Introduction to botany
Cell Structure
Parts of a plant cell
Cell Wall
The cell wall encloses and protects the cell contents and plays a vital role in cell division and
cell expansion. Composed of overlapping cellulose microfibrils, other polysaccharides and varying amounts of lignin, the cell wall is a relatively rigid structure in mature cells. It may vary in thickness and has pits that that function in communication between cells. The region between the primary walls of adjacent cells, the middle lamella, is composed of a cementing substance called pectin. Other substances that may be present in the cell wall are gums, resins, silica, calcium carbonate, waxes and
cutin, and both structural protein and enzymes (which are also proteins). There may be intercellular spaces between walls of bordering cells.
Pits
Primary pit fields are thin areas in the cell wall with tiny strands of cytoplasm, called plasmodesmata connecting one cell with another. Pits are important in facilitating the flow of water and mineral nutrients between conducting cells in the xylem vascular tissue.
Plasma Membrane
A semipermeable membrane encloses the cytoplasm within a cell. It is composed of variable amounts of fat type molecules (lipids) and proteins, and has within it channels for the movement of ions such as potassium (K), calcium (Ca), and hydrogen (H).
Cytoplasm
The cytoplasm is a liquid, gel-like substance and contains several types of organelles; smooth (g) or rough endoplasmic reticulum, rough referring to attached ribosomes on the ER (endoplasmic reticulum) and free ribosomes.
Vacuole
In a mature plant cell, one large vacuole usually occupies most of the space within the cell. It is surrounded by a single-layered membrane, the tonoplast, and contains cell sap composed of water, sugars, and various organic and inorganic solutes. It may, in some cells such as in beet roots and flower petals, contain water-soluble pigments. The vacuole functions in regulation of osmotic balance and turgidity of the cell, and it stores secondary metabolites.
Organelles
Within the cytoplasm are mitochrondria, dictyosomes (Golgi bodies), microbodies, and microtubules. Microtubules are represented by an array of parallel tubular tracks and facilitate movement of proteins and organelles within the cell. The ER is a system of tubes and sacs, that work with dictyosomes to produce and secrete compounds and deliver specific proteins and membrane lipids to their proper locations within a cell. Also, there may be plastids such as chloroplasts, leucoplasts, and chromoplasts; and non-living substances of water-soluble products or reserve substances such as oil droplets, protein bodies, and crystals.
Nucleus
The nucleus is enclosed by a double membrane that has pores in it to allow communication
with the cytoplasm. Within the nucleus are chromosomes, which contain DNA needed to create proteins within the cell. Chromosomes are only visible during cell division. Also present in the nucleus are one or more nucleoli containing RNA. The rest of the nucleus is filled with nucleoplasm. The information needed to create the entire plant is within the nucleus, mitochondria, and chloroplasts of each cell.
a-Cell wall
b-middle lamella
c-Intercellular area
d-Pit field
e-Plasma membrane
f-Cytoplasm
g-Smooth ER
h-Rough ER
i-Ribosome
j-Vacuolar membrane
k-Mitochondrion
l-Dictyosome
m-Microtubules
n-Chloroplast
o-Nuclear membrane
p-Pore
q-Nucleolus
r-Nucleoplasm
s-Plasmodesmata
Cell Organelles
The cytoplasm (cytosol) of a plant cell contains various organelles with specific functions. With the use of a transmission electron microscope (TEM), which shows structures magnified thousands of times, these organelles can be viewed individually. The organelles represented here are not shown in proportion to one another.
Plastids
Plastids are classified by the primary pigment they contain. Young cells have undifferentiated plastids (proplastids), which can multiply by simple division. They develop into the various kinds of plastids
characteristic of mature cells.
Chloroplast
This plastid is where photosynthesis takes place. In higher plants it is usually oval-shaped and is surrounded by a double membrane. Within it are sac-like structures (shown cut in half) called thylakoids. A stack of thylakoids is called a granum , and this is where green chlorophyll pigments are located. Chlorophyll and proteins bound to the thylakoids use light energy to make simple sugars from carbon dioxide and water. Extensions from some thylakoid membranes form interconnections between grana. The thylakoids also contain accessory pigments, carotenoids, and xanthophylls. In the stroma of the chloroplast are DNA, RNA, oil droplets, ribosomes, and other materials such as starch grains, found in chloroplasts of green plant tissues that have been actively photosynthesizing.
Leucoplast
Such colorless plastids contain storage products, which include oils, protein bodies, or starch grains. In plant parts with a high starch content, such as potato tubers or rice grains, a leucoplast that contains starch grains is called an amyloplast.
Chromoplast
These plastids are colored red, orange, or yellow, depending on the pigments they contain. In the changing colors of ripening fruit such as a tomato or a red pepper, the chloroplasts (green) differentiate into chromoplasts (orange to red). As fruit color changes, so do plastid structure, pigment types, and content.
Mitochondria
These organelles are surrounded by a double membrane. The inner of these two membranes has infoldings, called cristae, that protrude into the cavity within. Mitochondria are the primary sites of enzymes controlling cell respiration (a chemical release of energy from sugar or other metabolites).
They can replicate by simple division and, like chloro- plasts, they contain DNA.
Ribosomes
Ribosomes contain ribosomal RNA and function in protein synthesis. They are also found in
cytoplasm, and associated with the endoplasmic reticulum in the cytoplasm.
Dictyosomes (Golgi Bodies)
These organelles appear as a stack of flattened sacs (s) and associated vesicles (t). Dictyosomes produce and secrete cell wall polysaccharide precursors and complex carbohydrate substances that are secreted out of root cap cells. This results in less injury to the growing root as it penetrates
the soil.
Chlorophylls
The chlorophylls are oil-soluble. Chlorophyll a (greenish-yellow in solution) is the primary
photosynthetic pigment in green plants for the transfer of light energy to a chemical acceptor. Light that is absorbed provides the energy for photosynthesis. A green leaf absorbs blue light (mostly at 430 nm) and red light (mostly at 660 nm). It reflects the green wavelengths so as to appear green to us. Chlorophyll a, alone, is found in blue-greens and in some red algae. Accessory pigments in photosynthesis transfer light energy to chorophyll a. One of these is chlorophyll b (blue-green solution), found in higher plants and green algae with chlorophyll a. Chlorophyll c is also an accessory pigment found with chlorophyll a in brown algae and diatoms. Chlorophyll d, together with chlorphyll a, is found in some red algae.
Carotenoids
These pigments are considered as accessory pigments in photosynthesis, when found in chloroplasts associated with chlorophyll, and as color pigments when found in chromoplasts. Carotenoids, like the chlorophylls, are not water-soluble. They absorb mainly violet and blue light between 400 and 500 nm, and they reflect red, orange, yellow, brown, and the green color of avocado fruit. They give color to carrot roots, tomato fruit, and many yellow flowers. Carotenoids of red and yellow are revealed in autumn leaves (h, I) after the green chlorophyll pigments begin to break down. This occurs when daylengths start to shorten and cooler temperatures prevail in temperate regions of the world. Carotenes (yellow or orange non-oxygen containing pigments) seem to function in the prevention of chlorophyll destruction in the presence of light and oxygen. β-carotene is the most important of the carotines. Xanthophyll pigments are oxygen-containing carotenoids. They transfer energy to chlorophyll from light. Some of the xanthophylls include lutein, zeaxanthin, violxanthin, and fucoxanthin, which is found in brown algae.
Phycobilins
These pigments are water-soluble. As accessory pigments they absorb light and transfer excitation energy to chlorophyll a. The red pigment, phyoerythrin, is found in red algae. The blue pigment phycocyanin, is present in blue-greens and some red algae.
Phytochrome
This pigment plays an important role in regulating many processes of plant growth and development. Phytochrome is found in two reversible forms. One form, (Pr) , absorbs red light (mostly at 660 nm) and reflects a blue-green color. The other form, (Pfr) , absorbs far-red light (mostly at 730 nm) and reflects a light-green color. The highest amounts of phytochrome are found in meristermatic tissues.
Flavinoids
These water-soluble pigments accumulate in cell vacuoles. They absorb ultraviolet wavelengths of light. Anthocyanins are phenolic pigments that are found in most fruits and in many flowers. These
red-, purple-, and blue-reflecting pigments are seen in the red color of apple (Malus) fruit and geranium (Pelargonium) flowers; the blue of cornflowers (Centaurea) and larkspur (Delphinium); and the red and purple of Fuchsia flowers. The function of these pigments can only be hypothesized, but study of plants having these pigments is useful in determining evolutionary relationships. Flavins often appear to us as yellow or ivory-colored flowers. Some flavins, such a riboflavin (vitamin B 2 ), act as co-factors in enzyme reactions and some are thought to be receptor pigments in the bending of plants toward light.
Betalains
These are water-soluble, nitrogen-containing reddish pigments that are found in only 9 families
of the flowering plant subclass, Carophyllidae. These pigments are water-soluble and found in the vacuoles of cells. Betacyanin appears as blue-violet to red and is found in beet (Beta) roots and red cactus flowers. Betaxanthin pigments reflect yellow, orange, and orange-red.
Plastids
Plastids are classified by the primary pigment they contain. Young cells have undifferentiated plastids (proplastids), which can multiply by simple division. They develop into the various kinds of plastids
characteristic of mature cells.
Chloroplast
This plastid is where photosynthesis takes place. In higher plants it is usually oval-shaped and is surrounded by a double membrane. Within it are sac-like structures (shown cut in half) called thylakoids. A stack of thylakoids is called a granum , and this is where green chlorophyll pigments are located. Chlorophyll and proteins bound to the thylakoids use light energy to make simple sugars from carbon dioxide and water. Extensions from some thylakoid membranes form interconnections between grana. The thylakoids also contain accessory pigments, carotenoids, and xanthophylls. In the stroma of the chloroplast are DNA, RNA, oil droplets, ribosomes, and other materials such as starch grains, found in chloroplasts of green plant tissues that have been actively photosynthesizing.
Leucoplast
Such colorless plastids contain storage products, which include oils, protein bodies, or starch grains. In plant parts with a high starch content, such as potato tubers or rice grains, a leucoplast that contains starch grains is called an amyloplast.
Chromoplast
These plastids are colored red, orange, or yellow, depending on the pigments they contain. In the changing colors of ripening fruit such as a tomato or a red pepper, the chloroplasts (green) differentiate into chromoplasts (orange to red). As fruit color changes, so do plastid structure, pigment types, and content.
Mitochondria
These organelles are surrounded by a double membrane. The inner of these two membranes has infoldings, called cristae, that protrude into the cavity within. Mitochondria are the primary sites of enzymes controlling cell respiration (a chemical release of energy from sugar or other metabolites).
They can replicate by simple division and, like chloro- plasts, they contain DNA.
Ribosomes
Ribosomes contain ribosomal RNA and function in protein synthesis. They are also found in
cytoplasm, and associated with the endoplasmic reticulum in the cytoplasm.
Dictyosomes (Golgi Bodies)
These organelles appear as a stack of flattened sacs (s) and associated vesicles (t). Dictyosomes produce and secrete cell wall polysaccharide precursors and complex carbohydrate substances that are secreted out of root cap cells. This results in less injury to the growing root as it penetrates
the soil.
Cell Pigments
Chlorophylls
The chlorophylls are oil-soluble. Chlorophyll a (greenish-yellow in solution) is the primary
photosynthetic pigment in green plants for the transfer of light energy to a chemical acceptor. Light that is absorbed provides the energy for photosynthesis. A green leaf absorbs blue light (mostly at 430 nm) and red light (mostly at 660 nm). It reflects the green wavelengths so as to appear green to us. Chlorophyll a, alone, is found in blue-greens and in some red algae. Accessory pigments in photosynthesis transfer light energy to chorophyll a. One of these is chlorophyll b (blue-green solution), found in higher plants and green algae with chlorophyll a. Chlorophyll c is also an accessory pigment found with chlorophyll a in brown algae and diatoms. Chlorophyll d, together with chlorphyll a, is found in some red algae.
Carotenoids
These pigments are considered as accessory pigments in photosynthesis, when found in chloroplasts associated with chlorophyll, and as color pigments when found in chromoplasts. Carotenoids, like the chlorophylls, are not water-soluble. They absorb mainly violet and blue light between 400 and 500 nm, and they reflect red, orange, yellow, brown, and the green color of avocado fruit. They give color to carrot roots, tomato fruit, and many yellow flowers. Carotenoids of red and yellow are revealed in autumn leaves (h, I) after the green chlorophyll pigments begin to break down. This occurs when daylengths start to shorten and cooler temperatures prevail in temperate regions of the world. Carotenes (yellow or orange non-oxygen containing pigments) seem to function in the prevention of chlorophyll destruction in the presence of light and oxygen. β-carotene is the most important of the carotines. Xanthophyll pigments are oxygen-containing carotenoids. They transfer energy to chlorophyll from light. Some of the xanthophylls include lutein, zeaxanthin, violxanthin, and fucoxanthin, which is found in brown algae.
Phycobilins
These pigments are water-soluble. As accessory pigments they absorb light and transfer excitation energy to chlorophyll a. The red pigment, phyoerythrin, is found in red algae. The blue pigment phycocyanin, is present in blue-greens and some red algae.
Phytochrome
This pigment plays an important role in regulating many processes of plant growth and development. Phytochrome is found in two reversible forms. One form, (Pr) , absorbs red light (mostly at 660 nm) and reflects a blue-green color. The other form, (Pfr) , absorbs far-red light (mostly at 730 nm) and reflects a light-green color. The highest amounts of phytochrome are found in meristermatic tissues.
Flavinoids
These water-soluble pigments accumulate in cell vacuoles. They absorb ultraviolet wavelengths of light. Anthocyanins are phenolic pigments that are found in most fruits and in many flowers. These
red-, purple-, and blue-reflecting pigments are seen in the red color of apple (Malus) fruit and geranium (Pelargonium) flowers; the blue of cornflowers (Centaurea) and larkspur (Delphinium); and the red and purple of Fuchsia flowers. The function of these pigments can only be hypothesized, but study of plants having these pigments is useful in determining evolutionary relationships. Flavins often appear to us as yellow or ivory-colored flowers. Some flavins, such a riboflavin (vitamin B 2 ), act as co-factors in enzyme reactions and some are thought to be receptor pigments in the bending of plants toward light.
Betalains
These are water-soluble, nitrogen-containing reddish pigments that are found in only 9 families
of the flowering plant subclass, Carophyllidae. These pigments are water-soluble and found in the vacuoles of cells. Betacyanin appears as blue-violet to red and is found in beet (Beta) roots and red cactus flowers. Betaxanthin pigments reflect yellow, orange, and orange-red.
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