Chapter 5: Capturing and Releasing Energy

Part 2

The Light-Independent Reactions

The ATP (from Photosystem II) and the NADPH (from Photosystem I) from the light-dependent reactions power the energy-requiring/energy-storing reactions of the second stage of photosynthesis.

In this second stage, carbon dioxide is converted to carbohydrates such as glucose.

The reactions of the second stage of photosynthesis are also called the Calvin-Benson cycle and occur in the stroma of the chloroplast.

It is so called this because it is a cyclic reaction: the end products of one reaction serve as the beginning reactants of the next reaction.

The Light-Independent Reactions During this part of photosynthesis, carbon fixation

occurs. Carbon fixation is a process in which carbon atoms

from an inorganic source (such as carbon dioxide) are incorporated into an organic molecule (such as glucose).

The enzyme that performs carbon fixation in the light-independent reactions we will call rubisco.

The Light-Independent Reactions Rubisco attaches a carbon dioxide molecule to a 5-

carbon molecule called RuBP. This forms a 6-carbon molecule that is immediately

cleaved into two 3-carbon molecules called PGA. This process has to occur six times to fix the 6 carbon

dioxide molecules necessary to form one 6-carbon molecule of glucose and store the energy from the high-energy products (ATP and NADPH) of the light-dependent reactions.

The Light-Independent Reactions (Calvin-Benson Cycle)

The Light-Independent Reactions The glucose produced during these reactions may

either be used immediately by the plant or stored for future use in the form of sucrose or starch.

For example, excess glucose may be stored as starch granules in the stroma of the chloroplast.

Photosynthesis

Plant Adaptations for Photosynthesis Plants are able to live and continue to perform

photosynthesis in regions where water is scarce or only available at certain times due to an adaptation called the cuticle.

The cuticle is a thin, waxy, waterproof coating that prevents water loss by evaporation from plant parts that are above ground.

However, this coating also prevents the exchange of gases (that are important for photosynthesis- CO2 and O2) across the cells on the plant’s surface.

Plant Adaptations for Photosynthesis

To overcome this obstacle, plants have evolved tiny, closable “pores” on the surface of their photosynthetic parts called stomata.

When the stomata are open, carbon dioxide can diffuse into the plant’s photosynthetic tissues (so that the light-independent reactions can occur) and oxygen (from the light-dependent reactions) can diffuse out.

Plant Adaptations for Photosynthesis Plants that use the Calvin-Benson cycle are called C3 plants

(because the first intermediate in a 3-carbon molecule). These plants can conserve water on dry days by closing their

stomata. However, when the stomata are closed, oxygen gas from the

light-dependent reactions cannot get out and so accumulates in the photosynthetic cells.

The problem with this is that, when oxygen concentrations are high, rubisco binds oxygen to RuBP instead of carbon dioxide and so carbon fixation does not occur.

This is called photorepspiration and is very inefficient and undesirable.

Photosynthesis vs. Photorespiration

Plant Adaptations for Photosynthesis However, some plants (such as corn and bamboo)have evolved

adaptations to help them minimize photorespiration. For example, C4 plants (so called because their first intermediate in

the light-independent reactions is a 4-carbon molecule) fix carbon twice in two different kinds of cells.

In the first kind of cell, carbon is fixed by a different enzyme that doesn’t use oxygen at all, even at high concentrations.

An intermediate carbon molecule that is produced from this process is then transported to the second kind of cell and converted back to carbon dioxide, where it then enters the Calvin-Benson cycle.

The extra initial step keeps levels of carbon dioxide high in the cells that do the Calvin-Benson cycle, thus minimizing photorespiration.

C4 Plants

Plant Adaptations for Photosynthesis

In another kind of plant, called CAM plants, the extra reaction of the C4 plants runs at different times of day rather than in different cells.

The C4 reactions run during the day (when oxygen levels are high because the stomata are closed to prevent water loss) and the Calvin-Benson cycle (C3 reactions) run at night (when the oxygen levels are lower because the stomata are open due to lower environmental temperatures and less water loss by evaporation).

CAM stands for Crassulacean Acid Metabolism and plants such as succulents and cacti use this pathway.

CAM Plants

C4 Plants vs.CAM Plants

PHOTOSYNTHESIS HOMEWORK 1. Using the graph on slide 10 in the Part 1 Powerpoint, list

the colors that are absorbed and reflected by each of the following pigments according to the graph. Also, tell what color each pigment would appear to the human eye. B-carotene Chlorophyll a Chlorophyll b Phycoerythrin Phycocyanin

PHOTOSYNTHESIS HOMEWORK

2. Use the picture below to explain the names of each of the pigments on the previous question:

PHOTOSYNTHESIS HOMEWORK

3. Explain the problem with rubisco.

4. Compare and contrast C3, C4, and CAM plants.