how cells acquire energy
DESCRIPTION
How Cells Acquire Energy. Chapter 7. Photosynthesis: The Big Picture. Source of BOTH matter and energy for most living organisms Captures light energy from the sun and converts it into chemical energy Synthesized organic molecules from inorganic molecule BOTTOM LINE: Makes FOOD. - PowerPoint PPT PresentationTRANSCRIPT
How Cells Acquire Energy
Chapter 7
Photosynthesis: The Big Picture
Source of BOTH matter and energy for most living organisms
Captures light energy from the sun and converts it into chemical energy
Synthesized organic molecules from inorganic molecule
BOTTOM LINE: Makes FOOD
Autotroph: Organisms that make their own food (energy-rich organic
molecules) from simple, inorganic molecules
Photoautotroph: Organisms that make their own food through
photosynthesis; obtain energy from the sun Type of autotroph
Heterotroph: Get carbon and energy by eating autotrophs or one
another
Definitions
Photoautotrophs
Capture sunlight energy and use it to carry out photosynthesisPlantsSome bacteria
cyanobacteriaMany protistans
algae
Plants
Algae (spirogyra)
Cyanobacteria
Algea (Kelp)
Linked Processes
Photosynthesis
Energy-storing pathway
Releases oxygen
Requires carbon dioxide
Aerobic Respiration
Energy-releasing pathway
Requires oxygen
Releases carbon dioxide
Photosynthesis Equation
12H2O + 6CO2 6O2 + C6H12O6 + 6H2O
Water Carbon Dioxide
Oxygen Glucose Water
LIGHT ENERGY
In-text figurePage 115
Where Atoms End Up
In-text figurePage 116
Chloroplast Structure
two outer membranes
inner membrane system(thylakoids connected by channels)
stroma
Figure 7.3d, Page 116
Light dependent reactions Converts light energy into chemical energy (ADP ATP) Gathers e- and H+ from water (NADP+ NADPH) Occurs in thylakoid membranesLight independent reactions (Calvin-Benson Cycle) Reduces CO2 to synthesize glucose using energy and
hydrogens (i.e. ATP and NADPH) generated in the light dependent reaction
Occurs in Stroma
Notice that these reactions do not create NADH, but rather NADPH
Two Stages of Photosynthesis
Two Stages of Photosynthesis
sunlight water uptake carbon dioxide uptake
ATP
ADP + Pi
NADPH
NADP+
glucoseP
oxygen release
LIGHT-INDEPENDENT
REACTIONS
LIGHT-DEPENDENT REACTIONS
new water
In-text figurePage 117
Electromagnetic Spectrum
Shortest Gamma rays
wavelength X-rays
UV radiation
Visible light
Infrared radiation
Microwaves
Longest Radio waves
wavelength
Visible Light Electromagnetic energy with a wavelength of
308-750nm Light energy is organized into packets called
photons The shorter the wavelength the greater the
energy carried by the photons
Properties of Light
White light (from the sun) contains all of the wavelengths of light
When light hits matter, it can be reflected (transmitted) or absorbedWhite substances reflect all lightBlack substances absorb all light
Pigments
A substance that absorbs light We see the color that is transmitted by
pigment The absorbed color disappears into
pigment
Plant pigments
Plant use a variety of pigments during photosynthesis:Chlorophylls a and bCarotenoidsAnthocyaninsPhycobilins
The main photosynthetic pigment is Chlorophyll a
ChlorophyllsW
avel
eng
th a
bso
rpti
on
(%
)
Wavelength (nanometers)
chlorophyll b
chlorophyll a
Chlorophyll a absorbs red and blue light, and reflects green light (what we see)
Note: The colors that are absorbed are used for photosynthesis
Figure 7.6a Page 119
Figure 7.7Page 120
Effect of Light on Pigments
What happens when light hits pigments? The color disappears, but the energy does not Absorbing photons of light excites electrons (e-),
thus adding potential energy Ground state: normal pigment Excited state: pigment absorbing light (e- excited)
e-e-
Photon of light:
Atom in pigment:Ground state
Atom in pigment:Excited state
Photosystems
In thylakoid membrane, pigments are organized in clusters called photosystems
These clusters contain several hundred pigment molecules
Two types of photosystems Photosystem I = P700 (absorbs light at 700nm)Photosystem II = P680 (absorbs light at 680nm)
Reaction Center Chlorophyll
One of the pigments in each photosystem is known as the reaction center chlorophyll (RCC)
if any pigment within the photosystem gets hit by a photon, the energy is transferred to the RCC
The RCC will then transfer its excited e-
into an electron transport chain
Pigments in a Photosystem
reaction center
Figure 7.11Page 122
Light Dependent Reactions
Location: the thylakoid membranes Function: to generate ATP (energy!) and
NADPH (reducing power!) that will be used in the light independent reaction
Two processes: Non-cyclic electron flow
Generates ATP and NADPH Cyclic electron flow
Generates only ATP
Noncyclic Electron Flow
Two-step pathway for light absorption and electron excitation
Uses two photosystems: type I and type II
Produces ATP and NADPH Involves photolysis - splitting of water
Machinery of Noncyclic Electron Flow
photolysis
H2O
NADP+ NADPH
e–
ATP
ATP SYNTHASE
PHOTOSYSTEM IPHOTOSYSTEM II ADP + Pi
e–
first electron transfer chain
second electron transfer chain
Figure 7.13aPage 123
Steps of Non-cyclic electron flow
Photosystem II gets hit by a photon; electron of RCC gets excited
The excited (high energy) e- gets picked up by an electron carrier and taken into an electron transfer chain (ETC)
The excited e- provides energy to pump protons (H+) into the thylakoid (tiny space)
Through chemiosmosis, ATP is generated
Chemiosmotic Model of ATP Formation Electrical and H+ concentration gradients
are created between thylakoid compartment and stroma
H+ flow down gradients into stroma through ATP synthase
The energy driven by the flow of H+ powers the formation of ATP from ADP and Pi
Chemiosmotic Model for ATP Formation
ADP + Pi
ATP SYNTHASE
Gradients propel H+ through ATP synthases;ATP forms by phosphate-group transfer
ATP
H+ is shunted across membrane by some components of the first electron transfer chain
PHOTOSYSTEM II
H2Oe–
acceptor
Photolysis in the thylakoid compartment splits water
Figure 7.15Page 124
Non-cyclic electron flow: Photolysis While Photosystem II gets hit by light, etc.,
water is split:
H2O ½ O2 + 2H+ + 2e-
This process is called photolysis The H+ are pumped into the thylakoid to
create the proton gradient The e- replace the excited e- that was
taken away from the RCC
Non-Cyclic Electron Flow:The saga continues Photosystem I gets excited at the same
time as photosystem II Its excited e- gets taken into a second
electron transfer chain that attaches the excited e- and the leftover H+ to NADP+ to make NADPH:
NADP+ + H+ + e- NADPH
Non-cyclic electron flow
The “electron hole” in photosystem I is then filled with the used up, low energy e- from photosystem II
Now everything is back to normal, and we can start all over again
Energy Changes in Non-cyclic electron flow
Figure 7.13bPage 123
Po
ten
tial
to
tra
nsf
er e
ner
gy
(vo
lts)
H2O 1/2O2 + 2H+
(Photosystem II)
(Photosystem I)
e– e–
e–e–
secondtransfer
chain
NADPHfirst
transferchain
Non-cyclic electron flow: Summary After two excited photosystems, two ETCs
and the splitting of water, both ATP and NADPH are generated!!!
Cyclic electron flow
The light independent reactions require more ATP than NADPH
Cyclic electron flow is like a short cut to making extra ATP
Involves only Photosystem I
Cyclic electron flow
Photosystem I gets excited Excited e- is carried into the first ETC;
energy goes to pump H+ into thylakoid compartment
Chemiosmosis powers formation of ATP The same e- (now low energy) replaces
itself in the “electron hole” in Photosystem I
Cyclic electron flow
photolysis
H2O
NADP+ NADPH
e–
ATP
ATP SYNTHASE
PHOTOSYSTEM IPHOTOSYSTEM II ADP + Pi
e–
first electron transfer chain
second electron transfer chain
Figure 7.13aPage 123
Light dependent reactions:Summary Non-cyclic electron flow
Generates ATP
Synthesis part of
photosynthesis
Can proceed in the dark
Take place in the stroma
Calvin-Benson cycle
Light-Independent Reactions
Calvin-Benson Cycle
Overall reactants
Carbon dioxide
ATP
NADPH
Overall products
Glucose
ADP
NADP+
Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated
Calvin-Benson Cycle
Three Phases:
1. Carbon Fixation
2. Reduction
3. Regeneration of RUBP
Calvin-Benson Cycle: Carbon Fixation
Capturing atmospheric (gaseous) CO2 by attaching it to RuBP, a 5-carbon organic molecule
This process forms two 3-carbon molecules
The enzyme that catalyzes this process is called Rubisco
Calvin Benson Cycle:Reduction The captured CO2 has very little energy and no
hydrogens In order to make sugar, energy and hydrogens
need to be added to the molecules formed by Carbon fixation
ATP and NADPH (made in the light dependent reactions) break down to form ADP and NADP+ and, in the process, transfer energy and hydrogens to the 3-carbon compounds formed by carbon fixation, resulting in sugar formation
Calvin-Benson Cycle:Regeneration Some of the sugar created by reduction
leaves the Calvin cycle, and is used to build up glucose and other organic molecules
The rest of the sugar is used to remake (regenerate) RuBP
This process requires ATP (which was made in the light dependent reactions)
Calvin-Benson Cycle:Summary
The cycle proceeds 6 times to form each molecule of glucose
In the process, ATP and NADPH is used up 6CO2 are converted into C6H12O6 - glucose
In Calvin-Benson cycle, as described, the first stable intermediate is a three-carbon PGA
Because the first intermediate has three carbons, the pathway is called the C3 pathway
The C3 Pathway
Photorespiration in C3 Plants
On hot, dry days stomata (holes in the leaf) close to prevent evaporation of water
As a result, within the leaf oxygen levels rise, and Carbon dioxide levels drop
Rubisco attaches RuBP to oxygen instead of carbon dioxide
Results in a VERY wasteful process known as Photorespiration – uses up ATP without generating sugar
C4 and CAM Plants
To avoid photorespiration, plants that live in hot, dry climates evolved mechanisms to separate carbon fixation from the Calvin Cycle
The CO2 that enters the Calvin cycle is derived from the breakdown of previously synthesized organic acids
In this way, the enzyme that catalyzes the reaction that attaches CO2 to RuBP is not exposed to atmospheric oxygen
C4 and CAM Plants C4 plants do carbon fixation in a different
location (cell type) than the Clavin cycle CAM plants do carbon fixation at a different
time (night) that the Calvin cycle (day)
Summary of Photosynthesis
Figure 7.21Page 129
light6O2
12H2O
CALVIN-BENSON CYCLE
C6H12O6
(phosphorylated glucose)
NADPHNADP+ATPADP + Pi
PGA PGAL
RuBP
P
6CO2
end product (e.g., sucrose, starch, cellulose)
LIGHT-DEPENDENT REACTIONS
6H2O
LIGHT-INDEPENDENT REACTIONS