Photosynthesis Lecture Notes

Photosynthesis is the process by which plants use carbon dioxide, water, and light to produce sugar and oxygen.

Photosynthesis occurs inside chloroplasts within plant cells.
Chloroplasts contain thylakoids, which are membranous sacs where the light reactions of photosynthesis take place.
The stroma is the thick fluid outside the thylakoids where the Calvin cycle occurs.

Light energy is converted to chemical energy during the light reactions.
Photosystems, containing chlorophyll, capture light energy.
An electron transport chain connects photosystems.
Water is split to replace electrons in photosystem II, producing oxygen as a byproduct.
Energy from the electron transport chain pumps hydrogen ions into the thylakoid, creating a concentration gradient.
ATP synthase uses the hydrogen ion gradient to produce ATP.
Electrons are captured by NADPH.
The light reactions produce ATP and NADPH, which power the Calvin cycle.

The Calvin cycle occurs in the stroma.
Carbon dioxide combines with RuBP (ribulose-1,5-bisphosphate).
ATP and NADPH power a series of reactions that produce G3P (glyceraldehyde-3-phosphate).
Most G3P is recycled to regenerate RuBP.
Excess G3P is used to build glucose, starch, cellulose, or sucrose.
Cellular respiration in plant mitochondria breaks down some sugar, generating ATP for plant work.

Electromagnetic energy comes in different wavelengths; short wavelengths produce gamma rays.
Visible light ranges from approximately 380 to 740 nanometers.
Amplitude affects brightness and light intensity.
Chlorophyll a, chlorophyll b, and carotenoids are pigments that absorb light at different wavelengths.
Red and blue light are most useful for photosynthesis.
Absorbed light excites electrons, which can then release energy as light or heat or be passed from one pigment to another.
Carotenoids protect the plant from excessive light by capturing energy and dissipating it as heat.

Photosystems I and II work together to generate NADPH and ATP, connected by an electron transport chain.
Linear electron flow involves electrons moving from one photosystem to the other.
Cyclic electron flow involves electrons being recycled; it generates ATP but not NADPH.
Some bacteria use just one photosystem to generate ATP.

Chloroplasts and mitochondria both generate ATP by chemiosmosis.
ATP synthases are similar in both organelles.
In chloroplasts, electrons come from water; in mitochondria, they come from organic molecules.
Proton gradients are opposite in chloroplasts and mitochondria.

The Calvin cycle is anabolic, building sugars from smaller molecules using ATP and NADPH.
Carbon enters as carbon dioxide and leaves as glyceraldehyde-3-phosphate.
For the synthesis of one glyceraldehyde-3-phosphate, three molecules of carbon dioxide must be fixed; one molecule for each turn of the cycle.
Carbon fixation is catalyzed by the enzyme Rubisco.

Sometimes called the C3 pathway because carbon is fixed to form a three-carbon molecule.
For every three carbon dioxide molecules, six molecules of G3P are generated, but five are recycled.
The cycle regenerates ribulose bisphosphate.

When it gets hot, plants close their stomata, limiting carbon dioxide availability.
Rubisco can bind to oxygen instead of carbon dioxide, leading to photorespiration.
Photorespiration generates a two-carbon molecule instead of a three-carbon molecule and uses ribulose bisphosphate without generating useful products, draining energy.

C4 plants use a different molecule to fix carbon dioxide, with a higher affinity for carbon dioxide than Rubisco when carbon dioxide concentrations are low.
C4 plants use a four-carbon compound as the first product of the carbon cycle instead of a three-carbon compound.
Examples include corn and sugarcane.

Photosynthesis begins in mesophyll cells.
PEP carboxylase has a higher affinity for carbon dioxide than Rubisco.
Four-carbon compounds are transferred to bundle sheath cells.
Carbon dioxide is released in bundle sheath cells and undergoes the normal Calvin cycle.