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

Current assessment of practical work in England


1. In England practical work is often seen as central both to the appeal and effectiveness
of science education and to the development of practical skills that will be of use in
Higher Education and/or the workplace. Indeed, The House of Commons Science and
Technology Committee (2002) reported that:

In our view, practical work, including fieldwork, is a vital part of science
education. It helps students to develop their understanding of science,
appreciate that science is based on evidence and acquire hands-on skills that are
essential if students are to progress in science.
(para. 40)

2. By ‘practical skills’ we mean those skills the mastery of which increases a student’s
competence to undertake any type of science learning activity in which they are
involved in manipulating and/or observing real objects and materials.

3. In a report on the testing of practical skills in science for ages 11, 13 and 15, Welford,
Harlen and Schofield (1985) suggested, that “the assessment of practical skills may
be possible from pupils’ reports or write-ups – provided that they have actually
carried out the practical or investigation prior to putting pen to paper” (p. 51).
However, it is our opinion that practical skills are, in some cases, best assessed
directly. For example, whilst a conceptual understanding of the topology of knots
and manifolds might well be assessed by a written task the most effective means of
assessing whether a student is competent in tying their shoe laces is, we would
argue, to watch them as they attempt to tie them.

4. As such, we feel that a useful distinction can be made between what we refer to as
direct assessment of practical skills (DAPS) and indirect assessment of skills (IAPS)1
.
The former, DAPS, refers to any form of assessment that requires students, through
the manipulation of real objects, to directly demonstrate a specific or generic skill in
a manner that can be used to determine their level of competence in that skill. An
example of this would be if a student was assessed on their skill in using an ammeter
and this was determined by requiring them to manipulate a real ammeter and use it
within a circuit to take readings and for these readings to need to be within an
acceptable range for the student to be credited.

5. In contrast, IAPS relates to any form of assessment in which a student’s level of
competency, again in terms of a specific or generic skill, is inferred from their data
and/or reports of the practical work that they undertook; for example, when a
student writes up an account of the reaction between hydrochloric acid and calcium
carbonate chips in a way that the marker would not be certain if the student is
faithfully writing what they have just done or simply remembering what they have
previously done or been told about this reaction.

6. A common example of the use of both DAPS and IAPS to assess practical skill and 

conceptual understanding respectively, and one that we consider provides a useful 

analogy, is the UK Driving Test. In this example not only does the candidate have to 

demonstrate a sufficient level of competency in terms of practical driving skills out 

on the road (DAPS) but they must also pass an on-line test to assess their 

understanding of how to drive a car safely and competently (IAPS). Table 1 shows a 

comparison between DAPS and IAPS.

Improving the assessment of practical work in school science 

Professor Michael Reiss 

Institute of Education, University of London 

Dr Ian Abrahams 

Department of Education, University of York 

Rachael Sharpe 

Department of Education, University of York 

 October 2012 

http://www.gatsby.org.uk/~/media/Files/Education/Improving%20the%20assessment%20of%20practical%20work%20in%20school%20science.ashx

.... source

RESISTANCE (CONDUCTIVITY).

THE three electrical quantities which the physical chemist has most frequently to measure are resistance or its reciprocal, conductivity, current strength, and electromotive force. In other words, the three quantities involved in the equation :

I = E / R

Conductors of electricity are usually divided into two classes, though there is much doubt as to whether there is any true distinction between them: (1) those which conduct the current without suffering chemical decomposition, and (2)those which undergo chemical change when traversed by the electric current. To the first class belong the metals and carbon, while to the second belong the solutions of many substances which undergo decomposition at the poles. It is with the second class of conductors that we are chiefly concerned. These conductors are known as electrolytes, and include chiefly the solutions of acids, bases, and salts. There are many substances which in solution do not conduct the electric current, and these are known as non-electrolytes; among these may be mentioned the alcohols, the ketones, and the hydrocarbons.

Specific and Molecular Conductivity. The specific resistance of a conductor is the electrical resistance of a centimeter cube of it when the current flows through it from one face to the face opposite. Specific resistance is wholly dependent upon the nature of the conductor. Denoting the specific resistance by s', and the length and cross-sectional area of the conductor by I and a respectively, then the resistance is

Since conductivity is the reciprocal of resistance, it follows that the specific conductivity of the conductor is

 Conductors of the second class, as has been said, consist of solutions of an electrolyte in some solvent, and since liquids have no definite form it is obvious that the above definition of specific conductivity does not apply. Since the conductivity of solutions depends upon the dissolved electrolyte, we select the gram-molecular weight of dissolved substance in a litre as the basis of a definition which shall render the resistances of all solutions comparable. Consider a litre of solution containing a gram-molecular weight placed between two electrodes which are separated by a distance of 1 cm. The cross-section will be 1000 cm². This will have 1/1000 the resistance or 1000 times the conductivity of a centimetrecube of the same solution.

If v denotes the number of cubic centimetres of any solution containing a gram-molecule of dissolved substance, and s represents the specific conductivity of a centimeter cube of the solution, the molecular conductivity µ is

Where g gram-molecules of dissolved substance are contained in a litre of solution, we have as a perfectly general expression

If the specific conductivity be referred to a cylinder of solution 100 cms. in length and 0.1 cm. in cross-section, then obviously (1) and (2) become

Thus when solutions of the same concentration are employed their molecular conductivities are directly comparable.

Wheatstone's Bridge.

For the measurement Of all but very high or very low resistances the Wheatstone's bridge

 

is the most convenient. It consists of a combination of resistances. It is obvious that in the divided circuit from C to A there must be a point on the branch CDA which will have the same potential as a point on the branch CEA. Let us imagine that by means of the galvanometer G two such points have been found, and let these points be denoted by D and E. Then we have the following proportion:

From this equation it is evident that if the values of any three of the four resistances are known the other one is determined. Let us imagine the resistance-box to be inserted hi the arm R and the unknown resistance to be placed in the arm X; then we can alter the position of the point E until the galvanometer shows no deflection, and thus determine the lengths of CE and AE. Since resistance is directly proportional to the length of the conductor, it follows that the values of r₃ and r₄ are proportional to the lengths AE=l₁ and CE=l₂, or

The most convenient form of the Wheatstone 's bridge is the slide-wire-metre bridge, Fig. 67. In this form of bridge

the conductor AEC, corresponding to the similarly lettered portion of Fig. 66, is made of a thin uniform wire one metre long, the point E being determined by a sliding contact which moves over a millimetre scale. The arms CD and DA of the bridge consist of heavy copper straps which offer inappreciable resistance. The lettering in the two diagrams being the same, theMatter becomes self-explanatory. A single determination of the position of the index is not reliable owing to variations in the size of the wire and to lack of precision hi determining the point of balance. For these reasons the mean of a series of observations should be taken. When a direct current is passed through the solution of an electrolyte bubbles of gas appear on the electrodes after a very short time, or, as we say, polarization sets in. Polarization causes a counter E.M.F., which makes the accurate measurement of conductivity an impossibility. This difficulty has been overcome by Kohlrausch, who introduced the use of the alternating current.

The alternating current is furnished by a small inductorium, the wires from the secondary of which are connected with the ends of the bridgewire. Since the galvanometer cannot be used with the alternating current, it is replaced by a telephone. The inductorium is best placed in another room from that in which the bridge is placed, so that the sound of the coil can only be heard through the telephone. The sliding contact is then moved along the bridge-wire until a point is found where the sound of the coil either entirely vanishes or attains a minimum of intensity. This point is the position of balance between the arms of the bridge.

Before the Wheatstone's bridge is used the wire should be carefully calibrated. Of the several methods in use for this, that of Strouhal and Barus is best adapted to the physico-chemical laboratory.

LABORATORY EXERCISES PHYSICAL CHEMISTRY

BY

FEEDEKICK H. GETMAN, PH.D.,

Fellow by Courtesy of The Johns Hopkins University,

Carnegie Research Assistant.

Handbook of Chemistry and Physics, CRC Press, Inc., 2000 Corporate Blvd. N. W., Boca Raton, FL 33431.Comments may also be sent by electronic mail to drlide@post.harvard.edu.

ENERGY EQUIVALENT