This photochemical energy is stored ultimately in carbohydrates which are made using ATP from the energy harvesting , carbon dioxide and water. In most cases, a byproduct of the process is oxygen, which is released from water in the capture process. Photosynthesis is responsible for most of the oxygen in the atmosphere and it supplies the organic materials and most of the energy used by life on Earth. The steps in the photosynthesis process varies slightly between organisms.
In a broad overview, it always starts with energy capture from light by protein complexes, containing chlorophyll pigments, called reaction centers. Energy from the light is used to strip electrons away from electron donors usually water and leave a byproduct oxygen, if water was used.
Chloroplasts are found in almost all aboveground plant cells, but are primarily concentrated in leaves. The interior of a leaf, below the epidermis is made up of photosynthesis tissue called mesophyll, which can contain up to , chloroplasts per square millimeter. Within the inner chloroplast membrane is the stroma, in which the chloroplast DNA and the enzymes of the Calvin cycle are located.
Also within the stroma are stacked, flattened disks known as thylakoids which are defined by their thylakoid membranes. The space within the thylakoid membranes are termed the thylakoid spaces or thylakoid lumen. The protein complexes containing the light-absorbing pigments, known as photosystems, are located on the thylakoid membrane. Besides chlorophylls, carotenes and xanthophylls are also present, allowing for absorption of light energy over a wider range.
The same pigments are used by green algae and land plants. Brown algae and diatoms add fucoxanthin a xanthophyll and red algae add phycoerythrin to the mix. In plants and algae, the pigments are held in a very organized fashion complexes called antenna proteins that help funnel energy, through resonance energy transfer, to the reaction center chlorophylls.
A system so organized is called a light harvesting complex. The electron transport complexes of photosynthesis are also located on the thylakoid membranes.
Image by Aleia Kim. This will be discussed elsewhere in the section on metabolism HERE. The chloroplasts are where the energy of light is captured, electrons are stripped from water, oxygen is liberated, electron transport occurs, NADPH is formed, and ATP is generated.
The thylakoid membrane does its magic using four major protein complexes. There are three fundamental similarities between oxidative phosphorylation and photophosphorylation. In this chapter we first consider the process of oxidative phosphorylation. We begin with descriptions of the components of the electron transfer chain in mitochondria, the sequence in which these carriers act, and their organization into large functional complexes in the mitochondrial inner membrane.
We then look at the chemiosmotic mechanism by which electron transfer is used to drive ATP synthesis, and the means by which this process is regulated in coordination with other energy-yielding pathways. The evolutionary origins of mitochondria, touched upon in Chapter 2, are further considered.
With this understanding of mitochondrial oxidative phosphorylation, we turn to photophosphorylation. Light-absorbing pigments in the membranes of chloroplasts and photosynthetic bacteria transfer the energy of absorbed light to reaction centers where electron flow is initiated. Electron flow occurs through a series of carriers and, as in mitochondria, this flow drives ATP synthesis by the chemiosmotic mechanism.
The discovery in by Eugene Kennedy and Albert Lehninger that mitochondria are the site of oxidative phosphorylation in eukaryotes marked the beginning of the modern phase of studies of biological energy transductions.
Mitochondria are organelles of eukaryotic cells, believed to have arisen during evolution when aerobic bacteria capable of oxidative phosphorylation took up symbiotic residence within a primitive, anaerobic, eukaryotic host cell see Fig.
Mitochondria, like gramnegative bacteria, have two membranes Fig. The outer mitochondrial membrane is readily permeable to small molecules and ions; transmembrane channels composed of the protein porin allow most molecules of molecular weight less than 5, to pass easily. The inner membrane bears the components of the respiratory chain and the enzyme complex responsible for ATP synthesis. Figure Biochemical anatomy of a mitochondrion.
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