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Introduction to Biology: Photosynthesis

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BIO 101 - Introduction to Biology
Lecture 5: Photosynthesis

Learning outcomes

By the end of this lecture you should be able to:
- write the overall equation for photosynthesis and say where each product comes from
- name the parts of a leaf and a chloroplast where each stage happens
- explain why leaves are green, using absorption of light by pigments
- describe what the light-dependent reactions make and how
- describe the three stages of the Calvin cycle and what each one uses
- explain photorespiration and how C4 and CAM plants avoid it

1. Why photosynthesis matters

Almost every food chain on Earth starts with photosynthesis. Organisms that make their own food from an energy source and carbon dioxide are called autotrophs. Plants, algae and cyanobacteria are photoautotrophs: their energy source is light. Everything else, including animals and fungi, is a heterotroph and gets its energy by eating organic molecules that an autotroph made first.

Photosynthesis does two jobs at once. It turns light energy into chemical energy stored in the bonds of sugars, and it releases the oxygen that aerobic organisms breathe.

The overall equation:

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

Exam point: the oxygen released comes from the water, not from the carbon dioxide. In 1941 Ruben and Kamen gave algae water labelled with the heavy isotope oxygen-18, and the labelled oxygen turned up in the O2 gas the algae gave off.

2. Where it happens

Leaf
- Most photosynthesis happens in the mesophyll, the layer of cells in the middle of the leaf.
- CO2 enters and O2 leaves through stomata, small pores mostly on the underside of the leaf.
- Each stoma is opened and closed by a pair of guard cells. Closing the stomata saves water but also cuts off the supply of CO2.

Chloroplast
- Surrounded by a double membrane (outer and inner membrane).
- Inside are flattened membrane sacs called thylakoids. A stack of thylakoids is a granum (plural grana).
- The space inside a thylakoid is the thylakoid lumen.
- The fluid around the thylakoids is the stroma.
- Do not confuse stroma (fluid in the chloroplast) with stoma (pore in the leaf).

Stage 1, the light-dependent reactions, happens in the thylakoid membranes.
Stage 2, the Calvin cycle, happens in the stroma.

3. Light and pigments

Light travels as waves. The shorter the wavelength, the more energy it carries. Plants use light between about 400 nm (violet) and 700 nm (red), roughly the same range our eyes can see.

A pigment absorbs some wavelengths and reflects the rest. The colour we see is the light that was reflected.
- Chlorophyll a is the main pigment. It absorbs blue and red light strongly and reflects green, which is why leaves look green.
- Chlorophyll b absorbs slightly different blue and red wavelengths and passes the energy on to chlorophyll a.
- Carotenoids (yellow, orange) absorb blue-green light. They also protect the chloroplast by getting rid of excess energy as heat in very bright light.

An absorption spectrum shows which wavelengths a pigment absorbs. An action spectrum shows how fast photosynthesis runs at each wavelength. In 1882 Engelmann shone a spectrum of light along a strand of alga and found that oxygen-loving bacteria gathered at the blue and red parts, which matches the absorption peaks of chlorophyll.

Having several pigments widens the range of light a plant can use, which matters for plants growing in shade.

4. The light-dependent reactions

Purpose: turn light energy into chemical energy in two carriers, ATP and NADPH, for the Calvin cycle to spend.

Pigments sit in photosystems in the thylakoid membrane. Each photosystem has an antenna complex of a few hundred pigment molecules that collect light and pass the energy inward to a reaction centre, a special pair of chlorophyll a molecules.

The two photosystems are numbered in the order they were discovered, not the order they work in.

Step by step (linear electron flow):
1. Light energy reaches the reaction centre of photosystem II (P680). An excited electron leaves it and is caught by a primary electron acceptor.
2. The lost electron is replaced by splitting water: 2 H2O → 4 H+ + 4 electrons + O2. This is where the oxygen comes from.
3. The electron passes along an electron transport chain: plastoquinone, the cytochrome b6f complex, then plastocyanin. As it moves, energy is used to pump H+ from the stroma into the thylakoid lumen.
4. At photosystem I (P700) the electron is energised again by more light.
5. It passes to ferredoxin, and the enzyme NADP+ reductase uses it to turn NADP+ into NADPH.

Making ATP
- H+ builds up inside the lumen, from pumping and from splitting water. This creates a concentration gradient across the thylakoid membrane.
- H+ can only flow back out into the stroma through ATP synthase. The flow drives ATP synthase to add a phosphate to ADP, making ATP.
- Making ATP this way is called chemiosmosis. Because light powers it, it is also called photophosphorylation.

Cyclic electron flow: electrons from photosystem I can loop back to the cytochrome complex instead of making NADPH. This makes extra ATP but no NADPH and no oxygen. It tops up ATP, because the Calvin cycle uses more ATP than NADPH.

Summary of the light-dependent reactions
In: light, H2O, ADP + phosphate, NADP+
Out: O2 (released), ATP and NADPH (sent to the stroma)

5. The Calvin cycle (light-independent reactions)

Purpose: use the ATP and NADPH to build sugar from CO2.

Older books call these the "dark reactions". That name is misleading: they do not need light directly, but they need a constant supply of ATP and NADPH from the light reactions, and several of their enzymes are switched on by light, so in practice they run during the day.

Stage 1, carbon fixation
The enzyme RuBisCO attaches CO2 to a five-carbon sugar, ribulose bisphosphate (RuBP). The six-carbon product splits at once into two molecules of 3-phosphoglycerate (3-PGA), which has three carbons. RuBisCO is probably the most abundant protein on Earth.

Stage 2, reduction
ATP and NADPH from the light reactions turn 3-PGA into glyceraldehyde 3-phosphate (G3P), a three-carbon sugar. It is called reduction because 3-PGA gains electrons from NADPH.

Stage 3, regeneration
Most of the G3P is used, with more ATP, to rebuild RuBP so the cycle can keep going.

Counting it up
- Each turn fixes one CO2.
- Three turns fix three CO2 and give one G3P to the rest of the plant. This costs 9 ATP and 6 NADPH.
- One glucose (six carbons) needs two G3P, so six turns: 18 ATP and 12 NADPH.

What the plant does with G3P: makes glucose, sucrose to transport around the plant, starch to store, and cellulose for cell walls.

6. When RuBisCO gets it wrong: photorespiration

RuBisCO can grab O2 instead of CO2. When it does, the plant uses energy and releases CO2 without making any sugar. This is photorespiration.

It gets worse on hot, dry days. The plant closes its stomata to save water, so CO2 inside the leaf falls while O2 from the light reactions builds up.

Two adaptations reduce it:
- C4 plants (maize, sugarcane) separate the steps in space. In the mesophyll, an enzyme called PEP carboxylase, which ignores O2, fixes CO2 into a four-carbon acid. This is moved to bundle-sheath cells deeper in the leaf and broken down to release CO2 right next to RuBisCO.
- CAM plants (cacti, pineapple) separate the steps in time. They open their stomata at night, when it is cooler and less water is lost, and store the CO2 as malic acid. During the day the stomata stay shut and the stored CO2 is released for the Calvin cycle.

7. What limits the rate

The rate of photosynthesis is set by whichever factor is in shortest supply, called the limiting factor:
- Light intensity: more light raises the rate, until something else runs out.
- CO2 concentration: more CO2 raises the rate, until something else runs out.
- Temperature: the Calvin cycle is run by enzymes, so the rate rises with warmth up to an optimum and then falls.

8. Photosynthesis and respiration

Photosynthesis stores energy in sugar in the chloroplasts. Cellular respiration releases it as ATP in the mitochondria. The products of one are the raw materials of the other: photosynthesis uses CO2 and H2O and gives off O2, while respiration uses O2 and gives off CO2 and H2O.

Common mistake: plants do not photosynthesise instead of respiring. They respire all the time, day and night, because every living cell needs ATP. In the light they simply photosynthesise faster than they respire, so overall they take in CO2 and give off O2.

This set covers photosynthesis from its basic significance and anatomical setting in the leaf and chloroplast through the detailed biochemistry of the light-dependent reactions and the Calvin cycle, concluding with the complications and adaptations that shape real-world photosynthetic efficiency. You should come away able to describe chloroplast structure and pigment function, trace electron and proton flow through the thylakoid membrane, tally the ATP and NADPH needed to build sugar through carbon fixation, reduction, and regeneration, and explain photorespiration, C4/CAM adaptations, limiting factors, and the relationship between photosynthesis and cellular respiration.

Topics in this set

Autotrophs and heterotrophs
The overall equation for photosynthesis
Source of released oxygen
Leaf structures involved in photosynthesis
Chloroplast structures involved in photosynthesis
Pigments and why leaves are green
Absorption spectrum versus action spectrum
Photosystems and reaction centres
Linear electron flow
Chemiosmosis and photophosphorylation
Cyclic electron flow
Calvin cycle carbon fixation
Calvin cycle reduction and regeneration
Calvin cycle ATP and NADPH costs
Photorespiration
C4 adaptation
CAM adaptation
Limiting factors
Relationship between photosynthesis and respiration

Key terms

Autotroph: An organism that makes its own food from an energy source and carbon dioxide.
Photoautotroph: An autotroph whose energy source is light, such as plants, algae and cyanobacteria.
Heterotroph: An organism that gets energy by eating organic molecules made by an autotroph.
Stomata: Small pores, mostly on the underside of a leaf, through which CO2 enters and O2 leaves; each is opened and closed by a pair of guard cells.
Stroma: The fluid around the thylakoids inside a chloroplast, where the Calvin cycle happens.
Thylakoid: A flattened membrane sac inside the chloroplast; stacks of thylakoids are called grana, and the light-dependent reactions happen in the thylakoid membranes.
Pigment: A molecule that absorbs some wavelengths of light and reflects the rest; the reflected light is the colour we see.
Absorption spectrum: A graph showing which wavelengths of light a pigment absorbs.
Action spectrum: A graph showing how fast photosynthesis runs at each wavelength of light.
Photosystem: A complex in the thylakoid membrane containing an antenna complex and a reaction centre; photosystem II (P680) and photosystem I (P700) are numbered by order of discovery, not order of action.
Reaction centre: A special pair of chlorophyll a molecules at the core of a photosystem where excited electrons leave to be caught by a primary electron acceptor.
Chemiosmosis: The process by which H+ flowing back across the thylakoid membrane through ATP synthase drives the addition of phosphate to ADP, making ATP.
Photophosphorylation: Another name for chemiosmotic ATP production in the chloroplast, so called because light powers the H+ gradient that drives it.
Cyclic electron flow: A pathway where electrons from photosystem I loop back to the cytochrome complex instead of reducing NADP+, producing extra ATP but no NADPH or oxygen.
RuBisCO: The enzyme that attaches CO2 to ribulose bisphosphate (RuBP) in carbon fixation; described as probably the most abundant protein on Earth.
3-phosphoglycerate (3-PGA): A three-carbon molecule formed when the six-carbon product of CO2 and RuBP splits immediately after fixation.
Glyceraldehyde 3-phosphate (G3P): The three-carbon sugar produced when ATP and NADPH reduce 3-PGA; most is used to regenerate RuBP, the rest goes to make glucose, sucrose, starch, or cellulose.
Ribulose bisphosphate (RuBP): The five-carbon sugar that CO2 is attached to by RuBisCO at the start of the Calvin cycle, and which must be regenerated from G3P to keep the cycle running.
Photorespiration: A process where RuBisCO fixes O2 instead of CO2, using energy and releasing CO2 without producing sugar.
PEP carboxylase: An enzyme in C4 plants' mesophyll cells that fixes CO2 into a four-carbon acid and ignores O2.
C4 plants: Plants such as maize and sugarcane that separate CO2 fixation and the Calvin cycle in space, using mesophyll and bundle-sheath cells, to reduce photorespiration.
CAM plants: Plants such as cacti and pineapple that separate CO2 fixation and the Calvin cycle in time, opening stomata at night and storing CO2 as malic acid.
Limiting factor: Whichever factor (light intensity, CO2 concentration, or temperature) is in shortest supply and so sets the rate of photosynthesis.

Why photosynthesis matters

Almost every food chain on Earth starts with photosynthesis.
Photosynthesis converts light energy into chemical energy stored in sugar bonds and releases oxygen that aerobic organisms breathe.
The overall equation is $6 CO X_{2} + 6 H X_{2} O light energy C X_{6} H X_{12} O X_{6} + 6 O X_{2}$
The oxygen released in photosynthesis comes from the water, not from the carbon dioxide.
In 1941, Ruben and Kamen gave algae water labelled with oxygen-18, and the labelled oxygen appeared in the O2 gas the algae released, showing the oxygen's source.

Where it happens: the leaf

Most photosynthesis happens in the mesophyll, the layer of cells in the middle of the leaf.
CO2 enters and O2 leaves through stomata, small pores mostly on the underside of the leaf.
Each stoma is opened and closed by a pair of guard cells.
Closing the stomata saves water but also cuts off the supply of CO2.

Where it happens: the chloroplast

The chloroplast is surrounded by a double membrane, consisting of an outer and inner membrane.
Inside the chloroplast are flattened membrane sacs called thylakoids, and a stack of thylakoids is called a granum (plural grana).
The space inside a thylakoid is called the thylakoid lumen.
The fluid surrounding the thylakoids is called the stroma.
Stroma (fluid in the chloroplast) should not be confused with stoma (pore in the leaf).
The light-dependent reactions (Stage 1) happen in the thylakoid membranes.
The Calvin cycle (Stage 2) happens in the stroma.

Light and pigments

Light travels as waves, and the shorter the wavelength, the more energy it carries.
Plants use light between about 400 nm (violet) and 700 nm (red), roughly the same range visible to human eyes.
A pigment absorbs some wavelengths of light and reflects the rest, and the colour we see is the reflected light.
Chlorophyll a is the main pigment; it absorbs blue and red light strongly and reflects green, which is why leaves look green.
Chlorophyll b absorbs slightly different blue and red wavelengths and passes the energy on to chlorophyll a.
Carotenoids are yellow or orange pigments that absorb blue-green light and also protect the chloroplast by releasing excess energy as heat in very bright light.
An absorption spectrum shows which wavelengths a pigment absorbs, while an action spectrum shows how fast photosynthesis runs at each wavelength.
In 1882, Engelmann shone a spectrum of light along a strand of alga and found that oxygen-loving bacteria gathered at the blue and red parts, matching chlorophyll's absorption peaks.
Having several pigments widens the range of light a plant can use, which matters for plants growing in shade.

Purpose and structure of the light-dependent reactions

The light-dependent reactions turn light energy into chemical energy stored in two carriers, ATP and NADPH, which the Calvin cycle then spends.
Pigments are organized into photosystems in the thylakoid membrane, each with an antenna complex of a few hundred pigment molecules that collect light and pass energy to a reaction centre.
The reaction centre of each photosystem is a special pair of chlorophyll a molecules.
Photosystem II and photosystem I are numbered by the order they were discovered, not the order in which they act in the pathway.

Linear electron flow, step by step

Light energy excites the reaction centre of photosystem II (P680), and the excited electron is caught by a primary electron acceptor.
The electron lost from photosystem II is replaced by splitting water, described as $2 H X_{2} O 4 H X^{+} + 4 e X^{-} + O X_{2}$ , which is the source of the oxygen released by photosynthesis.
The electron travels along an electron transport chain made of plastoquinone, the cytochrome b6f complex, and plastocyanin, and this movement powers the pumping of H+ from the stroma into the thylakoid lumen.
At photosystem I (P700), the electron is re-energized by more light.
The electron passes to ferredoxin, and the enzyme NADP+ reductase uses it to convert NADP+ into NADPH.

Making ATP: chemiosmosis and photophosphorylation

H+ builds up inside the thylakoid lumen both from the electron transport chain pumping and from the splitting of water, creating a concentration gradient across the thylakoid membrane.
H+ can only flow back into the stroma through ATP synthase, and this flow drives ATP synthase to add a phosphate to ADP, making ATP.
This method of making ATP is called chemiosmosis, and because light powers it, it is also called photophosphorylation.

Cyclic electron flow

In cyclic electron flow, electrons from photosystem I loop back to the cytochrome complex instead of being used to make NADPH.
Cyclic electron flow produces extra ATP but no NADPH and no oxygen.
Cyclic electron flow tops up the ATP supply because the Calvin cycle uses more ATP than NADPH.

Summary of the light-dependent reactions

Inputs to the light-dependent reactions are light, water, ADP plus phosphate, and NADP+.
Outputs of the light-dependent reactions are oxygen, which is released, and ATP and NADPH, which are sent to the stroma for use by the Calvin cycle.

Purpose of the Calvin cycle

The purpose of the Calvin cycle is to use the ATP and NADPH from the light reactions to build sugar from CO2.
Older books call these reactions the "dark reactions," but this name is misleading because although they do not need light directly, they depend on a constant supply of ATP and NADPH from the light reactions and several of their enzymes are switched on by light, so they run during the day in practice.

Stage 1: carbon fixation

The enzyme RuBisCO attaches CO2 to the five-carbon sugar ribulose bisphosphate (RuBP).
The resulting six-carbon product splits immediately into two molecules of the three-carbon compound 3-phosphoglycerate (3-PGA).
RuBisCO is probably the most abundant protein on Earth.

Stage 2: reduction

ATP and NADPH from the light reactions convert 3-PGA into the three-carbon sugar glyceraldehyde 3-phosphate (G3P).
This stage is called reduction because 3-PGA gains electrons from NADPH.

Stage 3: regeneration

Most of the G3P produced is used, along with more ATP, to rebuild RuBP so that the Calvin cycle can continue.

Counting the Calvin cycle

Each turn of the Calvin cycle fixes one CO2 molecule.
Three turns of the cycle fix three CO2 molecules and yield one G3P for the rest of the plant, at a cost of 9 ATP and 6 NADPH.
Because one glucose molecule (six carbons) requires two G3P molecules, building one glucose takes six turns of the cycle, costing 18 ATP and 12 NADPH.
The plant uses G3P to make glucose, sucrose for transport around the plant, starch for storage, and cellulose for cell walls.

Photorespiration

RuBisCO can fix O2 instead of CO2; when it does, the plant uses energy and releases CO2 without making sugar, a process called photorespiration.
Photorespiration gets worse on hot, dry days because the plant closes its stomata to save water, causing CO2 inside the leaf to fall while O2 from the light reactions builds up.

Adaptations that reduce photorespiration

C4 plants such as maize and sugarcane separate the steps of carbon fixation in space: PEP carboxylase in the mesophyll fixes CO2 into a four-carbon acid, which ignores O2.
In C4 plants, the four-carbon acid is moved to bundle-sheath cells deeper in the leaf and broken down there to release CO2 right next to RuBisCO.
CAM plants such as cacti and pineapple separate the steps in time: they open their stomata at night, when it is cooler and less water is lost, and store CO2 as malic acid.
In CAM plants, stomata stay shut during the day and the stored CO2 is released for use in the Calvin cycle.

Limiting factors

The rate of photosynthesis is set by whichever factor is in shortest supply, called the limiting factor.
Increasing light intensity raises the rate of photosynthesis until some other factor becomes limiting.
Increasing CO2 concentration raises the rate of photosynthesis until some other factor becomes limiting.
Because the Calvin cycle is run by enzymes, the rate of photosynthesis rises with temperature up to an optimum and then falls.

Photosynthesis and respiration

Photosynthesis stores energy in sugar in the chloroplasts, while cellular respiration releases that energy as ATP in the mitochondria.
The products of photosynthesis and respiration are each other's raw materials: photosynthesis uses CO2 and H2O and releases O2, while respiration uses O2 and releases CO2 and H2O.
Plants respire all the time, day and night, because every living cell needs ATP; they do not photosynthesise instead of respiring.
In the light, plants photosynthesise faster than they respire, so overall they take in CO2 and give off O2.

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