)
• Photosynthesis converts solar energy into the chemical bond energy that holds the glucose
molecule together
• What is a chemical bond?
• Remember that atoms are made up of a nucleus with protons (+ charge) and neutrons (no
charge), surrounded by cloud of electrons (- charge)
• Atom as a whole is electrically neutral - no net + or - charge
• Visualize a whirling cloud of atoms and molecules - atoms are always in motion
• Constantly bumping into one another
• Mostly just bounce away, bump into another molecule - but not always
• Energy of motion is kinetic energy
• Moving atoms have kinetic energy - temperature = average kinetic energy
• Atoms or molecules bump into one another with sufficient speed, lots of kinetic energy
• May overcome the natural repulsion of their electron clouds, cause them to bond together
• Some of the energy of their collision is stored as potential energy in the chemical bond they
form
• When atoms bond together, one or more of the electrons in their outer shell is exchanged or
becomes shared
> One atom acquires net positive charge (loses one or more electron - oxidation)
> Other atom acquires net negative charge (gains one or more electrons - reduction)
• Electrically charged atoms (or groups of atoms) are called ions
• Ions of opposite charge will stick together (opposite charges attract, like charges repel)
• Chemical bonds are formed by exchanging or sharing of electrons between atoms
• Chemical bonds are a type of potential energy
• Can later release that stored energy by breaking the chemical bond
• In photosynthesis, the captured chemical energy comes from sunlight
• Light has kinetic energy
• Light strikes the outer electrons of the chlorophyll molecule, “excites” its electrons
• Electrons release the energy they absorbed as they go back to usual level of energy
• Cells capture that tiny bit of released energy, use it to:
> Break water into hydrogen and oxygen
> Combine the hydrogen with carbon dioxide to form glucose (sugar)
• Energy stored in glucose powers the cell
• As glucose is burned, the energy released by the chemical bonds is transferred to ADP
(adenosine diphosphate)
• The released energy goes into forming a new high energy bond to add a third phosphate to
ADP = ATP (triphosphate)
• Cells can then break that bond at will, changing ATP back to ADP, and using the energy
released by the broken bond to power the cell
• When used for “fuel”, a single glucose molecule makes 36 ATP “power packs”
• Glucose also used as a building block to make smaller molecules like amino acids and nucleic
acids
• These in turn form DNA, proteins
• Ecosystems are composed of organisms that produce their own energy (autotrophs) and
organisms that use the energy made by others (heterotrophs)
• Autotroph = producers, “self-feeder” - autotrophic organisms produce their own energy
(photosynthesis, chemosynthesis)
• 6CO2 + 12H2O + light => C6H12O6 + 6H2O + 6O2
• Heterotrophs = consumers, fed by others, eat other organisms to survive
• Heterotrophs burn (oxidize) glucose (and other compounds) to recover the energy stored in
the chemical bonds (respiration)
• C6H12O6 + 6O2 + 6H2O => 6CO2 + 12H2O + energy
• There are over 100 different kinds of atoms in nature, but only ~20 found in cells, and only a
few of these are common
• Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorous
• Where did these elements come from?
• Atoms which make up living things were all formed from simpler atoms in the heart of a
vanished star
• Younger suns made of hydrogen - nuclear fusion creates the higher elements, from helium, on
to carbon, oxygen etc…
• When aging suns explode (nova), they disperse atoms of higher elements which go into the
next generation of stars
• We are all made of stardust
• Lets look at the way these important elements cycle through natural systems
• Start with the water cycle
• Water is the essence of life
> Living things made mainly of water
> Water is an essential ingredient in photosynthesis
> Chemical and physical properties of water make life possible
• Water has a high specific heat - it requires a huge amount of energy to heat water, and it gives
off much energy when it cools
• Water expands just before the freezing point - so ice is lighter than water, floats to the top -
makes aquatic life possible
• Rain falls from the sky
• Some rain enters surface water as runoff
• Some percolates down through the soil, until it reaches a layer it can’t penetrate
• Water under the ground is called groundwater
• Top of this layer of groundwater is the water table
• Porous layer of saturated rock in which water moves (or pools up) is an aquifer
• Groundwater eventually finds its way towards the surface (springs, wells…)
• Part of the rain that enters the soil is captured by plant roots
• Pulled up through the plant by capillary action (wet paper towel)
• Water exits the leaves as water vapor - transpiration
• Some water held by capillary action in the soil evaporates directly from the soil
• Combination of evaporation from the soil and transpiration from plants called
evapotranspiration
• Surface waters also evaporate, return water vapor to the atmosphere
• Water vapor in the atmosphere condenses into clouds, falls back as rain
• Carbon is another element with unusual properties that make it highly suitable for life
• Bonds to itself very firmly, forms long chains that are the backbone of most biomolecules
(C/H/O)
• Carbon enters living systems through photosynthesis
• Atmospheric CO2 fixed into glucose
• CO2 exhaled by plants and animals in respiration (break glucose down into CO2, oxygen and
water)
• Lots of carbon in plant bodies as glucose, or starch, or other biomolecules made from glucose
• Some carbon locked up as dead tissue, finds its way into the soil when organisms decompose
• Photosynthesis is also important in aquatic habitats
• Atmospheric CO2 dissolves in water to make bicarbonate ions (HCO3-)
• Carbon used by many sea creatures to make shells of calcium carbonate (Ca CO3 )
• Deep part of the carbon cycle
> Marine shells pool in bottom sediments
> Form limestone and marble
> Return to the surface over millions of years through volcanic eruptions (CO2 ) and
weathering of rocks
• Deep part of the carbon cycle
> Organic sediments converted over millions of years into coal, oil, natural gas (fossil fuels)
> Carbon returned to the atmosphere when these fuels are burned by autos, industry etc…
> Renewable resource, but only on a geological time scale!!
• Nitrogen cycle is more complex
• Nitrogen and phosphorous are essential for all organisms, especially plants
• Nitrogen is a limiting nutrient
• But our atmosphere is 78% Nitrogen
• ???
• Nitrogen in the air is N2 - plants can’t use it directly in this form
• Some bacteria can use N2 , convert to form plants can use = nitrogen fixation
• Plants take up nitrogen as ammonium (NH4+) or nitrate (NO3-) ions
• Some bacteria make N2 into ammonium (nitrogen fixing bacteria)
• Some bacteria convert ammonium in soil to nitrate (nitrifying bacteria)
• Lightning also causes nitrogen fixation
• Humans also fix nitrogen (fertilizers)
• Humans now dominate global cycling of nitrogen!!
• Plants take up nitrogen as ammonium or nitrate, incorporate it into biomolecules
• Nitrogen of plants consumed by herbivores, carnivores
• Animals excrete large amounts of nitrogen as ammonia, urea, uric acid - recycled to the soil
when they die
• If soils become anaerobic, soil bacteria start using the oxygen locked up in nitrates, release N2
back to the air
• Farmers must plow ASAP in spring to aerate the soil before this happens, or soil loses some
fertility
• Phosphorous is another essential nutrient, commonly found in mineral form as phosphate
(PO4-3)
• Plants get phosphate from soil or water, goes into compounds like ATP
• Some phosphate excreted by animals
• Some phosphate returned to soil when organisms decompose
• Unlike water, carbon, and nitrogen - phosphorous doesn’t exist as a gas
• When nitrogen or carbon enter the air or water, they can be transported anywhere on Earth
• When phosphorous is added, it stays put
• When phosphorous is removed, it does not readily return (ex. tropical deforestation)
• Phosphorous stays in or on the ground
• Deep part of the phosphorous cycle
> Finds its way into many rocks or minerals
> Released millennia later via weathering or mining (Polk County, Fla.)
• Energy and matter flow through ecosystems in many complex and interdependent ways
• Cyclic processes regulated by feedback
• Humans have altered these natural cycles in many ways - changing amount and timing of
inputs or out puts, sources and sinks…