Unit 10 - Biological Molecules and Metabolic Pathways
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Learning aim C: Explore the factors that can affect the pathways and the rate of Photosynthesis in plants
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Unit 10 - Biological Molecules and Metabolic Pathways
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PEARSON (PEARSON)
the document focuses on the stages of photosynthesis in detail, from light dependent reactions to light independent reactions. In the second section of the document, the three descriptive practical data and evaluation is written, from hypothesis to comparing the data with secondary data. The effect...
Unit 10 - Biological Molecules and Metabolic Pathways
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Name: Ransley Fernandes Admin no. 40202
Learning aim C: Explore the factors that can affect the pathways and the rate of
Photosynthesis in plants
Stages of Photosynthesis CO2
H20 NADP+
Light ADP
+ P
Light dependent Calvin cycle in
reactions in stroma (light
thylakoids independent
reactions)
ATP
O2 NADPH
SUGAR- G3P
Light dependent reactions
The light dependent reactions need sunlight. The energy from the sunlight is taken in by
chlorophyll and it is converted into stored chemical energy, in the form of electron carrier
molecule NADPH and ATP in light dependent reactions. The light dependent reactions occur
in thylakoid within the chloroplast.
The process which converts light energy into the chemical energy occurs in a photosystem.
There are 2 types of photosystems that are embedded in the thylakoid membrane:
photosystem I (PSI) and photosystem II (PSII). Each photosystem in the light dependent
reactions has an important role in taking energy from sunlight by exciting the electrons. The
electrons which are energised are carried by energy carrier molecules, which control the
light independent reactions. Photosystems contain light harvesting complex and a reaction
centre. The two special chlorophyll a molecule in reaction centre receives light energy from
the pigments in the light harvesting complex. An electron from the chlorophyll a pair is
excited by light and transferred to the primary electron acceptor. After that, the excited
electrons must be substituted. The electrons in photosystem II comes from splitting water,
which gives oxygen as a waste product. The electron in photosystem I comes from
chloroplast electron transport chain.
These two photosystems oxidise different low-energy electron sources, deliver energised
electrons to different locations, and reacts to different light wavelengths.
,Name: Ransley Fernandes Admin no. 40202
Role of photosynthetic pigments (chlorophylls and carotenoids) in absorbing
different wavelengths of light.
Chlorophylls
There are 5 main types of chlorophylls: chlorophylls a, b, c and d, plus bacteriochlorophylls
found in prokaryotes. The chlorophyll a and chlorophyll b are key photosynthetic pigments
in plants. The chlorophyll molecules absorb blue and red wavelengths. Chlorophyll molecule
contains a hydrophobic tail which places into the thylakoid membrane and has a porphyrin
ring head which absorbs light. Chlorophyll a has a hydrophobic tail which inserts in the
thylakoid membrane and porphyrin head which takes light energy.
Despite the fact that both chlorophyll a and b absorb light, chlorophyll a play an important
role in transforming the light energy into the chemical energy. Chlorophyll a is found in all
photosynthetic plants, algae, and cyanobacteria, while chlorophyll b is found in only plants
and green algae, and few types of cyanobacteria. The accessory pigments enable a broader
range of wavelengths to be absorbed, and therefore, more light energy is taken from the
sunlight.
Carotenoids
Carotenoids are group of pigments which absorbs violet and blue-green light. The bright
coloured carotenoids can be found in fruits like, red of tomatoes, the yellow of corn seeds,
or the orange of an orange peel, which are often used to attract animals that can help
disperse the plant’s seeds. In photosynthesis, the carotenoids help to take light, however
carotenoids also have a crucial role in removing excess light energy. If a leaf is exposed to a
full sun, it gets a lot of energy, if this energy is not controlled properly, it can cause damage
to the photosynthetic mechanism. In chloroplasts, carotenoids help absorb the excess
energy and to disperse it as heat.
Light independent reaction/ Calvin cycle
The light independent reactions occur in stroma. The energised electrons from light
dependent reaction supply energy in order to produce carbohydrates from the carbon
dioxide molecules in Calvin cycle. The light independent reactions are also known as Calvin
cycle due to the cyclical nature of the process. While light independent reaction does not
involve the presence of light as a reactant, but they do need the products of light dependent
reactions in order to function. The energy carrier molecule ATP and NADHP drive the
construction of new carbohydrate molecules in light independent molecules. The energy
carrier molecules then return to the light dependent reactions to get more energised
electrons after energy is transferred. In addition, the light activates many enzymes involved
in light independent reactions.
Reactions of the Calvin cycle
Carbon fixation: one carbon dioxide molecule joins with 5-carbon acceptor
molecule, ribulose-1, 5-biphosphate (RuBP). This step produces 3-phosphoglyceric
acid, a six-carbon compound that divides into two molecules of a three-carbon
, Name: Ransley Fernandes Admin no. 40202
compound (3-PGA). The enzyme RuBP carboxylase/ oxygenase, also known as
rubisco, catalyses this reaction.
Reduction: In the second step of Calvin cycle, ATP and NADPH are used to transform
the 3-PGA molecules into molecules of a 3-carbon sugars, glyceraldehyde-3-
phosphate (G3P). The name of this step comes from the fact that NADPH provides
electrons to, or reduces, a three-carbon intermediate to produce G3P.
Regeneration: some G3P molecules are recycled to regenerate the RuBP acceptor,
while other G3P molecules are used to form glucose. Regeneration needs ATP and it
includes a complex network of reactions.
For G3P to leave the cycle, the three-carbon dioxide molecule are required to enter the
cycle, giving three new atoms of fixed carbon. Six G3P molecules are formed when three
carbon dioxide molecules enter the cycle. One exits the cycle, and it is used to form glucose,
while other five are recycled to regenerate the three molecules of the RuBP acceptor.
C2 Describe the effect of factors on the rate of photosynthesis e.g., temperature, carbon
dioxide concentration, light intensity.
Investigating the effect of light intensity on rate of photosynthesis
Equation for photosynthesis
Carbon dioxide + water glucose + oxygen
6 CO 2+6 H 20 →C 6 H 12 O6+ 6 O2
Hypothesis
My hypothesis states that, as the distance of the lamp from elodea decreases, the length
and the volume of the bubble increase, therefore, increasing the rate of photosynthesis. The
light intensity is inversely proportional to the distance of the lamp to the plant. When the
light intensity will increase, the light dependent reaction and also the rate of photosynthesis
will increase, generating more ATP and NADPH2. When the light intensity is low, the light
dependent reaction and rate of photosynthesis will be low, producing less ATP and NADPH2.
Independent variable
Distance of the lamp from the elodea (cm)
Dependent variable
Rate of photosynthesis (mm3/min)
Control variable
The length of the elodea, the volume of water used and its temperature.
Risk assessment
Do not touch the lamp because it is hot and can burn or clot the area you touched.
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