Topic 3.4 · Photosynthesis

48 expectation lines from 14 lessons · 16 glossary terms

The reaction, written out

• Read the equation for photosynthesis, carbon dioxide + water → glucose + oxygen with light energy above the arrow: carbon dioxide and water are the reactants, glucose and oxygen are the products, and light is the energy input.
• Say whether a given organism or cell photosynthesizes.

Where a tree’s mass comes from

• Justify, from the willow’s masses, the claim that the tree’s new mass did not come from the soil.
• Justify, from the labeled-carbon result, the claim that the sugar’s carbon comes from carbon dioxide and its hydrogen from water.
• Say that light supplies the energy to build the sugar, and no atoms.
• Say where the oxygen photosynthesis gives off comes from, water, and cite the labeled-water result that shows it.

What a leaf does with its sugar

• State the three fates of the glucose a leaf makes: broken down in respiration for ATP, built into new molecules, or stored as starch.
• Read an iodine test on a leaf: a blue-black patch shows where starch is, so where photosynthesis has been happening.

A plant in a sealed jar, day and night

• Predict whether the oxygen or the carbon dioxide in a sealed jar rises, falls or stays level in the dark, in bright light and in dim light.
• Calculate a plant’s rate of photosynthesis from sealed-jar readings: add the dark reading, respiration alone, to the change measured in the light, assuming respiration uses oxygen at the same rate in light and dark.

What a chloroplast is, and its six places

• Say what goes into a chloroplast, carbon dioxide and water, and what comes out, glucose and oxygen, with light energy driving the work.
• Say that the photosynthesizing cells of plants, and algae, hold chloroplasts.
• Say that a cyanobacterium holds no chloroplast.
• Identify, on a drawn chloroplast, the outer and inner membranes, the stroma, a thylakoid with its thylakoid membrane and thylakoid space, and a granum.

Two rooms, two jobs

• Say where the light reactions capture light energy: in the thylakoid membranes of the grana.
• Say where the Calvin cycle fixes carbon dioxide and builds sugar: in the stroma.
• Trace ATP and NADPH from the thylakoid membranes into the Calvin cycle, and ADP and NADP⁺ back.

Light and color

• Say that white light is a mix of colors, and that each color is light of a different wavelength.
• Name the color of visible light from its wavelength in nanometers, from violet near 400 nm to red near 700 nm.
• Explain the color of an object from the light it reflects: the object absorbs the other colors.
• Say that a molecule that absorbs some colors of light and reflects or transmits the rest is called a pigment.

Why leaves are green

• Say that chlorophyll, the main pigment of photosynthesis, is held in the thylakoid membranes.
• Say which colors chlorophyll absorbs strongly, blue and red, and which it reflects or transmits, green.
• Explain why green light drives a plant’s photosynthesis least: chlorophyll reflects most green light, and only absorbed light drives photosynthesis.
• Read from an absorption graph which wavelengths a pigment absorbs strongly and which weakly.
• Predict from a pigment’s absorption graph which color of light drives photosynthesis fastest in the organism that holds it.

One electron’s journey

• Describe a photosystem: a cluster of chlorophyll, other pigment molecules and proteins in the thylakoid membrane, in which light boosts an electron in chlorophyll to a higher energy level and the photosystem passes that electron to a neighboring molecule.
• Name the two kinds of photosystem in the thylakoid membrane, photosystem II and photosystem I, and say which one the electron’s path starts at.
• Explain where photosystem II gets its replacement electrons: water is split, the water’s electrons refill the photosystem, the water’s protons enter the thylakoid space, and the water’s oxygen leaves as O₂.
• Explain that electrons leaving photosystem II pass along an electron transport chain in the thylakoid membrane, and that the energy they release pumps protons from the stroma into the thylakoid space, building a proton gradient.
• Explain that light boosts the electrons again at photosystem I, and that photosystem I passes them to NADP⁺, reducing NADP⁺ to NADPH, the carrier that supplies electrons to the Calvin cycle.

The same machine, powered by light

• Explain how the light reactions make ATP: the chain pumps protons into the thylakoid space, and the protons flowing back into the stroma through ATP synthase drive the formation of ATP from ADP and inorganic phosphate, which is photophosphorylation.
• Trace one electron from water to NADPH, naming what each step makes.

Building sugar in the stroma

• Describe the Calvin cycle by what goes in (carbon dioxide, ATP and NADPH) and what comes out (a three-carbon sugar, with ADP, Pi and NADP⁺ sent back to the light reactions).
• Say that the Calvin cycle uses no light directly.
• Read where the labeled carbon turns up, and when, as evidence that carbon dioxide is fixed into a three-carbon molecule first and built up into sugar afterwards.

Break one room and the other fails

• Predict what cutting the light reactions, by herbicide or by darkness, does to the stroma’s ATP and NADPH and to carbon fixation.
• Explain why carbon fixation falls when the light reactions are cut, although carbon dioxide is plentiful.
• Predict what closing the stomata does to a lit leaf’s sugar output and to the ATP and NADPH in its stroma.
• Trace the energy of photosynthesis from light to boosted electrons, to the proton gradient, to ATP and NADPH, to sugar, naming where each step happens.
• Trace the carbon from carbon dioxide to sugar and the oxygen from water to O₂, citing the labeled-atom results.
• Support the claim that photosynthesis captures light energy and stores it in sugar, with labeled-atom, gas or mass evidence.

One machine, three places

• Compare the electron transport chains of the mitochondrion, the chloroplast and a prokaryote’s plasma membrane by where their electrons come from, where they end and where their protons are pumped.
• Say that photosynthesis first evolved in prokaryotes and that eukaryotic photosynthesis was built on those prokaryotic pathways.
• Support, from the DNA comparison and the matching machinery, the claim that the chloroplast descends from a cyanobacterium by endosymbiosis.
• Support with evidence the claim that oxygen-releasing photosynthesis by cyanobacteria began to build up oxygen in the air and sea about 2.4 billion years ago.

Shared pathways, shared ancestry

• Classify an organism into one of the three domains: Bacteria, Archaea or Eukarya.
• Explain how the finding that glycolysis and oxidative phosphorylation occur in all three domains of life supports common ancestry: a complex pathway shared by every group is most simply explained as inherited from a shared ancestor that already had it.

Glossary

absorption graph

A graph of the percentage of light a pigment absorbs, plotted against wavelength in nanometers. Read it to find which colors the pigment absorbs strongly, and so which colors drive photosynthesis fastest in the organism that holds it.

Calvin cycle

The cycle of reactions in the stroma that fixes carbon dioxide and, using ATP and NADPH from the light reactions, builds it into sugar.

carbon fixation

Attaching carbon dioxide to an organic molecule, so that its carbon becomes part of a larger carbon-containing molecule; the step that begins the Calvin cycle, in the stroma.

chlorophyll

The main pigment of photosynthesis, held in the thylakoid membranes; it absorbs blue and red light strongly and reflects or transmits green, which is why leaves look green.

cyanobacteria

Bacteria that photosynthesize the way plants do, releasing oxygen: they capture light energy and make sugar from carbon dioxide and water with no chloroplast, on membranes folded inside the cell itself.

domain

One of the three largest groups of living things: Bacteria, Archaea and Eukarya (the eukaryotes: every organism whose cells have a nucleus).

endosymbiosis

One organism living inside another. A eukaryotic cell took a cyanobacterium inside itself and kept it, and the cyanobacterium’s descendants became chloroplasts.

light reactions

The reactions of photosynthesis that capture light energy, in the thylakoid membranes of the grana; they make ATP and NADPH.

NADP⁺ / NADPH

An electron carrier of the chloroplast, a close relative of NAD⁺: NADP⁺ picks up electrons in the light reactions and becomes NADPH, which supplies them to the Calvin cycle in the stroma.

nanometer (nm)

A billionth of a meter: 1 nm is 0.000 000 001 m. The wavelengths of visible light are measured in nanometers, from violet near 400 nm to red near 700 nm.

net change

What is left after one process has taken its share from another: in a sealed jar, the oxygen the leaves made minus the oxygen respiration used. Net means what remains after the subtraction.

photophosphorylation

Making ATP from ADP and inorganic phosphate with a proton gradient that light energy built: protons pumped into the thylakoid space by the chain flow back into the stroma through ATP synthase.

photosystem

A cluster of chlorophyll and other pigment molecules with proteins in the thylakoid membrane, in which absorbed light boosts an electron in chlorophyll to a higher energy level. There are two kinds, photosystem II and photosystem I.

pigment

A molecule that absorbs some colors (wavelengths) of light and reflects or transmits the others. A thing looks the color its pigment reflects.

stroma

The clear fluid inside a chloroplast’s inner membrane, around the thylakoids.

thylakoid, thylakoid membrane, thylakoid space, grana (granum)

A thylakoid is a flattened sac of membrane floating in the stroma. Its membrane is the thylakoid membrane; the space it encloses is the thylakoid space. Thylakoids sit in stacks called grana (one stack is a granum).