Photosynthesis – review
Nutrition
Nutrition is the overarching point of photosynthesis. Mostly it is plants that undergo
photosynthesis, though some other organisms do as well.
Autotrophs are organisms which produce organic molecules from carbon dioxide and from other
inorganic molecules from the environment. They are called producers because these organic
molecules are then utilised by other organisms.
Photosynthesis
Heterotrophs are organisms which obtain organic molecules through consumption of other
organisms. Humans are an example of heterotrophs. Heterotrophs are referred to as consumers.
Photoautotrophs are organisms that use sunlight as their energy source and synthesise organic
molecules from carbon dioxide and (mostly) water.
Photosynthetic organisms are not just plants. Other organisms perform photosynthesis, such as
algae, unicellular eukaryotes, cyanobacteria1 and purple sulfur bacteria.
All the green parts of a plant undergo photosynthesis, including young fruit and stems.
Photosynthesis is comprised of two processes: a light-dependent reaction and the Calvin cycle,
which is light-independent. It is a series of redox reactions with the overall equation:
6CO2 + 6H2O ⟶ C6H12O6 + 6O2
Here, carbon dioxide is reduced and then later, glucose will be oxidised through oxidative
phosphorylation.
Chloroplasts
The chloroplast (found in mesophyll cells, mostly in leaves) contains an inner and an outer
membrane, separated by an inter-membrane space.
The interior is a dense fluid called the stroma, and within the stroma are membranous sacs calls
thylakoids.
Inside them is the thylakoid space.
The stacks of thylakoids are called grana.
Chlorophyll is contained in the thylakoid membranes.
1
Cyanobacteria do not have chloroplasts; they undergo photosynthesis without the organelles.
Metabolism
Metabolism is defined as the set of chemical reactions that occur in chemical organisms to maintain
life. These processes allow organisms to grow and reproduce, maintain their structures and respond
to environmental changes. It is the totality of an organism’s chemical reactions, consisting of
catabolic and anabolic pathways, which manage the material and energy resources of a cell. Note
that metabolic pathways do not exist in isolation.
Anabolism and catabolism
Catabolism
Anabolism
Break down complex molecules into simpler
compounds.
Combine simpler compounds into complex
molecules.
Release energy.
Require energy.
Break chemical bonds.
Form new chemical bonds.
e.g. cellular respiration
e.g. protein synthesis
Structure and function of ATP
Full name: adenosine triphosphate. It is composed of ribose, adenine and three phosphate groups.
There are very high energy, relatively unstable phosphoanhydride bonds or phosphate bonds.
These are broken when ATP is used to release their energy.
For the purposes of examination, remember the following structure:
Phosphate groups
Adenine
Ribose
The bonds between the phosphate groups of the ATP tail can be broken by hydrolysis. This requires
the addition of H2O. After one of these groups breaks off, the resultant molecule is called ADP
(adenosine diphosphate) plus an inorganic phosphate.