Section 9.2 Photosynthesis: Trapping the Sun’s Energy

p. 225 - 230

Trapping Energy From Sunlight

• • • Plants and other

green

organisms must

trap light energy

from the sun to be able to use it.

• The

energy

must then be

stored

that can be used by the cells.

in a way

ATP

is the form!

Autotrophs

trap and make energy with photosynthesis

Trapping Energy From Sunlight

• During photosynthesis, plants use the sun’s energy to make

simple sugars

.

• These sugars are then made into

complex carbohydrates

, such as starch.

•

Starches

store energy.

Trapping Energy From Sunlight

• There are two parts to photosynthesis:

light-dependent

reactions and

light independent

reactions.

• Light-dependent reactions change

light energy

into

chemical energy

, splitting water and

releasing oxygen

.

• Light-independent reactions

produce simple sugars

.

Where Does Photosynthesis Take Place?

• Photosynthesis takes place in the

chloroplast

.

• Chloroplasts contains

pigment

.

• Pigments take in

specific wavelengths

of sunlight.

Where Does Photosynthesis Take Place?

• Wavelengths of sunlight

transfer

energy.

• The most common

pigment

chloroplast is

chlorophyll

.

in the • Chlorophyll is the main

energy-trapping

molecule in the plant.

Chlorophyll a

• Every place that doesn’t have a letter telling you the element, is a carbon.

• Hydrogens are attached to all of the empty spots on the carbons.

The red, yellow, and purple pigments are visible in the autumn.

Why can’t you see the colors during the summer?

During the summer, the plant cells manufacture chlorophyll, which hides the other pigments that are present in leaves.

Where Does Photosynthesis Take Place?

• Chlorophyll is a plant pigment that absorbs most wavelengths of sunlight except

green

.

• Since it cannot absorb green, it

reflects green

.

• This makes leaves look green!

• In fall the leaves

stop chlorophyll

so producing

other pigments

are now

visible

.

Wavelengths of light

• Light that we see and plants use for photosynthesis is a small part of the spectrum of electromagnetic radiation, from just below 400 nm to just above 700 nm. • Our peak sensitivity is in the middle of this range at about 550 nm or in the green part of the spectrum.

• Coincidentally this is the part of the spectrum which plants do not use or "see". • They absorb light in the blue and in the red.

Chlorophyll-a absorption spectrum

Wavelengths of light

• • The sunlight is absorbed by

pigments.

• Chlorophyll absorbs red and blue light and

reflects green

so it appears green.

Carotenoids

are other pigments which absorb in the blue and appear yellow.

Light-Dependent Reactions

• The first phase of photosynthesis

needs sunlight

.

• This first phase takes place in the chloroplasts, more specifically, in the

thylakoid membranes

: membranes within the chloroplast that contain enzymes.

•

Energy

from sunlight is trapped by chlorophyll in this

thylakoid membrane

.

Light-Dependent Reactions

• When sunlight strikes the

chlorophyll

the plant’s leaves the energy from the in sunlight is transferred to

electrons

in the chlorophyll.

• The

electrons

move

from the chlorophyll

to an electron transport chain.

Chlorophyll molecules absorb light energy and energize electrons for producing ATP and NADPH.

NADP is nicotinamide adenine dinucleotide phosphate The H is a hydrogen ion that gets combined with the NADP along with two excited electrons.

Light-Dependent Reactions

• *** The electrons move from the chlorophyll to an electron transport chain.

• An electron transport chain is a line of

proteins

embedded in a

membrane

along which the

electrons are passed

down.

Light-Dependent Reactions

• Each protein in the line

passes the electron

to the next protein.

• As the electrons pass along this line, they

lose some

of their energy.

Light-Dependent Reactions

• If you filled a bucket with water and passed it along a line of people very quickly, some of the water would spill.

• This is similar to how the electrons lose their energy.

Electron transport chain

What Happens to the Lost Energy?

• The energy the electrons lose can be used to form

ATP

from ADP.

• Energy that is

not used

to form ATP

can be stored

for use in the light-independent reactions.

What Happens to the Lost Energy?

• The energy is stored in an

electron carrier

called

NADPH

.

• NADPH

carries

the energy to the

light independent

reaction.

NADP

• NADP = an

electron

and proton “

carrier

” • Carries the electrons and protons to

make energy.

This is what NADP+ looks like:

Conversion of NADP + to NADPH

•

And here is a closer look at the conversion of NADP + to NADPH

Photololysis – still in phase one , light dependent In photolysis, a molecule of water is split to

replace electrons

lost from chlorophyll.

It splits water into

oxygen

and

hydrogen

.

Light-Independent Reactions

• The second phase of photosynthesis

does not

need sunlight.

• It also takes place in the chloroplasts and is called the

Calvin Cycle

.

• More specifically, it takes place in the

stroma

.

• The stroma is the

liquid-y

part of the

chloroplast

.

Light-Independent Reactions

• The Calvin Cycle uses

carbon dioxide

sugars.

to form • The sugars then become

stored energy

.

• You could say it is a series of reactions that synthesize simple sugars from carbon dioxide and hydrogen.

• It’s a big job to make sugars from

carbon dioxide

!

Light-Independent Reactions

• The ATP and NADPH produced in the

light dependent

reactions are used.

• The

chloroplasts

breaks down this large task into very small steps.

• The end result is energy

stored

in the plant as

sugar

.

Light-Independent Reactions

• Organisms that

eat plants

use these sugars to give them energy. • The energy is used in

cellular respiration

.

The stroma in chloroplast is where the Calvin Cycle takes place.

The Calvin Cycle takes the carbon in CO2, adds it to one molecule of RuBP, and forms sugars through a series of reactions in the stroma of the chloroplasts.

The NADPH and ATP produced during the earlier light-dependent reactions are important molecules for this series of reactions.

Why is the Calvin cycle in plants directly and indirectly important to animals?

The energy in the bonds of the sugars synthesized by plants is the source of energy for animals. Indirectly, Plants also provide shelter and a home to many animals

Calvin Cycle

• At the beginning, one molecule of CO 2 added to one molecule of a

five-carbon

is sugar RuBP to form a six-carbon sugar.

• After a series of reactions, the sugar breaks down to two three-carbon sugars called phosphoglyceraldehyde, or

PGAL

.

Calvin Cycle

• After 3 rounds of this cycle, 6 molecules of PGAL are produced.

• 5 PGALs are rearranged to make 3 new RuBP molecules

Calvin Cycle

• The sixth PGAL is available to make sugars elsewhere in the body.

• Calvin cycle takes 6 rounds to make one glucose molecule