Plants and Photosynthesis

Plant biology on this subtest is photosynthesis plus a small amount of structure. The equation and its location carry most of the questions.

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The equation

Carbon dioxide + water, with light energy, produces glucose + oxygen.

In symbols: 6 CO2 + 6 H2O + light gives C6H12O6 + 6 O2.

Three things to take from it:

The plant makes its own food. That is what "autotroph" means, and it is the defining feature of a plant.

Oxygen is a waste product. The plant does not make oxygen for anyone's benefit; it is what is left over after the carbon dioxide has been stripped for its carbon. That the atmosphere depends on it is a consequence, not a purpose.

Light is an input, not a reactant. It supplies the energy that drives the reaction.

Where it happens

In the chloroplasts, which are found mostly in leaf cells.

Chlorophyll is the green pigment inside them that absorbs light. It absorbs red and blue light well and reflects green, which is why leaves look green - the green is the light the plant is not using.

The two stages

Photosynthesis runs in two steps, and the questions ask what each makes.

The link between them is the pair ATP and NADPH: made in the first stage, spent in the second. The oxygen comes from the water, not from the carbon dioxide, which is a favorite trick question.

Photosynthesis against respiration

The mirror pair, and the source of the topic's best question.

Plants do both, all the time. Photosynthesis stops in the dark because it needs light; respiration continues day and night, which is why a plant consumes oxygen at night.

During the day a healthy plant photosynthesizes faster than it respires, so the net effect is oxygen released and carbon dioxide taken in. At night only respiration runs and the direction reverses.

Plant structure

Roots come in two patterns: a taproot is one thick main root going deep, like a carrot or a dandelion; a fibrous root system is a mat of thin roots near the surface, like grass.

Xylem up, phloem food. Xylem carries water in one direction only; phloem moves sugars in whichever direction they are needed.

Stomata are the plant's trade-off. They must open to admit carbon dioxide, and whenever they are open water escapes. That loss is transpiration, and it is what pulls water up the xylem from the roots.

In dry conditions the guard cells close the stomata to save water, which also stops carbon dioxide entering and so stops photosynthesis. That trade-off is a question: a plant conserving water cannot photosynthesize at the same time.

Reproduction

Flowering plants reproduce sexually through flowers.

Pollination is pollen moving from anther to stigma, by wind, water or animals. Fertilization follows when the pollen reaches the ovule. The ovule becomes a seed and the ovary becomes the fruit.

A fruit is a ripened ovary, which is why tomatoes, cucumbers and peppers are botanically fruits. That is a favorite question.

Asexual reproduction also occurs: runners, tubers and cuttings produce offspring genetically identical to the parent.

Pollinators - bees, butterflies, birds, bats - carry pollen between flowers in return for nectar, which is why flowers are colored and scented at all.

Seeds and germination

A seed is an embryo plant with a food supply. The cotyledon is the seed leaf that provides nutrition to the embryo until it can photosynthesize. When a seed germinates - sprouts - the first thing out is the radicle, the embryonic root, followed by the shoot.

Monocots have one cotyledon (grasses, corn, lilies); dicots have two (beans, oaks, roses). Mono is one, di is two.

The plant kingdom

Four groups, in the order they evolved, and each is defined by one thing:

Vascular tissue is what lets a plant grow tall, because it can carry water up against gravity. A moss cannot, which is why mosses are never more than a few centimeters high. Gymno means naked; angio means vessel - the seed in a container.

Tropisms

A plant's growth responses, and the vocabulary gets asked.

• Phototropism: growth toward light. Stems are positively phototropic.
• Gravitropism: growth in response to gravity. Roots grow down, stems grow up.
• Thigmotropism: growth in response to touch, as in a climbing vine.

Plant hormones

Tropisms are steered by hormones, and five are asked:

Ethylene is why one ripe banana ripens the rest of the bunch, and why a paper bag speeds ripening.

Legumes - beans, peas, clover - carry nitrogen-fixing bacteria in root nodules, which turn nitrogen from the air into a form the plant can use. The ecology lesson covers the whole nitrogen cycle.

What you can skip

Across all 2,104 General Science questions in our bank, none mentions:

• The machinery inside the chloroplast. Photosystems, rubisco and photorespiration never appear, and the thylakoid is never named. What the two stages make, and where the oxygen comes from, is the depth that is asked.
• C4 plants never appear. CAM plants, the desert variation, come up once at the hardest difficulty, and the whole answer is that they open their stomata only at night to save water.

And a handful of words come up exactly once - entomophily (insect pollination), sori, cork cambium - which makes them poor value against the tables above.

Where people lose points

Saying plants do not respire. They do, constantly.

Thinking plants release oxygen at night. Photosynthesis needs light; at night they consume oxygen like any other organism.

Saying chlorophyll absorbs green light. It reflects it.

Swapping xylem and phloem.

Thinking a plant's mass comes from the soil. It comes mostly from carbon dioxide in the air.

Calling a tomato a vegetable in a botany question. It is a fruit.

Work one in under a minute

A sealed container holds a healthy plant and is left in the dark for two days. What happens to the oxygen and carbon dioxide levels?

Without light there is no photosynthesis, but respiration continues.

So the plant consumes oxygen and releases carbon dioxide: oxygen falls, carbon dioxide rises.

In the light the same container would show the reverse, because photosynthesis would outrun respiration.

Where this leads

Photosynthesis is the entry point for energy into almost every food chain, which is where the ecology lesson begins.