There are four basic steps to the aerobic Cycle:

1. Glycolysis - the splitting of glycogen/glucose into a smaller sugar called pyruvate. (Glycolysis itself yields two ATP's and two NADPH's)

2. Pyruvate is joined to coenzyme A to form a complex called Acetyl coenzyme A.

3. Citric Acid (Krebs) Cycle --> acetyl coA is partially broken down to yield water and carbon dioxide, releasing energy which is packaged as ATP.

4. Electron Transport Chain --> excited electrons/protons are passed along a series of enzymes; the energy released at each transfer is packaged as ATP.

(Steps 3 and 4 yield 32 ATPs via chemiosmosis)

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WHY DO PLANTS MAKE SUGAR?

FOR YOU?

Yeah. Right.

NO. Plants make sugar for their own use! As they collect solar energy and store it as sugar, they are also *burning* that sugar and using the energy to run their own chemical reactions.

Fortunately for the consumers (heterotrophs), plants usually have some energy left over after photosynthesis. This is what becomes the BIOMASS (dry weight) of the plant, and it's what the heterotrophs eat, stealing the plant's hard-won energy! Energy and matter cycle among the organisms via photosynthesis, celllular respiration and other processes.

All organisms interact with one another in ECOSYSTEMS (the living and non-living components of a habitat, considered collectively) and one of the most fundamental ways in which they interact is by eating and being eaten.

Different levels of feeding, or TROPHIC LEVELS ("trophism" - from the Greek word troph, meaning "to eat") are named on the basis of how many levels they are from the first level, plants

PRIMARY PRODUCERS - Organisms that can perform photosynthesis, harnessing the energy of sunlight by placing it into the chemical bonds of sugar, which they manufacture from water (from the soil and surroundings) and carbon dioxide (from the atmosphere).

PRIMARY CONSUMERS (1^o) - (first trophic level) - Organisms that feed on primary producers. Also called herbivores, frugivores (fruit eaters), seed eaters, etc.

SECONDARY CONSUMERS (2 ^o) - (second trophic level) - Organisms that feed on primary consumers. These are carnivores, insectivores, etc.

TERTIARY CONSUMERS (3 ^o) - (third trophic level) - You guessed it.

QUATERNARY CONSUMERS (4 ^o) - (fourth trophic level) You guessed it again.

Special note should be taken of

DECOMPOSERS - digest organic molecules and break them down into their inorganic components. Depending on what dead thing they're eating, their trophic levels can be just about anywhere above primary producer.

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As you might guess, most organisms eat more than one kind of food, and a particular species isn't always eating at the same trophic level (Can you think of an example of this, using yourself?).

This means that the trophic levels do not form a straight line, or "chain" from one level to the next. Rather, they interlace to form more of a WEB. Ecologists call these complex feeding relationships the FOOD WEB, and every ecosystem is characterized by a specific type of food web.

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PRODUCTIVITY OF ECOSYSTEMS

The total amount of light energy converted by producers into chemical energy (organic molecules, such as sugars) is known as GROSS PRIMARY PRODUCTION (GPP).

Not all of this is stored as plant biomass, of course. What's left over after the plants have used the sugars they've made for themselves is called NET PRIMARY PRODUCTION (NPP). This can be calculated by subtracting the energy required for cellular respiration from the GPP.

NPP = GPP - respiration

The productivity of various ecosystems can be calculated by measuring the biomass (dry weight) of vegetation per unit area per unit of time. There's a tremendous amount of variation in productivity among the various terrestrial and aquatic ecosystems, as shown HERE

Now we get back to that pesky Second Law of Thermodynamics. Because unfortunately for us consumers, energy transfers between trophic levels are NOT 100% efficient. In fact, about 90% of the energy is LOST AS ENTROPY when transferred to the next higher trophic level! (This varies among ecosystems, with some being more efficient or less efficient. The 90% loss per trophic level is a rough average).

In other wordsŠ It takes 1,000,000 Joules (a Joule (J) is a unit of work or energy in the International System (SI) of Units; it is equal to the work done by a force of one newton acting through one meter. Ah, heck. Don't worry about it. Take Physics 101.) of sunlight for a plant to store 10,000J worth of biomass.

If grasshoppers (primary consumers) eat those 10,000J of plant biomass, they will convert it into only 1000J of grasshopper biomass.

If field mice eat 1000J of grasshopper biomass, they will convert it into only 100J of field mouse biomass.

If foxes eat 100J of field mouse biomass, they will be able to convert only 10J into fox biomass.

...and so on!

It's this type of PRODUCTIVITY PYRAMID that shows us why environmentalists urge us to "eat low on the food chain!" The higher you go in trophic levels, the more of the energy captured at lower trophic levels is wasted (lost as entropy). The pyramid in the diagram is an idealized one, but it does show the general pattern of energy loss as one goes higher up the trophic levels. It also is a reflection of why there are, in most ecosystems, far greater producer biomass than primary consumer biomass, and far greater secondary consumer biomass than primary consumer biomass, and so on. (Predators will never outnumber prey for long!)

To make 1 kg of human biomass, it takes approximately 10kg of grain. But if you eat beef, it takes 10kg of beef to make 1 kg of human, and 100kg of grain to make that 10kg of beef! So if you skip the cow step, you can feed more people!