Xylem and Phloem Functions

Xylem and Phloem

Xylem

  • Transports water and mineral ions from roots to leaves.
  • Provides structural support to plants.
  • Xylem vessels are dead and have no cell contents. Their hollow, continuous tubes have lignified walls, giving low resistance to water flow and support.

Phloem

  • Transports sucrose and amino acids from leaves to other parts
  • Living cells
  • Moves substances from sources (leaves) to sinks (growing tissues)

Identification in Plant Parts

Plant Part Xylem Phloem
Roots Present in vascular bundles Present in vascular bundles
Stems Present in vascular bundles Present in vascular bundles
Leaves Veins contain xylem Veins contain phloem

Root, stem, and leaf vascular bundles showing xylem and phloem labelled

Root Hair Cells

Structure and Function

Root hair cells are specialized epidermal cells extending into soil. They have a long projection that increases surface area significantly.

Feature Description Function
Long projections (root hairs) Elongated extensions from epidermis cells Increases surface area 50-200 times for faster water/mineral uptake
Thin cell wall Short diffusion distance between soil solution and cell membrane Allows faster water uptake by osmosis
Large central vacuole Filled with concentrated cell sap Creates a lower water potential, drawing water in by osmosis
Mitochondria Abundant organelles present Provide energy for active transport of mineral ions

Root hair cell showing long projection, thin cell wall, large vacuole, and mitochondria

Water Uptake Process

  1. Water enters root hair cells via osmosis (soil water potential > cell sap water potential).
  2. Moves through the cortex by osmosis and enters xylem vessels.
  3. Moves through the xylem to the mesophyll of the leaf via transpiration pull.

Investigation: Water Pathway Using Stain

Purpose

  • Trace water movement through root to stem

Method

  1. Cut stem of plant (e.g., celery, daffodil)
  2. Place in water with red dye/stain
  3. Observe over time
  4. Cut across stem at different heights
  5. Observe stained vessels

Results

  • Xylem vessels show red staining.
  • Phloem does not stain.
  • This supports the conclusion that xylem transports water.

Dye tracing water from root hair through cortex and xylem to leaf mesophyll

Transpiration

Definition and Mechanism

Transpiration is the loss of water vapour from leaves to the atmosphere. Transpiration

Process

  1. Water evaporates from mesophyll cell surfaces into air spaces
  2. Water vapour diffuses through stomata to outside air
  3. Water vapour concentration is higher inside leaf than outside air

Stomata and Water Loss

Stomata Structure

  • Pores on leaf surface (mostly on lower epidermis)
  • Regulated by guard cells
  • Open for CO₂ entry, O₂ exit, and water vapour exit

Factors Affecting Transpiration Rate

Factor Effect Explanation
Temperature Higher temperature = faster rate Increased kinetic energy, more evaporation
Wind Speed Higher wind speed = faster rate Removes water vapour, maintains concentration gradient
Humidity Lower humidity = faster rate Larger concentration gradient between leaf and air
Light intensity — Enrichment Higher light can increase the rate Stomata may open more for CO₂ uptake

Investigation: temperature and wind speed

A potometer estimates transpiration rate by measuring water uptake by a leafy shoot.

  1. Cut a leafy shoot under water and fit it into a sealed potometer.
  2. Remove air bubbles and check that the apparatus does not leak.
  3. Change one factor, such as temperature or wind speed, while keeping leaf area, light intensity, humidity, shoot length, and time constant.
  4. Measure the distance moved by an air bubble in a fixed time, or calculate water uptake per unit time.
  5. Repeat each condition and calculate a mean rate.

A higher temperature or wind speed should usually increase water uptake and estimated transpiration rate. Wind removes moist air near the leaf, maintaining a water-vapour concentration gradient. A potometer measures water uptake, which is used as an estimate of transpiration rather than a direct measurement of water-vapour loss.

Potometer measuring water uptake by a leafy shoot under controlled temperature and wind conditions

Extended Understanding

Transpiration Pull

  • Water evaporates from mesophyll cells creating negative pressure
  • Creates tension pulling water up xylem from roots
  • Water molecules stick together via cohesion (hydrogen bonds)
  • Forms continuous column of water from roots to leaves

Water Vapour Loss Relation

  • Air spaces in spongy mesophyll hold water vapour
  • Stomata provide exit pathway
  • Rate depends on stomatal opening and environmental conditions.

Leaf transpiration showing evaporation from mesophyll surfaces into air spaces and diffusion through stomata

Wilting

  • Occurs when plant loses water faster than it can absorb
  • Cells lose turgor pressure
  • Plant becomes limp
  • Can be temporary (recoverable) or permanent (damaged)

Translocation

Definition

Translocation is the movement of sucrose and amino acids in phloem from sources to sinks.

Sources and Sinks

Sources

  • Organs that produce or export organic substances
  • Examples: Mature leaves (photosynthesis), storage organs breaking down reserves

Sinks

  • Organs that consume or store organic substances
  • Examples: Growing shoot tips, root tips, developing fruits, storage roots

Movement in Phloem

Loading (at source)

  1. Sucrose produced in leaves during photosynthesis
  2. Sucrose actively transported into companion cells
  3. Moves into sieve tube elements
  4. Lowers water potential in sieve tube
  5. Water enters by osmosis from xylem
  6. Creates high pressure pushing sap down phloem

Translocation

  • Mass flow of sap from sources to sinks.
  • Driven by pressure differences.

Unloading (at sink)

  1. Sucrose actively transported out of sieve tube
  2. Water potential increases
  3. Water leaves by osmosis
  4. Sap volume decreases

Sources and Sinks Dynamics Over Time

Daytime

  • Leaves are sources (photosynthesis produces sucrose)
  • Growing tissues are sinks (consuming sucrose for growth)

Nighttime

  • Leaves may become sinks (using stored reserves)
  • Storage organs may become sources (releasing stored sucrose)

Seasonal Changes

  • Spring: Roots are sources (mobilizing stored food).
  • Summer: Leaves are primary sources.
  • Autumn: Storage organs become sinks (accumulating reserves).

Sources and Sinks

Summary Table: Xylem vs Phloem

Feature Xylem Phloem
Substances Transported Water, mineral ions Sucrose, amino acids
Direction Roots → Leaves Sources → Sinks
Cell Type Dead cells Living cells
Energy Required No Yes (for loading/unloading)
Structure Hollow tubes with lignin Sieve tubes with companion cells
Function Water/mineral uptake, support Food distribution

Enrichment: practical applications

Agriculture

Irrigation Management

  • Understanding transpiration helps optimize watering
  • Water lost through transpiration must be replaced
  • Drought conditions reduce transpiration rate

Fertilizer Use

  • Nitrate ions absorbed by root hairs support protein production
  • Phloem transports these amino acids to growing tissues
  • Proper nutrient balance affects plant growth

Greenhouse Growing

  • Temperature and humidity control affects transpiration
  • Higher temperature increases transpiration rate
  • Humidity control prevents excessive water loss

Glossary

Term Definition
Xylem Tissue transporting water and minerals from roots
Phloem Tissue transporting sugars and amino acids
Root hair cells Specialized epidermal cells for water uptake
Transpiration Loss of water vapour from leaves
Mesophyll Inner leaf tissue where evaporation occurs
Stomata Pores on leaf surface for gas exchange
Guard cells Cells controlling stomatal opening
Translocation Movement of food in phloem from sources to sinks
Source Organ exporting organic substances
Sink Organ consuming or storing organic substances
Transpiration pull Tension pulling water up xylem
Cohesion Water molecules sticking together
Wilting Loss of turgor causing limp plant