How Cells Acquire Energy

1
How Cells Acquire Energy

• Chapter 7

2
Carbon and Energy Sources

• Photoautotrophs
• Carbon source is carbon dioxide
• Energy source is sunlight
• Heterotrophs
• Get carbon and energy by eating autotrophs or one
  another

3
Photoautotrophs

• Capture sunlight energy and use it to carry out
  photosynthesis
• Plants
• Some bacteria
• Many protistans

4
Linked Processes

• Photosynthesis
• Energy-storing pathway
• Releases oxygen
• Requires carbon dioxide

• Aerobic Respiration
• Energy-releasing pathway
• Requires oxygen
• Releases carbon dioxide

5
Chloroplast Structure
two outer membranes
stroma
inner membrane system
(thylakoids connected by channels)
Figure 7.3d, Page 116
6
Photosynthesis Equation
LIGHT ENERGY
12H2O 6CO2
6O2 C2H12O6 6H2O
Water
Carbon Dioxide
Oxygen
Glucose
Water
In-text figurePage 115
7
Where Atoms End Up
In-text figurePage 116
8
Two Stages of Photosynthesis
sunlight
water uptake
carbon dioxide uptake
ATP
ADP Pi
LIGHT-INDEPENDENT REACTIONS
LIGHT-DEPENDENT REACTIONS
NADPH
NADP
glucose
P
oxygen release
new water
In-text figurePage 117
9
Electromagnetic Spectrum

• Shortest Gamma rays
• wavelength X-rays
• UV radiation
• Visible light
• Infrared radiation
• Microwaves
• Longest Radio waves
• wavelength

10
Visible Light

• Wavelengths humans perceive as different colors
• Violet (380 nm) to red (750 nm)
• Longer wavelengths, lower energy

Figure 7.5aPage 118
11
Photons

• Packets of light energy
• Each type of photon has fixed amount of energy
• Photons having most energy travel as shortest
  wavelength (blue-violet light)

12
Pigments

• Color you see is the wavelengths not absorbed

13
Variety of Pigments

• Chlorophylls a and b
• Carotenoids
• Anthocyanins

14
Chlorophylls

• Main pigments in most photoautotrophs

chlorophyll a
Wavelength absorption ()
chlorophyll b
Wavelength (nanometers)
Figure 7.6a Page 119
Figure 7.7Page 120
15
Accessory Pigments
Carotenoids, Phycobilins, Anthocyanins
beta-carotene
phycoerythrin (a phycobilin)
percent of wavelengths absorbed
wavelengths (nanometers)
16
Pigments in Photosynthesis

• Bacteria
• Pigments in plasma membranes
• Plants
• Pigments and proteins organized into photosystems
  that are embedded in thylakoid membrane system

17
Arrangement of Photosystems
water-splitting complex
thylakoid compartment
H2O
2H  1/2O2
P680
P700
acceptor
acceptor
pool of electron carriers
stroma
PHOTOSYSTEM II
PHOTOSYSTEM I
Figure 7.10Page 121
18
Light-Dependent Reactions

• Pigments absorb light energy, give up e-, which
  enter electron transfer chains
• Water molecules split, ATP and NADH form, and
  oxygen is released
• Pigments that gave up electrons get replacements

19
Photosystem Function Harvester Pigments

• Most pigments in photosystem are harvester
  pigments
• When excited by light energy, these pigments
  transfer energy to adjacent pigment molecules
• Each transfer involves energy loss

20
Photosystem Function Reaction Center

• This molecule (P700 or P680) is the reaction
  center of a photosystem

21
Pigments in a Photosystem
reaction center
Figure 7.11Page 122
22
Electron Transfer Chain

• Adjacent to photosystem
• As electrons pass along chain, energy they
  release is used to produce ATP

23
Cyclic Electron Flow

• Electrons
• are donated by P700 in photosystem I to acceptor
  molecule
• flow through electron transfer chain and back to
  P700
• Electron flow drives ATP formation
• No NADPH is formed

24
Synthesis of ATP(chemiosmotic phosphorylation)
H2O
second electron transfer chain
photolysis
e
e
ATP SYNTHASE
NADPH
first electron transfer chain
NADP
ATP
ADP Pi
PHOTOSYSTEM I
PHOTOSYSTEM II
Figure 7.13aPage 123
25
Chemiosmotic Model of ATP Formation

• Electrons within the membrane of the chloroplast
  attract H protons
• The H protons are pumped inside the chloroplast
  membranes
• The Protons are allowed to pass out of the
  membrane through the CF1 particle that is rich in
  ADP P plus phosphorylating enzymes.

26
Chemiosmotic Model for ATP Formation
Gradients propel H through ATP synthases ATP
forms by phosphate-group transfer
H is shunted across membrane by some components
of the first electron transfer chain
Photolysis in the thylakoid compartment splits
water
H2O
e
acceptor
ATP SYNTHASE
ATP
ADP Pi
PHOTOSYSTEM II
Figure 7.15Page 124
27
Light-Independent Reactions

• Synthesis part of photosynthesis
• Can proceed in the dark
• Take place in the stroma
• Calvin-Benson cycle

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Calvin-Benson Cycle

• Overall reactants
• Carbon dioxide
• ATP
• NADPH

• Overall products
• Glucose
• ADP
• NADP

Reaction pathway is cyclic and RuBP (ribulose
bisphosphate) is regenerated
29
Calvin- Benson Cycle
6
CO2 (from the air)
CARBON FIXATION
6
6
RuBP
unstable intermediate
12
PGA
6 ADP
12
ATP
6
ATP
12
NADPH
4 Pi
12 ADP 12 Pi 12 NADP
10
PGAL
12
PGAL
2
PGAL
Pi
P
Figure 7.16Page 125
glucose
30
The C3 Pathway

• In Calvin-Benson cycle, the first stable
  intermediate is a three-carbon PGA
• Because the first intermediate has three carbons,
  the pathway is called the C3 pathway

31
Photorespiration in C3 Plants

• On hot, dry days stomata close
• Inside leaf
• Oxygen levels rise
• Carbon dioxide levels drop
• The plant is in trouble because it does not
  enough Carbon dioxide to undergo photosynthesis
• The plant still needs energy so it taps its own
  store of glucose

32
C4 Plants

• Carbon dioxide is fixed twice
• In mesophyll cells, carbon dioxide is fixed to
  form four-carbon oxaloacetate
• Oxaloacetate is stored as a crystal
• When times get bad (drought conditions), the
  plant can now convert the crystalline form of
  oxaloacetate back to Carbon dioxide and undergo
  photosynthesis.

33
Summary of Photosynthesis
LIGHT-INDEPENDENT REACTIONS
Figure 7.21Page 129
34
Satellite Images Show Photosynthesis
Atlantic Ocean
 Photosynthetic activity in spring
Figure 7.20Page 128