Photosynthesis

Photosynthesis

Process by which plants use light energy to make food.

A reduction process that makes complex organic molecules from simple

molecules.

Autotrophs

Organisms that can manufacture their own food from inorganic molecules

Self-feeding organisms

PSN General Equation

6 CO2 + 6 H2O + light C6H12O6 + 6 O2

Requires:

Chlorophyll

PSN: a redox process

Hydrogens are added to Carbons.

Water is a source for the Hydrogens.

Complex covalent bonds are made.

PSN

Has two chemical reactions:

1. Light Dependent Reactions

2. Light Independent Reactions

Names are from “light” as a requirement, not where or when

they occur.

Light

A form of electromagnetic radiation.

Visible light has the necessary energy for use in

PSN.

Too Hot Too Cold

Just Right

Action Spectrum

Not all colors are useable to the same degree for PSN.

Red and Blue light - absorbed and used in PSN.

Green light - reflected or transmitted.

Photosynthesis Pigments

1. Chlorophylls

2. Accessory Pigments

Chlorophylls Has CHON and Mg.

Several types possible. Molecule has a lipophilic tail

that allows it to dissolve into membranes.

Contains Mg in a reaction center.

Accessory Pigments

Absorb light energy and transfer it to chlorophyll.

Ex: Carotene (orange). Xanthophyll (yellow)

Fall Leaf Colors Chlorophyll breaks down.

N and Mg salvaged and moved into the stem for next

year. Accessory pigments remain behind, giving the various fall

leaf colors.

Chloroplast Structure

Double outer membrane. Inner membrane folded and

stacked into grana. Stroma - liquid that

surrounds the thylakoid membranes.

stroma

thylakoid sacs

Photosystems Collection of pigments that serve as a

light trap. Made of chlorophyll and the accessory

pigments. Two photosystems are known:

Photosystem I ,which uses P700 ( a type of chlorophyll a energized by the frequency 700 nm), and Photosystem II which uses P680 ( a type of chlorophyll a energized by the frequency 680 nm)

Noncyclic Photophsphorylation

Uses Photosystem I and Photosystem II.

Splits water, releasing H+, a pair of e-, and O2.

Produces ATP and NADPH. (e- carrier similar to NADH)

Light Dependent Reactions

Photosystem II: Electrons trapped by P680 are

energized by light and accepted by the “primary”

electron acceptor

As electrons move down the Electron Transport Chain

they lose energy… this energy is used to produce ATP. Some transport

proteins, like ferredoxin and

cytochrome, include nonprotien parts containing iron.

Light Dependent Reactions

Light Dependent Reactions

Electrons are again energized

by light and accepted by a

primary acceptor

Light Dependent Reactions

Electrons lose energy as they move down the ETC … the energy is

used to make NADPH…. The 2

electrons lost to Photo- system II are replaced when water is split….

This is called “Photolysis”.

Light Dependent Reaction

Same thing as Noncyclic Photophsphorylation.

Location - grana of the chloroplast.

Function - to split water and produce ATP and NADPH.

Light Dependent Reaction

Light Water ADP + Pi NADP+

O2 (released into the environment)

ATP NADPH

Requirements Products

H2O + ADP + Pi + NADP+ + light ATP + NADPH + O2 + H+

Chemiosmosis Model The chloroplast produces ATP

in the same manner as the mitochondria in Respiration.

Light energy is used to pump H+ across a membrane.

When the H+ diffuses back, ATP is generated.

Chemiosmosis Model

H+ are pumped into the thylakoid space.

ATP and NADPH are made when the H+ diffuse into the

stroma.

Calvin-Benson Cycle…. Light Independent Reactions

How plants actually makes food (carbohydrates).

Don't require light … energy supplied by ATP made during Light Dependent Reactions

Also known as the Calvin cycle or C3 Ps.

Calvin-Benson Cycle…. Light Independent Reactions

Function - to use ATP and NADPH to build food from CO2

Location - stroma of the chloroplast.

Rubisco (RuBP)

Ribulose BisPhosphate Carboxylase.

Enzyme that adds CO2 to an acceptor molecule.

Most important enzyme on earth.

Calvin-Benson Cycle… Light Independent Reactions

CO2 (1C) combines with RuBP (5C) to form 2 PGA (3C)

Carboxylation: 6CO2 + 6 RuBP produce 12 PGA

The energy from ATP and the H+ from the NADPH

are used to convert 12 PGA to 12 PGAL (G3P)

6 ATP are used to convert 10 PGAL to 6 RuBP… the remaining 2 PGAL are used to make Glucose.

C3 PSN

6 CO2

18 ATP 12 NADPH

C6H12O6

18 ADP + 18 Pi 12 NADP+

Requirements Products

6CO2 + 18 ATP + 12 NADPH+ + H+ 18 ADP + 18 Pi12 NADP+ + 1 Glucose

Cyclic Photophosphorylation

Uses Photosystem I only. Requires light Electrons from Photosystem I are

used to produce ATP…. the electrons then return to Photosystem 1…. They are not incorporated into NADPH as in Non-Cyclic Photophosphorylation

e-

Photorespiration Rubisco is the most abundant protein

on earth. Occurs when Rubisco accepts O2

instead of CO2 as the substrate. Generates no ATP. Decreases PSN output by as much as

50%. Products of Photorespiration are

broken down by peroxisomes… found near the chloroplasts.

Photorespiration

May reflect a time when O2 was less plentiful and CO2

was more common.

Alternate PSN Methods

1. C4 PSN

2. CAM PSN

C4 PSN C4 plants high rate of

photosynthesis reduces to overall loss of water….

Therefore they are found in dry, hot climates…. This

gives them an advantage over C3 plants.

C4 PSN

Uses a different enzyme, PEP, to initially capture CO2

Still uses C3 PSN to make sugar, but only does so in the bundle

sheath cells…. The bundle cells are near the water source… this makes the C4 plants extremely

efficient.

PEP Carboxylase

Enzyme used for CO2 capture in C4 PSN.

Can use CO2 down to 0 ppm. Prevents photorespiration.

C4 PSN

Found in 19 plant families. Characteristic of hot regions

with intense sunlight. Examples - sugarcane,

Bermuda grass, crab grass

C3 Ps vs C4 Ps

Photorespiration

Shade to full sun

High water use

Cool temperatures

Slow to moderate growth rates

Cool season crops

No Photorespiration

Full sun only

Moderate water use

Warm temperatures

Very fast growth rates

Warm season crops

CAM PSN Crassulacean Acid Metabolism

Found in plants from arid conditions where water stress

is a problem. Examples - cacti, succulents,

pineapples, many orchids.

CAM PSN Open stomata at night to take

in CO2.

The CO2 is stored as a C4 acid. During the day, the acid is broken down and CO2 is fixed

into sugar.

CAM plants

Tissues decrease in pH over night, rise in pH during day.

Avoid H2O stress by keeping stomata closed during the

day. Generally have slow growth.

Factors That Affect Photosynthesis

1. Light - quantity and quality.

2. Temperature - too hot or too cold.

3. CO2 - often limits C3 plants.

4. Minerals - especially NPK and Mg.

Importance of Photosynthesis

1. Food - either directly or indirectly comes from plants.

2. Oxygen in the air.

3. CO2 balance.

4. Plant products.

5. Life on Earth.