Transport In Plants Flashcards
What substances are required by a plant
Carbon dioxide, water, oxygen, organic molecules, minerals ions
Why don’t plants require a transport system for oxygen and carbon dioxide
Plants have a low metabolic rate, oxygen and carbon dioxide can be transported due to a small diffusion distance (opening in the stomata)
What does the xylem transport
Water and minerals
What does the phloem transport
Sugars
What does the term ‘sources’ mean
Leafs
What does the term ‘sinks’ mean
Storage organs
What do plants require a mass transport system for
Water, mineral ions and organic molecules
What are cotyledons
Leaves that come out of the seed that provide nutrients to the plant till it can photosynthesise
What are some characteristics of monocotyledonous plants
One cotyledon leaf, parallel leaf veins
What are some characteristics of dicotyledonous plants
Two cotyledon leaves, net like leaf veins
Describe how vascular tissue is arranged in a leaf of a dicotyledonous plant
The veins branch from a central vein to form a branching network
What’s the epidermis
tissue that forms the outer layer of root and contains root hair cells for the absorption of mineral ions
What’s the phloems function
Transports sucrose up or down
What’s the xylems function
Transports water and mineral ions upwards from the roots
What’s the cortex
tissue that lies beneath epidermis and contains unspecialised cells that play a role in storage
What’s the endodermis
A layer of cells that surrounds the vascular bundle in the root, and plays a key role in transporting water to the xylem
What’s the pericycle
tissue that is made of meristem cells, and lies just inside the endodermal layer in the root
What’s the vascular cambium
tissue that lies between the xylem and phloem and is made up of meristem cells
Describe how vascular tissue is arranged in a root of a dicotyledonous plant
Xylem forms an ‘X’ at centre of root and phloem lies between arms
Describe how vascular tissue is arranged in a stem of a dicotyledonous plant
Vascular bundles arranged in a ring towards outside of stem. Xylem nearer centre of stem.
Identify a suitable stain to show the position of xylem tissue and explain why it is used.
Toluidine. Acts as a differential stain.
Stains different cells different colours so they can be distinguished.
Non-lignified tissue is stained pink/purple. Lignified tissue (xylem) is stained green/blue.
What specialised cells does a xylem have and what’s their function
dead xylem vessels to transport water and mineral ions upwards from the roots;
dead sclerenchyma with thickened cell walls for support and living parenchyma cells that act as packing tissue.
What specialised cells does the phloem have and what’s their functions
sieve tube elements for the transport of the products of photosynthesis and companion cells that are important in loading assimilates e.g. sucrose into the phloem sieve tubes.
Identify three features of xylem vessels that ensure the flow of water is not impeded.
No end cell walls
No cell contents
Thickening and lignification of cell wall prevents the wall from collapsing
What is the role of lignification?
Waterproofs wall to reduce water loss from xylem vessel
Supports and strengthens wall to prevent collapse under tension
Increases adhesion of water molecules to vessel walls to allow capillary action
What are bordered pits and what is their role?
Non-lignified areas of cell wall
Permeable to water to allow water to leave a xvlem vessel and enter an adiacent vessel e.g. to by-pass an air lock or to pass into neighbouring (living) cells.
Suggest why young plants have spiral, annular or reticulate thickening but older plants contain more vessels with pitted walls.
Allows xylem to stretch as plant grows and enables stem to bend so it doesn’t break
Explain why companion cells contain large numbers of mitochondria and ribosomes?
Mitochondria to produce ATP for active loading of sucrose into sieve tube
Ribosomes on RER to synthesise transport proteins e.g. proton pump, sucrose H* cotransporter protein that are required for active loading
Describe the apoplast pathway
Water moves through cell walls and extracellular spaces.
No membranes are crossed so mineral ions can be carried by water.
Describe the Symplast pathway
Water crosses the plasma membrane into the cytoplasm and can move from one cell to another through the plasmodesmata
Describe the Vacuolar pathway
Water crosses the plasma membrane into the cytoplasm and vacuoles and can move from one cell to another through the plasmodesmata
How are root hair cells adapted?
Large surface area
Many mitochondria to produce
ATP for active transport
Many carrier proteins for active transport
What is the Casparian strip? Describe its role.
Cells in endodermis contain a band of suberin in cell wall that forms a waterproof barrier called the Casparian strip.
This blocks the apoplast pathway. Water and mineral ions must cross the plasma membrane and enter the endodermal cells to follow the symplast pathway.
How are mineral ions carried across the root cortex?
Via apoplast pathway until endodermis when both water and mineral ions must enter endodermal cells
How is the movement of mineral ions into the xylem controlled?
Mineral ions require carrier proteins to cross the plasma membrane and enter endodermal cells. The type / number of carrier proteins can be controlled by the cell.
What is transpiration?
Transpiration is the loss of water by evaporation from the aerial parts of a plant (mainly the leaves)
explain why transpiration is an inevitable consequence of gas exchange?
Stomata open during the day to allow gas exchange for photosynthesis but water is also lost via transpiration.
explain how water vapour lost through open stomata is replaced with water taken from the xylem.
Water evaporates from the cell walls of mesophyll cells into the air spaces in the leaf
Water vapour diffuses down a water vapour potential gradient out of the leaf through an open stoma
Lost water is replaced by water taken from the xylem vessel, that moves through the mesophyll cells of leaf by osmosis down a water potential gradient via either the symplast or apoplast pathways.
State the three processes that contribute to water movement up a stem
Root pressure / Transpiration pull / Capillary action
True or false “Mineral ions diffuse into xylem lowering the water potential”
False
True or false “Mineral ions are actively transported into xylem lowering the water potential”
True
True or false “Water enters xylem by osmosis via the apoplast pathway”
False
True or false “Water enters xylem by osmosis down the water potential gradient”
True
True or false “Water entry increases the hydrostatic pressure at the base of the xylem vessel”
True
True or false “Water entry decreases the hydrostatic pressure at the base of the xylem vessel”
False
Is the movement of water up the xylem a passive or active process
Passive
Explain the cohesion tension theory
The water molecules are attracted to each other by forces of cohesion, due to the hydrogen bonds between them.
Water is pulled up xylem vessels as a continuous column by mass flow, under tension due to transpiration.
What is capillary action
Water molecules are also attracted to lignin in the walls of the xylem vessel - this is called adhesion and is responsible for capillary action
Capillary action is increased by narrow xylem vessels and helps prevent the water column dropping back down.
What happens if a water column is broken
Water can move sideways through the bordered pits to another xylem vessel to by-pass the air lock
What is the transpiration stream
Movement of water up stem in xylem vessels from roots to leaves
Why is the transpiration stream useful in plants
Supplies water to cells in leaf for photosynthesis
Enables plant cells to elongate
Keeps plant cells turgid supporting the plant
Transport of mineral ions from soil
Evaporation cools leaf
Does the number of leaves increase or decrease the rate of transpiration
Increase as it increases the surface area available for evaporation
Does a high number and large size of the stomata increase or decrease transpiration
Large stomata and lots of them increase diffusion
What is the role of the stomata
Stomata open during the day to allow gas exchange for photosynthesis but water is also lost via transpiration
If water uptake from the soil is sufficient: guard cells are turgid and the stomata are open
If water loss in transpiration is greater than uptake via the roots: guard cells become flaccid and the stomata close to reduce water loss
Comment on the number/size of stomata found in the upper and lower epidermis.
More and larger stomata in lower epidermis.
Cooler (not in direct sunlight) and less air movement so less water loss
Why do deciduous plants lose their leaves in winter?
Light intensity is lower for photosynthesis so leaves are lost to reduce water loss by transpiration when less water may be available if the ground is frozen
What is a potometer
A simple apparatus used to measure the rate of water uptake from a cut leafy shoot and so estimate the rate of water loss by transpiration
Why is it important to form an airtight seal? How can this be achieved? (Potometer)
The airtight seal is necessary, or water will not be drawn up the capillary tube and no air bubble movement will be possible.
Use Vaseline / Plumber’s Mate to help make a tight seal
Why does the apparatus only provide an estimate of transpiration?
Potometer measures water uptake not water loss. We assume all water taken up is lost in transpiration, but some is used by cells e.g. in photosynthesis, to maintain turgor
How could water loss be estimated directly in this apparatus?
Weigh the potometer at the beginning and then after a set period of time. The fall in mass will be due to water loss (assuming no leaves etc are lost).
Compare the use of bubble and mass potometers.
A potometer where movement of a bubble is measured is more difficult to set up since it must be air-tight but quicker to obtain data.
A mass potometer must be left longer (e.g. 2 days) to produce a measurable change in mass.
How do you calculate water uptake
volume = distance moved by bubble x pieR2
