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 in a way that can be used by the cells.

• 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 in the chloroplast is chlorophyll.

• 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 producing chlorophyll so 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 in the plant’s leaves the energy from the 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

In photolysis, a molecule of water is split to replace electrons lost from chlorophyll.

It splits water into oxygen and hydrogen.

Photololysis – still in phase one , light dependent

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 to form sugars.

• 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 CO2 is added to one molecule of a five-carbon 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