The light-dependent reactions, or light reactions, are the first stage of photosynthesis, the process by which plants capture and store energy from sunlight. In this process, light energy is converted into chemical energy, in the form of the energy-carrying molecules ATP and NADPH. In the light-independent reactions(also called dark reactions, by convention, as they are driven by products of light, ATP and NADPH-it should not be misunderstood that they occur in dark or they are independent of the need of light), the formedNADPH and ATP drive the reduction of CO₂ to more useful organic compounds, such as glucose.

The light-dependent reactions take place on the thylakoid membrane inside a chloroplast. The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place. The thylakoid membrane contains some integral membrane protein complexes that catalyze the light reactions. There are four major protein complexes in the thylakoid membrane: Photosystem I (PSI), Photosystem II (PSII), Cytochrome c6f complex and ATP synthase. These four complexes work together to ultimately create the products ATP and NADPH.

The two photosystems absorb light energy through proteins containing pigments, such as chlorophyll. The light-dependent reactions begin in photosystem II. When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, an electron in this molecule attains a higher energy level. Because this state of an electron is very unstable, the electron is transferred from one to another molecule creating a chain of redox reactions, called an electron transport chain (ETC). The electron flow goes from PSII to cytochrome b6f to PSI. In PSI the electron gets the energy from another photon. The final electron acceptor is NADP. In oxygenic photosynthesis, the first electron donor is water, creating oxygen as a waste product. Inanoxygenic photosynthesisvarious electron donors are used.

Cytochrome b6f and ATP synthase work together to create ATP. This process is called photophosphorylation, which occurs in two different ways. In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump protons from the stroma to the lumen. The proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from not only PSII but also PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light-independent reactions.

The net-reaction of all light-dependent reactions in oxygenic photosynthesis is:
2H₂O + 2NADP⁺ + 3ADP + 3P_i→ O₂ + 2NADPH + 3ATP

Light to chemical energy

The two photosystems are protein complexes that absorb photons and are able to use this energy to create an electron transport chain. Photosystem I and II are very similar in structure and function. They use special proteins, called light-harvesting complexes, to absorb the photons with very high effectiveness. If a special pigment molecule in a photosynthetic reaction center absorbs a photon, an electron in this pigment attains the excited state and then is transferred to another molecule in the reaction center. This reaction, called photoinduced charge separation, is the start of the electron flow and is unique because it transforms light energy into chemical forms.

The light-harvesting system

A common misconception is that photosynthesis relies only on chlorophyll pigments. The truth is that photosynthesis would be rather inefficient using only chlorophyll molecules. Chlorophyll molecules absorb light only at specific wavelengths (see image). A large gap is present in the middle of the visible regions between approximately 450 and 650 nm. This gap corresponds to the peak of the solar spectrum, so failure to collect this light would constitute a considerable lost opportunity. That's why photosynthesis organisms have developed a light-harvesti

A climate cycle is a type of recurring climate pattern that involves natural cyclic variations in the Earth's surface temperature, as indicated by temperature proxies found in glacier ice, sea bed sediment, tree ring studies or otherwise.

One difficulty in detecting climate cycles is that the Earth's climate has been changing in non-cyclic ways over most scales of time. For instance, we are now in a period of global warming that seems to be anthropogenic. In a larger time frame, the Earth is emerging from the latest ice age, which means that climate has been changing over the last 15,000 years or so. And the Pleistocene period, dominated by repeated glaciations, has developed out of a more stable climate in the Miocene and Pliocene. All of these changes complicate the task of looking for cyclical behavior in the climate.

There are nevertheless several climate cycles which have been identified or hypothesized. These range from the cyclic behaviour of the Earth's orbital parameters (called Milankovich cycles) which are reflected in the long-term climatic record spanning several ice ages, through the Atlantic Multidecadal Oscillation, to short cycles such as:

• the El Niño Southern Oscillation
• the Pacific decadal oscillation
• the Interdecadal Pacific Oscillation
• the Arctic oscillation
• the North Atlantic Oscillation
• the North Pacific Oscillation.

The 11-year Sun spot cycle (the Hale cycle) may also be discernible in the climate record (see Solar variation).

Climate cycles are popular with media. One example is a 2003 study on the correlation between wheat prices and sunspot numbers..

There is also a 1,500-year climate cycle claimed from ice core samples, and used in the global warming controversy.

Other than the Milankovich cycles (and perhaps the Hale cycle), no climate cycle is found to be perfectly periodic and a Fourier analysis of the data does not give a sharp spectrum.