Biology

By Gerard ChretienBiology
Photosynthesis:Energy from the Sun
Identifying Photosynthetic Reactants And Products
A. The water for photosynthesis in land plants comes primarily from the soil and must travel from the roots to leaves.


B. Carbon dioxide is taken in, and water and O2 are released, through tiny openings in leaves , called stromata.

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C.Light is absolutely necessary for the production of oxygen and carbohydrate.


The photosynthetic production of oxygen by green plants is an important source of atmospheric oxygen, which most organisms including plants themselves require in order to complete their respiratory chains and obtain the energy for life.


The TWO PATHWAYS OF PHOTOSYNTHESIS: AN OVERVIEW
The overall photosynthetic reaction takes place in the chloroplasts of photosynthetic cells, which in most plants are found in the leaves. Photosynthesis consists of many reactions of many reactions that can be divided into two pathways:
A. The first pathway, called the light reactions, driven by light energy. It produces ATP and a reduced electron carrier (NADPH + H+).


B.The second pathway, called the Calvin-Benson cycle, does not use lightly. It uses ATP, NADPH + H+, and CO2 to produce sugar. In the first pathway of photosynthesis —-the light reactions—light energy is captured by pigments molecules and is used to produce ATP from ADP and Pi. The light reactions are mediated by molecular assemblies called photosystems.


The NADH++H+ andn Atp produced by the light reactions are used in the second pathway, the CalvinBenson cycle, whose reactions trap CO2, and reduce the resulting acid to sugar. This pathway is also known as the photosynthesis carbon reduction cycle, or simply the dark reactions. The reactions of both pathways proceed within the chloroplast, but they reside in different parts of that organelle.


PROPERTIES OF LIGHT AND PIGMENTS
Light is a source of both energy and information.


Light comes in packects called photons
Light is a form of electromagnetic radiation. It comes in discrete packects called photons. Light also behaves as if it were propagated in waves. The wavelength of light is the distance from the peak of one wave to the peak of the next. The colors relate to the wavelenghts of the light. The speed of light in a vacuum is one of the universal constants of nature. In a vacuum, light travels at 3 x 10 to 10 centimeters per second. The amount of energy, E, contained in a single photon is directly proportional to to its frequency. Thus shorter wavelenghts mean greater energies; that is, energy is inversely proportional to wavelength. The brightness, or intensity, of light at a given point is the amount of energy falling on a defined area.


ABSORPTION OF A PHOTON PUTS A PIGMENT IN A EXCITED STATE
A. The photon may bounce off the molecule—it may be reflected.

B. The photon may pass through the molecule— it may be transmitted .

C. The photon may be absorbed by the molecule.

When a molecule absorbs a photon, it acquires the energy of that photon. It is thereby raised from a ground state to an excited state. The difference in energy between he excited state and the ground state is precisely equal to the energy of the absorbed photon. All molecules absorb electromagnetic radiation. Molecules that absorb wavelenghts in the visible region of the spectrum are called pigments.


LIGHT ABSORPTION AND BIOLOGICAL ACTIVITY VARY WITH WAVELENGTH
A given type of molecule can absorb radiant of only certain wavelengths. A plot of the effectiveness og light as a function of wavelength called an action spectrum. All wavelength of visible are at least somewhat effective in causing photosynthesis, but the blue and orange-red wavelength are the most effective.


Photsynthesis uses chlorophylls and accessory pigments
Certain pigments are important in biological processes
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