Plant Structure & Function
digestibility of plants (i.e. how easy they are to digest) affects the
efficiency of animals' digestive processes, and is related to the
structure of plant cells and tissues.
| Digestive efficiency
of ruminants | Digestive
efficiency of hindgut fermenters | Environmental
effects of low digestibility
easily digest the contents of plant cells, but not their cell walls. All plant cells have an outer cell wall
of cellulose or lignin.. This
reduces the amount of energy animals can extract from plants, because
most animals lack the specific enzymes that would allow them to digest
the amount of cellulose
present in the plant. In general, older and woodier
plants have higher levels of cellulose and lignin. Lignin
cannot be digested no matter how long it remains in the digestive
system. In addition,
lignin interferes with gut microbes that do have the necessary enzymes
to digest cellulose, because it both acts as a physical barrier to
digestion, and contains chemical bonds that cannot be broken
down by normal stomach
'Digestibility" describes how readily the plant can be digested and its
energy released for use by an animal. The digestible part of a plant
includes the cell contents
and the small amount of fibre that can be broken down. The amount of
energy available to an animal eating a plant is determined by the
proportion of fibre
to cell contents in that plant.
of plant structures
normally have higher fibre content than legumes
such as clover. The amount of fibre in leaves of grass is
twice that of legume leaves, and grass leaves are harder to
those of legumes. The leaves of grasses typically have a midrib made of
to provide support for the leaf, which adds to the higher fibre levels
lower digestibility of the leaves. The stems of most plant species have
fibre levels compared to the leaves, and grass stems usually contain
than legumes. Digestibility not only declines down the stem, but also
rapidly than leaf digestibility with increasing plant age (Buxton
& Redfearn, 1997).
efficiency of ruminants
better than nonruminants at digesting high-fibre diets. Leaves have a
rumen retention time than stems due to both faster rates of fibre
higher rates of passage of undigested material. Ruminants can digest
legume fibre and 60-70% of grass fibre (Buxton & Redfearn, 1997). Small
particles of food are digested faster than large particles because they
more surface area exposed relative to the total food volume. This is
regurgitate and re-chew (ruminate) their food after eating, to reduce
of the food particles. Because high fibre plants are harder to break
spend more time regurgitating and chewing grasses than legumes and more
chewing mature than immature plants.
efficiency of hindgut fermenters
fermenters such as horses do not re-chew their food. They
rely on eating more
food to extract enough nutrients. This means that at certain times of
nonruminants must eat lower quality feed in order to sustain their
requirements. So when food becomes limited, animals such as horses and
eat whatever is available, while cattle may pick and choose. In both
and nonruminants efficiency of digestion drops off as dietary fibre
certain levels. Above 60% fibre, the efficiency of digestion in both
horses drops to the point whereby not enough energy can be extracted
from the food
to make it worth eating. This is about the same level of fibre as the
effects of low digestibility
Due to the
comparatively low digestibility of grass and other vegetative matter,
and hindgut fermenters must consume proportionally more food than
omnivores. The high food intake and low nutrient extraction of
hindgut fermenters means that they concentrate and excrete large
quantities of plant
nutrients such as phosphorus and nitrogen. This leads to
problems such as pollution and eutrophication
of lakes and rivers when large numbers of cattle are concentrated on a
comparatively small amount of farmland. Phosphorus and nitrogen from
is washed in to lakes and waterways
and causes blooms of algae that reduces
Cellulose | Lignin
The plant cell wall surrounds the
cell membrane. It is made up of multiple layers of cellulose which are
into primary and secondary walls. Cellulose is the most common organic
compound on Earth. About 33% of all
plant matter is cellulose - the cellulose content of cotton is
90% and wood is 50% cellulose.
The cell walls of all vascular plants
also contain a polymer called lignin. Lignin is
It reinforces cell walls, keeping them from collapsing. This is
particularly important in the xylem, because the column of water in the
hollow xylem cells is under tension (negative pressure) and without the
lignin reinforcement the cells would collapse.
molecules attached end to end (Thus cellulose is an example of
polysaccharide.). A cellulose molecule may be from several
to over 10,000 glucose units long. Cellulose from wood pulp has typical
between 300 and 1700 units; cotton and other plant fibres have chain
ranging from 800 to 10,000 units (Klemm et al. 2005).
is similar in form to complex carbohydrates like starch and glycogen.
These polysaccharides are
also made from multiple subunits of glucose. The difference
between cellulose and other complex carbohydrate molecules is how the
molecules are linked together. In addition, cellulose is a straight
polymer, and each cellulose molecule is long and rod-like. This differs
from starch, which is a coiled molecule. A result of these differences
in structure is that, compared to starch and other
carbohydrates, cellulose can not be broken down into its glucose
subunits by any enzymes produced by animals.
provides the mechanical support for stems and leaves and supplies the
and rigidity of plant walls. Lignin provides the structural strength
large trees to reach heights in excess of 100 m. Without lignin these
would collapse on themselves. Also, lignin along with other cell wall
constituents provides resistance to diseases, insects, cold
temperatures, and other
stresses. Lignin plays a crucial part in conducting water in plant
structure of lignin has not been properly determined as it usually
upon extraction and there appears to be no consistent structure to it.
polysaccharide components of plant cell walls are highly hydrophilic
permeable to water, whereas lignin is more hydrophobic. The
polysaccharides by lignin is an obstacle for water absorption to the
Thus, lignin makes it possible for the plant's vascular
tissue to conduct water efficiently. Lignin is
present in all
vascular plants, but not in bryophytes,
supporting the idea that the original function of lignin was restricted
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Cells and Tissues
smallest functional units of life. A living organism may comprise a
single cell e.g. an alga; or it may be a multicellular organism made up
of of billions of cells e.g a kauri tree. An individual plant
different cell types, each adapted to perform a particular function.
However, each living plant cell is made up of the same basic
components: a cell
wall, plasma membrane, nucleus, and mitochondria and other organelles.
is self-contained and at least partially self-sufficient. This
plant cells have a number of organelles that perform functions
such as storing food, processing waste, fixing and releasing
chemical energy, and copying their genetic material. The nucleus, the
mitochondrion, the plastids, the Golgi apparatus, and the endoplasmic
are all examples of organelles. Some organelles, such as mitochondria
chloroplasts, have their own genetic material separate from that found
nucleus of the cell. Such organelles are thought to have originated
through the process of endosymbiosis.
Other structures such as the cell wall and the cell membrane
serve to protect the cell and maintain the shape, structure, and
functioning of the cell.
The plant cell wall
cells plant cells have an outer cell wall composed of polysaccharides,
particular cellulose. Cell walls provide rigidity and
mechanical support, maintain cell shape and the direction of cell
cell wall also prevents expansion when water enters the cell (Brooker et al. 2008).
of a plant cell is made up of primary and secondary cell walls. When
one cell divides into two, a primary cell wall forms around each two
cells. The primary cell wall is laid down just outside the cell
membrane, and is flexible, allowing the new cells to grow.
new cell wall is mostly made of cellulose molecules, arranged
thin hair-like strands called
microfibrils. The microfibrils are arranged in a meshwork pattern along
other components such as hemicellulose, glycans and pectins, which link
together and help strengthen the cell wall.
The secondary cell wall
is constructed between the plasma membrane and the primary cell wall
cell has finished growing. It is made by laying down successive layers
cellulose microfibrils and lignins. Mature xylem cells are heavily
lignified - they make up the 'wood' of woody plants (Raven et al. 2005).
courtesy of Roberta Farrell
Cell plasma membrane
cell is bounded by an outer membrane called the cell plasma membrane,
or plasmalemma. The cell membrane separates the cell's contents from
the surrounding environment and controls
the movement of materials into and out of the cell.
The cell membrane is made up
of two phospholipid layers (a bi-layer)which spontaneously arrange so
that the hydrophobic "tail"
regions are protected from
surrounding water, causing the more hydrophilic "head" regions to be pointed towards either
of the cell. Substances such as amino acids, nucleic acids,
proteins, and many ions are unable to diffuse across the membrane but
must enter or leave the cell through active transport, under the
control of the cell membrane.
involves some of the many different proteins and lipids embedded in
the plasma membrane. Other proteins are involved in
cellular processes such as cell linkage, ion channel flow and
communication. The plasma membrane also serves as the attachment point
the intracellular cytoskeleton
(found in all eukaryote cells) and the cell wall.
is the gooey, semi-transparent fluid in which the other organelles are
suspended. Processes such as breaking down food molecules into smaller
molecules happens in the cytoplasm.
consists mostly of water, dissolved ions, small molecules, and
large water-soluble molecules such as proteins. It also has a high
concentration of potassium ions and a low concentration of sodium ions,
help in controlling the amount of water in the cell. If you make a wet
mount of a leaf from the pondweed Elodea, you may be able to see the cell's
organelles, especially the chloroplasts, moving
as the cytoplasm flows through the cell (Raven et al.
cells the nucleus is
surrounded by a double membrane called the nuclear envelope.
This separates the
genetic material from the rest of the cell. The nuclear envelope has a
number of holes or ‘pores’ that allow the cell to
move molecules across the
nuclear envelope and in and out of the nucleus.
These molecules include RNA and ribosomes moving
from nucleus to the cytoplasm,
and proteins, carbohydrates, and lipids moving into the
nucleus. You could say that these molecules provide a means of
communication between the cytoplasm and
the nucleus. The nucleus performs two important functions: it controls
activity of the cell by determining what proteins are produced and when
are produced by the cell; and it also stores the genetic information of
which is then passed on to daughter cells during cell division.
of most plant cells are large, filling much of the space inside the
cell wall and pushing the cytoplasm out to the periphery of the
A single, large
vacuole is found in the cytoplasm
of most mature plant cells. (Animal cells have much smaller, multiple
vacuoles.) The vacuole is bounded by a membrane that keeps the
vacuole's contents separate from the cytoplasm. Young plant cells
typically contain a number of small independent vacuoles.
These merge together
to form one large vacuole as the cell matures, so that in a fully
the vacuole may occupy more than 90% of the cell volume.
contain salts, sugars, and sometimes proteins,a nd have a variety of
secretory, excretory, and storage functions. They are usually
acidic. Some of them, like those in grapefruit and lemons are very
acidic – which
is where the sour taste of these fruit comes from (Raven et al. 2005). The vacuole may also be
involved in the breakdown of
old organelles or macromolecules and then recycling the products back
rest of the cell. For example, an entire mitochondrion
may be engulfed by the vacuole, so that it is surrounded by a 'bubble'
of the vacuole membrane. This 'bubble' is then pinched off, forming
suspended in the vacuole. After the mitochondrion has been
endoplasmic reticulum (often abbreviated to ER) is an extensive network
membranous sack-like folds, tubes and vesicles, held together
by the cytoskeleton.
This network is continuous with the nuclear membrane. Plant cells
contain both rough and smooth ER. The relative
proportions of rough and smooth endoplasmic reticulum in a
can change quickly, depending on
the cell's metabolic needs. The endoplasmic reticulum provides a site
for the synthesis of lipids and proteins and for the storage and
of those molecules.
of the rough endoplasmic reticulum is studded with ribosomes,
giving it a "rough"
appearance (Campbell & Reece, 2005) .
This means that the rough ER is a site of protein synthesis. Once
produced, the proteins are packaged into vesicles and transported to
the Golgi apparatus,
where they are modified and packaged for export from the cell or
transport to where they will be used.
endoplasmic reticulum is also made up of tubules and vesicles that
branch forming a
network attached to the nuclear membrane. The smooth endoplasmic
reticulum provides an increased surface area for the action or storage
enzymes and the products of these enzymes. The smooth endoplasmic
produces lipids, such as the phospholipids that make up the cell membrane
- in fact, it is the source of new cell membrane material.
It also metabolises carbohydrates breaking them down into glucose
energy for the cell.
smooth ER is also involved in the breakdown and detoxification
drugs and chemicals e.g. in human liver cells smooth
ER is involved in the metabolism of ethanol. Long-term use of
leads to a increased amount of smooth ER in the cells, which increases
rate of alcohol metabolism. This explains why people who regularly
drink large amounts
of alcohol often have a high tolerance for alcohol (Brooker et al. 2008).
apparatus is a collective term for Golgi bodies: flattened,
membrane-enclosed compartments arranged into a stack of five or six
often with globs or cisternae at the ends. The Golgi bodies are
processing and sorting centres for cellular materials such as proteins,
and lipids. Cellular materials are transported from production areas
the endoplasmic reticulum to the Golgi bodies and, after sorting, to
at the ends of the Golgi bodies. At the cisternae the molecules are
enclosed in a phospholipid membrane. This membrane is then pinched off
to form a vesicle, which transports the molecules to their destination
and fuses with a membrane there to release
its contents. In plants, cellulose sub-units are packaged
into vesicles by the Golgi bodies, which then travel to and fuse with
plasma membrane. The vesicles discharge their contents to the outside
of the cell and
the cellulose is incorporated in the cell
- the cell's 'power plants' - generate most of the cell's
supply of the energy-carrying molecule adenosine triphosphate (ATP).
MItochondria are also involved in a range of other processes:
signalling, cellular differentiation, cell death, control of the cell
cycle, and cell growth. They evolved as the
result of endosymbiosis
from ancient bacteria, which were engulfed by the ancestors of
eukaryote cells more than a billion years ago.
Mitochondria are much smaller than plastids
- around 0.5µm in diameter i.e. too small to observe with a
light microscope. The number of mitochondria in a cell varies
widely: from one to several thousand, depending on organism and tissue
type. Mitochondria replicate (by binary fission) mainly in
response to the energy needs of the cell. In other words, their
growth and division is not linked to the cell cycle. When the energy
needs of a
cell are high, mitochondria grow and divide. When energy use is low,
mitochondria are destroyed or become inactive (Raven et al. 2005).
is composed of inner and outer membranes separated by a region called
intermembrane space. The inner membrane is highly folded, forming
called cristae. The membrane folds greatly increase the surface area of
the inner membrane, which is studded with the enzymes involved in
making ATP. The area inside the inner membrane is called
the matrix (Brooker et
matrix contains enzymes that produce molecules used in the production
of ATP on
the inner membrane. The mitochondrial matrix also contains the
DNA and ribosomes.
Mitochondria and aerobic
role of the mitochondrion is the production of ATP, used to power
cellular processes. Glycolysis
in the cytoplasm produces pyruvate
Once in the mitochonrion, pyruvate is oxidised in the Krebs cycle to
produce CO2, a small amount of ATP, and hydrogen
ions (H+). These then enter the Electron
Transport Chain, which generates a large amount of ATP, and water
(through the combination of the H+ ions with
oxygen). In other words, if oxygen is not available other less
anaerobic processes, such as fermentation,
must be used to generate ATP.
respiration has a much higher yield of ATP than anaerobic respiration
example, one molecule of glucose will produce between 32 to 36
molecules of ATP
under aerobic conditions; compared to 2 molecules of ATP under
and cell walls,
plastids are characteristic of plant cells. Plastids are responsible
products like starch and for the synthesis of many classes of
molecules, such as
fatty acids and terpenes,
which are needed as cellular building blocks and for normal functions
plant. Like mitochondria, plastids originated through endosymbiosis.
In plants, plastids
may differentiate into several forms, depending upon which function
to play in the cell.
are give plants their characteristic green colour and are the site of
photosynthesis. Like a mitochondrion, a chloroplast has an
outer and inner membrane, with an
intermembrane space between them. Within the inner membrane lies the
stroma, which contains the
thylakoid membranes - the site of photosynthesis. The
thylakoid membranes form a series of flattened,
fluid-filled, interconnected tubules that are often stacked on top of
each other to form a structure
called a granum. The photosynthetic pigments, such as chlorophyll,
are located in the
Chromoplasts - 'coloured bodies' - synthesise and
store pigments. They are found in coloured
organs of plants such as fruit and flower petals, giving them their
distinctive colours. When fruit such as tomatoes ripen, the
to are converted to chromoplasts that mostly contain red carotenoids.
During the day, when plants are photosynthesising, amyloplasts store glucose
and convert it into a form of starch for storage. They can also convert
the starch back into sugar, when the plant
are a specialised form of amyloplast used by plants in detecting and
responding to gravity.
Statoliths are denser than the cytoplasm and tend to move towards the
bottom of the cell, indicating which direction is 'down'. They
are found mainly in root tissues.
In plants, the
processes of differentiation and specialisation allow groups
of cells that are structurally or
functionally distinct to form tissues that have specific functions.
composed of only one type of cell are called simple tissues, while
those made up of two or more types of cells are called complex tissues.
and collenchyma are simple tissues; xylem, phloem, and epidermis are
tissues. Tissues can also be grouped into tissue systems, and in plants
these are (1) ground tissue, (2) vascular tissue, and (3) dermal
tissue. Gground tissue is made up of parenchyma,
collenchyma and sclerenchyma. Vascular tissue system consists of the
two conducting tissues –
xylem and phloem. Dermal tissue comprises the epidermis: pavement
guard cells, and trichomes. (In older woody plants the epidermis is
replaced by the periderm, or bark.) (Raven et al. 2005).
Most of a
plant’s body is made up of ground tissue, which has a variety
including photosynthesis, storage of carbohydrates and mechanical
the plant. Ground tissue can be divided into three tissue types;
collenchyma and sclerenchyma (Brooker et al.
the most common and versatile ground tissue: it forms the cortex and pith of stems, the cortex of
roots, the mesophyll of
leaves, the pulp of fruits, and the endosperm of seeds. Parenchyma cells are living cells, and may
still be able to divide when they are mature. Because of
this, parenchyma cells are important for
regeneration and wound healing. They have thin but flexible cell walls
the secondary cell wall is usually absent. Photosynthesis usually
occurs in parenchyma cells (the mesophyll), and they function in the
carbohydrates. They are generally polygonal when packed close together,
nearly spherical when separated from their neighbours. They have large
central vacuoles, which
allow the cells to store and regulate ions, waste products and water.
Cross-section of a
dicot root, showing the vascular tissue surrounded by large, rounded,
thin-walled parenchyma cells.
Image courtesy of Wendy Paul.
cells are elongated and have unevenly thickened walls.
Like parenchyma cells, they are still alive when mature.
Collenchyma provides structural
particularly in growing shoots and leaves. The stringy pieces in the
celery, for example, are composed of collenchyma.
Mature sclerenchyma is composed of
extremely thick secondary walls cell walls composed of cellulose and
lignin: these walls make up to 90% of the whole cell volume.
Sclerenchyma tissue is a
tissue. There are two types of
cells, fibres and sclereids. Fibres are generally long, slender cells,
commonly occur in strands or bundles, and can be seen in the diagram
below as dense red caps to the outside of the vascular tissue. Fibres
are of great
importance, since they make up the source material for many
flax, hemp and jute. Sclereids are variable in shape and usually
shorter than fibre cells. They make up the seed coats of seeds, the
of nuts and the stone of stone fruits.
of a dicot stem, showing cortex, pith, and vascular tissue.
Image courtesy of Wendy Paul.
tissue is a complex tissue and characteristic of vascular plants.
The two main
components of vascular tissue are the xylem and phloem. The vascular
plants is arranged in long, discrete strands called vascular bundles.
bundles include xylem and phloem, as well as supporting and protective
cells. The arrangement of vascular tissue differs in monocot and dicot
plants, as you can see by comparing the two previous images with the
two that follow below.
of a monocot root, showing cortex, pith and vascular tissue.
Image courtesy of Wendy Paul.
through a monocot stem, showing pith and vascular bundles.
Image courtesy of Wendy Paul.
a continuous system that transports water and dissolved mineral
throughout the plant from the roots. There are two types of conducting
cells in the xylem: tracheids and vessel elements. (Xylem also
contains parenchyma and sclerenchyma cells.) Both tracheids
and vessel elements are dead when mature and have thick, lignified cell
with pits in their walls, through which the water passes as it moves up
the plant. Vessels have a larger diameter than tracheids and their end
walls have disappeared. They are stacked end-on-end to form a
continuous series of tubes up the stem, allowing for more efficient
water movement than in tracheids
(Villee et al. 1989).
Section through woody
tissue, showing xylem tracheids.
Image courtesy of Roberta Farrell.
Phloem transports sugars, proteins and minerals around the
Unlike the xylem where flow is only one way, from the roots up, phloem
can move sugars both up and down the stem. In
flowering plants, phloem is contains four cell types: sieve tube
companion cells, fibres and parenchyma. Sieve tube members are very
specialised. The ends of the cell wall are perforated by sieve
plates, and sap passes through the sieve plates from one
member to another. Mature sieve tube
are not dead, but they do lose their nuclei
and most of their cytoplasm in order to accommodate the flow of phloem
they mature. For this reason, mature sieve tube members have
companion cells which supply the RNA and proteins that keep the sieve
member cell alive. Companion cells are also involved in the off-loading
on-loading of sugars from the phloem sap. As with the xylem tissue the
cells provide structural support to the tissue and parenchyma cells
storage for various substances.
tissue isthe boundary between the plant and the outside world. It
functions in protection against water loss, regulation of gas
secretion, and (especially in roots) absorption
and mineral nutrients. There are two main types of dermal tissue:
epidermis and periderm. The epidermis is usually transparent
(epidermal cells lack chloroplasts) and coated on its
outer surface with a waxy cuticle that prevents water loss. Epidermal
includes several differentiated cell types, including pavement
cells, and trichomes (epidermal hairs). The other dermal tissue type is
periderm, or bark. It replaces the epidermis of stems and roots once a
plant has developed secondary thickening.
are the most numerous epidermal cells, irregularly shaped and the least
specialised. They do not contain chlorophlasts. Because they are
transparent, light can penetrate into
the interior of stems and leaves, where photosynthesis
cells are tightly packed together to
water loss. The epiderms if leaves and stems is covered by a waxy
cuticle, which also helps prevent water loss. However, the
epidermis is also perforated by openings called stomata, which permit
gas exchange. Specialised cells called guard cells control the opening
and closing of these stomata, regulating the exchange of
water vapour between the outside air and the interior of the leaf. A
stoma is open when the guard cells surrounding it are swollen (turgid)
with water, and closed when the guard cells lose water and become
Image courtesy of Lynne Baxter.
Trichomes are minute
on the epidermis. On leaves these hairs can interfere with the
some herbivores, prevent frost forming on and damaging the leaf, reduce evaporation due to wind, and reflect light.
locations where plants get most of their water from cloud drip, leaf
hairs maximise this process by acting as points of condensation. Hairs on the roots increase the surface
available to the plant for uptake of water and minerals.
trichomes and stoma. Image courtesy of Lynne Baxter.
Widmaier E.P., Graham L.E. & Stiling P.D. 2008: Biology.
McGraw-Hill. New York.
Buxton D.R. & Redfearn D.D. 1997. Plant limitations to fibre
digestion and utilisation. Journal
Heublein B., Fink, H. & Bohn, A. 2005: Cellulose: fascinating
and sustainable raw material. ChemInform 36:
P.H., Evert, R.F. & Eichhorn S.E. 2005: Biology
of Plants (7th
Edition). W.H. Freeman & Company. New York.
Campbell, N. A.
& Reece, J.B 2005. Biology (7th
Benjamin Cummings Publishing,
Rich, P.R. 2003.
The molecular machinery of Keilin's
respiratory chain. Biochemical Society
Transactions 31: 1095-1105.
Solomon, E.P., Martin, C.E., Martin, D.W.,
Berg L.R. and Davis, R.W. 1989. Biology
(2nd Edition). Saunders College Publishing. Fort
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