CORE Biology (9221) Study Notes: Exchange and Transport in Plants

A Friendly Introduction: Getting the Ingredients!

Welcome to a fascinating chapter! We’ve already learned that plants are amazing energy factories (photosynthesis). But how do they get all the raw materials needed to run this factory?

This chapter, which is key to understanding Bioenergetics, focuses on how plants perform two essential jobs:
1. Exchange: Swapping gases (getting \(CO_2\), releasing \(O_2\)).
2. Transport: Moving water, minerals, and food around the plant.

Don't worry if some terms look new; we will break them down step-by-step!

1. The Need for Exchange in Plants

Plants are constantly exchanging substances with their environment. They need specific things for photosynthesis and respiration, and they need to get rid of waste.

Key Exchanges Needed:
  • Input: Carbon Dioxide (\(CO_2\)): Essential raw material for making glucose (photosynthesis).
  • Input: Water (\(H_2O\)) and Minerals: Absorbed from the soil. Water is needed for photosynthesis and to keep the plant rigid.
  • Output: Oxygen (\(O_2\)): A waste product of photosynthesis (but also needed for respiration!).
  • Output: Water Vapour: Lost through transpiration (a necessary side effect of gas exchange).

2. Gas Exchange: The Role of Stomata

Plants take in \(CO_2\) from the air and release \(O_2\) mainly through tiny openings found primarily on the underside of leaves. These openings are called stomata (singular: stoma).

The Structure of a Stoma (The Plant’s Door)

Each stoma is surrounded by two specialised cells called guard cells.

  • When the plant has enough water, the guard cells become turgid (swollen) and curve, pulling the stoma open. This allows \(CO_2\) to rush in.
  • When the plant is losing too much water (or is dehydrated), the guard cells become flaccid (limp) and close the stoma. This stops water loss, but it also stops \(CO_2\) uptake, halting photosynthesis.

Analogy: Think of the stoma like a bank vault door. The guard cells are the security guards. They open the door to let \(CO_2\) (money) in, but they slam it shut if there’s a risk of losing too much water (a crucial resource).

Quick Review: Plants have a tricky trade-off. They must open the stomata to get \(CO_2\) for energy production, but opening them causes water loss.

3. Transport Systems: Xylem and Phloem

Once water, minerals, and glucose are acquired or created, they need to be moved around. Plants use two major types of tubes (or vessels) for transport. These vessels run all the way from the roots up to the leaves.

3a. The Xylem: Water and Mineral Transport

The xylem vessels are responsible for transporting water and dissolved mineral ions (like nitrates and magnesium) from the roots up to the stem and leaves.

  • Structure: Xylem vessels are long, hollow tubes made of dead cells. They have thick, woody walls (lignin) which provide structural support for the plant.
  • Direction of Flow: Always one way—from the roots UP to the leaves.
  • Function: Transporting water (essential for photosynthesis and maintaining cell turgor) and providing support.

Memory Tip: Xylem sounds like "Hi-lem" – it carries water **HIGH** up the plant.

3b. The Phloem: Food Transport

The phloem vessels transport the products of photosynthesis (mainly sugars, like sucrose) from where they are made (usually the leaves) to where they are needed for energy or storage (like the roots, fruits, or growing tips).

  • Structure: Phloem vessels are made of living cells. They contain sieve tubes and companion cells.
  • Direction of Flow: Two ways—up or down, depending on where the sugars are needed. This process is called translocation.
  • Function: Moving sugars for respiration and growth throughout the whole plant.

Key Takeaway Summary:
Xylem: Water, Upwards, Dead cells.
Phloem: Food (Sugars), Up and Down, Living cells (Translocation).

4. How Water and Minerals Enter the Plant

The roots are the plant's absorption system. They have specialised structures to maximise the uptake of water and vital minerals from the soil.

Root Hair Cells (Maximising Surface Area)

The roots are covered in thousands of tiny projections called root hair cells. These are simply extensions of epidermal cells.
Their function is crucial: they provide a massive surface area for absorption, making water and mineral uptake extremely efficient.

Water Uptake (Osmosis)

Water moves into the root hair cells by osmosis.

  1. The concentration of water molecules in the soil is usually higher than inside the cytoplasm of the root hair cell.
  2. Because of this concentration difference, water moves across the partially permeable cell membrane.
  3. Water moves from the high water potential (soil) to the low water potential (root cell).

Remember: Osmosis is a passive process—it doesn't require the plant to use energy (ATP).

Mineral Uptake (Active Transport)

Plants need specific mineral ions (like nitrates for proteins, magnesium for chlorophyll). Sometimes the concentration of these minerals is much lower in the soil than in the root cells.

To move minerals against their concentration gradient (from low concentration in soil to high concentration in the root), the plant must use active transport.

  • Requirement: Active transport requires energy (ATP), which is supplied by respiration in the root cells.

Analogy:
Water uptake (Osmosis) is like floating downstream—no energy needed.
Mineral uptake (Active Transport) is like swimming upstream—requires lots of energy (ATP) and effort!

5. Transpiration: The Engine of Water Movement

Once water is in the xylem, how does it get all the way up to the highest leaves? The answer is transpiration.

What is Transpiration?

Transpiration is the loss of water vapour from the surfaces of the plant, mainly through the stomata in the leaves.

Even though the plant tries to conserve water, this loss creates a vital force called the transpiration stream.

The Mechanism (The Transpiration Stream)
  1. Water evaporates from the moist surfaces of the spongy mesophyll cells inside the leaf, turning into water vapour.
  2. This water vapour diffuses out of the leaf through the open stomata.
  3. The loss of water from the leaf cells creates a lower pressure (a 'pull' or suction).
  4. This pulling force is transmitted down the xylem vessels, drawing water continuously up from the roots.
  5. This movement of water (root uptake + xylem transport + leaf evaporation) is known as the transpiration stream.

Did you know? Transpiration is so powerful that it can lift water up the trunk of the tallest trees, acting like a giant straw!

Factors Affecting the Rate of Transpiration

The rate at which water evaporates and leaves the plant is affected by the surrounding environment. If the rate increases, the plant needs to absorb water faster.

Factor Effect (If Factor Increases) Reason
Temperature Increases rate Higher heat provides more energy for water molecules to evaporate (turn to vapour).
Humidity Decreases rate High humidity means the air already holds lots of water vapour, reducing the concentration gradient between the leaf and the air.
Air Movement (Wind) Increases rate Wind sweeps away the saturated layer of air around the leaf, maintaining a steep concentration gradient for water vapour loss.
Light Intensity Increases rate Higher light intensity causes the stomata to open wider (to get \(CO_2\) for photosynthesis), leading to more water loss.

Common Mistake to Avoid: Students often confuse transpiration (water loss) with translocation (sugar movement). Remember: Transpiration = Removal of water. Translocation = Transport of food.

Key Takeaway: Transpiration drives the movement of water and minerals needed for photosynthesis and structure, linking root absorption directly to leaf exchange.


You've reached the end of the chapter notes! Take a deep breath—you have covered the entire system plants use to fuel their growth and energy needs. Well done!