Ch 2 · 7.2 Transport mechanisms · AS & A Level Biology 9700

Topic 2 of 2 in Transport in plants — AS & A Level Biology 9700 — 59 lessons.

Introduction

Water moves from soil to air through a plant with no pump, while sugars move from leaves to roots in separate tubes. Both rely on physical mechanisms that examiners expect in named stages: the two pathways across the root, cohesion-tension in the xylem, and active loading followed by mass flow in the phloem. (Syllabus section 7.2.)

Key ideas — What is carried, and where

Plants transport substances dissolved in water. Xylem carries water with dissolved mineral ions (such as nitrate and magnesium) up from the roots. Phloem carries assimilates, the organic compounds made by the plant, mainly sucrose and amino acids, dissolved in water, from sources (where they are made or released, e.g. photosynthesising leaves or a storage organ in spring) to sinks (where they are used or stored, e.g. growing tips, roots, fruits). This movement in phloem is translocation.

🧑‍🏫 Teacher's explanation

Think of a plant like a massive city with two distinct plumbing systems. The xylem is like a one-way water main that pulls water up from the ground to the skyscrapers (leaves) using suction. The phloem is like a two-way delivery service, moving food (sugars) from the kitchens (leaves) to the warehouses (roots or fruits) wherever they are needed most.

📖 Definitions

🔍 Worked example — Easy

A plant cell has a concentration of sucrose of 0.5 mol dm^-3 and the surrounding phloem sap has a concentration of 0.2 mol dm^-3. Calculate the percentage change in sucrose concentration if the phloem sap concentration increases to 0.4 mol dm^-3.

🔍 Worked example — Standard

Describe the pathway of water from the soil into the xylem, specifically mentioning the role of the endodermis.

🔍 Worked example — Exam-style

Explain how the 'cohesion-tension' mechanism allows water to move up the xylem.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — From soil to xylem: apoplast and symplast

Water enters root hair cells by osmosis, then crosses the root cortex by two pathways:

🧑‍🏫 Teacher's explanation

Imagine a large, busy city where water is like a delivery truck. One route is like driving through the open streets and alleyways between buildings (the apoplast); it is fast and direct, but anyone can wander in. The other route is like walking through the hallways inside the buildings (the symplast); it is slower because you have to pass through security checkpoints (cell membranes) at every door, but it allows the city to control exactly what enters the central hub.

📖 Definitions

🔍 Worked example — Easy

A student observes water moving through a root section. They note the water is moving through the cellulose cell walls and the spaces between cells without crossing any plasma membranes. Identify the pathway and state one characteristic of this route.

🔍 Worked example — Standard

Explain why the presence of the Casparian strip in the endodermis is essential for plant mineral uptake.

🔍 Worked example — Exam-style

Compare the composition and function of the xylem walls to the endodermis walls in terms of waterproofing.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Name the waterproof substance found in the Casparian strip. [1] (b) Describe how the Casparian strip affects the movement of water and mineral ions from the soil to the xylem. [3] [4 marks]

Key ideas — Transpiration and the transpiration pull

Transpiration is the loss of water vapour from a plant. Water evaporates from the wet cell walls of mesophyll cells into the air spaces inside the leaf, then water vapour diffuses out through open stomata to the atmosphere, down a water vapour potential gradient.

🧑‍🏫 Teacher's explanation

Imagine drinking a thick milkshake through a straw. When you suck on the straw, you create a low pressure (tension) in your mouth. This tension pulls the liquid up the straw. In a plant, the 'straw' is the xylem, and the 'mouth' is the atmosphere. As water evaporates from the leaf surfaces, it creates a negative pressure that pulls the entire water column upwards.

📖 Definitions

🔍 Worked example — Easy

A plant leaf has a surface area of 50 mm^2. If the rate of transpiration is 0.2 mg/mm^2/h, calculate the total mass of water lost in 5 hours.

🔍 Worked example — Standard

Explain the role of hydrogen bonding in the movement of water through the xylem.

🔍 Worked example — Exam-style

A student observes that a plant in a very dry environment (low humidity) loses water much faster than a plant in a humid environment. Explain this observation in terms of water vapour potential.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Xerophytes: leaves that save water

Xerophytes are plants adapted to dry habitats, such as marram grass (Ammophila) on sand dunes and cacti. Look for these features on a transverse section, and annotate each with how it reduces transpiration:

Key ideas — Loading the phloem: companion cells

Sucrose is loaded into sieve tubes against its concentration gradient, using energy from ATP in the companion cells:

🧑‍🏫 Teacher's explanation

Imagine a crowded nightclub where everyone wants to get in. The door is narrow, and it is already packed inside. To get more people in, the bouncer (the proton pump) has to physically push people out of the club into the street, creating a huge crowd outside. Because there is such a massive crowd outside, people are desperate to get back in, so they rush through the door, but they can only enter if they grab a friend (sucrose) and squeeze through together.

📖 Definitions

🔍 Worked example — Easy

A student observes that companion cells have a significantly higher density of mitochondria compared to sieve tube elements. Explain the functional necessity of this observation.

🔍 Worked example — Standard

Calculate the percentage increase in sucrose concentration in a sieve tube if the concentration in the companion cell increases from 0.2 mol/dm^3 to 0.8 mol/dm^3.

🔍 Worked example — Exam-style

Describe the mechanism by which sucrose is loaded into the phloem sieve tube element from the companion cell.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Mass flow in the phloem

Loading sucrose into the sieve tube at the source lowers the water potential there, so water enters by osmosis from the nearby xylem. This raises the hydrostatic pressure at the source end. At the sink, sucrose is removed (to be used in respiration or stored as starch), the water potential rises, water leaves by osmosis and the hydrostatic pressure falls. The solution flows by mass flow down this hydrostatic pressure gradient, from source to sink, carrying all the dissolved assimilates with it. Because sources and sinks can change (a storage root is a sink in summer and a source in spring), phloem can carry solution in either direction.

🧑‍🏫 Teacher's explanation

Think of the phloem like a pressurized plumbing system in a house. In a kitchen (the source), you turn on a tap and fill a bucket with thick syrup; the pressure in the pipes rises. In the living room (the sink), someone is constantly scooping syrup out of a bucket. Because the pressure is high in the kitchen and low in the living room, the syrup is pushed through the pipes from one room to the other.

📖 Definitions

🔍 Worked example — Easy

A plant cell in the phloem sieve tube has a sucrose concentration of 0.5 mol dm^-3. The surrounding xylem has a water potential of -100 kPa. If the sucrose concentration in the sieve tube lowers the water potential to -150 kPa, describe the direction of water movement.

Ch 2 · 7.2 Transport mechanisms · AS & A Level Biology 9700

All chapters in Transport in plants