Source: Wikipedia, some random lecture, and some random websites
Definition
Alcoholism is the consumption of or preoccupation with alcoholic beverages to the extent that this behavior interferes with the alcoholic's normal personal, family, social, or work life.
Consequence
The chronic alcohol consumption caused by alcoholism can result in psychological and physiological disorders. Estimates of the economic costs of alcohol abuse, collected by the World Health Organization, vary from one to six per cent of a country's GDP.
Alcohol Levels
One standard drink = 10 grams alcohol.
1 standard drink = pot of beer, alcoholic shot, port/sherry, restaurant wine = 0.8 S.D.
1.5 standard drinks = sparkling wine, champagne
Bottle of wine = 7 Standard Drinks
Bottle of spirits = 22 SDs (which is why you take only a nip)
Drinking and driving legal limits: 0.05 (0.05 grams per 100 ml blood = one more drink than the number of hours for men, and drinks equal the number of hours for women). For taxi, bus, heavy vehicle, learner and P plate drivers, the limit is 0.02 grams per 100 ml. At 0.05, the risk of an accident is doubled than that of someone who has not consumed alcohol. At 0.10, the risk is 7 times higher.
Low-risk drinking guidelines
Men: No more than 2 standard drinks in the first hour, and one per hour after that. No more than 4 standard drinks a day on average (never more than 6) and at least one or two alcohol-free days per week.
Women: No more than 1 standard drink per hour. No more than 2 standard drinks per day (on average) and never more than four and one or two alcohol-free days per week.
4 L’s of Alcohol’s harmful effects:
· Liver (injuries, brain disease)
· Lover (domestic violence)
· Livelihood (poor performance)
· Law (drinking and driving, assault)
Almost 100,000 deaths per year in USA are attributable to alcohol.
Alcohol Addiction
While alcohol use is required to trigger alcoholism, the biological mechanism of alcoholism is uncertain. For most people, moderate alcohol consumption poses little danger of addiction. Other factors must exist for alcohol use to develop into alcoholism. These factors may include a person's social environment, emotional health and genetic predisposition. In addition, an alcoholic can develop multiple forms of addiction to alcohol simultaneously such as psychological, metabolic, and neurochemical. Each type of addiction must be treated individually for an alcoholic to fully recover.
Alcohol Screening
The CAGE questionnaire, named for its four questions, is one such example that may be used to screen patients quickly in a doctor's office.
Two "yes" responses indicate that the respondent should be investigated further. The questionnaire asks the following questions:
Have you ever felt you needed to Cut down on your drinking?
Have people Annoyed you by criticizing your drinking?
Have you ever felt Guilty about drinking?
Have you ever felt you needed a drink first thing in the morning (Eye-opener) to steady your nerves or to get rid of a hangover?
Diagnosis of Alcohol Levels
There are reliable tests for the actual use of alcohol, one common test being that of blood alcohol content (BAC).
Alcohol content in blood can be directly measured by a hospital laboratory. More commonly in law enforcement investigations, BAC is estimated from breath alcohol concentration (BrAC) measured with a machine commonly referred to as a Breathalyzer.
1/13/07
1/11/07
The Medical Interview
Studies have shown that 60-80% of diagnosis are made on the medical history one takes from a patient, therefore it is a very important skill to perfect.
All of these skills interweave throughout the discussion of the presenting illness:
1. Eliciting the patient's narrative about the presenting problem
- Asking Open Questions: invite the patient to "open up" by saying, "Could you tell me about this pain from the beginning?" Open questions encourage the patient to express the presenting problem in their own words and bring in all the relevant factors from the patient's POW.
- Establish Motivation: identify the reason(s) for attenting today
- Facilitation techniques: "Uh-huh," nods, and paraphrasing info encourage patient's opening up.
- Responding to patient's emotional reactions: Either reflect on the patient's feelings as you have understood them ("You have been feeling sad since that event") or legitimize their feelinds ("most people worried when they have chest pain," or "It is natural to be concerned when your baby is ill").
- Summarizing and Feedback
- Probing to Completeness: "Is there anything else concerning you?"
2. The skill to assemble the informaiton obtained from a medical (disease) perspective
A neat trick to remember the diagnostic questions one can ask:
WWQQAAB:
- Where: The location and radiation of a symptom
- When: When it began, fluctuation over time, duration
- Quality: What it feels like
- Quantity: Intensity, extent, degree of disability
- Aggravating and Alleviating factors: what makes it worse/better
- Associated symptoms: other symptoms that have occurred with the main symptom
- Beleifs: the patient's beliefs about the symptoms
3. The skill to enable patients to talk about how the illness is impacting on their life circumstances.
The patient's perspective includes ideas, feelings, expectations and effects. In a patient centred approach to the medical interview it is just as important to explore this area as the Medical perspective in part 2 above.
All of these skills interweave throughout the discussion of the presenting illness:
1. Eliciting the patient's narrative about the presenting problem
- Asking Open Questions: invite the patient to "open up" by saying, "Could you tell me about this pain from the beginning?" Open questions encourage the patient to express the presenting problem in their own words and bring in all the relevant factors from the patient's POW.
- Establish Motivation: identify the reason(s) for attenting today
- Facilitation techniques: "Uh-huh," nods, and paraphrasing info encourage patient's opening up.
- Responding to patient's emotional reactions: Either reflect on the patient's feelings as you have understood them ("You have been feeling sad since that event") or legitimize their feelinds ("most people worried when they have chest pain," or "It is natural to be concerned when your baby is ill").
- Summarizing and Feedback
- Probing to Completeness: "Is there anything else concerning you?"
2. The skill to assemble the informaiton obtained from a medical (disease) perspective
A neat trick to remember the diagnostic questions one can ask:
WWQQAAB:
- Where: The location and radiation of a symptom
- When: When it began, fluctuation over time, duration
- Quality: What it feels like
- Quantity: Intensity, extent, degree of disability
- Aggravating and Alleviating factors: what makes it worse/better
- Associated symptoms: other symptoms that have occurred with the main symptom
- Beleifs: the patient's beliefs about the symptoms
3. The skill to enable patients to talk about how the illness is impacting on their life circumstances.
The patient's perspective includes ideas, feelings, expectations and effects. In a patient centred approach to the medical interview it is just as important to explore this area as the Medical perspective in part 2 above.
Cellular phsyiology
Physiology is the study of the mechanical, physical, and biochemical functions of living organisms.
The Fundamentals
Solutions: a homogenous mixture of solute within solvent (e.g. salt in water, oxygen in water)
Colloids: s a substance with components of one a homogeneous mixture (a solution) and a heterogeneous mixture with properties also intermediate between the two. Typical membranes restrict the passage of dispersed colloidial particles more than they restrict the passage of dissolved ions or molecules; i.e. ions or molecules may diffuse through a membrane through which dispersed colloidal particles will not. The dispersed phase particles are largely affected by the surface chemistry existent in the colloid. Examples: butter, milk, cream, asphalt, ink, glues, etc.
Molarity: Molarity (M) denotes the number of moles of a given substance per litre of solution.
Electrolytes: An electrolyte is a substance containing free ions which behaves as an electrically conductive medium.
Osmosis: is the net movement of water through a selective permeable membrane from a region of low solute potential to a region of high solute potential (or equivalently, from a region of high solvent potential to a region of low solvent potential). The partially permeable membrane must be permeable to the solvent, but not to the solute, resulting in a pressure gradient across the membrane.
Osmolarity: osmoles/litre
Osmolality: osmoles/kilogram
Tonicity: is the ability of a solution to cause water movement. It is in reference to hypertonic, hypotonic and isotonic cellular states.
The Medical Physiology: water balance, electrolyte balance
Clinical Insights: dehydration, diarrhoea, oedema
Appropriate regulation of membrane water, eg permeability and body solutes, is a fundamental requirement of all living organisms. The distribition of fluids and their electrolyes, proteins and macromolecules is normally controlled by a wide range of mechanisms. These occur at the cellular level to maintain a physiological balance of fluids content between the cells themselves, adn the extracellular environment. Broadly, the latter includes the extra- or intercellular spaces (sometimes called the interstitial space), blood plasma, lymphatic fluid, lung airspace, the luminal contents of the guy and numerous other 'extra-tissue' compartments in organs. Control of fluid distribution, its hydrostatic pressure and hte process of solute transport is a major function of the vascular sstem and the kidneys. These in turn maintain the correct fluid compositions of organs as diverse as the brain (cerebrospinal fluid), cartilage (fluid-rich matrix), cornea and the lens of the eye (cellular coverings of these tissues).
Body fluid composition is a vital component of the maintenance of internal stability. This is not constant but changes within a controlled physiological range that contributes to homeostasis.
Important Lecture Content:
- Total Body Water (TBW) is approximately 60% of body mass
- Extracellular Fluid (ECF) is approximately 20% of body mass
- ECF is composed of plasma water contained in the blood vessels, intersitial fluid in the extracellular matrix, water in lymph vessels and transcellular water in spaces such as the joints, cerebrospinal fluid and eye chambers
- Intracellular Fluid (ICF) is approximately 40% of body mass
- The capillary membrane barrier is highly permeable to water and electrolytes
- The cell membrane is highly permeable to water, not electrolytes
The Fundamentals
Solutions: a homogenous mixture of solute within solvent (e.g. salt in water, oxygen in water)
Colloids: s a substance with components of one a homogeneous mixture (a solution) and a heterogeneous mixture with properties also intermediate between the two. Typical membranes restrict the passage of dispersed colloidial particles more than they restrict the passage of dissolved ions or molecules; i.e. ions or molecules may diffuse through a membrane through which dispersed colloidal particles will not. The dispersed phase particles are largely affected by the surface chemistry existent in the colloid. Examples: butter, milk, cream, asphalt, ink, glues, etc.
Molarity: Molarity (M) denotes the number of moles of a given substance per litre of solution.
Electrolytes: An electrolyte is a substance containing free ions which behaves as an electrically conductive medium.
Osmosis: is the net movement of water through a selective permeable membrane from a region of low solute potential to a region of high solute potential (or equivalently, from a region of high solvent potential to a region of low solvent potential). The partially permeable membrane must be permeable to the solvent, but not to the solute, resulting in a pressure gradient across the membrane.
Osmolarity: osmoles/litre
Osmolality: osmoles/kilogram
Tonicity: is the ability of a solution to cause water movement. It is in reference to hypertonic, hypotonic and isotonic cellular states.
The Medical Physiology: water balance, electrolyte balance
Clinical Insights: dehydration, diarrhoea, oedema
Appropriate regulation of membrane water, eg permeability and body solutes, is a fundamental requirement of all living organisms. The distribition of fluids and their electrolyes, proteins and macromolecules is normally controlled by a wide range of mechanisms. These occur at the cellular level to maintain a physiological balance of fluids content between the cells themselves, adn the extracellular environment. Broadly, the latter includes the extra- or intercellular spaces (sometimes called the interstitial space), blood plasma, lymphatic fluid, lung airspace, the luminal contents of the guy and numerous other 'extra-tissue' compartments in organs. Control of fluid distribution, its hydrostatic pressure and hte process of solute transport is a major function of the vascular sstem and the kidneys. These in turn maintain the correct fluid compositions of organs as diverse as the brain (cerebrospinal fluid), cartilage (fluid-rich matrix), cornea and the lens of the eye (cellular coverings of these tissues).
Body fluid composition is a vital component of the maintenance of internal stability. This is not constant but changes within a controlled physiological range that contributes to homeostasis.
Important Lecture Content:
- Total Body Water (TBW) is approximately 60% of body mass
- Extracellular Fluid (ECF) is approximately 20% of body mass
- ECF is composed of plasma water contained in the blood vessels, intersitial fluid in the extracellular matrix, water in lymph vessels and transcellular water in spaces such as the joints, cerebrospinal fluid and eye chambers
- Intracellular Fluid (ICF) is approximately 40% of body mass
- The capillary membrane barrier is highly permeable to water and electrolytes
- The cell membrane is highly permeable to water, not electrolytes
Internal Organization of the Cell
Inside the nucleus:
- Nuclear membrane: separates contents from the cytoplasm
- Chromatin: in two forms, heterochromatin that is DNA not being transcribed and euchromatin where DNA is transcribed
- Nucleolus of DNA where ribosomal genes make rRNA that is combined with proteins to produce the ribosomes.
Cytosol: a sol-gel liquid in which metabolic reactions and protein synthesis occur
Rough Endoplasmic Reticulum: a membrane compartment for protein synthesis
Smooth Endoplasmic Reticulum: a membrane compartment for steroid hormone synthesis and for detoxification of drugs
Mitochondrion: organelle that makes ATP which serves as energy source for chemical/mechanical work
Golgi apparatus: organelle that functions to process and package proteins and lipids
Lysozymes: formed by Golgi and contain enzymes to break down unwanted or foreign ingested material
Peroxisomes: exudate toxic materials, and can kill bacteria
Inclusion bodies are inert, stored materials
- Fat or lipid droplets: stored triglycerides/cholesterol for energy source and biosynthetic reactions
- Glycogen granules: major form of stored carbohydrate
- Pigment/lipofuscin: undigested waste materials
Cytoskeleton: internal scaffolding of the cell, contains:
- Microtubules: rigid protein cylinders for cell shape and to guide organelles/molecules to destinations
- Microfilaments: assist contraction, stabilize shape
- Intermediate filaments: resist tension and connect cells through special junctions
Surface specializations are linked to cytoskeleton
- Microvilli: slender extensions of plasma membrane
- Cillia: hairlike and motile at the surface
- Flagella: a much longer cilium for propulsion
- Nuclear membrane: separates contents from the cytoplasm
- Chromatin: in two forms, heterochromatin that is DNA not being transcribed and euchromatin where DNA is transcribed
- Nucleolus of DNA where ribosomal genes make rRNA that is combined with proteins to produce the ribosomes.
Cytosol: a sol-gel liquid in which metabolic reactions and protein synthesis occur
Rough Endoplasmic Reticulum: a membrane compartment for protein synthesis
Smooth Endoplasmic Reticulum: a membrane compartment for steroid hormone synthesis and for detoxification of drugs
Mitochondrion: organelle that makes ATP which serves as energy source for chemical/mechanical work
Golgi apparatus: organelle that functions to process and package proteins and lipids
Lysozymes: formed by Golgi and contain enzymes to break down unwanted or foreign ingested material
Peroxisomes: exudate toxic materials, and can kill bacteria
Inclusion bodies are inert, stored materials
- Fat or lipid droplets: stored triglycerides/cholesterol for energy source and biosynthetic reactions
- Glycogen granules: major form of stored carbohydrate
- Pigment/lipofuscin: undigested waste materials
Cytoskeleton: internal scaffolding of the cell, contains:
- Microtubules: rigid protein cylinders for cell shape and to guide organelles/molecules to destinations
- Microfilaments: assist contraction, stabilize shape
- Intermediate filaments: resist tension and connect cells through special junctions
Surface specializations are linked to cytoskeleton
- Microvilli: slender extensions of plasma membrane
- Cillia: hairlike and motile at the surface
- Flagella: a much longer cilium for propulsion
1/10/07
Gram Staining
History:
In 1884, Hans Christian Gram, a Danish bacteriologist, developed a method for distinguishing between two major classes of bacteria. This technique, the Gram Stain, continues to be a standard procedure in medical microbiology. Gram was a modest man, and in his initial publication he remarked "I have therefore published the method, although I am aware that as yet it is very defective and imperfect; but it is hoped that also in the hands of other investigators it will turn out to be useful."
Defintion:
Gram staining is an empirical method of differentiating bacterial species into two large groups (gram-positive and gram-negative) based on the chemical and physical properties of their cell walls.
Importance:
Gram staining is probably the single most useful staining procedure in a bacteriological laboratory. The technique is widely used as a tool for the differentiation of gram-positive and gram-negative bacteria, as a first step to determine the identity of a particular bacterial sample.
Gram stains are performed on body fluid or biopsy when infection is suspected. It yields results much more quickly than culture, and is especially important when infection would make an important difference in the patient's treatment and prognosis; examples are cerebrospinal fluid for meningitis and synovial fluid for septic arthritis.
Gram Staining Procedure:
1. Make a slide of tissue or body fluid that is to be stained. Heat the slide for few seconds until it becomes hot to the touch so that bacteria are firmly mounted to the slide.
2. Add the primary stain crystal violet and incubate 1 minute, then wash with water for a maximum of 5 seconds to remove the unbound crystal violet.
3. Add Gram's iodine for 1 min. It is not a stain it is a mordant. It doesn't give color directly to the bacteria but it fixes the crystal violet to the bacterial cell wall.
4. Wash with Acid Alcohol. If the bacteria is gram positive it will retain the primary stain. If it is gram negative it will loose the primary stain.
5. Add the secondary stain, safranin, and incubate 1 min, then wash with water for a maximum of 5 seconds. If the bacteria is gram positive then it will retain the primary stain and will not take the secondary stain. It will look black-violet in a pink background. If it is gram negative then it will loose the primary stain and take secondary stain making it pink-red.
Gram Stain is 2 g of 90% crystal violet dissolved in 20 ml of 95% ethyl alcohol.
Gram's iodine is 1 g of iodine, 2 g of potassium iodide, dissolved in 300 ml of distilled water.
Acid alcohol is 70% ethyl alcohol containing either 0.5% or 1.0% HCl. Quite often acetone or an acetone-alcohol mixture is substituted.
In addition it now common to use basic fuchsin inistead of safranin.
Why?
The decolorizing mixture causes dehydration of the multilayered peptidoglycan in the gram-positive cell wall, thus decreasing the space between the molecules and causing the cell wall to trap the crystal violet-iodine complex within the cell. But in gram-negative bacteria, the decolorizing mixture acts as a lipid solvent and dissolves the outer membrane of the gram-negative cell wall. The thin layer of peptidoglycan is unable to retain the crystal violet-iodine complex and the gram-negative cell is decolorized. The decolorisation step is the crucial one, and requires some degree of skill, as being gram positive is not an all-or-none phenomenon.
Results
Gram-positive bacteria (thick peptidoglycan layer): Black-violet
Gram-negative bacteria (thin peptidoglycan layer, and an outer membrane): Pink-Red
As a general rule of thumb (which has exceptions), gram-negative bacteria are more dangerous as disease organisms because:
- Outer membrane hidden by capsule or slime layer (which hides its antigens, or "foreign markers")
- Gram-negative bacteria have lipopolysaccharide in their outer membrane, an endotoxin which increases the severity of inflammation (so severe it may lead to septic shock).
Gram-positive bacteria infections are less severe because of human body's production of lysozyme, an enzyme that attacks the open peptidoglycan layer of gram-positive bacteria. Gram-positive bacteria are also much more susceptible to beta-lactam antibiotics, such as penicillin.
Other
After the staining distinction, the bacteria are further distinguished via their shape - as rods or cocci.
Cell Wall - a medical target
Source: Wikipedia + same lecture as the "Cell Specializations" - lecture slides
Cell Wall - a medical target
The cell wall is an important and common target for combating disease. This is because cell walls are not found in eukaryotes (so human cells remain safe).
Some antibiotics work in this way. Some antibiotics specifically interfere with synthesis of peptidoglycan. Consequently, there is little if any effect on the cells of humans and other eukaryotes.
Peptidoglycan: a polymer consisting of sugars and amino acids that forms a homogeneous layer outside the plasma membrane of eubacteria. Peptidoglycan serves a structural role in the bacterial cell wall, giving the wall shape and structural strength, as well as counteracting the osmotic pressure of the cytoplasm. Peptidoglycan is also involved in binary fission during bacterial cell reproduction.
Antibacterial drugs such as penicillin interfere with the production of peptidoglycan by binding to bacterial enzymes, transpeptidases. Transpeptidases form the bonds between oligopeptide crosslinks in peptidoglycan. For a bacterial cell to reproduce through binary fission, more than a million peptidoglycan subunits (NAM-NAG+oligopeptide) must be attached to existing subunits. Mutations in transpeptidases that lead to reduced interactions with an antibiotic are a significant source of emerging antibiotic resistance.
Cell Wall - a medical target
The cell wall is an important and common target for combating disease. This is because cell walls are not found in eukaryotes (so human cells remain safe).
Some antibiotics work in this way. Some antibiotics specifically interfere with synthesis of peptidoglycan. Consequently, there is little if any effect on the cells of humans and other eukaryotes.
Peptidoglycan: a polymer consisting of sugars and amino acids that forms a homogeneous layer outside the plasma membrane of eubacteria. Peptidoglycan serves a structural role in the bacterial cell wall, giving the wall shape and structural strength, as well as counteracting the osmotic pressure of the cytoplasm. Peptidoglycan is also involved in binary fission during bacterial cell reproduction.
Antibacterial drugs such as penicillin interfere with the production of peptidoglycan by binding to bacterial enzymes, transpeptidases. Transpeptidases form the bonds between oligopeptide crosslinks in peptidoglycan. For a bacterial cell to reproduce through binary fission, more than a million peptidoglycan subunits (NAM-NAG+oligopeptide) must be attached to existing subunits. Mutations in transpeptidases that lead to reduced interactions with an antibiotic are a significant source of emerging antibiotic resistance.
1/9/07
Cell Specialization
Source: Wikipedia and some random week 2 notes
Prokaryotes:
Prokaryotes are organisms without a cell nucleus. Most prokaryotes are bacteria, and the two terms are often treated as synonyms. In 1977, Carl Woese proposed dividing prokaryotes into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) because of the significant genetic differences between the two. This arrangement of Eukaryota (also called "Eukarya"), Bacteria, and Archaea is called the three-domain system replacing the traditional two-empire system.
Ev0lution of Prokaryotes:
It is generally accepted that the first living cells were some form of prokaryote. Fossilized prokaryotes approximately 3.5 billion years old have been discovered, and prokaryotes are perhaps the most successful and abundant organism even today. In contrast the eukaryote only appeared between approximately 1.7 and 2.2 billion years ago.
Eukaryotes:
A eykaryote is an organism with complex cells that have nuclei containing DNA. Eukaryotes include animals, plants and fungi, as well as various unicellular organisms such as protists.
The Special Features of Eukaryotic cells
Unlike prokaryotic cells (such as bacteria), eukaryotic cells are larger and show internal compartments, the nucleus and the cytoplasm. The existence of these compartments allows a eukaryote to separate various metabolic functions inside the cell, for example the regulation of DNA synthesis within the nucleus, and the production of proteins within the cytoplasm. This function, where specialised activities occur within defined regions of a single cell, is characteristic of special multicullular organisms.
The Symbiotic Theory
A critical step for the specialization of eukaryotic cells is beleived to be the symbiotic acquisiton of mitochondria from aerobic bacteria and in the case of plants, or chloroplasts from photosynthetic bacteria. The theory supposes that the separation of both respiration and energy metabolism into a mitochondrion may have provided the ancestral eukaryote with an abundant source of energy.
Cell Specialization
Eukaryotic cells have developed internal membranes that provide an enormous surface area for metabolic reactions that perform hundreds or thousands of different functions inside the cell:
- Cytoskeleton: allows for arrangement of membranes, alterations of cell shape and motility
- Nucleus: packages genetic material, separating DNA and RNA sequences from cytoplasm and provides for independant control of the chromosomes
- Cell plasma membrane: specialized to provide for adhesion, receptors for external stimuli, and control over the entry and exit of substances
Multicellular Organisms
Increasing cell specialization, uncluding the ability to control cell multiplication, has led to the complexity and diversity of many different types of cells that are reorganized as tissues, organs and organ systems. Unicellular organisms can form colonies, e.g. Volvox (green algae), where around 50,000 individual cells form colonies consisting of hollow balls in which the cells are embedded in a gelatinous matrix. The colony is structurally and functionally polarised - it can swim towards light and its reproductive cells reside at one region of the colony. Volvox displays specialized cells that cooperate in function, an essential feature of multicellular organisms.
Additional key features of multicellularity:
- Cohesions between cells and with extracellular molecules
- Complete cell division, or incomplete cytokinesis through cytoplasmic bridges
- Formation of multicellular sheets, or epithelium
- Inner and outer layers of epithelium
- Cell-cell communication, cell responds to signals
Gene Expression
Cenral to the developnment of multicellular organisms and the specialized cell types which they produce, is the process of differentiation. This is where single cells or groups of cells undergo structure and metabolic changes which distinguish them functionally from other cells in the developing organism. The instructions are contained in the DNA, and when activated, this gene expression induces important phases of biochemical reactions throughout development.
Prokaryotes:
Prokaryotes are organisms without a cell nucleus. Most prokaryotes are bacteria, and the two terms are often treated as synonyms. In 1977, Carl Woese proposed dividing prokaryotes into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) because of the significant genetic differences between the two. This arrangement of Eukaryota (also called "Eukarya"), Bacteria, and Archaea is called the three-domain system replacing the traditional two-empire system.
Ev0lution of Prokaryotes:
It is generally accepted that the first living cells were some form of prokaryote. Fossilized prokaryotes approximately 3.5 billion years old have been discovered, and prokaryotes are perhaps the most successful and abundant organism even today. In contrast the eukaryote only appeared between approximately 1.7 and 2.2 billion years ago.
Eukaryotes:
A eykaryote is an organism with complex cells that have nuclei containing DNA. Eukaryotes include animals, plants and fungi, as well as various unicellular organisms such as protists.
The Special Features of Eukaryotic cells
Unlike prokaryotic cells (such as bacteria), eukaryotic cells are larger and show internal compartments, the nucleus and the cytoplasm. The existence of these compartments allows a eukaryote to separate various metabolic functions inside the cell, for example the regulation of DNA synthesis within the nucleus, and the production of proteins within the cytoplasm. This function, where specialised activities occur within defined regions of a single cell, is characteristic of special multicullular organisms.
The Symbiotic Theory
A critical step for the specialization of eukaryotic cells is beleived to be the symbiotic acquisiton of mitochondria from aerobic bacteria and in the case of plants, or chloroplasts from photosynthetic bacteria. The theory supposes that the separation of both respiration and energy metabolism into a mitochondrion may have provided the ancestral eukaryote with an abundant source of energy.
Cell Specialization
Eukaryotic cells have developed internal membranes that provide an enormous surface area for metabolic reactions that perform hundreds or thousands of different functions inside the cell:
- Cytoskeleton: allows for arrangement of membranes, alterations of cell shape and motility
- Nucleus: packages genetic material, separating DNA and RNA sequences from cytoplasm and provides for independant control of the chromosomes
- Cell plasma membrane: specialized to provide for adhesion, receptors for external stimuli, and control over the entry and exit of substances
Multicellular Organisms
Increasing cell specialization, uncluding the ability to control cell multiplication, has led to the complexity and diversity of many different types of cells that are reorganized as tissues, organs and organ systems. Unicellular organisms can form colonies, e.g. Volvox (green algae), where around 50,000 individual cells form colonies consisting of hollow balls in which the cells are embedded in a gelatinous matrix. The colony is structurally and functionally polarised - it can swim towards light and its reproductive cells reside at one region of the colony. Volvox displays specialized cells that cooperate in function, an essential feature of multicellular organisms.
Additional key features of multicellularity:
- Cohesions between cells and with extracellular molecules
- Complete cell division, or incomplete cytokinesis through cytoplasmic bridges
- Formation of multicellular sheets, or epithelium
- Inner and outer layers of epithelium
- Cell-cell communication, cell responds to signals
Gene Expression
Cenral to the developnment of multicellular organisms and the specialized cell types which they produce, is the process of differentiation. This is where single cells or groups of cells undergo structure and metabolic changes which distinguish them functionally from other cells in the developing organism. The instructions are contained in the DNA, and when activated, this gene expression induces important phases of biochemical reactions throughout development.
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