Module 3: Exchange And Transport 2 Flashcards
Why do plants need a transport system??
Plants need to exchange CO2 in and O2 out
Water and minerals in
Waste products out
Why is there no need for specialised gas exchange surfaces in plants??
Leaves are adapted to allow gas exchange by diffusion
What are the 2 vascular tissues in a plant?
Xylem and phloem
Xylem
Upwards transport of water and mineral ions
Phloem
Bidirectional transport of organic food molecules
Vascular tissue structures of stems and roots
Share similar vascular tissue structures
But arrangement and position differs
Root structure: epidermis
Single layer of cells
Often have long extensions called root hairs
Increase surface area
A single plant may have 10^10 root hairs
Root structure: cortex
Thick layer of packaging cells often containing stored starch
Root structure: endodermis
Single layer of tightly packed cells containing a waterproof layer called the casparian strip
Prevents movement of water between cells
Root structure: pericycle
Layer of undifferentiated menstematic (growing) cells
Root structure: vascular tissue
Contains phloem and xylem cells
Continuous with stem vascular bundles
Arrangement is different
Xylem usually forms a star shape with 2-6 arms
Stem structure: epidermis
One cell thick
In young plants the epidermis cells may secrete a waterproof cuticle
In older plants, the epidermis may be absent replaced by bark
Stem structure: cortex
Composed of various ‘packaging’ cells
Give young plants strength and flexibility
Source of plant fibres such as sisol and hemp
Stem structure: vascular tissue
Contains phloem and xylem tissue
Grows out from the combium
In young plants the vascular tissue is arranged in vascular bundles
Phloem on the outside and xylem on the inside
In older plants the xylem bundles fuse together to form the bulk of the stem
Stem structure: parenohyma
The central region of a stem
Used for food storage in young plants
May be absent in older plants
Stem structure: cambium
Unspecialised meristem that can divide to form vascular tissues
Vascular tissue in the leaf: parenchyma
Relatively unspecialised cells
Make up the bulk of soft internal tissues of leaves, stem, roots and fruits
Thin flexible cell walls
Large water-filled vacuole
Vascular tissue in the leaf: Collenchyma
Living at maturity, never lignified
Support tissue in growing plant parts
Cell walls strengthened by cellulose and pectin but distortable when stretched
Found in strands beneath the epidermis
Vascular tissue in the leaf: schlerenchyma
Thick walled dead cells, heavily lignified
Fibres frequently grouped in bundles:
- elongated cells, often with pointed ends
- associated with vascular tissue
- high load-bearing capacity and easily processed fibres
What is transported in xylem vessels
Water
Soluble mineral ions
What is a tissue
A group of cells working together to perform a particular function
What is the dual function of xylem tissue?
Has the dual functions of support and transport
Xylem as a tissue - what does it do?
Transport water and other minerals from the soil up to the roots and stem of a plant
Contain long elongated cells called Tracheids and shorter and wider cells called vessel elements
What are xylem vessels made up of?
Vessels are made up of elongated vessel elements arranged end to end
How does the xylem structure form?
Each element began as a normal plant cell
Substance called lignin is laid down
As lignin builds up around the cell, the cell dies which leaves an empty space inside
The end walls of neighbouring vessel elements would break down completely to form a continuous tube
What is lignin??
A hard strong substance which is impermeable to water
What are pits in the xylem??
In the area where plasmodesmata is present, no lignin would be laid down
These non-lignified areas can be seen as gaps in the walls of xylem vessels
Water movement in a plant
In the root, water crosses the cortex, endodermis and pericycle and moves into the xylem through the pits in their walls
Will then move towards the leaves and up the vessels
Water and dissolved minerals only move in one direction through the plants
Similarities between tracheids and xylem vessel elements
Both conduct water
Both are dead
Both develop lignin
What are tracheids
Spindle shapes cells with pits through which water flows from cell to cell
They have tapered ends and a function in support as well as water transport
What are pits
Thinner regions where water can pass through from cell to cell
What does TS mean
Transverse section
What does LS mean
Longitudinal section
Why is xylem a tissue?
Because it contains a variety of specialised, water conducting cells known as tracheory elements
What is the role of parenchyma cells
Fibres lend support
What is the role of fibres in the xylem??
Lend support to tissues
What is the difference between a tracheid and a xylem vessel??
Tracheids are thinner than xylem vessels
What is the purpose of lignin in xylem vessel cell walls?
Plays a crucial part in conducting water in plant stems
Gives support
Prevents collapse of xylem vessel
Prevents escape of water
What is the effect of the lignin on the xylem cells?
Prevents water loss from xylem cells as it is impermeable
What are the 4 basic ways that molecules can be transferred??
Diffusion
Osmosis
Active transport
Bulk transport
What is water potential??
Water potential is a measure of the ability of water molecules to move freely in solution
How does water move?
Water moves from a solution with high water potential to one of lower water potential
How is water potential decreased?
Decreased by the presence of solutes
How is water transported from the root hair to the xylem?
Water is taken up by the root hair and crosses the cortex
Then it enters the xylem in the centre of the root
Why does water move from the root hair to the xylem??
There is a lower water potential in the xylem vessels than there is in the root hairs
Water moves down the water potential gradient
Does soil have a high or low water potential??
High
Does the root hair cell have a high or low water potential??
Low
The apoplast pathway
The cells of a cortex are surrounded by cell walls made of several layers of cellulose fibres criss crossing one another
Water can soak into these walls
Water can then deep across the root cortex from cell wall to cell wall until it reaches the xylem
The symplast pathway
Water moves into the cytoplasm of a cortical cell and then into adjacent cells through their interconnecting plasmodesmata
The continuum of cytoplasm within a plant tissue requires only the crossing of a plasma membrane
The vascular pathway
Similar to the symplast pathway except fluid can move into and out of the vacuoles
What are the three pathways?
The apoplast pathway
The symplast pathway
The vascular pathway
What is a casparian strip??
A bond which forms an impenetrable barrier to water in cell walls
At the stele how does water cross the endodermis
Through the cytoplasm of cells
What happens as the endodermis cells get older??
The Suberin deposits become more extensive
What happens inside the stele
Once across the endodermis, water continues to move down the water potential gradient across the pericycle and towards the xylem vessels
What is transpiration?
Loss of water by evaporation from the aerial parts of the plant
In leaves where does water evaporate from?
Cell walls
In leaves how does water evaporate?
The vapour accumulates in the air spaces and then diffuse out through open stomata
What happens to the leaf cells as water evaporates??
The leaf cells are cooled
Transpiration process
Water evaporates from the leaf cells
Water is drawn by osmosis from the xylem vessels of the leaf veins to replace the lost water
As a consequence water is drawn up through the plant
What 3 processes does Transpiration involve??
1) osmosis from the xylem to the mesophyll cells
2) evaporation from the surface of the mesophyll cells into intercellular spaces
3) diffusion of water vapour from the intercellular spaces out through the stomata
What is cohesion?
Attraction of molecules for other molecules of the same kind
Why do water molecules have strong cohesive forces??
Because of the ability to form hydrogen bonds with one another
What is Adhesion
Attraction of molecules of one kind, to molecules of a different kind
Why is the attraction of molecules quite strong for water
Because other molecules bear positive or negative charges
In what ways is the movement of water useful to the plant??
Water is required in the leaves for photosynthesis
Water is needed to enable cells to grow and elongate
Water keeps cells turgid so that stomata will open
The flow of the water passively carries minerals up the plant
Evaporation of water keeps the plants cool
What is the transpiration pull
As water leaves the xylem in the leaf it must be replaced from below
Water moves up the xylem from the roots to replace the lost water
This is because it is under tension
How is water moved up the xylem from the roots to the leaves?
Low pressure at the top, water is under tension
Adhesion of water molecules to the xylem
Water moves up through the xylem by mass flow
Are the cells living or dead in a xylem??
Dead
Are phloem sieve tube elements living or dead??
Living
Do xylem vessels have bordered pits?
Yes
Do phloem sieve tube elements have bordered pits??
No
Is lignin present or absent in a xylem vessel
Present
Is lignin present or absent in phloem sieve tube elements??
Absent
What does the xylem vessel transport??
Water
Mineral ions
What does the phloem sieve tube elements transport??
Organic solutes such as sugars and amino acids
What direction is transport in a xylem vessel??
One way
What direction of transport is the phloem sieve tube element??
Either direction
What evidence the scientist might expect to see in companion cells, using an electron microscope??
Many mitochondria
Plasmodesmata between companion cell and sieve tube
Many ribosomes
What are the 5 environmental factors that affect the rate of transpiration
Light
Temperature
Humidity
Wind
Soil water
Affect of light on rate of transpiration
Plants transpire more rapidly in the light than in the dark
This is because light stimulates the opening of the stomata
What is the affect of temperature on the rate of transpiration??
Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises
What is the affect of humidity on the rate of transpiration??
The rate of diffusion of any substance increase as the difference in concentration of the substances in two regions increases
When the surrounding air is dry diffusion of the water out of the leaf goes on more rapidly
What is the affect of wind on transpiration??
When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration
What occurs when a breeze is present
The humid air is carried away and replaced by drier air
Effect of soil water on rate of transpiration
When absorption of water cannot equal the transpiration rate, the stomata close which reduces transpiration rate.
Therefore it is important that there is water present in the soil
Describe two xeromorphic features
Hairs on the lower surface trap water vapour and reduce water potential gradient
Thick waxy cuticle reduces evaporation of water
What is the cohesion tension theory??
As leaves transpire, the continual removal of water from the xylem vessels generates a pulling force that allows for water to be drawn through the root tissues and into the xylem
Three possible routes for water to get into the xylem
The apoplast pathway
The symplast pathway
The vacuolar pathway
What happens at the stele
As the endodermal cells get older, the Suberin deposits become more extensive, except in certain passage cells
What is a xerophyte??
A plant that is adapted to very dry conditions
Examples of xerophytes
Succulents
Bromeliads
Cacti
How do cacti reduce water loss in their leaves
Their leaves are reduced to spines which reduces the surface area of the leaf in water loss
Thick, waxy cuticle covers the plants surfaces which reduces transpiration
How do cacti reduce water loss in their stem
Tick and fleshy with numerous water storage cells to retain water absorbed by roots
How do cacti reduce water loss in their root
Shallow, spreading root system to quickly absorb any water from rainfall
What is the effect of a round and compact shape
Reduces surface area for water loss
What is the effect of a shiney surface
Reflect heat and light
What is the effect of stomata in reducing water loss
Closed during the day, reduces water loss
Open at night, absorb CO2, used for photosynthesis during the day
What is the effect of rolling up leaves??
Protects the lower epidermis from the outside helps to trap air within the leaf
As this region becomes saturated with water, the water vapour potential is reduced
What is the effect of hairy leaves??
Trap moist air next to leaf surface
Reduces water potential gradient between outside and inside the leaf
Less water loss by transpiration
What is the effect of stomata in pits??
Traps water vapour in pits reducing water vapour potential gradients
What are hydrophytes??
Plants which grow submerged or partially submerged in water
Adaptations of a hydrophytes
Extensive system of air spaces in stems and leaves
Gases diffuse quickly
Provide buoyancy to keep the plants close to the light and are a resource of O2 and CO2
Which vessels in the vascular bundle will water be moving in?
The xylem
What is the purpose of the reservoir of water
Reset the apparatus so that multiple readings can be taken
Other than transpiration, what may a very small percentage of water be used for in the plant?
Photosynthesis
Maintaining all turgour
What precautions must be taken in setting up a potometer to ensure your results are valid?
Set up the apparatus under water to eliminate air bubbles
Avoid wetting leaves, ensure leaves are dry before taking readings
Consistent external environment conditions
