1. Xylem & Phloem Structure Flashcards
see slide 2 for dia of plant vascular system
xylem, phloem, root hair cells
Why do plants require a transport system?
- A transport system ensures that all the cells of a plant receive a sufficient amount of nutrients (eg. Water, minerals, sugars to live)
- Plants are fairly big & have a relatively small SA:V ratio, but have a high metabolic rate
- This is achieved by the combined action of XYLEM tissue and PHLOEM tissue
Plants that have specialised transport systems are known as VASCULAR PLANTS
3 reasons why multicellular plants require specialised mass transport systems
- Increasing transport distances (due to size)
- SA:V ratio
- Increasing levels of activity (metabolic rate)
3 reasons why multicellular plants require specialised mass transport systems: Large transport distances (due to size)
- Every cell in a plant requires water, glucose & mineral ions
- The roots of a plant take in water & mineral ions while the leaves produce glucose by photosynthesis
- These molecules need to be transported to other parts of the plant (glucose is transported as SUCROSE in plants)
- This large transport distance makes simple diffusion a non-viable method for transporting substances all the way from the exchange site to the rest of the organism
- Diffusion wouldn’t be fast enough to meet the metabolic requirements of cells
3 reasons why multicellular plants require specialised mass transport systems: SA:V ratio
- As the size of a plant increases, its SA:V ratio decreases (bc volume increases much more rapidly than SA as size increases)
- This means is has relatively less SA available for substances to diffuse through, so the rate of diffusion may not be fast enough to meet its cells’ requirements
- Large plants therefore cannot rely on diffusion alone to supply their cells with substances & to remove waste products
- Therefore, they require specialised transport systems
How are plants adapted to increase SA:V ratio
- Plants have a branching body shape
- Leaves are flat and thin
- Roots have root hairs
3 reasons why multicellular plants require specialised mass transport systems: Metabolic rate (increasing activity levels)
- Larger organisms are not only more physically active but they also contain more cells than smaller organisms
- A larger no. of cells results in a higher level of metabolic activity
- As a result, the demand for oxygen & nutrients is greater & more waste is produced - Plant cells & tissues have a much lower metabolic rate than animal cells
- Therefore their demand for oxygen for anaerobic respiration is reduced
What are Cotyledons
Organs that act as food stores for the developing embryo
What are dicot plants
- Plants that make seeds that contain 2 cotyledons
There are 2 main groups of dicots:- Herbaceous dicots (non-woody stem) eg daisies
- Woody dicots eg oak
What are monocot plants
Plants that make seeds that contain 1 cotyledon
see slide 11 for comparison between monocot and dicot plants
What is the vascular system in plants
- A plant has a series of transport vessels running through the roots, stems & leaves
- This system of vessels is known as the vascular system
What are the types of transport vessels that make up herbaceous dicot vascular systems
Xylem & Phloem
- They are arranged tg in vascular bundles in the stem, roots & leaves
What are the 2 different plant transport systems
- Transpiration system
- Translocation system
What is the transpiration system
- The movement of water molecules & dissolved mineral ions
- Xylem vessels
- Passive process
What is the translocation system
- The movement of sugars (sucrose) & amino acids
- Phloem vessel - sieve & companion cells
- Active process
Overall water movement in plants
- Water is absorbed from soil, but is needed all over the plant
- Water & mineral ions move up stem in xylem
- Water is lost from the leaves
Overall sugar (sucrose) movement in plants
- Sugars (glucose) are made in leaves but are needed all over the plant
- Sugars (as sucrose) move down stem in phloem
What is a vascular bundle
- Xylem & Phloem are arranged in vascular bundles in the roots, stems & leaves.
- The arrangement of xylem & phloem is different in different organs
Vascular bundle structure
- There is a layer of cambium in between xylem & phloem, that is meristem cells which are involved in production of new xylem & phloem tissue
(see slide 17, 18 for dia)
Location of vascular bundles
The location of vascular bundles is dependent on which organ they are in as the different organs are under different stresses
Location of vascular bundles: In roots
- In roots, the vascular bundle is found in the centre
XYLEM: in the centre core
EFFECT: This helps the roots withstand the pulling strain they are subjected to as the plant transport water upwards & grows
PHLOEM: on the edges of the centre core
Location of vascular bundles: In stems
- In stems, the vascular bundles are located around the outside
XYLEM: on the inside (closest to centre of the stem) to help support the plant
PHLOEM: on the outside (closest to the epidermis)
Location of vascular bundles: In leaves
In leaves, the vascular bundles form the midrib & veins and therefore spread from the centre of the leaf in a parallel line.
XYLEM: on the upper side of the bundles (closest to upper epidermis)
PHLOEM: on the lower side of the bundles (closest to lower epidermis)
see slide 21 for dia of xylem, phloem, cambium
Vascular system in a root
In the endodermis around the vascular bundle of the root, there is a layer of meristem cells that form the pericyclic.
- This provides a ‘drill’ like structure, enabling the plant to push down into soil
- Xylem tissue is strongest so is in centre ( as an ‘X’)
- Phloem in 4 separate sections surrounding the xylem
(slide 22, pg88)
Vascular system in a leaf
- Xylem located on top of phloem
- The xylem & phloem make up a network of veins which support the thin leaves
- This only applies to to dicotyledonous plants (other plant types have a diff structure)
(slide 23, pg88)
Vascular system in the stem
- Xylem located on the inside (in non-wooded plants) & phloem on the outside
- The xylem & phloem are near the outside in stems, to provide support that reduces bending
- The cambium layer contains meristem cells
(slide 25, pg88)
Differences in location of vascular bundles in leaves, stem, root
slide 26, pg88
Vascular bundles in dicot and monocot STEM
slide 27, 28
Function of Xylem tissue
- Transports water & dissolved minerals upwards from the root hair cells to the leaves
- This is called the transpiration stream
Structure of Xylem tissue
- Long, tube-like structures formed from cells joined end to end
- A dead tissue - no cytoplasm, no nuclei
- No end walls making an uninterrupted tube - allows water to pass through easily
- Walls contains spiralised lignin - gives xylem tissue support, prevents from collapsing inwards (amount of lignin increases as cell gets older)
-
Small pits in walls (non-lignified areas) - water & ions move into/out of vessels through these non-lignified areas
(pg88)
Function of Phloem tissue
Transports solutes (dissolved substances), mainly sugars like sucrose, up & down plants
Structure of Phloem tissue
Like xylem, phloem is formed from cells arranged in tubes. BUT, unlike xylem, its purely a trasnport tissue - isn’t used for support as well
- Contains phloem fibres, phloem parenchyma, sive tube elements & companion cells
Structure of Phloem tissue: Sieve tube elements
- These are living cells that form the tube for transporting solutes through the plant
- They are joined end to end to form sieve tubes
- The “sieve’ parts are the end walls, which have lots of holes in them - allowing solutes to pass through
- Unusually for living cells, sieve tube elements have no nucleus, a very thin layer of cytoplasm & few organelles
- The cytoplasm of adjacent cells is connected through the holes in the sieve plates
(pg89)
Structure of Phloem tissue: Companion cells
- The lack of a nucleus & other organelles in sieve tube elements, means they cant survive on their own. So theres a companion cell for every sieve tube element
- Companion cells carry out the living functions for both themselves & their sieve cells.
Eg. they provide the energy for the active transport of solutes
What is a plant’s vascular system
Xylem & Phloem make up a plant’s vascular system. They are found throughout a plant & transport materials to all parts.
Other way of referring to plants with wooded stems
Herbaceous dicotyledonous plants
What are transverse cross-sections
- The position of the xylem & phloem in the root, leaf & stem are shown in transverse cross-sections.
- This refers to sections cut through each structure at a right angle to its length
What are longitudinal cross-sections
- These are taken along the length of a structure.
- For eg, theyre used to show where the xylem & phloem are located in a typical stem (see pg88)
Method to dissect plant stems
- Use a scalpel to cut a cross-section of the stem (transverse or longitudinal). Cut the sections as thinly as possible - thin sections are better for viewing under a microscope
- Use tweezers to gently place the cut sections in water until you come to use them. This stops them from drying out
- Transfer each section to a dish containing a stain, eg toluidine blue (TBO), & leave for 1min. TBO stains the lignin in the walls of the xylem vessels blue-green. This will let you see the position of the xylem vessels & examine their structure
- Rinse off the sections in water & mount each one onto a slide
- Examine the plant tissue under a microscope & draw
