Glossary: Nutrition in Plants

This chapter's terms in context

Chapter 18 works with four terms describing how a plant makes its own food: photosynthesis, chlorophyll, chloroplast and limiting factor. Photosynthesis is the process by which green plants and some other organisms convert light energy into chemical energy stored in glucose, using carbon dioxide and water as raw materials and releasing oxygen as a by-product; the overall word equation, carbon dioxide plus water, in the presence of light and chlorophyll, yields glucose plus oxygen, summarises a process that actually happens in two linked stages, the light-dependent reactions and the light-independent reactions, although SPM-level treatment focuses mainly on the overall inputs, outputs and the factors that affect the rate.

Chlorophyll is the green pigment responsible for absorbing light energy for photosynthesis, and it absorbs red and blue light most strongly while reflecting green light, which is exactly why most plant leaves appear green to the eye. Chlorophyll itself is not scattered freely throughout a plant cell; it is contained specifically within the chloroplast, an organelle found in plant cells (and some protists) that is the actual site where the whole process of photosynthesis takes place. A chloroplast contains internal membrane stacks called grana, where chlorophyll is concentrated and the light-dependent reactions occur, surrounded by a fluid stroma where the light-independent reactions take place; confusing chlorophyll (the pigment molecule) with chloroplast (the organelle containing it) is a very common and avoidable vocabulary error, since a chloroplast can exist and be seen under a microscope even before considering whether the chlorophyll within it is functioning normally.

A limiting factor is whichever single environmental factor, light intensity, carbon dioxide concentration, or temperature are the three tested at SPM level, is, at a given moment, in shortest supply relative to the plant's needs, and it is this one factor alone that restricts the rate of photosynthesis at that time, regardless of how favourable the other factors are. This principle explains a graph shape seen repeatedly in this chapter: as one factor (such as light intensity) increases, the rate of photosynthesis rises steadily until it reaches a plateau, at which point a different factor (such as carbon dioxide concentration or temperature) has become limiting instead, and increasing the first factor further has no additional effect until the new limiting factor is also increased. A frequent exam error is describing all three factors as limiting simultaneously, when the definition specifically requires identifying the one factor currently in shortest supply.

Paper 3 questions on this chapter typically ask a candidate to interpret a graph of photosynthesis rate against light intensity, carbon dioxide concentration or temperature, and full marks depend on correctly identifying the plateau region as evidence that a different factor has become limiting, then naming a plausible alternative factor, rather than simply describing the shape of the curve without this reasoning.

Because commercial greenhouse growers deliberately raise carbon dioxide concentration or supplement lighting to remove a limiting factor and increase crop yield, this chapter also connects photosynthesis theory directly to agricultural application, and examiners occasionally frame a limiting factor question around a greenhouse scenario rather than a laboratory graph alone.

A related application question asks a candidate to explain why a plant kept in complete darkness for several days turns pale (a condition called etiolation) even though it is otherwise healthy, and the correct explanation links chlorophyll production itself to the presence of light: without light, a plant cannot synthesise chlorophyll normally, so its leaves lose their green colour and photosynthesis cannot proceed even once light becomes available again until new chlorophyll has been produced. A second frequent scenario tests a candidate's understanding of variegated leaves, leaves with naturally occurring white or pale patches lacking chlorophyll, by asking why only the green parts of such a leaf test positive for starch after a destarching and light exposure experiment, a result that directly demonstrates chlorophyll's essential role in the process rather than merely stating that 'light is needed'.