Lecture 13: Photosynthesis: Carbon Fixation & Cell Division

10/1/25

How do weedkillers disrupt photosynthesis?

Weedkillers interfere with the chloroplasts or ETC, preventing the plant from producing sugars for energy and leading to cell damage or death. These inhibitors act by blocking the photosystem II complex. 

Examples of weedkillers that block PSII

• DCMU

• Paraquat

What would cyanide do if it entered the mitochondrial ETC?

Cyanide would bind to complex 4 in the mitochondrial ETC, altering the shape of its active site. Consequently, electrons cannot be released to oxygen and the ETC would shut down. The result would be death (suffocation)

Where do the dark reactions (Calvin Cycle) take place?

The stroma of the chloroplasts

What are the Calvin Cycle inputs?

• CO2

• ATP

• NADPH

What are the Calvin cycle outputs?

• Sugars

• ADP

• NADP+

Why is the Calvin cycle a cycle?

Because what goes in comes out - a transformation - no net usage of plant material

Diagram the Calvin cycle showing the carbon inputs and outputs, where energy is used, and the fixation, reduction, and regeneration stages of the biochemistry.

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Explain why photosynthetic carbon fixation is called C3 photosynthesis, and how it can lead to the production of 6-carbon sugars.

Photosynthetic carbon fixation is called C3 photosynthesis because the first stable organic product formed from the enzyme RuBisCO fixing carbon dioxide is 3-phosphoglycerate (3-PGA), a three-carbon compound

Diagram the Cell Cycle

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Diagram and explain the phases of Mitosis

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Define Ribulose bisphosphate carboxylase

An enzyme that catalyzes the first step of the Calvin cycle, where CO2 is fixed by combining it with ribulose bisphosphate (RuBP)

• The most abundant enzyme on Earth

Define RuBisCO

Short for Ribulose - 1,5-bisphosphate carboxylase/oxygenase, it’s the enzyme that initiates carbon fixation in the Calvin Cycle

Define Ribulose bisphosphate (RuBP)

A 5-carbon sugar that acts as the CO2 acceptor in the Calvin Cycle. It reacts with CO2 in a reaction catalyzed by RuBisCO

Define Calvin Cycle

The set of light-independent reactions in photosynthesis that take place in the stroma of chloroplasts. It uses ATP and NADPH to fix CO2 into glucose. It has three phases

1. Fixation

2. Reduction

3. Regernation

Define Fixation

The first phase of the Calvin Cycle, where CO2 is attached to RuBP, forming an unstable 6-carbon compound that splits into two 3-carbon molecules (3-PGA)

Define Reduction

The second phase of the Calvin cycle, where ATP and NADPH are used to convert 3-PGA into G3P (glyceraldehyde-3-phosphate), a sugar

Define regeneration

The third phase of the Calvin cycle, where some G3P is used to regenerate RuBP so the Calvin cycle can continue

Define cell cycle

The entire life cycle of a cell, including the stages of growth (interphase), a preparation for division, and cell division (mitosis and cytokinesis)

Define centriole

A cylindrical structure involved in forming the spindle fibers during mitosis in animal cells

Define chromatin

the uncondensed, thread-like form of DNA in the nucleus during interphase 

Define chromosome

condensed form of chromatin, visible during cell division; contains genetic information

Define centromere

the region where two sister chromatids are joined together; attachment point for spindle fibers

Define kinetochore

a protein structure on the ceontromere where spindle fibers attach during mitosis

Define sister chromatid

one of two identical copies of a chromosome connected at the centromere

Define spindle

A structure made of microtubules that separates chromosomes during mitosis

Define Interphase

The longest part of the cell cycle, consisting of G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis). The cell performs normal functions and duplicates its DNA

Define Prophase

The first phase of mitosis

• Chromatin condenses into visible chromosomes

• Nuclear envelope begins to break down

• Spindle fibers start to form

Define Metaphase

The second phase of mitosis

• Chromosomes align at the cell’s equator (metaphase plate)

• Spindle fibers attach to centromeres via kinetochores

Define Anaphase

The third phase of mitosis

• Sister chromatids are pulled apart by the spindle and move to opposite poles

Define Telephase

The fourth phase of mitosis

• Where the separated chromosomes arrive at opposite ends of the cell

• The nuclear envelop reforms around each set of chromosomes to create two new nuclei

• The chromosomes decondense, and the mitotic spindle disassembles

Define Cytokinesis

• division of the cytoplast, resulting in two separate daughter cells

• In plant cells, a cell plate forms during this process

Define cell plate

a structure that forms in plant cells during cytokinesis. It eventually develops into a new cell wall separating the two daughter cells