4.2 Energy Flow

Use theories to explain natural phenomena—the concept of energy flow explains the limited length of food chains.

Energy flow is governed through the laws of thermodynamics.  The first law (and I’m paraphrasing) essentially says that energy cannot be created or destroyed but merely transformed from one form to another.  The second law (the one about entropy) says that energy transfer is never 100% efficient and some energy is always lost as heat, which can not be regained or reused.

Therefore, in a food chain, light energy is transformed into chemical energy by producers and then into additional chemical forms as it passes through to consumers. As energy is being transformed at each trophic level, some of this energy is lost to the system (mostly as heat from respiration, but also in the form of undigested parts, excretion etc).  Thus while the total energy remains the same as the amount put in by the sun,  the amount that is actually available to consumers decreases with each increase in trophic level.  We use the figure of 10-20% as a rough rule of thumb – that is, at each successive trophic level, only 10-20% of the energy from the previous level is available. So 10% of the energy in a producer is available to a primary consumer, but only 10% of this energy is available to a secondary consumer – 1% of the original energy in the producer. Thus the higher the trophic level, the less energy is available and the limited length of most food chains and why many organisms can function at multiple trophic levels.



2.2 Water

Use theories to explain natural phenomena—the theory that hydrogen bonds form between water molecules explains the properties of water.

Theories in biology are explanations for how the natural word works. They can be broad statements that incorporate facts and laws and must be testable through experimentation or observation.  In our everyday conversation we may use theory as a synonym for “educated guess” but in the scientific context theories are far more rigorous and comprehensive.

Consider these observations of water in the lab (you may well have done all of these at some point!):

  1. We can use a pipette to pile drops of water onto a coin.  The water does not spill off immediately but piles higher and higher.
  2. With great care, we can float a paper clip on top of a glass of water. Adding a drop of detergent causes the paperclip to immediately sink.
  3. If we heat and cool samples of ethanol and water, the water heats up more slowly, boils at a higher temperature and cools down more slowly than the ethanol.


Observations in the natural world, such as insects that seem to walk across water or the absorption of water by a plant add to the idea that water appears to be a rather unusual liquid and must have particular properties in order to explain these features.

Based on our observations and experiments, we need to review other scientific theories and ideas to help develop a theory – in this case, atomic theory and the properties and behaviour of electrons.  This then develops into a coherent theory explaining our observations and results as a consequence of hydrogen bonding that takes place between water molecules.

The red lines show the attraction between the electron-rich Oxygen atom and the electron-poor Hydrogen atom (Gould)

We cannot “see” a hydrogen bond and cannot prove absolutely that they exist.  However, the theory of hydrogen bonds and how they function explains all of the above observations and more about the properties of water and has withstood experimental and observational testing.  We can accept this (or any) theory as correct if there is evidence for it, if it has predictive power, if it has not yet been falsified, and if it explains natural processes.



Allott, Andrew, and David Mindorff. Biology: Course Companion. Oxford, Oxford University Press, 2014.

Gould, S.E. “Hydrogen Bonds: Why Life Needs Water.” Scientific American Blog Network, Scientific American, 6 Aug. 2013, blogs.scientificamerican.com/lab-rat/httpblogsscientificamericancomlab-rat20110802hydrogen-bonds-why-life-needs-water/. Accessed 20 Apr. 2017.

Purvis, David. “Water Drops on a Penny.” Dr. Dave’s Science, 2015, drdavesscience.com/free-science-activities/. Accessed 20 Apr. 2017.

VILLANUEVA, A. “Floating Paperclip on Water.” Understanding Biology, Blogspot, 27 Jan. 2010, understanding-biology.blogspot.com/2010/01/floating-paper-clip-cohesion-surface.html. Accessed 20 Apr. 2017.


5.2 Natural selection

Use theories to explain natural phenomena—the theory of evolution by natural selection can explain the development of antibiotic resistance in bacteria. (2.1)

Evolution occurs at both the micro and macro levels.  Macroevolution is the eye-catching form, where we see species changing into dramatically new ones. This process though takes time and is not directly observable. 

Microevolution, while less “glamorous” is no less interesting. Indeed, it has applications that are amongst the most serious concerns in health, medicine and agriculture. This is the ability of populations of bacteria, protists, fungi, insects or plants to evolve resistance to antibiotics, drugs, pesticides and other chemicals used to control them. 

The resistance of bacteria to antibiotics has occurred at an incredible rate, as the image below shows:

Image from CDC.gov

What is particularly concerning about this is shown in the following graph – the number of antibiotics being developed approved continues to decline, which leaves fewer options for treatment.

Understanding the process of evolution is critical to estimating the number and type of new drugs that are needed to combat them.  It is thus necessary to understand that antibiotic use represents a very strong selection pressure. Given the reproductive potential of bacteria (more offspring are born than can survive) and the variation that is possible (through both mutation and horizontal gene transfer) it should therefore come as no surprise that populations rapidly evolve resistance.  Evolution and natural selection are thus not the dated musings of a 19th-century naturalist, but of critical importance to health problems of the 21st-century: in the US alone, over 2 million illnesses and 23,000 deaths per year are directly attributed to evolved resistance.

From an assessment perspective, antibiotic resistance in bacteria is a great example to use when responding to and extended response question on evolution/natural selection.


“About Antimicrobial Resistance | Antibiotic/Antimicrobial Resistance | CDC “. Centers for Disease Control and Prevention. Cdc.gov., 2016. Web. 11 Dec. 2016.

“Microevolution”. Understanding Evolution. University of California Museum of Paleontology.Evolution.berkeley.edu. 2016. Web. 11 Dec. 2016.

9.4 Reproduction in plants

Paradigm shift—more than 85% of the world’s 250,000 species of flowering plant depend on pollinators for reproduction. This knowledge has led to protecting entire ecosystems rather than individual species.

Albert Einstein is claimed to have said: “If the bee disappears from the surface of the earth, man would have no more than four years to live.” (Quote Investigator). While it appears he did not actually ever say this, it is a remarkably prescient observation about the role that pollinators play in continuing the survival of plants.

While wind, water and explosive propulsion do work for some flowering species, the majority rely on animal pollinators. The co-evolution of pollinators with the plants they pollinate means that, in many cases, species may be pollinated by only a select few animals.  Should the animals decline in population, so will the plants.

A recent example in New Zealand reminds us of this (Biello). The endemic flowering shrub Rhabdothamnus solandri, or New Zealand gloxinia, relies primarily on the bellbird (Anothornis melanura) and stitchbird (Notiomystis cincta) to pollinate its flowers.  These birds have long beaks and tongues to access the long, tubular flowers of the shrub.  However, the bellbird and stitchbird have recently become extinct on New Zealand’s North island. To investigate this impact on the flower, researchers conducted a study on three smaller offshore islands, where the birds were still present. The results were alarming – in the absence of the two birds on the North island, just 22% of of flowers produced fruit and had only 37 seeds per flower. This compares to the the islands that still have the birds, where they produced 232 seeds per flower and 58% produced fruit. In order to save this flower, we must also save the birds.


New Zealand Gloxinia – Rhabdothamnus solandri. 


A paradigm shift represents a radical change in thinking based on new evidence.  Understanding that protecting birds, for example, will also protect plants, is an important change in thinking from purely looking at the conservation of single species. Conservation methods that focus on ecosystems as a holistic unit reflect our increased understanding of the way animals and plants in particular are inter-related.


Biello, David. “For Want Of A Pollinator, A Flower May Be Lost–Or A Forest”. Scientific American. N. p., 2016. Web. 13 Oct. 2016.

“If The Bee Disappeared Off The Face Of The Earth, Man Would Only Have Four Years Left To Live | Quote Investigator”. Quoteinvestigator.com. N. p., 2013. Web. 13 Oct. 2016.

“T.E.R:R.A.I.N – Taranaki Educational Resource: Research, Analysis And Information Network – Rhabdothamnus Solandri (Taurepo) “. Terrain.net.nz. N. p., 2016. Web. 14 Oct. 2016.

8.2 Cell Respiration

Paradigm shift—the chemiosmotic theory led to a paradigm shift in the field of bioenergetics.

Paradigm shifts are something we see from time to time in biology but we also discuss them in TOK – they represent a way of looking at a problem from a completely new angle (van de Lagemaat).  In doing so, paradigm shifts can often be controversial and may take several years, or even decades, before they are accepted by the scientific community.

In 1961 Peter Mitchell proposed the chemiosmotic coupling theory to account for the production of ATP in oxidative phosphorylation. This theory went against the prevailing view that there were “energy-rich” chemical intermediates that explained the resulting ATP formation. As he writes in his landmark publication, from 1966,

“the study of the question of the coupling mechanism has continued to be ruled by the well-trodden and familiar tenets of the chemical coupling conception, no matter how fantastic the resulting tissue of hypothesis.” (Mitchell,1507)

Terms like “well-trodden” and “familiar” refer to the accepted theory that, despite results to the contrary, remains the only accepted explanation.  A paradigm shift must counter such ingrained views, which is why it can take time for the new explanation to become accepted.  Mitchell was awarded the Nobel Prize in Chemistry in 1978.  Part of his acceptance speech illustrates the challenges for the scientist trying to overturn entrenched theory:

 “…the originator of a theory may have a very lonely time, especially if his colleagues find his views of nature unfamiliar, and difficult to appreciate.” (“Peter Mitchell – Banquet Speech”)

There are many examples of paradigm shifts in biology – they make useful reference points for TOK discussion and analysis. Key terms to consider include bias, justification, subjective, objective and verification.


Mitchell, Peter. “Chemiosmotic coupling in oxidative and photosynthetic phosphorylation”.  Biochimica et Biophysica Acta (BBA) – Bioenergetics, Volume 1807, Issue 12, December 2011, Pages 1507-1538. Web. 19 April, 2016.

“Peter Mitchell – Banquet Speech”. Nobelprize.org. Nobel Media AB 2014. Web. 19 Apr 2016.

van de Lagemaat, R. Theory of Knowledge for the IB Diploma. Cambridge, Cambridge University Press. 2011. Print.