Energy Flow, Ecosystems and the Environment

Edexcel International A-Level Biology· Statements 5.1–5.26· 60 min read

1. Photosynthesis Reactions & Core Practical 10★★★☆☆⏱ 15 min

Photosynthesis splits water to release oxygen, combines hydrogen with CO₂ to store energy in glucose, and produces ATP via photophosphorylation for immediate energy use. The reaction occurs in two linked stages: light-dependent and light-independent (Calvin cycle).

The light-dependent stage uses light to excite chlorophyll electrons, producing ATP and reducing NADP. Both cyclic and non-cyclic photophosphorylation produce ATP, while only non-cyclic produces reduced NADP and splits water (photolysis) to release oxygen. The light-independent stage uses RUBISCO to fix CO₂, producing GP, which is converted to GALP using ATP and reduced NADP. GALP is used to synthesise glucose, or regenerate RuBP to continue the cycle.

📘 Definition

Rf Value

Ratio of the distance a pigment travels up chromatography paper to the distance the solvent front travels, always ≤1.

Example:

If a pigment travels 2cm and solvent travels 5cm, Rf = 0.4

R_f = \frac{\text{Distance travelled by pigment}}{\text{Distance travelled by solvent front}}

📐 Worked Example

A student runs a leaf pigment chromatography experiment. The solvent front travels 7.5cm, and carotene travels 6.75cm. Calculate the Rf value of carotene.

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Step 1: Recall the Rf formula
2
$R_f = \frac{\text{Pigment distance}}{\text{Solvent distance}}$
3
Step 2: Substitute the values from the question
4
$R_f = \frac{6.75}{7.5} = 0.9$
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Step 3: Confirm the value is ≤1, which it is, so the calculation is valid.

Core Practical 10 investigates the effect of light intensity, wavelength, temperature and CO₂ concentration on photosynthesis rate, measured via oxygen bubble count or CO₂ uptake. Control variables include plant species, volume of water, and experimental duration.

Exam tip:

Only use the spec-approved names for Calvin cycle intermediates: GP, GALP, RuBP, RUBISCO. Alternative names like G3P will not be awarded marks. Always state oxygen comes from photolysis of water, not CO₂.

2. Ecosystem Energy Flow & Core Practical 11★★☆☆☆⏱ 12 min

Energy flows through ecosystems via trophic levels (producers → primary consumers → secondary consumers → decomposers). Only ~10-20% of energy is transferred between levels, with most lost as heat via respiration, waste, or unused biomass.

📘 Definition

Net Primary Production (NPP)

NPP = GPP - R

Energy stored in plant biomass available to the next trophic level, calculated as GPP minus respiratory losses (R).

\text{Energy Transfer Efficiency (\%)} = \frac{E_{\text{current trophic level}}}{E_{\text{previous trophic level}}} \times 100

📐 Worked Example

A wheat field has GPP of 32000 kJ m⁻² yr⁻¹, and plant respiratory losses (R) are 14000 kJ m⁻² yr⁻¹. Aphids (primary consumers) absorb 2700 kJ m⁻² yr⁻¹ from wheat. Calculate NPP and percentage energy transfer from wheat to aphids.

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Step 1: Calculate NPP using the formula
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$NPP = 32000 - 14000 = 18000 \text{ kJ m}^{-2} \text{yr}^{-1}$
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Step 2: Use NPP as the producer energy value for transfer calculation
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$\text{Efficiency} = \frac{2700}{18000} \times 100 = 15\%$
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Step 3: Add units to all final values for full marks.

Core Practical 11 uses quadrats and transects to measure species abundance and distribution, paired with measurements of abiotic factors like pH, temperature and light intensity. Random quadrats are used for uniform habitats, while belt transects are used for gradient habitats like seashores.

Exam tip:

Always show full working for energy calculations, and include correct units for every value. Marks are awarded separately for working, units, and final answer.

3. Climate Change Evidence & Core Practical 12★★★☆☆⏱ 15 min

Evidence for climate change includes long-term CO₂ records from ice cores, tree ring growth (dendrochronology), and pollen preserved in peat bogs. Correlations between CO₂ levels and temperature do not confirm causation, but multiple lines of evidence support anthropogenic (human-caused) climate change via greenhouse gas emissions.

📘 Definition

Q10

Q10 = Rate(T+10°C) / Rate(T°C)

Measure of how much reaction rate increases with a 10°C temperature rise, used to quantify temperature effects on enzyme-controlled biological processes. Typical Q10 values for biological reactions are 1-4.

📐 Worked Example

The rate of caterpillar growth is 1.2 g per week at 18°C, and 2.7 g per week at 28°C. Calculate the Q10 value for caterpillar growth.

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Step 1: Recall the Q10 formula
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$Q_{10} = \frac{\text{Rate}(T+10^\text{C})}{\text{Rate}(T^\text{C})}$
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Step 2: Substitute the values from the question
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$Q_{10} = \frac{2.7}{1.2} = 2.25$
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Step 3: Confirm the value falls within the expected 1-4 range for biological reactions.

Core Practical 12 investigates the effect of temperature on organism development (e.g. seed germination, insect larval growth). Ethical considerations include minimising harm to test organisms and returning them to their natural habitat after the experiment.

Exam tip:

You will regularly be asked to critique climate change model limitations: common limitations include incomplete historical data, unaccounted variables, and uncertainty about future human emissions.

4. Succession, Speciation & Climate Change Mitigation★★★★☆⏱ 12 min

Succession is the gradual change in community structure over time, starting with pioneer species colonising bare ground, and ending with a stable climax community. A species' niche (its role in the ecosystem, including biotic and abiotic interactions) determines its distribution and abundance.

📘 Definition

Speciation

Formation of new species via mutation and natural selection, leading to reproductive isolation between populations. There are two key types: allopatric and sympatric.

📐 Worked Example

Two populations of beetle are separated by a newly formed canyon for 200,000 years. When reintroduced, they can no longer interbreed to produce fertile offspring. Name the type of speciation and justify your answer.

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Step 1: Identify the isolation mechanism: a geographic barrier (canyon) separates the two populations.
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Step 2: Match the isolation mechanism to speciation type: allopatric speciation.
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Step 3: Justify: Allopatric speciation occurs when geographic isolation prevents gene flow between populations, leading to accumulated genetic differences and eventual reproductive isolation.

Climate change mitigation strategies include reforestation (increasing carbon sequestration) and sustainable biofuel production (reducing fossil fuel emissions). Sympatric speciation, in contrast to allopatric, occurs without geographic isolation, e.g. when populations develop different mating seasons or feeding preferences within the same habitat.

Exam tip:

Do not confuse allopatric and sympatric speciation: always look for the presence or absence of a geographic barrier first, as this is the key distinguishing feature.

5. Common Pitfalls

Wrong move:

Stating oxygen released in photosynthesis comes from CO₂

Why:

Common misconception about the source of oxygen in photosynthesis

Correct move:

Always state oxygen is produced by photolysis of water in the light-dependent reaction

Wrong move:

Using non-spec names for Calvin cycle intermediates (e.g. G3P instead of GALP)

Why:

Mark schemes only award marks for exact spec-approved terminology

Correct move:

Use only GP, GALP, RuBP, RUBISCO when referring to Calvin cycle components

Wrong move:

Using GPP instead of NPP when calculating producer to primary consumer energy transfer

Why:

GPP includes energy lost to plant respiration, which is not available to consumers

Correct move:

Always use NPP as the producer energy value for transfer efficiency calculations

Wrong move:

Confusing allopatric and sympatric speciation by linking reproductive isolation only to sympatric speciation

Why:

Both types of speciation result in reproductive isolation, the difference is the initial isolation mechanism

Correct move:

Allopatric = geographic barrier first, sympatric = reproductive isolation without geographic barrier

Wrong move:

Calculating Rf as solvent distance divided by pigment distance, leading to values >1

Why:

Mixing up the numerator and denominator of the Rf formula

Correct move:

Rf = pigment travel distance / solvent travel distance, values are always ≤1

Wrong move:

Omitting units for NPP, energy transfer, and Q10 calculations

Why:

Mark schemes award separate marks for correct units

Correct move:

Include appropriate units for every calculated value in your answer

6. Quick Reference Cheatsheet

Concept	Formula/Key Definition	Exam Reminder
Rf Value	Pigment distance / Solvent distance	Always ≤1, no units
NPP	GPP - R	R = producer respiratory loss, units kJ m⁻² yr⁻¹
Energy Transfer Efficiency	(Current level energy / Previous level energy) × 100	Use NPP for producer level, units %
Q10	Rate(T+10°C) / Rate(T°C)	Values ~1-4 for biological reactions, no units
Allopatric Speciation	Speciation via geographic isolation	Look for mountain, river, canyon barrier in question
Sympatric Speciation	Speciation without geographic isolation	Reproductive isolation arises first e.g. different mating seasons
Photosynthesis O₂ Source	Photolysis of water	Common multiple choice distractor is CO₂

7. Frequently Asked

What is the difference between absorption and action spectra?

An absorption spectrum shows the wavelengths of light absorbed by each photosynthetic pigment, while an action spectrum shows the rate of photosynthesis at each wavelength of light. The two spectra are closely matched, demonstrating that absorbed light drives photosynthesis.

What is the difference between a population and a community?

A population is all individuals of the same species living in a defined habitat at the same time. A community is all populations of all species living in that same habitat at the same time, plus their interactions.

What's Next

Now that you have mastered Topic 5 content for Edexcel IAL Biology Unit 4, you are ready to move to Topic 6: Microbiology, Immunity and Forensics, the second half of the WBI14 exam content. You should first practice past paper questions focused on photosynthesis calculations, energy transfer, and climate change evaluation to consolidate your knowledge, paying special attention to high-weight command terms like 'evaluate' and 'justify'. Ensure you can describe all three core practicals in full, including variables, methodology, and common limitations, as these appear in at least 1 in 3 Unit 4 papers. You can revisit IAS enzyme and genetics content if you struggle with Q10 calculations or speciation concepts.