DP Biology HL C1.3 Photosynthesis

DP Biology HL C1.3 Photosynthesis - Full notes

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DP Biology HL | C1.3 Photosynthesis

DP BIOLOGY · THEME C: INTERACTION AND INTERDEPENDENCE

C1.3

Photosynthesis

Full notes for HL · IB-style

Contents

1	Photosynthesis: transforming light energy
2	Photosynthetic pigments and absorption
3	Separating pigments by chromatography
4	Limiting factors
5	Carbon dioxide enrichment and controlled environments
6	Photosynthesis and the atmosphere
7	The light-dependent reactions (HL only)
8	The light-independent reactions: the Calvin cycle (HL only)
9	Photosynthesis: the overall process
10	Light and wavelength
11	Absorption and action spectra
12	Chromatography of photosynthetic pigments
13	Limiting factors
14	Measuring the rate of photosynthesis
15	Carbon dioxide enrichment
16	The light-dependent reactions (overview)
17	The light-independent reactions (overview)
18	Photosynthesis and global carbon
19	Photosystems and chemiosmosis (HL)
20	Mixed review: worked problems
21	Additional exam-style practice
22	Photosynthesis versus respiration
23	Adaptations of leaves for photosynthesis
24	C4 and CAM plants
25	Common misconceptions and links
26	Exam-style question with markscheme
27	Check your understanding
28	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
explain photosynthesis as the conversion of light energy to chemical energy
write the word and balanced equations
explain absorption and action spectra and the role of pigments
separate pigments by chromatography and calculate Rf values
explain the effects of limiting factors
evaluate carbon dioxide enrichment experiments
explain the source of oxygen produced
explain the light-dependent reactions, photolysis, photosystems and chemiosmosis
explain the Calvin cycle and the role of rubisco

Sections marked HL only (Sections 7 and 8) are Additional Higher Level content. Every other section is common to SL and HL.

Guiding questions

How is light energy used to produce organic compounds? What limits the rate of photosynthesis?

How this topic is examined

C1.3 is examined through equations, absorption and action spectra, chromatography (Rf values), and limiting factor graphs. At HL, the light-dependent and light-independent reactions are tested in detail.

Limiting factor graphs need careful explanation: identify which factor limits each part of the curve.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Photosynthesis: transforming light energy

Light energy becomes chemical energy in sugars.

Think about it

A tree's trunk weighs several tonnes, yet the soil around it has barely changed. Where did all that mass come from?

Photosynthesis is the process by which autotrophs (plants, algae and cyanobacteria) use light energy to make organic compounds (glucose and other carbohydrates) from carbon dioxide and water, releasing oxygen as a by-product. It transforms light energy into chemical energy stored in the bonds of organic molecules. Almost all food chains depend on it.

Photosynthesis takes place in chloroplasts. Most of the mass of a plant comes from carbon dioxide taken from the air, not from the soil.

carbon dioxide + water → glucose + oxygen 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The oxygen released comes from the water molecules, which are split during the light-dependent reactions (photolysis). This was demonstrated using water labelled with the heavy isotope ¹⁸O: the oxygen released contained ¹⁸O.

Quick check

1. What are the raw materials of photosynthesis?

2. Where does the oxygen released come from?

Answers: 1. carbon dioxide and water 2. water (photolysis)

2 Photosynthetic pigments and absorption

Pigments absorb some wavelengths and reflect others.

Light is absorbed by pigments in the thylakoid membranes. The main pigment is chlorophyll a; accessory pigments include chlorophyll b and carotenoids, which absorb other wavelengths and pass the energy on.

Chlorophyll absorbs mainly blue (about 430 to 450 nm) and red (about 640 to 680 nm) light and reflects green light, which is why plants look green. Carotenoids absorb blue and violet light and look yellow-orange.

An absorption spectrum shows how much light of each wavelength a pigment absorbs. An action spectrum shows the rate of photosynthesis at each wavelength. The two have very similar shapes, which is evidence that the pigments are responsible for capturing light used in photosynthesis.

Figure 1. Absorption and action spectra closely match.

Quick check

1. Which colours does chlorophyll absorb most?

2. Distinguish absorption and action spectra.

Answers: 1. blue and red 2. absorption: light absorbed per wavelength; action: rate of photosynthesis per wavelength

3 Separating pigments by chromatography

Different pigments travel different distances.

Leaf pigments can be separated by chromatography. Leaves are ground with a solvent to extract the pigments; a concentrated spot is placed on a line near the bottom of chromatography paper (or a thin-layer plate); the bottom is dipped in a solvent, which moves up the paper. Pigments that are more soluble in the solvent and less attracted to the paper travel further.

Each pigment is identified by its Rf value:

Rf = distance moved by pigment ÷ distance moved by solvent front

WORKED EXAMPLE 1 Calculating Rf values

The solvent front moved 80 mm. A yellow-orange pigment moved 76 mm and a blue-green pigment 36 mm. Calculate their Rf values and suggest their identities.

Solution

Yellow-orange: 76 ÷ 80 = 0.95: caroteneA1

Blue-green: 36 ÷ 80 = 0.45: chlorophyll aA1

Pigment	Colour	Typical Rf (paper)
Carotene	yellow-orange	about 0.95
Xanthophyll	yellow	about 0.70
Chlorophyll a	blue-green	about 0.45
Chlorophyll b	yellow-green	about 0.35

Quick check

1. Define Rf.

2. Why do pigments separate?

Answers: 1. distance by pigment ÷ distance by solvent front 2. different solubility in solvent and attraction to paper

4 Limiting factors

The factor in shortest supply sets the rate.

The rate of photosynthesis is affected by light intensity, carbon dioxide concentration and temperature. A limiting factor is the factor in shortest supply relative to need: increasing it increases the rate, while increasing other factors does not.

Light intensity. At low light, rate increases in proportion to light, because light provides energy for the light-dependent reactions. At higher light, the rate levels off because another factor (CO₂ or temperature) becomes limiting.

Carbon dioxide concentration. CO₂ is the substrate for carbon fixation. Increasing CO₂ increases the rate up to a point where light or temperature becomes limiting. Atmospheric CO₂ (about 420 ppm) is often limiting.

Temperature. Affects the enzymes of photosynthesis, especially rubisco. Rate increases with temperature to an optimum, then falls as enzymes denature.

Figure 2. At low light, light limits the rate; on the plateau, CO₂ limits it.

WORKED EXAMPLE 2 Interpreting a limiting factor graph

Using the graph, identify the limiting factor at light intensity 2 and at light intensity 8 for the low CO₂ curve. Justify your answers.

Solution

At 2: light intensity: increasing light increases rate; both curves overlapA1 R1

At 8: CO₂ concentration: rate has plateaued, and the high CO₂ curve is higherA1 R1

Quick check

1. Define limiting factor.

2. Why does rate level off at high light intensity?

Answers: 1. the factor in shortest supply that limits the rate 2. another factor (CO₂ or temperature) becomes limiting

5 Carbon dioxide enrichment and controlled environments

Raising CO₂ can boost growth, within limits.

Growers increase yields in greenhouses by controlling limiting factors: artificial lighting, heating to an optimum temperature, and adding CO₂ (often by burning propane or from industrial sources) to about 1 000 ppm. This CO₂ enrichment can increase yields of crops such as tomatoes by 20% to 30%.

FACE (free-air carbon dioxide enrichment) experiments test the effects of higher CO₂ on plants growing in natural conditions: pipes release CO₂ around plots of crops or forest, raising the concentration to levels expected later this century. Results show increased photosynthesis and growth in many plants, but the effect is often smaller than in greenhouses, because other factors (nitrogen, water) become limiting, and food quality can fall (lower protein and mineral content). This is relevant to predicting how ecosystems and agriculture will respond to rising atmospheric CO₂.

Quick check

1. Why do growers add CO₂ to greenhouses?

2. What is a FACE experiment?

Answers: 1. CO₂ is often limiting; raises photosynthesis and yield 2. raising CO₂ around plants growing outdoors to study effects

6 Photosynthesis and the atmosphere

Photosynthesis made the oxygen we breathe.

Photosynthesis by cyanobacteria from about 2.4 billion years ago gradually added oxygen to the atmosphere and oceans (the Great Oxidation Event), allowing aerobic respiration and the formation of the ozone layer. Today, photosynthesis removes carbon dioxide from the atmosphere and is a key part of the carbon cycle (C4.2). Deforestation reduces this uptake.

7 The light-dependent reactions (HL only)

Light splits water and generates ATP and reduced NADP.

HL only

This section is Additional Higher Level content. SL students do not need it.

The light-dependent reactions take place in the thylakoid membranes. Pigments are organized into photosystems: arrays of chlorophyll and accessory pigments with a reaction centre containing a special pair of chlorophyll a molecules. Absorbed light energy is passed to the reaction centre, where it excites electrons to a higher energy level.

In photosystem II, excited electrons leave the reaction centre and are passed to an electron transport chain. They are replaced by electrons from water, which is split by photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂. Oxygen is released as a by-product.

As electrons pass along the chain, energy is used to pump protons into the thylakoid space, creating a proton gradient. Protons flow back to the stroma through ATP synthase, producing ATP by chemiosmosis (photophosphorylation).

Electrons then reach photosystem I, where they are re-excited by light and passed to NADP, which is reduced to reduced NADP (NADPH). The ATP and reduced NADP are used in the Calvin cycle.

Figure 3. The light-dependent reactions.

8 The light-independent reactions: the Calvin cycle (HL only)

CO₂ is fixed and reduced to sugar.

HL only

This section is Additional Higher Level content. SL students do not need it.

The Calvin cycle takes place in the stroma. It does not use light directly, but depends on ATP and reduced NADP from the light-dependent reactions.

Carbon fixation: CO₂ combines with ribulose bisphosphate (RuBP), a 5C compound, catalysed by the enzyme rubisco (the most abundant protein on Earth). The unstable 6C product splits into two molecules of glycerate 3-phosphate (GP), 3C.

Reduction: GP is reduced to triose phosphate (TP) using ATP and reduced NADP.

Regeneration: of every six TP molecules, one is used to make glucose and other organic compounds (starch, sucrose, amino acids, lipids); the other five are used to regenerate RuBP, using ATP, so the cycle continues.

Six turns of the cycle fix six CO₂ and produce one glucose (from two TP).

Figure 4. The Calvin cycle.

Quick check

1. Where is the Calvin cycle?

2. What enzyme fixes CO₂?

3. What is RuBP regenerated from?

Answers: 1. stroma 2. rubisco 3. triose phosphate, using ATP

9 Photosynthesis: the overall process

Light energy to chemical energy.

Photosynthesis converts light energy into chemical energy in carbon compounds: 6CO2 + 6H2O → C6H12O6 + 6O2. It occurs in chloroplasts. Oxygen is a by-product from the splitting of water (photolysis).

WORKED EXAMPLE 3 Equation (basic)

Write the balanced equation for photosynthesis.

Solution

6CO2 + 6H2O → C6H12O6 + 6O2A2

Quick check

1. Where does the oxygen released in photosynthesis come from?

Answers: 1. water (photolysis)

10 Light and wavelength

Which colours are used?

Visible light ranges from about 400 nm (violet) to 700 nm (red). Chlorophyll absorbs mainly red and blue light and reflects green, which is why leaves look green. Accessory pigments (carotenoids) absorb other wavelengths and pass energy to chlorophyll.

WORKED EXAMPLE 4 Green leaves (standard)

Explain why leaves appear green.

Solution

Chlorophyll absorbs red and blue lightR1

Green light is reflectedR1

11 Absorption and action spectra

Comparing curves.

An absorption spectrum shows how much light pigments absorb at each wavelength. An action spectrum shows the rate of photosynthesis at each wavelength. Their similarity shows that pigments absorbing those wavelengths drive photosynthesis.

WORKED EXAMPLE 5 Comparing (exam level)

Explain why the action spectrum closely matches the absorption spectrum of chlorophyll.

Solution

Wavelengths absorbed by chlorophyll provide the energy for photosynthesisR1

High absorption corresponds to high rateR1

Try this

Engelmann's experiment: bacteria that need oxygen clustered around parts of a filamentous alga lit by red and blue light, showing where photosynthesis was fastest.

12 Chromatography of photosynthetic pigments

Separating pigments.

Pigments can be separated by paper or thin-layer chromatography using a non-polar solvent. Each pigment has a characteristic Rf value: carotenes travel furthest, then xanthophylls, chlorophyll a, chlorophyll b.

WORKED EXAMPLE 6 Rf (standard)

A pigment moves 4.2 cm when the solvent moves 9.0 cm. Calculate its Rf.

Solution

4.2/9.0M1

= 0.47A1

Quick check

1. Why does carotene travel further than chlorophyll b?

Answers: 1. it is more soluble in the non-polar solvent and less attracted to the paper

13 Limiting factors

The slowest factor controls the rate.

The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity, CO2 concentration or temperature. Increasing a non-limiting factor has no effect on the rate.

Figure 5. Light intensity and CO2 as limiting factors.

WORKED EXAMPLE 7 Interpreting (exam level)

Explain the shape of the low CO2 curve.

Solution

At low light, rate increases with light: light is limitingR1

At high light, rate plateaus: CO2 is limitingR1

14 Measuring the rate of photosynthesis

Practical methods.

Methods: counting oxygen bubbles or measuring gas volume from pondweed (Elodea or Cabomba); floating leaf discs; colour change of hydrogencarbonate indicator; CO2 uptake using data loggers. Control temperature (water bath), CO2 (NaHCO3 solution) and light intensity (distance from lamp).

WORKED EXAMPLE 8 Light distance (exam level)

A lamp is moved from 10 cm to 20 cm from a plant. By what factor does the light intensity change (inverse square law)?

Solution

I ∝ 1/d2M1

(10/20)2 = 1/4A1

Quick check

1. Why is a heat shield (water tank) placed between the lamp and the plant?

Answers: 1. to prevent the lamp from changing the temperature

15 Carbon dioxide enrichment

Greenhouses and FACE experiments.

Raising CO2 concentration in greenhouses increases crop yields when CO2 is limiting. Free-Air CO2 Enrichment (FACE) experiments study the effect of future CO2 levels on crops and ecosystems in the field.

WORKED EXAMPLE 9 FACE (standard)

Why are FACE experiments more realistic than greenhouse studies?

Solution

Plants grow in natural field conditions with natural variationR1

In the real world

Commercial tomato growers often raise greenhouse CO2 to around 1000 ppm, more than double atmospheric levels, to boost yields.

16 The light-dependent reactions (overview)

Thylakoids.

In the thylakoid membranes, light energy excites electrons in chlorophyll. Water is split (photolysis), releasing O2, protons and electrons. Electron flow generates ATP (photophosphorylation) and reduced NADP.

WORKED EXAMPLE 10 Products (standard)

State the products of the light-dependent reactions.

Solution

ATP, reduced NADP, O2A2

17 The light-independent reactions (overview)

The Calvin cycle in the stroma.

In the stroma, CO2 is fixed by the enzyme Rubisco onto RuBP, forming glycerate-3-phosphate. ATP and reduced NADP from the light-dependent reactions convert it into triose phosphate, used to make glucose and other organic compounds and to regenerate RuBP.

WORKED EXAMPLE 11 Rubisco (standard)

State the role of Rubisco.

Solution

Catalyses fixation of CO2 with RuBPA1

18 Photosynthesis and global carbon

Producers fix carbon.

Photosynthesis removes about 120 billion tonnes of carbon from the atmosphere each year. Forests, grasslands and phytoplankton are major carbon sinks. Deforestation reduces carbon uptake and releases stored carbon.

Quick check

1. Why do atmospheric CO2 levels show seasonal fluctuations?

Answers: 1. more photosynthesis in Northern Hemisphere summer lowers CO2

19 Photosystems and chemiosmosis (HL)

Detailed light-dependent reactions.

Photosystem II absorbs light; excited electrons pass along the electron transport chain to photosystem I, pumping protons into the thylakoid lumen. Photolysis replaces electrons in PSII. PSI re-excites electrons, which reduce NADP. Protons flow through ATP synthase (chemiosmosis). Cyclic photophosphorylation uses PSI only and produces ATP but no NADPH.

WORKED EXAMPLE 12 Chemiosmosis (exam level)

Explain how ATP is produced in the thylakoid.

Solution

Electron transport pumps protons into the lumenR1

Protons diffuse back through ATP synthase, driving ATP synthesisR1

20 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 13 Mixed problem (exam level)

A student measures oxygen production by pondweed at increasing light intensities. Explain why the rate eventually stops increasing, and suggest how to increase it further.

Solution

Another factor (CO2 or temperature) becomes limitingR1

Add NaHCO3 to increase CO2 or raise temperature (below optimum)R1

21 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 14 Paper 1 question (standard)

Which wavelengths are absorbed least by chlorophyll? A. blue B. red C. green D. violet

Solution

CA1

WORKED EXAMPLE 15 Paper 2 question (exam level)

Outline how to separate photosynthetic pigments by chromatography.

Solution

Extract pigments by grinding leaves with solventR1

Spot concentrated extract on line; run in non-polar solventR1

Calculate Rf values and compare with referencesR1

22 Photosynthesis versus respiration

Opposite processes.

Feature	Photosynthesis	Respiration
location	chloroplasts	cytoplasm and mitochondria
energy	absorbed (light)	released (ATP)
reactants	CO2, H2O	glucose, O2
products	glucose, O2	CO2, H2O
occurs in	photosynthetic cells, in light	all living cells, always

Quick check

1. What is the compensation point?

Answers: 1. light intensity at which photosynthesis and respiration rates are equal

23 Adaptations of leaves for photosynthesis

Structure for function.

Leaves are broad and thin (large surface area, short diffusion distance), palisade mesophyll cells are packed with chloroplasts near the upper surface, spongy mesophyll has air spaces, stomata allow gas exchange, and veins supply water and remove sugars.

Quick check

1. Why are palisade cells near the upper surface of the leaf?

Answers: 1. to receive the most light

24 C4 and CAM plants

Alternative pathways.

C4 plants (maize, sugar cane) concentrate CO2 around Rubisco, reducing photorespiration in hot conditions. CAM plants (cacti, pineapple) open stomata at night to fix CO2, then close them by day to save water.

Quick check

1. Why do CAM plants open their stomata at night?

Answers: 1. cooler, more humid air reduces water loss

25 Common misconceptions and links

Students often think…	Actually…
plants get their mass from soil	most comes from CO₂ in the air
oxygen comes from CO₂	oxygen comes from water
increasing light always increases rate	only when light is limiting
the Calvin cycle needs light directly	it needs ATP and NADPH from the light reactions

How C1.3 connects to the rest of the course

Topic	Connection
B2.2 Organelles	chloroplast structure
C1.2 Respiration	chemiosmosis and ATP synthase
C4.2 Energy and matter	producers and the carbon cycle
D4.3 Climate change	CO₂ levels
B3.1 Gas exchange	stomata

26 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 6. Effect of light intensity on photosynthesis at two CO₂ concentrations.

(a) Calculate the Rf of a pigment that moved 28 mm when the solvent moved 80 mm. [1]

(b) Identify the limiting factor at low light intensity. [1]

(d) Explain why the action spectrum resembles the absorption spectrum. [2]

(e) Outline the source of oxygen released in photosynthesis. [2]

(f) Explain the roles of ATP and reduced NADP in the Calvin cycle. [3]

Markscheme

Part	Answer	Marks
(a)	0.35	A1
(b)	light intensity	A1
(c)	another factor becomes limiting	R1
	e.g. CO₂ concentration (higher curve with more CO₂)	R1
(d)	light absorbed by pigments drives photosynthesis	R1
	wavelengths absorbed most give highest rates	R1
(e)	water split by photolysis	R1
	evidence from ¹⁸O labelling	R1
(f)	reduce GP to triose phosphate	R1
	NADPH provides hydrogen / electrons	R1
	ATP provides energy, also to regenerate RuBP	R1

Examiner's comments. In (c), naming the factor without evidence from the graph scored one mark only.

27 Check your understanding

Section A: multiple choice

1. Chlorophyll absorbs mostly

A. blue and red light B. green light C. infrared D. ultraviolet only

2. The oxygen produced comes from

A. carbon dioxide B. glucose C. water D. chlorophyll

3. Rf is calculated as

A. solvent ÷ pigment B. distance moved by pigment ÷ distance moved by solvent C. pigment × solvent D. pigment − solvent

4. At low light intensity, the limiting factor is usually

A. CO₂ B. temperature C. water D. light intensity

5. CO₂ enrichment in greenhouses

A. always doubles yield B. increases photosynthesis if CO₂ is limiting C. reduces photosynthesis D. replaces light

6. An action spectrum shows

A. rate of photosynthesis at different wavelengths B. absorption by pigments C. pigment colours D. Rf values

7. Rubisco catalyses

A. photolysis B. ATP synthesis C. reduction of NADP D. the fixation of CO₂ to RuBP

8. Photolysis occurs at

A. photosystem I B. the stroma C. photosystem II D. the Calvin cycle

Section B: structured

9. Explain the effect of temperature on the rate of photosynthesis. [3]

10. Describe how to separate pigments by chromatography. [4]

11. Evaluate CO₂ enrichment as a way to increase crop yields. [4]

12. Explain the light-dependent reactions. [5]

Answers

1. A 2. C 3. B 4. D 5. B 6. A 7. D 8. C

9. Enzymes (rubisco) faster to an optimum; then denature; rate falls.

10. Extract with solvent; spot on line; solvent rises; measure distances; calculate Rf.

11. Raises rate if CO₂ limiting; cost; other factors limit; FACE shows smaller gains and lower nutritional quality.

12. Light excites electrons in PSII; photolysis; ETC pumps H⁺; chemiosmosis makes ATP; PSI re-excites; NADP reduced.

28 Summary and topic map

Idea	Remember
Equation	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Pigments	chlorophylls, carotenoids; absorb blue and red
Spectra	absorption and action spectra match
Chromatography	Rf = pigment ÷ solvent distance
Limiting factors	light, CO₂, temperature
CO₂ enrichment	greenhouses; FACE experiments
HL	photosystems, photolysis, chemiosmosis; Calvin cycle, rubisco, RuBP, GP, TP

Before the exam, make sure you…

•give balanced equations
•identify limiting factors from graph evidence
•calculate Rf values
•state oxygen comes from water

Photosynthesis on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 7. Topic map for Photosynthesis.

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