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What is photosynthesis?
All life on Earth depends on photosynthesis. The process is described by the following word and symbol equations:
These equations are summaries of a complex two-step process that takes place in the chloroplasts of green plants. The end products are not just glucose, but complex organic molecules such as carbohydrates, amino acids, lipids and nucleic acids.
carbondioxide + water oxygen + glucose
light energy
6CO2 + 6H2O 6O2 + C6H12O6
light energy
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Why do plants photosynthesize?
Photosynthesis is an essential biological process. This is because it produces:
energy for processes in the organism
complex organic molecules needed for growth
oxygen, which is then used for respiration.
The oxygen produced is released into the atmosphere and is available for other organisms.
If the plant is eaten, the organic molecules are used to provide energy to organisms higher up the food chain.
There are not just advantages for the plant itself:
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Leaves and photosynthesis
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Structure of the chloroplast
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Plant photosynthetic tissues
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Plant anatomy
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Functions of photosynthetic structures
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In plants there are two types of pigments: chlorophylls and carotenoids. They are coloured because they absorb particular wavelengths of light and reflect others.
A photosynthetic pigment is a coloured biological compound that is present in chloroplasts and photosynthetic bacteria, and which captures light energy for photosynthesis.
Chlorophyll is the pigment that gives plants their green colour by reflecting green light. Carotenoids reflect red, orange or yellow light.
Photosynthetic pigments
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Photosynthetic pigments
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Photosynthetic pigments
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Adenosine triphosphate
Adenosine triphosphate, or ATP, is an important molecule found in all living cells. It readily diffuses around the cell and provides energy for cellular processes.
ATP releases energy in the light-independent reaction when a bond between inorganic phosphate groups is broken, producing ADP and an inorganic phosphate group.
adenine
ribose
3 phosphate groups
ATP is made in the light-dependent reaction in photosynthesis from adenosine diphosphate (ADP) and an inorganic phosphate group (Pi). This requires energy.
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Adding a phosphate molecule is phosphorylation. In this case, light is used as an energy source so the process is called photophosphorylation.
In the light-dependent reaction:
+ ®+ energy
In the light-independent reaction:
The use of water makes this a hydrolysis reaction.
Photophosphorylation and hydrolysis
ADP ATP
+ energy+®H2O ADPATP
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NADP and NADPH
Nicotinamide adenine dinucleotide phosphate (NADP) is a coenzyme involved in the photosynthesis reactions.
The compound is a dinucleotide, containing an adenine base and a nicotinamide base. The nucleotides are joined through their phosphate groups. In addition there is an extra phosphate on the ribose of the adenine-containing nucleotide.
NADP can accept electrons to be reduced to reduced NADP, often called NADPH. NADPH can be oxidized back to NADP, releasing electrons.
nicotinamide
adenine
ribose
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Which molecule?
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Location of the reaction
The light-dependent reaction takes place in the thylakoid membranes of the chloroplasts.
Situated in these membranes are light-harvesting systems called photosystems. There are two types, photosystem I (PSI) and photosystem II (PSII). Both have chlorophyll at their centres.
chlorophyll
lightphotosystem
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Light-dependent reaction
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Light-dependent reaction: summaryCyclic photophosphorylation
Non-cyclic photophosphorylation
photolysis
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Products of the light-dependent reaction
Products of the light-dependent reaction that pass into the light-independent reaction:
Products of the light-dependent reaction that leave the plant:
Products of the light-dependent reaction that are re-used in another part of the light-dependent reaction:
reduced NADP
ATP
oxygen
H+ ions
electrons
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How much do you know?
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Location of the reaction
The light-independent reaction can also be called the “dark reactions”, or the Calvin Cycle.
The light-independent reaction takes place in the stroma of the chloroplasts.
The reaction consists of a cycle of enzyme-controlled processes making complex molecules such as carbohydrates.
stroma
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The light-independent reaction
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Products of the Calvin cycle
Products of the light-independent reaction that pass back into the light-dependent reaction:
Products of the light-independent reaction that are used in other processes:
NADP
ADP
triose phosphate – used to build complex carbohydrates, amino acids and lipids.
inorganic phosphate
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Making complex molecules
Pairs of TP molecules combine to form hexose sugars, such as glucose, some of which may isomerize to form fructose.
Triose phosphate is a three-carbon sugar that can be used to make a variety of complex biological molecules.
TP can be converted to glycerol and this may be combined with fatty acids to make lipids.
These monosaccharides can combine to form disaccharides such as sucrose, and polysaccharides such as cellulose and starch.
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How much can you remember?
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Summary of photosynthesis
light- dependent reactions
light- independent
reactions
light
carbohydrates, other complex molecules
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Photosynthesis: true or false?
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Glossary
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What’s the keyword?
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Inputs and outputs to the reactions
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Multiple-choice quiz
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