Transport in Plants Flashcards
What is the xylem?
vascular tissue that transports water and mineral salts, from roots to leaves in the transpiration stream
what is the phloem?
vascular tissue, transports organic solutes from sources to sinks by translocation
what is transpiration?
the evaporation of water from aerial parts of the plant, i.e leaves, petals, and lenticels
what is translocation?
the bidirectional movement of organic solutes like sucrose and amino acids in the phloem
what is a source?
where carbohydrate is made via photosynthesis. Leaves
what is a sink?
where carbohydrate is stored or used for growth. Roots
What are angiosperms?
flowering plants
What cell types does xylem consist of?
vessels (angiosperms)
tracheids (ferns, conifers, angiosperms)
Fibres
Parenchyma
What cell types do Phloem consist of?
Sieve tube elements
Companion cells
Fibres
Parenchyma
Transfer Cells
What is the structure of xylem vessels?
they are straight tubes, so water moves up fast
what is the structure of tracheids?
they are spindle shaped so water takes a slower twisting route up
What is lignification?
Vessels and tracheids have their cellulose cell walls layered with lignin, it laid down in different patterns
What are the properties of lignin?
It is rigid and waterproof
What is the purpose of lignification?
to thicken cell walls
to strengthen cell walls
How does thickening of cell walls in lignification help the plant?
it gives the plant extra support
How does strengthening of cell walls during lignification help the plant?
prevents their collapse
through cohesion tension theory
What is the cohesion tension theory?
Transpiration pulls produce water tension in the xylem which pull the water upward and the cohesive and adhesive forces maintain the water column in the xylem.
How does water enter/leave vessels and tracheids?
Gaps in the lignin thickening
Bordered pits (reinforced pores)
What is callose?
plant polysaccharide acts as a temporary cell wall and is laid down in areas of stress or damage
what are sieve tube elements?
no nucleus
end cell walls break to form sieve plates to allow cell-to-cell transport of organic solutes
what are companion cells?
have chloroplasts, large nucleus, mitochondria) retain cytoplasmic connections with sieve tube elements via plasmodesmata
What are sites of water loss in plants?
Leaves (most via stomata and some across cuticle
Flowers
Stems or Lenticels
What are environmental factors that can affect transpiration rate?
Humidity
Wind
Temperature
Sunlight
What are internal factors that can affect transpiration rate?
Stomatal density
Thickness of waxy cuticle
Leaf surface area
How does water move through a plant (4 stages)?
- Uptake by roots
- Across cortex and into xylem
- From roots to leaves in xylem
- Loss from leaves
How is water taken up by roots?
by Osmosis via apoplastic pathway via root hairs which increase surface area
What is the apoplastic pathway?
non-living route across cortex along cellulose cell walls, between cells, and through intercellular air spaces
What is the symplastic pathway?
living route across the cortex cell to cell through cytoplasm, and across cell walls via plasmodesmata
what is the vacuolar pathway?
route across the cortex vacuole to vacuole by osmosis
What is the plasmodesmata?
cytoplasmic connections between plant cells
What is casparian strip?
composed of suberin, is waterproof, only allows water to cross the endodermis by symplastic pathway
What is the cohesion-tension theory?
cohesion means water molecules stick to each other and adhesion water molecules stick to the walls of the xylem vessels, the tension from evaporation of water from leaves pulls water upwards. Pull of transpiration
what is root pressure?
pull of transpiration aided by push from roots
the endodermis actively pumps salts into the xylem this lowers the water potential in the xylem
therefore water molecules ‘follow’ the salts into the xylem by osmosis down a water potential gradient
How is water lost from the leaves?
transpiration, water diffuses from intercellular spaces into the air via stomata down a water potential gradient
what are the adaptations of sieve tube elements?
no nucleus ,mitochondria, ribosomes or vacuole - so less resistance
sieve plates - allows cell-to-cell movement
what are the adaptations of companion cells?
dense cytoplasm, many mitochondria for atp supply, plasmodesmata - to supply enzymes, proteins, and energy to sieve tubes
what are the adaptation of transfer cells?
modified companion cells that actively transfer organic solutes from sources into sieve tube elements
many mitochondria for atp supply
inner walls folded for increased surface area for uptake
what do organic solutes flow along?
a hydrostatic pressure gradient hypothesis
do sieve tubes in sources have a high or low hydrostatic pressure?
high
do sieve tubes in sinks have a high or low hydrostatic pressure?
low
what is the term for the flow of solutes from sources to sinks?
mass flow hypothesis
What does the mass flow hypothesis not account for in the phloem?
movement of sucrose in opposite directions in the same tube
movement of sucrose at different rates
what other hypotheses have been proposed for movements of sucrose in the phloem?
Diffusion of sucrose and cytoplasmic streaming
What have radioactive tracer studies shown about phloem?
shows phloem translocates carbohydrates produced by photosynthesis, autoradiography studies using radioactive CO2 or radioactive sucrose
shows that labelled carbohydrates entered the phloem for redistribution inside the plant, radioactive label first appears in the sieve tube elements of the phloem
Is sugar transport in the phloem regulated by gravity?
no, its bi-directional
How were aphids used in the theory of translocation?
aphids are small insects with stylets (needles like mouths) the positive pressure inside the sieve elements pumps the sap into the aphids who secrete drops of sucrose-rich ‘honeydew’ from their posterior. if aphids aphids are separated form their stylets the phloem sap will continue to flow from the stylet and can be collected and analysed. confirms sucrose is most commonly transported sugar and there is pressure in the phloem
What are mesophytes?
land plants growing in temperate regions that have an adequate water supply
What do mesophytes do during times of the year when water availability is decreased?
shed their leaves before winter (reduce transpiration water loss)
aerial parts of non woody plants die so they are not exposed to frost or cold (underground organs e.g. bulbs survive)
over-winter as dormant seeds with a low metabolic rate (require almost no water)
What are xerophytes?
plants that have adapted to survive in environments with very little water availability
What are some adaptations of maram grass (a type of xerophyte)?
Rolled leaves
Sunken Stomata
Stiff interlocking Hairs
Thick waxy cuticle
Stiff fibres of sclerenchyma
What do rolled leaves of maram grass (xerophyte) do?
reduces the leaf area exposed to air and so reduces transpiration
What do sunken stomata of maram grass (xerophyte) do?
As they are in pits/depressions humid air is trapped in the pit outside the stomata. this reduces the water potentail gradient between the inside of the leaf and the outside so reduces the rate of diffusion of water out through the stomata
What do the stiff interlocking hairs of maram grass (xerophyte) do?
trap water vapour and reduce the water potential gradient between the inside of the leaf and the rate of diffusion of water out
What does the thick waxy cuticle of maram grass (xerophyte) do?
wax is waterproof and so reduces water loss, the thicker the the cuticle the lower the rate of transpiration through the cuticle
What do the stiff fibres of sclerenchyma of maram grass (xerophyte) do?
stiff maintains the leaf shape even when cells become flaccid
What are hydrophytes?
plants that grow partially or fully submerged in water
How are hydrophytes adapted?
little or no lignified support tissues
poorly developed xylem
little or no cuticle on leaves
stomata on upper surface of floating leaves
large air spaces continuous down to roots
why do hydrophytes have little or no lignified support tissues?
water is a supportive medium
Why do hydrophytes have a poorly developed xylem?
they are surrounded by water so there is little need for water transport system
Why do hydrophytes leaves have little or no cuticle?
no need to prevent water loss, surrounded by water
Why are stomata on the upper surface of floating leaves of hydrophytes?
because the lower surface is in the water, therefore can’t do gas exchange
Why do hydrophytes have large air spaces that are continuous down to roots?
forms a reservoir of oxygen and carbon dioxide which provide buoyancy
