Transport in Plants

Why are transport systems needed in multicellular plants

diffusion is too slow over long distances due to large size, low SA:V ratio and metabolic demands

Why does large size require transport systems in plants

increases diffusion distance making diffusion inefficient

Why does a low SA:V ratio require transport systems

less surface area available for exchange relative to volume

What does the xylem transport

water and mineral ions

What is the direction of flow in xylem

unidirectional (from roots to leaves)

Are xylem vessels living or dead

dead

Why are xylem vessels hollow

no cytoplasm to reduce resistance to water flow

Why are end walls absent in xylem vessels

to form a continuous column for water transport

What is the role of lignin in xylem

strengthens walls, waterproofs and prevents collapse

What are pits in xylem

regions without lignin that allow lateral movement of water

What does the phloem transport

assimilates such as sucrose and amino acids

What is the direction of flow in phloem

bidirectional

Are sieve tube elements living or dead

living

Why do sieve tube elements lack a nucleus

to provide more space for transport

What are sieve plates

perforated end walls that allow flow between sieve tube elements

What is the role of companion cells

provide ATP and metabolic support to sieve tube elements

How are companion cells linked to sieve tube elements

via plasmodesmata

Where is xylem located in roots

in the centre (often star-shaped)

Where is phloem located in roots

between arms of xylem

How are vascular bundles arranged in dicot stems

in a ring

Where is xylem located in stems

on the inside of vascular bundles

Where is phloem located in stems

on the outside of vascular bundles

What is the cambium

a layer between xylem and phloem that produces new cells

Where are xylem and phloem found in leaves

in the veins

What is transpiration

loss of water vapour from plant leaves

Where does transpiration occur

through stomata

What causes transpiration

evaporation from mesophyll cell walls followed by diffusion out of stomata

Why is transpiration a consequence of gas exchange

stomata open for CO₂ allowing water vapour to escape

How does light affect transpiration

increases it by opening stomata

How does temperature affect transpiration

increases evaporation and diffusion rate

How does humidity affect transpiration

high humidity decreases transpiration

How does air movement affect transpiration

increases transpiration by removing water vapour

What does a potometer measure

rate of water uptake

How is transpiration rate estimated using a potometer

by measuring movement of an air bubble over time

What is a limitation of a potometer

water uptake does not equal water loss exactly

Why might a potometer give inaccurate results

leaks or air bubbles

How does water enter root hair cells

by osmosis

What is the apoplast pathway

movement of water through cell walls and spaces

Does the apoplast pathway cross membranes

no

What is the symplast pathway

movement of water through cytoplasm via plasmodesmata

Does the symplast pathway cross membranes

yes (once)

What is the Casparian strip

waterproof barrier in endodermis made of suberin

What is the function of the Casparian strip

blocks apoplast pathway and forces water into symplast

What is cohesion

attraction between water molecules via hydrogen bonding

What is adhesion

attraction between water molecules and xylem walls

What is the transpiration stream

movement of water up xylem due to evaporation at leaves

What creates the transpiration pull

tension caused by evaporation of water from leaves

What are xerophytes

plants adapted to dry environments

Give xerophyte adaptation

- thick cuticle to reduce water loss - sunken stomata to trap moisture - reduced leaf surface area (spines)

What are hydrophytes

plants adapted to aquatic environments

Give one hydrophyte adaptation

large air spaces for Buoyancy - thin cuticle

Where are stomata located in hydrophytes

mainly on upper surface

What is translocation

movement of assimilates in phloem

What is the main sugar transported in phloem

sucrose

What is a source in translocation

region where sucrose is produced (e.g. leaves)

What is a sink in translocation

region where sucrose is used or stored (e.g. roots)

What happens during active loading

sucrose is actively transported into phloem using ATP

How does active loading affect water potential

lowers it in phloem

Why does water enter phloem at the source

by osmosis from xylem due to low water potential

What pressure is created at the source

high hydrostatic pressure

What happens at the sink

sucrose is removed increasing water potential

Why does water leave phloem at the sink

by osmosis

What pressure is created at the sink

low hydrostatic pressure

What is the mass flow hypothesis

movement of phloem sap down a pressure gradient

Does translocation require energy

yes (ATP for active transport)