Photosynthesis Flashcards
Heterotrophs
must get energy from the food they
eat,
autotrophs
can make their own food.
Photoautotrophs take light energy and convert it to chemical energy using photosynthesis.
Photosynthesis
reduces atmospheric carbon
dioxide, releases oxygen, and creates chemical energy that can be transferred through food chains. Photons (light energy) are used to synthesize sugars (glucose) in photosynthesis.
Carbon fixation
is the process by which inorganic
carbon (CO2) is converted into an organic molecule (glucose). Photosynthesis takes electrons released from photolysis (the process of splitting water molecules) and excites them using solar energy. These excited electrons are then used to power carbon fixation.
Photosynthesis vs Cellular Respiration
Photosynthesis and cellular respiration are reverse processes in terms of their overall reactions:
6CO2+6H2O<=(photosynth forward, cellular resp going reverse)=====>C6H12O6 + 6O2
Photosynthesis is non-spontaneous and
endergonic, producing glucose after an input of
solar energy.
Cellular respiration is spontaneous and exergonic, breaking down glucose to generate energy in the form of ATP.
Epidermis
- an outer layer of cells that provides
protection and prevents water loss.
Palisade mesophyll cells
- located right below the
upper epidermis, has many chloroplasts; this is
where most photosynthesis occurs.
Spongy mesophyll cells
- found at the bottom of the leaf where there is space for gas exchange, allowing these cells to facilitate movement of gases within the leaf; has some chloroplasts for moderate amounts of photosynthesis.
Stomata
- pores on underside of leaf where gas
can enter and exit.
Guard cells
- surround stomata and control their
opening/closing.
Chloroplasts
are organelles found in plants and
photosynthetic algae, but not in cyanobacteria.
They are similar to mitochondria and contain the structures listed below (outermost to innermost).
Chloroplast Structure includes:
1) Outer membrane
2) intermembrane
3) inner membrane
4) stroma
5) thylakoids
6) thylakoid lumen
Outer membrane
Outer plasma membrane made of
phospholipid bilayer.
Intermembrane space
Space between the outer and inner
membranes
Inner membrane
Inner plasma membrane made of
phospholipid bilayer.
Stroma
The fluid material fills the area inside the inner membrane. The Calvin cycle occurs here.
Thylakoids
A phospholipid bilayer structures
organelle suspended within the
stroma, and where the light
dependent reactions occur. The
individual membrane layers are
thylakoids, while an entire stack is
called granum. A junction between
grana is called lamella.
Thylakoid lumen
Interior of the thylakoid and H+
ions accumulate here, making it acidic.
light dependent reactions
occur in the thylakoid membrane and harness light energy to produce ATP and NADPH (an electron carrier) for later use in the Calvin cycle (ATP generated here is not used to power the cell - it is consumed in the
Calvin cycle).
Photosystems
contain special pigments, such as
chlorophyll and carotenoids, that absorb photons.
reaction center
is a special pair of chlorophyll molecules in the center of these proteins.
Chlorophyll has a porphyrin ring
structure with a magnesium atom bound in its
center. Photosystem II (P680) and Photosystem I(P700) are used in photosynthesis.
Non-cyclic photophosphorylation
is carried out by the light-dependent reactions.
Steps of Non-cyclic photophosphorylation
- Water is split (photolysis), passing electrons
to photosystem II and releasing protons into the thylakoid lumen. - Photons excite electrons in the reaction
center of photosystem II, passing the
electrons to a primary electron acceptor. - The primary electron acceptor sends the
excited electrons to the electron transport
chain (ETC). During the redox reactions within
the ETC, protons are pumped from the stroma
to the thylakoid lumen. The electrons are
then deposited into photosystem I. - Photons excite pigments in photosystem I,
energizing the electrons in the reaction center
to be passed to another primary electron
acceptor. - The electrons are sent to a short electron
transport chain that terminates with NADP+
reductase, an enzyme then reduces NADP+
into NADPH using electrons and protons. - The accumulation of protons in the thylakoid
lumen generates an electrochemical gradient
that is used to produce ATP using an ATP
synthase, as H+ moves from the thylakoid lumen back into the stroma.
Cyclic photophosphorylation
happens when photosystem I passes its electrons back to the first ETC instead of the second ETC. This causes more proton pumping and more ATP production,
while no NADPH is generated.
The Calvin cycle
- made up of reactions known as
light-independent reactions because they do not directly use light energy, but can only occur if the light-dependent reactions are providing ATP and NADPH. - takes place in the chloroplast
stroma of plant mesophyll cells. It fixes carbon
dioxide that enters stomata.
6 CO2 + 18 ATP + 12 NADPH + H+ →
18 ADP + 18 Pi +12 NADP+ + 1 glucose
Steps in Carbon Cycle:
1) Carbon Fixation
2) Reduction
3) Regeneration
4) Carb synthesis
Carbon fixation (Calvin Cycle)
- carbon dioxide combines
with five-carbon ribulose-1,5-bisphosphate
(RuBP) to form six-carbon molecules, which
quickly break down into three-carbon
phosphoglycerates (PGA). This reaction is
catalyzed by RuBisCo.
Reduction (Calvin Cycle)
- PGA is phosphorylated by ATP and
subsequently reduced by NADPH to form
glyceraldehyde-3-phosphate (G3P).
Regeneration (Calvin Cycle)
- Most of the G3P is converted
back to RuBP.
Carbohydrate synthesis(Calvin Cycle)
- some of the G3P is
used to make glucose.
RuBisCo
in addition to fixing carbon dioxide into
RuBP, can also cause oxygen to bind to RuBP in a process called photorespiration.
Photorespiration
- occurs in the stroma, producing a two-carbon molecule phosphoglycolate that is shuttled to
peroxisomes and mitochondria for conversion
into PGA. However, fixed carbon is lost as carbon dioxide in the process. Overall, there is a net loss of fixed carbon atoms and no new glucose is made. - Also called C2 photosynthesis, since two-carbon phosphoglycolate is produced.
What happens to stomata during hot and dry weathers/seasons:
- stomata are closed to minimize
water loss, oxygen accumulates inside the leaf
while carbon dioxide is used up. RuBisCo binds oxygen and photorespiration occurs.
C3 photosynthesis
- normal photosynthesis,
where three-carbon PGA is produced.
C4 photosynthesis
- produces four-carbon oxaloacetate; occurs in plants living in hot environments. Carbon dioxide is spatially isolated to prevent photorespiration.
Steps of C4 Photosynthesis
- PEP carboxylase fixes CO2 into a three carbon PEP molecule, producing oxaloacetate, which is converted into malate in the mesophyll cell.
- Malate is transferred to bundle sheath cells,
which have lower concentrations of oxygen. - Malate is decarboxylated to release CO2,
spatially isolating where CO2
is fixed by RuBisCo. The only drawback is that pyruvate is also produced and needs to be shuttled back to mesophyll cells using ATP energy. - Pyruvate is converted back into PEP.
Crassulacean acid metabolism (CAM)
photosynthesis
uses temporal isolation of
carbon dioxide to prevent photorespiration in hot environments.
Steps to Crassulacean acid metabolism (CAM) photosynthesis
- During the day, stomata are closed to prevent transpiration (evaporation of water from plants).
- During the night, stomata are open to let
carbon dioxide in. Just like in C4
photosynthesis, PEP carboxylase fixes CO2
into PEP, producing oxaloacetate and
afterwards malate. However, malate is stored
in vacuoles instead of being shuttled to
bundle sheath cells. - During the next day, the stomata are closed
again and malate is converted back into
oxaloacetate, which releases CO2 and PEP.
Thus, CO2 accumulates in the leaf for use in the Calvin cycle through temporal isolation.