Ch. 10 Photosynthesis

• Autotrophs:biotic producers; photoautotrophs; chemoautotrophs;

obtains organic food without eating other organisms

• Heterotrophs:biotic consumers;

obtains organic food by eating other organisms or their by-products (includes decomposers)

Photosynthesis

• What is photosynthesis?The process in which a cell will convert light energy

into chemical energy.• What type of cells do this?Plant cells!!! And some protista!• Where does it occur in the cell?In any green part of the plant: leaves, stems, or even

unripened fruit!

But mainly…the leaf!• Structures:1. Cuticle – outside waxy

protective layer; prevents dehydration

2. Epidermis- single layer of cells on the outside of the cell.

3. Mesophyll cells- spongy cells responsible for carrying on photosynthesis

4. Stomata- openings on the underside of the leaf that allow gas exchange (O2 & CO2)

In the mesophyll cells are chloroplasts.

• Chloroplasts contain a pigment called Chlorophyll.

• This is the major pigment responsible for capturing all light wavelengths EXCEPT green.

• As a result, the green wavelength is reflected and we see the plant as green.

Here is a chloroplast up close:Structures:1. Granum (grana)-

stacks of thykaloids

2. Thykaloids- coin looking structure. Where chlorophyll is found.

3. Stroma- liquid portion (cytoplasm of the chloroplast)

4. Thykaloidmembrane- where the absorption/ PS takes place.

Mesophyll cell

Mesophyll

Vein

Stomata

CO2 O2

t

5 µm

1 µm

The process of photosynthesis

6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2

Carbon Dioxide + water+ light glucose + oxygen

This process is a redox reaction

Redox reactions are reactions that deal with the transfer of electrons between molecules.

• Oxidation describes the loss of electronsby a molecule, atom or ion (LEO)

• Reduction describes the gain of electronsby a molecule, atom or ion

Photosynthesis has 2 steps (reactions)

1. Light Reactionsrequires light and waterNoncyclic vs. Cyclic Cycles

2. The Calvin Cycle (Dark Reactions)requires CO2 DOES NOT depend on light. Can occur

in dark!!!

Photosystems• Light harvesting units of

the thylakoid membrane• Antenna pigment

molecules are struck by photons

• Energy is passed to reaction centers (redox location)

• Excited e- from chlorophyll is trapped by a primary e- acceptor

• Photosystem II (P680) and Photosystem I (P700)

Light Reactions

1. Light is absorbed by the chlorophyll in the thykaloid membrane of photosystem II.

2. The energy absorbed causes water to split. This releases an H+ and electron.

3. The H+ is picked up by a hydrogen carrier called NADP+ and it becomes NADPH.

4. This is then taken to the Calvin cycle and O2 is released into the atmosphere

The Calvin Cyclea metabolic pathway found in the stroma of the chloroplast in

which carbon enters in the form of CO2 and leaves in the form of sugar.

There are 3 Phases of the cycle:1. Carbon FixationCO2 is incorporated into a five-carbon sugar

named ribulose bisphosphate (RuBP). RuBP carboxylase or rubisco is the enzyme that

helps this process. The product then splits to form 2 molecules of 3-

phosphoglycerate

2. ReductionATP and NADPH2 from the light reactions

are used to convert 3-phosphoglycerate to G3P, glyceraldehyde 3-phosphate which will become glucose later.

3. Regeneration more ATP is used to convert some of the of

the pool of G3P back to RuBP, the acceptor for CO2, thereby completing the cycle

The Calvin cycle

Carbon Fixation methods1. C3 Plants: Photorespiration

On hot/dry days, stomata close to prevent water loss. BUT CO2decrease

O2 increase in leaves; O2 added to rubisco; no ATP or food generated

2. C4 plants: Have 2 photosynthetic cells: bundle-sheath & mesophyll; PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll;

new 4C molecule releases CO2(grasses)

Alternative carbon fixation methods

• 3. CAM plants: open stomata during night, close during day (crassulacean acid metabolism);

cacti, pineapples, etc.

A review of photosynthesis