Photosynthesis. u Process by which plants use light energy to make food. u A reduction process that...
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Transcript of Photosynthesis. u Process by which plants use light energy to make food. u A reduction process that...
Photosynthesis
Photosynthesis
Process by which plants use light energy to make food.
A reduction process that makes complex organic molecules from simple
molecules.
Autotrophs
Organisms that can manufacture their own food from inorganic molecules
Self-feeding organisms
PSN General Equation
6 CO2 + 6 H2O + light C6H12O6 + 6 O2
Requires:
Chlorophyll
PSN: a redox process
Hydrogens are added to Carbons.
Water is a source for the Hydrogens.
Complex covalent bonds are made.
PSN
Has two chemical reactions:
1. Light Dependent Reactions
2. Light Independent Reactions
Names are from “light” as a requirement, not where or when
they occur.
Light
A form of electromagnetic radiation.
Visible light has the necessary energy for use in
PSN.
Too Hot Too Cold
Just Right
Action Spectrum
Not all colors are useable to the same degree for PSN.
Red and Blue light - absorbed and used in PSN.
Green light - reflected or transmitted.
Photosynthesis Pigments
1. Chlorophylls
2. Accessory Pigments
Chlorophylls Has CHON and Mg.
Several types possible. Molecule has a lipophilic tail
that allows it to dissolve into membranes.
Contains Mg in a reaction center.
Accessory Pigments
Absorb light energy and transfer it to chlorophyll.
Ex: Carotene (orange). Xanthophyll (yellow)
Fall Leaf Colors Chlorophyll breaks down.
N and Mg salvaged and moved into the stem for next
year. Accessory pigments remain behind, giving the various fall
leaf colors.
Chloroplast Structure
Double outer membrane. Inner membrane folded and
stacked into grana. Stroma - liquid that
surrounds the thylakoid membranes.
stroma
thylakoid sacs
Photosystems Collection of pigments that serve as a
light trap. Made of chlorophyll and the accessory
pigments. Two photosystems are known:
Photosystem I ,which uses P700 ( a type of chlorophyll a energized by the frequency 700 nm), and Photosystem II which uses P680 ( a type of chlorophyll a energized by the frequency 680 nm)
Noncyclic Photophsphorylation
Uses Photosystem I and Photosystem II.
Splits water, releasing H+, a pair of e-, and O2.
Produces ATP and NADPH. (e- carrier similar to NADH)
Light Dependent Reactions
Photosystem II: Electrons trapped by P680 are
energized by light and accepted by the “primary”
electron acceptor
As electrons move down the Electron Transport Chain
they lose energy… this energy is used to produce ATP. Some transport
proteins, like ferredoxin and
cytochrome, include nonprotien parts containing iron.
Light Dependent Reactions
Light Dependent Reactions
Electrons are again energized
by light and accepted by a
primary acceptor
Light Dependent Reactions
Electrons lose energy as they move down the ETC … the energy is
used to make NADPH…. The 2
electrons lost to Photo- system II are replaced when water is split….
This is called “Photolysis”.
Light Dependent Reaction
Same thing as Noncyclic Photophsphorylation.
Location - grana of the chloroplast.
Function - to split water and produce ATP and NADPH.
Light Dependent Reaction
Light Water ADP + Pi NADP+
O2 (released into the environment)
ATP NADPH
Requirements Products
H2O + ADP + Pi + NADP+ + light ATP + NADPH + O2 + H+
Chemiosmosis Model The chloroplast produces ATP
in the same manner as the mitochondria in Respiration.
Light energy is used to pump H+ across a membrane.
When the H+ diffuses back, ATP is generated.
Chemiosmosis Model
H+ are pumped into the thylakoid space.
ATP and NADPH are made when the H+ diffuse into the
stroma.
Calvin-Benson Cycle…. Light Independent Reactions
How plants actually makes food (carbohydrates).
Don't require light … energy supplied by ATP made during Light Dependent Reactions
Also known as the Calvin cycle or C3 Ps.
Calvin-Benson Cycle…. Light Independent Reactions
Function - to use ATP and NADPH to build food from CO2
Location - stroma of the chloroplast.
Rubisco (RuBP)
Ribulose BisPhosphate Carboxylase.
Enzyme that adds CO2 to an acceptor molecule.
Most important enzyme on earth.
Calvin-Benson Cycle… Light Independent Reactions
CO2 (1C) combines with RuBP (5C) to form 2 PGA (3C)
Carboxylation: 6CO2 + 6 RuBP produce 12 PGA
The energy from ATP and the H+ from the NADPH
are used to convert 12 PGA to 12 PGAL (G3P)
6 ATP are used to convert 10 PGAL to 6 RuBP… the remaining 2 PGAL are used to make Glucose.
C3 PSN
6 CO2
18 ATP 12 NADPH
C6H12O6
18 ADP + 18 Pi 12 NADP+
Requirements Products
6CO2 + 18 ATP + 12 NADPH+ + H+ 18 ADP + 18 Pi12 NADP+ + 1 Glucose
Cyclic Photophosphorylation
Uses Photosystem I only. Requires light Electrons from Photosystem I are
used to produce ATP…. the electrons then return to Photosystem 1…. They are not incorporated into NADPH as in Non-Cyclic Photophosphorylation
e-
Photorespiration Rubisco is the most abundant protein
on earth. Occurs when Rubisco accepts O2
instead of CO2 as the substrate. Generates no ATP. Decreases PSN output by as much as
50%. Products of Photorespiration are
broken down by peroxisomes… found near the chloroplasts.
Photorespiration
May reflect a time when O2 was less plentiful and CO2
was more common.
Alternate PSN Methods
1. C4 PSN
2. CAM PSN
C4 PSN C4 plants high rate of
photosynthesis reduces to overall loss of water….
Therefore they are found in dry, hot climates…. This
gives them an advantage over C3 plants.
C4 PSN
Uses a different enzyme, PEP, to initially capture CO2
Still uses C3 PSN to make sugar, but only does so in the bundle
sheath cells…. The bundle cells are near the water source… this makes the C4 plants extremely
efficient.
PEP Carboxylase
Enzyme used for CO2 capture in C4 PSN.
Can use CO2 down to 0 ppm. Prevents photorespiration.
C4 PSN
Found in 19 plant families. Characteristic of hot regions
with intense sunlight. Examples - sugarcane,
Bermuda grass, crab grass
C3 Ps vs C4 Ps
Photorespiration
Shade to full sun
High water use
Cool temperatures
Slow to moderate growth rates
Cool season crops
No Photorespiration
Full sun only
Moderate water use
Warm temperatures
Very fast growth rates
Warm season crops
CAM PSN Crassulacean Acid Metabolism
Found in plants from arid conditions where water stress
is a problem. Examples - cacti, succulents,
pineapples, many orchids.
CAM PSN Open stomata at night to take
in CO2.
The CO2 is stored as a C4 acid. During the day, the acid is broken down and CO2 is fixed
into sugar.
CAM plants
Tissues decrease in pH over night, rise in pH during day.
Avoid H2O stress by keeping stomata closed during the
day. Generally have slow growth.
Factors That Affect Photosynthesis
1. Light - quantity and quality.
2. Temperature - too hot or too cold.
3. CO2 - often limits C3 plants.
4. Minerals - especially NPK and Mg.
Importance of Photosynthesis
1. Food - either directly or indirectly comes from plants.
2. Oxygen in the air.
3. CO2 balance.
4. Plant products.
5. Life on Earth.