Plant structure and function (parts of chapters 35, 36 and 37)

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Plant structure and function(parts of chapters
35, 36 and 37)

• Unlike animals, plants remain in one place and
  produce food through photosynthesis.
• In the process of photosynthesis plants (and
  other photosynthetic organisms such as algae,
  other protists, and cyanobacteria) trap the
  energy in sunlight and store it in chemical
  bonds.
• The energy stored in chemical bonds can then be
  used to fuel metabolic processes.

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Figure 10.2
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Plants and photosynthesis

• This process is called photosynthesis.
• In this class we will not discuss the process of
  photosynthesis in detail. It is covered in depth
  in Bio 101.

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Photosynthesis

• In photosynthesis carbon dioxide (CO2) and water
  (H20) and the energy provided by light are used
  to make glucose.
• 6 CO2 12 H20 energy ? C6H12O6 6O2 6 H20

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Chloroplasts

• The organelle plants use to carry out
  photosynthesis is the chloroplast.
• In plants chloroplasts are concentrated in the
  leaves, which generally are thin and flat to
  allow maximum exposure to light.

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Fig 10.3
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Basic structure of plants

• To carry out photosynthesis plants must obtain
  water and minerals from the soil, CO2 from the
  air, and light from the sun.
• The structure of plants reflects their need to
  carry out these tasks.

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Basic structure of plants

• Plants have three basic organs
• Roots
• Stems
• Leaves
• These organs are organized into two systems the
  largely below-ground root system and the
  above-ground shoot system (stems and leaves).

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35.2
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Roots

• Roots perform several tasks. They
• Anchor the plant in place
• Absorb minerals and water
• Store organic nutrients such as sugars (e.g.
  carrot, sugar beet, turnip).

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Roots

• Roots systems may have a central taproot with
  lateral roots branching off from it (e.g.
  dandelion).
• Alternatively, a root system may have no obvious
  main root, but instead be a fibrous system with
  many small roots growing from the stem, each of
  which has its own lateral roots (e.g. grasses).

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Roots

• The entire root system anchors a plant in place,
  but absorption of water and minerals occurs
  mainly at the root tips.
• At the root tips huge numbers of root hairs
  increase the surface area enormously.

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Root hairs

• Root hairs are extensions of individual epidermal
  root cells and are not multicellular structures
• (as lateral roots are).

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Roots

• Root hairs are permeable to water and adhere
  closely to soil particles allowing efficient
  absorption of water and nutrients.
• Most plants forms mutually beneficial
  relationships with fungi, which facilitate
  absorption of water and minerals.

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Mycorrhizae

• The plants and fungi form mycorrhizae symbiotic
  associations of plant roots united with fungal
  hyphae (hyphae are tiny filaments that form the
  bulk of a fungus).
• Most plants form these symbiotic mycorrhizal
  relationships and they greatly enhance the plants
  growth. a symbiotic relationship is a close,
  mutually beneficial relationship

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Mycorrhizae (white) growing on a root
36.10
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Mycorrhizae

• The fungal hyphae grow over the root and
  penetrate into it and may in some cases form a
  mantle or layer over the root.
• The fungus benefits from a steady supply of sugar
  donated by the host plant.

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37.12
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Mycorrhizae

• Plant receives numerous benefits
• Fungus greatly increases surface area for
  absorption (can be as much as 3 meters of hyphae
  per cm of plant root length).
• Fungus selectively absorbs phosphate and other
  nutrients and supplies them to plant.
• Fungus may secrete growth factors that promote
  root growth.
• Fungus may produce antibiotics that protect the
  plant from pathogenic bacteria and fungi in the
  soil.

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Mycorrhizae

• Plant-fungus symbiosis may have been one of the
  early adaptations that allowed plants to colonize
  the land, which probably initially was quite
  nutrient poor.
• Fossils of some of the earliest plants show
  mycorrhizae.

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Shoot Systems

• Shoot systems consist of stems and leaves.
• Stems are elongated structures comprised of nodes
  and internodes.
• Nodes are where leaves are attached and
  internodes are the sections in between.

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35.2
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Shoot Systems

• Stems have a terminal bud at the tip and this is
  where elongation takes place, enabling the stem
  to reach upwards towards the light.
• If the tip of the stem is eaten or shaded,
  however, axillary buds (buds on the side) will
  begin to grow.

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Shoot Systems

• Gardeners shape plants by pruning them.
• By removing terminal buds a bushier plant can be
  produced or by removing lateral flower buds a
  single large flower can be produced.

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Shoot Systems

• Stems have been greatly modified in many plants
  to perform a variety of functions.
• Rhizomes, bulbs, tubers, and stolons are all
  modified stems although they are often mistaken
  for roots.

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Modified stems

• Bulbs vertical shoots that grow underground.
  The flesh of a bulb (e.g. an onion) consists of
  leaves modified for food storage.
• Stolons and rhizomes are stems that grow on
  (stolons) or just under (rhizomes) the soil
  surface. New plantlets form periodically along
  the length of these stems (asexual reproduction).

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35.5
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Modified stems

• Tubers are enlarged ends of rhizomes specialized
  for storing food (e.g. potato).
• The eyes of a tuber are axillary buds.

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35.5
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Leaves

• Leaves are the main photosynthetic organ of
  plants, although green stems also perform
  photosynthesis.
• Leaves vary in form, but usually have a flat
  blade and a stalk (petiole) that joins the leaf
  to the stem.

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Leaves

• Leaves are generally flat to maximize the area
  exposed to the sun and minimize the distance
  gases must be transported to and from
  photosynthesizing cells.
• However, in many cases leaves have been
  substantially modified by natural selection to
  perform other functions.

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Modified leaves

• Tendrils of climbing plants such as clematis are
  often modified leaves.
• Spines of cacti are modified leaves (most
  photosynthesis being carried out by the fleshy
  stem.
• Some leaves are modifed as storage leaves to
  store water.
• Some leaves called bracts look like petals (e.g.
  in dogwoods) being brightly colored and enlarged
  to attract pollinators to the flowers they
  surround.
• Some leaves produce plantlets that drop off the
  plant and take root in the soil.

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35.7
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Plant vascular system

• Plants contain two vascular systems that
  transport water, minerals, and sugars around the
  plant.
• Xylem transports water and dissolved minerals
  from the roots into the shoots.
• Phloem transports sugars from the leaves to where
  they are needed in the plant.

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36.2
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Plant vascular system

• Xylem cells are dead at functional maturity and
  form thin elongated tubes that water moves
  through.
• Phloem cells are alive.

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Plant secondary growth

• Primary growth is stem elongation, secondary
  growth refers to the thickening of woody plants
  over time.
• Xylem and phloem cells are both produced by a
  plant tissue called vascular cambium that is
  located under the bark.
• This cambium produces xylem cells on the inside
  and phloem on its outside.

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Plant secondary growth

• As the plant grows older inner xylem tissue forms
  the heartwood of the tree. This tissue no longer
  transports liquid.
• Xylem cells have thick lignified walls (lignin is
  a complex cross-linked polymer) that provide
  structural support for the plant.
• The outer (more recently produced) xylem is
  called sapwood and this carries liquid.

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35.20
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Plant secondary growth

• On the outside of the cambium layer phloem is
  produced. Phloem is produced more slowly than
  xylem and older phloem is sloughed off the tree
  so it does not accumulate as xylem does.

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Plant secondary growth

• Because the outer layer of phloem is essential to
  transportation, a tree that is ringed by
  grazers (i.e., has its outer bark removed around
  the circumference of the plant) will die.
• In contrast, a tree may be hollowed out and still
  survive.