Transport in plants Flashcards
Why do plants need a transport system
-every cell of multicellular plants need a regular supply of oxygen, water, nutrients and minerals
-plants are not very active and respiration rate is low therefore demand for oxygen is low - can be met by diffusion
-demand for water and sugar is high - plants can absorb water and minerals at roots but sugar can’t be absorbed from soil
-leaves can perform gaseous exchange and manufacture sugars by photosynthesis but can’t absorb H20 from air therefore water and minerals from roots up to leaves and sugars from leaves to rest of plant
What are the different types of vascular tissue
-xylem tissue in which water and soluble mineral ions travel upwards
-phloem tissue in which assimilates such as sugars travel up or down
-there is no pump and respiratory gases are not carried by these tissues
Describe how xylem and phloem are distributed in young root
-vascular bundle found at centre of young root
-central core of xylem, often in shape of X
-phloem found in between arms of x-shaped xylem tissue
-this arrangement provides strength to withstand pulling forces to which roots are exposed
-around vascular bundle is special sheath of cells called endodermis - key role in getting water into xylem vessels
-just inside endodermis is layer of meristem cells, that can divide, called pericycle
Describe distribution of xylem and phloem in the stem
-vascular bundles are found near the outer edge of the stem
-non-woody plants have bundles separate and discrete
-woody plants the bundles are separate in young stems but become a continuous ring in older stems - means there is a complete ring of vascular tissue just under bark of tree; this arrangement provides strength and flexibility to withstand bending forces to which stems and branches are exposed
-xylem is found towards inside each vascular bundle and phloem towards outside
-in between phloem and xylem is layer of cambium - layer of meristem cells that divide to produce new xylem and phloem
Describe distribution of xylem and phloem in the leaf
-vascular bundle form the midrib and veins of a leaf
-a dicotyledonous leaf has a branching network of veins that gets smaller as spread away from midrib
-within each vein, xylem located on top of phloem
Structure and function of xylem
-transports water and mineral ions from roots up to leaves
-vessels to carry water and dissolved mineral ions
-fibres to help support plant
-living parenchyma cells which act as packing tissue to separate and support vessels
Describe the xylem vessels
-as xylem vessels develop, lignin impregnates walls of cells, killing them and making them waterproof
-end walls and contents decay leaving column of dead cells with no contents - tube called xylem vessels
-lignin strengthens the vessel walls and prevents collapse - keeps them open even during short water supply
-the patterns of lignin prevent vessel from being too rigid and allows some flexibility of stem or branch
-when lignification not complete, gaps left in cell wall - bordered pits form
-bordered pits in 2 adjacent vessels are aligned to allow water to leave one vessel and pass to another - also allow water to leave xylem and pass into living parts
How are xylem vessels adapted to carry water and mineral ions from roots to top of plant
-made from dead cells, end to end, forming continuous column
-narrow tubes, water column doesn’t break easily and capillary action effective
-bordered pits in lignified walls allow water move sideways from one vessel to another
-lignin deposited in walls in spiral, annular or reticulate patterns allows xylem to stretch as plant grows and enables stem/branch to bend
How is flow of water not impeded (prevented) in xylem
-no cross walls
-no cell contents, nucleus or cytoplasm
-lignin thickening prevents walls from collapsing
Structure and function of phloem
-phloem is a tissue used to transport assimilated around the plant
-sucrose is dissolved in water to form sap
-phloem tissue consists of sieve tubes, made up of sieve tube elements, and companion cells
Describe sieve tube elements
-elongated sieve tube elements are lined up, end to end, to form sieve tubes - contain no nucleus and very little cytoplasm, which leaves space for mass flow of sap to occur
-ends of sieve tube elements are perforated cross walls - sieve plates, which allow movement of sap from one element to another
-sieve plates have pores which very rapidly become blocked by deposition of callose to prevent loss of sap and inhibits transport of pathogens after injury or infection
Describe companion cells
-between sieve tubes are small cells, each with a large nucleus and dense cytoplasm - these are companion cells
-have numerous mitochondria to produce ATP needed for active processes - companion cells carry out metabolic processes needed to load assimilated actively into sieve tubes
-companion cells and sieve tube elements in phloem linked by strands of cytoplasm - allows communication and flow substances between the cells
Compare xylem and phloem
XYLEM
-impregnaned with lignin - supports, prevents collapse, waterproofs the walls
-end walls decay so water can flow unimpeded
-small diameter, narrow, allows for capillary action
-leaves long column of continuous, hollow, dead cells - minimises obstruction to water
PHLOEM
-made of sieve tube elements, have very little cytoplasm and no nucleus
-have companion cells which have large nucleus and dense cytoplasm and lots of mitochondria to produce ATP to load sucrose
-sieve plates with pores to allow flow of sap
-plasmodesmata links cytoplasm of companion cells to phloem
Define hydrophyte
-plant adapted to living water or where the ground is very wet
Define xerophyte
-plant adapted to living in dry conditions
How are terrestrial plants (land) adapted to reduce water loss (behavioural and structural)
-waxy cuticle on leaf - reduce water loss due to evaporation through epidermis
-stomata often found on under surface of leaves not top - reduces evaporation due to less evaporation by direct heating from sun
-most stomata close at night when no light for photosynthesis
-deciduous plants lose leaves in winter when ground may be frozen (and water less available) and when temperatures too low for photosynthesis
Why is marram grass adapted
-ammophila specialises in living on sand dunes
-conditions are particularly harsh because water in sand drains away quickly, sand may be salty and leaves are exposed to windy conditions
-marram grass is a xerophyte
How is marram grass adapted
-leaf rolled longitudinally so air is trapped inside - this air becomes humid. reducing water loss from leaf
-thick waxy cuticle on outer side of rolled leaf to reduce evaporation
-stomata on inner side of rolled lead so are protected by enclosed air space
-stomata are in pits in lower epidermis, also folded and covered by hairs - reduce air movement and therefore loss of water vapour
-spongy mesophyll is very dense with few air spaces so less surface area for evaporation
How are cacti adapted
-cacti are succulents - they store water in their stems which become fleshy and swollen
-stem often ribbed or fluted so it can expand when water available
-leaves are reduced to spines - reduces surface area of leaves therefore less water lost be transpiration
-stem green to allow photosynthesis
-roots very widespread in order to take advantage of any rain that does fall
Describe other features of xerophytes
-closing stomata when water availability low which reduces water loss and reduces need to take up water
-some plants have low water potential inside their leaf cells which is achieved by maintaining high salt concentration in the cells
-low water potential reduces evaporation of water from cell surface as water potential gradient reduced
-very long tap root that can reach water deep underground
Why and how are hydrophytes adapted
-hydrophytes have easy access to water however they struggle to get oxygen to their submerged tissues and keep afloat
-they have many large air spaces in leaf to keep them afloat - therefore in air and can absorb sunlight for photosynthesis
-stomata on upper epidermis so they’re exposed to air allowing for gaseous exchange
-leaf stem has many large air spaces - helps with buoyancy but allows oxygen to diffuse quickly to roots for aerobic respiration
Describe water uptake and movement across the root
-outermost layers of cells (epidermis) of a root contains root hair cells - cells with a long extension, increasing surface area
-these cells absorb mineral ions and water from soil
-water moves across root cortex down a water potential gradient to endodermis of vascular bundle
-water may also travel through apoplast pathway as far as endodermis but then must enter symplast pathway as apoplast blocked by casparian strip
Summarise how it root absorption works
-mineral ions actively absorbed from soil making water potential of cytoplasm more negative
-water enters root hair cell by osmosis
-water move across root cortex by osmosis via apoplast pathway
-mineral ions actively transported into medulla making water potential more negative so water follows by osmosis
