Section 5 ( Energy transfer in between Organisms ), Topics 11
All eukaryotic organisms require energy to supply their metabolic pathways to sustain life's
requirements to function accordingly. Plants require energy for photosynthesis, mineral
absorption ( active transport in roots ), DNA replication, cell division, and protein synthesis.
Animals require energy for muscle contraction, maintenance of body temperature
( homeostasis ), active transport, DNA replication, cell division ( Mitosis ) and protein
synthesis.
The chemical reaction ( though incredibly simplified ) for photosynthesis is represented as:
6CO2 + 6H2O ---> C6H12O6 + 6O2 ( + Energy )
Carbon Dioxide + Water ---> Glucose + Oxygen ( + Energy )
Photosynthesis is the process in which light energy is used to produce glucose from water
and carbon dioxide. The light energy is converted into chemical energy in the form of
glucose. In other words, energy is conserved in plants, until the energy is released by the
decomposition of glucose via respiration. Animals obtain their glucose demands by
consuming plants ( or other animals ), and respire the glucose obtained to release energy.
Role of ATP
Once energy is obtained via glucose, cell usage requires an intermediate to transfer the
glucose energy to where it is needed in cells. As a product, ATP is produced for that very
reason. It is represented as:
Cells require an intermediate, ATP.
In respiration, the energy released by the breakdown of glucose is used to make ATP.
ATP carries the energy to where it is needed.
ATP is synthesised via a condensation reaction between ADP ( Adenosine
Diphosphate ) and an inorganic Phosphate. The energy is stored in the phosphate
bond as chemical energy. The enzyme ATP synthase catalyses this reaction.
Once ATP is required, it is hydrolysed back into ADP and P. The bond broken released
energy once formed by the breakdown of glucose. ATP hydrolase catalyses this
reaction.
ATP has specific properties that make it a good energy storage unit for cells.
ATP stores in small, manageable amounts at a time, so energy is wasted as heat.
It's small and soluble, so can be transported.
Easily broken down, energy is released instantaneously.
Quickly resynthesised.
Can provide energy for other pathways, transferring phosphate groups to others
( phosphorylation )
ATP cannot leave the cell by passive transport, so cell always has an immediate
supply of energy.
, Coenzymes
A coenzyme is a molecule that aids the function of an enzyme. They work by transferring a
chemical group from one molecule to another. A coenzyme used in photosynthesis is NADP,
which transfers hydrogen from one molecule to another. This means it can reduce ( give
hydrogen to ) or oxidise ( take hydrogen from ) another molecule.
Photosynthesis
Photosynthesis occurs in the chloroplasts of plant cells, flattened organelles surrounded by a
double membrane. Thylakoids ( fluid filled sacks ) are stacked up in chloroplasts called
Grana. Chloroplasts contain photosynthetic pigments called Chlorophyll, which are coloured
substances that absorb light energy needed for photosynthesis. The pigments are found in
thylakoid membranes, and attached to proteins. The protein and pigment attached is called
a Photosystem. There are two Photosystems being used to capture light. Contained within
the inner membranes of the chloroplast is the Stroma, where enzymes, sugars and organic
acids needed are stored in.
Photosynthesis is split into two different reactions:
Light Dependent Reaction
o Requires light
o Photoionisation of the Chlorophyll. Chlorophyll absorbs light energy, exciting
electrons leading to the eventual release of electrons.
o Energy released from the released electrons is used to add phosphate groups
to ADP to form ATP.
o Some energy is also used reduce NADP to form NADPH. ATP transfers energy
and NADPH transfers hydrogen to the light dependent reaction.
o During this process, H2O is oxidised to O2.
Light Independent Reaction
o Also known as the Calvin cycle, does not require light.
o Relies on the products of the light dependent reaction.
o Takes place in the Stroma
o ATP and NADPH from the light dependent reaction supply energy and
hydrogen to make simple sugars from CO2.
Light Dependent Reaction
In this reaction, energy resulting from the photoionisation of chlorophyll is used for three
things:
Making ATP from ADP and inorganic phosphate. This reaction is called
photophosphorylation.
Making reduced NADP ( NADPH ), reduction.
