8: Photosynthesis and Respiration

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Plant Physiology and Regulation

Plant physiology focuses on the chemistry and physics of how plants function. Plants capture light energy and produce sugars through photosynthesis and break down these sugars through aerobic cellular respiration. They respond to a variety of environmental conditions through growth changes, life events like germination or flowering, and even, in special cases, through movement. Plants rely on soils for mineral nutrients and water, and biogeochemical cycles replenish soils with these nutrients. Once water and minerals are absorbed, they must be transported through the xylem, and this movement is driven by the loss of water vapor from leaves (transpiration) and the cohesive and adhesive properties of water. Likewise, sugar-rich assimilate must be moved, or translocated, through the phloem. Five main types of hormones in plants are responsible for relaying messages throughout the plant body. Throughout this unit are examples of how plants regulate their internal conditions whether it the concentration of carbon dioxide in the leaves; the positioning of stems, roots, and leaves; or the movement and retention of water (Figure \(\PageIndex{1}\)).

Microscope image of an open stoma (top) and closed stoma (bottom). Both are surrounded by transparent, thick-walled guard cells. — Figure \(\PageIndex{1}\): Plants balance water loss with their need for carbon dioxide through stomatal opening (top) and closure (bottom). Image by KuriPop (CC-BY-SA).

Attribution

Melissa Ha (CC-BY-SA)

Photosynthesis and Respiration

Photosynthesis and aerobic cellular respiration are key metabolic pathways. Photosynthesis is essential to all life on earth; both plants and animals depend on it (Figure \(\PageIndex{2}\)). It is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that most organisms use to power their metabolism through aerobic cellular respiration or other pathways. In brief, the energy of sunlight is captured and used to energize electrons, which are then stored in the covalent bonds of sugar molecules. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis almost 200 million years ago.

View underneath an oak tree, showing lobed leaves with light penetrating them — Figure \(\PageIndex{2}\): The leaves of this oak tree capture light energy from the sun through photosynthesis. (The dark spheres are oak apple galls, induced by the California gall wasp.) Image by Melissa Ha (CC-BY).

Attribution

Curated and authored by Melissa Ha using the following sources:

8.1 Overview of Photosynthesis from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
3 Photosynthesis from Introduction to Botany by Alexey Shipunov (public domain)

8.1: Energy and ATP
8.2: Aerobic Cellular Respiration
Through aerobic cellular respiration, organisms break down sugars to produce usable energy in the form of ATP. This process consumes gaseous oxygen and releases carbon dioxide and water. There are four steps: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.
8.3: Photosynthesis Overview and Equation
Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. In brief, the energy of sunlight is captured and used to energize electrons, which are then stored in the covalent bonds of sugar molecules.
8.4: Discovery of Photosynthesis
The history of the studies done on photosynthesis dates back into the 17th century with Jan Baptist van Helmont. He rejected the ancient idea that plants take most of their biomass from the soil.
8.5: The Light-dependent Reactions
Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to build carbohydrate molecules. However, autotrophs only use a few specific components of sunlight.
8.6: Light-independent Reactions
The enzymatic stage has many participants. These include carbon dioxide, hydrogen carrier with hydrogen (NADPH), ATP, ribulose biphosphate (RuBP), and RuBisCO along with some other enzymes. Everything occurs in the matrix (stroma) of the chloroplast.
8.7: Photorespiration and Photosynthetic Pathways
Photorespiration occurs when RuBisCO binds to gaseous oxygen rather than carbon dioxide. It undoes the good anabolic work of photosynthesis, reducing the net productivity of the plant. Plants in different environments have adaptations to reduce photorespiration while minimizing water loss.
8.8: Chapter Summary

Thumbnail: Plant cells with visible chloroplasts (from a moss, Plagiomnium affine). (CC BY SA 3.0 Unported; Kristian Peters).