PROKARYOTIC CELLS

PROKARYOTIC CELLS

Cells that lack a membrane-bound nucleus are called prokaryotes (from the Greek meaning before nuclei). These cells have few internal structures that are distinguishable under a microscope. Cells in the monera kingdom such as bacteria and cyanobacteria (also known as blue-green algae) are prokaryotes.

A cell having no nuclear membrane and hence no separate nucleus.

BACTERIA: 

Single-celled microorganisms that can exist either as independent (free-living) organisms or as parasites (dependent on another organism for life). The plural of bacterium.

 

SIZE AND MORPHOLOGY:

Most bacteria are 0.2 um in diameter and 2-8 um in length.

The general appearance of individual cell as seen under microscope is known as cellular morphology.

SHAPES AND ARRANGEMENT:

The three basic bacterial shapes

·         Coccus (spherical)

·         Bacillus (rod-shaped)

·         Spiral (twisted)

·         Pleomorphic bacteria can assume several shapes.

Cocci

Cocci may be oval, elongated, or flattened on one side.
Cocci may remain attached after cell division. These group characteristics are often used to help identify certain cocci.

 Bacilli

Since bacilli only divide across their short axis there are fewer groupings.

Bacillus is a shape (rod shaped) but there is also a genus of bacteria with the name Bacillus.

Spiral bacteria

Spiral bacteria have one or more twists.

Other shapes:

Star shaped and square shaped

		COCCI ARRANGEMENT		Cocci that remain in pairs after dividing are called diplococci.  Cocci that remain in chains after dividing are called streptococci. Cocci that divide in two planes and remain in groups of four are called tetrads. Cocci that divide in three planes and remain in groups cube like groups of eight are called sarcinae.  Cocci that divide in multiple planes and form grape like clusters or sheets are called staphylococci.
			Most bacilli appear as single rods. Diplobacilli appear in pairs after division. Streptobacilli appear in chains after division.  Some bacilli are so short and fat that they look like cocci and are referred to as coccobacilli.
		BACILLUS ARRANGEMENT
	SPIRAL ARRANGEMENT		Vibrios look like curved rods. Spirilla have a helical shape and fairly rigid bodies. Spirochetes have a helical shape and flexible bodies. Spirochetes move by means of axial filaments, which look like flagella contained beneath a flexible external sheath.

OTHER SHAPES

Bacterial Cell Structure:

Internal Structure: Bacteria have a very simple internal structure, and no membrane-bound organelles.

Nucleoid	Dna in the bacterial cell is generally confined to this central region. Though it isn't bounded by a membrane, it is visibly distinct (by transmission microscopy) from the rest of the cell interior.
Ribosomes	Ribosomes give the cytoplasm of bacteria a granular appearance in electron micrographs. Though smaller than the ribosomes in eukaryotic cells, these inclusions have a similar function in translating the genetic message in messenger rna into the production of peptide sequences (proteins).
Storage granules	Nutrients and reserves may be stored in the cytoplasm in the form of glycogen, lipids, polyphosphate, or in some cases, sulfur or nitrogen.
Endospore	Some bacteria, like clostridium botulinum, form spores that are highly resistant to drought, high temperature and other environmental hazards. Once the hazard is removed, the spore germinates to create a new population.

Surface Structure: Beginning from the outermost structure and moving inward, bacteria have some or all of the following structures:

Capsule	This layer of polysaccharide (sometimes proteins) protects the bacterial cell and is often associated with pathogenic bacteria because it serves as a barrier against phagocytosis by white blood cells.
Outer membrane	this lipid bilayer is found in gram negative bacteria and is the source of lipopolysaccharide (lps) in these bacteria. Lps is toxic and turns on the immune system of, but not in gram positive bacteria.
Cell wall	Composed of peptidoglycan (polysaccharides + protein), the cell wall maintains the overall shape of a bacterial cell. The three primary shapes in bacteria are coccus (spherical), bacillus (rod-shaped) and spirillum (spiral). Mycoplasma are bacteria that have no cell wall and therefore have no definite shape.
Periplasmic space	(not shown) this cellular compartment is found only in those bacteria that have both an outer membrane and plasma membrane (e.g. Gram negative bacteria). In the space are enzymes and other proteins that help digest and move nutrients into the cell.
Plasma membrane	This is a lipid bilayer much like the cytoplasmic (plasma) membrane of other cells. There are numerous proteins moving within or upon this layer that are primarily responsible for transport of ions, nutrients and waste across the membrane.

Appendages: Bacteria may have the following appendages:

pili	These hollow, hairlike structures made of protein allow bacteria to attach to other cells. A specialized pilus, the sex pilus, allows the transfer of plasmid DNA from one bacterial cell to another. Pili (sing., pilus) are also called fimbriae (sing., fimbria).
flagella	The purpose of flagella (sing., flagellum) is motility. Flagella are long appendages which rotate by means of a "motor" located just under the cytoplasmic membrane. Bacteria may have one, a few, or many flagella in different positions on the cell.

Bacterial cell walls

As briefly noted before, fundamental differences in ultrastructure of the cell wall are responsible for the reaction (+ or -) of bacteria towards the Gram stain. In both types of cell, the cytoplasmic membrane is surrounded and supported by a cell wall, which provides strength, rigidity and shape.

GRAM-POSITIVE:

• Relatively thick and featureless (electron microscope)

• Major component (~50%) is peptidoglycan

• No lipid and often no protein

• Accessory polymers (teichoic acid and/or teichuronic acid) covalently linked to

peptidoglycan

GRAM-NEGATIVE

• The cell envelope consists of a pair of membranes (cytoplasmic and outer) with a thin, intermediate layer of peptidoglycan

• The outer membrane contains lipopolysaccharide (LPS) as well as lipids and proteins. LPS is located exclusively in the outer leaflet: lipid embedded in the membrane, polysaccharide protruding. This makes the bacteria appear rather fuzzyunder an electron microscope.

TYPES OF FLAGELLA

There are basically four different types of flagellar arrangements:

1. A single flagellum can extend from one end of the cell - if so, the bacterium is said to be monotrichous.
2. Several flagella (tuft) can extend from one end or both ends of the cell – lophotrichous
3. A single flagellum (or multiple flagella; see below) can extend from both ends of the cell - amphitrichous.
4. Multiple flagella may be randomly distributed over the entire bacterial cell - peritrichous.

EUKARYOTIC CELL:

(Eu- true, Karyote-Nucleus)

Eukaryotic cells (from the Greek meaning truly nuclear) They can be easily distinguished through a membrane-bound nucleus and other organelles.

Eukaryotic cell are typically 10-100 micrometers or 10 times larger than the size of prokaryotic cell.

Examples of eukaryotic cell:

Animals, Plants,algae and 

Fungi.

ORGANELLES OF EUKARYOTIC CELL AND THEIR FUNCTIONS:

Organelle	Function
Nucleus	The “brains” of the cell, the nucleus directs cell activities and contains genetic material called chromosomes made of DNA.
Mitochondria	Make energy out of food
Ribosomes	Make protein
Golgi Apparatus	Make, process and package proteins
Lysosome	Contains digestive enzymes to help break food down
Endoplasmic Reticulum	Called the "intracellular highway" because it is for transporting all sorts of items around the cell.
Vacuole	Used for storage, vacuoles usually contain water or food. (Are you are thirsty? Perhaps your vacuoles need some water!)
Plant cells also have:
Chloroplasts	Use sunlight to create food by photosynthesis
Cell Wall	For support

Eukaryotic Cell Envelope & External Structures

• Cell Wall: The cells of plants, algae and fungi have thick, protective cell walls, which provide support, help maintain the shape of the cell, and prevent the cell from taking in too much fresh water and bursting.
• Plasma Membrane: All cells, both prokaryotic and eukaryotic, have a plasma membrane, made mainly of phospholipids and proteins, which functions as a barrier, regulating the movement of materials between the inside and the outside of the cell.
• Cilia & Flagella: These extensions of the cell are covered with plasma membrane and supported internally with a structural system of microtubules—kind of like a bone covered in skin. Flagella, which are longer, and cilia, which are shorter, aid in cell movement. Cilia, which are able to beat together in a coordinated manner, can also help direct materials around the outside of the cell.

Eukaryotic Membrane-bound Organelles

The main distinction between prokaryotic and eukaryotic cells is the presence of membranous organelles, a feature that only eukaryotes have. Organelles separate function within the eukaryotic cell, like a bunch of tiny, specialized factories that work together to help the cell run.

• The Endomembrane System: Organelles that are membranous have an additional handy feature, a built-in internal transportation system. Because membranous organelles are enclosed by the same type of material as the plasma membrane is made of (phospholipids and proteins), cellular supplies can easily be shipped when a piece of one membrane-bound organelle breaks off, forming a vesicle that travels within the cell, and then fuses with a different membrane-bound organelle. Material can also enter (endocytosis) or exit (exocytosis) the cell via this method.
• Nucleus: The nucleus is typically the largest and most visible organelle in a eukaryotic cell. Bound by a double-layer nuclear membrane, the nucleus contains the cell’s genome—the main genetic instructions in the form of DNA (deoxyribonucleic acid).
• Endoplasmic Reticulum: Functioning mainly as a factory for making and shipping proteins and lipids, the ER is network of hollow tubes, extending off of the nuclear membrane. There are two types of ER, rough and smooth. Rough endoplasmic reticulum is covered with ribosomes, non-membrane-bound organelles which are the sites of protein synthesis within the cell. Smooth endoplasmic reticulum is not associated with ribosomes, and specializes in the synthesis and transport of lipids.
• Golgi Body: These pancake-like stacks of vesicles as another type of factory within the cell. The Golgi body modifies cellular molecules and coordinates the packaging and shipment of materials out of the cell. It is the only organelle that can generate lysosomes.
• Lysosomes: This specialist vesicle contains lysozymes, enzymes that can degrade organic materials. They function in cellular digestion and the recycling of materials within the cell.
• Peroxisomes: A type of specialist vesicle required by cells that use aerobic respiration (oxygen to extract energy from food) and made by the endoplasmic reticulum. Peroxisomes are armed with enzymes that break down dangerous oxygen free radicals.

Other Eukaryotic Cell Components and Organelles

• Mitochondria: These tiny powerhouses of the cell, are double-membrane bound organelles which extract energy from food to produce ATP (adesnosine-5’- triphosphate), a multi-purpose molecule that carries energy for use within the cell.
• Cytoplasm: The inside of the cell, between the nucleus and plasma membrane, is filled with a gel-like fluid in which the organelles are suspended. Cytoplasm includes both the liquid (called cytosol) and the suspended organelles.
• Cytoskeleton: Composed of microtubules, intermediate filaments and microfilaments, this network of fibers provides an inner framework for the cell. The cytoskeleton supports the cells structure, anchors and helps transport organelles, and aids in cell division.
• Microtubule Organizing Center (MTOC): This eukaryotic structure is where microtubules are assembled and anchored. In animal cells the MTOC is called the centrosome, which consists of two centrioles. In plant cells, the nuclear envelope appears to function as the main MTOC.

PROKARYOTIC CELLS

Cells that lack a membrane-bound nucleus are called prokaryotes (from the Greek meaning before nuclei). These cells have few internal structures that are distinguishable under a microscope. Cells in the monera kingdom such as bacteria and cyanobacteria (also known as blue-green algae) are prokaryotes.

A cell having no nuclear membrane and hence no separate nucleus.

BACTERIA: 

Single-celled microorganisms that can exist either as independent (free-living) organisms or as parasites (dependent on another organism for life). The plural of bacterium.

 

SIZE AND MORPHOLOGY:

Most bacteria are 0.2 um in diameter and 2-8 um in length.

The general appearance of individual cell as seen under microscope is known as cellular morphology.

SHAPES AND ARRANGEMENT:

The three basic bacterial shapes

·         Coccus (spherical)

·         Bacillus (rod-shaped)

·         Spiral (twisted)

·         Pleomorphic bacteria can assume several shapes.

Cocci

Cocci may be oval, elongated, or flattened on one side.
Cocci may remain attached after cell division. These group characteristics are often used to help identify certain cocci.

 Bacilli

Since bacilli only divide across their short axis there are fewer groupings.

Bacillus is a shape (rod shaped) but there is also a genus of bacteria with the name Bacillus.

Spiral bacteria

Spiral bacteria have one or more twists.

Other shapes:

Star shaped and square shaped

		COCCI ARRANGEMENT		Cocci that remain in pairs after dividing are called diplococci.  Cocci that remain in chains after dividing are called streptococci. Cocci that divide in two planes and remain in groups of four are called tetrads. Cocci that divide in three planes and remain in groups cube like groups of eight are called sarcinae.  Cocci that divide in multiple planes and form grape like clusters or sheets are called staphylococci.
			Most bacilli appear as single rods. Diplobacilli appear in pairs after division. Streptobacilli appear in chains after division.  Some bacilli are so short and fat that they look like cocci and are referred to as coccobacilli.
		BACILLUS ARRANGEMENT
	SPIRAL ARRANGEMENT		Vibrios look like curved rods. Spirilla have a helical shape and fairly rigid bodies. Spirochetes have a helical shape and flexible bodies. Spirochetes move by means of axial filaments, which look like flagella contained beneath a flexible external sheath.

OTHER SHAPES

Bacterial Cell Structure:

Internal Structure: Bacteria have a very simple internal structure, and no membrane-bound organelles.

Nucleoid	Dna in the bacterial cell is generally confined to this central region. Though it isn't bounded by a membrane, it is visibly distinct (by transmission microscopy) from the rest of the cell interior.
Ribosomes	Ribosomes give the cytoplasm of bacteria a granular appearance in electron micrographs. Though smaller than the ribosomes in eukaryotic cells, these inclusions have a similar function in translating the genetic message in messenger rna into the production of peptide sequences (proteins).
Storage granules	Nutrients and reserves may be stored in the cytoplasm in the form of glycogen, lipids, polyphosphate, or in some cases, sulfur or nitrogen.
Endospore	Some bacteria, like clostridium botulinum, form spores that are highly resistant to drought, high temperature and other environmental hazards. Once the hazard is removed, the spore germinates to create a new population.

Surface Structure: Beginning from the outermost structure and moving inward, bacteria have some or all of the following structures:

Capsule	This layer of polysaccharide (sometimes proteins) protects the bacterial cell and is often associated with pathogenic bacteria because it serves as a barrier against phagocytosis by white blood cells.
Outer membrane	this lipid bilayer is found in gram negative bacteria and is the source of lipopolysaccharide (lps) in these bacteria. Lps is toxic and turns on the immune system of, but not in gram positive bacteria.
Cell wall	Composed of peptidoglycan (polysaccharides + protein), the cell wall maintains the overall shape of a bacterial cell. The three primary shapes in bacteria are coccus (spherical), bacillus (rod-shaped) and spirillum (spiral). Mycoplasma are bacteria that have no cell wall and therefore have no definite shape.
Periplasmic space	(not shown) this cellular compartment is found only in those bacteria that have both an outer membrane and plasma membrane (e.g. Gram negative bacteria). In the space are enzymes and other proteins that help digest and move nutrients into the cell.
Plasma membrane	This is a lipid bilayer much like the cytoplasmic (plasma) membrane of other cells. There are numerous proteins moving within or upon this layer that are primarily responsible for transport of ions, nutrients and waste across the membrane.

Appendages: Bacteria may have the following appendages:

pili	These hollow, hairlike structures made of protein allow bacteria to attach to other cells. A specialized pilus, the sex pilus, allows the transfer of plasmid DNA from one bacterial cell to another. Pili (sing., pilus) are also called fimbriae (sing., fimbria).
flagella	The purpose of flagella (sing., flagellum) is motility. Flagella are long appendages which rotate by means of a "motor" located just under the cytoplasmic membrane. Bacteria may have one, a few, or many flagella in different positions on the cell.

Bacterial cell walls

As briefly noted before, fundamental differences in ultrastructure of the cell wall are responsible for the reaction (+ or -) of bacteria towards the Gram stain. In both types of cell, the cytoplasmic membrane is surrounded and supported by a cell wall, which provides strength, rigidity and shape.

GRAM-POSITIVE:

• Relatively thick and featureless (electron microscope)

• Major component (~50%) is peptidoglycan

• No lipid and often no protein

• Accessory polymers (teichoic acid and/or teichuronic acid) covalently linked to

peptidoglycan

GRAM-NEGATIVE

• The cell envelope consists of a pair of membranes (cytoplasmic and outer) with a thin, intermediate layer of peptidoglycan

• The outer membrane contains lipopolysaccharide (LPS) as well as lipids and proteins. LPS is located exclusively in the outer leaflet: lipid embedded in the membrane, polysaccharide protruding. This makes the bacteria appear rather fuzzyunder an electron microscope.

TYPES OF FLAGELLA

There are basically four different types of flagellar arrangements:

1. A single flagellum can extend from one end of the cell - if so, the bacterium is said to be monotrichous.
2. Several flagella (tuft) can extend from one end or both ends of the cell – lophotrichous
3. A single flagellum (or multiple flagella; see below) can extend from both ends of the cell - amphitrichous.
4. Multiple flagella may be randomly distributed over the entire bacterial cell - peritrichous.

EUKARYOTIC CELL:

(Eu- true, Karyote-Nucleus)

Eukaryotic cells (from the Greek meaning truly nuclear) They can be easily distinguished through a membrane-bound nucleus and other organelles.

Eukaryotic cell are typically 10-100 micrometers or 10 times larger than the size of prokaryotic cell.

Examples of eukaryotic cell:

Animals, Plants,algae and 

Fungi.

ORGANELLES OF EUKARYOTIC CELL AND THEIR FUNCTIONS:

Organelle	Function
Nucleus	The “brains” of the cell, the nucleus directs cell activities and contains genetic material called chromosomes made of DNA.
Mitochondria	Make energy out of food
Ribosomes	Make protein
Golgi Apparatus	Make, process and package proteins
Lysosome	Contains digestive enzymes to help break food down
Endoplasmic Reticulum	Called the "intracellular highway" because it is for transporting all sorts of items around the cell.
Vacuole	Used for storage, vacuoles usually contain water or food. (Are you are thirsty? Perhaps your vacuoles need some water!)
Plant cells also have:
Chloroplasts	Use sunlight to create food by photosynthesis
Cell Wall	For support

Eukaryotic Cell Envelope & External Structures

• Cell Wall: The cells of plants, algae and fungi have thick, protective cell walls, which provide support, help maintain the shape of the cell, and prevent the cell from taking in too much fresh water and bursting.
• Plasma Membrane: All cells, both prokaryotic and eukaryotic, have a plasma membrane, made mainly of phospholipids and proteins, which functions as a barrier, regulating the movement of materials between the inside and the outside of the cell.
• Cilia & Flagella: These extensions of the cell are covered with plasma membrane and supported internally with a structural system of microtubules—kind of like a bone covered in skin. Flagella, which are longer, and cilia, which are shorter, aid in cell movement. Cilia, which are able to beat together in a coordinated manner, can also help direct materials around the outside of the cell.

Eukaryotic Membrane-bound Organelles

The main distinction between prokaryotic and eukaryotic cells is the presence of membranous organelles, a feature that only eukaryotes have. Organelles separate function within the eukaryotic cell, like a bunch of tiny, specialized factories that work together to help the cell run.

• The Endomembrane System: Organelles that are membranous have an additional handy feature, a built-in internal transportation system. Because membranous organelles are enclosed by the same type of material as the plasma membrane is made of (phospholipids and proteins), cellular supplies can easily be shipped when a piece of one membrane-bound organelle breaks off, forming a vesicle that travels within the cell, and then fuses with a different membrane-bound organelle. Material can also enter (endocytosis) or exit (exocytosis) the cell via this method.
• Nucleus: The nucleus is typically the largest and most visible organelle in a eukaryotic cell. Bound by a double-layer nuclear membrane, the nucleus contains the cell’s genome—the main genetic instructions in the form of DNA (deoxyribonucleic acid).
• Endoplasmic Reticulum: Functioning mainly as a factory for making and shipping proteins and lipids, the ER is network of hollow tubes, extending off of the nuclear membrane. There are two types of ER, rough and smooth. Rough endoplasmic reticulum is covered with ribosomes, non-membrane-bound organelles which are the sites of protein synthesis within the cell. Smooth endoplasmic reticulum is not associated with ribosomes, and specializes in the synthesis and transport of lipids.
• Golgi Body: These pancake-like stacks of vesicles as another type of factory within the cell. The Golgi body modifies cellular molecules and coordinates the packaging and shipment of materials out of the cell. It is the only organelle that can generate lysosomes.
• Lysosomes: This specialist vesicle contains lysozymes, enzymes that can degrade organic materials. They function in cellular digestion and the recycling of materials within the cell.
• Peroxisomes: A type of specialist vesicle required by cells that use aerobic respiration (oxygen to extract energy from food) and made by the endoplasmic reticulum. Peroxisomes are armed with enzymes that break down dangerous oxygen free radicals.

Other Eukaryotic Cell Components and Organelles

• Mitochondria: These tiny powerhouses of the cell, are double-membrane bound organelles which extract energy from food to produce ATP (adesnosine-5’- triphosphate), a multi-purpose molecule that carries energy for use within the cell.
• Cytoplasm: The inside of the cell, between the nucleus and plasma membrane, is filled with a gel-like fluid in which the organelles are suspended. Cytoplasm includes both the liquid (called cytosol) and the suspended organelles.
• Cytoskeleton: Composed of microtubules, intermediate filaments and microfilaments, this network of fibers provides an inner framework for the cell. The cytoskeleton supports the cells structure, anchors and helps transport organelles, and aids in cell division.
• Microtubule Organizing Center (MTOC): This eukaryotic structure is where microtubules are assembled and anchored. In animal cells the MTOC is called the centrosome, which consists of two centrioles. In plant cells, the nuclear envelope appears to function as the main MTOC.