Green Plants as Organisms
Contents
Photosynthesis
 Photosynthesis vs. Respiration
 Plant Hormones
 Commercial Uses of Plant Hormones
 Transport and Water Relations
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Photosynthesis
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Photosynthesis is the chemical change which happens in
the leaves of green plants. It is the first step towards
making food. During this reaction, carbon dioxide and
water are converted into glucose and oxygen. The
reaction requires energy in the form of sunlight, and
chlorophyll must also be present.
Plants are producers – they produce their own food
Plants produce their food via photosynthesis
Photosynthesis
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Occurs in the leaf
Needs:
Chlorophyll – not used up
CO2 (from the air)
Water (from the soil)
Sunlight energy (any light except green light) – not used up
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Produces:
Glucose
Oxygen (waste product)
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chlorophyll + light energy
carbon dioxide + water
6CO2
+ 6H2O
glucose + oxygen
C6H12O6 + 6O2
Factors Affecting Photosynthesis
Three factors limit photosynthesis from going any faster:
1.
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Light level
Without enough light a plant cannot photosynthesise very fast, even if
there is plenty of water and carbon dioxide. Increasing the light intensity
will make photosynthesis faster.
2. Carbon dioxide level
Photosynthesis can be limited by the level of carbon dioxide. Even if
there is plenty of light a plant cannot photosynthesise if it has run out of
carbon dioxide.
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3. Temperature.
If it gets too cold the rate of photosynthesis will slow right down. If it
gets too hot then plants cease to be able to photosynthesise.
Night-time vs. Daytime
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Daytime:
Photosynthesis & Respiration
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Night-time: (no sunlight therefore no photosynthesis)
Only respiration
Plant Hormones
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Stem  always grows towards light
Roots  always grow away from light
controlled by auxins
Auxins are produced in the growing tips of shoots and roots
Auxins respond to light, gravity and moisture
Auxins cause some cells to elongate or grow at a faster rate
Tropism = plant’s response to light, gravity or moisture
Positive tropism = response towards stimulus
Negative tropism = response away from stimulus
Plant Hormones
Geotropism = plant’s response to gravity
 Positive geotropism = roots grow towards direction of gravity
 Negative geotropism = shoots grow away from direction of gravity
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Hydrotropism = plant’s response to water
 Positive hydrotropism = shoots always follow direction of water
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Phototropism = plant’s response to light
Light destroys auxins
Auxins = plant hormones controlling its growth
Auxins on sunlit side of plant are destroyed,
growth slows…
Growth of plant on shaded side continues
Commercial Uses of Plant Hormones
1)
Growing cuttings: cuttings are dipped into powder
containing growth hormones. Plants are cloned quickly and
cheaply
2)
Producing fruit without seeds: Growth hormones sprayed on
unpollinated flowers  grow fruit without fertilisation
3)
Ripening fruit: Plant hormone ethene ripens fruit. Sprayed
onto unripened fruit during transportation to supermarkets
4)
Increasing size of fruit: e.g. grapes
Transport & Water Relations
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Plants require water for growth, temperature regulation and
to hold themselves up
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Transpiration is the loss of water from the leaves by
evaporation
Transpiration system:
Water absorbed into roots through root hair cells
Water carried to leaves through xylem vessels
Water evaporates from underside of leaves
Water escapes through holes called stomata
As water evaporates, more is sucked up xylem
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Stomata (tiny holes under a leaf allowing it to breathe) open
and close to control water loss
Xylem and Phloem Vessels
Xylem are vein like tissues that transport water and minerals up a plant
Phloem are vein like structures through which food is transported around a plant
Xylem
Phloem
Made of
Dead cells
Living cells
Cell wall thickness
Thick
Thin
Cell wall material
Lignin (rigid)
Cellulose
Permeability
Impermeable
Permeable
Cytoplasm?
None
Cytoplasm lining
Transports…
Water & minerals
Food
Carried to…
Leaves
Growing parts & storage organs
Direction of flow
Upwards
Up and down
Tissue also has…
Fibres
Companion cells
Water Provides Support
Green plants rely on cell turgor which is the
stiffness given to cell walls to hold themselves up
 Cell walls become turgid with water
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In woody plants, they do not need cell turgor, but
instead rely on the support of the strong,
impermeable xylem vessels
Summary
Photosynthesis: carbon dioxide + water  glucose + oxygen
 Limiting factors: temperature, CO2, light intensity
 Daytime: respiration & photosynthesis; night-time: respiration
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Plant hormone, auxin, responds to light, gravity and moisture
Light destroys auxin
Auxin speeds up plant growth
Water travels around the plant via the transpiration system
 Xylem and phloem vessels transport water, minerals and food
 Water is needed in green plants for structure and support
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