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1.2: Cellular Organization - Prokaryotic and Eukaryotic Cells - Biology

1.2: Cellular Organization - Prokaryotic and Eukaryotic Cells - Biology


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Learning Objectives

  1. Briefly describe why, in terms of differences in cell size, a eukaryotic cell is structurally more complex and compartmentalized than a cell that is prokaryotic.
  2. When given a description, determine whether a cell is prokaryotic or eukaryotic and explain why.
  3. Briefly state why viruses are not considered as prokaryotic nor eukaryotic.

According to the cell theory, the cell is the basic unit of life. All living organisms are composed of one or more cells. Based on the organization of their cellular structures, all living cells can be divided into two groups: prokaryotic and eukaryotic (also spelled procaryotic and eucaryotic). Animals, plants, fungi, protozoans, and algae all possess eukaryotic cell types. Only bacteria have prokaryotic cell types.

Prokaryotic cells are generally much smaller and more simple than eukaryotic (Figure (PageIndex{1})). Prokaryotic cells are, in fact, able to be structurally more simple because of their small size. The smaller a cell, the greater is its surface-to-volume ratio (the surface area of a cell compared to its volume).

The surface area of a spherical object can be calculated using the following formula:

[S = 4, pi, r^2]

The volume of a spherical object can be calculated using the formula:

[V = dfrac{4}{3}, pi, r^3 ]

For example, a spherical cell 1 micrometer (µm) in diameter - the average size of a coccus-shaped bacterium - has a surface-to-volume ratio of approximately 6:1, while a spherical cell having a diameter of 20 µm has a surface-to-volume ratio of approximately 0.3:1.

A large surface-to-volume ratio, as seen in smaller prokaryotic cells, means that nutrients can easily and rapidly reach any part of the cells interior. However, in the larger eukaryotic cell, the limited surface area when compared to its volume means nutrients cannot rapidly diffuse to all interior parts of the cell. That is why eukaryotic cells require a variety of specialized internal organelles to carry out metabolism, provide energy, and transport chemicals throughout the cell. Both, however, must carry out the same life processes. Some features distinguishing prokaryotic and eukaryotic cells are shown in Table (PageIndex{1}). All of these features will be discussed in detail later in Unit 1.

Table (PageIndex{1}): Eukaryotic Versus Prokaryotic Cells

Nuclear Body

eukaryotic cell

a. The nuclear body is bounded by a nuclear membrane having pores connecting it with the endoplasmic reticulum (see Figure (PageIndex{2}) and Figure (PageIndex{3})).
b. It contains one or more paired, linear chromosomes composed of deoxyribonucleic acid (DNA) associated with histone proteins ).
c. A nucleolus is present. Ribosomal RNA (rRNA) is transcribed and assembled in the nucleolus.
d. The nuclear body is called a nucleus.

An electron micrograph of a cell nucleus, showing the darkly stained nucleolus. (Public Domain; US National Institute of General Medical Sciences/National Institutes of Health)

prokaryotic cell

a. The nuclear body is not bounded by a nuclear membrane (see Figure (PageIndex{4})).
b. It usually contains one circular chromosome composed of deoxyribonucleic acid (DNA) associated with histone-like proteins.
c. There is no nucleolus.
d. The nuclear body is called a nucleoid .

Cell Division

eukaryotic cell

a. The nucleus divides by mitosis .
b. Haploid (1N) sex cells in diploid or 2N organisms are produced through meiosis .

prokaryotic cell

a. The cell usually divides by binary fission . There is no mitosis.
b. Prokaryotic cells are haploid. Meiosis is not needed.

Cytoplasmic Membrane - also known as a cell membrane or plasma membrane

eukaryotic cell

prokaryotic cell

a. The cytoplasmic membrane (Figure (PageIndex{4})) is a fluid phospholipid bilayer (Figure (PageIndex{5})) that usually lacking sterols. Bacteria generally contain sterol-like molecules called hopanoids (Figure (PageIndex{7})).

b.The membrane is incapable of endocytosis and exocytosis.

Cytoplasmic Structures

eukaryotic cell

prokaryotic cell

a. The ribosomes are composed of a 50S and a 30S subunit that come together during protein synthesis to form a 70S ribosome . See Figure (PageIndex{8}).

- Ribosomal subunit densities: 50S and 30S

b. Internal membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, vacuoles, and lysosomes are absent (see Figure (PageIndex{4}))
c. There are no chloroplasts. Photosynthesis usually takes place in infoldings or extensions derived from the cytoplasmic membrane.
d. There is no mitosis and no mitotic spindle.
e. The various structural filaments in the cytoplasm collectively make up the prokaryotic cytoskeleton. Cytoskeletal filaments play essential roles in determining the shape of a bacterium (coccus, bacillus, or spiral) and are also critical in the process of cell division by binary fission and in determining bacterial polarity.

Respiratory Enzymes and Electron Transport Chains

eukaryotic cell

- The electron transport system is located in the inner membrane of the mitochondria. It contributes to the production of ATP molecules via chemiosmosis.

-Electron micrograph of a mitochondrion from the Biology Department at the University of New Mexico.

prokaryotic cell

Cell Wall

eukaryotic cell

a. Plant cells, algae, and fungi have cell walls, usually composed of cellulose or chitin. Eukaryotic cell walls are never composed of peptidoglycan (see Figure (PageIndex{3})).
b. Animal cells and protozoans lack cell walls (see Figure (PageIndex{2})).

prokaryotic cell

a. With few exceptions, members of the domain Bacteria have cell walls composed of peptidoglycan (see Figure (PageIndex{4})).
b. Members of the domain Archae have cell walls composed of protein, a complex carbohydrate, or unique molecules resembling but not the same as peptidoglycan.

Locomotor Organelles

eukaryotic cell

prokaryotic cell

Representative Organisms

  • eukaryotic cell: The domain Eukarya: animals, plants, algae, protozoans, and fungi (yeasts, molds, mushrooms).
  • prokaryotic cell: The domain Bacteria and the domain Archae.

Since viruses are acellular- they contain no cellular organelles, cannot grow and divide, and carry out no independent metabolism - they are considered neither prokaryotic nor eukaryotic. Because viruses are not cells and have no cellular organelles, they can only replicate and assemble inside a living host cell. They turn the host cell into a factory for manufacturing viral parts and viral enzymes and assembling the viral components.

Viruses, which possess both living and nonliving characteristics, will be discussed in Unit 4. Recently, viruses have been declared as living entities based on the large number of protein folds encoded by viral genomes that are shared with the genomes of cells. This indicates that viruses likely arose from multiple ancient cells.

Summary

  1. There are two basic types of cells in nature: prokaryotic and eukaryotic.
  2. Prokaryotic cells are structurally simpler than eukaryotic cells.
  3. The smaller a cell, the greater its surface to volume ratio.
  4. The smaller the surface to volume ratio, the more structurally complex (compartmentalized) a cell needs to be in order to carry out life functions.
  5. There are fundamental differences between prokaryotic and eukaryotic cells.
  6. Bacteria are prokaryotic cells; fungi, protozoa, algae, plants, and animals are composed of eukaryotic cells.
  7. Viruses are not cells so they are neither prokaryotic nor eukaryotic. They can replicate only inside a living cell.

Ultra-structure of Prokaryotic Cells

Some of the major cell organs involved in ultra-structure of prokaryotic cell are as follows:

1. Cell envelope 2. Cytoplasm 3. Nucleoid 4. Appendages.

Prokaryotic cells are the simplest of most primitive cells. The records of microfossils suggest that they have evolved 2.5 billion years ago and existed as the only organisms on earth for the next one billion years until eukaryotes evolved about 1.5 billion years ago. Earlier claims that oldest prokaryotic microfossils, the stromatolites (i.e., giant colonies of ancient cyanobacteria or BGA) of 3.5 billion years ago are actually lifeless mineral artifacts.

The Prokaryotic ceil is the structural unit of two microbial groups: the archaebacteria and the eubacteria. Despite variations in shape and size, the fundamental structures of prokaryotic cells are same. Each prokaryotic cell is essentially a one envelop system that consists of protoplasm encased within cell envelope. The ultrastructure of a prokaryotic cell, particularly a typical bacterial cell consists of cell envelope, cytoplasm, nucleoid, plasmids and surface appendage.

1. Cell envelope:

It is the protective covering of bacterial cell that has three basic layers: the outermost glvcocalyx, middle cell wall and innermost cell membrane (plasma membrane),

It is the outermost layer of cell envelope which chemically composed of polysaccharides with or without proteins. When glycocalyxis thick and tough, it is called capsule, and when it forms a loose sheath it is called slime layer.

Though not essential for bacterial survival, glycocalyx has many functions:

(a) Protects cell from desiccation, toxins and phagocytes,

(b) Helps in adhesion, immunogenicity and virulence.

It is the rigid middle layer of cell envelope that provides shape and prevents a bacterium from osmotic bursting in a hypotonic solution. In Gram-positive bacteria, cell wall is single layered and almost uniform in thickness (10 to 80nm).It is composed of peptidoglycan (murein or mucopeptide), which consists of a three dimensional network of glyean strands cross linked by peptide chains. Each glycan strand is 20-130 units long consists of two alternating amino sugars, N-acetylglucosamine (NAG) and N-acetylmuramicacid (NAM).

Certain antibiotics (penicillin) lysozyme prevents cross linking and kills bacteria. The walls of Gram- positive bacteria also contain teichoic acids (Polyphosphate Polymery but proteins are almost absent. Teichoic acids act as surface antigen. In Gram-negative bacteria cell wall is two- layered and only 7.5-12 nm thick. The inner layer in a thin peptidoglycan and the outer layer is a unit membrane called outer membrane. The outer membrane is a lipid bilayer consists of phospholipids, proteins and a unique lipid, lipopolysaccharide (LPS).

The LPS molecules present only in the outer face of the outer membrane. The outer membrane also contains channel like proteins called porins, through which low molecular weight sub­stances enter or exit. In some Gram-negative bacteria (.Mycobacterium, Noccardia) the wall contains long chain fatty acids called mycolic acids. In the walls of Gram- negative bacteria, a fluid- filled space present on either side of the peptidoglycan layer i.e. between the outer membrane of cell wall and the cell membrane. This space is called periplasm or periplasmic gel. It acts like bacterial lysosome.

Many species of bacteria are wall-less, either develop spontaneously or induced by cell wall degrading agents. These bacteria are called L-forms, after the Lister Institute, London. The L- forms of bacteria may develop cell wall but mollicutes will never develop cell wall.

It is the innermost layer of cell envelope. It is a semi-permeable, quasi-fluid, dynamic membrane similar to that of eukaryotic membrane. But the only difference is that, in bacteria they lack sterols instead hopanoids present. The hopanoids are pentacyclic sterol-like molecules that stabilize the bacterial cell membrane. In Gram-negative bacteria at certain places, the outer face of plasma membrane is continuous with the inner face of the outer membrane to form Bayer’s junctions. There are about 200-400 Layer’s junctions present in a Gram- negative cell.

In a bacterial cell, plasma membrane performs many functions:

(a) It retains the cytoplasm

(b) Prevent loss of essential components through leakage

(c) Aids in the movement of nutrients, wastes and secretions across the membrane

(d) Holds receptor molecules that detect and respond chemicals in their surroundings. Such as respiration, photosynthesis, synthesis of lipids and cell wall constituents

(f) It invaginates to form mesosome and thylakoids of cyanobacteria.

2. Cytoplasm:

It is granular, crystallo-colloidal complex that fills the whole prokaryotic cell excluding nucleoid. The cytoplasm contains mesosome, chromatophores, ribosomes, inclusion bodies and plasmids.

(i) Mesosome (or chondrioids):

It is a convoluted membranous infolding of the plasma membrane. Mesosome when connected with nucleoid is called septal mesosome and when not connected called lateral mesosome.

(b) Chromosome replication and distribution to daughter cells

(d) Increase the surface area of plasma membrane

(e) Mesosome can be considered analogous to mitochondria of eukaryotic cells as these are the sites of respiratory enzymes. Hence, mososomes are also called as “mitochondria of prokaryotic cells” or “bacterial mitochondria”.

(ii) Chromatophores:

These are internal membrane systems of prokaryotic cells. These are very extensive and complex in photosynthetic forms like cyanobacteria and purple bacteria where they are called as thylakoids. In nitrifying bacteria the chromatophores increase metabolic area.

Prokaryotic ribosomes are 70S in nature that consist of larger 5OS and smaller 30S subunits. During protein synthesis, about 4-8 ribosomes attach to a single mRNA to form polyribosomes or polysomes. Non functional ribosomes present in separated subunits.

(iv) Inclusion bodies:

These are non-living structures present freely in the cytoplasm. Inclusion bodies may be organic or inorganic. They include mainly food reserve and special prokaryotic organelles like gas vacuoles, chromosomes, carboxysomes, and magnetosomes. Except food reserve other inclusion bodies are surrounded by a single layer non-unit membrane which is 2-5 nm thick.

These are reserve materials or storage granules which are not bounded by any membrane system. Generally, a given bacterial species store only one kind of reserve material. Further, the cellular content of reserve material is low in actively growing cells but them increase when cells are short of nitrogen.

The volutin granules (= polyphosphate granules) and sulphur granules are inorganic inclusion bodies which store phosphate and sulphur respectively. These granules are also called meta-chromatic granules because of their ability to take different colours to basic dyes. The organic- food reserves present in some bacteria are glycogen granules, protein granules (=cyanophycin), cyanophycean granules (= starch granules) and PHB granules. The PHB (Poly β-hydroxybutyrate) granules are storage reservoir of fatty acids in case of Pseudomonas, Bacillus, Azotobacter species. The PHB is commercially used to prepare biodegradable plastics.

These are the organic inclusions of most aquatic, free floating forms. Each gas vacuole is an aggregate of variable number of hollow, cylindrical gas vesicles. Gas vacuoles help in floating, for proper positioning in water to trap sunlight for photosynthesis and protect against harmful radiations.

These are cigar-shaped vesicles that enclose photosynthetic pigments like bacteriochlorophyll c, d, or e. Chlorosomes are distinct structures found just below the plasma membrane but tightly joint to it by a basal plate. These are found in the green bacteria.

These are principal sites of CO2 fixation in case of autotrophic prokaryotes like cyanobacteria, purple bacteria, nitrifying bacteria etc.

These are the vesicles filled with crystals of magnetite (Fe3O4). Magneto-somes help the bacteria to orient themselves in a magnetic field and determine the direction of swimming.

Laderberg and Hays (1952) introduced the term ‘plasmid’ to those ring-like self replicating extra chromosomal double stranded DNA that are found in the cytoplasm of prokaryotes. They are also found in eukaryotes (yeast) and their organelles. Plasmids are used as in ideal vector for indirect gene transfer in recombinant DNA technology (Genetic Engineering).

Plasmids are generally double stranded closed circles of D.N A with sizes vary from 1-3 00 kilobase pairs (1 Kbp = 1000bp) and carries 5-100 genes. Hence, plasmids are often called as minichromosomes. They are also known as dispensable autonomous elements because the genes they carry have no role in viability and bacterial growth.

The average number of plasmids per bacterial cell is called copy number. Plasmids with low copy number (1-2) are called single copy plasmids, while those with high copy number (10-30) are called multi-copy plasmids. Some plasmids can temporarily integrate or can detach from main chromosome and are called as episomes, e.g., F-plasmid. It is to be noted that all episomes are plasmids but all plasmids are not episomes. The term episome was coined by Jacob and Wollman 1958).

On the basis of function, plasmids are of following types –

It carries fertility factor (F-factor) responsible for the formation of sex-pili and conjugation. Hence, often called F-plasmid.

It carries resistance (R) factor which provide resistance again si antibiotics, heavy metals, UV-radiation etc., e.g. Rl.R4G etc.

It carries colicinogenic factor that, produce colicins (bacteriocins) to kill other bacterial.

(d) Degradative plasmid:

It decompose hydracarbon in petroleum, e g. present in Pseudomonas putida [A genetically engineered bacterium which would degrade all the four types of substrates i.e. OCT (Octane, Hexane, Dcene), XYL (Xylene, Toluene), CAM (camphor) and NAH (naphthalene)>

These are tumour including plasmids carried by the Agrabacterium tumefaciens. Ti-plasmid carries

T-DNA (transforming DNA) which is 200 Kbp long and causes crown gall disease in plants, T- DNA is an ideal vector for gene transfer in plants.

A subground of Ti-plasmids inducing the hairy root tumours e.g.,-A-rhizogene.

Plasmid can be categorised on the basis of number of copies per cell

(a) Released Plasmid: It normally maintain multiple copies per cell

(b) Stringent Plasmid: It has limited number of copies per cell

3. Nucleoid

Nucleoid is the genetic material of a prokaryotic cell that occupies up to 1/5 of the interior of the bacterial cell. It is represented by a single circular naked ds DNA which is highly looped and super coiled with the help of polyatnines (nucleoid proteins) and RNA. Nucleoid is a compact structure hat nuclear envelope and nucleolus, and therefore it is not an organized nucleus rather an incipent nucleus. In Escherichia coli the compact DNA is 1.2 mm in length which is about 250-700 times the length of the cell. The nucleoid is attached to the plasma membrane directly or by mesosomes.

4. Appendages:

The surface appendages present on bacterial cell may be motile flagellum or non-motile pili and fimbriae.

(a) Flagella (sing. Flagellum):

These are long (1-71m) fine hairy locomotary appendages present on bacterial surface for swimming. Their number and arrangement is called flagellation which is characteristic features of different genera of bacteria

Some bacteria bear sheath flagella surrounded by extension of cell membrane. In Vibrio, flagellation is mixed type where polar sheathed flagellum present along with many peritrichously arranged unsheathed flagella.

The ultrastructure of each flagellum shows 3 parts – basal body, hook and filament. The Basal body consists of a central root with one two pairs of rings. In Gram positive bacteria the basal body possesses two pairs of rings the outer pair (L and P rings) remains attached to the outer membrane, whereas the inner pair (S and M rings) remains connected to the cell membrane. In Gram- positive bacteria, on the other hand, only the inner pair (Sand M) is present and the remains attached to the cell membrane (fig. 2.4).

The hook is slightly wider and curved structure about45 nm long that connects, basal body with the filament. The filament is hollow cylindrical structure about 1-70 nm long and 20 nm in diameter. Filament composed of 3-8 spiral of flagellian proteins. But hook is made up of a different kind of proteins.

The bacterial flageilum rotate by 360″ rather than a whip like back and forth movement. As a result the bacterial cell spins in the opposite direction and pushes the bacterium in forward direction.

(b) Pili (sing. Pilus) and fimbriae:

These two terms have often been interchangeably used. The pili are tubular outgrowths of about 18-20 nm made up of pilin protein. They are reported only in donor cell Gram negative bacteria where they help in conjugation. Hence, pili are also called sex-pili or F-pili. Their number is 1 -4 per cell. Fimbriae are bristle like surface appendages help in adhesion and mutual clinging length varies from 6.1 – 7.5 nm and diameter 3-10 nm.


Compare and contrast prokaryotic and eukaryotic cells and the impact viruses have on them.

Write a 4000 word illistrated report on cell biology.

Unit Introduction
1.1: Discuss selected characteristics of living cells
1.2: Compare and contrast prokaryotic and eukaryotic cells and the impact viruses have on them
1.3: Discuss eukaryotic sub-cellular structure and organelles
2.1: Explain the role of the cell membrane in regulating nutrients and waste products
2.2: Explain how animal cells use nutrients to provide energy for growth, movement and cell division
2.3: Discuss the synthesis of proteins
2.4: Explain the role of nucleic acids in the nucleus and cytoplasm
3.1: Explain the generation of specialised tissues from embryonic stem cells
3.2: Explain the importance of interphase and factors that initiate cell division
3.3: Explain how the same genetic information is received by each daughter cell
3.4: Compare and contrast cancer cells with normal cells

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Transcription: from DNA to mRNA

Both prokaryotes and eukaryotes perform fundamentally the same process of transcription, with the important difference of the membrane-bound nucleus in eukaryotes. With the genes bound in the nucleus, transcription occurs in the nucleus of the cell and the mRNA transcript must be transported to the cytoplasm. The prokaryotes, which include bacteria and archaea, lack membrane-bound nuclei and other organelles, and transcription occurs in the cytoplasm of the cell. In both prokaryotes and eukaryotes, transcription occurs in three main stages: initiation, elongation, and termination.


Cells

The first cells that arose about 3.5 billion years ago most likely resembled Bacteria or Archaea they had relatively simple structures and lacked nuclei or internal organelles. Most phylogenetic trees of life show Archaea and Bacteria diverging first from the Last Universal Common Ancestor (LUCA). We infer therefore, that the LUCA had a simple cell structure, with cytoplasm bounded by some type of phospholipid bilayer membrane, and no nuclei or internal membrane compartments or organelles.

Phylogenetic Tree of Life with 3 Domains, based on 16S rRNA sequences, from Wikimedia Commons

Bacteria and Archaea are classified as prokaryotes, meaning cells without nuclei, although some modern biologists dislike the term because prokaryotes appear not to form a monophyletic group.

Methanococcus janaschii, with many flagella, image courtesy of UC Museum of Paleontology, www.ucmp.berkeley.edu.

Bacteria and Archaea have diverse cell morphologies, but they all have some common structural features.

Prokaryotic cell structure, from Wikipedia

  • a single circular chromosome (a few species have two circular chromosomes)
  • a nucleoid region that contains the chromosomal DNA, with no surrounding membrane to separate it from the cytoplasm
  • small circular DNA molecules called plasmids dispersed in the cytoplasm.

In addition to their phospholipid bilayer cell membrane, they have cell walls that differ in composition between Bacteria and Archaea. Prokaryotic cells are generally smaller than eukaryotic cells. They have a rudimentary cytoskeleton and can have flagella for motility.

Relative scale of cell sizes, from Wikipedia

Evolution of eukaryotes

About 2.1-2.4 billion years ago, the first eukaryotic cells appear in the fossil record. This coincides with, or occurs soon after, the Great Oxygenation Event. Eukaryotic cell membranes have sterols, whose synthesis requires molecular oxygen. How did eukaryotes arise? One clue is that eukaryotic genes for proteins that replicate DNA and synthesize RNA in the nucleus are similar to Archaeal genes, whereas eukaryotic genes for energy metabolism and lipid biosynthesis in the cytoplasm resemble Bacterial genes. This observation led to the current hypothesis that eukaryotes evolved from an ancient endosymbiosis or cell fusion event between an Archaeon and a Bacterium.

Eukaryotic evolution required many innovations. One is endocytosis (taking in molecules bound to the plasma membrane by forming a small vesicle, a bubble-like structure made by a lipid bilayer sac enclosing internal fluid). Modern prokaryotes lack endocytosis or phagocytosis (taking particles into the cell by forming a large vesicle). But endocytosis or phagocytosis is essential for taking in and harboring endosymbionts within a membrane enclosure, and leads to formation of vesicles inside the cell. Invagination of the plasma membrane deep into the cytoplasm to surround the cell’s chromosomes can lead to the formation of a membrane envelope that separates the nuclear compartment from the rest of the cell, and simultaneous development of an endomembrane system.

Proteins required for endocytosis share structural similarities with nuclear pore proteins, suggesting a common evolutionary origin for the endomembrane system and the nucleus. Fig. 5 from Devos et al. 2004, PLoS Biology doi:10.1371/journal.pbio.0020380

Therefore, phagocytosis/endocytosis can account for the formation of the nucleus enclosed by a nuclear envelope, the endomembrane system, and the evolution of mitochondria and chloroplasts from endosymbiosis of aerobic bacteria and cyanobacteria, respectively.

Eukaryotic cell structure

Eukaryotic cell from Wikipedia

What should students in freshman biology know about the structure of a eukaryotic cell? Rather than trying to memorize details about the various organelles and cell structures, students should think about major cell systems.

Cytoplasm

The cytoplasm is the internal region of the cell bounded by the plasma membrane, excluding the interior of the nucleus and the interior regions of organelles and the endomembrane system. The cytoplasm contains ribosomes, tRNAs and mRNAs for protein synthesis, the cytoskeleton, many metabolic enzymes, and proteins that function in cell signaling. The cytoplasm is so crowded with macromolecules that it has the consistency of a hydrated gel much of the water molecules are associated with other molecules.

Endomembrane system

The endomembrane system includes the nuclear envelope, the endoplasmic reticulum (ER), the Golgi complex, lysosomes, transport vesicles, secretory vesicles, endosomes, and the plasma membrane. The double membrane of the nuclear envelope is contiguous with the ER.

Endomembrane system from Wikipedia. The rough ER has ribosomes bound to the ER membrane. ER-bound ribosomes synthesize proteins into the ER membrane or lumen (internal space). Other ribosomes remain in the cytoplasm and synthesize proteins that remain in the cytoplasm or go to the nucleus or the mitochondria or chloroplasts.

That all these membranes comprise a single system becomes clear when we think about membrane biogenesis. For cells to grow, they have to make more membrane lipids and membrane proteins.

Membrane proteins for the endomembrane system and proteins for secretion are made in the rough ER (rER) by ribosomes docked to protein channels in the ER membrane. The polypeptide chain emerging from the ribosome passes through the channel into the ER lumen (the interior space of the ER) and begins to fold. Any parts of the chain that form hydrophobic alpha-helices remain embedded in the ER membrane, as transmembrane domains. The newly synthesized proteins in the rER membrane or lumen move to the smooth ER, where they are partially glycosylated (oligosaccharide groups are covalently bonded to particular amino acids). Membrane lipids (phospholipids, sterols) are also made in and added to the smooth ER. Transport vesicles containing membrane proteins and secreted proteins bud from the smooth ER and travel to the Golgi. These vesicles fuse with the Golgi, adding their membrane lipids and membrane proteins, as well as their internal contents, to the Golgi vesicles. In the Golgi, the membrane proteins and secreted proteins are sorted and processed via additional glycosylation. Lysosomal proteins are segregated to vesicles that pinch off and become lysosomes. Secreted proteins are packaged into secretory vesicles that pinch off and are transported to the cell periphery, where the secretory vesicles fuse with the plasma membrane, adding their lipid and protein to the plasma membrane and dumping their internal contents to the outside of the cell.
rough ER –> smooth ER –> transport vesicles –> Golgi –> secretory vesicles –> PM
Note that the endomembrane system does not include mitochondria nor chloroplasts, which are independent organelles and will be discussed later in the context of energy metabolism. Proteins destined for mitochondria or chloroplasts, as well as proteins destined for the interior of the nucleus, are made by free cytoplasmic ribosomes (undocked to any membrane). These proteins are then imported into the respective organelles via specialized protein import systems (mitochondria and chloroplasts) or via the nuclear pore complexes (nuclei). Of course, proteins that function in the cytoplasm are also made by free cytoplasmic ribosomes.

Cytoskeleton

The cytoskeleton is another cellular system. It consists of actin microfilaments, several types of intermediate filaments, and microtubules. These are dynamic structures required for cell shape, cell mobility, and organization and movement of materials inside the cell. Microfilaments are thinner, and form networks near the plasma membrane to either stabilize or change the shape of the cell, especially when parts of the membrane are extended outward. Microtubules (polymerized from dimers of alpha- and beta-tubulin) serve as tracks for movement of transport vesicles and secretory vesicles by motor proteins, and also for movement of chromosomes during cell division. In brief, microfilaments are for cell shape, microtubules are for moving stuff around inside the cell.

Extracellular matrix

Outside the cell, overlying the plasma membrane, is the extracellular matrix. In plants and yeast, this is the cell wall. In animal cells, this consists of collagen and other polymers of protein and polysaccharides.

Nucleus

The nucleus contains the cell’s chromosomes. All chromosomal DNA replication and transcription to make RNA occurs in the nucleus, as well as RNA processing. The enzymes that perform these tasks, the proteins that bind to DNA to form chromatin, indeed all proteins in the nucleus, are made by ribosomes in the cytoplasm, and then imported into the nucleus through the nuclear envelope pore complexes. Conversely, ribosomal and messenger RNAs are made in the nucleus and exit the nucleus via the same pore complexes, so they can function in cytoplasmic protein synthesis.

Cellular dynamics: Inner Life of the Cell molecular animation

Watch the Inner Life of the Cell video below, and see if you can identify the various components of the endomembrane system and narrate what is going on. This video is for more advanced students, but the middle of the video, starting with the plasma membrane, beautifully illustrates the dynamic interconnections between the cell structures.

The video begins with leukocytes (white blood cells) rolling along a blood vessel. Endothelial cells are the cells that form the inner lining of the blood vessel. Cell surface proteins on the white blood cell interact and bind to the cell surface proteins on the lining of the blood vessel to slow down and stop the white blood cell. From here the video dives into the cell.

The key parts to watch for:

  • The plasma membrane is a fluid mosaic of phospholipids and proteins.
  • Sphingolipids and cholesterol make parts of the plasma membrane rigid – these rigid parts are called lipid rafts, that are important for cell signaling.
  • The cell contains different types of cytoskeletal elements – the video shows spectrin, an intermediate filament actin microfilaments and microtubules. Let’s not worry about additional details mentioned in the video.
  • Motor proteins “walk” along the microtubules, transporting vesicles back and forth. The “walking” of these motor proteins is powered by ATP hydrolysis.
  • The nuclear envelope contains pores, and mRNA molecules exit the nucleus into the cytoplasm through the nuclear pores.
  • Free ribosomes in the cytoplasm translate and make proteins that stay in the cytoplasm, or partner with special proteins that deliver them to mitochondria and other organelles that are independent of the endomembrane system.
  • Free ribosomes also initiate translation of endomembrane system proteins and secreted proteins, but they stall until they are docked to a protein complex in the rER. The rER is “rough” because all the ribosomes located there gives this portion of the ER a rough appearance in electron micrographs. Membrane proteins are embedded in the ER membrane, whereas secreted proteins end up in the lumen.
  • The membrane and secreted proteins are transported in vesicles to the Golgi.
  • The Golgi completes the glycosylation of these proteins.
  • Secretory vesicles are transported from the Golgi to the plasma membrane, where they fuse.
  • You can ignore the rest of the video, although it’s really cool. It shows how white blood cells squeeze between the cells that line the blood vessel to get into the tissues at a site of infection and inflammation.


Cell Structure Flashcards Preview

What name is given to the basic functional and structural unit of all living organisms?

The basic structural and functional unit of all known life-forms is the cell.

For the AP Biology exam, you'll want to be familiar with animal and plant cells, as well as the general structure of bacterial cells.

The cell theory, originally composed in 1838, includes three primary tenets. Name them.

  1. The cell is the basic unit of life.
  2. All living things are composed of cells, whether one or many.
  3. All cells arise from other cells.

All cells can be categorized into which two broad groups?

Prokaryotic and eukaryotic cells

Prokaryotic cells are generally simpler and include bacterial species. Eukaryotes can range from single-celled organisms (like yeast) to complex animals (like humans).

What main features characterize eukaryotic cells?

Eukaryotic cells have membrane-bound organelles, including nuclei, and linear chromosomes. They are also larger than prokaryotic cells and differ in specific aspects like flagellum structure.

Eukaryotic cells can comprise either unicellular or multicellular organisms.

What main features characterize prokaryotic cells?

Prokaryotic cells lack membrane-bound organelles. They generally contain one circular chromosome within a nucleoid region, but can also possess circular plasmids outside the genome.

Prokaryotic cells always comprise unicellular organisms.

How do eukaryotic and prokaryotic cells differ with respect to organelles?

Unlike eukaryotes, prokaryotes lack a nucleus, as well as all membrane-bound organelles.

Note that membrane-bound organelles include mitochondria, lysosomes, the ER, and the Golgi apparatus, but not ribosomes. Prokaryotes do contain ribosomes, a fact that may appear on the AP Biology exam.

How do eukaryotic and prokaryotic organisms differ in their cellular organization?

Prokaryotes are always unicellular, while eukaryotes can be either unicellular or multicellular.

One common example of a unicellular eukaryote is yeast, a fungus. Most other single-celled eukaryotes are classified as protists.

Determine if an organism with the following traits is a prokaryote or a eukaryote:

This organism is a eukaryote.

Only a eukaryote would possess mitochondria, since prokaryotes lack membrane-bound organelles. Eukaryotes also have linear, not circular, chromosomes. Note that both eukaryotes and prokaryotes can be unicellular.

The cytosol is the fluid contained within a cell.

In contrast, the cytoplasm includes both the intracellular fluid and all of the extranuclear organelles.

The cytoplasm includes both the intracellular fluid, or cytosol, and the organelles.

The only organelle that is not included in the cytoplasm is the nucleus.

An organelle is a separate, specialized structure within a cell.

Many organelles are enclosed by lipid bilayers, but some, including ribosomes, are not membrane-bound.

What is the cellular role of the plasma membrane?

The plasma membrane, also called the cell membrane, protects the interior of the cell from its environment. It also limits the movement of specific materials into and out of the cell.

Describe the composition of the plasma membrane.

The plasma membrane consists of a phospholipid bilayer, with polar heads on the exterior (pointing toward the extracellular fluid and cytoplasm) and nonpolar tails on the interior.

The membrane also contains cholesterol, associated large proteins, and sphingolipids, among other components.

Explain the fluid mosaic model.

The fluid mosaic model is used to describe the plasma membrane. It is composed of lipids with a "mosaic" of embedded proteins and other components, and its "fluidity" allows these macromolecule components to move laterally within the membrane.

In animal cells, which organelle serves as the location for DNA in the form of linear chromosomes?

The nucleus holds the cell's linear chromosomes. It is also the site of DNA replication and transcription.

While the mitochondria also include DNA, mitochondrial DNA is found in small circular chromosomes, not linear ones.

What structural features are present in the nucleus?

The nucleus is encased in a double membrane, known as the nuclear envelope. This membrane is marked by channels called nuclear pores. Inside, a fluid (the nucleoplasm) surrounds linear chromosomes.

In what part of the cell is the nucleolus located, and what function does it serve?

The nucleolus is located within the nucleus. It serves as the site of ribosomal RNA transcription and synthesis of ribosomal subunits.

Which organelle has two subunits and serves as the location for protein synthesis?

The ribosome

Ribosomes are small organelles found in both eukaryotic and prokaryotic cells. At these organelles, proteins are synthesized (translated). A typical ribosome includes a small and a large subunit, although the sizes of these subunits vary depending on the type of cell.

How do eukaryotic and prokaryotic organisms differ in the composition of their ribosomes?

Eukaryotic ribosomes are slightly larger, with a 40S and a 60S subunit combining to yield 80S. Prokaryotic ribosomes have a 30S and a 50S subunit, which combine to form 70S.

The abbreviation "S" refers to the rate at which a molecule settles in a centrifuge.

Describe the structural characteristics of the endoplasmic reticulum (ER).

The ER is a folded membrane divided into two regions: rough ER and smooth ER.

Rough ER contains ribosomes bound to its surface, while the smooth ER does not.

Within the cell, what is the role of the endoplasmic reticulum (ER)?

The ER is involved in a variety of processes, with smooth ER and rough ER performing different functions. The smooth ER is involved in lipid anabolism and detoxification, while the rough ER, with its many ribosomes, is the site of protein translation.

Both types of ER help synthesize macromolecules and shuttle them to the Golgi apparatus to be secreted from the cell.

What biological products are synthesized in the rough endoplasmic reticulum?

The rough ER synthesizes proteins.

These can include enzymes and peptide hormones, among other examples.

What biological products are synthesized in the smooth endoplasmic reticulum?

The smooth ER synthesizes lipids.

These include steroid hormones and phospholipids, among other examples.

What is the cellular role of the Golgi apparatus?

The Golgi apparatus modifies molecules that arrive from the ER. It has the ability to break off into vesicles and can thus facilitate the exocytosis of these modified products.

What main cellular function is performed by the mitochondria?

Mitochondria are involved in cellular metabolism, specifically the production of energy via aerobic respiration.

In the mitochondria, the Krebs cycle produces electron carriers, while the electron transport chain facilitates the formation of a proton gradient. This gradient is used to produce ATP.

What membranes and spaces are present in a mitochondrion?

Mitochondria contain both an outer and an inner membrane. The intermembrane space is located between the two, while the mitochondrial matrix is the innermost space, bounded by the inner membrane.

Both membranes are phospholipid bilayers.

Binary fission is most relevant to the production of which organelle?

Binary fission is the method of replication for mitochondria.

Just like prokaryotic asexual reproduction, which produces identical daughter cells, this method of division yields identical organelles.

Which organelle found in animal cells may have arisen as a result of mutualism?

Mitochondria may have evolved from a symbiotic relationship between small bacteria and larger cells. This is known as the endosymbiotic theory.

Like all instances of mutualism, this situation is thought to have provided benefits to both organisms. The smaller bacterium was given a livable environment while providing energy for the larger host.

What is the cellular role of lysosomes?

Lysosomes break down engulfed pathogens, nutrient molecules, and components of the cell itself that are no longer functional.

Like the stomach, a lysosome contains enzymes and an acidic interior.

What are peroxisomes, and what cellular function do they perform?

Peroxisomes are small membrane-bound organelles that contain enzymes. They function in fatty acid breakdown, detoxification, and facilitation of the pentose phosphate pathway.

Peroxisomes are named for hydrogen peroxide (H2O2), which can be both formed and broken down within the organelle. This is important because H2O2 is a poisonous radical initiator.


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