Concept 10.1: Photosynthesis converts light energy to the chemical energy of food Flashcards
Photosynthetic enzymes and other molecules are grouped together in a
biological membrane
The process of photosynthesis most likely originated in a group of bacteria that had infolded regions of the
plasma membrane containing clusters of such molecules.
the original chloroplast was a photosynthetic prokaryote that lived inside an ancestor of eukaryotic cells
what has come to be known as the endosymbiont
the tissue in the interior of the leaf
mesophyll
Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores called
stomata
Water absorbed by the roots is delivered to the leaves in
veins.
A typical mesophyll cell has
about 30–40 chloroplasts, each measuring about 2–4 µm by 4–7 µm.
A chloroplast has two membranes surrounding a dense fluid called the
stroma
Suspended within the stroma is a third membrane system, made up of sacs called
thylakoids
which segregates the stroma from the
thylakoid space inside these sacs.
In some places, thylakoid sacs are stacked in columns called
grana (singular, granum).
the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast.
Chlorophyll
The internal photosynthetic membranes of some prokaryotes are also called
thylakoid membranes
In the presence of light, the green parts of plants produce organic compounds and oxygen from carbon dioxide and water. Using molecular formulas, we can summarize the complex series of chemical reactions in photosynthesis with this chemical equation:
6 CO2 + 12 H2 O + light energy ּּ→ C6 H12 O6 + 6 O2 + 6 H2 O
We use glucose (C6 H12 O6) here to simplify the relationship between photosynthesis and respiration, but the direct product of photosynthesis is actually a
three-carbon sugar that can be used to make glucose
Water appears on both sides of the equation because 12 molecules are consumed and 6 molecules are newly formed during
photosynthesis.
We can simplify the equation by indicating only the net consumption of water:
6CO2 + 6H2O + Light energyּּ→ C6H12 O6 + 6O2
Writing the equation in this form, we can see that the overall chemical change during photosynthesis is the reverse of the one that occurs during
cellular respiration
Now let’s divide the photosynthetic equation by 6 to put it in its simplest possible form:
CO2 + H2O ּּ→ [ CH2O] + O2
Here, the brackets indicate that CH2O is not an actual sugar but represents the general formula for a
carbohydrate
One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants is derived from H2O and not from .
CO2
Here is the chemical equation for photosynthesis in these sulfur bacteria:
CO2 + 2H2S ּּ→ [CH2O] + H2O +2S
Learn van Neil formulas
Let’s briefly compare photosynthesis with cellular respiration. Both processes involve
redox reactions
During cellular respiration, energy is released from sugar when electrons associated with hydrogen are transported by carriers to oxygen, forming
water as a by-product
The electrons lose potential energy as they “fall” down the electron transport chain toward electronegative oxygen, and the
mitochondrion harnesses that energy to synthesize ATP
Photosynthesis reverses the direction of
electron flow.
Water is split, and its electrons are transferred along with hydrogen ions (H+)
from the water to carbon dioxide, reducing it to
sugar.
Because the electrons increase in potential energy as they move from water to sugar, this process requires
energy—in other words, it is endergonic
Actually, photosynthesis is not a single process, but
two processes, each with multiple steps
These two stages of photosynthesis are known as the
light reactions and the Calvin cycle
The light reactions are the steps of photosynthesis that convert solar energy to
chemical energy
Water is split, providing a source of electrons and protons (hydrogen ions, H+) and giving off .
O2 as a by-product
Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called
NADP+ (nicotinamide adenine dinucleotide phosphate) where they are temporarily stored.
The light reactions use solar energy to reduce NADP+ to _________ by adding a pair of electrons along with an H+.
NADPH
The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called
photophosphorylation
Thus, light energy is initially converted to chemical energy in the form of two compounds:
NADPH and ATP. NADPH, a source of electrons, acts as “reducing power” that can be passed along to an electron acceptor, reducing it, while ATP is the versatile energy currency of cells.
Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the.
Calvin cycle
The Calvin cycle is named for ________________, who, along with his colleagues James Bassham and Andrew Benson, began to elucidate its steps in the late 1940s.
Melvin Calvin
The cycle begins by incorporating from the air into organic molecules already present in the
chloroplast.
This initial incorporation of carbon into organic compounds is known as
carbon fixation
The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of
electrons.
To convert to carbohydrate, the Calvin cycle also requires chemical energy in the form of
ATP, which is also generated by the light reactions.
Thus, it is the Calvin cycle that makes sugar, but it can do so only with the help of the
NADPH and ATP produced by the light reactions.
The metabolic steps of the Calvin cycle are sometimes referred to as the _________________________________, because none of the steps requires light directly.
dark reactions, or light-independent reactions
As Figure 10.6 indicates, the thylakoids of the chloroplast are the sites of the light reactions, while the Calvin cycle occurs in the
stroma.
On the outside of the thylakoids, molecules of NADP+ and ADP pick up electrons and phosphate, respectively, and NADPH and ATP are then released to the .
stroma, where they play crucial roles in the Calvin cycle
