Rubisco, Photorespiration, C3/C4/CAM, and Stomata Regulation.

Why is rubisco considered an inefficient enzyme?

It has a slow turnover rate and cannot reliably distinguish CO₂ from O₂, causing oxygenation (photorespiration).

Under what conditions does rubisco act as a carboxylase?

When CO₂ levels are high and O₂ levels are low

Under what conditions does rubisco act as an oxygenase?

When O₂ is high and CO₂ is low — especially in hot, dry conditions when stomata close.

What is photorespiration?

A wasteful process where rubisco binds O₂ instead of CO₂, producing CO₂ and using ATP without making sugar.

How does photorespiration affect photosynthesis?

t decreases sugar production by 25–50% and wastes energy.

What are the three cellular locations involved in photorespiration?

Chloroplast → peroxisome → mitochondrion.

What is the approximate composition of Earth’s atmosphere?

~78% N₂, 21% O₂, 0.04% CO₂.

Why does photorespiration happen more today?

Because O₂ is abundant and CO₂ is low compared to ancient Earth.

What do stomata do?

Control gas exchange: let CO₂ in, release O₂ and water vapo

How do stomata open?

Guard cells take up K⁺ ions → water enters → cells swell → pore opens.

How do stomata close?

Guard cells lose K⁺ → water leaves → cells shrink → pore closes.

What environmental conditions cause stomata to close?

Heat, dryness, high wind, water stress, abscisic acid (ABA).

How are plants adapted to dry, hot conditions?

Thick cuticles, sunken stomata, CAM/C4 pathways, hairy leaves, closing stomata during day.

What defines a C3 plant?

Uses rubisco to fix CO₂ directly into 3-carbon 3PGA.

What is a major vulnerability of C3 plants?

High photorespiration under heat/drought.

How do C4 plants reduce photorespiration?

Use PEP carboxylase to capture CO₂ into a 4-carbon compound, concentrating CO₂ around rubisco.

What is special about C4 anatomy?

Spatial separation: mesophyll cells (CO₂ capture) and bundle sheath cells (Calvin cycle).

How do CAM plants reduce water loss?

Temporal separation: stomata open at night, store CO₂ as malate; release CO₂ during day.

Name some CAM plants.

Cacti, succulents, pineapple, agave.

Compare C3, C4, and CAM plants briefly.

• C3: Direct fixation by rubisco; high photorespiration.

• C4: Spatial CO₂ concentration; low photorespiration; warm climates.

• CAM: Temporal CO₂ storage; stomata open at night; very dry climates.

How much light reaching Earth is used for photosynthesis?

About 1%.

What decreases the amount of usable light for photosynthesis?

Reflection, heat dissipation, chlorophyll absorption limits, leaf position, shading, clouds.