TRANSPORT IN PLANTS
DESCRIBE THE CAMPANION CELLS
- In between the sieve tubes are small cells, each with a larger nucleus and dense cytoplasm. These are companion cells - They have numerous mitochondria to produce the ATP needed for active processes - The companion cells carry out the metabolic processes need to load assimilated actively into the sieve tubes
EXPLAIN ADAPTATIONS OF THE XYLEM TO ITS FUNCTIONS
- Made of dead cells aligned end to end to form a continuous column - Tubes narrow, so that the water column does not break easily and capillary actions can be effective - Bordered pits in the lignified walls allow water to move sideways from one vessel to another - Lignin deposited in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the plant grows, and enables the stem or branch to bend
HOW WATER LEAVES THE LEAF
- Most water leaves the plant via the stomata - Only a tiny amount of water leaves the plant through the waxy cuticle - Water evaporates from the cells lining the cavity immediately above the guard cells (the sub-stomatal air space) - This lowers the water potential in these cells, causing water to enter them by osmosis from neighbouring cells. In turn, water is drawn from the xylem in the leaf by osmosis - Water may also reach these cells by the apoplast pathway from the xylem.
DESCRIBE THE STRUCTURE AND FUNCTION OF A PHLOEM
- Phloem is a issue used to transport assimilates (mainly sucrose and amino acids) around plants. - Sucrose is dissolved in water to form sap. - Phloem tissues consists of sieve tubes- made up of sieve tube elements-and companion cells
WHY IS THE FLOW OF WATER NOT IMPEDED IN THE XYLEM
- There are no cross-walls - There are no cell contents, nucleus or cytoplasm - Lignin thickening prevents the walls from collapsing
WHAT HAPPENS ALONG THE PHLOEM(movement of sucrose?)?
- Water entering the sieve tube at the source increases the hydrostatic pressure - Water leaving the sieve tube at the sink reduces the hydrostatic pressure - Therefore a pressure gradient is set up along the sieve tube, and the sap flows from higher pressure to lower pressure - This could be in either direction, depending upon where sucrose is being produced and where it is needed - It is even possible that sap could be flowing in opposite directions in different sieve tubes at the same time - Since the sap In one tube is all moving in the same direction, this is mass flow
THE ROLE OF ENDODERMIS
- movement of water across the root is driven by an active process that occurs at the endodermis the endodermis is a layer of cells(known as starch sheath as it contains granules of starch-sign that energy is being used) surrounding the medulla and xylem. - casparian strip blocks the apoplast pathway between the cortex and the medullar--> this ensures that water and dissolved mineral ions(eg nitrates) have to pass into the cell cytoplasm through the plasma membranes - plasma membranes contain transporter proteins, which actively pump mineral ions from the cytoplasm of the cortex cells into the medullar and xylem - this makes the water potential of the medulla and xylem more negative, so that water moves from the cortex cells into the medulla and xylem by osmosis - once the water has entered the medulla, it cannot pass back back into cortex, as the apoplast pathway of the endodermal cells is blocked by the casparian strip
DESCRIBE ACTIVE LOADING
-Sucrose is loaded into the sieve tube by an active process-->This involves the use of energy from ATP in the companion cells -The energy is used to actively transport hydrogen ions (H+) out of the companion cells -This increases their concentration outside the cells and decreases their concentration inside the companion cells-->As a result, a concentration gradient is created -The hydrogen ions diffuse back into the companion cells through special cotransporter proteins -These proteins only allow the movement of the hydrogen ions into the cell if they are accompanied by sucrose molecules(known as cotransport) -It is also called secondary active transport, as it results from the active transport of the hydrogen ions out of the cell and moves the sucrose against its concentration gradient -As the concentration of sucrose in the companion cell increases, it can diffuse through the plasmodesmata into the sieve tube.
DESCRIBE THE DISTRUBUTION OF XYLEM AND PHLOEM(IN VASCULAR BUNDLE) IN THE YOUNG ROOT
-Vascular bundle (found at centre of a young root) = consists of endodermis (gets water in) + pericycle (meristem cells) + xylem (all of it together forms X/star shape) + phloem(found in between the legs of the X/star shape)--> shown in picture this arrangement provides strength to withstand the pulling force to which roots are exposed -endodermis- around vascular bundle, is a special sheath of cells, key role - getting H2O into xylem vessels
WATER UPTAKE
-When a plant is in pure water, the plant will take up the water molecules by osmosis(water molecules will move down the water potential gradient into the cell) -this is due to water potential in the cell is more negative than(lower) than the water potential of the water -but you would think that it would take up the water until it bursts. But once the cell is full, the cell will not continue to absorb water until it bursts→ because the cell has a strong cellulose cell wall. -Once a cell is full, it is described as being turgid. Water inside the cell starts to exert pressure on the cell wall, called the pressure potential. -As the pressure potential builds up, it reduces the reflux of water
MOVEMENT OF WATER BETWEEN CELLS
-When plant cells are touching each other, water molecules can pass from one cell to another. -The water molecules will move from the cell with the less negative (meaning high) water potential to the cell with the more negative (meaning lower) water potential. -This is known as osmosis
WATER LOSS
-When the plant is in a salt solution, it will lose water by osmosis(water moves down the water potential gradient out of the cell). -As water loss continues, the cytoplasm and vacuole shrink→ eventually, cytoplasm no longer pushes against the cell wall, and the cell is no longer turgid. -If water continues to leave the cell, the plasma membrane will lose contact with the wall - condition known as plasmolysis. -This tissue is now flaccid
stomata
-is the easy way for water to exit. water lost through this must be replaced -stomata is always open during the day for photosynthesis
DESCRIBE THE MOVEMENT PF SUCROSE
-movement of sucrose along the phloem is by mass flow -a solution of sucrose, amino acids and other assimilates flows along the tube -the solution is called sap, and it can be made to flow either up or down the plant as required -the flow is caused by a difference in hydrostatic pressure between the two ends of the tubes, which produces a pressure gradient. -water enters the tube at the source, increasing the pressure, and it leaves the tube at the sink, reducing the pressure. -therefore the sap flows from the source to the sink
EHAT IS TRANSLOCATION AND WHERE DOES IT OCCUR
-occurs in the phloem -movement of assimilates(substances made by the plants, using substances absorbed from the environment(eg sugars and amino acids) throughout the plant -part of the plant the plant that removes assimilates from the phloem sieve tubes is called a sink
DESCRIBE THE WATER UPTAKE AND MOVEMENT ACROSS THE ROOT(TRANSPIRATION STREAM)
-outermost layer of cells(epidermis) of a root contains root hair cells(have long extensions (root hair) that increase surface area of root) -they absorb mineral ions and water from soil -water moves across the root cortex down a water-potential gradient to the endodermis of the vascular bundle -water may also travel through the apoplast pathway as far as the endodermis, but must then enter the symplast pathway(since apoplast pathway is blocked by the casparian strip
WHY DO PLANTS NEED TRANSPORT SYSTEM?
-plants need substances(eg sugar, water and minerals) and get rid of waste production -multicellular organism -low surface area to volume ratio -high metabolic rate -diffusion is too slow and thus need a transport system plants-not active→ respiration rate low → demand for O2 low → demand can be met by diffusion - but demand for H2O and sugars is high→ plants absorb water and minerals at the roots → but cannot absorb sugar from soil→ so leaves perform gaseous exchange and manufacture sugar by photosynthesis -plants need transport system to move: -water and minerals from roots up to leaves -sugars from the leaves to the rest of the plant
DESCRIBE A SINK
-sink is anywhere that removes sucrose from the phloem sieve tubes. -the sucrose could be used for respiration and growth in a meristem, or it could be converted to starch for storage in a root -where sucrose is being used in the cells, it may also be removed by active transport -the removals of sucrose from the sap makes the water potential less negative (higher), so that water moves out of the sieve tube into the surrounding cells. this reduces the hydrostatic pressure in the phloem at the sink
DESCRIBE A SOURCE:
-sucrose entering the sieve tube element makes the water potential inside the sieve tube more negative(lower). -water molecules move into the sieve tube element by osmosis from the surrounding tissues. this increases the hydrostatic pressure in the sieve tube at the source -a source is any part of the plant that loads sucrose into the sieve tube -in early spring, this could be the roots, where energy stored as starch is converted to sucrose and moved to other parts of the plant in order to enable growth in the spring. -most obvious source is a leaf-sugars made during photosynthesis are converted to and loaded into the phloem sieve tubes -this occurs during late spring, summer, early autumn(when leaves are green) -the sucrose is transported to other areas of the plants that may be growing(meristems), or areas such as the roots for storage
STRUCTURE AND FUNCTION OF XYLEM
-tissue used to transport water and mineral ions CONSISTS 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 vessels
WHY IS TRANSPIRATION IMPORTANT?
-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
DESCRIBE THE DISTRUBUTION OF XYLEM AND PHLOEM(IN VASCULAR BUNDLE) IN THE LEAF
-vascular bundle forms the midrib and veins of a leaf. -dicotyledonous leaf has a branching network of veins that gets smaller as the spread away from the midrib -within each vein, the xylem is located on top of the phloem
HOW ARE TERRESTRIAL PLANTS ADAPTED TO REDUCE WATER LOSS (structural and behavioural)
-waxy cuticle on leaf-reduce water loss due to evaporation through the epidermis -stomata often found on the under surface of leaves, not on top surface - reduces the evaporation due to direct heating from the sun -most stomata are closed at night, when there is no light for photosynthesis -deciduous plats lose their leaves in winter, when the ground may be frozen(making water less available) and when temp may be too low for photosynthesis
DESCRIBE THE TYPICAL PATHWAY TAKEN BY MOST WATER LEAVING THE LEAF
1. water enters the leaf through the xylem, and moves by osmosis into the cells of the spongy mesophyll. may also pass along the cell walls via the apoplast pathway 2. water evaporates from the cell walls of the spongy mesophyll 3. water vapour moves by diffusion out of the leaf through the open stomata-this relies on a difference in the concentration of water vapour molecules in the leaf compared with outside the leaf(known as water potential gradient). there must be a less negative(higher) water vapour potential inside the leaf that outside
DESCRIBE THE APOPLAST PATHWAY
Appoplast: water passes through the spaces in the cell walls and between the cells. It does not pass through any plasma membranes into the cells. This means that the water moves by mass flow rather than by osmosis. Also dissolved mineral ions and salts can be carried with the water -water continues along the apoplast route until it hits the casparian strip in endodermis(stele). The casparian strip consists of the waxy substance suberin Once water hits this strip, it enter the cytoplasm of the endormis and takes the symplast pathway
DESCRIBE CACTI AND ITS ADAPTATIONS
Cacti show other features to overcome arid conditions: • Cacti are succulents — they store water in their stems which become fleshy and swollen. The stem is often ribbed or fluted so that it can expand when water is available. • The leaves are reduced to spines. This reduces the surface area of the leaves. When the total leaf surface area is reduced, less water is lost by transpiration. • The stem is green for photosynthesis. • The roots are very widespread, in order to take advantage of any rain that does fall.
DESCRIBE THE CAPILLARY ACTION
Capillary action- The same forces that hold water molecules together also attract water molecules to the sides of the xylem. This is called adhesion - As xylem vessels are very narrow, these forces of attraction can pull water up the sides of the vessel.
WHAT ARE DICOTYLEDONOUS PLANTS?
Dicotyledonous plants- have 2 seed leaves and a very characteristic distribution of vascular tissue
TERRESTRIAL PLANTS/PLANTS LIVING ON LAND
For most plants living on land access to water can be a problem. Stomata must be open during the day to allow for photosynthesis and respiration. While the stomata are open, there is an easy route for water to be lost. This water must be replaced. Plants living on land must be adapted to: • reduce this loss of water • replace the water that is lost.
DESCRIBE HYDROPHYTES AND THEIR ADAPTATIONS
Hydrophytes are plants that live in water, e.g. water lilies (family Nymphaeales). These plants have easy access to water, but are faced with other issues such as getting oxygen to their submerged tissues and keeping afloat — they need to keep their leaves in the sunlight for photosynthesis. The adaptations of a water lily include: • Many large air spaces in the leaf. This keeps the leaves afloat so that they are in the air and can absorb sunlight. • The stomata are on the upper epidermis, so that they are exposed to the air to allow gaseous exchange. • The leaf stem has many large air spaces. This helps with buoyancy, but also allows oxygen to diffuse quickly to the roots for aerobic respiration.
DESCRIBE MARRAM GRASS AND ITS ADAPTATIONS
Marram grass (Ammophila) specialises in living on sand dunes. The conditions are particularly harsh, because any water in the sand drains away quickly, the sand may be salty and the leaves are often exposed to very windy conditions. Marram grass is a xerophyte — a plant adapted to living in arid conditions. The adaptations of marram grass include: • The leaf is rolled longitudinally so that air is trapped inside air becomes humid, which reduces water loss from the leaf. The leaf can roll more tightly in very dry conditions. • There is a thick waxy cuticle on the outer side of the rolled leaf (upper epidermis), to reduce evaporation. • The stomata are on the inner side of the rolled leaf (lower epidermis), so they are protected by the enclosed air space. • The stomata are in pits in the lower epidermis, which is also folded and covered by hairs. These adaptations help to reduce air movement and therefore loss of water vapour. • The spongy mesophyll is very dense, with few air spaces — so there is less surface area for evaporation of water.
PATHWAYS TAKEN BY WATER:
PATHWAYS TAKEN BY WATER: Many plant cells are joined together by special cytoplasmic bridges. These are cell junctions at which cytoplasm of one cell is connected to that of another through a gap in their cell walls → these junctions are called plasmodemata (plasmadesma singular) -there are 3 possible pathways taken by water: symplast apoplast and vacuolar
DESCRIBE ROOT PRESSURE
Root pressure- The action of the endodermis moving minerals into the medulla and xylem by active transport draws water into the medulla by osmosis. Pressure in the root medulla builds up and forces water into the xylem, pushing the water up the xylem. Root pressure can push water a few metres up a stem, but cannot account for water getting to the top of tall trees.
DESCRIBE THE SYMPLAST PATHWAY
Symplast: water enters the cell cytoplasm through the plasma membrane. It can then pass through the plasmodesmata from one cell to the next
DESCRIBE TRANSPIRATION IN HYDROPHYTES
Transpiration is the loss of water vapour from the surfaces of the leaves — but the water will not evaporate into water or into air that has a very high humidity. If water cannot leave the plant, then the transpiration stream stops and the plant cannot transport mineral ions up to the leaves. Many plants contain specialised structures at the tips or margins of their leaves called hydathodes. These structures can release water droplets which may then evaporate from the leaf surface. Most dew seen on grass in the early morning is actually water released through hydathodes.
DESCRIBE THE TRANSPIRATIONAL PULL
Transpirational pull- The loss of water by evaporation from the leaves, must be replaced by water coming up from the xylem - Water molecules are attracted to each other by forces of cohesion. These cohesion forces are strong enough to hold the molecules together in a long chain or column - As molecules are lost at the top of the column, the whole column is pulled up as one chain. The pull from above creates tension in the column of water - This is why the xylem vessels must be strengthened by lignin. The lignin prevents the vessel from collapsing under tension - Because this mechanism involves cohesion between the water molecules and tension in the column of water, it is called the cohesion—tension theory - It relies on the plant maintaining an unbroken column of water all the way up the xylem - If the water column is broken in one xylem vessel, then the water column can maintained through another vessel via the bordered pits.
DESCRIBE THE VACUOLAR PATHWAY
Vacuolar: this is similar to the symplast pathways, but the water is not confined to the cytoplasm of the cells. It is able to enter and pass through membrane-bound organelles - the vacuoles
DESCRIBE THE DISTRUBUTION OF XYLEM AND PHLOEM(IN VASCULAR BUNDLE) IN THE STEM
Vascular bundles (found near the outer edge of the stem) In non-woody plants the bundles are separate and discrete In woody plants, bundles are separate in young stems, but become a continuous ring in older stems → arrangement provides strength and flexibility to withstand the bending forces to which stems and branches are exposed Xylem found towards the inside of each vascular bundle and the phloem towards the outside. Between xylem and phloem is a layer of cambium → layer of meristem cells that divide to produce new xylem and phloem
HOW DOES TEMPERATURE EFFECT THE RATE OF TRANSPIRATION?
a higher temperature will increase the rate of transpiration in three ways. it will: - increase the rate of evaporation from the cell surfaces so that the water potential in the leaf rises - increase the rate of diffusion through the stomata because the water molecules have more kinetic energy - decrease the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf
DEFINE A SOURCE?
a part of the plant that loads materials into the transport system(eg leaves photosynthesis and the sugars made are moved to other parts of the plant
DEFINE A SINK
a part of the plant that removes material from the transport system(eg, the roots receive sugars and store them as starch. at another time of year, the starch may be converted back to sugars and transported to a growing stem-so the roots can also be a source)
DEFINE XEROPHYTE
a plant adapted to living in dry conditions
DEFINE HYDROPHYTE
a plant adapted to living in water or where the ground is very wet
HOW DOES AIR MOVEMENT(WIND) EFFECT THE RATE OF TRANSPIRATION
air movement outside the leaf will carry away water vapour that has just diffused out of the leaf. this will maintain a high water vapour potential gradient
DESCRIBE THE DISTRUBUTION OF VASCULAR TISSUES IN A DICOTYLEDONOUS PLANTS
he vascular tissues are found together in vascular bundles(bundles may also contain other types tissues-e.g collenchyma and sclerenchyma which gives bundles some strength and help to support the plant)
HOW DOES RELATIVE HUMIDITY EFFECT THE RATE OF TRANSPIRATION
higher relative humidity in the air will decrease the rate of water loss since there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside
HOW DOES WATER AVAILABILITY EFFECT THE RATE OF TRANSPIRATION
if there is little water in the soil, then the plant cannot replaces the water that is lost. if there is insufficient water in soil, then the stomata close and the leaves wilt.
HOW DOES LIGHT INTESTITY EFFECT THE RATE OF TRANSPIRATION?
in light, the stomata open to allow gaseous exchange for photosynthesis. higher light intensity increases the transpiration rate
DESCRIBE THE XYLEM VESSELS
o Lignin saturates the walls of xylem - makes walls waterproof. à this kills the cells o End walls and content of the cell decay leaving a xylem vessel o Xylem vessels - long column of dead cells with no contents o Lignin also strengthens the vessel walls and prevents the vessel from collapsing à keeps vessels open at all times o Lignin thickening forms patterns in the cell wall à e.g spirals, annular(rings) or reticulate(network of broken rings) à prevents the vessels from being too rigid and allows some flexibility of the stem or branch o In some places lignifications are not complete, leaving gaps in the cells wall. à these form pits/bordered pits o The bordered pits in two adjacent vessels are aligned to allow water to leave one vessel and pass into the next vessel. They also allow water to leave the xylem and pass into the living parts of the plant
WHAT IS A TRANSPIRATION STREAM?
the movement of water from the soil, through the plant, to the air surrounding the leaf
WHAT MUST PLANTS LIVING IN LAND BE ADAPTED TO?
to reduce AND replace water lost
WHAT IS TRANSPIRATION (and why is it needed?)
transpiration is the loss of water vapour from the upper parts of the plants(eg leaves) water may evaporate through upper leaf surface-but this is limited due to waxy cuticles -thus water vapour leaves through the stomata(which open to allow gaseous exchange for photosynthesis)
WHAT ARE VASCULAR TISSUES?
xylem and phloem
DESCRIBE THE SIEVE TUBE ELEMENTS
Ø Elongated sieve tube elements are lined up end to end to form sieve tubes. They contain no nucleus and very little cytoplasm, leaving space for mass flow of sap to occur. At the end of the sieve tube elements are perforated cross-walls called sieve plates. Ø The perforations in the sieve plate allow movement of the sap from one element to the next Ø The sieve tubes have very thin walls and when seen in transverse section are usually five or six sided
DESCRIBE OTHER XEROPHYTIC FEATURES AND THEIR ADAPTATIONS
• Closing the stomata when water availability is low will reduce water loss and so reduce the need to take up water. • Some plants have a low water potential inside their leaf cells. This is achieved by maintaining a high salt concentration in the cells. The low water potential reduces the evaporation of water from the cell surfaces as the water potential gradient between the cells and the leaf air spaces is reduced - A very long tap root that can reach water deep underground.
WATER POTENTIAL
→ measure of the tendentency of water molecules to move from one place to another. -water always moves from a region of higher water potential to a region of lower water potential. Water potential of pure water = 0 -in plant cells, cytoplasm contains mineral ion and sugars(solutes) that will reduce the water potential → this is because there are few free water molecules available than in pure water→ as a result, water potential in plant cells is always negative
