Most ecosystems rely on energy from sunlight that is harvested through photosynthesis by producer organisms. This chemical energy is then passed on to consumers through food chains and webs. There are losses of energy at each trophic level due to the inefficiencies of energy transfer between organisms and the conversion of useful energy to heat. These energy losses restrict the length and complexity of food chains and limit the biomass that can be supported at higher trophic levels.

1. 4.2. Energy flow
Miltiadis-Spyridon Kitsos
Platon IB Diploma
https://www.learner.org/courses/envsci/visual/img_med/energy_transfer.jpg

2. The official IB Diploma Biology guide
Essential idea: Ecosystems require a continuous supply of energy to fuel life processes and to replace energy lost as heat.
https://ibpublishing.ibo.org/server2/rest/app/tsm.xql?doc=d_4_biolo_gui_14
02_1_e&part=3&chapter=4

3. Most ecosystems rely on a supply of energy from sunlight.
Producers harvest the solar energy through photosynthesis.
http://www.clipartkid.com/images/
201/yellow-sun-icon-by-phidari-
PAjQHk-clipart.png
https://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::535::53
5::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron
%20Transport%20and%20ATP%20Synthesis
Revision photosynthesis:
definition, factors and
processes.
energy loses
http://www.life.illinois.edu/ort/images/MinEloss.jpg

4. Most ecosystems rely on a supply of energy from sunlight.
Heterotrophs use carbon compounds as a source of energy. These carbon compounds derive directly or
indirectly from photosynthesis
Herbivores directly use carbon compounds produced by plants
Carnivores use carbon compounds produced by plants indirectly through
the consumption of herbivores
http://www.theanimalfiles.com/images/carnivore.jpg
https://upload.wikimedia.org/wikipedia/commons/c/cb/White-
tailed_deer_(Odocoileus_virginianus)_grazing_-
_20050809.jpg
Sun
Producers
production of C
compounds
Consumers
Detritrivres Saprotrophs
losses
herbivory predation
What do the arrows indicate?

5. Most ecosystems rely on a supply of energy from sunlight.
The amount of sunlight reaching the surface of the earth varies
From: Photon management for augmented photosynthesis
http://www.nature.com/ncomms/2016/160901/ncomms12699/full/ncomms12699.html
http://66.media.tumblr.com/19db7505e304f11402f46f82e912cbc8/tumblr_nhz2454ngx1t
15h2ho1_1280.jpg
This map demonstrates the distribution of primary
production for the period from 1998 to 2011.
Study the diagram and then outline the most important
findings
https://gmao.gsfc.nasa.gov/researchhighlights/phytoplankton/figure1-628.png
See more at OceanColor WEB
http://oceancolor.gsfc.nasa.gov/cgi/l3

6. Light energy is converted to chemical energy in carbon compounds by photosynthesis.
https://upload.wikimedia.org/wikipedia/commons/thumb/4/49/Thylakoid_membrane_3.svg/2000px-
Thylakoid_membrane_3.svg.png
http://www.shmoop.com/images/biology/biobook_photosyn_3.png
Revision Task: Outline the light-dependent and
light-independent reactions
Producers
increase in organic biomass
solar energy
cellular respiration
Heat
http://www.intechopen.com/source/html/21498/media/image2.jpeg

7. Chemical energy in carbon compounds flows through food chains by means of feeding
Food chains are diagrammatic representations of the feeding relationships between species (“who
eats who”) . Arrows indicate the flow of energy among the species.
Slide from

8. Chemical energy in carbon compounds flows through food chains by means of feeding
Note that the feeding habits of species may change according
to
• season
• developmental stage
• availability of prey
Thus, in reality in all habitats we may study the
food webs, which demonstrate all the feeding
relationships within the habitat (all possible food
chains)
In food webs a species may not belong to only
one trophic level but to different ones
For example see the code in this marine food
web.
The cod is a secondary consumer and a primary
consumer at the same time
http://mdk12.msde.maryland.gov/assessments/high_school/look_like/2003/biology/images/52_q_a.gif

9. Chemical energy in carbon compounds flows through food chains by means of feeding
Food webs vs food chains
Homework:
In the following marine food web:
(a) identify all producers
(b) write all the possible food chains
(c) identify the trophic level of reef sharks, snappers and
groupers

10. Chemical energy in carbon compounds flows through food chains by means of feeding
Food webs vs food chains

11. Chemical energy in carbon compounds flows through food chains by means of feeding
Food webs vs food chains

12. Energy released by respiration is used in living organisms and converted to heat
Ingested food
C compounds
Glycolysis
(energy stored in bonds)
Link reaction
Final oxidation/
oxidative phosphorylation
Krebs cycle
Production of ATP
muscular contraction
(Kinetic energy)
formation of larger
molecules
(energy stored in bonds)
intracellular locomotion

13. Thermodynamics - the study of energy transformation
The Second Law of Thermodynamics - Every energy transfer or transformation increases the entropy of the
universe.
◦ Every system tends to randomness
◦ Energy must be spent to retain order - this spending of energy usually releases heat, which increases the
entropy elsewhere
Endergonic reaction
(requires free energy)
DNA replication
Exergonic reaction
(produces free energy)
e.g. hydrolysis of ATP
Energy coupling
Production of heat,
may not be used again,
increases entropy
Thus, the energy conversion in most metabolic pathways is not efficient and heat is produced.
Heat is not reusable and increases the entropy.
Energy released by respiration is used in living organisms and converted to heat
Living organisms cannot convert heat to other forms of energy.

14. http://coolcosmos.ipac.caltech.edu/image_galleries/ir_zoo/images/gecko_human.jpg
http://coolcosmos.ipac.caltech.edu/image_galleries/ir_zoo/images/scorpion_human.jpg
Heat produced by organisms is used to retain internal temperature
in relation to the external environment (warm- blooded)
Heat is lost from ecosystems
Infrared photos indicate that organisms emit/ radiate heat to their
environment and next to the atmosphere.
It is obvious that ecosystems loose heat
or to increase activity in organisms that do not have this
homeostatic mechanism (cold- blooded). In most cases cold -
blooded animals take on the temperature of their surroundings

15. Energy losses between trophic levels restrict the length of food chains and the biomass of
higher trophic levels.
Skill: Pyramids of energy
Pyramids of energy are graphical representations of the quantitative trophic relations between the
different trophic levels of an ecosystem. It is an estimate of the amount of energy becoming
available to the next trophic level
Slide from

16. Energy losses between trophic levels restrict the length of food chains and the biomass of higher
trophic levels.
Skill: Trophic pyramids
Slide from
1. First draw the trophic level of producers. Write the biomass or energy of this trophic level if
available
Producers
1000 kj m-2 y-1

17. Energy losses between trophic levels restrict the length of food chains and the biomass of higher
trophic levels.
Skill: Trophic pyramids
Slide from
2. Calculate the energy available to the next trophic level (a usual conversion rate is 10% unless
given otherwise)
3. Draw the next trophic level. Differences in the boxes should be proportional to the differences
in the energy content.
Producers
1000 kj m-2 y-1
Primary consumers 100 kj m-2 y-1

18. Energy losses between trophic levels restrict the length of food chains and the biomass of higher
trophic levels.
Skill: Trophic pyramids
Slide from
2. Calculate the energy available to the next trophic level (a usual conversion rate is 10% unless
given otherwise)
3. Draw the next trophic level.Write the name of the trophic level.
Differences in the boxes should be proportional to the differences in the energy content.
Producers
1000 kj m-2 y-1
Primary consumers 100 kj m-2 y-1

19. Energy losses between trophic levels restrict the length of food chains and the biomass of higher
trophic levels.
Skill: Trophic pyramids
Slide from
4. Proceed with the remaining trophic levels
Producers
1000 kj m-2 y-1
Primary consumers 100 kj m-2 y-1
Secondary consumers 10 kj m-2 y-1

20. Energy losses between trophic levels restrict the length of food chains and the biomass of higher
trophic levels.
Outline why energy and biomass reduce moving up the trophic levels.
• Loss of energy as heat during the energy coupling in cellular respiration.
• Organisms are not entirely consumed by their predators (examples here?).
• Presence of indigestible material in ingested food.
Use theories to explain natural phenomena—the concept of energy flow explains the
limited length of food chains.
Most food webs do not have more than three to four different trophic levels. Furthermore, there is
a gradual decrease in biomass moving to higher trophic levels.
These concepts may be explained by applying the concepts of thermodynamics in energy flow in
ecosystems.

21. Energy losses between trophic levels restrict the length of food chains and the biomass of higher
trophic levels.
Now your turn. Time for some thinking…
Read the following article and debate whether
world hunger may end if human switch to
veganism, that is sole consumption of plant-
based foods.
http://eol.org/data_objects/31405326?taxon_updates=true
http://media.eol.org/content/2013/08/24/03/38963_580_360.jpg
Humpback whale trophic strategy. Read about
the feeding strategy of humpback whales and
commend on their trophic level in relation to
their energy requirements.

22. www.stepsinbiology.com