how cells acquire energy
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How Cells Acquire Energy. Chapter 7. Overview: I. Places A. Chloroplasts : photosynthesis organelle B. Thylakoid: discs i. light-dependent: sunlight to chemical energy ii. ETC and water split = forming H+, e- , ATP: oxygen is by-product C. Stroma: fliud interior - PowerPoint PPT PresentationTRANSCRIPT
How Cells Acquire Energy
Chapter 7
Overview:I. PlacesA. Chloroplasts : photosynthesis organelleB. Thylakoid: discs
i. light-dependent: sunlight to chemical energyii. ETC and water split = forming H+, e- , ATP: oxygen is by-product
C. Stroma: fliud interiori. light-independent: makes glucoseii. carbon dioxide reduced to form glucose sunlight
12H2O + 6CO2 ————> 6O2 + C6H12O6 + 6H2O
• Photoautotrophs – “self-nourishing”
– Carbon source is carbon dioxide
– Energy source is sunlight– Capture sunlight energy and use it to carry out
photosynthesis
– Plants, Some bacteria, Many protistans
• Heterotrophs
– Get carbon and energy by eating autotrophs or one
another
II. SunlightA. photoautotrophs intercept only about
1% of solar energy B. wavelengthC. Electromagnetic spectrum
Short-----------------------------------------------LongGamma, x-ray, UV Visible infrared, microwaves, radioToo much energy just right energy not enough energy
VIBGYOR
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)
Chloroplast Structure
two outer membranes
inner membrane system(thylakoids connected by channels)
stroma
Figure 7.3d, Page 116
Photosynthesis Equation
12H2O + 6CO2 6O2 + C2H12O6 + 6H2O
Water Carbon Dioxide
Oxygen Glucose Water
LIGHT ENERGY
In-text figurePage 115
E. Pigments: molecules that absorb photonsi. the color you see is the color reflectedii. absorption spectrum
-chlorophylls: absorb all except green-carotenoids: accessory pigments: absorb blue-violet and blue-green
-xanthophylls and beta-carotene-abundant in fruits, flowers and vegetables-can be seen in leaves during autumn
-anthocyanins: blue or red accessory pigment -phycobilins: blue accessory pigment
iii. photosystems: organization of pigments-proteins and 200-300 pigment molecules-PSII (p680)-PSI (p700)
Pigments in Photosynthesis
• Bacteria– Pigments in plasma membranes
• Plants– Pigments and proteins organized into
photosystems that are embedded in thylakoid membrane system
Arrangement of Photosystems
water-splitting complex thylakoidcompartment
H2O 2H + 1/2O2
P680
acceptor
P700
acceptor
pool of electron carriers stromaPHOTOSYSTEM II
PHOTOSYSTEM I
Figure 7.10Page 121
Light-Dependent ReactionsI. General
A. ThylakoidB. 3 Basic steps
i. photosystems harvest sunlightii. solar energy converted to chemical energy ATPiii. NADP+ (coenzyme) picks up e- and H+
II. “Random Walk” of EnergyA. sunlight is trapped and randomly passed
from chlorophyll b and carotenoids to reaction center (chlorophyll a)
B. Low energy electrons become “excited” and is passed to e- acceptor
C. e- can follow 2 pathways
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
Photosystem Function: Reaction Center
• Energy is reduced to level that can be captured by molecule of chlorophyll a
• This molecule (P700 or P680) is the reaction center of a photosystem
• Reaction center accepts energy and donates electron to acceptor molecule
Pigments in a Photosystem
reaction center
Figure 7.11Page 122
Electron Transfer Chain
• Adjacent to photosystem • Acceptor molecule donates electrons
from reaction center
• As electrons pass along chain, energy they release is used to produce ATP
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
Cyclic Electron Flow
electron acceptor
electron transfer chain
e–
e–
e–
e–
ATP
Electron flow through transfer chain sets up
conditions for ATP formation at other membrane sites.
Figure 7.12Page 122
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
III. Cyclic (1 word) Pathway A. PSI (700) – one photosystem
B. ATP – one productC. oldest pathway
IV. Non-Cyclic (2 words) PathwayA. PSII to PSI (2 photosystems)B. ATP and NADPH (2 products)C. uses water (photolysis)D. Evolution of oxygen
V. Chemiosmotic model for ATP formationA. hydrogen ions released by photolysis
accumulate inside the thylakoidi. oxygen is given off as waste
B. more H+ accumulates from the ETC C. the build up of H+ ions inside the thylakoid results in 2 gradients: concentration and
electrici. these 2 gradients force H+ into the stroma through ATP synthase forming ATP
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
• Synthesis part of photosynthesis
• Can proceed in the dark
• Take place in the stroma
• Calvin-Benson cycle• need ATP (energy) and NADPH (H+ and e-)• Uses carbon dioxide
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
CARBON FIXATION
6 CO2 (from the air)
6 6RuBP
PGA
unstable intermediate
6 ADP
6
12
12ATP
ATP
NADPH
10
12PGAL
glucoseP
PGAL2
Pi
12 ADP12 Pi
12 NADP+
12
4 Pi
PGAL
Figure 7.16Page 125
II. Calvin-Benson CycleA. Capture of carbon dioxide (Carbon Fixation – 1st step)
i. stromaii. enzyme (RuBP Carboxylase – Rubisco) + RuBP (ribulosebisphosphate)
+ CO2 yields 6-C intermediate that splits into 2 , 3-C PGA (phosphoglycerate) molecules
B. Reduction of PGAi. PGA accepts one phosphate group from ATP plus hydrogen and electrons from the NADPH, resulting in the formation of the intermediate PGAL (phosphorglyceraldehyde)
C. Regeneration of RuBPi. to form 1 six carbon sugar requires 6 CO2
molecules and the formation of 12 PGAL’sii. most of the PGAL get recycled back to RuBP, but 2 of the PGAL combine forming glucose with a phosphate groupiii. when glucose is phosphorylated like this it is primed to enter reactions to become sucrose,
cellulose and starch (the main plant carbohydrates)-the ADP, NADP+ and released phosphate diffuse through the stroma back to the light-dependent reactions
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
C4, C3 and CAM plantsI. General
A. since environments differ, photosynthesis is not the same in all plants
B. We will compare the carbon-fixing adaptations in plants
i. compared to C3 plants (most plants), C4 and CAM plants have modified ways of fixing carbon for photosynthesis during hot, dry conditions
C. Stomata: tiny openings in leaf surface through which gases diffuse
i. closes on hot dry days to conserve water; when this happens CO2 gets used up and O2 builds upD. a high O2 concentration triggers photorespiration, a process that lowers a plant’s sugar making capacity
i. rubisco (the enzyme in the calvin-benson cycle that attaches to CO2 in carbon fixation) also attaches to oxygen
ii. competition between CO2 and O2 for rubiscoiii. sugars are still formed, but at a lower rate
• 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
• C3 plants
– bluegrass, beans, wheat, rice, oats (most plants)
• In hot, dry conditions photorespiration occurs
The C3 Pathway
III. C4 plantsA. corn, bermuda grass, sugar caneB. oxaloacetate (4 carbons) is the first
intermediate in these plantsC. these plants fix carbon twice, in 2 types of photosynthetic cells, so CO2 levels do not decline as much
D. 2 types of photosynthetic cellsi. mesophyll uses PEP (phenolpyruvate, 3-C) to fix CO2 ii. oxaloacetate forms, then malate which diffuses into adjoining bundle sheathsiii. in the bundle sheaths, pyruvate forms. CO2 is released and enters the C-B cycleiv. the pyruvate regenerates PEP for use in the C4 cycle in the mesophyll cells
E. Advantages: small stomata, lose less water, make more glucose than C3 plants during hot, bright, dry days
CAM Plants
• Carbon is fixed twice (in same cells)
• Night – Carbon dioxide is fixed to form organic
acids
• Day– Carbon dioxide is released and fixed in
Calvin-Benson cycle
IV. CAM (crassulacean acid metabolism) plantsA. desert plants (cacti)B. Type of C4
i. closes stomates during day, open at night (fix CO2 at night)ii. at night, mesophyll cells use C4 cycle and store malate and other products until the next dayiii. day time, stomata close: malate releases CO2 which enters the C-B cycle in the same cells
-this way photosynthesis continues without water loss
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