3.3 - transport in plants

why do plants require a transport system?

to ensure all cells receive a strong supply of the nutrients they require. especially important as a plant must be able to transport substances up their stem (against gravity).

relate the structure of xylem to its function

• long, continuous columns made of dead tissue, allowing transportation of water

• contain pits, allowing water to move sideways between vessels

• thickened with a tough substance, providing structural support

relate the structure of the phloem to its function

• sieve tube elements transports sugars around the plant

• companion cells designed for active transport of sugar into tubes

• plasmodesmata allows flow of substances between cytoplasm of different cells

describe the structure and function of the vascular system in the roots

• consists of xylem and phloem

• xylem arranged in an X shape to provide resistance against force. surrounded by endodermis, a water supply.

describe the structure and function of the vascular system in the stem

• consists of xylem and phloem

• xylem on the inside of the bundle to provide support and flexibility, phloem on the outside

• layer of meristem cells that produce new xylem and phloem tissue when required

describe the structure and function of the vascular system in the leaves

consists of xylem and phloem, forms the midrib and veins. involved in transport and support.

transpiration

• the evaporation of water from the leaves of a plant

• consequence of gaseous exchange; occurs when the plant opens the stomata to exchange oxygen and CO2.

name 5 factors that affect the rate of transpiration

• increased light increases transpiration

• increased temperature increases transpiration

• increased humidity decreases transpiration

• increased air movement increases transpiration

• waxy cuticle prevents transpiration

how measure transpiration rate?

• potometer

• plant cutting is placed in a water-filled tube that contains an air bubble

• rate of transpiration is calculated by measuring movement of air bubble over time

define water potential as a mechanism of movement in plants

• water potential = the tendency of water to move by osmosis, from high water potential to low. pure distilled water has the highest water potential of 0.

• this is the basis by which water moves to the areas it is needed within plants

what is the apoplastic pathway

water moves through the cell walls and intercellular spaces by mass flow. this pathway can only be used until water reaches the Casparian strip

what is the symplastic pathway

a method of osmosis through the root hair cells, where water moves through the cytoplasm via plasmodesmata. to begin this pathway, water must be actively transported into the cells.

explain the cohesion-tension theory

• water molecules form hydrogen bonds with each other, causing them to ‘stick’ together (cohesion)

• the surface tension of the water also creates a sticking effect

• therefore as water is lost through transpiration, more can be drawn up the stem from the roots

give adaptations of xerophytes that allow them to live in dry conditions

• small/rolled leaves

• densely packed mesophyll

• thick waxy cuticle

• stomata often closed

• hairs to trap moist air

give adaptations of hydrophytes that allow them to live in wet conditions

• thin or absent waxy cuticle

• stomata often open

• wide, flat leaves

• air spaces for buoyancy

summarise the mechanics of translocation

• sucrose produced in the leaves loaded into sieve tubes via active transport (requiring energy)

• lowers water potential, causing water to move in from xylem

• assimilates move along the sieve tube towards areas of lower hydrostatic pressure (sink). sucrose diffuses into surrounding cells where it is needed.