Fifth lecture

Energy Flow

• Energy flows through ecosystems, while matter cycles within them.

• Resources critical to human survival and welfare, ranging from the food we eat to the oxygen we breathe, are products of ecosystem processes.

Energy Flow• Energy in ecosystems follows the rules of

thermodynamics– Ecosystems are open systems where energy is

transformed, and is ultimately released as heat from organisms.

– Heat lost from organisms is disordered, unusable energy and therefore has increased entropy of the system.

– Energy transfers between organisms are not completely efficient

– Without energy input (sun) ecosystems collapse

Photosynthesis

• Is the process that captures light energy and transforms into the chemical energy of carbohydrates– Energy stored in chemical bonds

• Occurs in:– Plants– Algae– Certain Bacteria

– 1. Pigments capture energy from sunlight

– Water is split, O2 released

– 2. Using energy to make ATP and NADPH

– 3. Using ATP and NADPH to power the synthesis of carbohydrates from CO2

Light-dependent reactions

Light-independent reactions

The Calvin cycle

6 CO2

carbondioxide

+ 12 H2Owater

+ Light energy C6H12O6

glucose

+ 6 O2

oxygen

+ 6 H2Owater

Building New Molecules

• Light-dependent reactions provide the raw material– Conversion of light energy into immediate source of

chemical energy• ATP

• NADPH

• Light independent reactions: – Energy stored during light reactions used to drive the

Calvin Cycle (C3 photosynthesis)

– assembles the new molecules– It takes place in the stroma of the chloroplast

Chemiosmosis in a chloroplast

Thylakoidspace

Photosystem II ATP synthaseElectron transport system

Photon

H2O

½O2

e–

H+2

H+

H+

H+

H+

H+

H+

H+

ADP ATPMembrane is impermeable

to protons

Primary productivity

• Gross Primary Productivity (GPP): – total amount of photosynthetic energy captured in a

given period of time.

• Net Primary Productivity (NPP): – the amount of plant biomass (energy) after cell

respiration has occurred in plant tissues.

NPP = GPP – Plant respirationplant growth/ total photosynthesis/ unit area/ unit area/unit timeunit time

Secondary Productivity

• Secondary productivity: the rate at which consumers convert the chemical energy of the food they eat into their own new biomass

The Laws of Thermodynamics• A set of universal laws governing all energy

changes in the universe

• The First Law thermodynamics:

“Energy cannot be created or destroyed it can only change forms”.

– During each conversion, some energy is lost into the environment as heat energy.

The second Law of Thermodynamics:

“as energy changes form it becomes more disorganized. i.e., ENTROPY increases”

Biomass

• The total mass of the organic matter at each trophic level is called biomass.

• Biomass is just another term for potential energy (energy that is to be eaten and used).

• The transfer of energy from one level to another is very inefficient (10% Law).

Ecological Pyramid

• An ecological pyramid shows the relationship between consumers and producers at different trophic levels in an ecosystem.

• It shows the relative amounts of energy or matter contained at each trophic level.

• It shows which level has the most energy and the highest number of organisms.

Pyramid of number

Fairly large animals

Pyramid of energy

Pyramid of biomass

Found in larger numbers, but still contain 90% less energy

Inverted pyramid

Exceptions to the rule?

• Carnivorous plants capture and digest animal prey

• They are able to grow without animal prey.

• ~600 sp. of carnivorous plants have been described