3.3 Flashcards
Dicotyledonous plants
Plants with two seeds and a branching pattern of veins in the leaf
Meristem
A layer of diving cells
Phloem
Transports dissolved assimilates
Vascular tissue
Consist of cells specialised for transporting fluids by mass flow
Xylem
Transports water and minerals
Companion cell
Cells that help to load sucrose into sieve tubes
Sieve tube elements
Makes up the tubes in phloem tissue that carry sap up and down plant, sieve tubes separated by sieve plates
Xylem vessels
Tubes which can carry water up the plant
Plasmodesmata
Gaps in cell wall containing cytoplasm that connects two cells
Potometer
A device that can measure rate of water uptake as a leaf stem transpires
Transpiration
Loss of water vapour from aerial parts of a plant, mostly through stomata in the leaves
Adhesion
Attraction between water molecules and walls of xylem vessel
Cohesion
Attraction between water molecules caused by hydrogen bonds
Why do plants need a transport system
All living things need to take substances and return waste to their environment
Why do larger plants need specialised exchange surfaces and transport systems
Have a smaller SA:V ratio
What does every cell of a multicellular plant need
Regular supply of oxygen, water, nutrients and minerals
Why is plants oxygen demand low
Plants aren’t very active and their respiration rate is low
How can plants need for oxygen be met
By diffusion
Is plants need for water and sugars high or low
High
What can plants absorb from their roots and what can’t they
Can absorb water and minerals but can’t absorb sugars from the soil
What function can leaves perform
Can perform gaseous exchange and manufacture sugars by photosynthesis
What function can leaves not perform
Can’t absorb water from the air
What do plants needs a transport system to move and from where to where
Water and minerals form roots to leaves, and sugars from leaves to rest of the plant
What do transport systems in plants consist of
Specialised vascular tissues
What does they xylem transport and in what direction
Water and soluble mineral ions travel up the plant
What travels in the phloem vessel and what direction
Assimilates such as sugars and they can travel up or down the plant
How is plants specialised transport system different to humans
There is no pump(heart) and respiratory gases aren’t carried by these tissues
What are dicotyledonous plants
Plants with 2seed leaves, and a characteristic distribution of vascular tissue
How is vascular tissue distributed in dicotyledons
Throughout the plant
Where are phloem and xylem found in dicotyledons
Found together in vascular bundles, bundles also contain other tissue types
What tissues are in a dicotyledons vascular bundle
Xylem, phloem, …
collenchyma, sclerenchyma)
What is collenchyma and sclerenchyma role in vascular bundle
Gives bundle strength and helps support the plant
what is structure of vascular bundle in dicotyledons roots
At the centre of a young root, central core is xylem (often in X shape)
What does xylems X shape provide root with
Strength to withstand pulling forces to which roots are exposed
What is around the vascular bundle
A special sheath of cells called endodermis
What role does the endodermis of a dicotyledon root
Key role of getting water into xylem
What is just on the inside of endodermis
A layer of meristem cells (unspecialised cells) called pericycles
What is in each section of a X shape xylem
Phloem
What is another name for the whole vascular bundle
Root stele
What is the layer outside of the endodermis called in roots
Cortex
What is the outside layer of the plant root called
Epidermis
In dicotyledons stem where is the vascular bundle found
Near the outer edge of the stem
What is the difference between woody plants and non woody plants vascular bundles in the stem
In non-woody plants they are separate and clear and in woody plants they are less clear
What do vascular bundles look like in woody plants stem
They are separate in young stems but become continuous rings in older stems, meaning there is a complete ring of vascular tissue just under the bark a tree
What does the arrangement of dicotyledons vascular bundles in stem mean
Arrangement provides strength and flexibility to withstand bending forces which stems and branches are exposed to
Where is the xylem, and phloem found in vascular bundle of stem
Xylem found towards the inside and phloem towards the outside
What is in between the xylem and phloem in a stem vascular bundle
A layer of cambium
What is cambium
A layer of meristem cells that divide to produce new phloem and xylem
What is the area in the middle of the stem and all the vascular bundles called
The pith
What is found on the outside of the phloem near the epidermis of a stem
Sclerenchyma
Where is collenchyma located in a stem of a dicotyledons
In a ring round the outside of all the vascular bundles to provide strength
Where do vascular bundles form in a leaf
In the midrib and leaves veins
What type of veins does a dicotyledon have
A branched network of veins that get smaller as they spread away from the midrib
Where are the xylem and phloem vascular bundle located in leaves
The xylem is on top of the phloem, closer to the upper epidermis of the leaf and phloem is closer to the lower epidermis
What does dissection of a plant to examine its distribution of vascular tissue need
Staining
What plant is usually used the examine the distribution of vascular tissue
Celery stick
How would you carry out an experiment to look at vascular bundles in a celery
Cut a thin section and view it at low power, allow the leafy stem to take up water by transpiration, the stem can then be cut longitudinally or transversely and examined with a microscope
What is xylem
A tissue used to transport water and mineral ions from the roots to the leaves and other parts of the plant
What does xylem tissue consist of
Vessels to carry water and dissolved mineral ions, fibres to help support the plant, living parenchyma cells which act as packing tissue to separate and support the vessel
What happens as the xylem vessel develops
Lignin impregnates walls of cells, making them waterproof
What happens once lignin impregnates xylem
It is waterproof which kills it and so the end walls and contents of the cells decay, leaving a long column of dead cells with no contents- creating the xylem vessel
What is lignin role in the xylem
Strengthens the vessel walls and prevents them from collapsing which keeps vessel open even when water is in low supply
What does lignin thickening form
Patterns in the cell wall (may be spiral, annular(rings) or reticulate (network of broken rings) which prevents vessel from being too rigid and allows stem/branches some flexibility
What happens in some places where lignification isn’t complete
Leaves gaps in the cell wall, these gaps form boarded pits
What is the name of the process of lignin thickening
Lignification
What are bonded pits
Pits in 2 adjacent vessels which are aligned to allow water to leave the xylem and pass into living parts of the plant
How can xylem vessels carry water and mineral ions from the roots to the top of the plant
They are made from dead cells aligned end to end to form a continuous column, tubes are narrow so water column doesn’t break easily and capillary action can be effective, bordered pits in lignification walls allow water to move sideways from one vessel to another, lignin deposited in walls in spiral, annular or reticulate patterns allow xylem to stretch as plant grows enabling stem/branches to bend
Why is flow of water in xylem not impeded
As there are no cross walls, there are no cell contents (nucleus or cytoplasm), lignin thickening prevents walls from collapsing
What is there in xylem lumen
Nothing expect water and minerals
Where is parenchyma located around the xylem
On the outside in bundles
What is phloem
A tissue used to transport assimilates(sucrose and amino acids) around plant
What is sucrose dissolved in to form what in plants
Dissolved in water to form sap
What does phloem tissue consist of
Sieve-tubes made up of sieve tube elements and companion cells
How do sieve tube elements form sieve tubes
Elongated sieve tube elements are lined up end to end to form sieve tubes
What do sieve tube elements contain and what don’t they
Contain no nucleus and very little cytoplasm, leaving space for mass flow and sap to occur
What are at the end of each sieve tube element
Perforated cross-walls called sieve plates
What do perforations on sieve plates allow
Movement of sap from one element to the next
What are sieve tubes walls like
Very thin and when seen in transverse sections they are usually 5-6 sections
What are the small cells found between sieve tubes
Companion cells
What is components of companion cells
Each have a large nucleus and dense cytoplasm, they have numerous mitochondria to produce ATP needed for active processes
What is companion cells function
Carry out metabolic processes needed to load assimilates actively into sieve tube
Are cellulose cell walls fully permeable to water
Yes
How to water molecules move between cellulose cell wall and gaps in between cells
Can move freely
How can water also pass into the cytoplasm or even vacuole
It can pass across the cell wall and through partially permeable plasma membrane into cells cytoplasm/vacuole
How are many plants cells joined
Cytoplasmic bridges
What are cytoplasmic bridges
Cell junctions at which cytoplasm of one cell is connected to another through a gap in their cell walls
What are gaps in the cell wall connecting plant cells together called
Plasmodesmata
What are the three pathways for water molecules to travel cell to cell
Apoplast pathway, symplast pathway, vacuolar pathway
What is the apoplast pathway
Water passes through spaces in cell walls and between cells in intercellular space, doesn’t pass through any plasma membrane, water can move by mass flow instead of osmosis, dissolved mineral ions and salts can be carried with the water
What is the symplast pathway
Water enters cell cytoplasm through plasma membrane, it can then pass through plasmodesmata from one cell to the next
What is the vacuolar pathway
Similar to symplast pathway but water isn’t confined to the cytoplasm of cells, it can enter and pass through the vacuole too
What is water potential
Is a measure of the tendency of water molecules to move from one place to another
What way does water always move on a gradient
From a region of higher water potential to a region of lower water potential
What is the water potential of pure water
0kPa
What reduces the water potential in plants and how
Cells cytoplasm has mineral ions and sugars which reduce water potential as fewer free water molecules are available than in pure water
In plants what is always the value of water potential
Below 0 (negative)
What happens when a plant cell is placed in pure water and why
It will take up water molecules by osmosis as water potential in cell is lower blood than water potential of pure water, water molecules will move down a water-potential gradient into the cell
When a plant cell is placed in a solution with higher water potential will it fill to its burst
No as cells have strong cellulose cell wall, when cells are full of water they become turgid
What is it called when a turgid water in plant cell exerts pressure on cell wall
Pressure potential
What happens as pressure potential builds up
It reduces the influx of water
What happens when plant cell is placed in a salt solution with very low water potential
It will loose water by osmosis as water potential in the cell is higher than that of the salt solution, so water moves down the concentration gradient out of the cell
What happens as water leaves a plant cell
Cytoplasm and vacuole shrink and eventually cytoplasm no longer pushes against cell wall and cell is no longer turgid
What happens when water continuously leaves a plant cell
The plasma membrane will loose contact with the cell wall, this is known as plasmolysis and the tissue is now flaccid
What happens when plant cells touch each other
Water molecules can pass cell to cell, water molecules will move from the less negative water potential to the more negative one, known as osmosis
What is transpiration
Loss of water vapour from the upper parts of a plant, mostly leaves
Where does some water evaporate from on a plant and how is it prevented
Some evaporates through upper leaves surfaces but loss is limited by waxy cuticle
Where is most water lost by on plants
Through the stomata which open to allow gaseous exchange from photosynthesis
Why is most water vapour lost during the day
As photosynthesis occur only when there is sufficient sunlight (in the day)
What is a typical pathway taken by water leaving the leaf (osmosis)
Water enters xylem and moves by osmosis into cells of spongy mesophyll, it may also pass along cell walls via apoplast pathway
What is a typical pathway taken by water leaving the leaf (evaporates)
Water evaporates from cell walls of spongy mesophyll
What is a typical pathway taken by water leaving the leaf (diffusion)
Water vapour moves by diffusion out of the leaf through open stomata which relies on a difference in the concentration of water vapour molecules in the leave compared to outside the leaf
What is the difference in concentration of water vapour in to leaf to outside known as
Water vapour potential gradient, there must be a less negative water vapour potential inside the leaf than outside
Is transpiration good or bad
Transpiration may be inevitable consequence of gaseous exchange but also essential for plant survival
What happens as water vapour lost from leaf
It must be replaced from below, plant draws up water from roots to stem as a transpiration stream
What does the movement of water from roots to leaves provide
Transports useful mineral ions up the plant, maintains cell turgidity, supplies water for growth, cell elongation and photosynthesis, supplies water that, as it evaporates, can keep the plant cool on a hot day
How does light intensity effect water loss of a plant
When it’s light, stomata open to allow gaseous exchange for photosynthesis, so higher light intensity increases transpiration rate
How does temperature effect transpiration rate
Higher temp will increase rate of transpiration in 3ways, increase rate of evaporation for cells surfaces so water potential in leaf rises, increases rate of diffusion through stomata as water molecules have more kinetic energy and decreases relative water vapour potential in air, allowing a more rapid diffusion of molecules out of the leaf
How does relative humidity effect transpiration rate
Higher the relative humidity in the air the lower the rate of water loss we there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside
How does air movement (wind) effect transpiration rate
Air moving outside of the leaf will carry away water vapour that has just diffuser which maintains a high water vapour potential gradient
How does water availability effect transpiration rate
Is there is little water in the soil, then the plant can’t replace lost water, if the water in the soil is insufficient, then stomata close and leaf wilts
What is a potometer
Device to measure rate of transpiration, but looking at how much water a plant uptakes in a certain time at certain conditions
What is the transpiration stream
Movement of water from the soil through the plant to air surrounding the leaves
What is the main driving force of transpiration stream
The water potential gradient between the soil and the air in leaf air spaces
What does the outermost layer of a root (epidermis) have
Root hair cells
What are root hair cells
Cells with long extensions that increases the SA of the root, these cells absorb mineral ions and water from soil
What happens once root hair cells absorb water and mineral ions from the soil
Water then moves across the root cortex down water-potential gradient into epidermis of the vascular bundle
After water and minerals been absorbed by root hair cell it may also take the apoplast pathway as far as the endodermis but can’t go further. Why?
As apoplast pathway is blocked by casparian strip so it must enter symplast pathway
What happens after mineral ions pass casparian strip via symplast pathway
They actively transport into the medulla making the water potential lower so water follows by osmosis
Are mineral ions actively or passively taken up by roots
Actively
How does water move across roots cortex
By osmosis
What is the role of the endodermis
The movement of water across the root is driven by an active process that occurs at the endodermis
What is the endodermis
The layer of cells surrounding medulla and xylem such as starch sheath as it contains starch granules which is a sign energy is being used
What is casparian strip
Blocks the apoplast pathway between cortex of roots and the medulla which ensures water and dissolved mineral ions(esp nitrates) have to pass into cell cytoplasm via cell membrane
What does the roots plasma membranes contain and what does this mean
Transporter proteins which actively pump mineral ions from cytoplasm of cortex cells into the medulla and xylem which makes water potential of medulla and xylem more negative, so water moves from cortex cells into medulla and xylem by osmosis
What happens once water has entered the medulla
It can’t pass back into cortex as apoplast pathway of endodermal cells is blocked by casparian strip
How does water move through the xylem
By mass flow- a flow of water and mineral ions in the same direction
What are the 3 processes to help water move up the stem
Root pressure, transpiration pull and capillary action
What is root pressure
Action of the endodermis moving minerals into medulla and xylem by active transport draws water into medulla by osmosis, pressure in root medulla then builds up forcing water into xylem, root pressure can push water a few meter up the steam but not to the top of a tall tree
What is transpiration pull
Loss of water by evaporation from leaves must be replaced by water coming up from xylem, water molecules are attracted to each other by forces of cohesion, these cohesive forces are strong enough to hold water molecules together in a long chain/column
What happens as molecules are lost at the top of the column of a xylem due to transpiration pull
The whole column is pulled up as one chain and the pull from above creates tension in the column of water, so the xylem must be strengthened by lignin which prevents vessel collapsing under tension
What is another way of calling transpiration pull and why
Cohesion tension theory as this mechanism involves cohesion between water molecules and tension in the column of water
What does cohesion tension theory rely on and what happens is this isn’t the case
Plant maintaining an unbroken column of water all the way up the xylem, if the water column is unbroken in one xylem vessel then water column can still be maintained through other vessels via bordered pits
What is capillary action
Same forces that hold water molecules together also attract water molecules to sides of the xylem vessel known as adhesion
How does adhesion work in capillary action
As xylem vessels are narrow these forces of attraction can pull water up the sides of the vessel
What happens to pressure when water leaves xylem at the leaves
It creates a low hydrostatic pressure so there is tension
In what state does most water leave the leaf in
Most water that leaves the the leaf exists as vapour through stomata, only a small amount leave through the waxy cuticle
Where does water most evaporate the leaves from
From cells lining the cavity immediately above the guard cells known as sub-stomatal air space
What does water evaporating from sub-stomatal air space cause
Lowers the water potential in these guard cells causing water to enter them by osmosis from neighbouring cells, so in turn water drawn from xylem in leaf by osmosis but water may also reach these cells by apoplast pathway from xylem
What are terrestrial plants
Plants living on land
What do most land plants have as their main problem
There is often limited access to water
How is water lost from a terrestrial plant
Lost by transpiration as plants exchange gases with the atmosphere via their stomata
What happens to a plant during the day to do with gas exchange
Plants take up lots of CO2 for photosynthesis and remove oxygen which is a bi-product of photosynthesis, so the stomata must be open during the day and while the stomata are open there is an easy route for water loss
Lost water must be replaced in plants, what must terrestrial plants be adapted to
Reducing water loss and replace lost water by
How can most terrestrial plants reduce their water loss by structural and behavioural adaptation
A waxy cuticle on leaf will reduce water loss due to evaporation through epidermis, stomata often under leaf surface which reduces evaporation from direct heat from sun, most stomata close at night when no light for photosynthesis, deciduous plants lose their leaves in winter when ground may be frozen and less water available and when temp may be too low for photosynthesis
How is marram grass specialised
Specialises living in sand dunes where conditions are particularly harsh as any water in the sand drains away quickly and sand may be salty and leaves often exposed to very windy conditions
What is marram grass
A xerophyte
What is an xerophyte
A plant adapted to living in arid conditions
What are marram grasses adaptations so it can survive arid conditions
Leaves rolled longitudinally so air trapped inside comes humid which reduces water loss from leaves, they can roll tighter in dryer conditions. has thick waxy cuticle in outer side of rolled leaf to reduce evaporation. Stomata on inter side of rolled leaf (lower epidermis) so they are protected by enclosed air space. stomata are in pits in lower epidermis which is also folded and covered by hairs, this helps reduce air movement, spongy mesophyll is dense with few air spaces so less SA for water to evaporate
How are cactus adapted to living in arid conditions
They are succulents-store water in their stems which becomes fleshy and swollen. Stem is often ribbed or fluted so it can expand when water is available. Leaves reduced to spines which reduces SA so less water lost by transpiration, stem green for photosynthesis, roots widespread to take advantage of any rainfall
What are other adaption of other xerophyte plants
Closing stomata when water availability is low to reduce water loss and reduce need to uptake water, some have low water potential in their leaf cells by having high salt concentration in their cells which reduces evaporation of water from cells surfaces as water potential gradient between cell and leaf air spaces is reduced, some have a very long tap root that can reach water deep underground
What are hydrophytes
Plants what live In water such as water lilies
What is hydrophytes problems
They have access to water but struggle to get oxygen into their submerged tissues and keeping afloat
Why do hydrophytes need to keep afloat
They need to keep their leaves in sunlight for photosynthesis
What are adaptations of water lilies
They have many large air spaces in the leaf which keeps leaves afloat so they are in the air and can absorb sunlight, stomata are on upper epidermis so they are exposed to air to allow gas exchange, leaf stem has many large air spaces, which helps with buoyancy and also allows oxygen to quickly diffuse into the roots for aerobic respiration
What happens to transpiration if the surrounding water or air is too humid
Water won’t evaporate, so water can’t leave the plant so the transpiration stream stops and plant can’t transport mineral ions to the leaves
How do plants deal with the issue of high humidity
Many plants have specialised structures at the tip/margins of their leaves called hydathodes which can release water droplets which may then evaporate from leaf surface
What is translocation in plants
Occurs in the phloem and is the movement of assimilates throughout the plant
What are assimilates
Substances made by plant using substances absorbed by the environment usually include sugars which are mostly transported as sucrose and amino acids
What is a source
Part of the plant that loads assimilates into phloem sieve tubes
What is a sink
A part of the plant that removes assimilates from phloem sieve tube
How is sucrose loaded into sieve tube
By an active process, involving use of energy from ATP in companion cells
What is used to actively transport hydrogen ions out of companion cells
Energy
Why are hydrogen ions actively transported out of companion cells in translocation
It increases their concentration outside the cell and decreases their concentration inside the companion cells so a concentration gradient is created
How do hydrogen ions diffuse back into companion cells in translocation
Through special cotransporter proteins which allow movement of hydrogen ions into a cell if they are accompanied by a sucrose molecule
What is hydrogen ions being allowed into companion cell if they are accompanied by a sucrose called
Cotransport or secondary active transport as it results from active transport of hydrogen ions out of cell and moves sucrose against its concentration gradient
What happens a sucrose concentration in companion cell increases
It can diffuse through plasmodesmata into sieve tube
How is sucrose moved along phloem
By mass flow
What solution is in the phloem
Sap- a solution of sucrose, amino acids and other assimilates
What direction does sap flow in in the phloem
Can be made to flow in either direction depending on where it’s required
What is flow in phloem caused by
Flow is caused by a difference in hydrostatic pressure between 2 ends of the tube, producing a pressure gradient
Why does sap flow from source to sink
Water enters tube at the source increasing the pressure and leaves tube at the sink reducing the pressure
What happens as sucrose enters sieve tube element at the source
It makes the water potential inside sieve tube more negative (lower) which results in water molecules moving into sieve tube element by osmosis from surrounding tissue which increases hydrostatic pressure in the sieve tube at the source
What is the source of a plant
Any part of the plant that leads sucrose into sieve tube
In early spring what are examples of a source in plants
Roots-where energy stored as starch is converted to sucrose and moved to other part of the plant in order to enable growth in spring
What is an example of a source in late spring, summer and early autumn
The leaf-sugars made during photosynthesis are converted to sucrose and loaded into phloem sieve tubes, occurs when leaves are green
Where in the plant is sucrose transported in translocation
Sucrose is transported to other areas of the plant that may be growing such as meristems or to roots for storage
What is a sink in a plant
Anywhere that removes sucrose from the phloem sieve tube
What are examples of things sucrose is used for in a plant
Sucrose could be used for respiration and growth in meristem or it could be converted to starch for storage in a root
Where sucrose is being used in cells how does it move out of the phloem
It diffuses out sieve tube via plasmodesmata or removed by active transport
What does removal of sucrose from sap do to the water potential
Increases saps water potential, so water moves out of sieve tube into surrounding cells which reduces the hydrostatic pressure in phloem at the sink
What does water entering sieve tube at the source increases
Increases the hydrostatic pressure in the phloem
What does water leaving the sieve tube at the sink reduce
Reduces the hydrostatic pressure in the phloem
What does the difference in hydrostatic pressure in the phloem create
Pressure gradient is set up along sieve tube and sap flows from higher to lower pressure, could be in either direction depending on where sucrose is being produced and where it is needed
Is it possible for sap to be flowing in different directions in different sieve tubes
Yes
Why is sap described as moving in mass flow
As sap in one tube is all moving in the same direction
What is the process of translocation
1.sucrose actively loaded into sieve tube element and reduces water potential, 2.water flows by osmosis and increases hydrostatic pressure in sieve tube elements, 3.sap moves down sieve tube from higher hydrostatic pressure at the source to lower at the sink, 4.sucrose removed from sieve tube by surrounding cells and increases water potential in sieve tube, 5.water moves out of sieve tube and reduces hydrostatic pressure
What is stoma doing when it’s cells are turgid
Open
What happen when stoma cells are flaccid
Shut
How do you measure rate of transpiration
Volume of cylinder, V=pi x r^2 x h, then divide volume by time, V/T= mm s-1
How would you carry out a potometer experiment
Cut the shoot under water so no air enters the xylem and cut stem diagonally for larger SA, dry leaves before experiment and make sure potometer is air tight, if you want air bubble back to 0 you open up the reservoir
