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- July 19th, 2012, 04:27 PM #701
^i think its B isn't it?July 19th, 2012, 10:49 PM #702
The Cell Membrane |
The cell membrane functions as a semi-permeable barrier, allowing a very few molecules across it while fencing the majority of organically produced chemicals inside the cell. Electron microscopic examinations of cell membranes have led to the development of the lipid bilayer model (also referred to as the model). The most common molecule in the model is the , which has a polar () head and two nonpolar () tails. These phospholipids are aligned tail to tail so the nonpolar areas form a hydrophobic region between the hydrophilic heads on the inner and outer surfaces of the membrane. This layering is termed a bilayer since an electron microscopic technique known as freeze-fracturing is able to split the bilayer, shown in Figure 2.Cholesterol is another important component of cell membranes embedded in the hydrophobic areas of the inner (tail-tail) region. Most bacterial cell membranes do not contain cholesterol. Cholesterol aids in the flexibility of a cell membrane.
Figure 2. Cell Membranes from Opposing Neurons (TEM x436,740). This image is copyright Dennis Kunkel at , used with permission.
Proteins, shown in Figure 2, are suspended in the inner layer, although the more hydrophilic areas of these proteins "stick out" into the cells interior as well as outside the cell. These proteins function as gateways that will allow certain molecules to cross into and out of the cell by moving through open areas of the protein channel. These integral proteins are sometimes known as gateway proteins. The outer surface of the membrane will tend to be rich in glycolipids, which have their hydrophobic tails embedded in the hydrophobic region of the membrane and their heads exposed outside the cell. These, along with carbohydrates attached to the integral proteins, are thought to function in the recognition of self, a sort of cellular identification system.
The contents (both chemical and organelles) of the cell are termed protoplasm, and are further subdivided into (all of the protoplasm except the contents of the nucleus) and nucleoplasm (all of the material, plasma and DNA etc., within the ). #yesJuly 19th, 2012, 10:50 PM #703
The nucleus |
The nucleus, shown in Figures 6 and 7, occurs only in cells. It is the location for most of the nucleic acids a cell makes, such as DNA and RNA. Danish biologist Joachim Hammerling carried out an important experiment in 1943. His work (click for a diagram) showed the role of the nucleus in controlling the shape and features of the cell. Deoxyribonucleic acid, DNA, is the physical carrier of inheritance and with the exception of DNA (cpDNA and mDNA, found in the chloroplast and mitochondrion respectively) all DNA is restricted to the nucleus. Ribonucleic acid, RNA, is formed in the nucleus using the DNA base sequence as a template. RNA moves out into the cytoplasm where it functions in the assembly of proteins. The is an area of the nucleus (usually two nucleoli per nucleus) where are constructed.July 19th, 2012, 10:52 PM #704
contain their own DNA (termed mDNA) and are thought to represent bacteria-like organisms incorporated into eukaryotic cells over 700 million years ago (perhaps even as far back as 1.5 billion years ago). They function as the sites of energy release (following glycolysis in the cytoplasm) and ATP formation (by ). The mitochondrion has been termed the powerhouse of the cell. Mitochondria are bounded by two membranes. The inner membrane folds into a series of , which are the surfaces on which adenosine triphosphate (ATP) is generated. The matrix is the area of the mitochondrion surrounded by the inner mitochondrial membrane. Ribosomes and mitochondrial DNA are found in the matrix. The significance of these features will be discussed below. The structure of mitochondria is shown in Figure 18 and 19.Mitochondria and endosymbiosis
Figure 19. Muscle Cell Mitochondrion (TEM x190,920). This image is copyright Dennis Kunkel at , used with permission.
During the 1980s, Lynn Margulis proposed the theory of endosymbiosis to explain the origin of mitochondria and chloroplasts from permanent resident prokaryotes. According to this idea, a larger prokaryote (or perhaps early eukaryote) engulfed or surrounded a smaller prokaryote some 1.5 billion to 700 million years ago. Steps in this sequence are illustrated in Figure 20.Instead of digesting the smaller organisms the large one and the smaller one entered into a type of known as , wherein both organisms benefit and neither is harmed. The larger organism gained excess ATP provided by the "protomitochondrion" and excess sugar provided by the "protochloroplast", while providing a stable environment and the raw materials the endosymbionts required. This is so strong that now eukaryotic cells cannot survive without mitochondria (likewise photosynthetic eukaryotes cannot survive without chloroplasts), and the endosymbionts can not survive outside their hosts. Nearly all eukaryotes have mitochondria. Mitochondrial division is remarkably similar to the prokaryotic methods that will be studied later in this course.
Figure 20. The basic events in endosymbiosis. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates () and WH Freeman (), used with permission.July 20th, 2012, 06:47 PM #705
if any one have links of essential words of MCAT with their meaning and synonyms.please tell meJuly 20th, 2012, 07:18 PM #706
- Member Since
- Dec 2011
- 5 times
you mean english vocabularyJuly 20th, 2012, 07:25 PM #707
- Member Since
- May 2012
- 28 times
cJuly 20th, 2012, 08:18 PM #708July 21st, 2012, 01:24 AM #709July 21st, 2012, 01:25 AM #710July 22nd, 2012, 11:08 PM #711
yes afatima,,,, and thank you so much h.aJuly 23rd, 2012, 03:49 PM #712
- Member Since
- Jan 2012
- 7 times
According to uhs, The answer to the 1st question would be given then we will have to do 219 mcqs ?July 23rd, 2012, 07:57 PM #713
2. STATES OF MATTER
e) Suggest from quoted physical data the type of structure and bonding present in a
I want to know that which topics would be included in this line like give some example? #confusedJuly 23rd, 2012, 08:08 PM #714July 24th, 2012, 03:32 PM #7157. In general bacterial genes are regulated at the time of
d. CojugationJuly 24th, 2012, 04:28 PM #716
- Member Since
- Oct 2011
- 3 times
Instructions_for_candidates_MCAT_2012July 24th, 2012, 10:49 PM #717
hey anybody plz giude me about the test session of star academy gujaranwala??? about hostel or pick and drop to other cities?? fee? n schedule??July 25th, 2012, 11:25 AM #718
hey guys, how's everyone's preparation going?
all of you would have revised all of the books once by now, is that true?
I haven't yet...#sad
But I still think there's enough time.
Last edited by Rehan; July 25th, 2012 at 09:09 PM. Reason: Please post in proper English. Shorthand typing is not allowed.July 26th, 2012, 10:37 AM #719
from where did u get these mcq's n keys???July 27th, 2012, 05:07 AM #720
Lymphatic System Practice Questions
1. Which of the following is not directly associated with the lymphatic pathway?A. Lymphatic trunk
B. Collecting duct
C. Subclavian vein
D. Carotid arteries2. The thymus is responsible for secreting _____ from epithelial cells.A. Thymosin
B. Growth hormone
D. Plasma cells3. Which of the following types of immunoglobulins is the most responsible for promoting allergic reactions?A. IgA
D. IgE4. Which of the following types of immunoglobulins is located on the surface of most B-lymphocytes?A. IgA
5. Which of the following types of immunoglobulins does not cross the barrier between mother and infant in the womb?
6. Which of the following is not an autoimmune disease?
A. Graves disease
B. Myasthenia gravis
C. Insulin-dependent diabetes mellitus
D. Alzheimer's disease
7. T-cell activation requires a/an _______ cell.
8. The thymus is located with the _______.
9. Which of the following statements is false regarding the spleen?
A. Divided up into lobules
B. Similar to a large lymph node
C. Contains macrophages
D. Limited blood within the lobules
10. Which of the following is not considered a central location of lymph nodes?
11. Lymphocytes that reach the thymus become _____.
C. Plasma cells
D. Beta cells
12. Lymphocytes that do not reach the thymus become _____.
C. Plasma cells
D. Beta cells
13. Which of the following types of immunoglobulins binds complement?
14. Which of the following is a key component of cytotoxic T cells?
15. Which of the following is not a primary target group of T cells?
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