PHOTOSYNTHESIS. Photosynthesis process by which green plants & some organisms –s–seaweed, algae...
-
Upload
jackson-johnston -
Category
Documents
-
view
331 -
download
1
Transcript of PHOTOSYNTHESIS. Photosynthesis process by which green plants & some organisms –s–seaweed, algae...
PHOTOSYNTHESIS
Photosynthesis• process by which green
plants & some organisms – seaweed, algae & certain
bacteria• use light energy to convert
CO2 + water glucose
• all life on Earth, directly or indirectly, depends on photosynthesis as source of food, energy & O2
Autotrophs• self feeders
– organisms that make their own organic matter from inorganic matter
– producers• need inorganic
molecules such as CO2, H2O & minerals to make organic molecules
Heterotrophs• consumers
– other feeders
• depend on glucose as energy source
– cannot produce it
• obtained by eating plants or animals that have eaten plants
Carbon and Energy Flow
CO2 + H2O
CarbsProteinsLipids + O2
Photosynthesis
Cellular (Aerobic)Respiration(ATP Produced)
Food Chain• byproduct of
photosynthesis is O2
• humans & other animals breathe in oxygen
• used in cellular respiration
Other Benefits of Photosynthesis• humans depend on
ancient products of photosynthesis
• fossil fuels– natural gas, coal &
petroleum– for modern industrial
energy• represent remains of
organisms that relied on photosynthesis millions of years ago
Photosynthesis• plants produce
more glucose than they use
• Stored–starch & other
carbohydrates in roots, stems & leaves
Sites of Photosynthesis • leaves & green
stems • cell organelles
– chloroplasts• concentrated in
green tissue of leaf • mesophyll• green due to
presence of green pigment chlorophyll
Chloroplasts• each cell has 40-50 chloroplasts
– oval-shaped structures with double membrane
• inner membrane encloses compartment filled with stroma
• suspended in stroma are disk-shaped compartments-thylakoids– arranged vertically like stack
of plates• one stack-granum (plural,
grana) • embedded in membranes of
thylakoids are hundreds of chlorophyll molecules
Chlorophyll• light-trapping pigment
How Photosynthesis Works
• Requires
–CO2
–Water
–Sunlight
• Makes
–O2
–Glucose
How Photosynthesis Works• CO2 enters plant via
pores- stomata in leaves • water-absorbed by roots
from soil• membranes in
chloroplasts provide sites for reactions of photosynthesis
• chlorophyll molecules in thylakoids capture energy from sunlight
• chloroplasts rearrange atoms of inorganic molecules into sugars & other organic molecules
Photosynthesis• redox reaction
• 6CO2 + 12H2OC6H12O6 + 6O2 + 6H2O in presence of light
• must be an oxidation & a reduction
• water is oxidized– loses electrons &
hydrogen ions• carbon dioxide is
reduced – gains electrons &
hydrogens
Photosynthesis• relies on a flow of energy &
electrons initiated by light energy
• light energy causes electrons in chlorophyll pigments to boost electrons up & out of orbit
• hydrogens along with electrons are transferred to CO2sugar
• requires that H2O is split into H & O2
– O2 escapes to air• light drives electrons from H2O
to NADP+ which is oxidized NADPH which is reduced
Photosynthesis• 2 stages• light-dependent reactions
– chloroplasts trap light energy– convert it to chemical energy – contained in nicotinamide
adenine dinucleotide phosphate-(NADPH) & ATP
– used in second stage• light-independent reactions
– Calvin cycle– formerly called dark reactions– NADPH (electron carrier)
provides hydrogens to form glucose
• ATP provides energy
Light Dependent Reactions• convert light energy
to chemical energy & produce oxygen
• takes place- thylakoid membranes
• solar energy absorbed by chlorophyllATP + NADPH
Light Energy for Photosynthesis
• sun energy is radiation– electromagnetic energy
• travels as waves• distance between 2 waves-
wavelength• light contains many colors• each has range of
wavelengths measured in nanometers
• range of wavelengths is electromagnetic spectrum
• part seen by humans– visible light
Pigments• light absorbing molecules• built into thylakoid membranes• absorb some wavelengths & reflect
others• plants appear green because
chlorophyll-does not absorb green light– reflected back.
• as light is absorbedenergy is absorbed• chloroplasts contain several kinds of
pigments• different pigments absorb different
wavelengths of light• red & blue wavelengths are most
effective in photosynthesis• other pigments are accessory pigments• absorb different wavelengths• enhance light-absorbing capacity of a
leaf by capturing a broader spectrum of blue & red wavelengths along with yellow and orange wavelengths
Pigment Color & Maximum Absoption
• Violet: 400 - 420 nm • Indigo: 420 - 440 nm • Blue: 440 - 490 nm • Green: 490 - 570 nm • Yellow: 570 - 585 nm • Orange: 585 - 620
nm • Red: 620 - 780 nm
Chlorophylls• Chlorophyll A
– absorbs blue-violet & red light– reflects green– participates in light reactions
• Chlorophyll B– absorbs blue & orange light – reflects yellow-green – does not directly participate in
light reactions– broadens range of light plant
can use by sending its absorbed energy to chlorophyll A
Carotenoids• yellow-orange pigments• absorb blue-green
wavelengths• reflect yellow-orange• pass absorbed energy to
chlorophyll A• protective function
– absorb & dissipate excessive light energy that would damage chlorophylls
Light Energy• light behaves
as discrete packages of energy called photons
• fixed quantity of energy
Light Energy• when pigment absorbs a
photon• pigment’s electrons gains
energy• electrons are excited• unstable• electrons do not stay in
unstable state• fall back to original orbits• as electrons fall back to
ground heat is released• absorbed energy is passed
to neighboring molecules
Photosynthesis• Pigments
• Absorb light
• Excites electrons
• Energy passed to sites in cell
• Energy used to make glucose
Photosystems• chlorophyll &
other pigments are found clustered next to one another in a photosystem
• two participate in light reactions
Photosystems• photosystem I & II• each has specific
chlorophyll at reaction center
• photosystem II– chlorophyll P680
• photosystem I– chlorophyll P700
• named for type of light they absorb best
• P700 absorbs light in far red region of electromagnetic spectrum
Reaction Center• when photon strikes one
pigment molecule
• energy jumps from pigment to pigment until arrives at reaction center
• electron acceptor traps a light excited electron from reaction center chlorophyll
• passes it to electron transport chain which uses energy to make ATP & NADPH
Reaction Center
Light Reactions• during process of
making ATP & NADPH
• electrons are removed from molecules of water
• passed from photosystem II to photosystem I to NADP+
Photosystem II• water is split• oxygen atom combines
with oxygen from another split water forming molecular oxygen-O2
• each excited electron passes from photosystem II to photosystem I via electron transport chain
Photosystem I • primary electron acceptor
captures an excited electron• excited electrons are passed
through short electron transport chain to NADP+ reducing it to NADPH
• NADP+ is final electron acceptor
• electrons are stored in high state of potential energy in NADPH molecule
• NADPH, ATP and O2 are products of light reactions
ATP Formation-Chemiosmosis• uses potential energy of
hydrogen ion concentration gradient across membrane
• gradient forms when electron transport chain pumps hydrogen ions across thylakoid membrane as it passes electrons down chain that connects two photosystems
ATP Formation-Chemiosmosis
• ATP synthase (enzyme) uses energy stored by H gradient to make ATP
• ATP is produced from ADP & Pi when hydrogen ions pass out of thylakoid through ATP synthase
• photophosphorylation
ChemiosmosisH+H+
Chemiosmosis
pH 7
pH 8
Substrate-level Phosphorylation
Calvin Cycle• light independent reactions• depend on light indirectly to
obtain inputs for cycle-ATP & NADPH
• takes place in stroma of chloroplast
• cycle of reactions• makes sugar from CO2 &
energy• ATP provides chemical
energy• NADPH provides high energy
electrons for reduction of CO2 to sugar
Steps of Calvin Cycle• starting material-ribulose
bisphosphate (RuBP)• first step-carbon fixation• rubisco (an enzyme) attaches CO2 to
RuBP• Next-reduction reaction takes place• NADPH reduces 3-phosphoglyceric
acid (3-PGA) to glyceraldehye 3-phosphate (G3P) with assistance of ATP
• to do this cycle uses carbons from 3 CO2 molecules
• to complete cycle must regenerate beginning component-RuBP
• for every 3 molecules of CO2 fixed, one G3P molecule leaves cycle as product of cycle
• remaining 5 G3P molecules are rearranged using ATP to make 3 RuBP molecules
Calvin Cycle• regenerated RuBP is used
to start cycle again• process occurs repeatedly
as long as CO2, ATP & NADPH are available
• thousands of glucose molecules are produced
• used by plants to produce energy in aerobic respiration
• used as structural materials• stored
Photosynthesis Variations
• plants vary in the way they produce glucose and when
C3 Plants• use CO2 directly from air• first organic compound produced is a
3 carbon compound 3-PGA• reduce rate of photosynthesis in dry
weather• CO2 enters plants through pores in
leaves• on hot days stomata in leaves close
partially to prevent escape of water• with pores slightly open, adequate
amounts of CO2 cannot enter leaf• Calvin cycle comes to a halt• no sugar is made• in this situation rubisco adds O2 to
RuBP• 2-carbon product of this reaction is
broken down by plant cells to CO2 + H20
• Photorespiration• provides neither sugar nor
ATP
C4 Plants• have special adaptations
allowing them to save water without shutting down photosynthesis
• corn, sugar cane & crabgrass• evolved in hot, dry
environments • when hot & dry stomata are
closed• saves water• sugar is made via another
route• developed way to keep CO2
flowing without capturing it directly from air
C4 Plants• have enzymes that incorporate
carbon from CO2 into 4-C compound
• enzyme has an intense desire for CO2
• can obtain it from air spaces even when levels are very low
• 4-C compound acts as a shuttle• transfers CO2 to nearby cells -
bundle-sheath cells• found in vast quantities around
veins of leaves• CO2 levels in these cells remain
high enough for Calvin cycle to produce sugar
CAM Plants• pineapple, some cacti &
succulent plants• conserve water by opening
stomata & letting CO2 in at night
• CO2 is fixed into a 4-C compound
• saves CO2 at night & releases it in the day
• photosynthesis can take place without CO2 needing to be admitted during the day when conditions are hot and dry
Environmental Consequences of Photosynthesis
• CO2 makes up 0.03% of air• provides plants with CO2 to make sugars• important in climates• retains heat from sun that would otherwise radiate from Earth• warms the Earth• greenhouse effect
Global Warming• CO2 traps heatwarms air• maintains average temperature on
Earth about 10 degrees C warmer than without it.
• Earth may be in danger of overheating because of this greenhouse effect
• CO2 in air is increasing because of industrialization
• when oil, gas and coal are burned CO2 is released
• levels in atmosphere have increased 30% since 1850
• increasing concentrations have been linked to global warming
• slow & steady rise in surface temperature of Earth
• could have dire consequences for all life forms on Earth