Cellular Transport and Water Movement in Plants

What is Simple Diffusion and how does it work in plants?

Simple diffusion allows molecules like O₂, CO₂, and H₂O to move freely across membranes following their concentration gradients. Osmosis is the diffusion of water specifically.

What is the role of hydrogen ions (H⁺) in plant membrane transport?

H⁺ gradients across membranes drive some transport processes, known as chemiosmosis. Proton pumps (H⁺ pumps) use ATP to expel H⁺ ions, creating a higher concentration of H⁺ outside the plasma membrane

What is membrane potential?

Membrane potential is the charge differential between the cytoplasm and the outer fluid, created by the unequal distribution of ions across the membrane. It helps drive various transport processes in plants, such as cation uptake.

How does cation uptake occur in plants?

Ion channel proteins allow cations like K⁺ and Mg²⁺ to cross the plasma membrane, driven by electrostatic attraction to the negatively charged cytoplasm.

What is cotransport, and how does it work with nitrate (NO₃⁻) and sucrose?

Cotransport involves the coupling of ion movement (like H⁺) with the transport of other molecules like NO₃⁻ (nitrate) and sucrose. These molecules cross the plasma membrane through cotransporter proteins, enabling their movement across cells.

What is the apoplast in plants?

The apoplast is a continuum of cell walls and intercellular spaces that allows fluid movement through the plant, but it is interrupted by the Casparian strip in the endodermis, which prevents fluid from bypassing the plasma membrane.

What is the symplast in plants?

The symplast is a continuous network of cytoplasm connected by plasmodesmata, allowing cytoplasm to flow between plant cells, especially in tissues like phloem for sugar transport.

How does phloem transport work?

Phloem transports food (sucrose) from sugar sources (e.g., leaves) to sugar sinks (e.g., growing tissues). Transport occurs through sieve tubes using osmotic pressure created by sugar loading and water movement.

What is the role of companion cells in phloem?

Companion cells, which have highly convoluted membranes, assist in loading sucrose into sieve tubes by facilitating chemiosmotic processes for efficient sugar transport.

What is water potential in plants?

Water potential combines solute potential (tonicity) and pressure potential. Water moves from high to low water potential, with osmosis playing a key role in nutrient and water movement in plant tissues.

How does root pressure affect xylem transport?

Root pressure is osmotic pressure generated in the xylem of roots. It pushes water upwards, but it is limited to short distances and causes guttation (water exudation from leaf margins in small plants).

What is the Transpiration-Cohesion-Tension Mechanism?

Transpiration (evaporation from leaves) creates negative pressure that draws water upwards through the xylem. Cohesion (water's attraction to itself) and adhesion (water's attraction to xylem walls) aid in this process.

How does transpiration benefit plants?

Transpiration generates tension that pulls water and nutrients upward, and it also cools leaves via evaporative heat loss.

What adaptations help plants reduce transpiration in dry environments?

• Cuticle (waxy covering on leaves) reduces water loss.

• Stomata on the underside of leaves limit exposure to air.

• Epidermal hairs or sunken crypts reduce air flow over stomata.

How do guard cells regulate transpiration?

Guard cells control stomata opening and closing. Turgid guard cells bow inward to open stomata, while a loss of turgor (e.g., during drought) causes stomata to close.

What is the circadian rhythm in guard cells?

Guard cells operate on a circadian rhythm, opening stomata during the day (via K⁺ transport) and closing them at night, which helps regulate transpiration and conserve water.

What are CAM plants and how do they conserve water?

CAM (Crassulacean Acid Metabolism) plants open stomata at night to allow CO₂ diffusion while minimizing water loss, adapting to xeric (dry) conditions.