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Photosynthesis
Photosynthesis is the process by which carbohydrates (an organic nutrient) are synthesized from
inorganic sources using the energy of light. In order for photosynthesis to take place the following
conditions must be met:
1. light- plants use the visible light spectrum (ROY G BIV)
2. photosynthetic pigments- there are various photosynthetic pigments found in
chloroplasts. These include: chlorophyll A, chlorophyll B, chlorophyll C, carotene (an
orange pigment) and xanthophyll (a yellow pigment). The most important pigment is
chlorophyll A.
3. thylakoid membranes- specialized membranes of the chloroplasts that are
embedded with chlorophyll. The chloroplasts also contain stacks of small protein
discs, called grana. The grana are surrounded by a gel-like substance called the
stroma.
Photosynthesis occurs in two distinct phases: the light reactions and the carbon fixation reactions
(Calvin cycle). The light phase is also known as the photophase because it requires light energy.
During this phase, chlorophyll absorbs energy and is said to be ‘energized’. The energized
chlorophyll causes the following reactions to occur:
1. A water molecule is split into hydrogen protons and oxygen atoms. This split occurs
due to chlorophyll’s strong attraction for electrons. The hydrogen protons are then
held by the coenzyme NADP. The oxygen atoms are released to the atmosphere.
2. ADP adds a phosphate group to become ATP (phosphorylation). The energy stored in
ATP is used during the Calvin cycle.
The following figure depicts the light reactions:
During the Calvin cycle, CO2 is converted to glucose (a carbohydrate) and H2O is given off to the
atmosphere. During this phase, light is not required; however, the ATP and NADPH produced by
the light reactions are. During a series of reactions, ATP and NADPH help convert CO 2 into PGAL.
Glucose is produced when two PGAL molecules are combined. PGAL is also used to synthesize other
organic compounds (lipids and amino acids) and as an energy source for cellular respiration. The
following figure depicts the Calvin cycle:
Cellular Respiration
During cellular respiration, the chemical bonds of glucose are broken thus releasing energy. That
energy then goes to form ATP molecules, which are used in other cell processes like active
transport, muscle contraction and synthesis of other compounds. Cellular respiration has three
steps (glycolysis, Kreb’s cycle and the electron transport system) that occur in two phases
(anaerobic and aerobic).
The anaerobic phase of cellular respiration takes place in the absence of oxygen. Glycolysis is the
first step in cellular respiration. During glycolysis, glucose is broken down to lower energy
molecules (2 PGAL molecules). The energy required to do this is supplied by 2 ATP. The PGAL
molecules are then oxidized by the loss of 2 hydrogen atoms and converted into pyruvic acid. This
oxidation releases energy, which is then used to synthesize ATP. The released hydrogen atoms are
accepted by NAD to form NADH. The net gain from glycolysis is 2 ATP. The following figure
depicts glycolysis:
The aerobic phase of cellular respiration requires the presence of oxygen and starts with Kreb’s
cycle (citric acid cycle). Pyruvic acid is broken down into acetyl coenzyme A (coA). During Kreb’s
cycle, coA enters a series of chemical reactions which result in 2 H atoms and 2 CO 2 molecules. The
hydrogen atoms are picked up by NAD to form NADH. The net gain from Kreb’s cycle is 2 ATP (1
from each pyruvic acid). The following figure depicts Kreb’s cycle:
The final step in cellular respiration is the electron transport system. This system takes hydrogen
atoms and their electrons from NADH or FADH. The hydrogen and its electron are passed from
one compound to another. Each time hydrogen and its electron are passed, energy is released. The
energy released is used to make ATP.
Oxygen acts as the final electron acceptor for aerobic respiration, combining with two hydrogen
atoms and their electrons to form water. The following figure depicts the electron transport
system: