Plant Form & Function I

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Which enzyme, found in both C3 and C4 plants, is capable of binding to both CO₂​ and O₂​, potentially leading to photorespiration?

Rubisco

Why does the C4 photosynthetic pathway require more energy (ATP and NADPH) than the C3 pathway?

It involves additional metabolic steps to concentrate CO₂ in specific cells

In the CAM pathway, when are the stomata typically open, and what is the primary benefit of this timing?

At night; it minimizes water loss while allowing CO₂ uptake

Where do CAM plants obtain the energy required to run the C4-like cycle at night when light reactions are not occurring?

Cellular Respiration

Based on the provided material, what is the primary source of the dry matter (mass) found in a fully grown plant?

Carbon dioxide from the air

Which plant tissue is composed of dead cells that form a 'biological straw' to transport water from roots to leaves?

Xylem

According to the principles of water potential, in which direction will water naturally move?

From an area of higher potential to an area of lower potential

What is the primary evolutionary driver that favored the development of the C4 pathway?

Low atmospheric CO₂ levels

How do root hairs contribute to the plant's ability to survive in its environment?

They increase the surface area for the exchange of water and nutrients

If a C4 plant and a C3 plant are placed in an understory environment with very limited light, why might the C4 plant be 'in trouble'?

The energy cost to run the C4 cycle exceeds the energy harvested from limited light

Which enzyme is capable of binding to both CO2​ and O2​, leading to the process of photorespiration?

Rubisco

What is the primary trade-off for C4 plants compared to C3 plants?

C4 plants require more energy (ATP & NADPH) to fix one molecule of glucose

How does the CAM pathway differ from the C4 pathway in its method of separating carbon fixation?

CAM separates reactions across different times (day vs night), while C4 separates them across different cells

If a plant closes its stomata to conserve water, what happens to the CO2​ concentration inside the leaf?

The CO₂ concentration decreases as it is consumed by photosynthesis

What happens during the day in a CAM plant?

Stomata are closed, and stored carbon is converted back to CO₂ for the Calvin Cycle

In a hypothetical experiment, two plants (C3 and C4) are placed in sealed bags. Why would the bag containing the C3 plant likely show higher water vapor accumulation?

C3 plants must keep their stomata open wider and longer to maintain sufficient CO₂ levels for Rubisco

Which specific enzyme in the Calvin cycle has the ability to bind with both CO₂ and O2

Rubisco

What does the 'O' at the end of the name Rubisco signify regarding its chemical function?

It indicates the enzyme's capacity to bind with oxygen (O₂).

How does PEP Carboxylase (PEPC) differ from Rubisco in terms of gas binding specificity?

PEPC only binds to CO₂ and does not bind to O₂

In the C4 pathway, what is the role of PEP Carboxylase?

It captures inorganic CO₂ to fix it into a four-carbon organic molecule.

Which photosynthetic pathway is commonly referred to as the C3 cycle?

The Calvin cycle

What is the primary energetic cost associated with the extra processing steps in the C4 cycle?

It requires additional ATP and NADPH compared to the C3 cycle.

Under what atmospheric conditions did the C4 photosynthetic pathway likely evolve as an adaptation?

Periods with very low levels of carbon dioxide (CO_₂).

How does the C4 pathway achieve a higher concentration of CO₂ around the enzyme Rubisco?

By separating the initial carbon fixation and the Calvin cycle into different cells (spatial separation).

What effect does concentrating CO₂ in bundle sheath cells have on the rate of photorespiration?

It decreases the rate of photorespiration by reducing Rubisco's contact with oxygen.

Where do the light reactions of photosynthesis primarily occur within a C4 plant leaf?

In the cells near the top of the leaf where light is most abundant.

Why is C4 photosynthesis considered advantageous in climates with abundant sunlight?

The high energy cost of the pathway is easily covered by the surplus of light energy.

How does the distribution of C4 plants change as latitude increases away from the equator?

The percentage of C4 plants decreases as the climate becomes colder and light becomes less consistent.

What occurs to the internal concentration of CO₂ in a leaf when the stomata are closed during photosynthesis?

The CO₂ concentration begins to decrease as it is used up by the cell.

Why are C4 plants able to keep their stomata more closed than C3 plants in hot environments?

PEP Carboxylase remains efficient at capturing CO₂ even when internal concentrations are very low.

What is the trade-off for a C4 plant placed in the shaded understory of a forest?

It may fail to grow because it lacks the extra light energy required to run its high-cost photosynthetic pathway.

What is the monophyly status of C4 plants as a group?

They are non-monophyletic, as the C4 pathway evolved independently multiple times across different families.

Which photosynthetic pathway separates carbon fixation and the Calvin cycle by time rather than space?

The CAM (Crassulacean Acid Metabolism) pathway

During which time of day do CAM plants open their stomata to bring in CO₂?

At night

What is the primary benefit of CAM plants opening their stomata only at night?

It drastically reduces water loss because temperatures are lower and humidity is higher.

Where does a CAM plant obtain the energy (ATP) needed to fix carbon into organic acids at night?

From cellular respiration

What happens to the organic acids stored in CAM plants when the sun comes up?

They are converted back into CO₂ to be used in the Calvin cycle while the stomata are closed.

Why do CAM plants generally have slower growth rates compared to C3 or C4 plants?

Growth is limited by the fixed amount of CO₂ they can store overnight and the energy cost of the process.

What is the fundamental functional difference between photosynthesis and cellular respiration?

Photosynthesis creates sugar (fuel) using light energy, while respiration breaks down sugar to produce usable energy (ATP).

From which environmental source do plants derive the majority of their physical mass (matter)?

From the air (CO₂ taken in during photosynthesis).

How does the role of soil nutrients like Nitrogen (N) and Phosphorus (P) differ from the role of CO₂ in plants?

Soil nutrients are used as building blocks for molecules like DNA and enzymes, rather than as the primary fuel source.

What term describes the evaporative loss of water from a plant's leaves through the stomata?

Transpiration

Approximately what percentage of the water taken up by a plant's roots is typically lost to the atmosphere?

Up to 90%

Which type of vascular tissue is responsible for transporting water from the roots to the leaves?

Xylem

What is a defining structural characteristic of the cells that make up functional xylem?

They are dead at maturity and form hollow, rigid tubes.

Which vascular tissue transports sugars from the leaves to the rest of the plant?

Phloem

How do the cell walls of phloem compare to those of xylem?

Phloem cells are living and have much thinner cell walls.

What is the primary function of root hairs in a plant's root system?

To increase the surface area for the absorption of water and nutrients.

How does increasing surface area affect the rate of material exchange in biological systems?

It increases the rate of exchange by providing more space for molecules to move across boundaries.

Between a taproot system and a fibrous root system, which typically provides a higher surface area for water uptake?

A fibrous root system

In terms of water potential, in which direction does water naturally flow?

From an area of higher water potential to an area of lower water potential.

How does adding solutes to a solution affect its water potential?

It lowers the water potential (makes it more negative), encouraging water to move into that space.

Why is 'salting the earth' an effective historical tactic for preventing crop growth?

The high solute concentration in the soil lowers its water potential, preventing roots from taking up water.

How does the water potential of the atmosphere generally compare to the water potential inside a plant leaf?

The atmosphere has a much lower (more negative) water potential, which pulls water out of the leaf.

What is the approximate water potential value assigned to pure water at ambient pressure?

Zero

Which biological structure acts as a 'biological straw' to move water hundreds of feet up into trees?

Xylem vessels

How does the opening of stomata affect a plant's water status?

It allows CO₂ to enter for photosynthesis but results in significant water loss through transpiration.

What is the origin of the term 'stomata'?

It comes from the Latin word for 'mouth'.

In the context of the lecture, why is light energy referred to as 'free money' for most plants?

The availability of sunlight usually far exceeds the 1% that plants are actually able to capture and use

What is the specific name of the four-carbon storage molecule often mentioned in C4 and CAM cycles?

Malate (or malic acid)

How do CAM plants manage the energy requirement of fixing CO₂ without the immediate presence of light?

They use energy generated through cellular respiration from sugars produced during the previous day

What is the effect of a large surface area on the evaporation rate of a liquid?

It increases the evaporation rate because more molecules are at the surface and able to transition to gas.

Which plant organ is primarily responsible for acquiring the nitrogen needed to build proteins and DNA?

The roots

What is the relationship between the number of stomata and the amount of water vapor a plant releases?

Plants with more or wider open stomata release more water vapor into the atmosphere.

Why is the CAM pathway described as a 'two steps forward, one step back' business model?

The plant must spend some of its previously made sugar (via respiration) to fuel the process of making new sugar.

What is the driving force that allows a redwood tree to move water 379 feet into the air without muscles?

The gradient of water potential from the soil (high) through the plant to the atmosphere (low).

In a C4 plant, which cells perform the Calvin cycle?

Bundle sheath cells

How does the presence of root hairs mimic the function of villi in the human small intestine?

Both use small projections to maximize surface area for efficient absorption of materials.

If a C3 plant and a C4 plant are both in a bag, which one will cause more water accumulation via transpiration?

The C3 plant, because it must keep its stomata open wider to maintain CO₂ levels

Why is it important to distinguish between 'food' and 'nutrients' in plant biology?

Plants make their own food (carbon-based fuel) but must take up inorganic nutrients (N, P, Ca) from the soil.

What is the role of cellular respiration in the CAM pathway during the nighttime?

It provides the ATP necessary to run the C4-style carbon fixation cycle in the dark.

What term describes the movement of water across a semi-permeable membrane due to solute concentration differences?

Osmosis

What physical property of xylem allows it to provide structural support to a plant in addition to water transport?

The rigid, lignified cell walls of the dead xylem cells

What is the primary ecological trade-off of the C4 pathway?

Increased photosynthetic efficiency in heat and high light versus higher energy consumption

How do plants regulate the trade-off between CO₂ gain and water loss?

By opening and closing their stomata in response to environmental conditions

Why is the matter in a wooden table considered to have come primarily from the air?

The wood is composed of cellulose made from carbon fixed from atmospheric CO₂

What happens to the rate of photosynthesis in a C3 plant if CO₂ levels are low and O₂ levels are high?

The rate slows down due to increased photorespiration by Rubisco

What is the major cost of transpiration for a plant?

The risk of dehydration or 'wilting' due to losing too much water while trying to obtain CO₂

Which component of a plant acts as the 'primary organ' for creating carbon-based molecules?

The leaves

What is the significance of the fact that C4 evolved multiple times independently?

It suggests that C4 is a highly effective and repeatable evolutionary solution to low CO₂ and high heat

How does the energy conversion in the light reactions (Light to ATP/NADPH) relate to the Calvin cycle?

The light reactions provide the chemical energy required to power the fixation of carbon in the Calvin cycle