Mass transport in plants

• What is mass transport?

  Mass transport refers to the movement of substances in bulk over large distances within an organism. In plants, it involves the movement of water, minerals, and sugars.
• Why is mass transport important in plants?

  Delivering water and minerals from the soil to all parts of the plant.

  Transporting sugars (produced in photosynthesis) to areas where they’re needed or stored.
• What are the 2 main transport systems?

  Xylem and phloem
• What do xylem and phloem transport?

  Xylem - transports water and minerals

  Phloem - transports sugars and other organic substances
• What does the xylem do?

  Xylem moves water and dissolved minerals from the roots to the leaves.

  Water is needed for photosynthesis, cooling the plant (via transpiration), and keeping cells turgid (firm).
• Structure of the xylem

  Made up of dead cells.

  These cells form hollow tubes (like pipes) that are stacked end-to-end.

  Their walls are reinforced with lignin, a tough material that prevents collapse and makes the xylem waterproof.
• How does water move up the xylem?

  By transpiration, cohesion, adhesion and root pressure
• What is transpiration?


  Transpiration is the process by which plants release water vapor into the atmosphere from the stomata
• Describe transpiration


  Water is absorbed by the roots from the soil via osmosis.

  The water then moves into the xylem vessels and travels up through the plant to the leaves.

  Water moves from the xylem to the cells in the leaf.

  Once in the leaf cells, water moves to the spaces between the cells and eventually reaches the stomata.

  At the stomata, the water evaporates from the surface of the leaf cells into the air spaces within the leaf.

  The water vapor diffuses out of the stomata into the air outside the leaf.
• Why does transpiration happen?

  The plant needs to exchange gases with the atmosphere for photosynthesis.

  When the stomata open to let in carbon dioxide (CO₂), water vapor escapes as a side effect.
• What factors affect transpiration?

  Humidity, temperature, wind speed, and light intensity.
• • Light Intensity:
  • More light = stomata open wider for photosynthesis = more water loss.
  • Temperature:
  • Higher temperatures make water evaporate faster.
  • Humidity:
  • High humidity slows transpiration because the air is already full of water vapor.
  • Wind:
  • Wind removes water vapor near the leaf surface, increasing the diffusion gradient, so transpiration speeds up.
• The Importance of Transpiration

  • Pulling water up the plant:
  • As water evaporates, it creates a suction force (transpiration pull), which draws water up through the xylem.
  • Delivering minerals:
  • Dissolved minerals in water are transported from the roots to the leaves.
  • Cooling the plant:
  • Evaporation of water cools the plant, just like sweat cools humans.
  • Maintaining turgor pressure:
  • Keeps the plant cells firm, preventing wilting.
• How is the xylem adapted for its function?

  The cells form long, hollow tubes that allow uninterrupted water flow from the roots to the leaves.

  The cell walls of xylem vessels are reinforced with lignin, a strong, waterproof material. Stops water from leaking out of the xylem. Helps the plant stay upright, especially tall plants.

  No end walls between the cells; Creates a continuous column for water to move through without obstruction.

  Narrow tubes for cohesion and maintaining the water column; hydrogen bonding is stronger in narrower tubes.
• What Does the Phloem Do?

  • The phloem transports organic substances, like sugars (mainly sucrose), amino acids, and hormones.
  • These substances are moved from the places where they’re made (called sources) to where they’re needed or stored (called sinks).
  • Sources: Leaves (where photosynthesis occurs).
  • Sinks: Growing tissues (e.g., roots, flowers, fruits).
• Structure of the phloem

  made of living cells called sieve tube elements and companion cells:

  Sieve tube elements: Long tubes with little cytoplasm and no nucleus.

  They’re stacked to form a continuous path for transport.

  Companion cells: Help sieve tube elements by carrying out their metabolic functions.
• How does the phloem work?

  The movement of sugars in the phloem is called translocation, and it’s an active process (requires energy).

  Translocation occurs by the mass flow hypothesis
• Whats the cohesion tension theory?

  explains how water moves up through the xylem against gravity.
• Explain cohesion tension theory

  Transpiration: Evaporation of water from leaves creates low pressure (suction).

  Tension: Pulling force on the water column in the xylem.

  Cohesion: Water molecules stick together (hydrogen bonding), maintaining a continuous column.

  Adhesion: Water sticks to xylem walls, helping it resist gravity.

  Root Pressure (minor role): Osmosis at roots pushes water upward slightly.
• Does transpiration increase or decrease with light intensity?

  Increase as the stomata would open wider.
• Does transpiration increase or decrease with humidity?

  Decrease, In high humidity, the air around the plant is already saturated with water vapor, so the diffusion gradient is smaller.
• Does transpiration increase or decrease with temperature?

  Higher temperatures increase the kinetic energy of water molecules, so they evaporate faster.
• Does transpiration increase or decrease with wind speed?

  Wind removes the humid air near the leaf surface, maintaining a steep diffusion gradient, which increases transpiration.
• What is a potometre?

  A potometer measures the rate of water uptake by a plant, which is closely related to the rate of transpiration.
• Hows does a potometre work?

  A plant shoot is cut underwater and attached to the potometer to avoid air bubbles.

  As the plant transpires, it draws water up the capillary tube.

  The movement of a bubble in the tube is used to calculate the rate of water uptake.
• Why is the shoot cut at an angle?

  It increases surface area for water uptake and prevents air bubbles blocking the xylem
• what is the evidence for cohesion tension theory?

  Changes in the tree trunk diametre:

  During the day, when transpiration rates are high (due to sunlight opening stomata), the tension in the xylem increases.

  This causes the xylem vessels to contract, slightly reducing the diameter of the trunk.

  At night, when transpiration rates are lower, the tension decreases, and the trunk diameter increases slightly.

  Role of Adhesion in Narrow Tubes

  water rises in narrow capillary tubes (like xylem vessels) due to adhesion (water sticking to the tube walls)
• How does water enter the plant?

  Through the roots via osmosis
• What is the role of xylem in water transport?

  It moves water up through the plant
• How does water reach mesophyll cells in leaves?

  Through xylem in leaf veins
• What happens to water in mesophyll cells during photosynthesis?

  It is used in the photosynthesis process
• How does transpiration affect water movement in leaves?

  It creates negative pressure drawing water up
• What is cohesion in the context of water movement?

  Water molecules stick together
• What is adhesion in relation to water and xylem walls?

  Water sticks to xylem walls
• How do transpiration pull and cohesion work together in plants?

  They pull water up through the plant
• Why is water movement essential for photosynthesis?

  It provides necessary reactants for the process
• How does water movement help in cooling the plant?

  By evaporating from leaf surfaces
• How Water Moves Up the Xylem

  Cohesion refers to the property where water molecules stick to each other due to hydrogen bonds.

  This allows the water molecules to form a continuous column in the xylem. Water molecules also stick to the walls of the xylem vessels through adhesion.

  This helps to counteract gravity and maintain the water column in the xylem, ensuring the upward movement of water. As water evaporates from the leaf, it creates a pull or tension in the water column in the xylem. This transpiration pull draws more water up from the roots, creating a continuous flow.
• What type of preassure does tension create?

  Negitive
• How do you measure water uptake using an air bubble in a potometer?

  Prepare the plant: Cut the stem underwater and insert it into the potometer.

  Create an air bubble: Introduce a small air bubble into the tubing.

  Measure the bubble movement: As the plant transpires, water uptake moves the air bubble along the tube.

  Record the distance moved by the air bubble after a set time.

  Calculate the rate of uptake: Rate = Distance moved ÷ Time taken.