Photosynthesis
Chapter 10: Photosynthesis
What is Photosynthesis?
 Photosynthesis is the process that converts
 Directly or indirectly, photosynthesis nourishes almost
 Autotrophs sustain themselves by
 Heterotrophs obtain their organic material from
Auto versus Hetero
 Autotrophs are the
of the biosphere,
 Almost all plants are photoautotrophs, using the energy
of
 Heterotrophs are the
of the biosphere
 Almost all heterotrophs, including humans,
Overall Reaction

 What does this remind you of?

 Includes:
1)
 Light energy +
 Chemical Energy (ATP & NADPH) +
2)
 Chemical energy (ATP and NADPH) +

Photosynthesis Coenzyme
 NADP+

 It picks up a
 The H+ and electron move to
Photosynthesis
Reactants
Products





Photosynthesis in a Leaf
 Leaves are the major locations of
photosynthesis
 Their green color is from
chlorophyll,
absorbed by
chlorophyll drives the synthesis of
organic molecules in the chloroplast
 CO2 enters and O2 exits the leaf
through

 Chloroplasts are found mainly in cells
of the mesophyll,
 A typical mesophyll cell has
Chloroplast
Thylakoids

 Stacks of thylakoids are
called
 Contain

 Accessory pigments:
carotenoids, phycocyanins
Stroma

Overview in Words
 Photosynthesis is a redox process in which
 Chloroplasts split H2O into
 Electrons of hydrogen are put into
 Photosynthesis consists of the
 The light reactions (in the thylakoids)

 Reduce
 Generate ATP from ADP by
 The Calvin cycle (in the stroma)

 The Calvin cycle begins with carbon fixation,
Overview in Picture
Light Energy
 Light is a form of electromagnetic
energy, also called

 Wavelength determines the
 Wavelength is the distance
 The electromagnetic spectrum
is the
radiation
 Visible light consists of
, or
Photons & Pigments
 Light also behaves as though it
consists of discrete particles,
called
 Pigments are substances that

 Wavelengths that are not
absorbed
 Leaves appear green because
 A
measures a pigment’s ability to
absorb various wavelengths
Absorption Spectrum
 An absorption spectrum is a graph plotting a
pigment’s
 The absorption spectrum of chlorophyll a suggests
that
Action Spectrum
 An action spectrum profiles the relative effectiveness of
 Chlorophyll a is the
 Accessory pigments, such as chlorophyll b,
 Accessory pigments called carotenoids
Light-Dependent Reactions
 GOAL: To trap
and
convert it to
 ATP & NADPH (energy carriers)

 Light-dependent reaction animation
Photolysis

 H2O 
Water 
O
+
one oxygen atom
2H+
+
2 hydrogen ions
2e2 electrons
 Electrons from photolysis replenish ones taken by
NADP+ (to form NADPH)


Capturing Light
 When a pigment absorbs light, it goes from a
 When excited electrons fall back to the ground state,
 A photosystem consists of a reaction-center
complex
light-harvesting complexes
that funnel the
surrounded by

is the first step of the light reactions
Capturing Light
Photosystems
 There are two types of
photosystems in the thylakoid
membrane
 Photosystem II (PS II)
functions
and is
 The reaction-center
chlorophyll a of PS II is
called
 Photosystem I (PS I)
functions
and is
 The reaction-center
chlorophyll a of PS I is called
Excitation
 Linear electron flow, the primary
pathway,
 A photon hits a
energy is
and its
 An excited electron from P680 is
transferred to
 P680+ (P680 that is missing an electron)
is a
 H2O is split by
electrons are
 O2 is released as a
, and the
ETC
 Each electron “falls”
down an
 Energy released by the
fall drives the creation of
a
 Diffusion of H+
(protons) across the
membrane
Repeat (kinda)
 In PS I (like PS II),
 P700+ (P700 that is missing an
electron) accepts an electron
 Each electron “falls” down an
electron transport chain from the
primary electron acceptor of PS I to
the protein ferredoxin (Fd)
 The electrons are then
 The electrons of NADPH are
Cyclic Electron Flow
 Cyclic electron flow uses
 Cyclic electron flow generates
satisfying the higher demand in the Calvin
cycle
 Some organisms (such as purple sulfur
bacteria) have PS I but not PS II
 Cyclic electron flow
,
Cyclic Electron Flow
Chemiosmosis
 ATP is produced in
conjunction with electron
transport by the process of

 When water is broken
the
Chemiosmosis Differences
 Chloroplasts and mitochondria generate ATP by
chemiosmosis, but
 Mitochondria use
; chloroplasts use
 In mitochondria, protons are pumped to the
and drive ATP synthesis as
they
 In chloroplasts, protons are pumped into the
and drive ATP synthesis as they
Light-Dependent Reactions
Light-Dependent Reaction:
Summary
Location:
Reactants
Products

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


Light Dependent Reaction summary
Calvin Cycle
 The Calvin cycle, like the citric acid cycle,
 The cycle builds
from smaller molecules by
 Carbon enters the cycle as
and leaves as a sugar named
 For net synthesis of 1 G3P, the cycle must take place
 The Calvin cycle has three phases:

(catalyzed by rubisco)


(RuBP)
Carbon Fixation
 Each carbon dioxide (6 of them) is “fixed” (added) to a
 This creates an unstable six-carbon compound that
splits into
 The enzyme that catalyzes this first step is called RuBP
carboxylase (or Rubisco)
Reduction
 Each PGA receives a
become 1,2bisphosphoglycerate
 A pair of electrons
to
and loses a phosphate
to become glyceraldehyde3-phosphate


Regeneration of RuBP
 5 molecules of G3P are
 Cycle spends 3 more
 RuBP can now accept
another
Calvin Cycle Summary
Location:
Reactants
Products




Calvin Cycle Summary
Overview
Alternative Mechanisms
 Dehydration is a problem for plants, sometimes
requiring
 On hot, dry days, plants close
, which
 The closing of stomata
 These conditions favor a seemingly wasteful process
called
 In most plants (C3 plants), initial fixation of CO2,
via rubisco, forms a
Photorespiration
 In photorespiration, rubisco adds
 Photorespiration consumes O2 and organic fuel and
releases CO2
 Photorespiration
that build up in the absence of the Calvin
cycle
 In many plants, photorespiration is a problem because
on a hot, dry day it can drain as much as
C4 Plants
 C4 plants minimize the cost of
photorespiration by
 This step requires the enzyme
 Has a
rubisco does
 Can fix CO2 even when
than
 These four-carbon compounds are
exported to bundle-sheath cells
(surround leaf veins), where they

CAM Plants
 Some plants, including succulents,
use crassulacean acid
metabolism (CAM) to fix
carbon
 CAM plants
, incorporating CO2
into organic acids
 Stomata close during the day, and

Review
 The energy entering chloroplasts as sunlight
 Sugar made in the chloroplasts
and carbon skeletons to
synthesize the
 Plants store excess sugar as
 In addition to food production,
photosynthesis produces
Review Questions
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Define photosynthesis and state its chemical equation. Be able to list the light
reactions and Calvin cycle as separate equations as well.
Differentiate between autotrophs and heterotrophs.
Explain the anatomy of a leaf and how its parts pertain to photosynthesis.
Define the role of NADP+ in photosynthesis.
Detail the parts of the chloroplast and explain the events that occur in each part.
Briefly describe the electromagnetic spectrum and its influence on photosynthesis.
Define photons and pigments and state their role in photosynthesis.
Differentiate between the absorption spectrum and action spectrum of various
pigments.
State the goal, location, and steps of the light dependent reactions.
Define photosystem and differentiate between the 2 kinds used in photosynthesis.
Explain the events of the electron transport chain (ETC).
Trace the pathway that electrons from the hydrogen atom of water take in the light
dependent reactions.
Explain chemiosmosis and identify the differences in function in the chloroplast
versus the mitochondrion.
State the goal, location, and 3 steps of the Calvin cycle.
Differentiate between PGA, G3P, and RuBP.
Explain photorespiration.
Explain the differences between C4 plants and CAM plants, as oppossed to C3
plants.