Sunday, July 21, 2019
English Present: Simple And The Present Progressive
English Present: Simple And The Present Progressive Introduction: As a teacher of English to Arabic speaking students I have encountered a number of specific difficulties Arab students have in mastering the English language. In this paper, I would like to focus on a particular grammatical problem they have in the area of verb tenses because, of all the mistakes that my students make, mistakes with verbs and verb tenses impede communication to the greatest degree. The specific problem I will attempt to look at the area of verbs is the problem that Arabic speakers have in using and confusing the present progressive. I will base the evidence for these mistakes on actual writing errors that Arabic students have made. Mistakes such as I am live in Abu Dhabi. come up frequently in my students writing. This paper is basically a contrastive analysis since I feel that the majority of my students problems in this area come from mother tongue interference. However, as will be noted below, this does not mean I rule out other sources of errors such as intralingual errors. The following is the outline of this paper: In the first section of this paper, I will describe the various aspects of the grammatical structure of the present simple and the present progressive in the English language. In the second section of the paper, I will contrast the grammatical structure of the present simple and the present progressive with its Arabic counterparts. I will show how Arabic has structures that vary significantly and radically from their English counterparts. In the third section, I will introduce a number of examples takes from students written work and give an indepth analysis of the possible sources of the errors, mainly with respect to mother tongue interference, but also looking at some possible intralingual sources for these errors as well. Finally, in the last section, I will attempt to suggest a general theoretical approach to dealing with such problems Part One: A grammatical description of the English Present Simple and the Present Progressive: The simple present tense As we already know, the simple present of every verb (with the exception of the verb BE, which I will not be dealing with as a grammatical description since it is not the specific focus of this paper) is identical in every person with the basic unmarked base form of the verb except for the third person forms he, she and it to which we generally add s or es (Quirk 1985, p.98). However, numerous irregularities arise in the spelling and pronunciation of this third person form (Leicester 1998, 12.12)(Thomson 1986, p. 150). Questions are formed by using the auxiliaries do, does, in the present, and did in the past by putting all these before the subject. Negation is formed in the same way using dont (or do not) and doesnt, (or does not) in the present, and didnt (or did not) in the past. These forms go after the subject. In addition, the verb must be changed to the basic form. The simple present is used for statements that are always true, (e.g. The earth revolves around the sun.) (Azar 1989,p.2). The simple present is also used for events, actions or situations which are true in the present period of time and which, for all we know, may continue indefinitely, (e.g. Fatima goes to school at Zayed University.) (Azar 1989, p.2) What we are saying in these expressions is that this is how things stand at the present moment (Huddleston 1984, p.81). A further use of the simple present is for actions that are habitual, things that happen repeatedly, (e.g. We study a lot.) (Alexander 1988, p.163)(Quirke 1985, p.107). Observations and declarations are another use of the present simple, as in the sentence (It says here that there is a new night club opening.)(Alexander 19988, p.163). The present simple can also be used to express the future, especially when we want to express strong certainty, (e.g. When we graduate, we will get jobs.). Swan, Huddleston, Lewis, Thomson and Quirke, et. al. also add eight other functions of the present simple which might come up in other contexts such as: Demonstrations and commentaries (e.g. First, I take a bowl and break two eggs in it, thenÃ ¢Ã¢â ¬Ã ¦..) The structures here comes and there goes, (e.g. here comes your husband.) Promises and oaths (e.g. I promiseÃ ¢Ã¢â ¬Ã ¦Ã ¢Ã¢â ¬Ã ¦., I swear Ã ¢Ã¢â ¬Ã ¦Ã ¢Ã¢â ¬Ã ¦, He deniesÃ ¢Ã¢â ¬Ã ¦..) Formal correspondence (e.g. We write to advise you.) Instructions (e.g. You go left, turn rightÃ ¢Ã¢â ¬Ã ¦.) Stories (e.g. In act one, Hamlet meets the ghost of his father.), which Huddleston calls the historic present. In expressions of understanding such as hear, see, gather (e.g. I hear youre getting married.) Finally, the simple present can be used in newspaper headlines (e.g. RUSSIANS RAISE OBJECTIONS) Since there are so many instances of when to use the present simple, is there any way to summarize all of these? I concur with Lewis explanation that the present simple: 1-Expresses an event as a total single point in time. 2-Expresses an event as a matter of fact. 3-Expresses an event as immediate rather than remote. The present progressive tense Both the simple and progressive forms usually tell us that an action takes place. But the progressive forms also tell us that an activity is or was, or will be, etc. in progress, or thought of as being in progress. In other words, the present progressive tells us that the speaker sees an action as taking place over a period of time as opposed to a point in time. In addition researchers would add that the speaker sees the period as limited (Lewis 1986; Leech, 1975; Huddleston, 1984; Quirke, 1985). The present progressive tense is formed with the present of be (am/is/are) (which adds aspect and voice), said by Quirke to be the finite verb, plus the ing form (the non-finite form) (Quirke 1985, p. 120). There are no complications with the additional ing form; however the spelling of the ing has some irregularities and needs to be taught to students e.g. write, writing; run, running; begin, beginning; lie, lying). (Alexander 1988; Huddleston 1984; Quirke 1985). Question formation takes place by switching the place of the auxiliary be and the subject. Negation is achieved by inserting not between the subject and the auxiliary or by contracting nt with the auxiliary verb forms (with the exception of the first person singular form am) (Quirke, 1985). In the classroom, the classical reason given for why we use the present progressive is that it shows an uncompleted action in progress at the time of speaking. To emphasise this, we often use adverbials like now, at the moment, just, etc. For example, Hes not home at the moment, hes working. (Quirke 1985). The present progressive can also be used to describe actions which have not been happening for long, or are thought of as being temporary situations, and which are going on around now, e.g. Abdullah is living with his aunt until he can find a place of his own.. A further use of the present progressive is to refer to activities and events planned for the future. We generally use adverbials in such sentences unless the meaning is clear from context, e.g. Were spending next Thursday in Abu Dhabi. (Azar 1989; Huddleston 1984; Quirke 1985). The present progressive can also be used to talk about developing and changing situations, e.g. That child is getting bigger all the time. (Swan 1980). Sometimes the present progressive can be used to talk about feelings, such as I am feeling fine. or My back is hurting me.. The present progressive is used to show repeated actions along with adverbs such as always, constantly, continually, forever, perpetually, and repeatedly, such as He is always helping people.. In this sense it conveys not temporariness, but continuousness. (Leech 1975; Huddleston 1984). The present progressive also is used to show repeated actions that are happening around now, e.g. He is studying a lot of English these days. Why is he going to the library? (Swan 1980). Dynamic versus Stative Verbs in the present simple and the progressive tenses Dynamic/progressive verbs refer to verbs which show actions which are deliberate or voluntary, e.g. Im building a house., or changing situations, e.g. Hes becoming fat.. Dynamic verbs can be used in both the progressive as well as the simple forms e.g. I eat at 5:00 (everyday). as opposed to Im eating now.. Stative verbs (also known as non-progressive verbs) are verbs which indicate a state, condition or experience. Specifically, stative verbs fall into categories such as feelings (like, love), thinking/believing (think, know, realize), wants and preferences (need, want), perception and the senses (smell, see), and being, seeming, having, and owning (seem, look, appear). Stative verbs are generally not used in the progressive forms (Quirke 1985). However some stative verbs can be used in both the present simple and the progressive tenses, which results in a different meaning in each form, e.g. Im thinking of a solution. as opposed to I think he is the best man for the job. or These flowers smell good. as opposed to Latifa is smelling the flowers in the garden.) (Alexander 1988; Azar 1989; Azar 1986; Quirke 1985). The present simple versus the present progressive Swan makes note of a number of areas where students might confuse the present simple with the present continuous. A. We use the simple present to talk about things that are true for the present period of time, or, as was noted above, to say this is how things stand at the present moment for the foreseeable future. However, if the event is temporary and is taking place right now, we use the present progressive. Afrah studies at the Higher Colleges. Afrah is studying her English lesson. B. We use the present progressive to talk about habitual actions if these are happening around the moment of speaking. Fayrouz and Fatima are preparing for the Eid holidays. However, if the habitual action is not closely connected to the moment of speaking, we generally use the present simple. I go to Saudi Arabia once every three years. C. Verbs that refer to physical feelings can sometimes be used in either the simple present or the present progressive. I feel great! or Im feeling great! My head hurts. or My head is hurting. (Swan 1980). PART TWO A grammatical description of the Arabic present simple and the present progressive In this part of the paper, I would like to give readers a very brief background of the Arabic verb system in regard to the simple present and the present progressive. The Arabic verb system is very complicated. However, this does not mean that a teacher has to master the Arabic language before s/he is able to pinpoint errors that may be a result of the interference of Arabic in English. One can study the Arabic language with the goal of simply understanding the structure, rather than with the goal of speaking and writing in the language. Let us first look at the present simple, then the present progressive, and finally the verb to be since all of these grammatical items are specifically relevant to the particular problem at hand. A.The Present Simple In Arabic, the formation of the present simple is radically different from English, since Arabic uses a root system made up of the three most important consonants (though two or four consonant roots do sometimes occur). In Arabic the three basic consonants (the root) stay the same but it is by changes in the vowels, the suffixes and the prefixes that tense and number are indicated. It is vastly more complicated than the way some English verbs change tense by changing vowels, e.g. give, gave. For example, the sentence, he learns could be represented phonetically by ya-droo-soo. The d-r-s is the root, ya is the part that indicates this is a third person singular masculine verb (though this is not the pronoun). The pattern of the vowels and consonants (ya + c1 + c2+ oo + c3 + oo), lets the speaker know that this is the present tense. In contrast, the past could be represented by a different pattern; hence, he learned, dar-ah-sah has the pattern (c1 + ai or ah + c2 + ai or ah + c3 + ah) (and this is just one pattern out of ten!) From a sentence point of view the verb in Arabic is not necessarily treated as the nucleus of a sentence and, in the case of the copula verb BE, can be omitted entirely (as we shall see below). The verb can also be placed at the beginning of the sentence. Like its English counterpart, the present simple tense in Arabic expresses a habitual action. There are other functions, but they are not relevant to this discussion. B.The Present Progressive In general, the present simple form is also used in Arabic to express the idea of a continuous action occurring in the present. Hence, the English sentence He is working now. in Arabic becomes He works now. (represented phonetically by huwwah yaamaloo al eyn.) What is he doing? in Arabic becomes What does he do? (represented phonetically by mehzah yafaaloo al eyn?) Hence, in almost all cases, the present simple form is used to show the idea of continuous action in the present. However, there is a single verb form in Arabic called the ism-ul-fail which is the exact parallel to the idea of continuous action. However, the difference in Arabic is that the ism-ul-fail is used very sparingly compared to English and then only for some very specific verbs of movement, or verbs that indicate changing from one state to another (going up, going in, going down, walking to a place, leaving a place, etc.). Since the ism-ul-fail is radically different in form from the English progressive it is doubtful that any interference in form occurs. C.A Few Points About The Verb BE as a Copula Although BE as a copula is not the focus of this paper, it does deserve mention here for two specific reasons. The first point is that BE in Arabic, when it is the copula in the present tense, is unwritten and unspoken (although this is not true of the copula in the past tense or the future where it is written and spoken). (Kharma, 1989, p. 89). For example, the literal translation of the sentence Ahmed is a student. is Ahmed student.. So it is conceivable that students might leave BE out as a copula OR as the helping verb in the present progressive because it does not exist in the present tense in Arabic (although there are other additional reasons why students might forget to add it to the present progressive as we shall see). The second point is that BE is used so often in English, in so many different kinds of structures, and that it is so irregular, that it might simply add to the confusion of students (Kharma 1989, p. 161). Students who keep on being corrected for leaving out the verb to be when it is necessary, may for example, hypercorrect themselves and start to write it everywhere. Again, we shall explore this issue further below. PART THREE A look at some common written errors made by Arabic speaking students when using the English present simple and present progressive Finding the exact causes of any error can be a difficult and meticulous task. This is partly because there may be multiple reasons as to why students make one particular error and these causes may also overlap at any given time. In addition, it is extremely problematic, even for a native speaker of both Arabic and English, (which I am) to know exactly what is going on linguistically in the mind of a student when s/he makes such an error. However, having said that, even with these obstacles, we can at least make some good hypotheses and lists of possibilities as to why these errors occur with our own students. As a result, we will be able to generate classroom strategies and methods in order to correct and remedy these sorts of mistakes. The following categories of errors are the most common that I have found in students written work with regards to the simple present versus the present progressive. I will look at each category in turn, and offer an analysis of the sources for these types of error. Category One Fatima studies now. Ahmed does his homework now. In these sentences, the intention of the Arabic speaking writer seems to be to convey the meaning of what in English would be a present continuous action, expressed by the present continuous tense. This is clear by the use of the adverb now or in the case of other examples not shown here, from other adverbs or the context of the sentence. In examples one and two, the Arabic speaker seems to be transferring the rules of his native language into English. The Arabic speaker usually uses only the present simple to express events that would be expressed in English by both the present simple and the present continuous. Category Two Mariam cant talk, she eating now. This kind of mistake is a bit more problematic in terms of analysis. It could be that the Arabic speaker, feeling that the full meaning of the action is expressed in the verb with the ing, has decided that the am/are/is forms are redundant and unnecessary. It could also be the case that this mistake is a direct transfer of a particular grammatical form in Arabic. In certain cases Arabic speakers do express the present continuous with a verb and prefix change (called ism-ul-fail), but without the corresponding be form. For example, the literal translation of the sentence Ahmed is running. is Ahmed running. . Category Three Are you knowing the way to Dubai? I am wanting to see my family. In this case, the student has learned the present progressive form, but is over generalizing it to all verbs (or perhaps does not remember or has not been taught the rules for exceptions such as the above). These types of errors could very well be intralingual. This over generalization could also be found in sentences that have the function of explaining, demonstrating teaching or narrating such as: Next I am pouring the oil into the cooking pan. Ali is passing the ball to the goalkeeper. Category Four I am live in Abu Dhabi. We are study English. This category is probably the most difficult to analyze. This is because it is unclear whether the Arabic speaker is making the mistake of adding the additional am/is/are form while trying to use the present tense, or making the mistake of forgetting to use the present participle while trying to use the present progressive tense. That is, did the speaker intend to say I live in Abu Dhabi. and use the extra am form by mistake, or did s/he intend to say I am living in Abu Dhabi. and forget the correct present participle form? Of course, there are other possibilities but these seem like the two most likely. We must obviously look at the context of the paragraph to see if we can get the gist of what the speaker meant. The following is a more detailed analysis of these two possibilities from the standpoint of the students reasoning. 1. If we believe from the context that the student was trying to use the present simple and added the additional am in error, then the following analyses apply: A. The student may be confused by the lack of inflectional endings in English, since Arabic is a highly inflected language, and every personal pronoun has a distinct corresponding inflected verb form. The similarity of the verb forms in I live, you live, etc. may seem very awkward to the Arabic speaker. Hence, they may want to remedy the situation by distinguishing the verb forms in some way by, for example, adding an exceedingly familiar and overused verb form like am, are, or is. B. The student may be over generalising based on what they have learned about the present continuous. That is, they may have learned how to form the present continuous quite easily since there is no mother tongue interference from Arabic, (although they may not have mastered its use). They then may go on to conclude that every verb in the present simple or present continuous in English needs to be preceded by am/is/are. C. Similarly, the student may be hypercorrecting. They may have been corrected so many times for forgetting to use the verb BE in their sentences e.g. Ahmed happy, that they may start to feel that every sentence needs the verb BE. 2. However, if we believe that the student was trying to use the present continuous tense and used the present simple live (instead of the present participle living), then the following analyses apply: A. The student may not have correctly understood how to form the present participle by adding ing to the end of the verb. B. Perhaps students have simply forgotten to add the ing prefix because the structure is so different in their language. This is by no means an exhaustive analysis. However, these are, from my experience and collaboration with other colleagues, both native and non-native speakers, some of the major possibilities. PART FOUR Pedogogical implications of the above research for teaching the present simple and the present progressive to Arabic speaking students From the evidence I have presented here, I believe it is clear that many of the mistakes in using the present simple and the present progressive in form (such as omission of the verb to be in the simple present for Arabic speakers, e.g. I studying), as well as other mistakes in usage (e.g. using the simple present when the present progressive is required) seem to be traceable directly to Arabic mother tongue interference. Based on my analyses, reading and discussion with colleagues, I do feel that in this particular area, teachers of EFL to Arabic speakers must consider mother tongue interference as a major impediment to learning the present tense versus the present progressive. If we know that mother tongue interference is the cause of many errors, what should this imply for our teaching? One thing which I think it does not imply is that we teach English from the point of view of the mother tongue. For example, trying to get students to understand English grammar through word for word translations or using the grammatical structure of Arabic to help students to understand the grammatical structure of English are only useful in certain cases, and then only by someone who is a master of both languages. My experience in reading the research, being bilingual and talking to Arabic speaking students who are at the final stages of their English studies leads me to believe that, at least in the case of Arabic and English, that the two languages are sufficiently different that they are both best looked at in their own respective grammars. Students must be made, not only to think in English, but to understand English grammar in terms of English grammar without constantly switching back and forth to compare it with Arabic. Such practices are ineffective and will cause confusion among students. As Lewis says students should never expect the foreign language to be like their ownÃ ¢Ã¢â ¬Ã ¦..the fact that English has verb forms that contain [be] as an auxiliary does not suggest that other languages ought to have a corresponding formÃ ¢Ã¢â ¬Ã ¦.students should be positively encouraged to explore the foreign language within itself rather than through the expectations they bring from their own. (Lewis, 1986, pp. 164-165). In addition, I should add that intralingual factors can also be at work when students make such errors (in addition to context specific factors like student motivation, teaching style and competence, etc.). For example, on the intralingual side, we know that students of ESL from many different language groups and even children make common mistakes with the verb to be. Therefore, many such mistakes might be intralingual. (Mattar 1989). Hence, when we try to analyse our students errors we should not be prejudiced to any one theory and we should try to be open to looking at all possible sources of errors. What we as teachers should be doing in the classroom is continually collecting research on student errors and student learning styles in order to form hypotheses about why such errors occur and why such one approach worked and another didnt. We should then be trying to test these hypotheses to see if they are true or not, and afterwards share this information w ith other teachers in similar situations. Only then will we be able to understand why students make errors and what is the most effective way to correct them. Cell Membrane: Structure And Function Cell Membrane: Structure And Function A cell is a dynamic and a complex structure surrounded by a membrane known as the plasma membrane. This acts like a barrier between the inside of the cell and the outside resulting in different chemical environments on the two sides. The cell membrane is not restricted to the outer surface but is also present inside surrounding the organelles. These biological membranes have played a crucial role in the evolution from prokaryotes to multicellular eukaryotes. In prokaryotes, there is only one type of membrane present i.e. the plasma membrane but the unicellular eukaryotes have intracellular membranes compartmentalizing its contents into different functional chambers known as organelles. Each organelle though performs its own specific function, they cross-talk with each other via these membranes in order to work as a unit. Further, different cells in multicellular eukaryotes communicate with each other through these membranes. The membrane, therefore serves a dual purpose of both protecting the interior of the cell from its external environment and also provides a communication interface between the cell and its surroundings or other cells. The diverse functions performed by biological membranes can be attributed to the molecular composition and structure of these membranes. Models for Cell Membrane Structure It took almost a century to develop the present accepted model of a cell membrane based on various physiological and biophysical studies. Physiological experiments involving the transport of molecules and ions across the membrane by Overton in 1899 suggested that the membrane is composed of lipid molecules. Later, Langmuir (1917) showed that lipids when spread on water using Langmuir trough form a monomolecular layer on the surface of the water by calculating the area per lipid molecule. The hydrophobic tails of these lipids were bent and protruding out from the surface of the water. When this method for measuring the area per lipid molecule was applied to the lipids extracted from the known amount of erythrocyte membranes, Gorter and Grendel (1925) concluded that the lipids exist as a bilayer and not a monolayer in a membrane giving birth to the lipid- bilayer membrane model. In 1935, Danielli and Davson elaborated on the model based on the studies measuring the surface tension that membranes are made up of phospholipid bilayer sandwiched between two protein layers. Based on optical imaging of membrane morphology using electron microscopy, Robertson argued that the basic structure of all the membranes is same and proposed the Unit Membrane Model in 1959 . Several other studies [review or book] suggested that the lipid bilayer has fluid-like properties with lipids and proteins floating in it. Studies of proteins present in erythrocytes membrane and that extracted from other membranes led Singer and Nicolson to classify membrane proteins as peripheral and integral proteins; and finally proposing the Fluid Mosaic Model in 1972 . This is the most accepted model describing the structure of a cell membrane. According to this model, mosaic of protein molecules is embedded within the fluid of lipid bilayer which is supported by the freeze-fracture studies of the plasma membrane (Figure). Composition of Membranes Membrane Lipids The lipid bilayer is only 5 to 10nm thick organized in distinct regions primarily attributed to the hydrophobic effect caused due to the amphipathic nature of these molecules with both polar and the non-polar regions (Figure). The interactions of these regions with the aqueous environment have been studied with various techniques like x-ray reflectometry, neutron scattering[2HYPERLINK http://en.wikipedia.org/wiki/Lipid_bilayer#cite_note-Zaccai1975-1] and nuclear magnetic resonance. In order to exclude the non-polar regions from the aqueous environment, lipid molecules arrange in such a manner so that the hydrophobic tails point inwardly towards each other and the polar head groups are exposed on the outside facing the water. The outermost region on either side of the bilayer is completely hydrated and is typically around 8-9Ãâ¦ thick. The hydrophobic core of the bilayer is typically 3-4ÃâÃ nm thick. The intermediate region is partially hydrated and is approximately 3 Ãâ¦ thick. These lipid molecules arrange spontaneously naturally or artificially in solution to form structures like micelles and liposomes (Figure). Micelles are monolayer spherical structures formed by lipid molecules in aqueous environment. On the other hand, liposomes are concentric bilayer of fluid-filled vesicles surrounding the water compartment on both the surfaces. The membrane of the animal cells is composed primarily of three major types of lipids: phospholipids, glycolipids and cholesterol with phospholipids being the most abundant (Figure). The polar head groups of these phospholipids contain a phosphate group and either a glycerol (known as phosphoglycerides) or sphingosine. There are four major phospholipids present in the animal cells, three are phosphoglycerides namely phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine; and the fourth sphingomyelin is the only sphingolipid. The heads of glycolipids contain a sphingosine with one (known as cerebroside) or more sugars (known as ganglioside) attached to it. Cholesterol is a sterol molecule with a small hydrophilic hydroxyl group and a rigid ring structure that stabilizes the bilayer. Membrane Proteins The membrane consists of different types of proteins accounting for 25-75% of the mass of the membrane and are categorized based on their interactions with the lipid bilayer (Figure). Moreover, the manner in which a protein is associated with the membrane is indicative of its function. Integral or intrinsic proteins are embedded with in the lipid bilayer. These could be transmembrane proteins spanning the entire length of the bilayer and possess hydrophobic domains which are anchored to hydrophobic lipids and hydrophilic domain interacting with external molecules. They could have only one membrane-spanning (single pass transmembrane proteins, e.g. glycophorin) or multispanning (multi-pass transmembrane proteins, e.g. band3 protein of erythrocyte) segments. The transmembrane segments have helical e.g. bacteriorhodopsin or ÃÅ½Ã ²- barrel structures. These proteins can be extracted from the phospholipid bilayer only by disrupting the hydrophobic interactions by using detergents like S DS or Triton-X 100. Peripheral or the extrinsic proteins, on the other hand, are loosely bound to the hydrophilic lipid and protein groups on the surface of the membrane by weak ionic interactions. These can be easily removed with high salt or extreme pH without disrupting the phospholipid bilayer. Lipid-anchored proteins are covalently bound to lipid molecule which in turn anchors the protein in the cell membrane. The lipid can be phosphatidylinositol, a fatty acid or a prenyl group. Membrane Carbohydrates Carbohydrate moieties are present on the non-cytoplasmic surface of the membrane covalently attached to either protein or lipid molecules forming glycoproteins or glycolipids. These carbohydrates help in orientation of protein molecules on the cell surface and sorting in cellular compartments. The glycocalyx or the cell coat is the layer of carbohydrates on the cell surface that protects it and participates in the cell-cell interaction. The carbohydrates of the glycolipids of the erythrocytes membrane determine the ABO blood groups in human. Fluidity of Membranes Under physiological conditions, phospholipid molecules in the membrane are in the liquid crystalline state and the molecules are not physically attached to each other so, they can move within the bilayer. These movements could be within a monolayer i.e. rotational and lateral or between two layers i.e. flip-flop. Flip-flop movements are rare and slower compared to the other two as it requires energy for a lipid molecule to traverse from one layer to the other. Besides, some proteins also move in the membrane as concluded from studies based on human-mouse cell hybrids produced by fusion of human and mouse cells [Frye and Michael Edidin in 1970] and FRAP (fluorescence recovery after photobleaching) experiments (Figure). Fluidity in the cell membrane is attributed to its lipid composition. The cis-unsaturated fatty acids with kinks in their hydrocarbon tails and shorter lengths of the tails increase the fluidity by preventing the ordered packing of phospholipids in the bilayer. Cholesterol molecules present in the bilayer affects its fluidity differently at different temperatures because of its rigid ring structure. It reduces the fluidity by decreasing the movement of adjacent phospholipids but at low temperatures, it increases the fluidity by preventing solidification [Alberts]. Fluidity of the membrane allows different molecules like proteins to interact with each other to perform various processes like transport of molecules and cell signalling. Moreover, membrane fluidity is required for various cellular processes like cell movement and cell division. Asymmetry of Membranes The two leaflets, that is, the inner and the outer monolayer portions of the lipid bilayer differ in their physical and chemical properties. This is due to the asymmetric organization of the various components of the membrane. For example, glycolipids and glycoproteins are always present on the non-cytoplasmic surface of the plasma membrane. Membrane regions differ in their lipid composition. The outer leaflet contains predominantly phosphatidylcholine and sphingomyelin whereas, the inner leaflet contains phosphatidylethanolamine and phosphatidylserine. The inner leaflet also consists of phosphatidylinositol which play a key role in the transfer of stimuli from the plasma membrane to the cytoplasm [Cooper]. The membrane proteins also differ in their distribution in the two leaflets. For example, spectrin and ankyrin are present on the inner surface of the erythrocytes membrane forming a fibrillar membrane skeleton. GPI-anchored proteins ar present on the external surface of the membr ane. The asymmetry of the membrane suggested different roles played by the components of the membrane present on the two surfaces (Figure eg intestinal epithelial cell membrane:Tight junction , lateral movements). Lipid Rafts The plasma membrane of eukaryotic cells have specialized regions known as lipid rafts which differ in their composition from the rest of the membrane. These detergent-resistant and heterogeneous microdomains are rich in cholesterol, sphingolipids and certain proteins. Lipids in these rafts are more highly ordered and tightly packed as compared to the rest of the lipid bilayer. Various studies have attributed diverse roles like in transport of cholesterol, endocytosis, signal transduction, intracellular trafficking and neural development and function to these lipid rafts. Caveolae is an example of lipid rafts which are the invaginated domains in the plasma membrane. In caveolae, a protein caveolin is associated with the cholesterol in the lipid raft. It plays roles in membrane internalization and cell signaling. [Pike et al, 2002; Wary et al, 1998; Huang et al, 1999; Rothberg et al, 1992] (Review: Razani Lisanti, 2001. Exp. Cell Research 271: 36-44). Might not in endocytosis [Thompsen et al, 2002] see lipid rafts 4 references Functions of membranes: Membranes act as boundaries between the cell and its environment and are essential for maintaining the integrity of the cell and the various membrane-bound organelles within the cell, regulating the transport of materials into and out of the cell, responding to external and internal stimuli, and cell-to-cell recognition. The proteins present on the inner surface of the plasma membrane provides shape and maintains the integrity of the cell by anchoring the cytoskeleton found underlying the cell membrane in the cytoplasm. The major component of the cytoskeleton of the most well studied erythrocyte membrane is the fibrous protein spectrin. This protein interacts with other peripheral proteins like ankyrin, actin and tropomyosin. Dystophin, a member of the spectrin family is found in the membrane skeleton of muscle cells. The importance of these proteins is suggested by the fact that mutations in dystrophin leads to muscular dystrophy. Regulated transport of materials across the membrane is due to the amphipathic nature of the lipid bilayer. Therefore, the membranes are selectively permeable and the ability of a molecule or ion to traverse the bilayer depends majorly on its polarity and also on the size. Non-polar molecules like O2, N2 and benzene and small polar molecules like H2O, glycerol, urea and CO2 can pass the membrane but large uncharged (e.g. glucose), polar molecules (e.g. sucrose) and ions (e.g. H+, Na+, HCO3, ClÃ ¢Ã¢â ¬Ã¢â¬Ë) are not able to diffuse easily across membranes. Hence, various mechanisms are required for transport of materials across the membrane, including simple diffusion, facilitated diffusion and active transport for micromolecules and exocytosis and endocytosis for macromolecules. In simple diffusion, substances diffuse down their concentration gradient. In facilitated diffusion, movement of molecules down the concentration gradient is facilitated by channel and carrier proteins (e.g. glucose transporter). On the other hand, active transport requires energy to move solutes against their gradients and can be classified into primary or secondary active transport depending on the source of energy. The primary active transport depends on the hydrolysis of ATP and is of different types: P (e.g. Na+ K+ ATPase, Figure), F and V types and the ATP-binding cassette or ABC transporters. In secondary active transport, specific solute indirectly drives the active transport of another solute and does not involve the hydrolysis of ATP. Secondary active transport may include either symport (e.g. Na+/Glucose transporter) or antiport (ClÃâÃ -/HCO3 exchanger). The macromolecules such as proteins and polysaccharides are transported by endocytosis (from inside the cell to th e outside) and exocytosis (from outside into the cell) (Figure). Cell membrane is also involved in cell-cell communication. Specialized membrane structures like gap junctions in animals and plasmodesmata in plants provide the cytoplasmic continuity between cells. Tight junctions and desmosomes help in attachment of a cell to other cells or the extracellular matrix forming tissues. Membrane also maintains cell potential by creating chemical and electrical gradient. Cell signaling: Signals through chemical messengers (chemical or electrical stimuli) acting on the membrane receptors most of them being proteins. These signals are then transduced in the cell leading to a cascade of events in the cell. Specific for different cells like Gprotein, Tyrosine-kinase receptors Peripheral proteins act as enzymes e.g. and receptors In summary, biological membranes are the complex and dynamic structures composed of variety of proteins embedded in the fluid of the lipid bilayer. The amphipathic nature of the lipid bilayer and the diversity of membrane proteins are responsible for the involvement of biological membranes in large number of cellular processes.