Photosynthesis

*Photosynthesis – process of capturing light energy from the sun to convert water & CO2

into oxygen and high energy carbohydrates (food, ex: glucose, starch, & other sugars)

Equation: Energy (light) + 6CO2 + 6H2O 6O2 + C6H12O6

Investigating Photosynthesis

1. Van Helmont’s Experiment – do plant’s grow by taking material from the soil?• Found mass of dry soil• Planted a seedling, watered it at regular intervals

until it grew to a mass of 75kg.• Found mass of soil to be unchanged• Concluded the mass the plant gained came from the

water he added.• Partially correct, but did not determine where the

carbon in the carbohydrate comes from

2. Priestley’s Experiment – oxygen is produced by plants• Determined that oxygen was required to keep a

flame lit/burning.• Removed oxygen from a jar by placing a lit candle

under it until the flame went out.• Then placed a sprig of mint in the jar (empty of

oxygen)• After a few days, he found he could relight a

candle in this jar and it would remain lit for a while!

3. Jan Ingenhousz – light is essential to photosynthesis!• Showed the effect observed by Priestley occurred

only when the plant is exposed to light!• Together, Priestly and Ingenhousz showed the

plants need light and water to produce oxygen.

I. Photosynthesis Basics – occurs in the chloroplasts of plants, protists, and some bacteria cells.

A. Chloroplast – organelle where photosynthesis occurs

1. Surrounded by 2 membranes.2. Thylakoid – flattened sac made of membrane

inside the chloroplast3. Granum – stack of multiple thylakoids4. Stroma – fluid that surrounds the grana and

fills the chloroplast

B. Pigments – compound that absorbs light1. Chlorophyll – pigment on thylakoid membrane that absorbs light for photosynthesis• Chlorophyll a – absorbs less blue and more

red light; directly absorbs sunlight• Chlorophyll b – absorbs more blue and less

red light; helps chlorophyll a absorb light• Both chlorophyll a and b reflect green light• Caretenoid – another pigment that absorbs

blue and green light, but not orange; also helps chlorophyl a absorb light.

C. Photosynthesis is chemically the opposite of Respiration.

Respiration Photosynthesis Uses glucose to make ATP 1st converts light to ATP

2nd uses ATP to make glucose

**Equations are also the reverse!

II. Light Reactions – first step of photosynthesis, converts sunlight to ATP

1. Occurs on the thylakoid membrane.2. Light is absorbed by a chlorophyll.3. The light energy provides electrons for the

Electron Transport Chain. • The ETC splits water (H+ & O2 are released). • Some H+ is added to NADP+ and produces

NADPH.• The O2 is released to the atmosphere.

4. Chemiosmosis • Also happens on the membrane of the

thylakoids. • Rest of the H+ drive ATP Synthase proteins to

make ATP.

*Chemiosmosis and the ETC happen at the same time!!!

III. Calvin Cycle – the 2nd step of photosynthesis. Also called the Dark Reactions, as light does not play any direct role.

Uses ATP to make Glucose

Steps of the Calvin Cycle

1. RuBP (carbohydrate in plants) reacts with NADPH, CO2 (from the atmosphere), and ATP to make Glucose.

2. In the final step, RuBP is remade so the cycle can occur again.