Energy in living organisme cell

Cells Transform Different Types of Energy

Living organisms require a continual input of free energy for three major purposes: (1) the performance of mechanical work in muscle contraction and other cellular movements, (2) the active transport of molecules and ions, and (3) the synthesis of macromolecules and other biomolecules from simple precursors. The free energy used in these processes, which maintain an organism in a state that is far from equilibrium, is derived from the environment.

The First Law of Thermodynamics states that energy can be neither created nor destroyed. The amount of energy in the universe is constant. Nevertheless, energy can be converted from one form into another. Photosynthetic organisms, or phototrophs, use the energy of sunlight to convert simple energy-poor molecules into morecomplex energy-rich molecules that serve as fuels. In other words, photosynthetic organisms transform light energy into chemical energy. Indeed, this transformation is ultimately the primary source of chemical energy for the vast majority of organisms, human beings included. Chemotrophs, which include animals, obtain chemical energy through the oxidation
of foodstuffs generated by phototrophs. Chemical energy obtained from the oxidation of carbon compounds may be transformed into the unequal distribution of ions across a membrane, resulting in an ion gradient. This gradient, in turn, is an energy source that can be used to move molecules across membranes, that can be converted into yet other types of chemical energy, or that can convey
information in the form of nerve impulses. In addition, chemical energy can be transduced into mechanical energy. We convert the chemical energy of a fuel into structural alterations of contractile proteins that result in muscle contraction and movement. Finally, chemical energy powers the reactions that result in the synthesis of biomolecules.At any given instant in a cell, thousands of energy transformations are taking place. Energy is being extracted from fuels and used to power biosynthetic processes. These transformations are referred to as metabolismor intermediary
metabolism. (P:569)

ATP Is the Universal Currency of Free Energy in Biological Systems
Just as commerce is facilitated by the use of a common currency, the commerce of the cell metabolism is facilitated by the use of a common energy currency, adenosine triphosphate(ATP). Part of the free energy derived from the oxidation of foodstuffs and from light is transformed into this highly accessible molecule, which acts as the free-energy donor in most energy-requiring processes such as motion, active transport, or biosynthesis. ATP is a nucleotide consisting of an adenine, a ribose, and a triphosphate unit (Figure 14.3). The active form of ATP is usually a complex of ATP with Mg2+ or Mn2+. In considering the role of ATP as an energy carrier, we can focus on its triphosphate moiety. ATP is an energy-rich molecule because its triphosphate unit contains two
phosphoanhydride bonds. A large amount of free energy is liberated when ATP is hydrolyzed to adenosine diphosphate (ADP) and orthophosphate (Pi) or when ATP is hydrolyzed to adenosine monophosphate (AMP) and pyrophosphate (PPi).

The precise ∆G°for these reactions depends on the ionic strength of the medium and on the concentrations of Mg2+ and other metal ions. Under typical cellular concentrations, the actual ∆G for these hydrolyses is approximately -12 kcal mol-1 (-50 kJ mol-1).
The free energy liberated in the hydrolysis of ATP is harnessed to drive reactions that require an input of free energy, such as muscle contraction. In turn, ATP is formed from ADP and Pi when fuel molecules are oxidized in chemotrophs or when light is trapped by phototrophs. This ATP ADP cycle is the fundamental mode of energy exchange in biological systems. Some biosynthetic reactions are driven by hydrolysis of nucleoside triphosphates that are analogous to ATP namely, guanosine triphosphate (GTP), uridine triphosphate (UTP), and cytidine triphosphate (CTP). The diphosphate forms of these nucleotides are denoted by GDP, UDP, and CDP, and the monophosphate forms by GMP, UMP, and CMP.
Enzymes can catalyze the transfer of the terminal phosphoryl group from one nucleotide to another. The phosphorylation of nucleoside monophosphates is catalyzed by a family of nucleoside monophosphate kinases. The phosphorylation of nucleoside diphosphates is catalyzed by nucleoside diphosphate kinase, an enzyme with broad specificity. It is intriguing to note that, although all of the nucleotide triphosphates are energetically equivalent, ATP is nonetheless the primary cellular energy carrier. In addition, two important electron carriers, NAD+ and FAD, are derivatives of ATP. The role of ATP in energy metabolism is paramount. (P:572)

 Berg, Tymoczko, Stryer.
Biochemistry, fifth edition
© W. H. Freeman and Company and Sumanas, Inc.

from: center library of Syarif Hidayatullah State Islamic University, Jakarta

Assessment of laboratory/practical work

Description

The use of a laboratory situation to assess aspects of a student’s work that may not appropriately be assessed by regular paper-based tests. A wide variety of testing objectives are possible and Brown, Bull and Pendlebury (1997) offer a long list of potential objectives which may need to be included in assessment. As a result of deciding what exactly it is that needs to be assessed, the teacher must decide whether any simple paper-and-pencil test method is adequate, or whether the laboratory needs to be the venue for assessment. Typically, students are required to perform some experimental procedure, note the results and evaluate their findings

Variations

Many, including ‘dry practicals’, where the results of a laboratory experiment are presented and students are required simply to analyse them and to evaluate and comment on them. Group work is increasingly a feature of many undergraduate programmes, reflecting the importance of collaboration skills as a course aim. But the assessment of group work brings a variety of problems (Wood, 1991).

Marking systems

These vary, depending upon the approach. Main problems surround how to balance assessment of process versus outcome/analysis/evaluation and standardizing this. The classic science paradigm (methods, results, conclusion) may assist in producing assessment criteria. Work in the assessment of medical students using objective, standardized approaches may also be helpful (Harden and Gleeson, 1979).

Advantages

This method allows assessment of a uniquely important aspect of many subjects,the ‘real world’ of scientific enquiry.

Drawbacks

There are practical difficulties towards making assessment equitable for all students: what happens if a student’s experiment ‘goes wrong’. The cost of this method of testing may be high. It is also difficult to determine assessment priorities (eg, process versus outcome) and rewards.

Orals

Oral or viva voce examinations, though commonly used in professional and postgraduate assessment, are the subject of great concern to test developers and psychometricians – especially when loosely described criteria such as ‘sparkle’ are mentioned. Orals have their attractions, but are subject to all the well-known biases and problems of selection interviews, and should only be used in the full knowledge of these problems and how these effects may be minimized. The new practitioner in higher education is counselled to beware of and avoid orals, certainly until he or she has read some of the literature (eg, Wakeford, Southgate and Wass, 1995).

www.kogan-page.co.uk

© Individual contributors, 2003

Contextualised Media for Learning
Tim de Jong, Marcus Specht and Rob Koper Educational Technology Expertise Centre, Open University of the Netherlands, The Netherlands // tim.dejong@ou.nl // marcus.specht@ou.nl // rob.koper@ou.nl

ABSTRACT
In this paper, we analyse how contextualised media can be used to support learning. Additionally, the
advantages of contextualised learning and the types of learning that are fit to be supported are discussed. Our
focus throughout the paper will be on lifelong learning, and the integration of formal and informal learning
therein. However, we think, to this date, most of the research concerning contextualised and mobile learning has been focusing on technological issues. Therefore, as an attempt to shift the discussion to a more educational perspective, a generic technical framework is presented. The technical framework is based on a reference model that came about as the result of a literature analysis in a previous paper. The reference model should provide a foundation that leads to a flexible and generic technical framework that can be used in a range of different learning scenarios. Moreover, a generic technical approach should aim at an easier integration of contextualised learning appliances into current learning.

Keywords
Contextualised learning, Mobile learning, Mobile social software, Technical framework, Ubiquitous computing


read more...

ANALISIS KETERAMPILAN PROSES SAINS SISWA SMA PADA
“MODEL PEMBELAJARAN PRAKTIKUM D-E-H”


Oleh:
Susiwi1, Achmad A.Hinduan2, Liliasari2, Sadijah Ahmad3
1Jurusan Pendidikan Kimia FPMIPA UPI
2Sekolah Pascasarjana UPI
3FMIPA ITB


ABSTRAK
Tujuan penelitian ini adalah untuk memperoleh informasi tentang keterampilan proses sains siswa SMA pada Model Pembelajaran Praktikum Diskriptif - Empiris Induktif - Hipotetis Deduktif (MPP D–E–H). Keterampilan berpikir yang tergolong keterampilan proses sains dan merupakan komponen penting dalam suatu penyelidikan meliputi: keterampilan ”merumuskan hipotesis”, keterampilan ”mengendalikan variabel”, dan keterampilan ”merancang percobaan”. Penelitian ini dilakukan dengan studi deskriptif teoretik, dan dilanjutkan dengan studi eksperimental pada implementasi pembelajaran. Penelitian dilaksanakan dengan
melibatkan kelompok SMA-prestasi akademik tinggi, dan SMA-prestasi akademik sedang. Masing-masing kelompok SMA-Sedang maupun kelompok SMA-Tinggi dibagi menjadi kelas eksperimen dan kelas kontrol. Subyek penelitian seluruhnya berjumlah 216 siswa. Dalam penelitian ini digunakan subyek penelitian dari kelas eksperimen sebanyak 130 siswa, yang terdiri dari 43 siswa kelompok SMA-Sedang dan 87 siswa dari kelompok SMA-Tinggi. Adapun kelas kontrol sebanyak 86 siswa yang terdiri dari 43 siswa kelompok SMA-Sedang dan 43 siswa dari kelompok SMA-Tinggi. Untuk mengukur keterampilan proses sains tersebut diatas digunakan Lembar Kerja Siswa (LKS) yang berisi kemampuan-kemampuan yang
dikembangkan dalam praktikum, dan disusun dalam bentuk pertanyaan yang terdiri dari 9 kelompok pertanyaan dengan 15 percobaan. Hasil penelitian menunjukkan bahwa melalui pembelajaran MPP D–E–H: kemampuan “merumuskan hipotesis”, kemampuan “mengendalikan variabel” dan kemampuan “merancang percobaan” dapat dicapai secara tuntas baik pada kelompok SMA dengan prestasi akademik sedang maupun kelompok SMA dengan prestasi akademik tinggi. Untuk itu perlu diadakan diskusi dengan asisten untuk menindak lanjuti hasil rancangan yang dibuat siswa, terutama untuk mengevaluasi perencanaan alat dan bahan, serta cara kerja sehingga percobaan tersebut aman dan efisien untuk dilaksanakan.

Kata Kunci : MPP D-E-H (Model Pembelajaran Praktikum Diskriptif-Empiris Induktif-Hipotetis Deduktif), Keterampilan merumuskan hipotesis,Keterampilan mengendalikan variabel, Keterampilan merancang
percobaan.


SELENGKAPNYA, KLIK...

Toxic Responses

Toxic Responses
THE LIVER

The liver is often the first major metabolizing organ that an ingested toxicant encounters, and
it has very high metabolic activity. The major function of the liver is to metabolize, store, and
release nutrients, that is, to maintain nutrient homeostasis. When blood levels of nutrient molecules
are high, the liver can convert them to glycogen and fats and store them. When blood levels of
nutrients are low, the liver can convert some amino acids, pyruvate, and lactate to glucose, which
is released to blood. It uses amino acids to synthesize proteins that are released to blood, including
albumin, clotting factors, and transport proteins. The liver is the major site of fat metabolism and
releases fat to blood as needed. It produces bile, which acts to emulsify fats in the small intestine.
Materials enter and leave the liver as blood in arteries and veins. Nutrients, drugs, and ingested
xenobiotics absorbed from the small intestine go directly to the liver through the portal vein. The
liver has another mechanism for excretion in the form of bile discharged back to the intestines.
Bile discharge is a major route of elimination of xenobiotic compounds and their metabolites.
Insofar as toxicological chemistry is concerned, the major function of the liver is to metabolize
xenobiotic substances through phase I and phase II reactions. Because of this function, the liver is
a crucial organ in the study of toxicological chemistry. Since it processes xenobiotic chemicals,
the liver is often the organ that is damaged by such chemicals and their metabolites. Recall from
Chapter 8 that the livers of genetically susceptible individuals can be damaged by therapeutic doses
of some drugs, such as those shown in Figure 8.5. Other xenobiotics ingested accidentally can
damage the liver. Toxic effects to the liver are studied under the topic of hepatotoxicity, and substances that are toxic to the liver are called hepatotoxins . Much is known about hepatotoxicity from the many
cases of liver toxicity that are a manifestation of chronic alcoholism. Liver injury from excessive
alcohol ingestion initially hampers the ability of the organ to remove lipids, resulting in their
accumulation in the liver (fatty liver). The liver eventually loses its ability to perform its metabolic
functions and accumulates scar tissue, a condition known as cirrhosis. Inability to synthesize clotting
factors can cause fatal hemorrhage in the liver.
A wide range of substances can cause hepatotoxicity. Even an essential vitamin, vitamin A,

is hepatotoxic in overdoses, a fact that should be kept in mind by health food fans who drink large

amounts of carrot juice. Other hepatotoxins include toxins in hormones, tea (germander), and

drinking water infested with the photosynthetic cyanobacteria Microcystis aeruginosa . Each year

people are killed by eating toxic mushrooms, especially the appropriately named “death cap” mushroom,

Amanita phalloides . This fungus produces a mycotoxin consisting of seven amino acid residues, a heptapeptide called phalloidin. A great deal of information about hepatotoxicity has resulted from observed effects of pharmaceuticals, a number of which have been discontinued because of their damaging effects to the liver. An example of such a hepatotoxic compound tested as a pharmaceutical is fialuridine, which was tested during the mid-1990s as a treatment for viral chronic hepatitis B, a liver disease. Seven of 13 patients in the test developed debilitating hepatotoxicity with severe jaundice, alongbwith lactic acidosis due to accumulation of lactic acid, a metabolic intermediate. The seven patients were given liver transplants, but five of them died.

which was tested during the mid-1990s as a treatment for viral chronic hepatitis B, a liver disease.

Seven of 13 patients in the test developed debilitating hepatotoxicity with severe jaundice, along

with lactic acidosis due to accumulation of lactic acid, a metabolic intermediate. The seven patients

were given liver transplants, but five of them died. Steatosis, commonly known as fatty liver, is a condition in which lipids accumulate in the liver in excess of about 5%. It may result from toxicants that cause an increase in lipid synthesis, a decrease in lipid metabolism, or a decrease in the secretion of lipids as lipoproteins. An example of a substance that causes steatosis is valproic acid, once used as an anticonvulsant:

Other than ethanol, the xenobiotic chemical best known to cause steatosis is carbon tetrachloride,

CCl4 . This compound was once widely used in industry as a solvent, and even in consumer items

as a stain remover. As discussed in some detail in Chapter 16, it is converted by enzymatic action

in the liver to Cl3C· radical, then by reaction with O2 to Cl3COO· radical, which reacts with

unsaturated lipids in the liver to cause fatty liver. The general term hepatitis is used to describe conditions under which the liver becomes inflamed when liver cells that are damaged by a toxic substance, a substance that causes an immune response, or disease die, and their remnants are released to liver tissue. A number of toxicants can cause liver cell death. This is most damaging when it occurs through necrosis of liver cells, in which they rupture and leave remnants in the vicinity, which can lead to inflammation and other adverse effects. Dimethylformamide is a xenobiotic industrial chemical known to cause liver cell death:

A more orderly type of cell death is apoptosis, in which the cells become encapsulated and are

systematically removed from the organ. This essential housekeeping function is accomplished in

the liver by special cells called Kupffer cells. These cells perform phagocytosis , in which a solid

particle, such as a cell remnant or other foreign matter in the liver, becomes encapsulated in a

plasma membrane and incorporated into the Kupffer cell, which is then eliminated. Reduced bile output can result in an accumulation of bilirubin, a dark-colored pigment produced by the breakdown of blood heme. When this product is not discharged at a sufficient rate with bile, it accumulates in skin and eyes, giving the characteristic sickly color of jaundice. Impaired production and excretion of bile is known as canalicular choleostasis . It can be caused by a number of xenobiotic substances, such as chlorpromazine. Reduced bile output can also result from damage to bile ducts. Methylene dianiline used in epoxy resins is known to harm bile ducts.

Cirrhosis , which was mentioned in connection with chronic alcoholism above, is an often fatal

end result of liver damage. It is often the result of repeated exposure to toxic agents, such as occurs

with alcohol imbibed by heavy drinkers. Cirrhosis is characterized by deposition and buildup of

fibrous collagen tissue, which replaces active liver cells and eventually forms barriers in the liver

that prevent it from functioning. Liver tumors have been directly attributed to exposure to some toxicants. Androgens (associated with male sex hormones), aflatoxins (from fungi, see Chapter 19), arsenic, and thorium dioxide (administered as a suspension to many patients between 1920 and 1950 as a radioactive contrast agent for diagnostic purposes) are known to cause liver cancer. Arguably the most clearly documented human carcinogen is vinyl chloride, which has been shown to cause a type of liver tumor called

hemangiosarcoma , which is virtually unobserved except in workers heavily exposed to vinyl chloride.Up until about 1970, workers were exposed to high levels of up to several parts per thousand in air in the polyvinylchloride manufacturing industry. Poisoning was so common that reference was even made to “vinyl chloride disease,”characterized by damage to skin, bones, and liver. It is believed that hemangiosarcoma resulted from the action of the metabolically produced reactive epoxide generated by enzymatic oxidation

of vinyl chloride in the liver:

Stanley E. Manahan

Electrophilic Aromatic Substitution (SAr)

Electrophilic Aromatic Substitution (SAr)

In simple terms, electrophilic aromatic substitution proceeds in two steps. Initially, the electrophile E' adds to a carbon atom of the benzene ring in the same manner in which it would react with an alkene, but here the
n-electron cloud is disrupted in the process. However, in the second step the resultant carbocation eliminates a proton to regenerate the aromatic system (Scheme 2.1). The combined processes of addition and elimination result in overall substitution.

In the second step, a proton is abstracted by a basic species present in the reaction mixture. The attacked carbon atom reverts to sp2 hybridization and planarity and aromaticity are restored. This fast step is energetically favourable and is regarded as the driving force for the overall process. The product is a substituted benzene derivative. The energy changes that occur during the course of the reaction are related to the structural changes in the reaction profile shown in Figure 2.1. It should be noted that each step proceeds through a high-energy transition state in which partial bonds attach the electrophile and the proton to the ring and the n-cloud is incomplete. 

Most examples of electrophilic aromatic substitution proceed by this sequence of events: 

• Generation of an electrophile 
• The electrophile attacks the n-cloud of electrons of the aromatic ring
• The resulting carbocation is stabilized by resonance
• A proton is abstracted from the carbocation, regenerating the
• A substituted aromatic compound is formed phi-cloud

In the following sections, various examples are reviewed, highlighting the source of the electrophile and any variations in mechanistic detail.

Carbohydrate Digestion

Carbohydrate Digestion

A very simple example of a digestion process is the hydrolysis of sucrose (common table sugar),

to produce glucose and fructose monosaccharides that can be absorbed through intestine walls to

undergo metabolism in the body. Each digestive hydrolysis reaction of carbohydrates has its own

enzyme. Sucrase enzyme carries out the reaction above, whereas amylase enzyme converts starch

to a disaccharide with two glucose molecules called maltose, and maltose in turn is hydrolyzed to

glucose by the action of maltase enzyme. A third important disaccharide is lactose or “milk sugar,”

each molecule of which is hydrolyzed by digestive processes to give a molecule of glucose and one of galactose.

Digestion can be a limiting factor in the ability of organisms to utilize saccharides. Many adults

lack the lactase enzyme required to hydrolyze lactose. When these individuals consume milk

products, the lactose remains undigested in the intestine, where it is acted upon by bacteria. These

bacteria produce gas and intestinal pain, and diarrhea may result. The lack of a digestive enzyme

for cellulose in humans and virtually all other animals means that these animals cannot metabolize

cellulose. The cellulosic plant material eaten by ruminant animals such as cattle is actually digested

by the action of enzymes produced by specialized rumen bacteria in the stomachs of such animals.

Stanley E. Manahan

Copyright © 2003 by CRC Press LLC