Mass transport in plants Flashcards
What is transpiration?
The loss of water vapor from the stomata by evaporation, mainly occurring on the underside of leaves.
What are the four key factors that affect the rate of transpiration?
Light intensity, temperature, humidity, and wind.
How does light intensity affect transpiration?
Higher light intensity increases transpiration because more stomata open, allowing more water vapor to escape.
How does temperature affect transpiration?
Higher temperatures increase transpiration as water molecules gain more kinetic energy, evaporating faster.
How does humidity affect transpiration?
Higher humidity decreases transpiration because the air has more water vapor, reducing the water potential gradient.
How does wind affect transpiration?
More wind increases transpiration by removing humid air around the leaf, maintaining a strong water potential gradient.
What is the cohesion-tension theory?
A theory explaining how water moves up a plant against gravity using cohesion, adhesion, and root pressure.
What is cohesion in water transport?
The sticking together of water molecules due to hydrogen bonding, forming a continuous water column in the xylem.
What is adhesion in water transport?
The sticking of water molecules to the walls of the xylem, aiding capillary action and preventing water from falling back.
What is root pressure?
The pressure created when water enters roots by osmosis, pushing water upwards in the xylem.
How does transpiration contribute to water movement in plants?
Water evaporates from the stomata, creating a negative pressure that pulls water up through the xylem.
What happens to the xylem as water moves up?
The tension from water being pulled up causes the xylem walls to narrow, increasing capillarity.
Why do plants need transport systems?
Plants need transport systems because they are multicellular with a low surface area to volume ratio, diffusion is too slow, and substances must be moved over long distances.
What are the two main transport tissues in plants?
Xylem (transports water and minerals) and phloem (transports sugars and amino acids).
What is the function of xylem tissue?
Xylem transports water and mineral ions and provides structural support.
What are the adaptations of xylem vessels?
- Elongated, hollow tubes without end walls
- Lack organelles for an unobstructed flow
- Thick walls reinforced with lignin for support
- Non-lignified pits for lateral water and ion movement
What is the function of phloem tissue?
Phloem transports sugars and amino acids (assimilates) throughout the plant.
What are sieve tube elements in the phloem?
- Connected end-to-end to form sieve tubes
- Have sieve plates with pores to allow movement of sugars and amino acids
- Lack nuclei and most organelles
- Contain only a thin layer of cytoplasm
What are the adaptations of companion cells in phloem?
- Connected to sieve tube elements via plasmodesmata
- Contain a large nucleus and many mitochondria to provide energy for active transport
- Have many ribosomes for protein synthesis
How does water move through a plant?
Water enters root hair cells by osmosis, moves through the cytoplasm or cell walls toward the xylem, is transported up to the leaves, used in photosynthesis, and lost through transpiration.
What is transpiration pull?
The evaporation of water at the leaves creates tension, which pulls water up through the xylem due to cohesion between water molecules.
How does transpiration relate to gas exchange?
Stomata open for CO₂ absorption, allowing water vapor to escape. Transpiration occurs as a side effect of gas exchange.
Why does water vapor move out of leaves?
Water vapor moves down a water potential gradient from the air spaces in the leaf to the atmosphere.
What is a potometer used for?
A potometer measures the rate of transpiration by tracking water uptake in a plant.
How is a potometer set up?
The plant shoot is cut underwater, assembled in an airtight setup, acclimatized, and an air bubble is tracked in the capillary tube.
How do you calculate the rate of transpiration using a potometer?
Multiply the cross-sectional area of the capillary tube by the distance the air bubble moves, then divide by the time taken.
What is translocation?
Translocation is the mass flow of sucrose and amino acids from one part of a plant, the source, to another part of the plant, the sink.
What are the key features of translocation?
- It requires energy.
- It transports substances from sources (e.g., leaves) to sinks (e.g., roots).
- It occurs through the phloem.
- Water provides the medium in which substances dissolve for transport.
- Enzymes help maintain a concentration gradient.
What is the mass flow hypothesis?
The mass flow hypothesis proposes that translocation occurs due to pressure gradients between the source and the sink.
Describe the process of the mass flow hypothesis.
- At the source, sucrose is actively loaded into sieve tube elements from companion cells.
- Water enters sieve tube elements by osmosis, increasing hydrostatic pressure.
- At the sink, sucrose is actively removed, increasing water potential.
- Water leaves the phloem by osmosis, decreasing hydrostatic pressure.
- This creates a pressure gradient, pushing solutes from the source to the sink.
What happens at the sink in translocation?
Solutes are actively unloaded from the sieve tube element into companion cells and then into sink cells, where they are used for respiration or stored.
What evidence supports the mass flow hypothesis?
- Ringing experiments: Removing a ring of bark prevents downward sugar flow, causing a bulge above the cut.
- Radioactive tracers: Radioactive CO2 is traced from source to sink, proving movement.
- Aphids: Aphid feeding shows higher pressure at the top of the phloem than at the bottom.
- Metabolic inhibitors: ATP inhibitors stop translocation, showing that energy is required.
What are some arguments against the mass flow hypothesis?
- Sugars do not always move to the sink with the highest water potential.
- Pressure gradients may not be sufficient to drive flow through sieve plates, especially in large trees.
What are sieve tube elements?
Sieve tube elements are phloem cells that lack nuclei and have perforated end walls (sieve plates) to allow continuous flow of sugar solution.
What is the role of companion cells in translocation?
Companion cells provide ATP for active transport of sucrose into sieve tube elements.
How does water movement aid translocation?
Water moves in by osmosis at the source (increasing pressure) and out at the sink (decreasing pressure), creating a pressure gradient that drives translocation.
How do radioactive tracers help investigate translocation?
Radioactive CO2 is absorbed by the plant, incorporated into sugars, and traced using x-ray film to identify where sugars are transported.
What are ringing experiments, and what do they show?
Removing a ring of bark and phloem causes swelling above the cut due to sugar accumulation, proving that the phloem transports sugars.
What is the role of the xylem in translocation?
The xylem supplies water for osmosis into the phloem and helps maintain pressure gradients for translocation.
How does sucrose move from source to sieve tube elements?
Sucrose is actively transported from source cells into sieve tube elements via companion cells, using ATP from mitochondria in companion cells.
What happens to hydrostatic pressure during translocation?
- It increases at the source due to water intake by osmosis.
- It decreases at the sink as sucrose is removed and water exits.
- The difference in pressure drives mass flow.
Describe the cohesion tension theory of water transport in the xylem
- Water lost from leaf because of transpiration
- Transpiration through stomata
- Lowers water potential of leaf cells;
- Water pulled up xylem (creating tension);
- Water molecules stick together by hydrogen bonds (forming continuous) water column
- Adhesion of water (molecules) to walls of xylem;
