The Transport of Substances in Plants

TRANSPORT Transport of Substances in Plants

Necessity • Transport substances to all parts of plant  Water – for photosynthesis and cellular metabolism  Mineral ions – for healthy growth and development  Organic food materials – for cellular metabolism or storage

• Problem  Small TSA/V ratio  Long distances

• Overcome  Internal system of tubes = VASCULAR SYSTEM

The Vascular System • The vascular system involved in transporting substances in plants – except movement of gases (diffusion)

• Consist of 2 types of tissues: xylem and phloem – which function as two separate transport system.

Monocot plant leaf:

stem:

root:

Dicot plant

Xylem • Transports water and dissolved mineral ions from the roots to the upper parts of the plant – Against gravitational pull

• Gives mechanical support to woody plants

Xylem adaptation • Consists of 4 types of cells:  Xylem vessels    

Long, hollow, continuous tube Dead cell, no protoplasm Thickened by lignin (patterns: pits, spiral, ring) Numerous pits (holes), end walls open

 Tracheids  Dead cells, hollow  Longer and smaller than xylem vessels  Pointed end and pitted, end walls breaks down in pits

 Parenchyma – food storage  Fibres - support

Phloem • Transports organic substances (e.g. sucrose, amino acids) synthesised mainly in the leaves to other parts of plant – Used in cellular metabolism • Growth (shoot, root, buds, fruits) • Respiration

– Stored in in storage organ • e.g. tuber

Phloem adaptation • Consists of

 Sieve tubes

• Living cells (nucleus disintegrates during maturation, thin layer of cytoplasm, small mitochondria) • Cylindrical, elongated, end-to-end • End walls perforated to form sieve plates

 Companion cells

(transport food into tubes)

• Shorter than sieve tubes • Living cells (with nucleus, many mitochondria) • Adjacent to and support the function of sieve tube

 Parenchyma – food storage  Fibres - support

Transport of substances: Food (Translocation) • The movement of sucrose and other organic materials within the plant body • Primarily through phloem (sieve tube) • Drives by concentration (pressure) gradients • Importance: – Plant’s survival – Store and convert sucrose to other sugars

Bark Ringing • Removal of a ring of tissue external to the xylem from around the trunk of a woody plant. – Removal of phloem

Observation Tissue above the ring swells up Tissue below the ring withers

The upper part of plants lives normally

Inference • Tissue removed is phloem • Disrupt flow of food to root • Accumulation causes swelling at the upper part • Insufficient food cause the lower part to wither • Xylem is not removed • Flow of water is not disrupted • The upper part do not wilt

Transport of substances: Water and minerals • Water by osmosis • Minerals by active transport • Aided by three factors: – Root pressure – Capillary action • Cohesion and adhesion

– Transpiration pull

1) Root Pressure • The process that forces the water absorbed from the soil to move through the roots and up the stem of a plant • Caused by osmosis of water and active transport of mineral ions into the root xylem

The mechanism: • Cell sap of root hair cells is hypertonic to soil water • Water enter by osmosis • Cell sap is diluted, hypotonic to adjacent cells • Water moves to the next cells until crosses cortex and endodermis • Creates root pressure to push water into xylem • Helped by the active transport of minerals into xylem that increase osmotic pressure

Guttation • In small plants, water may be pushed out of special pores at leaf (hydathodes) • Usually seen early morning

2) Capillary action • Chemical forces that move water as a continuous column rather than as individual molecules. • Combination of 2 forces due to hydrogen bonding: cohesion & adhesion • The forces can be overcome by gravity.

Cohesion and Adhesion • Cohesion: attraction between water molecules, prevents break of water column • Adhesion: attraction between water molecules and xylem walls, prevents slipping of molecules

3) Transpiration pull • When water evaporates, it creates an empty space which is filled continuously. • As the water moves, more water is absorbed to replace the evaporated water (transpiration pull)

Conceptualise!

Transpiration • The process of water loss (as water vapour) from plants to the atmosphere due to evaporation (99% of water intake) – Through the stomata in leaves (90%) – Through cuticle (5%) or lenticels (5%)

• Importance – Creates transpiration pull – Provide cooling effect and turgor pressure – Maintain osmotic pressure

The process: • Water absorbed by roots to mesophyl cells • Heat from Sun cause water to evaporate to air spaces • Water vapour diffuse through stomata (traspiration)

Regulation by stomata: Open • During day – Photosynthesis produce glucose – K+ pumped into guard cells – Accumulation of glucose and K+ increase osmotic pressure – Water enters by osmosis

– Guard cells become turgid, curved outwards, stoma opens – Transpiration occurs

Regulation by stomata: Close • At night/dark – No photosynthesis, no glucose produced – K+ moves out to epidermal cells – Osmotic pressure decrease – Water diffuses out by osmosis

– Guard cells become flaccid, stoma closes – Transpiration stops

Factors affecting transpiration rate: Air Movement

Transpiration rate

• More air movement, easier evaporation

Air movement

Factors affecting transpiration rate: Temperature

Transpiration rate

• Higher temperature, higher rate of transpiration

Temperature

Factors affecting transpiration rate: Light intensity

Light intensity

Transpiration rate

• Higher intensity, higher rate of transpiration

Time

Light intensity Transpiration rate

Factors affecting transpiration rate: Relative humidity

Transpiration rate

• Higher surrounding humidity, lower rate of transpiration

Relative humidity