The Mechanisms of Transpiration and Translocation

Here is the step-by-step process of the transpiration stream:

1. Water enters the root hair cells from the soil via osmosis, moving from a dilute solution to a more concentrated solution.
2. Evaporation of water from the leaves creates a shortage of water, generating a suction force known as the transpiration pull.
3. Water molecules are held together in a continuous column by cohesion and stick to the xylem walls via adhesion.
4. This pulls water upwards in a strictly unidirectional (upwards only) flow.

Calculating the Rate of Transpiration

A potometer is used to estimate the rate of transpiration by measuring water uptake. The leafy shoot must be cut underwater to prevent air bubbles from breaking the continuous water column, and the apparatus must be sealed with Vaseline to keep it airtight.

$$\text{Rate} = \frac{\text{Distance moved by air bubble}}{\text{Time}}$$

Worked Example:

An air bubble in a potometer moves $40\text{ mm}$ in $20\text{ minutes}$. Calculate the rate of transpiration.

Step 1: Identify the values.

• Distance = $40\text{ mm}$
• Time = $20\text{ minutes}$

Step 2: Substitute into the equation.

• $\text{Rate} = \frac{40}{20}$

Step 3: Calculate the final answer with units.

• $\text{Rate} = 2\text{ mm/min}$

(Note: If asked for volume, use the volume of a cylinder formula: $V = \pi r^2 h$, where $h$ is the distance moved by the bubble).

Translocation (Phloem)

Just like humans have blood vessels to distribute food from the gut, plants have a dedicated pipeline to move sugars from their leaves to their roots.

• Phloem tissue is responsible for transporting dissolved sugars (specifically sucrose) and amino acids.
• Unlike xylem, phloem is made of living cells called sieve tube elements. These are long, thin cells with very little cytoplasm and no nucleus.
• The end walls between these cells have pores called sieve plates to allow cell sap to flow through easily.
• Phloem cells are supported by adjacent companion cells, which contain a high density of mitochondria to release energy for active transport.

Translocation is an active process (requiring energy from respiration) that moves substances in a bidirectional flow (both up and down the plant).

Here is the step-by-step process of translocation:

1. Sugars are produced or released at a source (where they are made, like photosynthesising leaves in summer).
2. These dissolved sugars are actively transported into the phloem tissue.
3. The sugars move through the sieve plates in a bidirectional flow (both up and down the plant).
4. The sugars are unloaded at a sink (where they are used or stored, like growing buds, fruits, or roots).
5. Stored sucrose is often converted into insoluble starch so it does not affect osmosis in the storage cells.

Comparing Xylem and Phloem

Feature	Xylem	Phloem
Substances transported	Water and mineral ions	Dissolved sugars and amino acids
Process name	Transpiration stream	Translocation
Direction of flow	Unidirectional (upwards only)	Bidirectional (up and down)
Energy requirement	Passive (driven by evaporation)	Active (requires energy from respiration)
Cell state at maturity	Dead cells	Living cells
Key structural features	Lignin, no end walls	Sieve plates, companion cells

(Note: In the plant stem, vascular bundles are arranged with phloem on the outside and xylem on the inside).

Stomata and Guard Cells

Plants face a constant dilemma: they must open pores to allow gas exchange—taking in the $CO_2$ needed for photosynthesis and releasing the oxygen produced as a byproduct, or taking in oxygen for respiration—but doing so inevitably lets their precious water vapour escape.

• A stoma (plural: stomata) is a tiny pore primarily found on the cooler, shaded underside of the leaf to reduce excess evaporation.
• Each stoma is controlled by two kidney-shaped guard cells. By changing shape to open and close the stomata, guard cells control the diffusion of carbon dioxide and oxygen, as well as the loss of water vapour.
• Guard cells have unevenly thickened walls: the inner wall (touching the pore) is thick and rigid, while the outer wall is thin and elastic.

Mechanism of Opening (Light/High Water):

1. Light triggers potassium ions ($K^+$) to enter the guard cells via active transport.
2. This lowers the internal water potential, causing water to enter by osmosis.
3. The cells swell and become turgid.
4. Because the thin outer walls stretch more than the thick inner walls, the cells curve outwards, opening the stoma.

Mechanism of Closing (Dark/Water Stress):

1. Potassium ions and water leave the cells.
2. The cells lose pressure and become flaccid (limp).
3. The guard cells straighten out, closing the stoma to prevent further water loss.

Calculating Stomatal Density

You can view stomata under a microscope using the nail varnish impression method.

$$\text{Density} = \frac{\text{Number of stomata counted}}{\text{Area of field of view}}$$

Worked Example:

A student counts $15$ stomata in a circular field of view with a radius of $0.25\text{ mm}$. Calculate the stomatal density per $\text{mm}^2$.

Step 1: Calculate the area of the field of view (Area = $\pi r^2$).

• $\text{Area} = \pi \times 0.25^2 \approx 0.196\text{ mm}^2$

Step 2: Calculate density.

• $\text{Density} = \frac{15}{0.196}$

Step 3: Final answer.

• $\text{Density} \approx 77\text{ stomata per mm}^2$

Factors Affecting the Rate of Transpiration

Every time you hang wet clothes on a washing line, you intuitively know what makes them dry faster—plants lose water following the exact same environmental rules.

• Temperature: Higher temperatures increase the rate. Water molecules gain more kinetic energy, speeding up evaporation and diffusion.
• Humidity: Higher humidity decreases the rate. It reduces the concentration gradient of water vapour between the inside and outside of the leaf.
• Air Flow (Wind): Increased air flow increases the rate. Wind blows evaporated water vapour away from the leaf surface, maintaining a steep concentration gradient.
• Light Intensity: Brighter light increases the rate. Stomata open wider to allow more carbon dioxide in for photosynthesis, which allows more water vapour to escape.