Chapter 10 - Photosynthes

Life on Earth is fuelled by the sun. Chloroplasts are cellular organelles found in plants and other photosynthetic organisms.

Chloroplasts include specialized molecular complexes that transform light energy that has traveled 150 million kilometers from the sun to chemical energy that is stored in sugar and other organic molecules. Photosynthesis is the name given to this conversion process. To begin, consider photosynthesis in its ecological context.

Photosynthesis directly or indirectly feeds nearly the whole living universe. An organism obtains the organic molecules it needs for energy and carbon skeletons through one of two mechanisms: autotrophic nutrition or heterotrophic nutrition.

• The term Autotrophs refers to being “self-feeders” (auto- means “self,” and trophos means “feeder”) as they sustain themselves without eating anything derived from other living beings.

Autotrophs synthesize organic compounds from CO2 and other inorganic raw materials found in the environment. Because autotrophs are the ultimate suppliers of organic molecules for all nonautotrophic species, scientists refer to them as the biosphere's producers.

Almost all plants are autotrophs, meaning that the only nutrients they need are water and minerals from the soil, as well as carbon dioxide from the atmosphere. Plants, in particular, are photoautotrophs, which are creatures that use light as a source of energy to create organic compounds.

The second primary route of feeding for heterotrophs in the acquisition of organic material. Because they are unable to create their own sustenance, they must rely on chemicals produced by other creatures (hetero means "other"). Heterotrophs are the consumers of the biosphere.

The most apparent example of "other-feeding" is when an animal consumes plants or other creatures. However, heterotrophic feeding might be more subtle.

Decomposers are heterotrophs that devour the remnants of other species by decomposing and feeding on organic litter such as dead organisms, excrement, and fallen leaves.

The majority of fungi and several types of prokaryotes obtain their nutrition in this manner. Almost all heterotrophs, including humans, are entirely dependent on the environment.

Photosynthesis turns light energy into chemical energy in the form of food. The extraordinary capacity of an organism to collect light energy and use it to fuel the synthesis of organic molecules arises from the cell's structural organization:

Photosynthetic enzymes and other molecules are clustered together in a biological membrane to efficiently carry out the required series of chemical reactions.

The photosynthesis mechanism most likely evolved in a population of bacteria with infolded portions of the plasma membrane harboring clusters of such molecules. Infolded photosynthetic membranes of extant photosynthetic bacteria operate similarly to the interior membranes of chloroplasts.

Light absorbed by chlorophyll causes electrons and hydrogen ions to be transferred from water to an acceptor termed NADP1 (nicotinamide adenine dinucleotide phosphate), where they are temporarily stored.

(The electron acceptor NADP+ is related to NAD+, which serves as an electron carrier in cellular respiration; the two molecules differ only by the inclusion of an additional phosphate group in the NADP+ molecule.)

Solar energy is used in the light processes to convert NADP+ to NADPH by contributing a pair of electrons and an H+.

The light processes also produce ATP by utilizing chemiosmosis to drive the addition of a phosphate group to ADP, a process known as photophosphorylation.

Different pigments absorb light of different wavelengths, and the absorbed wavelengths vanish.

When we shine white light on a pigment, we see the hue that is most reflected or transmitted by the pigment.

(A pigment appears black if it absorbs all wavelengths.) When we look at a leaf, we see green because chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light.

A spectrophotometer may be used to assess a pigment's capacity to absorb different wavelengths of light. This gadget sends light beams of various wavelengths through a pigment solution and measures the proportion of light transmitted at each wavelength.

Because light can only do work in chloroplasts if it is absorbed, the absorption spectra of chloroplast pigments give information on the relative efficiency of different wavelengths for promoting photosynthesis.

The absorption spectra of three types of pigments in chloroplasts are shown: chlorophyll a, the primary light-capturing pigment that participates directly in light processes; chlorophyll b, an accessory pigment; and carotenoids, a distinct group of auxiliary pigments.

The chlorophyll spectrum implies that violet-blue and red light are the most effective for photosynthesis because they are absorbed, whereas green is the least effective.

When a photon absorbs a photon, it raises an electron to an excited state, but the electron cannot stay there for long. The excited state is unstable, as are all high-energy states.

When isolated pigment molecules absorb light, their excited electrons return to the ground-state orbital in a billionth of a second, releasing surplus energy as heat.

On a bright day, this conversion of light energy to heat is what causes the top of a car to get extremely hot. (White automobiles are the coolest since their paint reflects all visible light wavelengths.)