Biology • Year 11 • Module 2 • Lesson 6
Autotrophs vs Heterotrophs
Apply autotroph and heterotroph concepts to real CO2 data, organism-classification scenarios, and a diagram critique.
1. Interpret CO2 concentration data over 24 hours
A sealed transparent chamber contains a healthy potted plant. CO2 concentration (in ppm) inside the chamber is measured automatically every two hours over a 24-hour period. The data are recorded below. Light is present from 06:00 to 18:00. 8 marks
| Time | CO2 (ppm) | Light condition |
|---|---|---|
| 00:00 | 610 | Dark |
| 02:00 | 618 | Dark |
| 04:00 | 626 | Dark |
| 06:00 | 634 | Light begins |
| 08:00 | 580 | Light |
| 10:00 | 492 | Light |
| 12:00 | 388 | Light (brightest) |
| 14:00 | 310 | Light |
| 16:00 | 290 | Light |
| 18:00 | 296 | Light ends |
| 20:00 | 340 | Dark |
| 22:00 | 398 | Dark |
| 24:00 | 460 | Dark |
1.1 Describe the trend in CO2 concentration during the light period (06:00–18:00) and the dark period (18:00–24:00). 2 marks
1.2 Explain, using lesson concepts, why CO2 decreases during daylight even though the plant is also respiring during that time. 2 marks
1.3 Between 20:00 and 24:00, CO2 increases from 340 ppm to 460 ppm. Identify the process responsible and explain what this data confirms about autotroph metabolism. 2 marks
1.4 Predict how the CO2 data would differ if the plant were replaced with an animal of similar mass. Justify your prediction. 2 marks
2. Interpret graph, net CO2 exchange at different light intensities
The graph below shows the net CO2 exchange rate of a leaf at increasing light intensities. Negative values indicate net CO2 uptake (more photosynthesis than respiration); positive values indicate net CO2 release. 6 marks
Stylised model based on typical C3 plant leaf-level gas exchange data.
2.1 At zero light intensity, the net CO2 exchange is positive (CO2 is being released). Identify the process responsible and explain why no photosynthesis is occurring. 2 marks
2.2 The point where the curve crosses zero is called the light compensation point. Explain in biological terms what is happening to photosynthesis and respiration at this point. 2 marks
2.3 A heterotroph (e.g. a mouse) placed in the same measuring apparatus would produce a graph that is a flat positive line. Explain why the heterotroph's graph would look fundamentally different from the plant's graph. 2 marks
3. Apply to a new scenario, classifying unfamiliar organisms
Three unfamiliar organisms are described below. For each, use the given clues to classify the organism as an autotroph or heterotroph, and justify your classification by referring to its energy source, nutrient source, and gas requirements. 9 marks, 3 each
Organism 1, Euglena
Euglena is a single-celled aquatic organism. Under bright light, it is green and produces bubbles of gas. In darkness, it can ingest bacteria and digest them for energy. It contains structures called chloroplasts.
3.1 Classify Euglena and justify using lesson concepts. Note any unusual feature. 3 marks
Organism 2, Nitrosomonas
Nitrosomonas is a bacterium found in soil. It does not use light. It obtains energy by oxidising ammonium ions (NH4+) to nitrite (NO2−). It uses this energy to fix CO2 from the air into organic compounds. It has no chloroplasts.
3.2 Classify Nitrosomonas and justify. Identify which type of autotroph it is. 3 marks
Organism 3, Venus flytrap
The Venus flytrap is a green flowering plant with chloroplasts in its leaf cells. It grows in nutrient-poor soils and supplements its mineral supply by trapping and digesting insects. It still requires light, CO2, and water to survive.
3.3 Classify the Venus flytrap and justify. Explain why consuming insects does not change its fundamental nutritional classification. 3 marks
Q1.1, Trend description
During the light period (06:00–18:00), CO2 concentration falls steadily from 634 ppm to a minimum of ~290 ppm, because photosynthesis consumes CO2 faster than respiration produces it (net CO2 uptake). During the dark period (18:00–24:00), CO2 rises from ~296 ppm to 460 ppm, because only cellular respiration is occurring and CO2 is released without being consumed by photosynthesis.
Q1.2, Why CO2 falls during daylight
During daylight, both photosynthesis and cellular respiration are occurring simultaneously in the plant [1]. The rate of photosynthesis exceeds the rate of respiration, so more CO2 is consumed (as a reactant in photosynthesis) than is produced (as a byproduct of respiration), resulting in a net decrease in chamber CO2 [1].
Q1.3, Night-time CO2 increase
The increase is caused by cellular respiration [1]. It confirms that autotrophs (plants) perform cellular respiration continuously, including at night when photosynthesis has stopped, the plant takes in O2 and releases CO2 just as a heterotroph does [1].
Q1.4, Prediction for animal
With an animal, CO2 would increase continuously throughout the 24 hours, regardless of light conditions [1]. Unlike the plant, the animal cannot photosynthesise, so CO2 is always produced by respiration and never consumed. There would be no decrease during the light period and no change in the rate between light and dark [1].
Q2.1, Zero light: process responsible
Cellular respiration [1]. No photosynthesis is occurring because photosynthesis requires light energy to drive the reactions; without light, chlorophyll cannot be activated and CO2 cannot be fixed, so the cell's only gas exchange is the consumption of O2 and release of CO2 by respiration [1].
Q2.2, Light compensation point
At the light compensation point, the rate of photosynthesis equals the rate of cellular respiration [1]. CO2 produced by respiration is exactly consumed by photosynthesis, so the net CO2 exchange is zero, there is no net gain or loss of CO2 [1].
Q2.3, Why the heterotroph graph is a flat positive line
The heterotroph cannot photosynthesise, it has no chloroplasts and no mechanism to fix CO2 [1]. At all light intensities, only cellular respiration occurs, releasing a steady amount of CO2. The flat positive line reflects constant CO2 release with no light-dependent process to counteract or exceed it [1].
Q3.1, Euglena
Classification: autotroph (by primary nutrition) / mixotroph. Euglena possesses chloroplasts and produces bubbles of O2 in light, evidence that it photosynthesises, producing its own organic molecules from CO2 and H2O using light energy [1]. The unusual feature is that in darkness it can also ingest other organisms (heterotrophic feeding), this makes it a mixotroph, able to switch between autotrophic and heterotrophic nutrition depending on light availability [1]. Its gas requirements when photosynthesising match an autotroph: CO2 in, O2 out (net daytime) [1].
Q3.2, Nitrosomonas
Classification: autotroph, specifically a chemoautotroph. Nitrosomonas uses the energy released from oxidising inorganic ammonium ions (chemical energy, not light) to fix inorganic CO2 into organic molecules [1]. It is therefore not a photoautotroph (no chloroplasts, no light used), but a chemoautotrophan autotroph that uses chemical reactions as its energy source [1]. It requires CO2 as its carbon source and produces organic molecules from inorganic inputs, which is the defining feature of all autotrophs [1].
Q3.3, Venus flytrap
Classification: autotroph. The Venus flytrap produces its own organic molecules through photosynthesis using light energy, CO2, and H2O, it has chloroplasts and the full autotrophic machinery [1]. It consumes insects only to supplement its mineral (inorganic nutrient) supply (e.g. nitrogen, phosphorus) that the poor soil cannot provide; it does not consume the insects as a primary source of organic molecules or energy [1]. The classification “autotroph vs heterotroph” is determined by how an organism acquires its organic molecules, the flytrap makes its own organics via photosynthesis, so it remains an autotroph regardless of its supplementary mineral acquisition strategy [1].