A-level Biology/Central Concepts/Photosynthesis

Photosynthesis is the method that plants and photoautotrophes utilize light energy to produce ATP via photophosphorylation in order to anabolise sugars. It is an energy transfer process, and almost all energy transferred to ATP in all organisms is derived from light energy trapped by autotrophs.

The equation for it is listed below;

6 CO_2(g) + 6 H₂O_(l) + photons → C₆H₁₂O_6(aq) + 6 O_2(g)

carbon dioxide + water + light energy → glucose + oxygen

Two reactions are involved in photosynthesis, the light dependent and independent.

Light Energy

Light energy is used to split H₂O into H and O in a process called photolysis, and is trapped by photosynthetic pigments. These pigments fall into 2 categories: chlorophylls and carotenoids. Chlorophylls absorb mainly red and blue-violet light, reflecting green light, whilst carotenoids absorb mainly the blue-violet light. These spectra, where the pigments can absorb light energy are known as absorption spectra (singular: spectrum). An action spectrum is a graph displaying the rates of photosynthesis at different wavelengths of light. See [1] for an action spectrum. The shorter the wavelength, the greater the energy contained.

Photosynthesis converts light energy to chemical energy through exciting electrons within the pigments. The 2 photosynthetic pigments fall into 2 sub-categories: i)primary pigments and ii)accessory pigments. The primary pigments comprises 2 types of "chlorophyll a" (with slightly different absorption peaks), whereas accessory pigments consists of other types of "chlorophyll a", "chlorophyll b" and carotenoids. All the pigments are arranged in photosystems, and several hundred accessory pigments surround a primary pigment, so that the light energy absorbed by accessory pigments can be transmitted to the primary pigments. The primary pigment is known as the reaction centre.

Light Dependent

Light-dependent reactions are the synthesis of ATP from ADP+Pi and the breakdown of H₂O using light energy to give protons.

Photophosphorylation

The Z-scheme - representing the relative energy levels of the photosystems

This is the process by which ATP is synthesised from ADP+Pi using light energy,and can be either cyclic or non-cyclic.

Cyclic

This type of photophosphorylation involves only photosystem I. When light is absorbed by photosystem I and passed to "chlorophyll a (P700)", an electron in this chlorophyll molecule is excited to a higher energy level and then captured by an electron acceptor. It is then passed back to a "chlorophyll a" molecule through a cycle of electron carriers (or electron transport chain/ETC), which at the meanwhile, release energy to synthesise ATP from ADP+Pi (phosphorylation) by a mechanism known as chemiosmosis. This ATP later enters the light independent stage.

Non-Cyclic

Non-cyclic photophosphorylation utilises both photosystems in a "Z-Scheme". See picture above. Light is absorbed by both photosystems I and II, and excited electrons are passed from both primary pigments to electron acceptors as well as electron transport chain before exiting the photosystems positively charged. Photosystem I receives electrons from photosystem II, which instead replenishes electrons from the photolysis of water. In this chain, as in cyclic, ATP is synthesised using the energy lost during the phase of electron transport chain.

Photolysis of water

Photosystem II has a water-splitting enzyme which catalyzes the breakdown of H₂O, producing O₂ as a waste product. The H⁺ combine with e_- from photosystem I and the carrier molecule NADP to give reduced NADP. This then passes to the light independent reactions, and is used in the synthesis of carbohydrates.

Light Independent

Calvin cycle

See picture

In the light-independent stage, RuBP (5-C) combines with one CO₂ molecule, that then splits into 2 glycerate-3-phosphate (GP) molecules (3-C), which is finally reduced to 2 triose phosphates (3-C). 1 triose phosphates (3-C) feed back in to the cycle to regenerate RuBP (5-C), 1 is polymerised into starch. The products of this cycle are used to form glucose, amino acids or lipids.

Leaf Structure/Function

The leaf is the main site of photosynthesis in most plants - it has a broad, thin lamina and an extensive network of veins. The functions of a leaf are best achieved by containing photosynthetic pigments, absorbing carbon dioxide (and disposing of oxygen) and have a water and solute supply/transport route. The leaf itself has a large surface area and arrangement such that it can absorb as much light as possible.The green color is from chlorophyll, where absorbs light energy to drive the synthesis of organic molecules in the chloroplast. Chlorophyll’s pigments absorb visible light.

Epidermis/Cuticle

The cuticle, on the upper epidermis provides a watertight layer for the top of the plant, and together with the epidermis (thin, flat, transparent cells) allow light through to the mesophyll below, and protect the leaf.

Palisade Cells

The palisade cells are the main site of photosynthesis, as they have many more chloroplasts than spongy mesophylls, and also have several adaptions to maximise photosynthetic efficiency;

• Large Vacuole - Restricts chloroplasts to a layer near the outside of the cell where they can be reached by light more easily.
• Cylindrical Arrangement - They are arranged at right angles to the upper epidermis, reducing the number of light-absorbing cross walls preventing light from reaching the chloroplasts. This also allows long-narrow air spaces between them, providing a large surface area for gaseous exchange.
• Movement of chloroplasts - Proteins can move the chloroplasts within cells to absorb maximum light.
• Thin cell walls - to allow gases to easily diffuse through them.

Spongy Mesophyll

These cells are the main site for gaseous exchange, and contain fewer chloroplasts, and will only photosynthesise at high light intensities. The irregular packing of the cells provides a large surface area for gaseous exchange and have a thin cell wall for enhanced gaseous exchange.

Stomata

CO2 enters and O2 exits the leaf through stomata.

Chloroplast ultrastructure 1. outer membrane 2. intermembrane space 3. inner membrane (1+2+3: envelope) 4. stroma (aqueous fluid) 5. thylakoid lumen (inside of thylakoid) 6. thylakoid membrane 7. granum (stack of thylakoids) 8. thylakoid (lamella) 9. starch 10. ribosome 11. plastidial DNA 12. drop of lipids)

The lower epidermis is basically the same as the upper, except that there are many stomata in the lower epidermis, which are pores in the epidermis through which gaseous exchange occurs. Each stomata is bounded by two guard cells, and changes in the turgidity of theses guard cells cause them to change shape so that they open and close the pore. If the guard cells gain water, the pore is open, and vice-versa.

Osmosis controls how much water is in the guard cells, and to have more end the water potential of the guard cells must belowered via the active removal of hydrogen ions, in an active transport process.

Chloroplast

The actual photosynthetic organelle is chloroplast - an image of a chloroplast is on the right. As you can see, 1,2 and 3 are the envelope of two phospholipid membranes. The system of membranes (4) running through the cell is the stroma, and provides space for the thylakoids, a series of flattened fluid-filled sacs (5,6), which form stacks called grana (7). This membrane system of the grana provides a large surface area for reactions, and as said before, the pigment molecules are also arranged in light-harvesting clusters with primary pigments and accessory pigments. Chloroplasts are found in cells of mesophyll, the interior tissue of the leaf. The chlorophyll is in the membranes of thylakoids. Thylakoids stack in grana

Stroma

The stroma is the site of the light-independent reactions, contain the Calvin cycle enzymes, sugars and organic acids. The ribosome (10), DNA (11) and some lipids (12) can also be seen.

Rate of Photosynthesis

Factors

The main factors that affect the rate of photosynthesis are light intensity, temperature and carbon dioxide concentration. at constant temperature, the rate of photosynthesis varies with light intensity, increasing at first but at higher light intensities this increase levels off.

The effect on the rate of photosynthesis at constant light intensities and varying temperatures - at high light intensities, the rate of photosynthesis increases as temperature does (to a limited range), but at low light intensities temperature does not make much difference.

Dehydration

Dehydration is one of the most common problems for plants, and it sometimes requires trade-offs with other metabolic processes, like photosynthesis. On hot and dry days, plants close stomata to conserve water but it then limits the ability for photosynthesis. The closing of stomata reduces access to the CO2 and causes O2 to build up. Plants have developed some mechanisms to solve this problem.

-In most plants (C3 plants), initial fixation of C02 via rubisco and it forms a three-carbon compound. Rubisco adds O2 instead of CO2 in the Calvin cycle during photorespiration. Photorespiration consumes O2 and releases C02 with no producing ATP and carbohydrate.

-C4 plants minimize the cost of photorespiration by incorporating CO2 into four carbon compounds in mesophyll cells. Enzyme PEP carboxylase is required during this process. PEP carbonxylase has a higher affinity for CO2 than rubisco, so it can fix CO2 even when CO2 concentrations are low.

-CAM plants are those that use CAM to fix carbon. They open their stomata at night, and this incorporates CO2 into organic acids. During the day, they close their stomata to reduce the chance of dehydration and CO2 is now released from organic acid in the calvin cycle.

References

Neil A. Campbell, Jane B. Reece "Biology 8th edition"