Comparison Of Prokaryotic And Eukaryotic Cells Essay

, Research Paper

Comparison of Plant and Animal Cells

Introduction

Eukaryotic cells are very complex; there are many organelles, each serving a distinct function, present in eukaryotic cells. We can divide the eukaryotic group of cells in to two main groups, according to the presence of these membrane bound organelles, and the structural differences amongst the cells and their organelles. The two groups of eukaryotic cells are plant and animal cells.

Nucleus

The reason that plant and animal cells are not divided in to two different types of cells, and instead are both grouped in the eukaryotic cell group lies in the definition of eukaryotic cells. The word is derived from the Greek eu meaning true, and karyon referring to the nucleus. This means the eukaryotic cells have true membrane-bound nuclei. Both plant and animal cells have a membrane-bound nucleus; hence, they are grouped as eukaryotic cells. The nucleus plays the same role and has the same structure in both plant and animal cells. You can see that the nucleus is present in both animal and plant cells by examining figure A and figure B.

Although the nucleus itself remains similar among both plant and animal cells, one difference lies in the positioning of the nucleus within the cell. Due to the central vacuole in a plant cell, the nucleus is usually not located in the center of the cell; rather, it is usually crowded nearer the plasma membrane. In most animal cells, however, the nucleus is located in the center of the cell, as this position is ideal in the process of mitosis, and there is no large central vacuole located in animal cells.

The nucleus contains the genes which control the entire cell, and, within the eukaryotic division of cells is uniform among all cells, that is, every eukaryotic cell has a nucleus which functions in the exact manner as other eukaryotic nuclei.

The nucleus of a eukaryotic cell performs three primary functions; these three functions make it possible for the cell to function, and to exist, as the building block of organisms. The Nucleus contains the genetic information of a cell. Encoded in this genetic information, deoxyribonucleic acid (DNA), are the instructions for all structures and organelles within the cell. This DNA instructs the cell as a whole in order for it to accomplish its primary function in a multicellular animal. The genetic information of a cell, through mitosis, is passed on to its daughter cells; therefore all of the cells in a multicellular animal contain the same DNA. This DNA is organized with proteins in to strands called chromosomes.

The second function of the nucleus within a cell is its control of protein synthesis. The nucleus controls protein synthesis through messenger RNA (ribonucleic acid), which is produced in the nucleus according to the instructions of the DNA and is transported to the ribosome through the cytoplasm or the endoplasmic reticulum, it then conveys the genetic message to the ribosome. Cells would not exist without the production of protein; hence, the role the nucleus plays in the protein production process is an important one.

As well as being the site of the control of protein synthesis, the nucleus is also the center in which the ribosome is produced. The ribosome is produced from RNA, which is produced in the nucleolus.

Nuclear Membrane

The nuclear membrane, is a semi-permeable membrane, which separates the nucleus from the cytoplasm of the cell. The membrane is a phospholipid bilayer, similar to the plasma membrane that bounds the cell. The phospholipid bilayer is visually represented in figure C. This semi-permeable membrane allows for the transport of necessary materials in to the nucleus as well as the export of materials such as mRNA. The nuclear envelope is actually made up of two nuclear membranes, separated by a space of about 20-30 nm.

Nuclear pores

Pore complexes cover the surface of the nuclear envelope; these pores assist in the transport of large molecules in to and out of the nucleus. Nuclear pores are each bordered by a ring of eight protein particles.

Nucleolus

The nucleolus is the most visible structure within the nucleus, during interphase. The nucleoli function as the producers of ribosomes, they produce the RNA which make up ribosomes. There are often two or more nuclei; this depends on the type of cell and the stage in the cell s reproductive cycle. The spherical nucleolus contains a large amount of RNA and proteins (representing ribosomes in multiple stages of production). The nucleolus of an actively growing cell can often produce about 10 000 ribosomes per minute. The nucleolus is formed from specialized regions of some chromosomes.

Energy Transducers

Mitochondria and chloroplasts are organelles that are the energy source for the cell, the energy converted by these organelles are needed by the eukaryotic cell in order for it to perform it s various functions. The mitochondria and chloroplasts grow and divide without the aid of the nucleus to increase their abundance within the cell.

Mitochondrion

Mitochondria (see figure A) provide the energy a cell needs to move, divide, produce secretory products, contract – in short, they are the power center of the cell. They are about the size of bacteria but may have different shapes depending on the cell type. The mitochondrion is found in both plant and animal cells, and serves the same function in both.

Mitochondria are membrane-bound organelles, and like the nucleus have a double membrane. The outer membrane is fairly smooth. But the inner membrane is highly convoluted, forming folds called cristae. The cristae greatly increase the inner membrane’s surface area. It is on these cristae that glucose is combined with oxygen to produce adenosine triphosphate (ATP). The mitochondrial matrix, which is the compartment of the mitochondrion enclosed by the inner membrane is where many of the metabolic steps of cellular respiration occurs:

Glucose + Oxygen  Carbon dioxide + Water + Energy (ATP)

Chloroplast

Chloroplasts, (see figure B) which are found only in plant cells, contain the green pigment chlorophyll, along with enzymes and other molecules that function in the process of photosynthesis. Photosynthesis is the process which a cell uses to produce it s own food, it involves the reaction of energy from the sun, carbon dioxide, water, and the catalyst to the process, chlorophyll. The reaction is as follows:

Energy + Water + Carbon dioxide  Oxygen + Glucose

The chloroplast consists of a double layered membrane, which encloses a third layer, of liquid called stroma. Inside of these layers is the thylakoid which is enclosed by it s own membrane. Thylakoids, when stacked, form grana.

Plastids

The chloroplast is one member of a family of plant organelles known as plastids. All three types of plastids develop from proplastids, which are found in unspecialized plant cells.

Amyloplasts (leucoplasts)

Amyoloplasts are colorless plastids, in plant cells only, which store starch, they are particularly more numerous in the cells of plant roots and tubers.

Chromoplasts

Chromoplasts, found only in plant cells, are enriched in pigments; these pigments give the orange and yellow hues of fruits, flowers, and autumn leaves.

Comparison of organelles/structures

Due to the different functions, and needs of plant and animal cells; there are many differences between them, in regards to the existence of some membrane bound organelles as well as to some structural differences between present organelles. There are also many organelles that are present in both plant and animal cells, with no structural or functional differences.

Ribosomes

Within eukaryotic cell types there are no structural or functional differences amongst the cell s ribosomes. The ribosomes of both plant and animal cells serve the same function, the assembly of enzymes and other proteins in cohesion with the genetic instructions sent to the ribosome by the nucleus.

The amount of ribosomes present within a cell is dependant upon the type of cell and the amount of protein needed by that cell, as well as the number of nucleoli present within the nucleus, which also depends upon the function of the cell.

The ribosome is produced inside the nucleolus, from RNA, which is synthesized within the nucleolus, and elsewhere within the nucleus, as well as protein. A eukaryotic ribosome is made up of two sub-units, the large and small sub-unit. These sub-units join together when they attach themselves to messenger RNA from the nucleus. This occurs after they are released in to the cytoplasm.

There are two different places within a eukaryotic cell that functioning ribosomes can be found, free ribosomes exist anywhere within the cytoplasm of the cell, however bound ribosomes are attached to the endoplasmic reticulum. Both types of ribosomes, in all eukaryotic cells, exist mostly in groups called polysomes. In these groups, several ribosomes are attached to one messenger RNA. This arrangement helps to increase the rate of protein production by the ribosome. The bound ribosomes usually produce proteins, which are needed within the membrane bound organelles of the cell, or are to be exported from the cell; this is why they are attached to the transport organelle known as the endoplasmic reticulum. The free ribosomes produce proteins that will function within the cytoplasm.

The ribosome receives its genetic messages from the nucleus in the form of mRNA. The large and small subunits of the ribosome attach to this mRNA and move along the genetic messenger. The genetic message from the nucleus is then translated into a protein; this protein has a specific amino acid sequence, which is read by the ribosome. This is how the genetic message of which proteins to synthesize is conveyed to the free or bound ribosome.

Endoplasmic Reticulum

Throughout the eukaryotic cell, especially those responsible for the production of hormones and other secretory products, is a vast amount of membrane called the endoplasmic reticulum. The Endoplasmic reticulum membrane is a continuation of the outer nuclear membrane and its function suggests just how complex and organized the eukaryotic cell really is. The membrane bound endoplasmic reticulum, which is found in both plant and animal cells and serves the same function in both, it acts as a transport mechanism for the cell. The endoplasmic reticulum consists of a network of cisternae; these are membranous tubules and sacs. The membrane of the endoplasmic reticulum separates the cisternal area of the endoplasmic reticulum from the cytoplasm of the cell. There are two separate regions of this organelle, they are known as the smooth, and the rough endoplasmic reticulum. The rough endoplasmic reticulum is entitled such, as the surface appears rough, this is because of the bound ribosomes, which are attached to the membrane of the rough endoplasmic reticulum. The smooth endoplasmic reticulum, in contrast, has a surface that does not possess bound ribosomes.

Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum has one primary purpose within all eukaryotic cells; however, dependent upon the cell, this function is used in different processes. The smooth endoplasmic reticulum functions in a broad area of metabolic processes, that is, it assists in such areas as, the synthesis of lipids, carbohydrate metabolism, and the detoxification of drugs and poisons, as well as other metabolic functions. The liver cell, which is diagrammed in figure A (see appendix), for example, uses the smooth endoplasmic reticulum in the process of carbohydrate metabolism.

Rough Endoplasmic Reticulum

Many specialized eukaryotic cells, for example white blood cells, secrete proteins produced by the rough endoplasmic reticulum, in the case of the white blood cell; antibodies produced by the rough endoplasmic reticulum are secreted. The proteins that are secreted are not produced by the rough endoplasmic reticulum itself, instead, by the ribosomes bound to this region of the endoplasmic reticulum.

A polypeptide chain, which grows from a bound ribosome, is threaded through the membrane of the endoplasmic reticulum and into the cisternal space. The protein then folds in to its regular formation. Most of the secretory proteins that are synthesized by this process are glycoproteins. Glycoproteins are proteins covalently bonded to carbohydrates. Once in the cisternal area of the rough endoplasmic reticulum the carbohydrate is attached to the protein by enzymes built in to the membrane of the endoplasmic reticulum. After these secretory proteins are produced, they are wrapped in vesicles and secreted from the cell, The endoplasmic reticulum separates them from the cytoplasm.

Vacuoles

A vacuole is a membrane-bound sac that plays roles in intracellular digestion and the release of cellular waste products. In animal cells, vacuoles are generally small. Vacuoles tend to be larger in plant cells and play a role in turgor pressure.

Food Vacuoles

The process of phagocytosis forms food vacuoles, when food comes in to contact with the cell membrane, the membrane forms around the food and then pinches off, this causes the food to be trapped in a vacuole. This food is later digested by the lysosome.

Contractile Vacuoles

The contractile vacuole exists in many freshwater plant cells. This is due to the cell s environment being hypotonic. Because there are more solutes within the cell then in it s environment, due to the concentration gradient, the water will diffuse in to the cell. In order to prevent the cell from bursting, the contractile vacuole pumps out the excess water.

Central Vacuole

A very significant, visual difference between plant and animal cells is the existence of a large central vacuole, the animal cell does not contain a central vacuole, however the plant cell does. The central vacuole can be seen in figure B.

The central vacuole, which is surrounded by a membrane called the tonoplast, is a versatile compartment. It is a place where the cell stores organic compounds, including proteins. Since the plant cell generally lacks the specialized lysosomes of animal cells, the vacuole functions as the plant cells lysosomal compartment. It contains hydrolytic enzymes, which digest stored macromolecules, and they recycle molecular components from organelles, just as the lysosome does in the animal cell.

Lysosome

Lysosomes (common in animal cells but rare in plant cells) contain hydrolytic enzymes necessary for intracellular digestion. The lysosome digests the contents of food vacuoles, as well as digesting damaged organelles through the process of autophagy.

In white blood cells that eat bacteria, lysosome contents are carefully released into the vacuole around the bacteria and serve to kill and digest those bacteria. Uncontrolled release of lysosome contents into the cytoplasm can also cause cell death (necrosis).

Microbodies

There are a variety of organelles, which are collectively called microbodies, bounded by single membranes, microbodies are compartments that are specialized for specific metabolic pathways, and each type has a particular group of enzymes. Nearly all eukaryotic cells have microbodies of one type or another; one common microbody is the peroxizome.

Peroxisomes

This organelle is responsible for protecting the cell from its own production of toxic hydrogen peroxide. As an example, white blood cells produce hydrogen peroxide to kill bacteria. The oxidative enzymes in peroxisomes break down the hydrogen peroxide into water and oxygen. Peroxisomes are present in both, plant and animal eukaryotic cells.

Vesicles

Vesicles are membrane surrounded areas like vacuoles, secretory vesicles are used in secretion, an example would be hormones, neurotransmitters are packaged in secretory vesicles at the Golgi apparatus. The secretory vesicles are then transported to the cell surface for release.

Golgi bodies (Golgi apparatus)

The golgi apparatus, which is found in both plant and animal cells, is

involved in the process of secretion. After leaving the endoplasmic reticulum, many transport vesicles travel to the golgi apparatus before being secreted. The golgi apparatus packages the products of the endoplasmic reticulum in to secretory vesicles.

The golgi apparatus is made up of flattened membrane bound sacs. They are stacked in piles that are called dictyosomes. Each of the stacked cisternae that make up these dictyosomess consists of a membrane, separating its internal space from the cytoplasm. The two ends of a golgi stack have a distinct polarity. These ends, called the cis face and the trans face act as the receiving and shipping departments of the golgi apparatus. The products of the endoplasmic reticulum are modified during their trip from the cis to the trans poles of the golgi apparatus. The products to be secreted then leave the trans face of the golgi apparatus inside of membranous secretory vesicles, which will eventually fuse with the plasma membrane.

The Cytoskeleton

As its name implies, the cytoskeleton helps to maintain cell shape. But the primary importance of the cytoskeleton is in cell motility. The internal movement of cell organelles, as well as cell locomotion and muscle fiber contraction could not take place without the cytoskeleton.

The cytoskeleton is an organized network of three primary protein filaments: microtubules, microfilaments, and intermediate fibers.

Microtubules

Microtubules are relatively straight hollow rods about 25 nm in diameter, and varying from 200 nm to 25 +m in length. The wall of the hollow tube is made up of two types of tubulin, a tubulin and b tubulin. These proteins spiral around the wall of the microtubule.

The microtubule helps to reinforce cell shape, concentrated groups of microtubules near the plasma membrane aid in this process. The microtubule is found in both plant and animal cells.

Microfilaments

The microfilament is made up of two strands of G actin. They are universal within eukaryotic cells. The microfilament functions in the following manners; muscle contraction, cytoplasmic streaming, cell division, as well as the maintenance and changes in cell shape.

Intermediate filaments

Intermediate filaments are hollow tubes about half of the size of a microtubule. They are made up of multiple proteins; the type depends on the cell. Like the microtubule and microfilament the intermediate filaments maintain the shape of the cell.

Centrioles

An animal cell has a pair of centrioles within its microtubule organizing center, located near the nucleus. The centrioles, which have a diameter of about 150 nm and exist at 90 degree angles to each other, are made up of nine sets of three microtubules. The centriole plays an important role in the process of mitosis within an animal cell.

Surface of the Cell

Plasma membrane (Cell membrane)

The plasma membrane is the structure that surrounds a eukaryotic cell; both the plant and animal cell possess a plasma membrane (see figures A and B). The plasma membrane is semi-permeable, like a filter, it allows some products to enter and exit the cell via diffusion, while disallowing other objects.

The plasma membrane is a phospholipid bilayer, this means that the membrane is made up of both phosphates and lipids. The outer wall and inner wall of the cell membrane are phosphates as they are hydrophilic; where as the interior of the cell is made up of lipids that are hydrophobic. This creates a region inside the bilayer cell membrane that does not contain liquids.

Although the plasma membrane is semi-permeable, it does not allow for the transport of some molecules that are too large to fit through the membrane. This is why the membrane also contains proteins, which participate in the active transport of large molecules, also known as endocytosis and exocytosis.

Cell Wall

The only eukarytoic cells in which cell walls are present are the plant cells. The plant cell wall is much thicker than the plasma membrane, it is made up, mainly, of polysaccharide cellulose that is embedded in a matrix of other polysaccharides as well as some small amounts of protein. A plant cell, when young usually possesses a thin flexible cell wall called the primary cell wall, after the cell matures it usually strengthens this main wall by adding pectin and other hardened substances to it, or by building a secondary cell wall.

The cell wall is used by the plant to maintain a more rigid, flat surface than the animal cell, it is needed by the cell as the central vacuole would make it harder for the cell to remain intact without the presence of a cell wall.

Intracellular Junctions

As the name suggests, intracellular junctions, are the connections which hold cells together, cells can communicate through these junctions. There are three types of intracellular junctions that exist between animal cells; they are tight junctions, desmosomes, and gap junctions. The plant cells connect with plasmodesmata.

Plasmodesmata

The plant cells only connect in one way. This is by their connecting cell walls, which are separated by a thin layer, called the middle lamella. The cell walls are perforated with channels called plasmodesmata, these channels connect the cytoplasm of two adjacent cells, hence making the entire plant one working unit.

Tight Junctions

The tight junction between animal cells, most often epithelial cells, holds cells together so tightly that they prohibit the transport of substances through this intracellular space. Specialized proteins in the plasma membrane bond to similar protein in the other plasma membrane allowing for a tight bond.

Desmosomes

A desmosome occurs when intracellular filaments puncture the surface of both plasma membranes. These filaments anchor themselves in both cells and keep them together by forming a junction.

Gap Junctions

The gap junction allows for the transfer of material between one cell and the other, acting like a tunnel joining the cells, just like the plasmodesmata of plant cells. The gap junction occurs when a patch of proteins called a connexon connects with a similar patch of proteins on the membrane of the other animal cell. The intracellular pores created by these gap junctions allows for the transport of inorganic ions, sugars, amino acids, vitamins, and other small molecules between the cytoplasm of both cells.