Science - Combined (0653) | Transport in plants

Transport in Plants (Biology B8): The Plant's Delivery System

Hello future scientists! This chapter is all about how plants manage their internal transport—moving essential ingredients like water and food from where they are absorbed or made to where they are needed. Think of it as studying the plant's plumbing and delivery network. Understanding this is key to grasping how plants grow, survive, and perform photosynthesis!

Section 1: The Plant's Vascular System - Xylem and Phloem (B8.1)

Plants have two main types of transport tissue, which are bundled together in structures called vascular bundles. These tissues work like parallel pipes running through the roots, stems, and leaves.

The Xylem: Water and Support

• Function: The primary role of xylem is the transport of water and mineral ions (like nitrates and phosphates) from the roots up to the leaves.
• Secondary Function: Xylem tissue also provides structural support to the plant, helping it stand upright.
• Structure Reminder: Xylem vessels are essentially dead, hollow tubes with thickened, lignified walls, making them strong and waterproof.

Memory Aid: Think of the letter X in Xylem. It sounds a bit like 'Aqua' (or just remember Xylem transports Water!

The Phloem: Food Delivery

• Function: The main role of phloem is the transport of dissolved sugars, primarily sucrose, and amino acids. This process is called translocation.
• Source to Sink: Sugars move from where they are made (the 'source', usually the leaves) to where they are stored or used (the 'sink', such as roots, buds, or growing fruits).
• Structure Reminder: Phloem tissue is made of living cells (sieve tubes and companion cells).

Memory Aid: Think of Phloem carrying Phood (sucrose and amino acids).

Location of Vascular Bundles in Dicotyledonous Plants (Core)

You need to be able to identify the position of xylem and phloem in cross-sections of non-woody dicotyledonous plants (like beans or sunflowers):

In diagrams or images, look for the following patterns:

1. In the Root: The vascular tissue is centrally located, often forming a star shape. The xylem is usually in the very centre, and the phloem is positioned between the arms of the xylem.
2. In the Stem: The vascular bundles are arranged in a distinct ring near the edge. The xylem is typically found towards the inside of the stem, and the phloem is towards the outside.
3. In the Leaf (Veins): The bundles are part of the veins. The xylem is on top (closer to the upper surface), and the phloem is on the bottom (closer to the lower surface).

Section 2: Getting Water In - Root Uptake (B8.2)

The journey of water into the plant begins in the roots. The key specialised cells responsible for absorbing water are the root hair cells.

Root Hair Cells and Absorption

• Structure: A root hair cell is a specialised epidermal cell found near the tip of the root. It has a long, narrow projection, which looks like a tiny hair.
• Core Function: Their primary function is the absorption of water and mineral ions from the soil.

The Importance of Surface Area (Supplement)

The "hair" structure is not just for decoration! It dramatically increases the surface area of the root that is exposed to the soil water. A larger surface area means the root can absorb water and mineral ions much faster and more efficiently.

The Pathway of Water (Core)

Once absorbed, water follows a specific path through the root to reach the transport system:

1. Water is absorbed by the root hair cells from the soil (mainly by osmosis, as the cell sap is usually more concentrated than the soil water).
2. The water moves through the root cortex cells (the main body of the root).
3. It then enters the xylem vessels in the centre of the root.
4. Water is pulled up the stem and into the leaves, finally reaching the mesophyll cells where photosynthesis occurs.

Did you know? A single rye plant can have 14 billion root hairs, creating a total surface area of over 400 square meters! That’s essential for drawing in enough water.

Section 3: Water Loss - Transpiration (B8.3)

Plants need water, but they also constantly lose it. This process is crucial for cooling and pulling new water up from the roots.

Defining Transpiration (Core)

Transpiration is defined as the loss of water vapour from the plant leaves (and sometimes stems) to the atmosphere. This usually happens through the tiny pores called stomata, which are mostly located on the underside of the leaf.

Why is Transpiration Important?

The evaporation of water from the leaves creates a "pull" or suction force that draws water upwards through the xylem vessels from the roots. This is called the transpiration stream.

Factors Affecting the Rate of Transpiration (Supplement)

The rate at which a plant loses water is influenced by environmental conditions. You need to know how two key factors affect the rate:

1. Temperature

• Effect: As temperature increases, the rate of transpiration increases.
• Explanation: Higher temperatures provide the water molecules inside the leaf with more kinetic energy. This means they evaporate faster (turn into vapour) from the moist surfaces of the mesophyll cells and diffuse out through the stomata faster.

2. Wind Speed

• Effect: As wind speed increases, the rate of transpiration increases.
• Explanation: When water vapour leaves the stomata, it forms a layer of humid air around the leaf surface. This layer reduces the concentration gradient, slowing down diffusion. Wind blows this humid air away, keeping the air around the leaf dry. This maintains a steep concentration gradient, meaning water vapour diffuses out much faster.

Analogy: Drying Clothes
Think about drying your clothes outside. They dry fastest on a hot (high temperature) and windy (high wind speed) day. Heat speeds up the evaporation, and wind carries the wet air away, just like in a plant leaf!

Investigating Transpiration Rate (Practical Context)

To measure the rate of transpiration, scientists often use an instrument called a potometer. Although the potometer measures the rate of water uptake by the shoot, we assume that almost all the water absorbed is lost through transpiration, so uptake rate is a good measure of transpiration rate.

• In an experiment, to test the effect of high wind speed, you would place a fan near the plant connected to the potometer.
• To test the effect of temperature, you might place the plant in a warm room or under a heat lamp.

Congratulations! You have mastered how plants move water and food throughout their bodies. Keep these functions and factors clear in your mind, and you'll ace your transport questions!