PHOTOSYNTHESIS AND PLANT TRANSPORT

What is C3 photosynthesis?

C3 photosynthesis is the most common and fundamental photosynthetic pathway, prevalent in virtually all eukaryotic photoautotrophs, including most trees, angiosperms, and food crops like rice, wheat, and soybeans.

Why is the C3 pathway considered universal?

No known organisms capable of photosynthesis do not, at some point, utilize the C3 pathway (the Calvin Cycle), making it a universal component of autotrophic carbon fixation.

Under what conditions do certain organisms employ additional photosynthetic processes like C4 and CAM?

Certain organisms, particularly those in challenging environments (e.g., hot, dry, or high-light conditions), employ additional processes like C4 and CAM to significantly enhance photosynthetic efficiency and overcome environmental limitations.

Define C3 photosynthesis in terms of carbon fixation.

C3 photosynthesis refers specifically to the biochemical pathway where carbon dioxide (CO_2) is initially fixed into a three-carbon compound, 3-phosphoglycerate (3-PGA), during the first stable intermediate step of carbon fixation in the Calvin Cycle. This occurs in the stroma of chloroplasts.

What is RuBisCO and why is it important?

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is arguably the most abundant enzyme on Earth and is crucial in the Calvin Cycle, catalyzing the rate-limiting step.

Describe RuBisCO's dual nature.

RuBisCO is a promiscuous enzyme with a dual nature: its primary function is carboxylation (fixing CO2 to ribulose-1,5-bisphosphate (RuBP) to produce 3-PGA and sugars), but it can also act as an oxygenase, fixing O2 instead of CO_2 to RuBP.

What is photorespiration and its consequences?

Photorespiration is a wasteful metabolic pathway initiated when RuBisCO fixes oxygen (O2) instead of carbon dioxide (CO2). It prevents efficient sugar production, consumes RuBP, ATP, and NADPH, and releases CO_2, reducing photosynthetic efficiency by up to 50% in C3 plants under certain conditions.

Under what conditions is photorespiration favored?

Photorespiration is favored under conditions of high temperature, high O2 concentration (e.g., when stomata close to conserve water, trapping O2), and low CO_2 concentration.

Why has RuBisCO not evolved to prevent photorespiration?

Despite millions of years of natural selection, no significant evolutionary modification of RuBisCO has been observed to circumvent photorespiration, possibly due to its fundamental role, complex metabolic costs of altering a core enzyme, and the difficulty of improving specificity without compromising catalytic speed.

What is C4 photosynthesis?

C4 photosynthesis serves as a supplementary pathway to C3, evolved to mitigate challenges of conventional photosynthesis under hot, dry, and high-light climates. It is common in grasses like corn, sugarcane, and sorghum.

List key features of C4 photosynthesis.

Key features include: initial fixation of CO2 into four-carbon molecules, counteracting photorespiration by concentrating CO2 around RuBisCO, and spatial separation of initial CO_2 fixation (mesophyll cells) from the Calvin Cycle (bundle sheath cells).

Explain the role of mesophyll cells in C4 photosynthesis.

Mesophyll cells are closer to the leaf surface and contain PEP carboxylase, an enzyme with a high affinity for CO2 that does not react with O2. They are responsible for initial CO_2 fixation and transport of 4-carbon compounds.

Describe the initial CO_2 fixation and transport process in C4 mesophyll cells.

Atmospheric CO_2 diffuses into mesophyll cells, where it is added to phosphoenolpyruvate (PEP) by PEP carboxylase, forming oxaloacetate. Oxaloacetate is converted to malate (or aspartate), which is transported to bundle sheath cells.

Explain the role of bundle sheath cells in C4 photosynthesis.

In bundle sheath cells, a four-carbon compound (malate) is decarboxylated, releasing CO2 at a high concentration directly to RuBisCO for the Calvin Cycle. This ensures RuBisCO operates in a high-CO2, low-O_2 environment to minimize photorespiration. Pyruvate is transported back to mesophyll cells.

What is Kranz anatomy?

Kranz anatomy is a special leaf structure in many C4 plants, involving enlarged bundle sheath cells forming a concentric ring around vascular bundles, surrounded by mesophyll cells. This aids in efficient CO_2 capture and delivery to bundle sheath cells.

What is CAM (Crassulacean Acid Metabolism)?

CAM is an adaptation predominantly found in succulents and desert plants (e.g., cacti, pineapples, agaves) that live in arid environments, characterized by a temporal separation of CO_2 fixation and the Calvin Cycle to conserve water efficiently.

Describe the key features of CAM photosynthesis.

CAM plants exhibit temporal separation: stomata open at night to fix CO2 into organic acids (stored in vacuoles); during the day, stomata close, and stored organic acids release CO2 for the Calvin Cycle using light energy.

Provide examples of CAM plants.

Examples include cacti (Opuntia), agaves (Agave americana), and many members of the Crassulaceae family (e.g., Kalanchoe).

Compare the efficiency and environmental preferences of C3, C4, and CAM plants.

C3 plants are efficient in cool, moist environments; C4 plants thrive in hot, dry, high-light conditions by spatially separating CO2 fixation; CAM plants are most water-efficient, ideal for extremely arid environments, by temporally separating CO2 uptake.

What are the practical implications of C4 and CAM pathways?

The C4 pathway significantly aids in photosynthesis and productivity in high light and high temperature conditions for agricultural grasses. CAM is crucial for survival in deserts and allows plants to colonize water-limited niches.

What is plant transport and its importance?

Plant transport involves the highly regulated movement of water, dissolved minerals, and sugars, crucial for photosynthesis, growth, structural support, and overall plant health.

How do roots absorb water and minerals?

Roots, especially root hairs (which increase surface area), absorb water and dissolved mineral nutrients from the soil. Root cells' respiration produces CO_2 for carbonic acid, aiding mineral ion uptake. Water is then transported upward through the xylem.

Define water potential (\Psi).

Water potential (\Psi) is a crucial physical property that predicts the direction of water movement; water always moves from an area of higher water potential to an area of lower water potential. It is measured in megapascals (MPa).

What is the overall water potential equation?

The overall water potential equation is ext{Ψ} = ext{Ψ}s + ext{Ψ}p , where ext{Ψ} is total water potential, ext{Ψ}s is solute potential, and ext{Ψ}p is pressure potential.

What is the water potential of pure water at standard conditions?

At room temperature (20^{ ext{o}}C) and standard atmospheric pressure, pure water has a water potential of 0 MPa.

Explain solute potential (\Psi_s).

Solute potential (\Psi_s) is caused by dissolved solutes, which lower the concentration of free water molecules, thereby reducing the water potential. It is always a negative value (or zero for pure water), becoming more negative with more solutes.

Explain pressure potential (\Psi_p).

Pressure potential (\Psi_p) refers to the physical pressure exerted on water. It can be positive (e.g., turgor pressure) or negative (e.g., tension in xylem). Positive pressure increases water potential, while negative pressure decreases it.

What is turgor pressure?

Turgor pressure is the positive hydrostatic pressure built up within plant cells due to osmotic water influx, causing the cell membrane to push outward against the rigid cell wall.

List vital functions of turgor pressure.

Turgor pressure is vital for maintaining plant structural integrity/rigidity, driving cell expansion/growth, regulating stomata opening/closing, and facilitating various movements.

How does water predominantly move throughout a plant?

Water movement throughout the plant is predominantly passive, driven by differences in water potential from the soil, through the plant, to the atmosphere.

Describe the symplastic route of water movement in roots.

In the symplastic route, water moves through the living cells of the root cortex, passing from cytoplasm to cytoplasm via plasmodesmata. This allows for selective uptake of solutes.

Describe the apoplastic route of water movement in roots.

In the apoplastic route, water moves through the non-living components of the root, including cell walls and intercellular spaces, without crossing cell membranes. This route is faster but blocked by the Casparian strip in the endodermis.

What is the Cohesion-Tension Theory?

The Cohesion-Tension Theory explains that water is pulled upward through the xylem by transpiration pull, which creates significant negative pressure, supported by water's cohesive and adhesive properties.

Explain the role of transpiration in water movement.

Transpiration, the evaporation of water vapor from the leaf surface, creates a steep water potential gradient between the leaf and the atmosphere, pulling water out of the xylem.

Explain the role of cohesion in water movement.

Cohesion refers to the strong attractive forces (hydrogen bonding) between water molecules, allowing them to stick together and form a continuous column of water within the xylem vessels.

Explain the role of adhesion in water movement.

Adhesion refers to water molecules sticking to the hydrophilic walls of xylem vessels, which helps to counteract gravity and resist the breaking of the water column.

What is root pressure?

Root pressure is a minor positive pressure push generated in the roots (mainly at night) by active transport of ions into the xylem, which then draws water in osmotically.

Photorespiration

A wasteful metabolic pathway initiated when RuBisCO fixes oxygen (O2) instead of carbon dioxide (CO2). It consumes RuBP, ATP, and NADPH, releases CO_2, and reduces photosynthetic efficiency.

C4 Pathway

A supplementary photosynthetic pathway that spatially separates initial CO_2 fixation (in mesophyll cells) from the Calvin Cycle (in bundle sheath cells) to counteract photorespiration in hot, dry, high-light conditions.

PEP Carboxylase

An enzyme found in mesophyll cells of C4 and CAM plants that has a high affinity for CO2 and does not react with O2, allowing for efficient carbon fixation even at low atmospheric CO_2 concentrations.

Mesophyll

Leaf cells located closer to the leaf surface where initial CO_2 fixation occurs in C4 plants, and where PEP carboxylase is active.

Bundle Sheath

Specialized cells forming a concentric ring around vascular bundles in C4 plants (Kranz anatomy). They concentrate CO_2 around RuBisCO for the Calvin Cycle, minimizing photorespiration.

"Physical Separation" (Spatial Separation)

A characteristic of C4 photosynthesis where different stages of carbon fixation happen in distinct physical locations within the plant (mesophyll cells vs. bundle sheath cells).

CAM (Crassulacean Acid Metabolism)

An adaptation in succulents and desert plants that involves temporal separation of CO_2 uptake (at night) and the Calvin Cycle (during the day) to conserve water efficiently.

"Temporal Separation"

A key feature of CAM photosynthesis where processes are separated by time, with CO_2 uptake and initial fixation occurring at night, and the Calvin Cycle occurring during the day.

Water Potential (\Psi)

A crucial physical property that predicts the direction of water movement, always from an area of higher water potential to one of lower water potential. Measured in megapascals (MPa).

Solute (osmotic) Potential (\Psi_s)

The component of water potential due to the presence of dissolved solutes, which lowers the water potential of a solution. It is always a negative value (or zero for pure water).

Pressure Potential (\Psi_p)

The component of water potential referring to the physical pressure exerted on water. Can be positive (e.g., turgor pressure) or negative (e.g., tension in xylem).

Turgor

The positive hydrostatic pressure built up within plant cells due to osmotic water influx, causing the cell membrane to push against the rigid cell wall. Essential for plant structural integrity and growth.

Wilt

The loss of turgor pressure in plant cells, leading to a drooping or limp appearance of stems and leaves.

Transmembrane

Pertaining to substances or processes that cross a biological membrane, such as water moving across cell membranes.

Symplast

A continuous network of plant cell cytoplasms connected by plasmodesmata, through which water and solutes can move directly from cell to cell.

Plasmodesmata

Microscopic channels through the cell walls of plant cells, allowing direct cytoplasmic connections between adjacent cells and facilitating symplastic transport.

Apoplast

The non-living components of a plant, including cell walls and intercellular spaces, through which water and solutes can move without crossing cell membranes.

Endodermis

An inner layer of cortical cells in plant roots that surrounds the vascular cylinder, playing a role in regulating water and solute movement into the xylem.

Casparian Strip

A waxy, suberin-rich band in the cell walls of endodermal cells that blocks apoplastic movement of water and solutes, forcing them into the symplast for selective passage.

Transpiration

The evaporation of water vapor from the leaf surface, primarily through stomata, which creates a negative pressure (tension) that pulls water up the xylem.

Adhesion

The tendency of water molecules to stick to hydrophilic surfaces, such as the walls of xylem vessels, helping to support the water column against gravity.

Cohesion

The strong attractive force between water molecules due to hydrogen bonding, allowing them to form a continuous, unbroken column within the xylem vessels during transpiration pull.