Unit 3 - Transport in plants Flashcards
Why do plants need a specialised transport system?
- to move products of photosynthesis, water, and oxygen around from their place of origin
- most plants are large and so have to transport substances huge distances from root to tip
- SA: vol is small in plants (even though for leaves SA:vol high)
Define vascular system
a system of transport vessels in plants or animals
Define herbaceous
having a fleshy/soft stem
Define vascular bundle
The vascular system of herbaceous dicots, made up of xylem and phloem tissue
Define dicotyledonous
plants that produce two seed leaves
How do you tell if a plant is mono- or di- cotyledonous from its leaves?
- Monocots have vessels parallel to the leave
- Dicots have vessels which branch out from a central vessel
What is the vascular bundle made up of?
Xylem and phloem tissue
What are the products of photosynthesis?
Carbohydrates, proteins, lipids, nucleic acids
What are the products of photosynthesis known as collectively?
Organic compounds
Why do plants need water? (5 reasons)
- to maintain turgidity of cells
- to transport nutrients around the plant
- to create an aqueous environment for reactions to occur
- to cool plants by evaporation
- for photosynthesis
How is water transported in the plant?
Via the xylem
From the inside to the outside what tissues are present in a dicot root section?
xylem phloem pericycle endodermis cortex epidermis- root hair cells
In which part of the root is water taken up?
Root hairs
Which part of the root do the lateral roots grow from?
The pericycle
Why does the root have so many root hairs?
Increases the surface area able to take up water
How does water enter the root?
By osmosis
What must the water potential of the soil be relative to the root in order for water to be taken up?
The water potential of the soil must be less negative than the water potential of the root
How does the plant ensure the water potential of the root is more negative than the soil?
-ions from the soil are ACTIVELY pumped into the root
What does active pumping mean?
-Energy is required for the movement to occur
What pathway is taken by the water to go from the soil to the xylem?
root hair cell cortex endodermis pericycle xylem
What are the 2 different routes the water can take?
Apoplast
Symplast
What is the function of the pericycle?
-it is meristematic and produces the lateral roots
What is the function of the endodermis?
-contains a ring of suberin which is impermeable to water
What is the function of the cortex?
-stores a large amount of starch
What shape is the xylem tissue?
-star-shaped
What is the apoplast pathway?
Water travels through the cellulose cell wall
What is the symplast pathway?
Water travels through the cytoplasm and from cell to cell via plasmodesmata
What is the advantage of travelling through the cellulose cell wall?
It offers the path of least resistance
What is the Casparian strip made of?
Suberin
What is the key property of the Casparian strip?
It is impermeable to water
What happens to water travelling via the apoplast pathway when it reaches the Casparian strip?
It cannot continue and so is diverted to the symplast pathway
What is the advantage of water being diverted into the symplast pathway?
It allows the plant to control the movement of water into the root via osmosis
How does water enter the xylem from the endoderm?
- endodermal cells acitively pump salts into the xylem
- this makes the water potential of the xylem more negative than the endodermal cells
- this means water enters the xylem via osmosis
- ensures a large water potential gradient is maintained
Where is the Casparian strip located?
In the endodermal cells
How do endodermal cells move ions into the xylem?
Active transport
Once water has entered the xylem how does it move?
As a continuous stream
What force is created by the bonds between water molecules?
Cohesion
What are force is created by the interaction between water molecules and the vessel holding the water?
Adhesion
How is the continuous stream of water created?
Cohesion and adhesion
What 3 processes are involved in the movement of water up the stem and through the leaf?
- root pressure
- capillarity
- cohesion-tension theory
What causes root pressure?
- Endodermal cells actively pump ions into the xylem vessels
- A water potential gradient is generated so water enters the xylem vessel by osmosis, as the water potential in the xylem is more negative than the water potential in endodermal cells
- Pressure in the xylem increases, forcing water upwards
What is the evidence for root pressure?
- If cyanide is added to the root sap is no longer exuded
- root pressure relies on the active pumping of ions which requires ATP
- If there is no ATP no ions are pumped actively and no water potential gradient is created
- therefore water cannot enter the xylem by osmosis
- so there is no root pressure
What causes capillarity?
- The adhesive forces between xylem vessels and the water molecules
- This pulls a water molecule up
- due to the cohesive forces between water molecules other water molecules are pulled up with it
Why is xylem a bundle of very narrow vessels rather than one wide vessel?
Greater heights of liquid are achieved in thinner tubes due to capillarity
- with a smaller tube there is a greater contact with the vessel wall compared with the volume of water in the centre
- so greater cohesive forces
What is transpiration?
The loss of WATER VAPOUR from leaves and stems as a result of evaporation from cell surfaces inside the leaf and diffusion down a concentration gradient out through the stomata
Why is transpiration pull referred to as the Cohesion-tension theory?
- The cohesive force between water molecules pulls other molecules upwards as it is a continuous stream
- this puts pressure on the column of water
- this inward pressure is called tension
What effect does tension have on the column of water?
It produces a narrower column of water
What happens if a xylem vessel is broken?
The continuous stream of water is broken so no water can be taken up
If a xylem vessel becomes blocked can water still reach the leaves?
Yes
Why can water still reach the leaves if a xylem vessel becomes blocked?
The pits in the xylem allows for the movement of water
What makes up the vascular bundle, from inwards out?
Xylem
Cambium
Phloem
Sclerenchyma
What is the function of the cambium?
It is meristematic and differentiates to form xylem and phloem as the plant grows
What is the advantage of xylem vessels not being completely lignified?
It allows for the stem to flex and move slightly (e.g in the wind)
What is the function of lignin in Xylem?
It strengthen the walls to resist the forces generated by water moving up the stem
Give an example of what a plant uses glucose for
Aerobic respiration
Give an example of what a plant uses lipids for
Cell membranes
Give an example of what a plant uses proteins for
Enzymes for reactions
Give an example of what a plant uses nucleic acids for
DNA replication
What are assimilates?
The products of photosynthesis/respiration
Describe how sucrose is transported from the mesophyll cells to the phloem
- companion cells actively pump H+ ions out of the cell and into surrounding mesophyll cells
- H+ diffuse back into the companion cell, down a diffusion gradient and co-transport sucrose into the companion cell
- sucrose diffuses down a concentration gradient into sieve tube elements
Describe how sucrose is moved along the phloem
- sucrose enters the phloem which lowers the water potential of the phloem
- water enters by osmosis
- water also enter from the xylem
- water entering the phloem forces the contents to flow
- this is known as mass flow
Describe how sucrose moves from the phloem to plant cells
- sucrose and other assimilates leave the phloem by diffusion
- the cells then use the sucrose
- keeping the sucrose concentration of the cell lower than the xylem and maintaining a concentration gradient
What do plant cells use sucrose for?
- converting it to starch for storage
- use it for respiration
How do guard cells differ from the lower epidermal cells?
- they contain chloroplasts
- they have extra cellulose thickening on the inner side of the cell
Outline how guard cells open
- cells surrounding guard cells actively pump K+ ions into the guard cells, making their water potential more negative
- water enters by osmosis as water potential of guard cells is more negative than surrounding cells
- guard cells swell but because inner wall is thicker than outer wall as cell swells a pore opens up
What is crucial about the cell walls of guard cells?
-the inner wall is thicker than the outer wall
Outline how guard cells close
- K+ ions diffuse out of the guard cells and back into epidermal cells
- no longer more negative water potential in guard cells so water leaves by osmosis
- guard cells become flaccid
Give 4 ways plants generally conserve water
- waxy cuticle
- stomata on underside of leaf
- closable stomata
- roots that grow down to the water in the soil
What is a xerophyte?
Plants with adaptions that enable them to survive in dry habitats or habitats where water is in short supply in the envrionment
Give two examples of xerophytes
Confiers and marram grass
In what kind of environment may water loss become a really problem for plants?
Hot, dry, breezy conditions
How do sunken stomata help to adapt a plant to hot dry conditions?
- reduce air movement
- creating a microclimate of still, humid air
- that reduces the water vapour potential gradient so reduces transpiration
How does a reduction in the number of stomata adapt xerophytes?
-reduces water loss by transpiration
What is a downside of reduced numbers of stomata?
-reduces gas exchange capillaries
How does a reduction in the number of leaves adapt xerophytes?
-water loss is greatly reduced by minimising the amount of water loss by transpiration
How do hairy leaves adapt xerophytes?
- create a microclimate of still, humid air
- this reduces the water vapour potential gradient
- minimises the loss of water by transpiration from the surface of the leaf
How do curled leaves adapt xerophytes?
- confine all the stomata within a microenvironment of still, humid air
- to reduce diffusion of water vapour from the stomata
How are succulents adapted to their envrionment?
- contain specialist parenchyma tissue in stems and roots
- water in stored in these and then used in times of drought
How can losing leaves adapt a xerophyte?
- leaves lost when water is not avaliable
- reduces water loss by transpiration
- trunk and branches turn green and photosynthesise
How do xerophytes have roots adapted for their environment?
- long tap roots grow deep into the ground below the surface
- mass of widespread shallow roots with large surface area are able to absorb surface water before a rain shower evaporates
How can some xerophytes ‘avoid the problem’ of their environments?
- plants die but leave seeds to germinate and grow rapidly when it rains again
- some survive as storage organs (e.g tubers, bulbs)
- some plants can be completely dehydrated and recover when it rains again
What is a hydrophyte?
-plants with adaptions that enable them to survive in very wet habitats or submerged at the surface of the water
Give two examples of hydrophytes
Water lilies and water cress
What are the problems faced by hydrophytes?
- water logging, air spaces of plants need to be full of air not water
- it is important that leaves float on water to enable photosynthesis
How does having a thin/no waxy cuticle adapt hydrophytes?
-allows water to be lost through transpiration
How does the waxy cuticle of hydrophytes and xerophytes differ?
Hydrophytes may have a very thin or no waxy cuticle whereas xerophytes have very thick waxy cuticles
How does having many stomata that are permanently open adapt hydrophytes?
maximises gas exchange
Why do hydrophytes have reduced structural support?
- water supports the leaves and flowers
- so no need for strong supporting structures
How do wide, flat leaves adapt hydrophytes?
- spread across the surface of the water
- capture as much light as possible
Why do hydrophytes have small roots?
- water can diffuse directly into stem and leaf tissue
- so less need for uptake by roots
How does have a large surface area of stems and roots under water adapt hydrophytes?
-maximises area for photosynthesis and for oxygen to diffuse into submerged plants
How do air sacs adapt hydrophytes?
-enable leaves and/or flowers to float on the surface of the water
What are aerenchyma?
Specialised parenchyma tissue which has many large air spaces
How do aerenchyma adapt hydrophytes?
- makes leaves and stems more buoyant
- form a low resistance pathway for the movement of substances such as oxygen to tissues below water
- this helps the plant to cope with extreme low oxygen conditions
