Modern Biology Chapter 6: Photosynthesis

Plant cell

6-1: Capturing the Energy in Light

Energy for life processes

• photosynthesis: process by which green plants convert solar energy into chemical energy– produces carbohydrates– produces oxygen

Energy for life processes

• chloroplast structure– double membrane surrounds entire organelle– thylakoids: flattened sacs inside double membrane– grana: stacks of thylakoids– stroma: fluid surrounding thylakoids inside double

membrane

Energy for life processes

• sunlight– provides heat and energy to earth

• white light from sun contains mixture of colors of light– wavelength of light determines its color– only a small portion of sun-light is visible to

humans

• The sun emits all visible wavelengths of light

• Green plants absorb red, orange, blue and violet

•They reflect yellow and green

• O2To air

• C6H12O6To plant

• CO2 From

Air

• H2OFrom Soil

• Light energy

From Sun

Overview of photosynthesis

Energy for life processes

• pigment: colored substance that reflects or absorbs light

Energy for life processes

• chlorophyll– type of pigment in thylakoid membranes

• two types of chlorophyll• chlorophyll a absorbs light in red end of spectrum• chlorophyll b absorbs light in blue end of the spectrum

(accessory pigment)• green light is not absorbed, but reflected giving the

leaves the appearance of being green

– by absorbing light pigments also absorb energy

Energy for life processes

• Cartenoids: other accessory pigments– absorb different colors depending on chemical

structure– become apparent when chlorophylls fade (fall

colors)

THE LIGHT REACTIONS

The light reactions consist of three basic components

• Photosystem 2• Photosystem 1• ATP synthase (chemiosmosis)

Photosystem 2

• water-plastoquinone oxidoreductase• Uses the energy from sunlight to split the

water molecule into three parts2H2O 4 H+ + 4 e- + O2

Photosystem 1

• plastocyanin: ferredoxin oxidoreductase• Uses the energy from sunlight to move the

electrons onto NADP+ for transport to the next phase of the process

ATP synthase

• Synthesizes ATP using a concentration gradient created by photosystem II

Light reactions

• Light and the energy associated with it are absorbed into photosystem I and photosystem II

Light-dependent reactions a.k.a. light reactions

• Electron transport occurs within membranes

Light-dependent reactions a.k.a. light reactions

• photosystem II (PSII)– accessory pigments absorb light and acquire

energy (E) (step 1)– energy is passed along membrane pigments until it

reaches a specific pair of chlorophyll a molecules

Light-dependent reactions a.k.a. light reactions

• photosystem II (PSII)– electron transport

• E forces e- to increase E level (e- are said to be “excited”)

• excited e- leave chlorophyll a• chlorophyll a is oxidized• PEA donates e-

• e- reduces primary e- acceptor (PEA) (step 2)• e- transported down ETC (step 3)• each transfer, the e- loses some E• E is used to move p+ into thylakoid

Light-dependent reactions a.k.a. light reactions

• photosystem I (PSI)– light absorbed by PSI (step 1b)– e- move from chlorophyll a to PEA (step 4)– e- lost are replaced by e- from PSII– PEA of PSI donates e- to NADP+ (step 5)

• brings e- to edge of thylakoid membrane by stroma• e- combine with p+ and NADP+

• NADP+ reduced to NADPH

Light-dependent reactions a.k.a. light reactions

• replacing e- (step 6)– recall e- from chlorophyll in PSII replace e-

that leave chlorophyll on PSI– e- from PSII need to be replaced or both ETCs

cease

Light-dependent reactions a.k.a. light reactions

• replacing e- (step 6)– replacement e- come from water

• enzyme in thylakoid splits water molecule• 2H2O 4 H+ + 4 e- + O2

• p+ (H+) remain in thylakoid• O2 diffuses out and leaves the plant

• replace e- lost by chlorophyll in PSII

Summary of Light Reactions

• Summary: what is produced during the light reactions– p+ concentration gradient– NADPH

Summary of Light Reactions

• Summary: what is produced during the light reactions– p+ concentration gradient– NADPH

Chemiosmosis

• potential E from gradient is harnessed by ATP Synthase in thylakoid membrane– ATP Synthase serves two functions

– Catalyzes ADP + (P) ATP– Acts as carrier protein for p+

• as H+ ions pass through ATP Synthase, ATP is produced

Section 6.2: The

Calvin cycle

Stomata

Open

Closed

Section 6.2: The Calvin cycle

• Light-independent reactions • Many names

– Calvin (or Calvin-Benson) cycle (men who first described cycle)

– Dark reactions (does not directly require light)– Carbon fixation (incorporation of C into organic

substances)

Section 6.2: The Calvin cycle

• sugars are long term energy storage (much more stable than ATP of NADP+)

• requires carbon dioxide (CO2 ) and water (H2O)– CO2 enters plants through stomata (little tiny pores

controlled by the plant)– H2O enters plant through osmosis, capillarity or

stomata

Step 1

• after diffusing into the stroma from the cytosol, CO2 joins with a 5-C sugar (RuBP) to produce 2 3-C molecules of PGA (process is known as carbon-fixation)

Step 2

• PGA is converted into PGAL– 2PGA + 2ATP + 2NADPH2PGAL + 2ADP +

2NADP+ +2 phosphate

Step 3 and 3B

• Most PGAL converted back into RuBP– 2PGAL + ATP RuBP + ADP + phosphate + some

fixed C

• Some PGAL leave Calvin cycle as fixed C (3B)

Balance Sheet

• Each turn of Calvin cycle results in fixation of 1 CO2

• Three times around Calvin cycle results in 1 PGAL– each turn requires 3 ATP and 2 NADPH

• 2 ATP from step 2• 1 ATP from step 3

– 3 turns requires 9 ATP and 6 NADPH

…

• PGAL and other organic molecules like carbohydrates are formed and then used all over the cell for a variety of functions.

• 6CO2+ 6H2O + energy C6H12O6 + 6O2

http://bcs.whfreeman.com/thelifewire/content/chp08/0802003.html

Alternative Pathways

• C4 pathway– when CO2 is low, enables plants to continue fixing

carbon – grasses, corn– uses less water, but moves much more slowly

Alternative Pathways

• CAM pathway– when very hot and dry, enables plant to continue to

fix carbon– cacti, pineapples– stomata open at night instead of during the day– uses less water, but moves much more slowly