Transport in Plants Flashcards
What are the 3 main reasons why multicellular plants need transport systems?
Metabolic Demands
Size
Surface Area: Volume ratio
What are dicotyledonous plants?
Plants that make seeds that contain two cotyledons
What is a cotyledon?
Organs that act as food stores for plants and form the first leaves when the seeds germinate
What are the characteristics of herbaceous dicots?
Soft tissues
Relatively short life cycle (dies down at the end of season to soil level)
What are the characteristics of woody dicots?
Hard lignified tissues
Long life cycle
What is the vascular system?
A series of transport vessels running through the stem, roots and leaves
What makes up the vascular system in herbaceous dicots?
Xylem and Phloem
Where are the vascular bundles located in the stem of a plant? Why?
Around the edge to give strength and support
Where are vascular bundles located in the root? Why?
Middle of the plant
Helps withstand the tugging strains that result as stem and leaves are blown in the wind
Where are vascular bundles located in the leaf of a plant? Why?
Midrib of dicot=main vein
Branching as well to help with transport and support
What are the two main functions of xylem?
Transport of water and mineral ions
Support
Which direction is the flow of material in the xylem?
Roots to the shoots and leaves
What are the xylem made up of?
Dead cells
What is the structure of the xylem?
Long hollow structures made by several columns of cells fusing together end to end
Which two tissues are also present in the xylem? What is there function?
Xylem parenchyma- stores food and tannin deposits
Xylem fibres- provide extra mechanical strength
What is tannin?
A bitter chemical that protects plant tissues from attack from herbivores
What is the role of lignin within the xylem?
Reinforce the xylem vessels so that they do not collapse under the transpiration pull
What is the role of the phloem?
Transports food in form of organic solutes around the plant from leaves specifically sugars and amino acids
Which direction is the flow of material in phloem?
Both directions, up and down the plant
What is the structure of the phloem?
Elongated sieve tube elements joined end to end with perforate walls called sieve tubes to form sieve plates
What is the phloem attached to to and how do they link?
Companion cells via plasmodesmata
What are the key features of companion cells?
A cell which maintains nucleus, and organelles. Contains a high proportion of mitochondria to provide enough ATP for metabolic needs and active transport
What supporting tissues does the phloem contain?
Fibres and sclereids (cells with extremely thick cell walls)
What is the role of turgor within a plant?
Provides a hydrostatic skeleton to support the stems and leaves
Drives cell expansion, force that allows the roots to penetrate hard surfaces
What is a root hair cell?
The exchange surface in plants where water is taken into the plant from the soil
What is a root hair?
A long thin extension from a root hair cell, it is a specialised epidermal cell
What 4 ways are root hair cells adapted as exchange surfaces?
Microscopic size so can penetrate the soil
Large SA:V ratio
Thin surface layer for effective diffusion
Concentration of solutes in cytoplasm in root hair cells maintains water potential gradient.
How and why does water move from soil into the root?
Soil has a very high water potential
Cytoplasm of root has low water potential as it has lots of solutes dissolved in it
Water potential gradient, so water moves in via OSMOSIS
What is the symplast?
The continuous cytoplasm of the living plants cells that is connected via plasmodesmata
How does water move via the symplast pathway?
Water moves via continuous cytoplasm
Root hair cell has higher water potential (as water moved into cell from soil) than next cell along so water moves into next cell
Continues until the xylem
How is a steep water potential maintained in the symplast pathway?
Water leaves root hair cell via osmosis
Water potential in the cytoplasm falls
Steep water potential then maintained
What is the apoplast?
The cell walls and intercellular spaces
How does water move through the apoplast pathway?
Water fills the space between the cellulose cell wall
Water molecules move into the xylem
More water molecules are forced through due to cohesive forces of the water molecules
This creates a tension meaning there is a continuous stream
When does water stop moving separately in the apoplast and symplast pathway?
Endodermis
What is the Casparian Strip?
A band of suberin, which is a waxy material that runs around endodermal cells acting as a waterproof layer
What happens to the apoplast pathway at the casparian strip?
Ends and molecules have to join symplast pathway
Pass into the cytoplasm via cell surface membranes which gets rid of any toxic materials
How do water molecules move from symplast pathway into the xylem?
Mineral ions are actively transported into the xylem to lower the water potential
Water then moves via osmosis into the xylem
Define ‘root pressure’
The active pumping of mineral ions into the xylem by root cells that produces a movement of water into the xylem by osmosis
Give 4 pieces of evidence that suggest that active transport is involved in root pressure
Cyanide application prevents active transport occurring
Changes in variations of temperature suggest chemical reactions are occurring
Levels of oxygen fall, root pressure falls
Guttation
What is guttation?
Sap being forced out of the cut end of the stems at certain times of the day
What is the role of the waxy cuticle?
Waterproofing to minimise water loss
Define ‘transpiration’
The loss of water from the leaves and stems of plants
Where does water evaporate from?
The mesophyll cells in the leaves
Define ‘transpiration stream’
The movement of water through a plant from the roots until it is lost by evaporation from the leaves
What is the ‘transpiration pull’?
Water drawn up the xylem in a continuous stream to replace water lost by evaporation
Describe what happens in the ‘cohesion-tension theory’
1) Water molecules evaporate from the mesophyll cells by diffusion into air
2) Water potential in the cell is lowered by the evaporation
3) Water moves into cell from adjacent cell via osmosis
4) Repeated across the leaf to the xylem
5) Forces of cohesion and adhesion force water up the xylem
What is ‘adhesion’?
Water molecules forming hydrogen bonds with the carbohydrates in the walls of the xylem
What is cohesion?
Water molecules forming hydrogen bonds to each other
What is ‘capillary action’?
Water rising up narrow tubes against the force of gravity
What evidence is there for cohesion-tension theory?
Changes in diameter of trees
Xylem vessel broken, no water loss just air drawn in
Air in xylem vessel prevents entrance of water
How do plants help prevent excess water loss?
Stomatal pores on the guard cells, which can open and close
What changes when the stomatal pores are open or closed?
Turgor, when the turgor is low the guard cell closes the pore to minimise water loss
What 5 factors affect rate of transpiration?
Light Humidity Temperature Air movement Soil-water availability
How does light affect the rate of transpiration?
Increasing light intensity, increases rate of transpiration Because: More open stomata Increases rate of water diffusing out Increases evaporation from the leaf
What is relative humidity?
A measure of the amount of water vapour in the air compared to the total concentration of water the air can hold
How does humidity affect the rate of transpiration?
High humidity= lower rate of transpiration because water potential gradient is lower
Low humidity= higher rate of transpiration because water potential gradient is higher
How does increase in temperature affect the rate of transpiration? (2 ways)
1) increases kinetic energy of water molecules and increases rate of evaporation from spongy mesophyll into air spaces of leaves, increasing rate of transpiration
2) increases the concentration of water vapour that the external air can hold before it becomes saturated, so decreases humidity, increasing transpiration
How does air movement affect rate of transpiration?
Wind increases the rate of transpiration because it prevents water gathering around the leaf which lowers water potential gradient, so the diffusion rate is faster a
How does soil water availability affect the rate of transpiration?
Lower levels of water availability decreases rate of transpiration because the plant is under water stress and doesn’t want to lose any more water
Define ‘translocation’
The movement of organic solutes from source to sink in a plant
What are assimilates? Give an example
The products of photosynthesis that are transported around a plant eg. sucrose
What are the 3 main sources of assimilates in a plant?
1) Green leaves and green stems
2) Storage organs
3) food stores in seeds when they germinate
What are the 3 main sinks in a plant?
1) Roots that are growing
2) Meristems that are actively dividing
3) Plant parts that are laying down food stores
How is sucrose moved into the cytoplasm from the apoplast pathway in phloem loading?
H+ ions are actively transported out of the cell using ATP
H+ ions return to companion cell down a concentration via a cotransport protein
Sucrose is cotransported with H+ ions into cytoplasm
How are companion cells adapted for phloem loading?
High proportion of mitochondria to provide energy for active transport
Increased surface area
What happens to the sucrose once it is in companion cell?
Water moves in via osmosis from xylem to increase water potential
Turgor pressure increases
Water carrying assimilates forced up/down sieve tube element to sinks
How is phloem unloaded?
Diffusion of sucrose from the phloem into surrounding cells
What happens to water once phloem has been unloaded?
Water potential in sieve tube element> water potential in surrounding cells
Water moves into surrounding cells
Some water draw into transpiration stream in xylem
What 4 pieces of evidence suggest translocation is occurring?
1) Companion cell adaptation can be seen under microscopes
2) Mitochondria poisoned, translocation stops
3) Active process must be occurring as too fast for diffusion
4) Aphids!
What are xerophytes?
Plants that are adapted to extremely dry conditions with limited access to water
Give 3 examples of xerophytes
Conifers
Marram Grass
Cacti
Give 8 examples of adaptations that xerophytes have
Thick waxy cuticle Sunken stomata Reduced numbers of stomata Reduced leaves Hairy leaves Curled leaves Leaf loss Root adaptations
How do sunken stomata prevent water loss?
Stomata located in pits
Reduces air movement
Microclimate of still air- reduces water potential gradient
Reduces transpiration
How does reduced number of stomata prevent water loss?
Less water loss as there is less ability for water to leave the plant
How does reducing the number of leaves prevent water loss?
Reduces SA: V ratio
Less area for diffusion so less transpiration
How do hairy leaves minimise water loss?
Creates a microclimate of still humid air
Reduces water vapour potential gradient
Minimises water loss by transpiration
How do succulents over come lack of water?
Store water in parenchyma
Water stored when in plentiful supply and used in times of drought
How does leaf loss prevent water loss?
Leaves are lost when water is not available so there is no transpiration
What rood adaptations do xerophytes tend to have?
Long tap roots which can get to underground water supply
Mass of widespread shallow roots which absorb available water before it evaporates
What are hydrophytes?
Plants with adaptations to enable them to survive in very wet habitats or be submerged or at the surface of water
What function do all hydrophyte leaves need to have?Why?
The ability to float to get light for photosynthesis
Give 8 adaptations hydrophytes may have
Very thin or no waxy cuticle Always open stomata Reduced structure to the plant Wide flat leaves Small roots Large surface areas of stems and roots under water Air sacs Aerenchyma
Why do hydrophytes have wide, flat leaves?
To spread across surface of water to capture as much light as possible
Why do hydrophytes have small roots?
Water can diffuse directly into steam and leaf tissue so don’t need large roots
What is the purpose of air sacs in hydrophytes?
To enable lead to float to surface of water
What is aerenchyma?
Specialised packing tissue with many air spaces
What are the 2 main functions of aerenchyma?
Make leaves more buoyant
Form low resistant internal pathway for movement of substances to areas below the surface
What are pneumatophores?
Special aerial roots which grow upwards into the air
