BIO Unit 2 Photosynthesis

Autotrophs

Organisms that make their own organic molecules from inorganic sources and do not rely on other organisms for energy.

Photoautotrophs

Autotrophs that use sunlight to manufacture carbohydrates; examples include plants, algae, and cyanobacteria.

Heterotrophs

Organisms that cannot produce their own food and must consume or absorb sugars and nutrients from outside sources; examples include animals, fungi, and most bacteria.

Goal of Photosynthesis

Convert solar energy into chemical energy by using light to transform carbon dioxide and water into carbohydrates and oxygen.

Overall Photosynthesis Equation

CO₂ + H₂O + light energy → (CH₂O)n + O₂.

Two Main Reactions of Photosynthesis

The light reactions and the Calvin cycle.

Light Reactions

Reactions that occur in the thylakoid membrane and use light and water to produce ATP, NADPH, and O₂.

Calvin Cycle

Reactions that occur in the stroma and use CO₂, ATP, and NADPH to synthesize carbohydrates.

Photosynthesis and Cellular Respiration

Photosynthesis is an endergonic process driven by light; it reduces CO₂ to carbohydrates and oxidizes H₂O to O₂.

Stomata

Pores on leaf surfaces through which CO₂ enters and O₂ and water vapor exit.

Guard Cells

Cells surrounding stomata that regulate the opening and closing of leaf pores.

Mesophyll

The internal tissue between the upper and lower epidermis; its cells contain many chloroplasts and are the main site of light absorption and carbohydrate synthesis.

Veins (Vascular Bundles)

Plant structures that transport water, nutrients, and carbohydrates throughout the plant.

Chloroplast Outer and Inner Membranes

Membranes that surround the chloroplast and are separated by an intermembrane space.

Thylakoids

Flattened, fluid-filled membrane sacs inside chloroplasts that contain chlorophyll pigments and electron transport chains.

Thylakoid Lumen

The internal space enclosed by the thylakoid membrane.

Granum

A stack of thylakoids; the plural is grana.

Stroma

The fluid-filled space outside the thylakoids where the Calvin cycle occurs.

Electromagnetic (EM) Radiation

Energy that travels in waves; visible light is a form of electromagnetic radiation.

Wavelength and Energy

Shorter wavelengths have more energy, while longer wavelengths have less energy.

Photons

Particles of light that carry energy.

Chlorophyll a and b

Light-absorbing pigments containing a porphyrin ring with a central Mg²⁺ ion and a hydrophobic tail that anchors them to the thylakoid membrane.

Porphyrin Ring

The light-absorbing ring structure in chlorophyll that contains a central magnesium ion.

Accessory Pigments

Pigments such as carotenoids and xanthophylls that absorb wavelengths chlorophyll cannot and extend the range of light used in photosynthesis.

Carotenoids

Pigments, including beta-carotene, that absorb additional wavelengths and help protect chlorophyll from free radical damage.

Xanthophylls

Accessory pigments that absorb light wavelengths chlorophyll cannot and help protect photosynthetic structures.

Absorption Spectrum

The wavelengths of light absorbed by a particular pigment.

Action Spectrum

The overall rate of photosynthesis at different light wavelengths.

Photosystem II (PSII)

The photosystem in which light excites the P680 reaction center, boosting electrons to an excited state.

P680

The reaction center of Photosystem II; absorbs light energy to excite electrons.

P680*

The excited state of P680 after it absorbs light energy and boosts an electron to a higher energy state.

Water Oxidation in Photosynthesis

Water is split to replace electrons lost by Photosystem II, producing protons, electrons, and oxygen: 2H₂O → 4H⁺ + 4e⁻ + O₂.

Source of Oxygen Released During Photosynthesis

The oxygen gas released during photosynthesis comes from the splitting of water during the light reactions.

Electron Transport Chain in Photosynthesis

Transfers electrons from Photosystem II toward Photosystem I through a series of carriers; released energy pumps H⁺ into the thylakoid lumen.

Photosystem I (PSI)

The photosystem in which light excites the P700 reaction center, allowing electrons to be transferred toward NADPH production.

P700

The reaction center of Photosystem I that absorbs light energy to excite electrons.

Ferredoxin (Fd)

A protein that receives excited electrons from Photosystem I and transfers them toward NADP⁺ reductase.

NADP⁺ Reductase

An enzyme that helps transfer electrons to NADP⁺ to produce NADPH in the stroma.

NADPH

A high-energy electron carrier produced during the light reactions; it supplies reducing power for the Calvin cycle.

Photophosphorylation

The production of ATP during photosynthesis using the proton gradient created by the light reactions.

How the Proton Gradient Forms in Photosynthesis

H⁺ accumulates in the thylakoid lumen through water splitting, proton pumping by the electron transport chain, and H⁺ consumption in the stroma during NADPH formation.

ATP Synthase in Photosynthesis

Uses the H⁺ gradient across the thylakoid membrane to produce ATP in the stroma.

Calvin Cycle Function

Fixes inorganic CO₂ into organic molecules using ATP and NADPH produced by the light reactions.

Calvin Cycle Energy Requirement

For every 3 CO₂ molecules fixed, the cycle consumes 9 ATP and 6 NADPH.

Direct Product of the Calvin Cycle

G3P (glyceraldehyde-3-phosphate), which can be used to make glucose, fructose, sucrose, or starch.

Three Phases of the Calvin Cycle

Carbon fixation, reduction, and regeneration of RuBP.

Carbon Fixation

The first phase of the Calvin cycle; CO₂ combines with the 5-carbon sugar RuBP in a reaction catalyzed by Rubisco.

RuBP (Ribulose 1,5-bisphosphate)

A 5-carbon sugar that combines with CO₂ at the beginning of the Calvin cycle.

Rubisco

An enzyme that catalyzes carbon fixation by combining CO₂ with RuBP; its full name is ribulose 1,5-bisphosphate carboxylase/oxygenase.

3-Phosphoglycerate (3PGA)

The 3-carbon product formed when the unstable intermediate produced during carbon fixation splits.

Reduction Phase of the Calvin Cycle

The second phase; ATP phosphorylates 3PGA, and NADPH supplies electrons to help convert it into G3P.

G3P (Glyceraldehyde-3-phosphate)

A 3-carbon sugar produced during the Calvin cycle that can leave the cycle to help synthesize carbohydrates.

G3P Production for Every 3 CO₂

For every 3 CO₂ fixed, 6 G3P molecules are produced, but only 1 exits the cycle for carbohydrate synthesis.

Regeneration of RuBP

The third phase of the Calvin cycle; the remaining 5 G3P molecules are reorganized using ATP to regenerate RuBP.

Purpose of RuBP Regeneration

Regenerates the CO₂-accepting molecule so the Calvin cycle can continue.

Photorespiration

A wasteful process in which Rubisco binds O₂ instead of CO₂, causing CO₂ release and energy consumption without producing sugars.

Conditions That Increase Photorespiration

Low internal CO₂ and high O₂, often occurring during hot, dry weather when stomata close to conserve water.

Why Rubisco Causes Photorespiration

Rubisco can function as an oxygenase and bind oxygen instead of carbon dioxide, interfering with carbon fixation.

Mesophyll Cells

Cells in the middle of a leaf that carry out photosynthetic processes; they are involved in initial carbon fixation in C3, C4, and CAM plants.

Bundle-Sheath Cells

Cells that surround leaf veins; in C4 plants, they contain chloroplasts and carry out the Calvin cycle using concentrated CO₂.

C3 Plants

Plants that directly fix CO₂ into a 3-carbon molecule using Rubisco; about 90% of plants use this pathway.

Examples of C3 Plants

Wheat, rice, trees, and soybeans.

Location of Photosynthesis in C3 Plants

The steps of carbon fixation and the Calvin cycle occur in mesophyll cells.

C3 Plants and Photorespiration

C3 plants are vulnerable to photorespiration in hot, dry conditions because closed stomata reduce CO₂ availability and allow O₂ to accumulate.

C4 Plants

Plants that reduce photorespiration by spatially separating initial CO₂ fixation and the Calvin cycle into different cell types.

Examples of C4 Plants

Corn, sugarcane, and crabgrass.

Spatial Separation in C4 Plants

Initial CO₂ fixation occurs in mesophyll cells, while the Calvin cycle occurs in bundle-sheath cells.

PEP Carboxylase

An enzyme used by C4 and CAM plants for initial carbon fixation; it binds CO₂ without binding oxygen.

Initial Carbon Fixation in C4 Plants

PEP carboxylase in mesophyll cells fixes CO₂ into a 4-carbon molecule.

Role of Bundle-Sheath Cells in C4 Plants

The 4-carbon molecule moves into bundle-sheath cells and releases CO₂, concentrating it around Rubisco.

How C4 Plants Reduce Photorespiration

They concentrate CO₂ in bundle-sheath cells, making it more likely that Rubisco binds CO₂ rather than O₂.

CAM Plants

Plants adapted to extremely hot and dry environments that separate initial CO₂ fixation and the Calvin cycle by time rather than cell type.

Examples of CAM Plants

Cacti, pineapples, and succulents.

Temporal Separation in CAM Plants

Initial CO₂ fixation occurs at night, while the Calvin cycle uses stored CO₂ during the day.

CAM Plants at Night

Stomata open, CO₂ enters mesophyll cells, and PEP carboxylase fixes it into malate, an organic acid stored in a vacuole.

CAM Plants During the Day

Stomata close to conserve water; stored malate is broken down to release CO₂ for the Calvin cycle.

How CAM Plants Conserve Water

They open their stomata at night when it is cooler and keep them closed during the hot daytime.

C3 vs. C4 vs. CAM Separation

C3 plants have no separation; C4 plants separate steps by cell type; CAM plants separate steps by time of day.

C3 Plants: Initial Carbon Fixation Cell

Mesophyll cells.

C3 Plants: Calvin Cycle Location

Mesophyll cells.

C3 Plants: Best Environment

Cool, wet, and cloudy environments.

C4 Plants: Initial Carbon Fixation Cell

Mesophyll cells.

C4 Plants: Calvin Cycle Location

Bundle-sheath cells.

C4 Plants: Best Environment

Hot, sunny, and dry environments.

CAM Plants: Initial Carbon Fixation Time and Location

Mesophyll cells at night.

CAM Plants: Calvin Cycle Time and Location

Mesophyll cells during the day.

CAM Plants: Best Environment

Extremely hot and arid environments, such as deserts.