LC Biology Notes 2023

Transport, Storage, and Gas Exchange in Plants

Root Hair

• Water Entry: Water enters root hairs by osmosis, as soil water is less concentrated than the cytoplasm.
• Movement to Xylem: Water travels to the xylem by diffusion (without a membrane).
• Adaptation: Root hairs are thin-walled and numerous, aiding in water intake.
• Mineral Intake: Minerals enter the root by diffusion.

The Cohesion-Tension Model

• Developers: Dixon & Joly (Irish scientists).
• Purpose: Explains how water moves against gravity in plants.

Process

1. Water enters the plant by osmosis, increasing pressure in the xylem (roots).
2. Transpiration from stomata in leaves causes evaporation of water.
3. Water molecules exhibit cohesion (cling to one another).
4. Transpiration and cohesion pull water upwards, creating tension in the xylem.
5. Water molecules also adhere to xylem walls (adhesion).

Additional Details

• Xylem contains lignin and is narrow, providing strength against pressure.
• Transpiration rate is highest in hot, sunny weather.

Disadvantages of Transpiration

• Excessive transpiration can lead to wilting.
• Plant Response: Guard cells close the stomata to prevent water loss.

The Leaf

Palisade Layer

• Function: Main site for photosynthesis.

Stomata

• Control: Guard cells regulate opening and closing.
• Location: Found on the underside of leaves to reduce water loss.
• Gas Exchange: Diffusion of gases occurs here.
• Environmental Influence: Conditions affect guard cell activity.

Stomata Open/Close Mechanism

• Open: Low CO2 concentration (daytime).
• Closed: High CO2 concentration (night) and drought conditions.

Carbon Dioxide in the Plant

• Entry: CO2 enters the leaf by diffusion and through lenticels in the stem.
• Production: CO2 is produced by respiration within the plant.
• Transpiration: More stomata increase the rate of transpiration.

Note

• Stomatal opening is influenced by CO2 levels, not light/darkness, although CO2 levels are linked to the day/night cycle.

Food Storage Organs in Plants

• Modified Roots: Carrots
• Modified Stems: Potatoes
• Modified Leaves: Onions

<span>An autotrophic organism makes its own food.</span>

<span>Plants make their own food in Photosynthesis.</span>

<span>Why do plants need a transport system?</span>

<span>To provide the materials needed for various plant metabolic processes including</span>

<span>____________ , __________, ______________and</span>

<span>________________</span>

<span>What materials are transported in plants?</span>

<span>1.</span>

<span>2.</span>

<span>3.</span>

<span>4.</span>

<span>Water Uptake</span>

<span>Water enters the root hairs by osmosis. Root hairs are</span>

<span>adapted for this process by having thin walls, no waxy</span>

<span>cuticle, and a large surface area.</span>

<span>Water moves by diffusion from the root hairs into the</span>

<span>ground tissue and eventually reaches the xylem.</span>

<span>Upward movement of water</span>

<span>Two mechanisms combine to cause upward movement of water through the stem in the</span>

<span>xylem.</span>

<span>1. Root pressure - As water moves into the root by osmosis it builds up a</span>

<span>pressure that pushes water up the xylem.</span>

<span>2. Transpiration - As water evaporates from the leaf by transpiration, more</span>

<span>water is pulled upwards through the xylem into the leaf.</span>

<span>Transpiration is the loss of water vapour from the leaves of a plant.</span>

<span>Control of water loss from leaf</span>

<span>1. Cuticle: a thick cuticle reduces transpiration.</span>

<span>2. Stomata: Mostly on lower surface of leaf. Helps to</span>

<span>reduce water loss by transpiration because rate of</span>

<span>evaporation is higher on the upper surface.</span>

<span>3. Stomatal opening /closing e.g. open as light intensity</span>

<span>increases and close at night - this reduces water loss.</span>

<span>Environmental conditions can cause stomata to close during the day – if a plant</span>

<span>loses too much water and high temperatures.Mineral uptake and transport</span>

<span>Needed for synthesizing plant components e.g. calcium (helps form cell walls),</span>

<span>magnesium (needed for formation of chlorophyll), nitrates, phosphates and potassium</span>

<span>ions. They enter root hairs, dissolved in water, by active transport and are transported</span>

<span>by xylem.</span>

<span>Carbon dioxide uptake and transport</span>

<span>CO2 is needed for photosynthesis; this takes place in the ground tissue of the leaf</span>

<span>(palisade and spongy mesophyll). There are two sources of CO2.</span>

<span>• Most diffuses in through the stomata from the atmosphere. It then diffuses</span>

<span>from the air spaces into the ground tissue.</span>

<span>• Produced in the leaf during respiration</span>

<span>The fate of the products of photosynthesis</span>

<span>Oxygen produced in photosynthesis diffuses into the airspaces. It can then be released</span>

<span>into the atmosphere or used in the leaf cells for respiration.</span>

<span>Glucose formed by photosynthesis can be used immediately in respiration,</span>

<span>stored as starch or translocated as sucrose in phloem sieve tubes (sap).Food storage organs in plants</span>

<span>• Modified root – in some plants the tap root can become swollen with stored food</span>

<span>e.g. carrots and turnips.</span>

<span>• Modified stem – potato plants have an underground stem system, the tips of the</span>

<span>stems become swollen with food. These are called stem tubers.</span>

<span>• Modified leaves – an onion and daffodil bulb is an underground stem, reduced in</span>

<span>size. Fleshy leaves surround a central bud.</span>

<span>The cohesion-tension model of water transport</span>

<span>• Put forward by Dixon and Joly, two Irish scientists.</span>

<span>• Main mechanism for upward movement of water in plants.</span>

<span>• Cohesion – sticking of similar molecules together, water molecules stick</span>

<span>together.</span>

<span>• Adhesion – when different molecules stick together. Water adheres to the walls</span>

<span>of xylem.</span>

<span>Outline of cohesion-tension model</span>

<span>1. Water evaporates from the xylem into the air spaces of the leaf. As transpiration</span>

<span>pulls each molecule out, the next is pulled too, due to high cohesion. This pull continues</span>

<span>down through the stem to the root.</span>

<span>2. When the water molecules are pulled, the column of water is stretched and is</span>

<span>described as under tension.</span>

<span>3. The cohesion-tension model accounts for how water is pulled up to great heights.</span>

<span>4. Stomata are open during daylight, so transpiration occurs. The tension in the water</span>

<span>column causes the xylem and also the stem to become narrower. Lignin stops the xylem</span>

<span>collapsing inwards.</span>

<span>5. When transpiration stops, the xylem return to their original shape.</span>

<span>Gas exchange in the leaf</span>

<span>• Plants need CO2 for photosynthesis.</span>

<span>• The CO2 diffuses from the atmosphere in through the stomata, to the</span>

<span>airspaces and then to the mesophyll cells.</span>

<span>• CO2 is used in photosynthesis. Food and O2 are produced.</span>

<span>• O2 diffuses out of the cells, into the airspaces and out through the stomata.</span>

<span>Though some may be used by the cell in respiration.</span>

<span>• H2O produced during respiration also diffuses out of the leaf (transpiration).Control of stomatal opening</span>

<span>• High levels of CO2 at night cause the stomata to close. No light at night means</span>

<span>rate of photosynthesis drops and less CO2 is absorbed, high levels of CO2 in the</span>

<span>air spaces.</span>

<span>• Low levels of CO2 by day cause the stomata to open. Photosynthesis occurs</span>

<span>during daylight; CO2 is absorbed and levels drop</span>