Transport in plants Flashcards
Phloem FUNCTION
ROLE ~ to transport the assimilates from the leaves to other parts of the plant
E.G roots , flowers
- MOVEMENT~ Molecules can be transported UP & DOWN the phloem
- PHLOEM SAP ~ fluid in the phloem
- LIVING TISSUE
- NO LIGNIN ~ phloem fibres & sclereids provide support
TWO TYPES OF TISSUE:
- Sieve tube element cells
- Companion cells
Vascular bundles
- consist of cells specialised for TRANSPORTING fluids by mass flow.
- Xylem vessels and phloem vessels are grouped together in vascular bundles.
- The arrangement is different in the:
- roots
- stem
- leaves
Vascular bundle in the ROOTS
- Root hair cells form a layer of external tissue called the EPIDERMIS
- CORTEX ~ thick layer of cells that contains parenchyma cells
VASCULAR BUNDLE ~ centre of root:
- Surrounded by a layer of cells called the EPIDERMIS
- XYLEM VESSELS:
- mechanically strong
- group together in the centre of the root
- helps prevent the root from being pulled out of the soil e.g by strong winds
Vascular bundle in the STEM
MEDULLA ~ centre of the plant stem which consists of parenchyma cells
- PHLOEM vessels located around the EDGE and XYLEM vessels found closer to the CENTRE.
- VASCULAR BUNDLE ~ found around the edge of the stem to help withstand bending due to wind.
- CAMBIUM ~ a layer of meristem cells that divide to produce new xylem and phloem.
Vascular bundle in LEAVES
Vascular bundle
- found in the CENTRE
- known as the MIDRIB
- Allows transport and gives support
- Leaf is also supported by smaller vascular bundles connected to the main one.
- XYLEM found in upper part of vascular bundle and PHLOEM found on lower part.
- PHOTOSYNTHESIS mainly takes place in the PALISADE MESOPHYLL which is the UPPER part of the leaf.
SUBSTANCES transported in plants
MINERAL IONS :
- cells in root tissue absorb mineral ions by active transport
- e.g NITRATE IONS used by plants to make amino acids
HORMONES:
- Transported from where they are synthesised to their target tissues.
XYLEM TISSUE
- Used to TRANSPORT water and mineral ions from the roots up to the leaves and other parts of the plant.
- The interior contents of the cell DIES.
Consists of:
- VESSELS to carry water and dissolved mineral ions
- FIBRES provide mechanical strength
- PARENCHYMA CELLS for the storage of starch and contain tannins which deter herbivores.
STRUCTURE related to FUNCTION of XYLEM VESSELS
LIGNIN :
- strengthens
- waterproof
- prevents collapsion
- Spiral shaped prevents vessel from becoming too rigid and allows some flexibility of the stem or branch.
NARROW:
- prevents water column from breaking easily allowing capillary action to be effective
BORDERED PITS:
- occur where lignification is incomplete
- allow water to move sideways from one vessel to another
HOLLOW:
- No cell contents, nucleus or cytoplasm
NO CROSS WALLS:
- One continuous tube so movement of substances is not disrupted.
How are ROOT HAIR CELLS adapted to allow the rapid uptake of water by osmosis?
DENSLEY PACKED:
- massively increases the SA:V of the root
THIN:
- Outer surface consists only of the cell wall and cell membrane
- short osmosis pathway
LOW WATER POTENTIAL:
- contains dissolved mineral ion and sugars
- the cell has a lower water potential than the soil
- water moves into the cell by osmosis down the water potential gradient
Symplast Pathway
- water moves from the CYTOPLASM of one cell to the cytoplasm of the adjacent cell.
- To do this , water moves through the PLASMODESMATA:
a microscopic channel through the cell wall connecting the cytoplasm of cells. - Relatively SLOW ~ obstructed by organelles in the cytoplasm
Driven by the WATER POTENTIAL gradient between the root hair cells & xylem:
- water continually moves into root hair
- the WP of the root hair rises above the WP in the cortex cells
- WP in xylem is relatively low, causing water to move across the cortex
Apoplast Pathway
- Water moves within the CELL WALLS and the SPACES BETWEEN cells
- Cellulose cell walls have a relatively OPEN STRUCTURE, allowing water to move easily between the cellulose fibres
- As water is carried away through xylem, more water moves along the apoplast pathway due to the COHESION of the water molecules.
- Offers much LESS RESISTANCE to water flow
The ENDODERMIS
- Before water enters xylem, it passes through a LAYER OF CELLS called endodermis
SUBERIN ~ a band of waterproof material which runs around the cell walls of these cells
CASPARIAN STRIP ~ formed from this band and prevents water moving through the apoplast pathway
- Instead the water passes through the cell membrane and into the CYTOPLASM , becoming part of the symplast pathway.
Passage of water into the xylem from the cytoplasm
- By forcing all the water through the cytoplasm, the cell membrane can CONTROL what substances can enter the xylem
- Cells in the endodermis use ACTIVE TRANSPORT to pump mineral ions into the xylem
- This LOWERS the WP of the xylem
- This TRIGGERS water to move IN to the xylem vessels by OSMOSIS
ROOT PRESSUE:
- Caused by water moving into the xylem vessels by osmosis
- The resulting force which helps to move water up the plant
- ACTIVE PROCESS ~ requires energy from respiration
INHIBIT RESPIRATION = ROOT PRESSURE STOPS:
- Using METABOLIC POISONS e.g cyanide
- Excluding OXYGEN
Movement of water through plants
Pathway water has to take:
- soil
- root hair cell
- root cortex
- xylem
THREE PATHWAYS:
- Symplast pathway ~ cytoplasm
- Apoplast pathway ~ cell walls
- Vacuolar pathway ~ similar to symplast , water can move through vacuole as well as cytoplasm
Xerophytes
- Plants that live in areas where water loss by transpiration is GREATER than taken up by the roots
- Have ADAPTIONS to live in hot & dry conditions
CACTI ~ adaptions
FIBROUS SPINES:
- Reduces the SA:V ~ reduces water loss
- Trap moist air ~ reduce rate of transpiration
- Provides shade for the stem
THICK WAXY CUTICLE:
- Water proof & impermeable
- reduces evaporation of water
STOMATA ARE SUNKEN INTO PITS:
- Traps a layer of moist air
- reduces water loss by transpiration
STOMATA ONLY OPEN AT NIGHT:
- absorb CO2 when conditions are relatively cool
- Transpiration rates are higher during the day when conditions are hotter, so having the stomata closed reduces water loss
DEEP ROOTS:
- access water from the lower levels of the soil
EXTENSIVE SHALLOW ROOTS:
- absorb water after a rain shower before the water evaporates
WATER STORAGE:
- Swollen stem
- Thick fleshy leaves ~ SUCCULENTS
MARRAM GRASS ~ adaptions
Found in SAND DUNES:
- water is scarce
- windy conditions increase evaporation
LEAVES ROLL UP INTO A TUBE:
- stomata on the inside
- moist air is trapped within the tube , rather than being blown away by wind
STOMATA IN SUNKEN PITS & FINE HAIRS:
- ensure the moist air is trapped around the stomata
- Reduces the CONCENTRATION GRADIENT for water vapour between the air and the internal spaces between the leaf cells
- Reduces rate of DIFFUSION of water vapour out of the stomata
THICK WAXY CUTICLE:
- reduces evaporation from the surface
LONG ROOTS:
- extend deep into the sand to find water
EXTENSIVE SHALLOW ROOTS:
- help the sand retain water
Assimilates
- In leaves, plants carry out PHOTOSYNTHESIS to produce GLUCOSE
- Glucose is used to form other compound , E.G amino acids & other sugars
- These are known as assimilates and are transported in the PHLOEM.
Sieve tube element cells ~ phloem
- Consist of a long line of cells arranged END to END.
NO ORGANELLES:
- leaves space for mass flow of sap to occur
SIEVE PLATES:
- Perforated cross-walls
- allows phloem sap to move between cells
Companion cells ~ phloem
ROLE ~ provide essential molecules to the sieve tube elements
MANY MITOCHONDRIA:
- Provide energy for active loading of sucrose into sieve tubes
PLASMODESMATA:
- Microscopic channels linking the companion cells to STEC
- ATP and proteins can move through to the STEC
Translocation
The movement of ASSMILATES around the plant , in the PHLOEM from SOURCES to SINKS.
SOURCES :
- Where assimilates are PRODUCES
- e.g photosynthesising leaves , storage organs (tubers)
SINKS:
- Where assimilates are REQUIRED
- e.g roots, storage organs, growing regions (shoots)
Translocation process 1 ~ ACTIVE LOADING
- ACTIVE TRANSPORT ~ large amounts of mitochondria provide ATP
- used in the companion cells to pump its H+ ions OUT into the cytoplasm & cell wall spaces
- Creates a CONCENTRATION GRADIENT of H+ ions , with more on the OUTSIDE of the cell membrane.
- H+ ions DIFFUSE back into companion cells ACCOMPANIED by SUCROSE
Translocation process 2 ~ MOVEMENT OF SUCROSE & WATER INTO STE
- Concentration of SUCROSE in companion cells is HIGH
- Sucrose diffuses through PLASMODESMATA into STEC
- LOWERS the water potential inside the SIEVE TUBE ELEMENT
- WATER from XYLEM VESSELS moves into STE by OSMOSIS
- INCREASES the HYDROSTATIC PRESSURE inside the STE.
Translocation process 3 ~ MASS FLOW & SINK
- Bulk movement of phloem sap UP or DOWN the STE towards the SINK
- At the sink, sucrose moves out of sieve tube element
Converted:
- GLUCOSE ~ respiration
- STARCH ~ storage
- INCREASEs the water potential inside STE
- Water moves OUT back into the xylem, and joins TRANSPIRATION STREAM.
EVIDENCE to SUPPORT & QUESTION the active model of movement into the phloem
SUPPORT:
- The RATE of flow of sucrose is much faster than could take place by DIFFUSION ALONE
- If we INHIBIT the companion cell MITOCHONDRIA, translocation STOPS.
QUESTION:
- SIEVE PLATES ~ function is unclear as they seem to hinder mass flow
- Sucrose is delivered at the SAME RATE to all regions , rather than going more quickly to more needed regions
Transpiration
The LOSS of WATER VAPOUR from AERIAL parts of the plant.
PROCESS:
- The SURFACE of cells in the leaf are covered with a THIN layer of WATER
- This water EVAPORATES from the surface of the cells
- INTERNAL LEAF SPACES ~ HIGH concentration of water vapour
- EXTERNAL AIR ~ LOW concentration of water vapour
- When STOMATA open , the water vapour DIFFUSES out of the leaf to the external air
Transpiration Stream & Tension
The MOVEMENT of water from the ROOTS, up the XYLEM, and out of the leaf.
- The CONTINOUS evaporation of water vapour from the surface of cells in leaves , LOWERS the water potential of these cells
- Water moves by OSMOSIS from ADJACENT cells
- WATER POTENTIAL of these cells LOWERS , causing water to move INTO them
- At some point, this reaches the XYLEM, with water passing OUT of the xylem to adjacent cells
TENSION ~ the continuous movement of water out of the xylem during transpiration.
Cohesion- Tension theory
Allows the TRANSPIRATION STREAM to take place , caused by:
TRANSPIRATION PULL ~ the combined effect of :
CAPILLARY ACTION ~
- water can move up very thin tubes
against the force of gravity
- This replaces water that has been
removed from xylem vessels due to
transpiration.
ADHESION ~
- water molecules form hydrogen bonds to molecules in the xylem vessel walls
- Water molecules ADHERE to LIGNIN in
xylem walls
COHESION ~
- water form hydrogen bonds to each other
EVIDENCE for the cohesion-tension theory
IF XYLEM VESSELS ARE BROKEN:
- air is sucked into the xylem, suggesting the vessel was under TENSION
- the air prevents COHESION between the water molecules, so water movement stops
CHANGE IN THE DIAMETER OF TREE TRUNKS:
- diameter reduces when transpiration is at its MAXIMUM
- shows that the transpiration pull generates a PRESSURE or TENSION in the xylem
The IMPORTANCE of transpiration
- TRANSPORTS useful mineral ions up the plant
- Maintains cell TURGIDITY
SUPPLIES WATER FOR:
- Growth
- Cell elongation
- Photosynthesis
- Evaporation of water keeps the plant COOL on a hot day
Effect of LIGHT INTENSITY on the rate of transpiration
INCREASING light intensity, INCREASES the rate of transpiration.
- Number OPEN STOMATA increases
- More water vapour can diffuse out of the leaf
- At HIGH light intensities, the rate of transpiration STOPS INCREASING
- Almost ALL of the stomata will be open
Effect of RELATIVE HUMIDITY on the rate of transpiration
RELATIVE HUMIDITY ~ the concentration of water vapour in the air as % of the maximum possible
INCREASING relative humidity:
- creates a SMALLER concentration gradient between inside of leaf & external air
- REDUCES rate of transpiration
Effect of TEMPERATURE on the rate of transpiration
INCREASING temperature, INCREASES rate of transpiration.
AT HIGHER TEMPERATURES:
- Water molecules have more KINETIC ENERGY
- GREATER rate of evaporation of water from the internal surfaces of the leaf
- The RELATIVE HUMIDITY of the external
air DECREASES
These TWO EFFECTS:
- Increase the concentration gradient of water between the inside of the leaf & external air
Effect of AIR MOVEMENT on the rate of transpiration
INCREASING air movement, INCREASES rate of transpiration.
- Air moving outside of the leaf can REMOVE water vapour that has just diffused out of stomata
- This INCREASES the concentration gradient of water vapour
Effect of WATER LEVELS IN THE SOIL on the rate of transpiration
INCREASING water levels , INCREASES the rate of transpiration
- In DROUGHT CONDITIONS, the roots produce a HORMONE
- This TRIGGERS the stomata to CLOSE
- REDUCES transpiration which REDUCES WATER LOSS by plant
The POTOMETER
- what it measures
- the apparatus
- assumptions
Measures the rate of WATER UPTAKE into a plant.
Consists of:
- CAPILLARY TUBE filled with water , connected to:
- PLANT cut from the stem
- SYRINGE filled with water
- NEEDLE used to place an air bubble at the
end of the capillary tube
ASSUMPTION:
- All water taken in by plant is used in transpiration
- However, some used in PHOTOSYNTHESIS
How the potometer works
- As water evaporates from the leaves of the PLANT, water is DRAWN IN to the stem
- Causes AIR BUBBLE to move towards the plant
- By measuring how far the air bubble MOVES in a given TIME , the RATE of water uptake can be calculated.
- Use different CONDITIONS to see how this effects the water uptake:
- Temperature ~ incubator
- Air movement ~ fan
- Light intensity ~ lamp
- Humidity ~ plastic bag
PRECAUTIONS when using the potometer
CUT PLANT UNDERWATER:
- prevents water being sucked into the
xylem vessels
- prevents breakage of the water column
CUT PLANT OBLIQUELY:
- Maximise surface area for water uptake
ALLOW APPARATUS TO EQUILBRATE:
- Allows any initial disturbances to settle
KEEP CONDITIONS CONSTANT:
- temperature, light intensity, humidity, water levels & water movement
- Unless one is the independent variable
FULLY SEALED
- smear petroleum jelly around the connection between stem & tube
PLACE POTOMETER UNDERWATER WHEN ATTACHING PLANT:
- prevents any air gaps
Guard cells & their FEATURES
- Surround the STOMA
- Determine whether the stoma is open or closed
CELLULOSE CELL WALL:
- prevents the guard cells from expanding
evenly when water moves into them by
osmosis
- Develop a CURVE SHAPE
- Allows the stoma to open between them
CELLULOSE MICROFIBRILS ARRANGED IN RINGS:
- Cause guard cells to expand lengthwise
rather than width wise
Why do plants need a transport system?
- SMALL surface area to volume ratio
- QUICKER than diffusion alone
- Ensures molecules E.G sucrose , are transported to tissues in the plant
TO TRANSPORT:
- water & minerals from the roots up to the leaves
- sugars from the leaves to the rest of the plant
PLANTS CAN:
- absorb water & mineral at the roots
- perform gaseous exchange
- manufacture sugars by photosynthesis
PLANTS CANNOT:
- absorb sugars from the soil
- absorb water from the air
