Chapter 10: Photosynthesis
Essential Knowledge 2.a.1 – All living systems require constant input of
free energy (10.1-10.3). 2.a.2 – Organisms capture and store free energy for
use in biological processes (10.1-10.3).
Photosynthesis Process by which plants use
light energy to make food A reduction process that makes
complex organic molecules from simple molecules
Ps General Equation6 CO2 + 6 H2O C6H12O6 + 6 O2
Requires: ♦Notice: Chlorophyll This is VERY Light similar to the
cell resp eq.
Ps: a redox process Hydrogens are added to
Carbons-reduces CO2 to sugar Water is split; becomes source
of hydrogens Complex covalent bonds made Requires energy
Ps Has two chemical reactions:1. Light Reaction2. Dark Reaction Names are from “light” as a
requirement, not where or when they occur
Light A form of electromagnetic
radiation/energy Visible light has the right
energy for use in Ps Travels in waves
Too Hot Too ColdJust Right
Detected by our eye-hence the term visible
Action Spectrum Not all colors are useable to
the same degree for Ps Red and Blue light - absorbed
and used in Ps Green light - reflected or
transmitted
Comment In oceans, red light is lost or
filtered out early because it has lower energy
Only blue light which has higher energy can reach the lower depths
Result Many deep ocean
fish are bright red in color.
Why? They can’t be seen
because there is no red light to reflect their color
Photosynthesis Pigments
Pigment: absorb light Different pigments absorb
different wavelengths Types:
Chlorophylls (a and b) Accessory Pigments
(carotenoids)
Chlorophylls Have C,H,O,N and Mg Two major types: a, b Molecule has a lipophilic tail
(allows it to dissolve into membranes)
Contains Mg in a reaction center.
Accessory Pigments Absorb light energy and
transfer it to chlorophyll Various shades of
yellow/orange Function: photoprotection 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
Parts of Plant: Ps function
Stomata Roots Mesophyll Veins
Site of Ps Chloroplast
Contain chlorophyll pigmentAll green parts of plants
contain chloroplastsMost are found in leaves
Why? Larger surface area
Chloroplast Structure Double outer membrane Inner membrane folded and
stacked into grana Stroma - liquid that
surrounds the thylakoid membranes
Photosystems Collection of pigments that
serve as a light trap Made of chlorophyll and the
accessory pigments Two types known:
PSI & PSII
Parts of Photosystem 1) Rxn center/e- acceptor
Chlorophyll uses light energy to move e- to next energy level
2) Pigment Light absorption, photon acceptor
3) Light-harvesting complex Proteins, contain pigment
Light Reaction Same thing as Noncyclic
Photophosphorylation Location - grana of the chloroplast Function - to split water and
produce ATP and NADPH (provide chem energy for next cycle)
Light Reaction
Requirements: Light Water ADP + Pi NADP+
Products: O2
ATP NADPH
Chemiosmosis Model The chloroplast produces
ATP in the same manner as the mitochondria in Rs
Light energy is used to pump H+ across a membrane
When the H+ diffuses back, ATP is generated
Chemiosmosis H+ are pumped into the
thylakoid space ATP and NADPH are made
when the H+ diffuse into the stroma
Both Rs and Ps use ETC to pump H+
Light Rxn
Tutorial
Dark Reactions How plants actually make food
(carbohydrates) Don't require light directly to
run Also known as the
Calvin cycle or C3 Ps
Calvin cycle/Dark reaction Anabolic – builds sugars from
smaller molecules Function - use ATP and
NADPH to build food from CO2
Location - stroma of the chloroplast
Calvin Cycle movie
Rubisco Ribulose BisPhosphate
Carboxylase Enzyme that adds CO2 to an
acceptor molecule Most important enzyme on earth Catalyzes first step of dark rxn
C3 Ps C3 plants are plants that DO use
Rubisco enzyme “normally” Keep stomata open during day MOST plants are C3 plants
When you “think of Ps”, you think of a C3 plant performing Ps
Adaptive value: Require less Less enzyme activity, no specialized
organs/anatomy
Photorespiration When Rubisco accepts O2
instead of CO2 as the substrate Generates no ATP
Actually uses ATP Decreases Ps output by as
much as 50%
Photorespiration
May reflect a time when O2 was less plentiful and CO2 was more common Important to evolution of autotrophs
Photo=occurs in light Respiration=consumes oxygen
while producing CO2
Alternate Ps Methods
1. C4 Ps2. CAM Ps
Intro to C4 and CAM
Remember: MOST plants are C3 plants that performs Ps “normally”
Both are adaptations to arid conditions CAM plants “idle” and save
energy while C4 plants go through Ps faster
C4 Ps Uses a different enzyme to
initially capture CO2
Separates CO2 capture from carbon fixation into sugar
Still uses C3 Ps to make sugar, but only does so in the bundle sheath cells
C4 Ps
Found in 19 plant families Characteristic of hot regions
with intense sunlight Examples - sugarcane,
Bermuda grass, crab grass
C3 Ps vs C4 Ps
Can use 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 plants Found in plants from arid
conditions (where water stress is a problem)
Avoid H2O stress by keeping stomates closed during the day
Generally have slow growth Ex: pineapple, cacti, orchids
CAM Plants, cont.
Store CO2 as C4 acid During the day, the acid is
broken down and CO2 is made into sugar.
Ps:Rs Ratios Reflect a plant’s balance in
making food and using food1. Ps > Rs, energy available
for growth and reproduction
2. Ps = Rs, no growth, but don’t die either
3. Ps < Rs, death by starvation
Comments - Ps:Rs Rs happens 24 hours a day Ps only in light Plants overwinter on stored
food when Ps > Rs If Ps < Rs, best solution is to
increase the amount of light
Factors Affecting Ps
1. Light - quantity and quality2. Temperature - too hot or too
cold3. CO2 - often limits C3 plants4. Minerals - especially NPK
and Mg
Importances of Ps
1. Food - either directly or indirectly comes from plants
2. Oxygen in the air3. CO2 balance4. Plant products5. Life on Earth
Summary Identify the basic equation of photosynthesis. Identify the main reactions of photosynthesis. Recognize the function, location, requirements
and products of the photosynthesis reactions. Recognize the role of light and pigments in the
photosynthesis process. Recognize the effects and importance of
photorespiration. Recognize various types of photosynthesis and
their importance. Identify the biological and economic importance
of photosynthesis.
Exclusion Statements You do NOT need to
memorize the steps in the Calvin Cycle, the structure of the molecules, and the names of the enzymes (except ATP synthase).