Structure and Function of Plants
Plants are as intricate and complicated as
animals. But you wouldn’t know that from looking at the SAT II Biology.
Though the test covers almost all aspects of animal organismal biology,
it covers plants in much less detail. This is not to say that you
can ignore plants completely while studying for the test, just that
you need to study the right things.
To begin with, you need to know what plants are, how plants
differ from animals on both the cellular and organismal level, and
the different categories of plants that exist. You should also know
about the structure and function of the three most important parts
of vascular plants: leaves, stems, and roots. In addition, it’s
a good idea to have a basic knowledge of how plants grow. Plants
also have various and unique means of reproducing themselves that
the SAT II may test. And finally, you should have some sense of
plant “behaviors,” which are called tropisms.
The General Plant Cell
We covered plant cells back in the chapter on Cell Processes,
but we’ll provide a summary here.
- Plants have all the organelles animal
cells have. Nucleus, ribosomes, mitochondria, endoplasmic
reticulum, Golgi apparatus, etc.
- Plants have chloroplasts. Chloroplasts are
special organelles that contain chlorophyll and allow plants to
carry out photosynthesis.
- Plant cells can sometimes have big vacuoles for
- Plants are surrounded by a rigid cell wall made
of cellulose, in addition to the cell membrane that surrounds
animal cells. These walls provide support.
Types of Plants
Just as there are many different kinds of animals, there
are also many different kinds of plants. The earliest plants had
no special vascular tissues devoted to transport, meaning they could
not grow to great heights because they couldn’t transport necessary
liquids and minerals over long distances. These plants are called nonvascular or nontracheophyte. Tracheophytes,
also called vascular plants, do have special vascular tissues for
Vascular tissues are composed of specialized cells that
create “tubes” through which materials can flow throughout the plant
body. These vessels are continuous throughout the plant, allowing
for the efficient and controlled distribution of water and nutrients.
In addition to this transport function, vascular tissues also support
the plant. The two types of vascular tissue are xylem and phloem.
Xylem consists of a “pipeline” of dead cells arranged
end to end for water and mineral transport. When the cells that
form xylem die at maturity, the nucleus and cytoplasm disintegrate,
leaving a hollow tunnel through which fluids can move. The xylem
carries water and dissolved minerals upward from the roots through
the stem and leaves of the plant. In larger seed plants, xylem cells
are specialized into tracheids and vessel elements. Vessels are wider
and better at conducting water than the tracheids.
In addition to distributing nutrients, xylem provides
structural support. In fact, the material commonly known as “wood”
is actually xylem. After a time, the xylem at the center of older
trees (woody dicots) ceases to function in transport and takes on
a purely supportive role.
Unlike xylem, the cells that make up phloem are living
at maturity and can carry materials both up and down
the plant body. Phloem consists of sieve elements, which are arranged end
to end to form passageways, and companion cells, which are closely
associated with the sieve elements, even though their exact function
is unknown. Mature companion cells have both a nucleus and cytoplasm,
while the mature sieve elements contain only cytoplasm; for this
reason it is thought that the nuclei of companion cells may control
the activities of neighboring sieve elements. Phloem is responsible
for distributing the products of photosynthesis, such as amino acids
and carbohydrates, from the leaves to the rest of the plant.
Types of Tracheophytes
There are also distinctions among the tracheophytes. Ferns
were the first tracheophytes to evolve. Ferns are notable because
they reproduce through the use of spores and do not use seeds. Otherwise,
in SAT II Biology world, you don’t really have to worry about them. The
two tracheophytes that are more important are gymnosperms and angiosperms.
Gymnosperms were the first tracheophytes to use seeds
for reproduction. The seeds develop in protective structures called
cones. A gymnosperm will contain some cones that are female and
some that are male. Female cones produce spores that, after fertilization, become
eggs enclosed in seeds that fall to the ground. Male cones produce
pollen, which is taken by the wind and fertilizes female eggs by
that means. Coniferous trees such as pines and firs are good examples
Angiosperms, the flowering plants, are the most highly
evolved plants and the most dominant in present times. Angiosperms
are also the type of plant that the SAT II mainly focuses on. In
fact, you don’t really need to know anything about nontracheophytes,
ferns, and gymnosperms beyond what we’ve already told you. The further
discussions of plant biology in this chapter will focus on angiosperms
(though what is true for angiosperms is also often true for the
other types of plants).
As for angiosperms, there are actually two kinds: monocots and dicots.
Monocots include grasses, grains, and other narrow-leaved angiosperms.
Monocots are named for the presence of a single cotyledon (seed
leaf) during embryonic development. In general, the veins of monocot
leaves are parallel, the flower parts occur in multiples of three,
and a fibrous root system is present. Bundles of vascular tissue
are scattered throughout the stem instead of appearing in a single
ring. Dicots, such as maples, oaks, elms, sunflowers, and roses,
originate from embryos with two cotyledons. They are further distinguished
from monocots by the branched network of veins in their leaves,
the occurrence of their flower parts in groups of four or five,
and the presence of a taproot, which is a single main root with
tributaries off it. The vascular bundles of dicots are arranged
in a tubular pattern in the stem.
Leaves are the sites of photosynthesis in plants. The
leaves’ broad, flattened surfaces gather energy from sunlight, while
apertures on their undersides bring in carbon dioxide and release
oxygen. On its two exteriors, the leaf has layers of epidermal cells
that secrete a waxy, nearly impermeable cuticle to
protect against water loss and fungal or bacterial attack. The only
way gases can diffuse in or out of the leaf is though stomata,
small openings on the underside of the leaf. The opening or closing
of the stomata occurs through the swelling or relaxing of guard
cells. If the plant wants to limit the diffusion of gases
and the transpiration of water, the guard cells swell together and
close the stomata.
The tissues between the epidermal cells are called mesophyll.
The mesophyll can be further broken down into two layers, the palisade
layer and the spongy layer. Both layers are packed
with chloroplasts, the factories of photosynthesis. In the palisade
layer, chloroplasts are lined in columns just below the epidermal
cells to facilitate the capture of light. In the spongy layer, cells
are less ordered and more diffuse, leaving large intracellular spaces that
facilitate the exchange of carbon dioxide and oxygen.
Overall, it is to the plant’s advantage to maximize the
gas exchange and sunlight trapping surface area while keeping leaf
thickness to a minimum so that once gases enter the leaf through
the stomata, they can diffuse easily throughout the leaf cells.
Like mitochondria, chloroplasts consist of a double membrane
layer with an intermembrane space between. The inner membrane is
folded into multiple stacks of flattened, disk-shaped compartments.
Each such compartment is called a thylakoid, and a
stack of thylakoids is called a granum. The thylakoid
membrane separates the chloroplast interior into two very different
compartments: the thylakoid space inside the thykaloids where the photosynthetic chlorophyll resides
and the stroma, a fluid that lies outside the stacked
disks and takes up the rest of the organelle.
In photosynthesis, plants (and other photosynthetic autotrophs)
use the energy from sunlight to create the carbohydrates necessary
for cell respiration. More specifically, plants take water and carbon
dioxide and transform them into glucose and oxygen:
6CO2 + 6H2O
+ light energyC6H12O6 +
This general equation of photosynthesis represents the
combined effects of two different stages. The first stage is called
the light reaction, since it is dependent on light. The dark reaction,
the second stage, does not need light.
The Light Reaction
The light reaction, also called the light-dependent reaction,
takes place within the thylakoid spaces of the grana in the chloroplast.
When sunlight strikes the chlorophyll contained within the thylakoid
spaces, electrons become excited and infused with energy, and they
are transferred down an electron transport chain similar to the
one found in aerobic respiration. The energy in the electrons is
used to set up a proton gradient across the membrane of the thylakoid
spaces. Protons flowing back across the thylakoid membrane according
to the concentration gradient are harnessed to produce ATP and NADPH
(the reduced form of NADP). As a by-product of this process, molecules
of water are split into molecular hydrogen and oxygen. The plant
needs the hydrogen to produce ATP. The oxygen you’re breathing right
now is some of that waste product.
The purpose of the light reaction is to make the usable
energy necessary to run the dark reaction.
The Dark Reaction
The dark reaction is also referred to as the light-independent
reaction, the Calvin cycle, or carbon fixation. The
reaction takes place in the stroma of the chloroplast.
In the dark reaction, the carbon from carbon dioxide is
added to the five-carbon sugar ribulose bisphosphate (RuBP) to produce
a six-carbon compound. This six-carbon sugar is immediately split
into two three-carbon molecules, which in a chain reaction using
the ATP and NADPH from the light reaction are modified to form glyceraldehyde
3-phosphate. The glyceraldehyde 3-phosphate can be synthesized into
carbohydrates such as glucose, and it can also be synthesized back
into ribulose biphosphate. One of the glyceraldehyde 3-phosphate
molecules is made into carbohydrates, while the other molecules
remain in the Calvin cycle to serve as raw materials for the next
round of production.
The roots of a plant draw water and minerals from the
soil and pass them upward through xylem and phloem to the stem and
leaves. Roots are also responsible for storing the plant’s organic
nutrients, which are passed downward from the leaves through the
phloem. Radiating from the roots is a system of root hairs, which
vastly increase the absorptive surface area of the roots. Roots
also anchor the plant in the soil.
Growth in Vascular Plants
Vascular plants undergo two kinds of growth, primary growth
and secondary growth. Primary growth occurs in the apical meristems,
located at the tip of both root and shoot, and is mainly a growth
of vertical length. The meristems are regions of rapid
mitotic division, churning out cells like a factory. When a cell
divides, one of its offspring moves down into the plant body, where
it elongates, and the other remains in the meristem to divide again.
Secondary growth is a growth of thickness. Secondary growth
is a product of two different, though related, tissues, which both
fall under the umbrella-term lateral meristems. Vascular cambium exists
between xylem and phloem: on its inside the cambium produces what
is known as secondary xylem; on its outside it forms secondary phloem.
The primary xylem and phloem are pushed farther inward and outward.
The vascular cambium is more productive during the growing seasons.
During the rest of the year it creates little growth. This phenomenon
creates distinct rings of growth, each ring representing a single
growing season. By studying these rings, it is possible to calculate
the age of a plant, and it’s even possible to determine the specific
conditions of a particular growing season. The second lateral meristem
is called cork cambium and is responsible for the formation of cork
(bark), which replaces the epidermis to form the protective covering
of shoot and root.
Controlling Growth: Plant Hormones
Plant growth is controlled by plant hormones, which influence
cell differentiation, elongation, and division. Some plant hormones
also affect the timing of reproduction and germination.
- Auxins. The primary function
of the auxin hormones is to elongate plant cells in the stem. Auxins
are also responsible for root development, secondary growth in the
vascular cambium, inhibition of lateral branching, and fruit development.
- Kinins promote cell division and tissue growth
in leaf, stem, and root. Kinins are also involved in the development
of chloroplasts, fruits, and flowers. In addition, they have been
shown to delay senescence (aging), especially in leaves, which is
one reason that florists use cytokinins on freshly cut flowers.
- Gibberellins stimulate growth, especially
elongation of the stem, and can also end the dormancy period of
seeds and buds by encouraging germination. Additionally, gibberellins
play a role in root growth and differentiation.
- Ethylene controls the ripening of fruits.
It also contributes to the senescence of plants by promoting leaf
loss and other changes. Ethylene can bring buds and seeds out of
dormancy, initiate flower development, and promote radial (horizontal)
growth in roots and stems.
- Inhibitors restrain growth and maintain the
period of dormancy in seeds and buds.
Plant Behavior: Tropisms
When people think about plants growing, they generally
think of them growing straight up or growing wider. But plants also
display other types of growth in response to the stimuli within
their environment. These responses to stimuli are called tropisms
and are controlled by plant hormones. There are three main tropisms:
- Phototropism is the tendency
of a plant to move toward light. Phototropism results from the rapid
elongation of cells on the dark side of the plant, which causes
the plant to bend in the opposite direction.
- Gravitropism refers to a plant’s tendency
to grow toward or against gravity. A plant that displays positive
gravitropism will grow downward, toward the earth. A plant that
displays negative gravitropism will grow upward, away from the earth.
Most plants are negatively gravitropic. Gravitropism is also controlled
by auxin. In a horizontal root or stem, auxin is concentrated in
the lower half, pulled by gravity. In a positively gravitropic plant,
this auxin concentration will inhibit cell growth
on that lower side, causing the stem to bend downward. In a negatively
gravitropic plant, this auxin concentration will inspire cell
growth on that lower side, causing the stem to bend upward.
- Thigmotropism, a reaction to touch, causes
parts of the plant to thicken or coil as they touch or are touched
by environmental entities. Tree trunks, for instance, grow thicker
when exposed to strong winds and vines tend to grow straight until
they encounter a substrate to wrap around.
Plants have a wide variety of flowering strategies involving
what time of year they will flower and, consequently, reproduce.
In many plants, flowering is dependent on the duration of day and
night; this is called photoperiodism.
All flowering plants have been placed in one of three
categories with respect to photoperiodism: short-day plants, long-day
plants, and day-neutral plants. Despite their names, however, scientists
have discovered that it is the uninterrupted length of night rather
than length of day that is the most important factor in determining
when and whether plants will bloom. Short-day plants begin to bloom
when the hours of darkness in a 24-hour period rise above a critical
level, as when days shorten in the autumn. These plants include poinsettias,
chrysanthemums, goldenrod, and asters. Long-day plants begin to
flower when the duration of night decreases past a critical point,
as when days lengthen in the spring and summer. Spinach, lettuce,
and most grains are long-day plants. Finally, many plants are day
neutral, which means that the onset of flowering is not controlled
by photoperiod at all. These plants, which are independent both
of night length and day length, include tomatoes, sunflowers, dandelions,
rice, and corn.
Plants can reproduce both asexually and sexually. Each
type of reproduction has its benefits and disadvantages.
Through asexual reproduction, many plants can produce
genetically identical offshoots (clones) of themselves, which then
develop into independent plants. This process is also called vegetative
propagation. The many modes of vegetative propagation include
the production of specialized structures such as tubers, runners,
and bulbs. Grafting is an artificial form of vegetative propagation.
The advantages to this kind of asexual reproduction, which can occur
either naturally or artificially, stem from the fact that it can
occur more rapidly than seed propagation and can allow a genetically
superior plant to produce unlimited copies of itself without variation.
As seen in potatoes, tubers are fleshy underground storage
structures composed of enlarged parts of the stem. A tuber functions
in asexual propagation as a result of the tiny scale leaves equipped
with buds that grow on its surface. Each of these buds can form
a new plant that is genetically identical to the parent.
Runners are slender, horizontal stems that spread outward
from the main plant, such as those found on strawberry plants. Entirely
new plants can develop from nodes located at intervals on the runners;
each node can give rise to new roots and shoots.
Bulbs such as onions and tulips are roughly spherical
underground buds with fleshy leaves extending from their short stems.
Each bulb contains several other buds that can give rise to new
In grafting, two young plants are joined, first by artificial
means and then by tissue regeneration. Typically, a twig or bud
is cut from one plant and joined to a rooted plant of a related
species or variety. The twig or bud is called the scion, and the
plant onto which is it grafted (and that provides the roots) is
called the stock. The scion eventually develops into an entire shoot
system. Grafting often allows horticulturalists to combine the best
features of two different plants into one plant. Sometimes the stock
and scion retain independent characteristics, and sometimes the
stock alters the characteristics of the scion in some desirable
Sexual Reproduction in Plants
All plants undergo a life cycle that takes them through
both haploid and diploid generations. The multicellular diploid
plant structure is called the sporophyte, which produces spores
through meiotic division. The multicellular haploid plant structure
is called the gametophyte, which is formed from the spore and gives
rise to the haploid gametes. The fluctuation between these diploid
and haploid stages that occurs in plants is called the alternation
of generations. The way in which the alternation of generations
occurs in plants depends on the type of plant. In nonvascular plants,
the dominant generation is haploid, so that the gametophyte constitutes
what we think of as the main plant. The opposite is true for tracheophytes,
in which the diploid generation is dominant and the sporophyte constitutes
the main plant. The SAT II Biology only deals with the specifics
of the tracheophyte alternation of generations, though nonvascular
plants have a similar life cycle.
The dominant phase in the tracheophyte life cycle is the
diploid (sporophyte) stage. The gametophytes are very small and
cannot exist independent of the parent plant. The reproductive structures
of the sporophyte (cones in gymnosperms and flowers in angiosperms) produce
male and female haploid spores: microspores (male) and megaspores
(female). These spores give rise to similarly sexually differentiated
gametophytes, which in turn produce gametes. Fertilization occurs
when a male and female gamete join to form a zygote. The resulting
embryo, encased in a seed coating, will eventually become a new
Reproduction in Flowering Plants
Angiosperms are special because they have developed special
reproductive systems, which are none other than the flowers you
should always take time to stop and smell. To understand angiosperm
reproduction, then, the first thing you have to do is know the structure of
Flowers, the reproductive structures of angiosperms, are
adaptations designed to attract insects and other pollen-bearing
animals to the plant to aid in pollen dispersal. For this reason,
flowers are most often colorful and showy; not surprisingly, plants
that rely on wind (instead of insects) for pollen dispersal have
flowers that are more likely to be small and drab.
The flower is composed of four organs: the sepal, petal, stamen,
and pistil. Sepals and petals are not directly involved
in reproduction, while the stamen and pistil are the male and
female reproductive organs. The stamen holds pollen,
which is the male gamete, and the stigma is the site where a pollen
grain must land in order to fertilize the female ovules that
are located in the ovary at the base of the pistil.
This ovary, an exclusive feature of angiosperms, encloses the ovules
and develops into a fruit after fertilization.
The female reproductive organ of angiosperms is the pistil,
located in the middle of the flower. As in gymnosperms, the male
gametophyte is the pollen grain. In order for fertilization to occur
in most flowering plants, insects or other animals must transport
the pollen to the pistil. A major distinguishing feature of angiosperms
is the process of double fertilization, in which an
angiosperm ovule contains an egg cell and a diploid fusion nucleus, which
is created through the joining of two polar nuclei within the ovule.
When a pollen grain comes into contact with the stigma,
or top of the pistil, it sends a pollen tube down into the ovary
at the pistil’s base. As the pollen tube penetrates the ovule, it releases
two sperm cells. One fuses with the egg to create a diploid zygote,
while the other joins with the fusion nucleus to form a triploid
nucleus. This triploid nucleus turns into an endosperm, which nourishes
the developing embryo (filling the role of gametophyte tissue in
the gymnosperm seed). As in gymnosperms, the ovule becomes a seed,
encasing the embryo and endosperm in a seed coat. But unlike gymnosperms,
in angiosperms, the ovary containing the ovules develops into a
fruit after fertilization. The fruit gives the embryos the double
benefit of added protection against desiccation and increased dispersal,
since it is eaten by far-ranging animals that then excrete the seeds.
Angiosperms either self-pollinate, in which a particular
plant fertilizes itself, or cross-pollinate, in which one plant
is fertilized by another of the same species. Cross-pollination generally
produces far more vigorous plants and is encouraged through differential
development of the male and female gametophytes on a flower or through
the positioning of these gametophytes so that self-pollination is