4.2.3.2 Plant Organ System

Structure of Xylem

• Dead cells (no organelles + cytoplasm) arranged end to end forming a continuous system of vessels -

  • free passage of water

• Hollow

  • more water can be drawn up

• Cell walls strengthened by lignin (impermeable)

What does the xylem do?

Carries water + mineral ions from the roots to the stems and leaves

• water drawn up osmosis

• minerals taken in by active transport

Structure of Phloem

• Columns of elongated living cells arranged end to end

• Small pores in the end of walls (sieve plates)

  • phloem sap + dissolved sugars to flow through

Sieve Tube Cells specialistion

• Sieve tube cells have perforated ends called sieve plates

  • phloem sap + dissolved sugars to move up and down

• Companion cells with mitochondria in tubes to provide energy

  • phloem needs energy to carry out translocation

• Very few organelles and no nucleus

  • reduce resistance and allow easier flow of sap

• Thin cell wall

  • helps with efficient translocation

What does the phloem do

Transports foods substances made by photosynthesis to the rest of the plant for:

• growing parts for immediate usage

• storage organs such as bulbs

• developing seeds

• respiration

What direction does the transport in the phloem go?

both directions

translocation

• movement of sugars + amino acids made in photosynthesis to the rest of the plant

• in phloem cells

• sources to sinks

cell sap

Consists of soluble products of photosynthesis

• sucrose

• amino cids

Winter transolcation transport

From storage organs → rest of plants (respiration)

Spring translocation transport

Roots (source) → growing areas (sinks)

Summer translocation transport

Leaves (source) → roots (sinks, storing sugars as starch)

Transpiration

Loss of water vapour from the plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata

Adaptations of xylem

• Lignin deposited in cell walls

  • so xylem cells die

  • become hollow (losing cytoplasm + organelles) and join end to end to form a continuous tube for water + mineral ions to travel through the roots

• Cell walls strengthened by lignin

  • withstand pressure of water movement

What does direction does the transport in xylem go?

One direction - from roots to leaves

Transpiration Stream

Continuous upward flow of water + dissolved materials from the roots to the leaves through the xylem to replace water lost due to transpiration

• Cohesion keeps water molecules stuck together (hydrogen bonding)

• Evaporation at the leaves creates a pulling force

Function of transpiration

• transporting mineral ions

• providing water to keep cells turgid

• provide water to leaf cells for photosynthesis

• cools leaf as water evaporates

Factors affecting transpiration

• Air movement

• Humidity

• Light Intensity

• Temperature

How does air movement affect rate of transpiration?

• High air movement gives more transpiration

• Good airflow - removes water vapour from air surrounding leaf

  • sets up conc. gradient between leaf + air, increasing water loss

Humidity

• High humidity gives less transpiration

• Measure of moisture in air - when air holds more water vapour

  • conc, gradient is weaker so less water is lost

Light Intensity

• High light intensity gives more transpiration

• Guard cells are responsive to light intensity, when it’s high they are turgid

  • stomata open allowing water to be lost

Temperature

• High temperature gives more transpiration

• Higher temp - more KE, water evaporates from mesophyll and diffuses away faster

  • transpiration occurs at faster rate

When lots of water, guard cells….

• Become turgid due to osmosis

  • Stomata they surround open and air can come in and out

• But water lost via transpiration

When less water, guard cells….

• Become flaccid as they lose water via osmosis

  • pull together and close stomata to reduce water loss (transpiration)

Why are stomata on the underside of the leaf?

• Cooler and shaded (less light intensity)

  • less transpiration - water loss

Role of stomata

Control gas exchange + water loss

At day, guard cells

stay open to get CO2 for food

At night, guard cells,

close as no need for CO2 (no light for photosynthesis) + save water

If plant loses too much water?

guard cells close to prevent transpiration though may affect food production

What are the steps of the transpiration stream?

1. water is absorbed in the roots through osmosis

2. water travels through the plant (xylem)

3. water evaporates from the surface of the cells (and becomes a gas)

4. water vapour diffuses out of the leaf through the stomata

differences of xylem + phloem

• xylem

  • dead cells

  • transports water and minerals

  • up from the roots to leaves (one direction)

• phloem

  • living cells

  • transports products of photosynthesis

  • both directions