Plant Structure & Function
Plant Structure & Function
The 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.
 
 
Digestibility of Plants
Digestibility of plant structures | Digestive efficiency of ruminants | Digestive efficiency of hindgut fermenters | Environmental effects of low digestibility
Animals can easily digest the contents of plant cells, but not their cell walls. All  plant cells have an outer cell wall composed 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 cellulose.
Fibre is the amount of cellulose and lignin 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 microbial flora .
'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.
Digestibility of plant structures
Grasses 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 digest than those of legumes. The leaves of grasses typically have a midrib made of lignin to provide support for the leaf, which adds to the higher fibre levels and lower digestibility of the leaves. The stems of most plant species have greater fibre levels compared to the leaves, and grass stems usually contain more fibre than legumes. Digestibility not only declines down the stem, but also declines more rapidly than leaf digestibility with increasing plant age (Buxton & Redfearn, 1997).
Digestive efficiency of ruminants
Ruminants are better than nonruminants at digesting high-fibre diets. Leaves have a shorter rumen retention time than stems due to both faster rates of fibre digestion and higher rates of passage of undigested material. Ruminants can digest 40-50% of legume fibre and 60-70% of grass fibre (Buxton & Redfearn, 1997). Small particles of food are digested faster than large particles because they have more surface area exposed relative to the total food volume. This is why ruminants regurgitate and re-chew (ruminate) their food after eating, to reduce the size of the food particles. Because high fibre plants are harder to break down, ruminants spend more time regurgitating and chewing grasses than legumes and more time chewing mature than immature plants.
Digestive efficiency of hindgut fermenters
Hindgut 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 the year, nonruminants must eat lower quality feed in order to sustain their energy requirements. So when food becomes limited, animals such as horses and pandas must eat whatever is available, while cattle may pick and choose. In both ruminants and nonruminants efficiency of digestion drops off as dietary fibre reaches certain levels. Above 60% fibre, the efficiency of digestion in both cattle and 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 wood of many trees.
Environmental effects of low digestibility
Due to the comparatively low digestibility of grass and other vegetative matter, ruminants and hindgut fermenters must consume proportionally more food than carnivores or omnivores. The high food intake and low nutrient extraction of ruminants and 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 cattle effluent is washed in to lakes and waterways and causes blooms of algae that reduces water quality.
 
Cellulose | Lignin
The plant cell wall surrounds the cell membrane. It is made up of multiple layers of cellulose which are arranged 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 water-resistant. 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.
Image courtesy of Roberta Farrell
Cellulose
Cellulose is a polymer made of repeating glucose molecules attached end to end (Thus cellulose is an example of a polysaccharide.). A cellulose molecule may be from several hundred to over 10,000 glucose units long. Cellulose from wood pulp has typical chain lengths between 300 and 1700 units; cotton and other plant fibres have chain lengths ranging from 800 to 10,000 units (Klemm et al. 2005) .
Cellulose 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 glucose molecules are linked together. In addition, cellulose is a straight chain 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.
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Lignin
Lignin provides the mechanical support for stems and leaves and supplies the strength and rigidity of plant walls. Lignin provides the structural strength needed by large trees to reach heights in excess of 100 m. Without lignin these trees 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 stems. The structure of lignin has not been properly determined as it usually fragments upon extraction and there appears to be no consistent structure to it. The polysaccharide components of plant cell walls are highly hydrophilic and thus permeable to water, whereas lignin is more hydrophobic. The crosslinking of polysaccharides by lignin is an obstacle for water absorption to the cell wall. 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 to water transport.
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Plant Cells and Tissues
Cells are the 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 contains many 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.
 
The plant cell wall | Plasma membrane | Cytoplasm | Nucleus | Vacuole | Endoplasmic reticulum | Golgi apparatus | Mitochondria | Plastids
Each plant cell is self-contained and at least partially self-sufficient. This is because 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 reticulum are all examples of organelles. Some organelles, such as mitochondria and chloroplasts, have their own genetic material separate from that found in the 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
Unlike animal cells plant cells have an outer cell wall composed of polysaccharides, in particular cellulose . Cell walls provide rigidity and mechanical support, maintain cell shape and the direction of cell growth. The cell wall also prevents expansion when water enters the cell (Brooker et al. 2008).
Cell wall structure
The cell wall 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 new cells. The primary cell wall is laid down just outside the cell membrane, and is flexible, allowing the new cells to grow. This new cell wall is mostly made of cellulose molecules, arranged into thin hair-like strands called microfibrils. The microfibrils are arranged in a meshwork pattern along with other components such as hemicellulose, glycans and pectins, which link them together and help strengthen the cell wall.
The secondary cell wall is constructed between the plasma membrane and the primary cell wall after the cell has finished growing. It is made by laying down successive layers of cellulose microfibrils and lignins. Mature xylem cells are heavily lignified - they make up the 'wood' of woody plants (Raven et al. 2005).
 
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Cell plasma membrane
Every living 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 the surrounding water, causing the more hydrophilic "head" regions to be pointed towards either the cytoplasm or the exterior of the cell. Substances such as amino acids, nucleic acids, carbohydrates, 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.
This involves some of the many different proteins and lipids embedded in the plasma membrane. Other proteins are involved in other cellular processes such as cell linkage, ion channel flow and cell communication. The plasma membrane also serves as the attachment point for both the intracellular cytoskeleton (found in all eukaryote cells) and the cell wall.
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Cytoplasm
The cytoplasm 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. 
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, which 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. 2005) .
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Nucleus
In eukaryote 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 large 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 the activity of the cell by determining what proteins are produced and when they are produced by the cell; and it also stores the genetic information of the cell which is then passed on to daughter cells during cell division.
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Vacuole
The vacuoles 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 cell. 
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 mature cell the vacuole may occupy more than 90% of the cell volume. 
Vacuoles typically contain salts, sugars, and sometimes proteins,a nd have a variety of secretory, excretory, and storage functions.  They are usually slightly 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 to the 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 a  vesicle suspended in the vacuole. After the mitochondrion has been digested the vesicle disappears.
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Endoplasmic reticulum
The endoplasmic reticulum (often abbreviated to ER) is an extensive network of 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 cell 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 transport of those molecules.
Rough endoplasmic reticulum
The surface 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.
Smooth endoplasmic reticulum
Smooth 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 of key enzymes and the products of these enzymes. The smooth endoplasmic reticulum also 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 providing energy for the cell. 
The smooth ER is also involved in the breakdown and detoxification of drugs and chemicals e.g. in human liver cells smooth ER is involved in the metabolism of ethanol. Long-term use of alcohol leads to a increased amount of smooth ER in the cells, which increases the 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).
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Golgi apparatus
The Golgi apparatus is a collective term for Golgi bodies: flattened, membrane-enclosed compartments arranged into a stack of five or six pancake-like structures,  often with globs or cisternae at the ends. The Golgi bodies are processing and sorting centres for cellular materials such as proteins, enzymes and lipids. Cellular materials are transported from production areas such as the endoplasmic reticulum to the Golgi bodies and, after sorting, to cisternae 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 the plasma membrane. The vesicles discharge their contents to the outside of the cell and the cellulose is incorporated in the cell wall   (Raven et al. 2005).
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Mitochondria
Mitochondria - 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) .
Mitochondria structure
A mitochondrion is composed of inner and outer membranes separated by a region called the intermembrane space. The inner membrane is highly folded, forming projections 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 al. 2008) . The matrix contains enzymes that produce molecules used in the production of ATP on the inner membrane. The mitochondrial matrix also contains the mitochondrion's DNA and ribosomes.
Mitochondria and aerobic respiration
The primary role of the mitochondrion is the production of ATP, used to power cellular processes. Glycolysis   in the cytoplasm produces  pyruvate and NADH . 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 efficient anaerobic processes, such as fermentation , must be used to generate ATP. 
Aerobic respiration has a much higher yield of ATP than anaerobic respiration (Rich, 2003) . For example, one molecule of glucose will produce between 32 to 36 molecules of ATP under aerobic conditions; compared to 2 molecules of ATP under anaerobic conditions. 
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Plastids
Like vacuoles and cell walls , plastids are characteristic of plant cells. Plastids are responsible for photosynthesis , storage of 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 of the plant. Like mitochondria, plastids originated through endosymbiosis . In plants, plastids may differentiate into several forms, depending upon which function they need to play in the cell. 
Chloroplasts
Chloroplasts 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 thylakoid membranes.
Chromoplasts
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 green chloroplasts to are converted to chromoplasts that mostly contain red carotenoids.
Amyloplasts
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 needs energy.
Statoliths
Statoliths 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.
Ground tissue | Vascular tissue | Dermal tissue
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. Tissues 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. Parenchyma and collenchyma are simple tissues; xylem, phloem, and epidermis are complex 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 cells, guard cells, and trichomes. (In older woody plants the epidermis is replaced by the periderm, or bark.) (Raven et al. 2005) .
Ground tissue
Most of a plant’s body is made up of ground tissue, which has a variety of functions, including photosynthesis, storage of carbohydrates and mechanical support of the plant. Ground tissue can be divided into three tissue types; parenchyma, collenchyma and sclerenchyma (Brooker et al. 2008) .
Parenchyma
Parenchyma is 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 because the secondary cell wall is usually absent. Photosynthesis usually occurs in parenchyma cells (the mesophyll), and they function in the storage of carbohydrates. They are generally polygonal when packed close together, but are 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.
Collenchyma
Collenchyma cells are elongated and have unevenly thickened walls. Like parenchyma cells, they are still alive when mature. Collenchyma provides structural support, particularly in growing shoots and leaves. The stringy pieces in the stalks of celery, for example, are composed of collenchyma. 
Sclerenchyma
Mature sclerenchyma is composed of dead cells with 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 supporting tissue. There are two types of sclerenchyma cells, fibres and sclereids. Fibres are generally long, slender cells, which 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 economical importance, since they make up  the source material for many fabrics such as flax, hemp and jute. Sclereids are variable in shape and usually shorter than fibre cells. They make up the seed coats of seeds, the shells of nuts and the stone of stone fruits.
Cross-section of a dicot stem, showing cortex, pith, and vascular tissue.
Image courtesy of Wendy Paul.
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Vascular tissue
Vascular tissue is a complex tissue and characteristic of vascular plants . The two main components of vascular tissue are the xylem and phloem. The vascular tissue in plants is arranged in long, discrete strands called vascular bundles. These 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.
Cross-section of a monocot root, showing cortex, pith and vascular tissue.
Image courtesy of Wendy Paul.
Cross-section through a monocot stem, showing pith and vascular bundles.
Image courtesy of Wendy Paul.
Xylem
The xylem forms a continuous system that transports water and dissolved mineral nutrients 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 walls.
Tracheids are long, tapering cells 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
Phloem transports sugars, proteins and minerals around the plant. 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 members, companion cells, fibres and parenchyma. Sieve tube members are very highly specialised. The ends of the cell wall are perforated by sieve plates, and sap passes through the sieve plates from one sieve tube member to another. Mature sieve tube members are not dead, but they do lose their nuclei and most of their cytoplasm in order to accommodate the flow of phloem sap once they mature. For this reason, mature sieve tube members have accompanying companion cells which supply the RNA and proteins that keep the sieve tube member cell alive. Companion cells are also involved in the off-loading and on-loading of sugars from the phloem sap. As with the xylem tissue the fibre cells provide structural support to the tissue and parenchyma cells provide storage for various substances.
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Dermal tissue
The dermal tissue isthe boundary between the plant and the outside world. It functions in protection against water loss, regulation of gas exchange, secretion, and (especially in roots) absorption of water 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 tissue includes several differentiated cell types, including pavement cells, guard cells, and trichomes (epidermal hairs). The other dermal tissue type is the periderm, or bark. It replaces the epidermis of stems and roots once a plant has developed secondary thickening.
Epidermis
Pavement cells 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 occurs. Pavement cells are tightly packed together to prevent 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 gases and 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 flaccid. 
Carmichaelia stoma. Image courtesy of Lynne Baxter.
Trichomes are minute hairs on the epidermis. On leaves these hairs can interfere with the feeding of some herbivores, prevent frost forming on and damaging the leaf, reduce evaporation due to wind, and reflect light. And in 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 area available to the plant for uptake of water and minerals.
Pittosporum trichomes and stoma. Image courtesy of Lynne Baxter.
References
Brooker R.J., 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 of Nutrition 127: 814S-818S
Klemm, D., Heublein B., Fink, H. & Bohn, A. 2005: Cellulose: fascinating biopolymer and sustainable raw material. ChemInform 36: 3358-3393.
Raven, 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 Edition). Benjamin Cummings Publishing, 
Rich, P.R. 2003. The molecular machinery of Keilin's respiratory chain. Biochemical Society Transactions 31: 1095-1105.
Villee, C.A., Solomon, E.P., Martin, C.E., Martin, D.W., Berg L.R. and Davis, R.W. 1989. Biology (2nd Edition). Saunders College Publishing. Fort Worth.