Photosynthesis Section 3.1

Overview of Photosynthesis General Equation: Light Energy 6 CO H 2 O  C 6 H 12 O O 2

The Leaf The leaf is the photosynthetic organ of green plants. mesophyll cells –The cells that perform photosynthesis –they contain the largest number of chloroplasts

Vein Leaf cross section Mesophyll CO 2 O2O2 Stomata Chloroplast Mesophyll 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid Granum Stroma 1 µm

Chloroplast Mesophyll 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid Space (thylakoid lumen) Thylakoid Granum Stroma 1 µm

Stomata (pores in leaves) Three functions: –Allow CO 2 in –Allow O 2 out –When water goes out, the leaves are cooled so they do not overheat (called evaporative cooling) CO2 in O2 out H2O out H2O in

Plants regulate size of stomata opening in response to environmental conditions in order to: –Maximize CO 2 uptake –Minimize water loss In general, stomata reduce in size: –Sunny, warm, dry and windy

Guard cells The guard cells control the stomata by opening and closing. This is controlled by osmosis: –When water is scarce, the guard cells lose H 2 O and deflate, closing the stomata. –When water is abundant, the guard cells swell, opening the stomata.

Also, in general, stomata are open in the daytime and closed at night

Two Types of Reactions in Photosynthesis Light Dependent Rxns (Light Reactions) –require light, which includes exciting electrons and splitting H 2 O. Light Independent Rxns (Dark Reactions) –do not directly require sunlight, includes Carbon fixation, the Calvin Cycle.

Light Reactions Occur in the grana Split water, release oxygen, produce ATP, and form NADPH H2OH2O CO 2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast Sucrose NADPH NADP  ADP + P O2O2 ATP starch

NADH NADPH

Dark Reactions The Calvin cycle –Occurs in the stroma –Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power H2OH2O CO 2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast Sucrose NADPH NADP  ADP + P O2O2 ATP starch

Electromagnetic Radiation Gamma rays X-raysUVInfrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm 10 6 nm 1 m 10 6 nm 10 3 m nm Visible light Shorter wavelength Higher energy Longer wavelength Lower energy

Pigments –Are substances that absorb visible light –The reflected light and transmitted include the colors we see –Absorbed light is important in driving photosynthesis Light Reflected Light Chloroplast Absorbed light Granum Transmitted light

Pigments Chlorophyll a –Is the main photosynthetic pigment Chlorophyll b –Is an accessory pigment C CH CH 2 C C C C C CN N C H3CH3C C C C C C C C C N C C C C N Mg H H3CH3C H C CH 2 CH 3 H C H H CH 2 H CH 3 C O O O O O CHO in chlorophyll a in chlorophyll b Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown

Spectrophometer Figure 10.8 White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. The low transmittance (high absorption) reading chlorophyll absorbs most blue light. Blue light

Absorption Spectra –Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis

They both have a double peak: in the red/orange and in the blue/violet areas. Between the two they absorb much of the V, B, O and R spectrums, leaving green and yellow (typical leaf colours).

Homework. Read Ch 3.1 Do p145 #4, 5, 6.