Photosynthesis Essay Example


Photosynthesis is the ultimate source of all of humankind's food and oxygen, whereas fossilized photosynthetic fuels provide ∼87% of the world's energy. It is the biochemical process that sustains the biosphere as the basis for the food chain. The oxygen produced as a by-product of photosynthesis allowed the formation of the ozone layer, the evolution of aerobic respiration and thus complex multicellular life.

Oxygenic photosynthesis involves the conversion of water and CO2 into complex organic molecules such as carbohydrates and oxygen. Photosynthesis may be split into the ‘light’ and ‘dark’ reactions. In the light reactions, water is split using light into oxygen, protons and electrons, and in the dark reactions, the protons and electrons are used to reduce CO2 to carbohydrate (given here by the general formula CH2O). The two processes can be summarized thus:

Light reactions:

Dark reactions:


The positive sign of the standard free energy change of the reaction (ΔG°) given above means that the reaction requires energy (an endergonic reaction). The energy required is provided by absorbed solar energy, which is converted into the chemical bond energy of the products (Box 1).

Box 1.

Standard free energy change

Photosynthesis converts ∼200 billion tonnes of CO2 into complex organic compounds annually and produces ∼140 billion tonnes of oxygen into the atmosphere. By facilitating conversion of solar energy into chemical energy, photosynthesis acts as the primary energy input into the global food chain. Nearly all living organisms use the complex organic compounds derived from photosynthesis as a source of energy. The breakdown of these organic compounds occurs via the process of aerobic respiration, which of course also requires the oxygen produced by photosynthesis.

Unlike photosynthesis, aerobic respiration is an exergonic process (negative ΔG°) with the energy released being used by the organism to power biosynthetic processes that allow growth and renewal, mechanical work (such as muscle contraction or flagella rotation) and facilitating changes in chemical concentrations within the cell (e.g. accumulation of nutrients and expulsion of waste). The use of exergonic reactions to power endergonic ones associated with biosynthesis and housekeeping in biological organisms such that the overall free energy change is negative is known as ‘coupling’.

Photosynthesis and respiration are thus seemingly the reverse of one another, with the important caveat that both oxygen formation during photosynthesis and its utilization during respiration result in its liberation or incorporation respectively into water rather than CO2. In addition, glucose is one of several possible products of photosynthesis with amino acids and lipids also being synthesized rapidly from the primary photosynthetic products.

The consideration of photosynthesis and respiration as opposing processes helps us to appreciate their role in shaping our environment. The fixation of CO2 by photosynthesis and its release during breakdown of organic molecules during respiration, decay and combustion of organic matter and fossil fuels can be visualized as the global carbon cycle (Figure 1).

Figure 1The global carbon cycle

The relationship between respiration, photosynthesis and global CO2 and O2 levels.

At present, this cycle may be considered to be in a state of imbalance due to the burning of fossil fuels (fossilized photosynthesis), which is increasing the proportion of CO2 entering the Earth's atmosphere, leading to the so-called ‘greenhouse effect’ and human-made climate change.

Oxygenic photosynthesis is thought to have evolved only once during Earth's history in the cyanobacteria. All other organisms, such as plants, algae and diatoms, which perform oxygenic photosynthesis actually do so via cyanobacterial endosymbionts or ‘chloroplasts’. An endosymbiotoic event between an ancestral eukaryotic cell and a cyanobacterium that gave rise to plants is estimated to have occurred ∼1.5 billion years ago. Free-living cyanobacteria still exist today and are responsible for ∼50% of the world's photosynthesis. Cyanobacteria themselves are thought to have evolved from simpler photosynthetic bacteria that use either organic or inorganic compounds such a hydrogen sulfide as a source of electrons rather than water and thus do not produce oxygen.

The site of photosynthesis in plants

In land plants, the principal organs of photosynthesis are the leaves (Figure 2A). Leaves have evolved to expose the largest possible area of green tissue to light and entry of CO2 to the leaf is controlled by small holes in the lower epidermis called stomata (Figure 2B). The size of the stomatal openings is variable and regulated by a pair of guard cells, which respond to the turgor pressure (water content) of the leaf, thus when the leaf is hydrated, the stomata can open to allow CO2 in. In contrast, when water is scarce, the guard cells lose turgor pressure and close, preventing the escape of water from the leaf via transpiration.

Figure 2Location of the photosynthetic machinery

(A) The model plant Arabidopsis thaliana. (B) Basic structure of a leaf shown in cross-section. Chloroplasts are shown as green dots within the cells. (C) An electron micrograph of an Arabidopsis chloroplast within the leaf. (D) Close-up region of the chloroplast showing the stacked structure of the thylakoid membrane.

Within the green tissue of the leaf (mainly the mesophyll) each cell (∼100 μm in length) contains ∼100 chloroplasts (2–3 μm in length), the tiny organelles where photosynthesis takes place. The chloroplast has a complex structure (Figure 2C, D) with two outer membranes (the envelope), which are colourless and do not participate in photosynthesis, enclosing an aqueous space (the stroma) wherein sits a third membrane known as the thylakoid, which in turn encloses a single continuous aqueous space called the lumen.

The light reactions of photosynthesis involve light-driven electron and proton transfers, which occur in the thylakoid membrane, whereas the dark reactions involve the fixation of CO2 into carbohydrate, via the Calvin–Benson cycle, which occurs in the stroma (Figure 3). The light reactions involve electron transfer from water to NADP+ to form NADPH and these reactions are coupled to proton transfers that lead to the phosphorylation of adenosine diphosphate (ADP) into ATP. The Calvin–Benson cycle uses ATP and NADPH to convert CO2 into carbohydrates (Figure 3), regenerating ADP and NADP+. The light and dark reactions are therefore mutually dependent on one another.

Figure 3Division of labour within the chloroplast

The light reactions of photosynthesis take place in the thylakoid membrane, whereas the dark reactions are located in the chloroplast stroma.

Photosynthetic electron and proton transfer chain

The light-driven electron transfer reactions of photosynthesis begin with the splitting of water by Photosystem II (PSII). PSII is a chlorophyll–protein complex embedded in the thylakoid membrane that uses light to oxidize water to oxygen and reduce the electron acceptor plastoquinone to plastoquinol. Plastoquinol in turn carries the electrons derived from water to another thylakoid-embedded protein complex called cytochrome b6f (cytb6f). cytb6f oxidizes plastoquinol to plastoquinone and reduces a small water-soluble electron carrier protein plastocyanin, which resides in the lumen. A second light-driven reaction is then carried out by another chlorophyll protein complex called Photosystem I (PSI). PSI oxidizes plastocyanin and reduces another soluble electron carrier protein ferredoxin that resides in the stroma. Ferredoxin can then be used by the ferredoxin–NADP+ reductase (FNR) enzyme to reduce NADP+ to NADPH. This scheme is known as the linear electron transfer pathway or Z-scheme (Figure 4).

Figure 4The photosynthetic electron and proton transfer chain

The linear electron transfer pathway from water to NADP+ to form NADPH results in the formation of a proton gradient across the thylakoid membrane that is used by the ATP synthase enzyme to make ATP.

The Z-scheme, so-called since it resembles the letter ‘Z’ when turned on its side (Figure 5), thus shows how the electrons move from the water–oxygen couple (+820 mV) via a chain of redox carriers to NADP+/NADPH (−320 mV) during photosynthetic electron transfer. Generally, electrons are transferred from redox couples with low potentials (good reductants) to those with higher potentials (good oxidants) (e.g. during respiratory electron transfer in mitochondria) since this process is exergonic (see Box 2). However, photosynthetic electron transfer also involves two endergonic steps, which occur at PSII and at PSI and require an energy input in the form of light. The light energy is used to excite an electron within a chlorophyll molecule residing in PSII or PSI to a higher energy level; this excited chlorophyll is then able to reduce the subsequent acceptors in the chain. The oxidized chlorophyll is then reduced by water in the case of PSII and plastocyanin in the case of PSI.

Figure 5Z-scheme of photosynthetic electron transfer

The main components of the linear electron transfer pathway are shown on a scale of redox potential to illustrate how two separate inputs of light energy at PSI and PSII result in the endergonic transfer of electrons from water to NADP+.

The water-splitting reaction at PSII and plastoquinol oxidation at cytb6f result in the release of protons into the lumen, resulting in a build-up of protons in this compartment relative to the stroma. The difference in the proton concentration between the two sides of the membrane is called a proton gradient. The proton gradient is a store of free energy (similar to a gradient of ions in a battery) that is utilized by a molecular mechanical motor ATP synthase, which resides in the thylakoid membrane (Figure 4). The ATP synthase allows the protons to move down their concentration gradient from the lumen (high H+ concentration) to the stroma (low H+ concentration). This exergonic reaction is used to power the endergonic synthesis of ATP from ADP and inorganic phosphate (Pi). This process of photophosphorylation is thus essentially similar to oxidative phosphorylation, which occurs in the inner mitochondrial membrane during respiration.

An alternative electron transfer pathway exists in plants and algae, known as cyclic electron flow. Cyclic electron flow involves the recycling of electrons from ferredoxin to plastoquinone, with the result that there is no net production of NADPH; however, since protons are still transferred into the lumen by oxidation of plastoquinol by cytb6f, ATP can still be formed. Thus photosynthetic organisms can control the ratio of NADPH/ATP to meet metabolic need by controlling the relative amounts of cyclic and linear electron transfer.

Box 2.

Relationship between redox potentials and standard free energy changes

Photosynthesis Essay example

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Green plants are producers. This means that they can survive without animals! They can make lots of organic chemicals from a few simple inorganic chemicals. They need simple things like carbon dioxide and water and can make complex things like sugar, starch, fat, and proteins.

Plants get their nutrients from the environment. Carbon dioxide comes from the air (unless they are aquatic plants, in which case they get it from the water surrounding them). They get water from the soil. They also need other inorganic nutrients like nitrate, sulphate and phosphate. A few plants cannot get nitrate out of the soil so they have to eat animals to get the nitrogen which they must have for…show more content…

In photosynthesis, light energy is converted into chemical energy. When animals and plants respire, the chemical energy in glucose can be converted into other forms of energy e.g. kinetic energy.

Plants can make enough glucose on a sunny day to last them through the night and through lots of cloudy dark days, but they cannot store up lots of glucose. What they do is convert the extra glucose into starch. When they need to use the energy, they can turn the starch back into glucose. Starch can be stored in leaves or other parts of the plant. they can turn glucose into sucrose: this is a sugar carried around the plant in special tubes called phloem.

Photosynthesis results in an increase in biomass; i.e. there is more carbohydrate in the plant. They can turn some of the glucose into fat or protein. They have to make lots of different chemicals to grow, but the two most important ones are fats and proteins. To do this they need energy (growth requires energy from glucose). Plants also have to make a very special chemical called DNA: this is the hereditary chemical of all animals and plants. They must also make lots of new chlorophyll.

Like carbohydrates, fats also contain three elements:

· Carbon

· Hydrogen

· Oxygen

Proteins contain four or five elements:

· Carbon

· Hydrogen

· Oxygen


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