anatomy of the plant cell

8/7/2019 Anatomy of the Plant Cell

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Anatomy of the Plant Cell

Like other eukaryotes, the plant cell is enclosed by a plasma membrane, which forms aselective barrier allowing nutrients to enter and waste products to leave. Unlike other

eukaryotes, however, plant cells have retained a significant feature of their prokaryoteancestry, a rigid cell wall surrounding the plasma membrane. The cytoplasm contains

specialized organelles, each of which is surrounded by a membrane. Plant cells differ

from animal cells in that they lack centrioles and organelles for locomotion (cilia and

flagella), but they do have additional specialized organelles. Chloroplasts convert light tochemical energy, a single large vacuole acts as a water reservoir, and plasmodesmata

allow cytoplasmic substances to pass directly from one cell to another. There is only one

nucleus and it contains all the genetic information necessary for cell growth andreproduction. The other organelles occur in multiple copies and carry out the various

functions of the cell, allowing it to survive and participate in the functioning of the largerorganism.

Thought to have evolved from the green algae, plants have been around since the early

Paleozoic era, more than 500 million years ago. The earliest fossil evidence of land

plants dates to the Ordovician Period (505 to 438 million years ago). By the

Carboniferous Period, about 355 million years ago, most of the Earth was covered byforests of primitive vascular plants, such as lycopods (scale trees) and gymnosperms

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(pine trees, ginkgos). Angiosperms, the flowering plants, didn't develop until the end of

the Cretaceous Period, about 65 million years agojust as the dinosaurs became

extinct.

Cell Wall - Like their prokaryotic ancestors, plant cells have a rigid wall

surrounding the plasma membrane. It is a far more complex structure, however,and serves a variety of functions, from protecting the cell to regulating the life

cycle of the plant organism.

Plant Cell Wall

One of the most important distinguishing features of plant cells is the presence of a cell

wall. The relative rigidity of the cell wall renders plants sedentary, unlike animals, whose

lack of this type of structure allows their cells more flexibility, which is necessary forlocomotion. The plant cell wall serves a variety of functions. Along with protecting the

intracellular contents, the structure bestows rigidity to the plant, provides a porous

medium for the circulation and distribution of water, minerals, and other nutrients, andhouses specialized molecules that regulate growth and protect the plant from disease.

Cell walls are significantly thicker than plasma membranes and were visible even to early

microscopists, including Robert Hooke, who originally identified the structures in asample of cork, and then coined the term cells in the 1660s. The thickness, as well as the

composition and organization, of cell walls can vary significantly. Many plant cells have

both a primary cell wall, which accommodates the cell as it grows, and a secondary cell

wall they develop inside the primary wall after the cell has stopped growing. The primarycell wall is thinner and more pliant than the secondary cell wall, and is sometimes

retained in an unchanged or slightly modified state without the addition of the secondary

wall, even after the growth process has ended.

The main chemical components of the primary plant cell wall include cellulose (in the

form of organized microfibrils; see Figure 1), a complex carbohydrate made up of

several thousand glucose molecules linked end to end. In addition, the cell wall contains

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two groups of branched polysaccharides, the pectins and cross-linking glycans.

Organized into a network with the cellulose microfibrils, the cross-linking glycans

increase the tensile strength of the cellulose, whereas the coextensive network of pectinsprovides the cell wall with the ability to resist compression. In addition to these networks,

a small amount of protein can be found in all plant primary cell walls. Some of this

protein is thought to increase mechanical strength and part of it consists of enzymes,which initiate reactions that form, remodel, or breakdown the structural networks of the

wall. Such changes in the cell wall directed by enzymes are particularly important for

fruit to ripen and leaves to fall in autumn.

The secondary plant cell wall, which is often deposited inside the primary cell wall as acell matures, sometimes has a composition nearly identical to that of the earlier-

developed wall. More commonly, however, additional substances, especially lignin, are

found in the secondary wall. Lignin is the general name for a group of polymers ofaromatic alcohols that are hard and impart considerable strength to the structure of the

secondary wall. Lignin is what provides the favorable characteristics of wood to the fiber

cells of woody tissues and is also common in the secondary walls of xylem vessels,which are central in providing structural support to plants. Lignin also makes plant cellwalls less vulnerable to attack by fungi or bacteria, as do cutin, suberin, and other waxy

materials that are sometimes found in plant cell walls.

A specialized region associated with the cell walls of plants, and sometimes consideredan additional component of them, is the middle lamella (see Figure 1). Rich in pectins,

the middle lamella is shared by neighboring cells and cements them firmly together.

Positioned in such a manner, cells are able to communicate with one another and share

their contents through special conduits. Termed plasmodesmata, these small passagespenetrate the middle lamella as well as the primary and secondary cell walls, providing

pathways for transporting cytoplasmic molecules from one cell to another.

Chloroplasts - The most important characteristic of plants is their ability to

photosynthesize, in effect, to make their own food by converting light energy intochemical energy. This process is carried out in specialized organelles called

chloroplasts.

Chloroplasts

One of the most widely recognized and important characteristics of plants are their abilityto conduct photosynthesis, in effect, to make their own food by converting light energy

into chemical energy. This process occurs in almost all plant species and is carried out inspecialized organelles known as chloroplasts. All of the green structures in plants,including stems and unripened fruit, contain chloroplasts, but the majority of

photosynthesis activity in most plants occurs in the leaves. On the average, the

chloroplast density on the surface of a leaf is about one-half million per squaremillimeter.

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Chloroplasts are one of several different types ofplastids, plant cell organelles that are

involved in energy storage and the synthesis of metabolic materials. The colorlessleucoplasts, for instance, are involved in the synthesis of starch, oils, and proteins.

Yellow-to-red colored chromoplasts manufacture carotenoids, and the green colored

chloroplasts contain the pigments chlorophyll a and chlorophyll b, which are able to

absorb the light energy needed for photosynthesis to occur. All plastids develop from tinyorganelles found in the immature cells of plant meristems (undifferentiated plant tissue)

termed proplastids, and those of a particular plant species all contain copies of the same

circular genome. The disparities between the various types of plastids are based upon theneeds of the differentiated cells they are contained in, which may be influenced by

environmental conditions, such as whether light or darkness surrounds a leaf as it grows.

The ellipsoid-shaped chloroplast is enclosed in a double membrane and the area between

the two layers that make up the membrane is called the intermembrane space. The outerlayer of the double membrane is much more permeable than the inner layer, which

features a number of embedded membrane transport proteins. Enclosed by the chloroplast

membrane is the stroma, a semi-fluid material that contains dissolved enzymes andcomprises most of the chloroplast's volume. Since, like mitochondria, chloroplasts

possess their own genomes (DNA), the stroma contains chloroplast DNA and special

ribosomes and RNAs as well. In higher plants, lamellae, internal membranes with stacks(each termed a granum) of closed hollow disks called thylakoids, are also usually

dispersed throughout the stroma. The numerous thylakoids in each stack are thought to be

connected via their lumens (internal spaces).

Light travels as packets of energy called photons and is absorbed in this form by light-absorbing chlorophyll molecules embedded in the thylakoid disks. When these

chlorophyll molecules absorb the photons, they emit electrons, which they obtain from

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water (a process that results in the release of oxygen as a byproduct). The movement of

the electrons causes hydrogen ions to be propelled across the membrane surrounding the

thylakoid stack, which consequently initiates the formation of an electrochemicalgradient that drives the stroma's production of adenosine triphosphate (ATP). ATP is the

chemical energy "currency" of the cell that powers the cell's metabolic activities. In the

stroma, the light-independent reactions of photosynthesis, which involve carbon fixation,occur, and low-energy carbon dioxide is transformed into a high-energy compound like

glucose.

Plant cells are remarkable in that they have two organelles specialized for energy

production: chloroplasts, which create energy via photosynthesis, and mitochondria,which generate energy through respiration, a particularly important process when light is

unavailable. Like the mitochondrion, the chloroplast is different from most other

organelles because it has its own DNA and reproduces independently of the cell in whichit is found; an apparent case of endosymbiosis. Scientists hypothesize that millions of

years ago small, free-living prokaryotes were engulfed, but not consumed, by larger

prokaryotes, perhaps because they were able to resist the digestive enzymes of theengulfing organism. According to DNA evidence, the eukaryotic organisms that laterbecame plants likely added the photosynthetic pathway in this way, by acquiring a

photosynthetic bacterium as an endosymbiont.

As suggested by this hypothesis, the two organisms developed a symbiotic relationshipover time, the larger organism providing the smaller with ample nutrients and the smaller

organism providing ATP molecules to the larger one. Eventually, the larger organism

developed into the eukaryotic cell, the smaller organism into the chloroplast.

Nonetheless, there are a number of prokaryotic traits that chloroplasts continue to exhibit.Their DNA is circular, as it is in the prokaryotes, and their ribosomes and reproductive

methods (binary fission) are more like those of the prokaryotes.

Endoplasmic Reticulum - The endoplasmic reticulum is a network of sacs that

manufactures, processes, and transports chemical compounds for use inside andoutside of the cell. It is connected to the double-layered nuclear envelope,

providing a pipeline between the nucleus and the cytoplasm. In plants, the

endoplasmic reticulum also connects between cells via the plasmodesmata.

Golgi Apparatus - The Golgi apparatus is the distribution and shipping

department for the cell's chemical products. It modifies proteins and fats built in

the endoplasmic reticulum and prepares them for export as outside of the cell.

Microfilaments - Microfilaments are solid rods made of globular proteins called

actin. These filaments are primarily structural in function and are an important

component of the cytoskeleton.

Microtubules - These straight, hollow cylinders are found throughout the

cytoplasm of all eukaryotic cells (prokaryotes don't have them) and carry out a

variety of functions, ranging from transport to structural support.

Mitochondria - Mitochondria are oblong shaped organelles found in thecytoplasm of all eukaryotic cells. In plant cells, they break down carbohydrate and

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sugar molecules to provide energy, particularly when light isn't available for the

chloroplasts to produce energy.

Nucleus - The nucleus is a highly specialized organelle that serves as theinformation processing and administrative center of the cell. This organelle has

two major functions: it stores the cell's hereditary material, or DNA, and it

coordinates the cell's activities, which include growth, intermediary metabolism,protein synthesis, and reproduction (cell division).

Peroxisomes - Microbodies are a diverse group of organelles that are found in the

cytoplasm, roughly spherical and bound by a single membrane. There are severaltypes of microbodies but peroxisomes are the most common.

Plasmodesmata - Plasmodesmata are small tubes that connect plant cells to each

other, providing living bridges between cells.

Plasmodesmata

Plasmodesmata (singular, plasmodesma) are small channels that directly connect the

cytoplasm of neighboring plant cells to each other, establishing living bridges betweencells. Similar to the gap junctions found in animal cells, the plasmodesmata, which

penetrate both the primary and secondary cell walls (see Figure 1), allow certainmolecules to pass directly from one cell to another and are important in cellular

communication.

The plasmodesmata is structured in an entirely different configuration than the animalcell gap junction because of the thick cell wall. Due to the presence of plasmodesmata,

plant cells can be considered to form a synctium, or multinucleate mass with cytoplasmiccontinuity. Accordingly, the tiny channels have caused a significant amount of debate

among scientists regarding cell theory, some suggesting that the cells of higher plants arenot really cells at all since they are not physically separated or structurally independent

from one another.

Somewhat cylindrical in shape, plasmodesmata are lined with the plasma membrane so

all connected cells are united through essentially one continuous cell membrane. A

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majority of plasmodesmata also contain a narrow tube-like structure called the

desmotubule, which is derived from the smooth endoplasmic reticulum of the connected

cells. The desmotubule does not completely fill the plasmodesma and, consequently, aring of shared cytoplasm is located between it and the inner surface of the membrane-

lined channel. Plasmodesmata typically form during cell division when parts of the

endoplasmic reticulum of the parent cell get trapped in the new cell wall that is producedto create daughter cells. Thousands of plasmodesmata may be formed that connect the

daughter cells to one another.

It is widely thought that by constricting and dilating the openings at the ends of the

plasmodesmata, plants cells regulate the passage of small molecules, such as sugars, salts,and amino acids, though this mechanism is not yet well understood. Yet, it is known that

in some cases the size restrictions on molecule passage between cells can be overcome.

By binding to parts of the plasmodesmata, special proteins and some viruses are able toincrease the diameter of the channels enough for unusually large molecules to pass

through.

Plasma Membrane - All living cells have a plasma membrane that encloses their

contents. In prokaryotes and plants, the membrane is the inner layer of protectionsurrounded by a rigid cell wall. These membranes also regulate the passage of

molecules in and out of the cells.

Ribosomes - All living cells contain ribosomes, tiny organelles composed ofapproximately 60 percent RNA and 40 percent protein. In eukaryotes, ribosomes

are made of four strands of RNA. In prokaryotes, they consist of three strands of

RNA.

Vacuole - Each plant cell has a large, single vacuole that stores compounds, helpsin plant growth, and plays an important structural role for the plant.

Plant Cell Vacuoles

Vacuoles are membrane-bound sacs within the cytoplasm of a cell that function in severaldifferent ways. In mature plant cells, vacuoles tend to be very large and are extremely

important in providing structural support, as well as serving functions such as storage,

waste disposal, protection, and growth. Many plant cells have a large, single central

vacuole that typically takes up most of the room in the cell (80 percent or more).

Vacuoles in animal cells, however, tend to be much smaller, and are more commonly

used to temporarily store materials or to transport substances.

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The central vacuole in plant cells (see Figure 1) is enclosed by a membrane termed the

tonoplast, an important and highly integrated component of the plant internal membranenetwork (endomembrane) system. This large vacuole slowly develops as the cellmatures by fusion of smaller vacuoles derived from the endoplasmic reticulum and Golgi

apparatus. Because the central vacuole is highly selective in transporting materials

through its membrane, the chemical palette of the vacuole solution (termed the cell sap)differs markedly from that of the surrounding cytoplasm. For instance, some vacuoles

contain pigments that give certain flowers their characteristic colors. The central vacuole

also contains plant wastes that taste bitter to insects and animals, while developing seed

cells use the central vacuole as a repository for protein storage.

Among its roles in plant cell function, the central vacuole stores salts, minerals, nutrients,

proteins, pigments, helps in plant growth, and plays an important structural role for theplant. Under optimal conditions, the vacuoles are filled with water to the point that they

exert a significant pressure against the cell wall. This helps maintain the structuralintegrity of the plant, along with the support from the cell wall, and enables the plant cell

to grow much larger without having to synthesize new cytoplasm. In most cases, the plant

cytoplasm is confined to a thin layer positioned between the plasma membrane and thetonoplast, yielding a large ratio of membrane surface to cytoplasm.

The structural importance of the plant vacuole is related to its ability to control turgorpressure. Turgor pressure dictates the rigidity of the cell and is associated with the

difference between the osmotic pressure inside and outside of the cell. Osmotic pressure

is the pressure required to prevent fluid diffusing through a semipermeable membraneseparating two solutions containing different concentrations of solute molecules. The

response of plant cells to water is a prime example of the significance of turgor pressure.When a plant receives adequate amounts of water, the central vacuoles of its cells swell

as the liquid collects within them, creating a high level of turgor pressure, which helps

maintain the structural integrity of the plant, along with the support from the cell wall. Inthe absence of enough water, however, central vacuoles shrink and turgor pressure is

reduced, compromising the plant's rigidity so that wilting takes place.

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Plant vacuoles are also important for their role in molecular degradation and storage.

Sometimes these functions are carried out by different vacuoles in the same cell, one

serving as a compartment for breaking down materials (similar to the lysosomes found inanimal cells), and another storing nutrients, waste products, or other substances. Several

of the materials commonly stored in plant vacuoles have been found to be useful for

humans, such as opium, rubber, and garlic flavoring, and are frequently harvested.Vacuoles also often store the pigments that give certain flowers their colors, which aid

them in the attraction of bees and other pollinators, but also can release molecules that are

poisonous, odoriferous, or unpalatable to various insects and animals, thus discouragingthem from consuming the plant.

Leaf Tissue Organization - The plant body is divided into several organs: roots, stems,

and leaves. The leaves are the primary photosynthetic organs of plants, serving as key

sites where energy from light is converted into chemical energy. Similar to the otherorgans of a plant, a leaf is comprised of three basic tissue systems, including the dermal,

vascular, and ground tissue systems. These three motifs are continuous throughout an

entire plant, but their properties vary significantly based upon the organ type in whichthey are located. All three tissue systems are discussed in this section.

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Plant cell

Like all eukaryotic cells, including animal cells, plant cells contain membrane-bounded

organelles. Some of these organelles, and other structures, are common to all eukaryotes,

while others are found in plant cells but not in animal cells. Among the distinguishingfeatures of plant cells are:

Like all eukaryotic cells, including animal cells, plant cells contain membrane-boundedorganelles. Some of these organelles, and other structures, are common to all eukaryotes,

while others are found in plant cells but not in animal cells. Among the distinguishing

features of plant cells are:

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Plant cell organelles

plastids : a group of organelles characterized by a double membrane envelope and

a complex of internal membranes. Plastids contain DNAand replicateautonomously

o

proplastids : specialized for dividing to form new plastid, usually found inmeristematic cells

o chloroplast : contains chlorophyll, internal membranes organized as grana,

specialized forphotosynthesis

o chromoplast : contains red, orange, or yellow carotenoid pigments, and

imparts color to fruits, etc.o leucoplast : specialized for storing starch (amyloplast), oil (elaioplast), or

protein (aleuroplast)

central vacuole : membrane bound organelle that typically occupies a large volumeof the cell cytoplasm. Vacuoles may contain:

o anthocyanins : red, blue or purple pigments that are water solubleo tannins : phenolic compounds that bind with protein; they function in plant

defense

o crystals: usually composed of calcium oxalate

Cell wall

primary cell wall : a cell wall layer deposited while a cell is growing; typicallyextensible

secondary cell wall: innermost layer of a cell wall deposited after cell enlargement

has ceased, often lignified

casparian strip : a band of suberin within the anticlinal walls of endodermal andexodermal cells

cuticle : a water-repellent layer that coats the outer cell walls of the epidermis on

aerial parts of plants, composed of cutin with a surface coating ofwax

mucigel : a slime sheath secreted by roots

polysaccharide : a polymer composed of sugars

o cellulose : the structural (microfibrillar) portion of the plant cell wall

o hemicellulose: the alkali-soluble portion of the cell wall matrix

o pectin : the hot-water-soluble portion of the cell wall matrix

lignin : an aromatic polymer that rigidifies may secondary cell walls. Ligin isstained red by phloroglucinol solutions

Intercellular connections

plasmodesmata : cytoplasmic channels lined with plasma membrane that connectthe protoplasts of adjacent cells across the cell wall

pit : a region where the secondary cell wall is absent, but the primary cell wall is

present

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o simple pit: a pit that is not bordered, may be round or slit-shaped

o circular-bordered pit: a round pit with a thickened margin

Plant Cell Anatomy

The cell is the basic unit of life. Plant cells (unlike animal cells) are surrounded by a

thick, rigid cell wall.

The following is a glossary of plant cell anatomy terms.amyloplast - an organelle in some plant cells that stores starch. Amyloplasts are found in

starchy plants like tubers and fruits.

ATP - ATP is short for adenosine triphosphate; it is a high-energy molecule used forenergy storage by organisms. In plant cells, ATP is produced in the cristae of

mitochondria and chloroplasts.

cell membrane - the thin layer of protein and fat that surrounds the cell, but is inside the

cell wall. The cell membrane is semipermeable, allowing some substances to pass intothe cell and blocking others.

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cell wall - a thick, rigid membrane that surrounds a plant cell. This layer of cellulose fiber

gives the cell most of its support and structure. The cell wall also bonds with other cellwalls to form the structure of the plant.

centrosome - (also called the "microtubule organizing center") a small body located nearthe nucleus - it has a dense center and radiating tubules. The centrosomes is where

microtubules are made. During cell division (mitosis), the centrosome divides and the

two parts move to opposite sides of the dividing cell. Unlike the centrosomes in animalcells, plant cell centrosomes do not have centrioles.

chlorophyll - chlorophyll is a molecule that can use light energy from sunlight to turn

water and carbon dioxide gas into sugar and oxygen (this process is calledphotosynthesis). Chlorophyll is magnesium based and is usually green.

chloroplast - an elongated or disc-shaped organelle containing chlorophyll.

Photosynthesis (in which energy from sunlight is converted into chemical energy - food)takes place in the chloroplasts.

christae - (singular crista) the multiply-folded inner membrane of a cell's mitochondrion

that are finger-like projections. The walls of the cristae are the site of the cell's energy

production (it is where ATP is generated).

cytoplasm - the jellylike material outside the cell nucleus in which the organelles are

located.

Golgi body - (also called the golgi apparatus or golgi complex) a flattened, layered, sac-

like organelle that looks like a stack of pancakes and is located near the nucleus. The

golgi body packages proteins and carbohydrates into membrane-bound vesicles for"export" from the cell.

granum - (plural grana) A stack ofthylakoid disks within the chloroplast is called agranum.

mitochondrion - spherical to rod-shaped organelles with a double membrane. The inner

membrane is infolded many times, forming a series of projections (called cristae). Themitochondrion converts the energy stored in glucose into ATP (adenosine triphosphate)

for the cell.

nuclear membrane - the membrane that surrounds the nucleus.

nucleolus - an organelle within the nucleus - it is where ribosomal RNA is produced.

nucleus - spherical body containing many organelles, including the nucleolus. Thenucleus controls many of the functions of the cell (by controlling protein synthesis) and

contains DNA (in chromosomes). The nucleus is surrounded by the nuclear membrane

photosynthesis - a process in which plants convert sunlight, water, and carbon dioxide

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into food energy (sugars and starches), oxygen and water. Chlorophyll or closely-related

pigments (substances that color the plant) are essential to the photosynthetic process.

ribosome - small organelles composed of RNA-rich cytoplasmic granules that are sites of

protein synthesis.

rough endoplasmic reticulum - (rough ER) a vast system of interconnected, membranous,

infolded and convoluted sacks that are located in the cell's cytoplasm (the ER is

continuous with the outer nuclear membrane). Rough ER is covered with ribosomes thatgive it a rough appearance. Rough ER transport materials through the cell and produces

proteins in sacks called cisternae (which are sent to the Golgi body, or inserted into the

cell membrane).

smooth endoplasmic reticulum - (smooth ER) a vast system of interconnected,

membranous, infolded and convoluted tubes that are located in the cell's cytoplasm (the

ER is continuous with the outer nuclear membrane). The space within the ER is called the

ER lumen. Smooth ER transport materials through the cell. It contains enzymes andproduces and digests lipids (fats) and membrane proteins; smooth ER buds off from

rough ER, moving the newly-made proteins and lipids to the Golgi body and membranes

stroma - part of the chloroplasts in plant cells, located within the inner membrane of

chloroplasts, between the grana.

thylakoid disk - thylakoid disks are disk-shaped membrane structures in chloroplasts that

contain chlorophyll. Chloroplasts are made up of stacks of thylakoid disks; a stack of

thylakoid disks is called a granum. Photosynthesis (the production ofATP moleculesfrom sunlight) takes place on thylakoid disks.

vacuole - a large, membrane-bound space within a plant cell that is filled with fluid. Mostplant cells have a single vacuole that takes up much of the cell. It helps maintain the

shape of the cell.

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anatomy of the plant cell

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