What you'll learn
- How photosynthesis converts light energy into chemical energy stored in biomass.
- How the light-dependent reactions and Calvin cycle fit together.
- How ecological succession changes communities and abiotic conditions over time.
- How to interpret practical data from photosynthesis and succession investigations.
The big picture: ecosystems need an energy input
An ecosystem is a community of organisms interacting with each other and with the abiotic factors in their environment, such as light intensity, temperature, water availability and soil mineral ion concentration.
A community is all the populations of different species living in the same place. A population is all the individuals of one species in an area. A habitat is the place where an organism lives.
The key link in this topic is simple: photosynthesis builds plant biomass, and plant biomass changes habitats. Over time, those habitat changes can drive succession, where one community is gradually replaced by another.
Photosynthesis: the energy input for most ecosystems
Photosynthesis
Photosynthesis is the process by which photoautotrophs, such as plants and algae, use light energy to convert carbon dioxide and water into organic molecules, releasing oxygen.
The overall equation is often summarised as:
6CO2+6H2O→C6H12O6+6O26\text{CO}_2 + 6\text{H}_2\text{O} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_26CO2+6H2O→C6H12O6+6O2This equation is useful, but remember that glucose is not usually the immediate first product. In plants, photosynthesis happens in chloroplasts, which contain chlorophyll, a green pigment that absorbs light energy. Chloroplasts contain stacks of membrane sacs called grana, made from thylakoid membranes, surrounded by fluid called the stroma.
The two stages of photosynthesis
Light-dependent reactions
The light-dependent reactions occur on the thylakoid membranes. Chlorophyll absorbs light energy, which excites electrons. This energy is used to make ATP, a molecule that transfers energy in cells.
Water is split by photolysis, producing protons, electrons and oxygen. The oxygen diffuses out of the plant as a waste product.
The electrons and protons are used to reduce NADP, forming reduced NADP. Some diagrams label reduced NADP as NADPH.
Oxygen source
The oxygen released in photosynthesis comes from water during photolysis, not from carbon dioxide.
Light-independent reactions: the Calvin cycle
The light-independent reactions happen in the stroma. They are also called the Calvin cycle. They do not use light directly, but they depend on ATP and reduced NADP from the light-dependent reactions.
Carbon dioxide combines with RuBP, a five-carbon compound. This reaction is catalysed by the enzyme rubisco. The products are converted into GP and then TP using ATP and reduced NADP. Some TP is used to make carbohydrates and other organic molecules; the rest is used to regenerate RuBP.
Photosynthesis in one sentence
The light-dependent reactions make ATP and reduced NADP; the Calvin cycle uses them to fix carbon dioxide into organic molecules.
Limiting factors and measuring photosynthesis
A limiting factor is the factor that prevents a process from happening faster. For photosynthesis, the main limiting factors are usually:
- light intensity
- carbon dioxide concentration
- temperature
At low light intensity, increasing light usually increases the rate of photosynthesis. Eventually the graph levels off because another factor, such as carbon dioxide concentration or temperature, becomes limiting.
In practical work, photosynthesis rate can be estimated by measuring oxygen production, carbon dioxide uptake, or the reduction of DCPIP. DCPIP is a blue dye that accepts electrons and becomes colourless when reduced, so it can be used to investigate the light-dependent reactions.
Calculating rate and identifying the limiting factor
An aquatic plant produces 1.8×10−6 m31.8 \times 10^{-6}\ \text{m}^31.8×10−6 m3 of oxygen in 300 s at low light intensity. When the lamp is moved closer, it produces 3.6×10−6 m33.6 \times 10^{-6}\ \text{m}^33.6×10−6 m3 in 300 s. Moving the lamp even closer gives no further increase.
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Use the rate formula: rate=ΔVoxygenΔt\text{rate} = \frac{\Delta V_\text{oxygen}}{\Delta t}rate=ΔtΔVoxygen.
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Calculate the low-light rate: r1=1.8×10−6 m3300 s=6.0×10−9 m3 s−1r_1 = \frac{1.8 \times 10^{-6}\ \text{m}^3}{300\ \text{s}} = 6.0 \times 10^{-9}\ \text{m}^3\ \text{s}^{-1}r1=300 s1.8×10−6 m3=6.0×10−9 m3 s−1.
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Calculate the higher-light rate: r2=3.6×10−6 m3300 s=1.2×10−8 m3 s−1r_2 = \frac{3.6 \times 10^{-6}\ \text{m}^3}{300\ \text{s}} = 1.2 \times 10^{-8}\ \text{m}^3\ \text{s}^{-1}r2=300 s3.6×10−6 m3=1.2×10−8 m3 s−1.
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Compare the pattern: the rate doubles when light intensity increases, so light was initially limiting. The later plateau suggests light is no longer limiting; carbon dioxide concentration or temperature may now be limiting.
Practical controls
When investigating photosynthesis, control variables such as temperature, carbon dioxide concentration, plant species, plant size, distance from the lamp and time allowed for equilibration.
Succession: how communities change over time
Succession
Succession is the gradual change in the species composition of a community over time, usually involving changes to both the organisms present and the abiotic conditions.
In primary succession, colonisation begins on a surface with no soil, such as bare rock after glacial retreat. In secondary succession, soil remains after a disturbance, such as fire, farming or woodland clearance, so succession usually happens faster.
A pioneer species is an early coloniser that can survive harsh conditions, such as low water availability, little shelter and few mineral ions. Lichens and mosses are common examples in primary succession.
A seral stage is one temporary community in a succession sequence. Over time, seral stages may progress from pioneer species to grasses, herbs, shrubs and eventually trees.
A climax community is the relatively stable community that develops if succession continues under the local environmental conditions.
Climax does not always mean woodland
A climax community is the stable community under local conditions. In much of the UK this may be deciduous woodland, but grazing, waterlogging, altitude or management can lead to grassland, heathland or a plagioclimax.
Why succession happens
Succession is not just “plants arriving in order”. The organisms already present change the habitat.
Pioneer species may trap particles, add organic matter when they die, and help form a thin soil. Decomposition produces humus, which improves water retention and mineral ion availability. These changes allow larger plants to survive.
As taller plants grow, they create shade and compete for light, water and mineral ions. Species that were successful early in succession may be outcompeted later.
The direction of succession
Succession happens because organisms modify abiotic conditions, making the habitat more suitable for some species and less suitable for others.
Investigating succession in the field
Succession often takes too long to watch from start to finish, so you may study different locations that represent different stages. For example, on sand dunes, distance from the shore can act as a rough time gradient.
A transect is a line used for systematic sampling across an environmental gradient. A belt transect uses quadrats placed along the line. A quadrat is a square frame used to sample organisms in a known area.
You might measure:
- percentage cover of each plant species
- species richness, meaning the number of species present
- abiotic factors, such as soil depth, soil moisture, pH and light intensity
Estimating percentage cover with a quadrat
A 0.50 m by 0.50 m quadrat is divided into 25 equal squares. A plant species is present in 17 squares.
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Calculate the total quadrat area: A=0.50 m×0.50 m=0.25 m2A = 0.50\ \text{m} \times 0.50\ \text{m} = 0.25\ \text{m}^2A=0.50 m×0.50 m=0.25 m2.
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Estimate percentage cover: 1725×100=68%\frac{17}{25} \times 100 = 68\%2517×100=68%.
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Estimate the area covered by that species: 0.68×0.25 m2=0.17 m20.68 \times 0.25\ \text{m}^2 = 0.17\ \text{m}^20.68×0.25 m2=0.17 m2.
Linking photosynthesis, productivity and succession
Gross primary productivity, or GPP, is the total chemical energy fixed by plants during photosynthesis per unit area per unit time.
Net primary productivity, or NPP, is the chemical energy stored as plant biomass after plant respiratory losses have been subtracted.
NPP=GPP−RNPP = GPP - RNPP=GPP−Rwhere RRR is energy lost through plant respiration.
Calculating net primary productivity
A grassland has a GPP of 18 000 kJ m−2 year−118\,000\ \text{kJ}\ \text{m}^{-2}\ \text{year}^{-1}18000 kJ m−2 year−1. Plant respiration losses are 11 500 kJ m−2 year−111\,500\ \text{kJ}\ \text{m}^{-2}\ \text{year}^{-1}11500 kJ m−2 year−1.
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Choose the productivity equation: NPP=GPP−RNPP = GPP - RNPP=GPP−R.
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Substitute the values with units: NPP=18 000 kJ m−2 year−1−11 500 kJ m−2 year−1NPP = 18\,000\ \text{kJ}\ \text{m}^{-2}\ \text{year}^{-1} - 11\,500\ \text{kJ}\ \text{m}^{-2}\ \text{year}^{-1}NPP=18000 kJ m−2 year−1−11500 kJ m−2 year−1.
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Calculate the stored energy: NPP=6 500 kJ m−2 year−1NPP = 6\,500\ \text{kJ}\ \text{m}^{-2}\ \text{year}^{-1}NPP=6500 kJ m−2 year−1.
Early in succession, total leaf area is low, so GPP is often low. As plants establish and biomass increases, productivity usually rises. In a mature community, GPP may still be high, but respiration by large plants is also high, so NPP may stabilise.
In the exam
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Link succession answers to changes in abiotic factors, such as soil depth, humus, water retention, mineral ions and light.
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For photosynthesis questions, separate the roles of the light-dependent reactions and the Calvin cycle clearly.
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In calculations, write the formula, substitute values with units, then give the final answer with units.
Check yourself
- Why does the oxygen released in photosynthesis come from water rather than carbon dioxide?
- How do pioneer species make a habitat more suitable for later seral stages?
- Why might NPP be lower than GPP in a mature woodland?
