Photosynthesis – Revision Pack (C4)
The Chemistry behind Photosynthesis:
+
Chlorophyll
This is taken from
the air
Taken from the
soil
(GLUCOSE) – Stored
as starch in the leaf
Released
back into the
environment
Simple sugars like glucose can be used in a number of ways; for example:
•
•
•
•
In respiration, releasing energy
Can be converted into cellulose to make cell walls
Can be used to make proteins for growth and repair
Can be converted into starch, fats and oils for storage
Starch is insoluble so it is used for storage. Glucose can affect the water
concentration of cells and cause osmosis; starch does NOT do this and doesn’t
move from where it is being stored.
Photosynthesis happens in a few simple steps:
STEP 1 – Water (H 2 0) is split up
STEP 2 – This releases oxygen gas and hydrogen ions
STEP 3 – Carbon dioxide (CO 2 ) combines with the hydrogen ions forming glucose
(and water)
The History of Photosynthesis:
Many Greek scientists just assumed that plants took
ALL nutrients and minerals out of the soil and this
helped them to grow and gain mass.
A man called Van Helmont (pictured above) planted a willow tree with 90kg of soil.
He let it grow and added water regularly but DID NOT change the soil. After 5 years,
the willow tree had increased in mass by 54kg and there was basically the same
amount of soil. Van Helmont concluded that the growth couldn’t just be due to the
uptake of soil minerals – he thought that it was due to the water alone!
Photosynthesis – Revision Pack (C4)
Priestley was another scientist who conributed to the understanding of
photosynthesis. His experiment showed that plants must produce oxygen.
A more modern experiment was conducted using a green alga (plant) and an
isotope of oxygen (O 18 ). This formed part of a water molecule. The experiment
showed that the light energy is used to split up the water, rather than the carbon
dioxide. The oxygen gas made was O 18 while the oxygen present in glucose was
normal oxygen (O 16 ). An isotope is a different form of a certain element.
The Rate of Photosynthesis and Limiting Factors:
Generally, three things can increase the rate of photosynthesis; these are: more
carbon dioxide, more light and a higher temperature (increases enzyme action).
Photosynthesis only happens in day time because it needs light. Respiration however
continues to happen at all times – this is because plants are living organisms; this
means they are releasing energy at all times. REMEMBER – During respiration takes in
oxygen and releases carbon dioxide.
During the day (in light), photosynthesis takes place – this is basically the same gas
exchange as respiration but in reverse. Photosynthesis takes in carbon dioxide and
releases oxygen. By comparison, the rate of gas exchange is a lot higher for
photosynthesis than it is for respiration. Respiration is only really noticed at night.
Here, light intensity is
the limiting factor
Since photosynthesis depends on light intensity, carbon dioxide concentration and temperature, if
one of these is lacking it limits the rate at which photosynthesis can take place. When one factor limits
the rate, we call it the limiting factor. For example, for the light intensity graph, when it begins to
plateau (level off) it means that the rate is being limited by either the temperature or the CO2 conc.
Photosynthesis – Revision Pack (C4)
Past Papers:
PPQ(1):
Photosynthesis – Revision Pack (C4)
PPQ(2):
Photosynthesis – Revision Pack (C4)
PPQ(3):
Photosynthesis – Revision Pack (C4)
PPQ(4):
Photosynthesis – Revision Pack (C4)
Photosynthesis – Revision Pack (C4)
Mark Schemes:
PPQ(1):
PPQ(2):
PPQ(3):
Photosynthesis – Revision Pack (C4)