Unit C – Cycling of Matter in Living Systems (pages 256 – 359 in your text)
9.2 Gas Exchange in Plants (pages 325 – 330)
Gas exchange animation: http://www.mhhe.com/biosci/genbio/animation_quizzes/graphics/plm2s3b.ram
 During photosynthesis plants need to consume carbon dioxide and water
gases.
 Oxygen gas produced in the process must be released from the leaf.
 Stomata in the lower epidermis to allow gases to diffuse in and out of the
leaf.
Once gases are in the leaf, spaces between the spongy tissue allow for passive
transport into and out of the cells.
During the day the process of photosynthesis and cellular respiration occurs at
the same time
 The plant consumes far greater quantities of carbon dioxide during
photosynthesis than it produces during cellular respiration.
 The plant produces far greater amounts of oxygen during photosynthesis
than it consumes during cellular respiration.
 Far greater volumes of gases are exchanged during photosynthesis than
during cellular respiration.
summary of metabolic activity in plants: day vs. night
DAY
NIGHT
photosynthesis
- CO2 consumed
- H2O consumed
- O2 produced
cellular respiration
- O2 consumed
- CO2 produced
- H2O produced
cellular respiration
- O2 consumed
- CO2 produced
- H2O produced
 low volumes of gas exchange due
to no photosynthesis
 net production of O2, net
consumption of CO2
 very high levels of gas exchange
due to photosynthesis
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Leaf and Lenticels
Leaves
 Leaves are the most
important gas-exchange
organ in a plant
 Leaves contain air spaces
that are connected to the
outside environment by
stomata
 Air diffuses through the
stomata and into the leaf
 Air circulates in the
spaces between the spongy and palisade tissue cells
 Carbon dioxide diffuses down its concentration gradient, dissolving into the
watery layer around the cells and diffusing into the cells themselves
 The chloroplasts use the carbon dioxide in the process of photosynthesis
 Oxygen produced during photosynthesis diffuses out of the cell and into
the air spaces.
 Oxygen diffuses out of the stomata and into the atmosphere.
Lenticels
 in woody plants (ie. trees and shrubs)
layer
of dead cork cells and waxy
substances prevent gas exchange.
between the external environment
and the cells just below the wood.
 Lens-shaped openings called lenticels
protrude from the bark of these
plants.
 Air can diffuse through the
lenticels
 Lenticels allow cells within the
roots and stems to exchange
oxygen and carbon dioxide with
the environment
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Gas Exchange is Tied to Water Loss
 Plants lose water during gas exchange.
 Palisade and spongy tissue cells are coated with a thin layer of water
 The water evaporates into the air spaces within the leaf
 As air diffuses from the leaf to the atmosphere some of the water is lost
 The evaporation of water from plants is called transpiration
 A plant can lose as much as 99% of the water absorbed by the roots due to
transpiration
 Plants that lose too much water from transpiration are in danger of drying
out and dying.
 Guard cells embedded in the lower epidermis control water loss by opening
and closing the stomata
Question 1. How is photosynthesis affected
by the opening and closing of the stomata?
Less photosynthesis occurs when the stomata
are closed.
 The opening and closing of the stomata
are determined by the amount of
water in the guard cells
 Water moves into and out of the guard
cells by osmosis
 As water moves into the guard cells the
water pressure inside the cells
increases and causes the cell to swell
 The high pressure, turgor pressure,
pushes the elastic cell membrane against the rigid cell wall.
 The swollen guard cell changes shape, opening the stomata.
 Water vapor then passes out of the leaf through transpiration
 As the amount of water in the guard cells decreases, the cells deflate and
change shape; the stomata closes.
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 If water within the soil becomes depleted the plant’s leaves droop and
wither; the stem softens and bends
wilted vs. turgid
 Once the plant is supplied with water, the limp cells have their turgor
pressure restored, the cells renew their shape and rigidity.
Question 2. Which gases are produced and consumed during photosynthesis?
- CO2 consumed
- H2O consumed
- O2 produced
Question 3. The cells of some plants are surrounded by a protective layer of cork
or waxy material. Describe how the cells of the stems exchange gases across this
barrier.
Lenticels, specialized structures, protrude from the woody plant. This allows a
break in the impermeable surface for gases to diffuse through.
Question 4. Why do plants require oxygen?
Plants require oxygen to make glucose and for cellular respiration.
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