SBI4U Photosynthesis: Notes and Practice

In the 1640s, Jan van Helmont grew a willow tree in a pot for five years. The tree gained about 74 kg, but the soil lost only about 60 g. He concluded that the mass came from the water he added. He was only partly right. Most of a plant's dry mass comes from the air: carbon dioxide, pulled out of the atmosphere and built into sugar using the energy of sunlight. Every carbon atom in your body, and almost every calorie you eat, passed through this process. Photosynthesis is the anabolic mirror image of respiration: it uses energy to reduce $CO_{2}$ into sugar, and it releases the oxygen that respiration later consumes.

6 CO_{2} + 6 H_{2} O light energy C_{6} H_{12} O_{6} + 6 O_{2} Δ G \approx + 2870 kJ/mol

Endergonic: the free energy comes from absorbed light. CO₂ is reduced; water is oxidized.

Where and with what. In plants and algae, photosynthesis happens in chloroplasts, mainly in the mesophyll cells of leaves. A chloroplast has a double outer membrane surrounding a fluid called the stroma. Suspended in the stroma are flattened membrane sacs called thylakoids, often stacked into grana (singular granum); the space inside each thylakoid is the thylakoid lumen. The thylakoid membranes hold the light-absorbing pigments. Chlorophyll a is the main pigment that drives the light reactions. Chlorophyll b and the yellow-orange carotenoids are accessory pigments: they absorb wavelengths chlorophyll a misses and pass the energy on, and carotenoids also protect the chlorophyll from damage by excess light. Chlorophylls absorb blue-violet and red light strongly but reflect green, which is why leaves look green. An absorption spectrum shows how much light a pigment absorbs at each wavelength. An action spectrum shows how fast photosynthesis runs at each wavelength. In 1882, Theodor Engelmann lit a strand of alga with a spectrum of colours and found that oxygen-seeking bacteria crowded around the parts lit by blue and red light. The action spectrum matches the combined absorption of all the pigments, not chlorophyll a alone, which is evidence that accessory pigments contribute energy.

The light-dependent reactions (thylakoid membranes)

The light-dependent reactions turn light energy into chemical energy in two forms, ATP and NADPH, and release $O_{2}$ . Pigments are organized into photosystems: an antenna of hundreds of pigment molecules that funnel absorbed energy to a reaction centre containing a special pair of chlorophyll a molecules. The two photosystems are named in the order they were discovered, but they work in the reverse order. (1) In photosystem II (reaction centre P680, best at absorbing 680 nm light), light excites an electron, which is passed to a primary electron acceptor. (2) The chlorophyll that lost the electron is now a powerful oxidizing agent and replaces it by splitting water: $2 H_{2} O \to 4 H^{+} + 4 e^{-} + O_{2}$ . This photolysis of water happens in the lumen and is the source of all the $O_{2}$ released. (3) The excited electrons move down an electron transport chain (plastoquinone, a cytochrome complex, plastocyanin), and the cytochrome complex uses the energy to pump $H^{+}$ from the stroma into the lumen. (4) In photosystem I (reaction centre P700), light re-energizes the electrons. (5) They pass through ferredoxin to the enzyme NADP⁺ reductase, which makes NADPH in the stroma. (6) The $H^{+}$ gradient (the lumen reaches about pH 5; the stroma is about pH 8) drives ATP synthase in the thylakoid membrane as protons flow back into the stroma. Making ATP this way is photophosphorylation, the same chemiosmosis you saw in mitochondria.

Fig 1The Z-scheme: light raises electrons from water in photosystem II (P680), they fall through an electron transport chain that pumps H+, light raises them again in photosystem I (P700), and they end on NADP+ to form NADPH.

ANoncyclic electron flow (noncyclic photophosphorylation)

Uses photosystem II and photosystem I
Electrons flow one way: $H_{2} O \to$ PSII $\to$ ETC $\to$ PSI $\to NADP^{+}$
Water is split, releasing $O_{2}$
Produces ATP and NADPH
The main pathway in most conditions

BCyclic electron flow (cyclic photophosphorylation)

Uses photosystem I only
Electrons from ferredoxin return to the cytochrome complex and back to PSI
No water split, no $O_{2}$ released
Produces ATP only (no NADPH)
Tops up ATP, because the Calvin cycle needs 3 ATP for every 2 NADPH, more than noncyclic flow supplies

The Calvin cycle (stroma)

The Calvin cycle uses the ATP and NADPH from the light reactions to build sugar from $CO_{2}$ . It has three phases. (1) Carbon fixation: the enzyme rubisco (ribulose bisphosphate carboxylase/oxygenase), probably the most abundant protein on Earth, attaches $CO_{2}$ to the 5-carbon sugar RuBP (ribulose bisphosphate). The unstable 6-carbon product immediately splits into two 3-carbon molecules of PGA (3-phosphoglycerate). (2) Reduction: each PGA is phosphorylated by ATP and reduced by NADPH to G3P (glyceraldehyde-3-phosphate), a 3-carbon sugar. This is where the energy and electrons are stored in the sugar. (3) Regeneration: most of the G3P is rearranged, using more ATP, back into RuBP so the cycle can continue. Only one G3P in every six made is a net gain. To export one G3P, the cycle fixes 3 CO₂ and uses 9 ATP (6 for reduction, 3 for regeneration) and 6 NADPH. Two G3P make one glucose, so one glucose needs 6 CO₂, 18 ATP and 12 NADPH. G3P is the actual product; the plant uses it to make glucose, sucrose (for transport), starch (for storage), cellulose, amino acids and fats.

Fig 2The Calvin cycle: rubisco fixes CO₂ to RuBP, PGA is reduced to G3P using ATP and NADPH, and most G3P is recycled to regenerate RuBP. Three turns export one G3P.

Why "dark reactions" is a misleading name

The Calvin cycle is often called the light-independent reactions, or the "dark reactions", because no step uses light directly. That does not mean it runs in the dark. It depends on a steady supply of ATP and NADPH from the light reactions, which run out within seconds to minutes of darkness, and several Calvin cycle enzymes (including rubisco) are switched on only by conditions created in the light, such as a higher stroma pH. In a real plant, the Calvin cycle runs mainly during the day.

Photorespiration, C4 and CAM plants

Rubisco has a flaw: it can bind $O_{2}$ instead of $CO_{2}$ . On hot, dry days, plants close their stomata to save water, so $CO_{2}$ inside the leaf falls while $O_{2}$ from the light reactions builds up. Rubisco then adds $O_{2}$ to RuBP, producing one PGA and a 2-carbon compound the plant must spend ATP to salvage, releasing $CO_{2}$ in the process. This photorespiration makes no sugar and no ATP; in hot conditions it can waste a quarter or more of the carbon a C3 plant (such as wheat, rice or soybeans) has fixed. Two adaptations reduce it. C4 plants (corn, sugarcane) separate the steps in space: mesophyll cells fix $CO_{2}$ with the enzyme PEP carboxylase, which ignores $O_{2}$ , into a 4-carbon acid; this is pumped into deeper bundle-sheath cells and broken down there to release a high concentration of $CO_{2}$ around rubisco. CAM plants (cacti, pineapple, jade plants) separate the steps in time: they open their stomata only at night, store the fixed $CO_{2}$ as organic acids in their vacuoles, then close their stomata by day and release the $CO_{2}$ to the Calvin cycle. Both cost extra ATP, so they pay off only in hot, bright or dry climates; in a cool Ontario summer, C3 plants usually do fine.

Feature	Photosynthesis	Aerobic respiration
Organelle	Chloroplast (plants and algae)	Mitochondrion (nearly all eukaryotes, including plants)
Energy change	Endergonic: light energy stored in sugar	Exergonic: energy in sugar released, partly captured as ATP
Carbon	$CO_{2}$ reduced to sugar	Sugar oxidized to $CO_{2}$
Role of oxygen	$O_{2}$ released, from photolysis of water	$O_{2}$ consumed as final electron acceptor, forming water
Electron carrier	NADPH (anabolic)	NADH and $FADH_{2}$ (catabolic)
Electron source and destination	From $H_{2} O$ to NADPH, then to sugar	From sugar to NADH, then to $O_{2}$
Where the $H^{+}$ gradient forms	Pumped into the thylakoid lumen	Pumped into the intermembrane space
ATP synthase faces	The stroma (ATP made in the stroma)	The matrix (ATP made in the matrix)

Matter cycles between the two processes; energy does not. It enters as light and leaves as heat. Both organelles descend from engulfed bacteria (endosymbiosis), which explains their shared chemiosmotic machinery.

Lab: detecting the products of photosynthesis

Oxygen, floating leaf disks: punch disks from a spinach leaf, use a syringe to pull the air out of them in a dilute sodium hydrogen carbonate (baking soda) solution, which supplies

CO_{2}

, so they sink. Under light, the

O_{2}

they produce fills their air spaces and they float. The time for half the disks to float (ET50) measures the rate; vary the light intensity or colour, and keep a control set in the dark. Oxygen, Elodea: count the bubbles released per minute by a sprig of pondweed at different lamp distances, and collect the gas to show that it relights a glowing splint. Starch: keep a plant in the dark for 48 h to use up its starch, cover part of a leaf with foil, expose it to light, then boil the leaf in water (to break the membranes) and in hot ethanol in a water bath (to remove the chlorophyll). Iodine solution turns blue-black only where light reached the leaf, showing that light is required for the sugar that is stored as starch. A variegated leaf shows starch only in its green (chlorophyll-containing) parts.

Photosynthesis

Key things to remember

The light-dependent reactions (thylakoid membranes)

The Calvin cycle (stroma)

Photorespiration, C4 and CAM plants

Worked examples

Where marks get lost

Flashcards 9

Practice