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Introduction to photosynthesis

Photosynthesis is the process that converts light energy into chemical energy (in the form of glucose).

Light energy (usually from the sun) is used to provide the energy for chemical reactions in photosynthesis. The overall chemical equation for photosynthesis is: $$$\ce{6CO2 + 6H2O ->[\text{light}] C6H12O6 + 6O2}$$$

$$$\text{Carbon dioxide + Water $\xrightarrow{\text{light}}$ Glucose + Oxygen}$$$

In plants, photosynthesis occurs in specialised cell organelles called chloroplasts.

Sugars produced by photosynthesis can be converted to starch for storage or used immediately for driving cell processes.

The energy plants get from glucose can be used to build new cells or transport substances around the plant.

Leaves absorb light energy and convert it into chemical energy.
Leaves absorb light energy and convert it into chemical energy.

Photosynthesis is a two-stage process. Only the first stage requires light energy.

Light-dependent stage Light-independent stage
Light energy is used to split water molecules. This process is called photolysis. Carbon dioxide is used to synthesise new sugars. This process is called carbon fixation.
The products of photolysis are oxygen ($$\ce{O2}$$) and hydrogen atoms. ($$\ce{H}$$) The products of the second stage are glucose and water.

Important! The light-independent reaction requires the products of the light-dependent reaction (i.e. oxygen and hydrogen atoms). It is therefore indirectly dependent on light.

Only the photolysis stage of photosynthesis requires light.
Only the photolysis stage of photosynthesis requires light.

Environmental factors affect the speed at which plants can photosynthesise. There are three important factors to consider:

  • Light intensity: photosynthesis increases with light intensity
  • Carbon dioxide: photosynthesis increases with carbon dioxide concentration
  • Temperature: photosynthesis increases with temperature until 35-40$$^\circ$$C. At this temperature the enzymes required for photosynthesis denature, stopping the process.
The rate of photosynthesis is affected by carbon dioxide concentration, light intensity and temperature.
The rate of photosynthesis is affected by carbon dioxide concentration, light intensity and temperature.

The rate of photosynthesis is also limited by internal factors (such as leaf surface area and chlorophyll levels).

Light levels vary with time of day, time of year and distance from the equator (latitude). As light levels increase, so does the rate of photosynthesis, since more light is exciting the chlorophyll.

Different plants are adapted to different light levels. Plants adapted to the shade reach photosynthetic capacity (maximum rate of CO$${_2}$$ assimilation) at a much lower light level than plants adapted to the higher light levels.

Shade plants also have a lower compensation point, i.e. the light level at which the rate of respiration exactly matches photosynthesis.

The limiting factor of a reaction is the factor that limits the rate of reaction.

The limiting factor for the burning of gas in a kitchen cooker is the supply of cooking gas (methane). You can regulate the flame by changing the gas flow.

In photosynthesis, the limiting factor will determine the rate at which glucose and oxygen are produced (the rate of photosynthesis).

The limiting factor of a process can change depending on the current conditions.
The limiting factor of a process can change depending on the current conditions.
  • At 1, the rate of photosynthesis increases with light intensity. Light is the limiting factor.
  • At 2, the rate of photosynthesis is no longer increasing with light intensity. This is because ($$\ce{CO2}$$ concentration) starts limiting the rate of photosynthesis.
  • At 3, $$\ce{CO2}$$ has been increased and light intensity has again become a limiting factor.

Importantly, only a change in the limiting factor can increase or decrease the rate of photosynthesis.

Significant parallels can be drawn between the processes of photosynthesis and respiration.

  • They are major metabolic processes.
  • They involve redox reactions confined to specialised organelles within the eukaryotic cell.
  • Both contain an electron transport chain that produces ATP through pumping hydrogen ions and employing ATP synthase enzymes.
  • They have cycles of reduction and oxidation of carbon compounds (Calvin cycle and Krebs cycle).

Chloroplasts and mitochondria are believed to have prokaryotic origins. They evolved an endosymbiotic relationship with cells. This theory explains many of their similar features, such as double membranes and circular DNA strands.

There are significant differences between photosynthesis and respiration, as the two processes serve different purposes:

  • Respiration breaks down organic compounds, releasing chemical energy. Photosynthesis creates organic compounds to store energy.

The processes have chemical differences.

  • In the electron transport chain, respiration uses reducing power (NADH) whereas photosynthesis creates reducing power.
  • Photosynthesis produces oxygen from water, releasing electrons. In respiration, oxygen acts as an electron acceptor, and produces water. Furthermore, the electron chain in respiration is never cyclic.
  • Although both processes produce ATP, ATP produced during photosynthesis cannot leave the chloroplast to be used in the cell.