Exploring Photosynthesis: An Educational Guide to Energy Production in Plants
Photosynthesis 3D Diagram
Tree producing oxygen using water and sunlight in photosynthesis process as nature green energy symbol explanation scheme concept. Colorful biology scheme for education in flat style.
This exciting model teaches the Light Reactions and Calvin Cycle in photosynthesis! It also explains the cyclic photophosphorylation and can be used to compare with our amazing ATP Synthase 3D model.
Photosynthesis
Photosynthesis is the process by which green plants, algae and some bacteria turn sunlight into energy-rich molecules like carbohydrates and oxygen. This is the reason that the Earth is blanketed with an oxygen-rich atmosphere, and it’s the source of most of our food and energy.
Photosynthesis occurs in regions of the cell called chloroplasts. Pigment molecules, including the green one that gives plant leaves their color (chlorophyll), absorb light energy to power two main stages of the reaction.
The light reactions of photosynthesis occur in the thylakoid membranes, and the dark reactions (also known as the Calvin cycle) occur in a region of the chloroplast stroma that contains enzymes to “fix” carbon dioxide from water into carbohydrates. Both the light and dark reactions are driven by energy from absorbed sunlight (an exergonic reaction). They produce the chemical energy-rich molecule ATP, which is used to power cellular respiration.
Light Reactions
The light reactions of photosynthesis take place in the thylakoid membranes of organelles called chloroplasts. They convert solar energy into adenosine triphosphate (ATP) and reduced electron carrier nicotinamide adenine dinucleotide phosphate (NADPH). The light reactions depend on large complexes of proteins and pigments that are optimized to absorb specific wavelengths of light, known as photosystems. There are two types of photosystems, PSI and PSII, both embedded in the thylakoid plasma membrane.
Each photosystem contains a special pair of chlorophyll a molecules called the reaction center. These molecules act as energy funnels, passing energy to other pigments through direct electromagnetic interactions known as resonance energy transfer. The excited pigments then pass the energy to the central region of the photosystem where it is used to generate ATP and NADPH.
Electron Transport
The final stage of photosynthesis involves a series of reactions that uses energy from sunlight to produce glucose and oxygen. The plant then releases the oxygen into the atmosphere, where it can be used by humans and animals to breathe.
These reactions are called the electron transport chain. They are a series of redox reactions that work together like a bucket brigade, passing electrons rapidly from one protein complex to the next. At the end of the chain, electrons are reduced to molecular oxygen by enzymes such as ATP synthase.
Electron carriers are small organic molecules that shuttle electrons between the two compartments where the light-dependent reactions and the carbon dioxide fixation occur. The 3D images reveal that these carriers have channels that connect the thylakoids to the pyrenoid.
Photophosphorylation
This part of photosynthesis generates ATP by adding a phosphate group to ADP. It is powered by the energy of light that strikes the photosynthetic reaction centers.
Electrons from water or a different electron donor are removed from the ETC and donated to the carrier, which is then accepted by NADP+ (generating an oxidation-reduction cycle). This generates a proton gradient across the thylakoid membrane and drives ATP synthesis.
Anoxygenic photophosphorylation can occur with oxygen or without it, but in either case it involves a noncyclic electron transport chain. Oxygenic photophosphorylation is used by cyanobacteria and algae, where the waste product is O2. Anoxygenic photophosphorylation is used by green bacteria and some plants. In the latter, it can involve diverse electron donors other than water or succinate and the waste product is never O2. Plants can switch between cyclic and noncyclic photophosphorylation to achieve the right ratio of ATPs to NADPH needed for carbon assimilation in their dark phase.
ATP Synthesis
The chemiosmotic proton gradient drives ATP synthase to combine ADP and inorganic phosphate (Pi) into adenosine triphosphate (ATP). This molecular motor complex consists of two molecular machines – the membrane-bound Fo complex and the soluble F1 complex – linked by a central stalk. The F1 complex is prevented from rotating by binding to a protein called the Stator.
The energy released by ATP synthesis is converted to a thermal free energy that can be dissipated as waste heat. Thus, a tradeoff exists between the speed of ATP synthesis and its efficiency.
The central stalk gamma subunit rotates 120o counterclockwise with each ATP synthesis or hydrolysis step. This rotation is coupled to the protonation-deprotonation cycles of three c subunits. These steps lead to the transition from state 1 to state 10, and the whole sequence is reversible.