Photosynthesis

Cellular Energy Unit Part I

Autotrophs Producers Photosynthesizing plants, bacteria, and

some eukaryotic single celled organisms.

The base of every food chain/web possible.

Overview Overall Reaction: 6CO2 + 6H2O + sunlight → C6H12O6 + 6O2

Affected by: Intensity of light, concentration of CO2 and temperature.

Chloroplasts

Two important sites of photosynthesis: thylakoid membranes and the stroma.

There are many pigments in the chloroplasts of plant cells. They are present in different proportions at different times of year which accounts for the many colors of leaves we see.

The two classes are chlorophylls and carotenoids. Chlorophylls are green and carotenoids run a spectrum from yellow to red but are most commonly called orange.

We see colors because these pigments absorb the white light and reflect the wavelength of light they do not absorb. For example, we see green because chlorophyll absorbs red, orange, yellow, blue, indigo and violet and reflects green back to us.

Plant Pigments

Light Reactions Light Reaction:

ADP + NADP+ + H2O+ Sunlight→ATP + NADPH + O2

Occurs in the Thylakoid Membranes of Chloroplasts

Includes Photosystem II where chlorophyll is excited by light and loses electrons. Water is split to provide more electrons to chlorophyll and oxygen is given off it diffuses out of the stoma.

The electrons travel to the electron transport chain where NADP + recharged to NADPH using one of the hydrogens from water.

Then Photosystem I where more chlorophyll is excited by light and more water is split to create a H+ gradient that ATP synthase uses to recharge ADP to ATP.

ATP Synthase

Dark Reactions ATP + NADPH + CO2 → C6H12O6 + ADP + NADP+

Occurs in the stroma of the chloroplasts. Does not depend on light Carbon dioxide diffuses into the stoma. Uses the calvin cycle for carbon fixation to create

a 6 carbon glucose from carbon dioxide. It takes 6 spins of the calvin cycle to make 1

glucose and it uses 18 ATP and 12 NADPH’s to power the process.

Glucose is stored in the vacuole as starch or it is used to feed the plant or make plant cell parts.

Calvin Cycle Also known as carbon

fixation. Uses ATP at two points in

each turn of the cycle. In the first point it uses 2 ATP’s and the second it uses 1 ATP for a total of 3 ATP’s for each turn of the cycle.

It uses 2 NADPH’s at one point in each cycle for a total of 12 NADPH’s used during the 6 cycles it takes to make one glucose.

It then feeds the used ADP and NADP+ back into the thylakoid for the light reactions recharge them back to ATP and NADPH.

Photosynthesis Review

Light

Occurs in the thylakoid

Uses water, sunlight and the recycled ADP and NADP+

Produces ATP, NADPH, and OXYGEN

Oxygen leaves the plant through the stoma.

Dark

Occurs in the stroma

Uses carbon dioxide, ATP, and NADPH

Produces glucose Recycles ADP and

NADP+ Glucose is food

and made into cell parts.