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.

 

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4.1 Species, Communities and Ecosystems

Looking for patterns, trends and discrepancies—plants and algae are mostly autotrophic but some are not. 

This is another NOS post that deals with the never-ending exceptions that arise whenever we seem to encounter a biological rule!

Back in Topic 5.3 (Classification) we learn that Kingdom Plantae is defined as containing organisms that are multicellular, photosynthetic and eukaryotic. Photosynthetic cells implies that the organism is autotrophic, but it turns out that this is not always the case!

The obvious exception would appear to be the carnivorous plants, such as the Venus Fly Trap (Dionaea muscipula) or the pitcher plant (such as Cephalotus sp.).

Cephalotus follicularis – the Albany Pitcher Plant

Venus Flytrap (Dionaea muscipula)

However, these plants retain their photosynthetic abilities and can even be raised successfully without providing any insects for them to eat (Rice). According to Rice (2007), this makes them only partially heterotrophic, as they still derive a large proportion of their energy from photosynthesis.

The true heterotrophic plants are those that are parasitic or saproptrophic.  A great example of a parasitic plant is Rafflesia arnoldii, the plant that produces the largest flower in the world.  It is a parasite of certain jungle vines and produces no roots, shoots or stem.  In fact, it is only visible when it flowers.  It gains all its nutrition through its parasitism of the host plant.

Similarly, there is a bewildering diversity amongst algae of organisms that are facultative and obligate heterotrophs. Many of these species have, like Rafflesia, become parasites, such as the genera Helicosporidium and Prototheca. Others, such as the fascinating genus Polytomella, have four flagella and are motile heterotrophs.

Polytomella

As always, biology refuses to be put into a box!

Sources:

Davis, Troy. “ Fully Open Flower of Rafflesia Arnoldii.” Rafflesia Arnoldii Robert Brown, Southern Illinois University, 29 Oct. 2010, parasiticplants.siu.edu/Rafflesiaceae/Raff.arn.page.html.

Figueroa-Martinez, F., Nedelcu, A. M., Smith, D. R. and Reyes-Prieto, A. (2015), When the lights go out: the evolutionary fate of free-living colorless green algae. New Phytol, 206: 972–982. doi:10.1111/nph.13279

“Polytomella.” Polytomella, EOL, http://www.eol.org/pages/90494/details.

Rice, Barry. “Are Carnivorous Plants Autotrophic or Heterotrophic?” The Carnivorous Plant FAQ: Autotrophic or Heterotrophic?, Jan. 2007, http://www.sarracenia.com/faq/faq1100.html.

“The Albany Pitcher Plant.” Cephalotus Follicularis – the Albany Pitcher Plant, Botanical Society of America, botany.org/Carnivorous_Plants/Cephalotus.php.

“Venus Fly Traps (Dionaea Muscipula).” The Mysterious Venus Flytrap, Botanical Society of America, botany.org/bsa/misc/carn.html.

Content, Practicals and the Nature of Science

As I have mentioned at various times on this blog, I think one of the challenges with the new syllabus is the idea that the NOS represents an “add-on” that will somehow impact teaching and learning.  Some of them certainly are new concepts and content, but some are also linked directly to either content, lab-work or both.  In these cases, making the connections is easy and can help reinforce what the students are already learning.  Some examples that would work here include (IBO,2014).:

1.4 Membrane transport Experimental design—accurate quantitative measurement in osmosis experiments are essential. (3.1)
This links to Practical 2:  Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions. 

2.5 Enzymes Experimental design—accurate, quantitative measurements in enzyme experiments require replicates to ensure reliability. (3.2)
This links to Practical 3 – Experimental investigation of a factor affecting enzyme activity.

2.9 Photosynthesis Experimental design—controlling relevant variables in photosynthesis experiments is essential. (3.1)

4.3 Carbon cycling Making accurate, quantitative measurements—it is important to obtain reliable data on the concentration of carbon dioxide and methane in the atmosphere. (3.1)
See my post on Carbon Database Analysis

6.1 Digestion and absorption Use models as representations of the real world—dialysis tubing can be used to model absorption in the intestine. (1.10)

9.1 Transport in the xylem of plants Use models as representations of the real world—mechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure to plant tissues.
This links to Practical 7 – Measurement of transpiration rates using potometers.

With a bit of planning, a single lesson can combine content, practical work and the Nature of Science.  Further, linking NOS to an experiment can help reinforce understanding in a much more effective way. As you begin to work towards the IA, these then provide additional inspiration for students to develop their own investigations.

In terms of preparing for examinations, students should draw on their experience with these practical experiments. This looks especially important for the new Paper 3, which includes a Section A with unseen data based on the core/AHL syllabus, but could also be important on the other papers as well.

I could envisage the following sorts of short-answer questions (to be clear, I have made these up myself!):

• Outline the use of models in:
  • measuring transpiration in plants
  • showing how absorption in the small intestine works
• Explain the need to control variables when designing experiments to measure photosynthesis
• Outline the importance of collecting adequate quantitative data when conducting osmosis experiments/measuring the rate of reaction in enzyme experiments.
• Explain the importance of quantitative data in providing evidence to support climate change

Remember to look back over your experimental notebooks or old lab reports here – this does not require so much in terms of memorisation of facts but rather the process and justification of experimental procedures.

Biology Guide: First Assessment 2016. Cardiff: IBO, 2014. Print.

4.3 Carbon Cycling and 4.4 Climate Change

Making accurate, quantitative measurements—it is important to obtain reliable data on the concentration of carbon dioxide and methane in the atmosphere. (IBO, 63) 

Assessing claims—assessment of the claims that human activities are producing climate change. (IBO, 65)

Climate change continues to be the subject of much debate in certain sections of the media and the world, despite the proliferation of evidence that supports both rapid climate change and the human role in driving it.  As these two NOS states, and as we learn in TOK, claims must be assessed and their evidence evaluated to determine their truth.

The best place to begin is to explore the website for the Intergovernmental Panel on Climate Change (IPCC), the global body that synthesises the climate research and produces reports at intervals of 5-6 years.  The Fifth Assessment Report was published between September 2013 and November 2014. The reports are provided in PDF and can be daunting!  However, the Summary for Policymakers  provides a nice overview of the main results of the review, including details on greenhouse gas emissions, temperature change, ocean acidification, snow and ice cover and changes in animal and plant behaviour and life-cycles.

A good activity for the students is adapted from Stephen Taylor’s page at i-biology.  Students can access a range of different databases from the CDIAC to examine carbon emissions.  I have the students collect data for both the last 5 years and for the entirety of the database they have chosen.  The longer the database, the more clearly the trend is displayed.  Students can then practice their graphing and analysis skills using spreadsheets.  With the new IA Guidelines allowing for database analysis, this could be a good starting point for an investigation.

Student Task Sheet

We finished our sequence of classes with a great discussion on the precautionary principle.  This is no longer explicitly required in the new syllabus but has great links to TOK and also to the idea of verifying data. We used the Visible Thinking Truth Routine – Claim, Support, Question– to examine two different articles with an opposing view of the PP. We ended up debating what certainty level is required for proof in the natural sciences – a nice end to the topic.

Source:

Biology Guide: First Assessment 2016. Cardiff: IBO, 2014. Print.

“Carbon Cycle.” BioNinja, 2017, http://www.ib.bioninja.com.au/standard-level/topic-4-ecology/43-carbon-cycling/carbon-cycle.html.