Post on 22-Jan-2018
transcript
TOPIC 6: PLANT STRUCTURES
Learning outcomes
• Iden=fy plant structure • Describe the func=ons for each plant structure • Explain the transporta=on of water and nutrients in xylem • Explain the transporta=on of nutrients in phloem
Plant Organs
• Flowering plants have two major components to their structure.
1. A root system; extends below ground
2. A shoot system; composed of the stem, leaves, and reproduc=ve organs.
• At the end of the root and shoot system is a terminal bud from which ver=cal growth, called primary growth, occurs.
Content
1. Leaf: Structure, func=on and adapta=on 2. Stems: Structure, func=on and adapta=on
3. Roots: Structure, func=on and adapta=on 4. Transporta=on
1. Xylem 2. phloem
1. Leaf
� plants make carbohydrates, occurs in the leaves via photosynthesis.
� To conduct photosynthesis, leaves need solar energy, water, and carbon dioxide.
� Photosynthe=c leaves share similar structural components. � The blade, the wide part of the
leaf � The pe=ole, the stalk
connec=ng leaf to stem.
The blade:
� the wide part of the leaf � Have maximum surface area for the collec=on of energy & absorb CO2
The pe5ole:
� the stalk connec=ng leaf to stem.
� receive H2O from the root by way of vascular =ssue that terminates in the leaves
• There is tremendous diversity in leaf structure between plant species.
• In some plant species, leaves may serve addi=onal func=ons, such as storage.
• Some plants are deciduous (they drop their leaves during certain seasons.)
Leaves adapta5ons
• Modified as tendrils-‐ that allow the plant to aZach to objects.
• The leaves of cactus are spines that reduce H2O loss & protect the plant from browsing animals.
• The leaves as traps for catching insects.
Organiza5on of Leaf
• The top and boZom of a typical eudicot leaf is composed of epidermis
– The epidermis o[en has hairs or glands.
– Stomata are located on the lower epidermis.
• The interior of the leaf is composed of photosynthe=c mesophyll cells.
– The spongy mesophyll is arranged randomly to increase surface area for gas exchange.
– The palisade mesophyll is comprised of elongated, ver=cally-‐oriented cells. Contain the most chloroplast.
2. Stems
• The stem is the main axis of the plant.
• Stems can produce side (lateral) branches from lateral (axillary) buds.
• Nodes are the points where leaves aZach to stems.
• An internode is the region between nodes.
• The stem also contains the vascular 5ssue that transports water and nutrients to leaves ( to support photosynthesis).
• In some plant species, stems may also carry out photosynthesis or serve as a storage organ (e.g. tuber)
Sweet potato- tuber plant
Monocotyledon Versus Dicotyledon Plants
Vascular 5ssue:
� The arrangement of the vascular =ssue differs between monocots and eudicots.
� Plants have two types of vascular =ssue. 1. The xylem transports water and minerals.
2. The phloem transports organic nutrients.
� The vascular =ssues serve as a type of circulatory system for plants.
Monocot vs. Dicot Vascular Tissue Arrangement
Scattered Forming a ring
3. Roots • Roots system supports
the plant by anchor plants to the soil.
• Roots also absorb water and nutrients from the soil.
• The surface area of roots is greatly increased by the produc=on of root hairs.
Root Hair
• There are different types of root systems.
1. Some plants have a single taproot. 2. Grasses have fibrous root systems.
3. Some plants have prop roots; a type of adven55ous root for support.
Root system
• For perennial plants, the roots act as a storage order that allows the shoot system to regrow each year.
grow and bloom over the spring and summer, die back every autumn and winter, and then return in the spring from their root-stock
TRANSPORTATION OF WATER & MINERALS : XYLEM
Xylem: Tracheids and Vessel Members
• Water and dissolved mineral ions flow through conduc=ng tubes of xylem
• Interconnected, perforated walls of tracheids and vessel members (dead cells) form the tubes
1. The water and nutrients taken up by roots and root hairs through soil water.
2. H2O enter the root by osmosis
3. H2O + nutrients are transported to leaves via the interconnected vessel elements of the xylem.
4. H2O + nutrients transported to leaves
1. Root Pressure
• The water and nutrients taken up by roots and root hairs through soil water.
• This movement is provided in part by root pressure, a posi5ve pressure created when water enters the root by osmosis.
• Soil hypotonic à root hypertonic Root pressure
2. Transpira5on
• Transpira=on – Evapora=on of water from plant parts (mainly though stomata) into air
– pulls water upward through xylem by causing con=nuous nega=ve pressure (tension) from leaves to roots
• root hypotonic ! leaves hypertonic
3. Cohesion-‐Tension Theory
• The cohesion-‐tension model explains how water travels up the xylem to leaves.
• leaves have numerous openings called stomata.
• When these stomata are open, water evaporates from the interior of the leaf to the outside air, a process called transpira=on.
Cohesion-‐Tension Theory 1. As plant leaves transpire water, a
tension is created that pulls water from roots to leaves.
2. This tension is maintained because water molecules display an aZrac=on to one another called cohesion.
3. Hydrogen bonds among water molecules resist rupturing (cohesion) so water is pulled upward as a con=nuous fluid column
4. Water also adheres to the xylem elements in a process called adhesion.
5. Hydrogen bonds break and water molecules diffuse into the air during transpira=on
Cohesion & adhesion
Transpiration
Root pressure created when water enter root cells via osmosis
Water rises through xylem vessels because :
1. Cohesion: Water molecules are attracted to each other 2. Adhesion: Water molecules form hydrogen bonds with the xylem cell wall
Transpiration Evaporation of water from stomata creating a tension that pulls the water column from root to leaf
** Because of cohesion, new water molecules is drawn from the xylem which is replaced by water from the roots
Opening and Closing of Stomata
� The opening and closing of the leaf stomata is controlled by turgor pressure within the guard cells.
� As water enters the guard cells, these cells swell, opening the stomata.
� As water exits the guard cells, the loss of turgor causes the stomata to close.
Water Conserva5on
• Cu5cle – Waxy covering that protects all plant parts exposed to surroundings
– Helps the plant conserve water
Water Conserva5on • Environmental signals cause stomata
to open and close
• Closed stomata limit water loss (but prevent gas exchange for photosynthesis and aerobic respira=on)
• Some plant like CAM plant adapt for water conserva=on and photosynthesis by opening their stomata at night to allow CO2 for photosynthesis and close stomata during the day to prevent dehydra=on.
The pineapple is an example of a CAM plant.
CAM= Crassulacean acid metabolism
TRANSPORTATION OF ORGANIC NUTRIENT: PHLOEM
Phloem: Sieve-‐Tube Members
perforated end plate of sieve-tube Cell (sieve plate)
one of a series of living cells that abut, end to end, and form a sieve tube
companion cell (in the background, pressed tightly against sieve tube)
Phloem: Sieve-‐Tube Members
Organic Nutrients in the Phloem
• As mature leaves photosynthesize, phloem load sucrose.
• Phloem is considered source of sugar (Source cell).
• The phloem transported sugar to =ssues that require sugars, called sink =ssues (Sink cell).
• Sugar are unloaded at sink region (ac=vely growing or storage parts of the plant )
Pressure flow theory
• Phloem
– Translocate photosynthe=c products down the gradient of pressure and solute concentra5on
• Transloca5on Process – Distributes sucrose and other
organic compounds throughout the plant
– An energy-‐requiring process – Can be elaborate by
Pressure-‐flow theory
Translocation
1. Source cells: produce glucose/ organic molecules (by photosynthesis)
2. Glucose is converted to sucrose for transport
3. Companion cell loads sucrose 4. Water follows from xylem by
osmosis 5. Sap volume and pressure
increased 6. Sap flow within phloem 7. Unload the organic molecules by
the companion cell 8. Sucrose is stored in sink cell/
=ssues (as starch) 9. Water diffuses into xylem 10. Water recycles as part of
transpira=on to re supply the sucrose loading
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Pressure flow theory
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glucose
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