Chapter 10: Photosynthesis

Essential Knowledge 2.a.1 – All living systems require constant input of

free energy (10.1-10.3). 2.a.2 – Organisms capture and store free energy for

use in biological processes (10.1-10.3).

Photosynthesis Process by which plants use

light energy to make food A reduction process that makes

complex organic molecules from simple molecules

Ps General Equation6 CO2 + 6 H2O C6H12O6 + 6 O2

Requires: ♦Notice: Chlorophyll This is VERY Light similar to the

cell resp eq.

Ps: a redox process Hydrogens are added to

Carbons-reduces CO2 to sugar Water is split; becomes source

of hydrogens Complex covalent bonds made Requires energy

Ps Has two chemical reactions:1. Light Reaction2. Dark Reaction Names are from “light” as a

requirement, not where or when they occur

Light A form of electromagnetic

radiation/energy Visible light has the right

energy for use in Ps Travels in waves

Too Hot Too ColdJust Right

Detected by our eye-hence the term visible

Action Spectrum Not all colors are useable to

the same degree for Ps Red and Blue light - absorbed

and used in Ps Green light - reflected or

transmitted

Comment In oceans, red light is lost or

filtered out early because it has lower energy

Only blue light which has higher energy can reach the lower depths

Result Many deep ocean

fish are bright red in color.

Why? They can’t be seen

because there is no red light to reflect their color

Photosynthesis Pigments

Pigment: absorb light Different pigments absorb

different wavelengths Types:

Chlorophylls (a and b) Accessory Pigments

(carotenoids)

Chlorophylls Have C,H,O,N and Mg Two major types: a, b Molecule has a lipophilic tail

(allows it to dissolve into membranes)

Contains Mg in a reaction center.

Accessory Pigments Absorb light energy and

transfer it to chlorophyll Various shades of

yellow/orange Function: photoprotection 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

Parts of Plant: Ps function

Stomata Roots Mesophyll Veins

Site of Ps Chloroplast

Contain chlorophyll pigmentAll green parts of plants

contain chloroplastsMost are found in leaves

Why? Larger surface area

Chloroplast Structure Double outer membrane Inner membrane folded and

stacked into grana Stroma - liquid that

surrounds the thylakoid membranes

Photosystems Collection of pigments that

serve as a light trap Made of chlorophyll and the

accessory pigments Two types known:

PSI & PSII

Parts of Photosystem 1) Rxn center/e- acceptor

Chlorophyll uses light energy to move e- to next energy level

2) Pigment Light absorption, photon acceptor

3) Light-harvesting complex Proteins, contain pigment

Light Reaction Same thing as Noncyclic

Photophosphorylation Location - grana of the chloroplast Function - to split water and

produce ATP and NADPH (provide chem energy for next cycle)

Light Reaction

Requirements: Light Water ADP + Pi NADP+

Products: O2

ATP NADPH

Chemiosmosis Model The chloroplast produces

ATP in the same manner as the mitochondria in Rs

Light energy is used to pump H+ across a membrane

When the H+ diffuses back, ATP is generated

Chemiosmosis H+ are pumped into the

thylakoid space ATP and NADPH are made

when the H+ diffuse into the stroma

Both Rs and Ps use ETC to pump H+

Light Rxn

Tutorial

Dark Reactions How plants actually make food

(carbohydrates) Don't require light directly to

run Also known as the

Calvin cycle or C3 Ps

Calvin cycle/Dark reaction Anabolic – builds sugars from

smaller molecules Function - use ATP and

NADPH to build food from CO2

Location - stroma of the chloroplast

Calvin Cycle movie

Rubisco Ribulose BisPhosphate

Carboxylase Enzyme that adds CO2 to an

acceptor molecule Most important enzyme on earth Catalyzes first step of dark rxn

C3 Ps C3 plants are plants that DO use

Rubisco enzyme “normally” Keep stomata open during day MOST plants are C3 plants

When you “think of Ps”, you think of a C3 plant performing Ps

Adaptive value: Require less Less enzyme activity, no specialized

organs/anatomy

Photorespiration When Rubisco accepts O2

instead of CO2 as the substrate Generates no ATP

Actually uses ATP Decreases Ps output by as

much as 50%

Photorespiration

May reflect a time when O2 was less plentiful and CO2 was more common Important to evolution of autotrophs

Photo=occurs in light Respiration=consumes oxygen

while producing CO2

Alternate Ps Methods

1. C4 Ps2. CAM Ps

Intro to C4 and CAM

Remember: MOST plants are C3 plants that performs Ps “normally”

Both are adaptations to arid conditions CAM plants “idle” and save

energy while C4 plants go through Ps faster

C4 Ps Uses a different enzyme to

initially capture CO2

Separates CO2 capture from carbon fixation into sugar

Still uses C3 Ps to make sugar, but only does so in the bundle sheath cells

C4 Ps

Found in 19 plant families Characteristic of hot regions

with intense sunlight Examples - sugarcane,

Bermuda grass, crab grass

C3 Ps vs C4 Ps

Can use 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 plants Found in plants from arid

conditions (where water stress is a problem)

Avoid H2O stress by keeping stomates closed during the day

Generally have slow growth Ex: pineapple, cacti, orchids

CAM Plants, cont.

Store CO2 as C4 acid During the day, the acid is

broken down and CO2 is made into sugar.

Ps:Rs Ratios Reflect a plant’s balance in

making food and using food1. Ps > Rs, energy available

for growth and reproduction

2. Ps = Rs, no growth, but don’t die either

3. Ps < Rs, death by starvation

Comments - Ps:Rs Rs happens 24 hours a day Ps only in light Plants overwinter on stored

food when Ps > Rs If Ps < Rs, best solution is to

increase the amount of light

Factors Affecting Ps

1. Light - quantity and quality2. Temperature - too hot or too

cold3. CO2 - often limits C3 plants4. Minerals - especially NPK

and Mg

Importances of Ps

1. Food - either directly or indirectly comes from plants

2. Oxygen in the air3. CO2 balance4. Plant products5. Life on Earth

Summary Identify the basic equation of photosynthesis. Identify the main reactions of photosynthesis. Recognize the function, location, requirements

and products of the photosynthesis reactions. Recognize the role of light and pigments in the

photosynthesis process. Recognize the effects and importance of

photorespiration. Recognize various types of photosynthesis and

their importance. Identify the biological and economic importance

of photosynthesis.

Exclusion Statements You do NOT need to

memorize the steps in the Calvin Cycle, the structure of the molecules, and the names of the enzymes (except ATP synthase).