Download Gas Exchange in Plants

S E C T I O N
8.5
Gas Exchange in Plants
E X P E C TAT I O N S
Describe the major processes
and mechanisms for gas
exchange in plants.
Describe how leaves are
modified to reduce water loss
while enabling gas exchange.
Compare gas exchange
mechanisms in plants and
animals.
Figure 8.28 Plants are complex
organisms — but they have no
lungs. How can such organisms
survive without a respiratory
system?
At the beginning of this chapter, you saw that
single-celled organisms such as protists do not
require any specialized respiratory structures. The
small size and moist habitat of these organisms
mean that a direct exchange of gases across the cell
membrane is sufficient to meet the organisms’
metabolic needs. You also saw that as animal
organisms became larger and more complex,
different kinds of respiratory systems evolved to
increase the respiratory surface area and facilitate
the exchange of gases.
Like animals, plants must respire to supply their
cells with oxygen and remove waste carbon
dioxide. Many terrestrial plants are large and
complex organisms. Although plants have
specialized structures for many functions,
including nutritive and reproductive functions,
they have neither a distinct respiratory surface area
nor a circulatory system that carries gases from one
part of a plant to the other. This means that each
plant organ must rely on a direct exchange of gases
with its environment. Like terrestrial animals,
terrestrial plants also face the challenge of
maintaining a moist respiratory surface for gas
exchange while preventing excessive water loss.
Gas Exchange in Roots
Like the soil-borne organisms you saw in section 8.1,
the roots and rhizoids of plants depend on gas
exchange with the air in the soil. The surface of the
root is covered with many outgrowths called root
hairs (Figure 8.29). These outgrowths add
considerable surface area to the root, and provide a
moist surface for gas exchange. As long as the soil
is sufficiently aerated and contains water, oxygen
will diffuse from the air into the air spaces of the
soil and then into the moisture film surrounding
the soil particles and root hairs. The dissolved
oxygen then enters the root hair cell and by
diffusion passes to the other cells of the root. At
the same time, carbon dioxide diffuses from the
cells of the root through the root hairs and out into
the soil. The rate of respiration in roots is relatively
low, so this mechanism is sufficient to meet the
roots’ gas exchange needs.
FAST FORWARD
To review plant anatomy turn to Chapter 15, Section 15.2.
Figure 8.29 Root hairs on the surface of the root of a
germinating radish.
274
MHR • Internal Systems and Regulation
Root cells can also obtain oxygen from the
intercellular spaces often found in plant tissues.
Some plants, including certain plants found in
swampy areas, have large air canals linking the root
and stem. These canals allow oxygen to diffuse
internally to the roots directly from other regions of
the plant.
Gas Exchange in Leaves
In most plants, leaves are the primary organs
responsible for photosynthesis, the process that
produces plant nutrients (in a few species, such as
cacti, the stem is the main site of photosynthesis).
During photosynthesis, plants absorb carbon
dioxide and give off oxygen. But plants also respire
constantly, absorbing oxygen and giving off carbon
dioxide. During the day, both photosynthesis and
respiration occur simultaneously in the leaf.
Both photosynthesis and respiration require the
leaf to exchange gases with its environment. At the
same time, the leaf must guard against losing too
much water. Most of the surface of the leaf is
covered with a waterproof coating, or cuticle. In
order to allow for the gases to pass into and out of
the leaf, the cuticle is perforated with small
structures called stomata (singular stoma). Each
stoma consists of a pore bordered by a pair of guard
cells. The guard cells allow the pore to be opened
to permit the exchange of gases, or closed to
prevent the loss of water. You will learn more
about how the stomata function in Unit 5.
As shown in Figure 8.30, the stomata open into
intracellular air spaces within the leaf. The
arrangement of different tissues within the leaf
ensures that all cells are close to, or directly in
contact with, these air spaces. The surfaces of the
leaf cells are moist to permit the gaseous exchange
to occur by diffusion across the cell membranes. As
the plant photosynthesizes, oxygen is released into
the intracellular air space and can be reused by the
cells for respiration. Similarly, carbon dioxide
released by the respiring leaf cells enters the
intracellular air space and can be taken up by cells
for photosynthesis. Additional oxygen reaches the
cells from the external environment through the
stomata, while excess carbon dioxide leaves the
plant through the same openings.
Gas Exchange in Stems
The stem of a plant — whether it is the flexible
stem of a tulip or grass, or the thick woody bark of
a giant sequoia — contains small pores. The
flexible green stems of grasses and other nonwoody plants perform photosynthesis and, like
leaves, contain stomata through which gases can
enter the leaf. On woody stems you can often see
the small white markings of lenticels. These are
arrangements of cells that provide openings from
the environment into the tissue of the stem.
leaf hair
cuticle
air space
leaf vein
O2 enters leaf
through stomata
cuticle
guard cell
CO2 exits leaf
through stomata
Figure 8.30 A cross section of a leaf. When the stomata
Figure 8.31 The bark of the giant sequoia tree can protect
are open, gases can enter and leave the air spaces within
the leaf.
the plant from forest fires. Even this thick bark has pores
through which the stem exchanges gases with the
environment.
The Breath of Life • MHR
275
p ant transport vesse s
stoma
CO2
out
Figure 8.32 A lenticel. On the woody stems of tree trunks,
lenticels usually appear as small white spots.
Lenticels often occur at the same place where a
stoma was once situated when the stem was
younger and capable of carrying on photosynthesis.
Oxygen diffuses through the stomata or lenticels
into the intercellular air space of the plant, and
from these air spaces can reach every cell in the
stem. The arrangement of cells in the stem is shown
in Figure 8.33. Inside the plant, oxygen dissolves in
the water of the moist cell membrane and then
diffuses across the cell membrane into the cell.
Carbon dioxide follows the opposite path, diffusing
across the cell membrane into the intracellular air
space and then out through the pores.
Looking at the stem of a large tree, it may be
hard to imagine that diffusion alone can allow gas
exchange to take place at a sufficient rate to meet
the needs of all the cells of the stem. In woody
stems, however, only a relatively thin layer at the
outer surface of the stem is made up of living cells.
The central portion of the stem is composed of
dead cells that do not require oxygen.
SECTION
276
100 µm
O2 in
air spaces
Figure 8.33 Portion of a cross section of the stem of a
young corn plant. Gases are exchanged through the
stomata. The air channels in the stem help respiratory
gases reach all the cells of the stem.
In this chapter you learned how different
organisms obtain oxygen from their environments.
As you saw, most animals need a mechanism to
deliver that oxygen to all the cells of the organism.
In most organisms, substances such as nutrients,
waste materials, and hormones must also be moved
from one tissue or organ to another. Both plants
and animals have intricate transport networks that
help maintain their vital processes. In the next
chapter, you will explore how some of these
transport systems operate.
REVIEW
1.
K/U How are plants adapted to reduce water loss
while still allowing for gas exchange?
2.
K/U What are some of the features of roots that
assist plants with the exchange of gases?
3.
With respect to water loss and gas exchange,
what are some of the features you would expect to
find in plants that live on the forest floor?
4.
MC Would you expect a shade-tolerant plant to have
more or fewer stomata per leaf than a plant that
grows best in full sunlight? Why?
MC
MHR • Internal Systems and Regulation
5.
MC You are planning a vegetable garden. What are
some of the things you should do, both as you
prepare the garden bed and as you care for the
plants, to make sure the plants can respire properly?
6.
MC Beaver dams often flood large areas of wooded
land. Knowing what you do about respiration in plant
stems and roots, what effect would you expect this
flooding to have on trees? Explain.