9.1 transport in the xylem of plants worksheet | PDF - Free Printable

Problem Overview:

The task involves explaining the role of guard cells in controlling water loss through transpiration, understanding how water moves from roots to leaves via xylem vessels, and exploring properties of water and its interaction with glass. Below is a detailed explanation for each part of the problem.

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2. Explain how the action of guard cells shows the control of water loss. (Slide 8)

#### Solution:
Guard cells are specialized cells located around the stomata (tiny pores) on the surface of plant leaves. They play a crucial role in regulating gas exchange and water loss through transpiration. Here's how they control water loss:

1. Structure of Guard Cells:
- Guard cells are kidney-shaped cells that surround each stoma.
- When turgid (full of water), they curve outward, opening the stoma.
- When flaccid (lacking water), they become straight, closing the stoma.

2. Control Mechanism:
- Opening Stomata: When guard cells take up water, they become turgid due to osmosis. This causes them to swell and curve outward, widening the stoma. This allows gases like carbon dioxide (for photosynthesis) and oxygen to enter or exit the leaf.
- Closing Stomata: When guard cells lose water, they become flaccid and straighten out, closing the stoma. This reduces the rate of water loss through transpiration.

3. Regulation of Water Loss:
- During conditions of high humidity or low light (e.g., at night), guard cells close the stomata to minimize water loss.
- During favorable conditions (e.g., bright sunlight and moderate humidity), guard cells open the stomata to facilitate gas exchange for photosynthesis while managing water loss.

#### Explanation:
Guard cells act as gatekeepers by dynamically adjusting the size of stomatal openings. This regulation ensures that plants can balance the need for gas exchange with the risk of excessive water loss, especially in dry environments.

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9.1 U.2 Plants transport water from the roots to the leaves to replace losses from transpiration.

#### Solution:
Water moves from the roots to the leaves through the xylem vessels, which are part of the plant's vascular system. The process is driven by several factors:

1. Transpiration Pull:
- Transpiration is the evaporation of water from the leaf surfaces through stomata.
- As water evaporates from the leaf, it creates a negative pressure (tension) in the xylem vessels.
- This tension pulls water upward from the roots through the continuous column of water in the xylem.

2. Cohesion and Adhesion of Water Molecules:
- Water molecules are cohesive (they stick together) and adhesive (they stick to the walls of xylem vessels).
- These properties allow water to move as a continuous column without breaking, even against gravity.

3. Root Pressure:
- Roots absorb water from the soil through osmosis.
- This absorption creates root pressure, which helps push water upward into the xylem.

4. Capillary Action:
- The narrow diameter of xylem vessels enhances capillary action, further aiding the upward movement of water.

#### Explanation:
Water moves from the roots to the leaves primarily due to transpiration pull, which creates a suction force that draws water upward through the xylem. Cohesion, adhesion, and root pressure also contribute to this process.

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9.1 U.3 The cohesive property of water and the structure of the xylem vessels allow transport under tension.

#### Solution:
Water's cohesive and adhesive properties, combined with the structure of xylem vessels, enable it to move under tension:

1. Cohesion:
- Water molecules are strongly attracted to each other due to hydrogen bonding.
- This cohesion allows water to form a continuous column in the xylem vessels, preventing air bubbles (embolisms) from disrupting the flow.

2. Adhesion:
- Water molecules are also attracted to the cell walls of xylem vessels.
- This adhesion helps maintain the integrity of the water column as it moves upward.

3. Structure of Xylem Vessels:
- Xylem vessels are long, hollow tubes made of dead cells with lignified walls.
- Their narrow diameter enhances capillary action, allowing water to move smoothly despite gravitational forces.

#### Explanation:
The cohesive and adhesive properties of water, along with the structural features of xylem vessels, ensure that water can be transported efficiently under tension, even over long distances within the plant.

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4. State one property of water which allows transpiration pull to be generated. (Slide 11)

#### Solution:
One key property of water that allows transpiration pull to be generated is cohesion.

- Cohesion refers to the strong attraction between water molecules due to hydrogen bonding.
- This property ensures that water molecules remain tightly bound together, forming a continuous column in the xylem vessels.
- As water evaporates from the leaf surfaces, the cohesive forces pull the entire column of water upward, creating the transpiration pull.

#### Explanation:
Cohesion is essential because it prevents the water column from breaking under tension, allowing water to move continuously from the roots to the leaves.

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5. Water is strongly attracted to glass. How does this explain the shape of the meniscus?

#### Solution:
The meniscus is the curved upper surface of a liquid in a container, such as a graduated cylinder. The shape of the meniscus is explained by the strong attraction of water to glass, known as adhesion:

1. Adhesion:
- Water molecules are attracted to the glass surface due to intermolecular forces (hydrogen bonding).
- This attraction causes the water to "climb" slightly up the sides of the glass container.

2. Surface Tension:
- Water molecules at the surface are also attracted to each other (cohesion), which creates surface tension.
- Surface tension causes the top layer of water to behave like a thin elastic membrane.

3. Resulting Shape:
- The combination of adhesion (water sticking to glass) and cohesion (water molecules sticking to each other) results in a concave meniscus (curved downward) in a graduated cylinder.
- If the liquid were not attracted to the glass (e.g., mercury), the meniscus would be convex (curved upward).

#### Explanation:
The concave shape of the meniscus in a graduated cylinder is due to the stronger adhesion between water and glass compared to the cohesion among water molecules at the surface.

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6. Liquid mercury, unlike water, is repelled by glass. If you filled your cylinder with mercury, what shape would the meniscus look like? Sketch your idea next to the graduated cylinder on the previous page.

#### Solution:
Mercury behaves differently from water because it is repelled by glass due to weak intermolecular forces between mercury and glass. This repulsion leads to a convex meniscus:

1. Repulsion:
- Mercury molecules are not attracted to glass; instead, they are repelled.
- This repulsion causes mercury to pull away from the glass surface.

2. Surface Tension:
- Mercury also exhibits surface tension due to cohesion among its molecules.
- However, the lack of adhesion to glass dominates, resulting in a convex meniscus.

3. Shape of Meniscus:
- The meniscus in a graduated cylinder filled with mercury would curve upward (convex) because the liquid is pulled inward due to its repulsion with the glass.

#### Explanation:
The convex shape of the mercury meniscus is a result of the repulsive forces between mercury and glass, causing the liquid to pull away from the container walls.

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Final Answer Summary:

1. Guard cells control water loss by dynamically opening and closing stomata based on turgor pressure.
2. Water moves from roots to leaves via xylem vessels due to transpiration pull, cohesion, adhesion, and root pressure.
3. Cohesion of water allows transpiration pull to be generated.
4. Adhesion of water to glass explains the concave shape of the meniscus in a graduated cylinder.
5. Mercury, being repelled by glass, forms a convex meniscus.

Boxed Final Answer:
\[
\boxed{\text{Guard cells control water loss by regulating stomatal openings; water moves via xylem due to transpiration pull; cohesion generates transpiration pull; adhesion explains the concave meniscus; mercury forms a convex meniscus.}}
\]