A Quick Glance Of Eukaryotic Cell

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Eukaryotic cells

  1. Contains true nucleus which consists of a membrane bound structure
  2. Also contains membrane bound organelles
  3. Thought to have evolved from prokaryotic cells

Ribosomes

  1. It translates genetic code into proteins
  2. See in rough endoplasmic reticulum and cytoplasm
  3. Consists of 60% RNA and 40% protein

Rough Endoplasmic Reticulum

  1. It consists of a network of continuous sacs
  2. It is associated with ribosome’s
  3. Concerned with the processing and transport of proteins
  4. It is continuous with the nuclear membrane

Smooth Endoplasmic Reticulum

  1. It is not associated with ribosomes
  2. It is concerned with the production of lipids, carbohydrate metabolism and detoxification of poisons
  3. It metabolizes calcium

Lysosome

  1. Single membrane structure
  2. It consists of digestive enzymes that break down cellular waste, debris and nutrients for use by the cell
  3. Aids in  cell death by lytic action with its acidic ph
  4. It’s the site of cellular digestion
  5. It consists of more than 40 enzymes ( e.g. acid hydrolas with ph 5 )

Golgi apparatus (dictyosomes)

  1. Attached to the RER and helps it in protein sorting
  2. It modifies proteins and lipids made by ER and prepares them for export
  3. It packs digestive enzymes in membrane making lysomes

Mitochrondria

  1. It’s the site of cellular respiration
  2. Its inner membrane is highly folded and the folding are called cristae
  3. Cristae consists of F1 particles which have a head and a tail. The head portion is the actual site of ATP synthesis
  4. It is present throughout the cytoplasm

Nucleus

  1. It is double membraned
  2. Separates the genetic material from the rest of the cell
  3. During prophase the DNA is present as chromatin
  4. It consists of nucleolus which is responsible for synthesis of ribosomes
  5. The nuclear envelope separates the nucleus from the cytoplasm

Plasma Membrane

  1. Made of phospho-lipid bilayer
  2. Its selectively permeable

Cilia and flagella

  1. External appendages that aid in locomotion
  2. Cilia can also move substances

Centrioles

  1. Found only in animal cells
  2. Self replicating unit
  3. Made of microtubules
  4. Aids in cell division

Microfilaments

  1. Are solid rods of globular protein
  2. It forms the major component of the cytoskeleton

Cell wall

  1. It is responsible for protection and rigidity
  2. Made of phospholipids and polysaccharides

Vacuole

  1. Plants have large central vacuole that stores water and nutrients
  2. In animals it is small and spread through the cytoplasm

A quick glance of Prokaryotic Cell

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Prokaryotic cell

  1. First cells to evolve
  2. Lack membrane bound organelles and nucleus
  3. Genetic material free floating in the cytoplasm and is called nucleoid
  4. Bacteria were the first cells to be studied under microscope
  5. Ribosome is the only organelle present

Cell wall

  1. Made of peptidoglycan. Is a polysaccharide coat that gives the cell, shape and rigidity.
  2. It surrounds the cytoplasmic membrane and offers protection from the environment.
  3. It is the basis of the gram staining procedure: prokaryotes with thick wall take up the stain and are called gram positive and those with thin walls do not take up the stain and are termed gram negative

Plasma membrane

  1. It’s a phospho-lipid bilayer. Lipids are arranged such that the hydrophilic hear is towards the outer end and the hydrophilic tails are inside and away from water.
  2. Consisted of intrinsic and extrinsic proteins and carbohydrates.
  3. It separates the cytoplasm from the external environment
  4. It regulates the flow of materials in and out of cell

Cytoplasm

  1. It is the location of growth, metabolism and replication.
  2. It’s a gel like matrix consisting of water, enzymes, nutrients, wastes, gases and contains cell structure.

Ribosome’s

  1. It translates genetic code into proteins
  2. They are free standing and distributed in cytoplasm

Nucleoid

  1. The place where chromosomal DNA is located
  2. Usually singular and linear. Sometimes circular.
  3. Smaller circles of DNA called plasmids are found in cytoplasm

Mesosome

  1. Infolding of cell membrane
  2. Its gives flexibility during cell division
  3. Increases surface area

Rutherford’s model of atom

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Introduction

Rutherford’s model is based on the experiment of scattering of alpha particles on passing through a thin gold foil.

The observation he recorded was that most of the particles passed through without any deflection, some of them were deflected at very small angles and a very few of them rebounded and retraced their own path. This occurs due to the repulsion between the positive charges in the alpha particles and the positive charges in the gold foil.

The large scale scattering is due to the whole mass of the positively charged centre which is nothing but the nucleus.

Rutherford proposed the following concepts

  1. The nucleus is positively charged where the whole of the mass of the atom is concentrated
  2. Around the nucleus at a relatively large distance are extra nuclear electrons equal to the number of the positive charge on the nucleus
  3. The electrons are constantly moving around the nucleus in different definite orbits.

He also postulated that the electrons do not fall into the nucleus as it has rapid rotation around the nucleus and the centrifugal force arising from this motion balanced the electrostatic attraction between the nucleus and the extra nuclear electrons

Drawback’s of Rutherford’s atomic model

The most fundamental objection arises from the electro-magnetic theory of radiation which predicts when a charged particles describes a circular orbit it should radiate energy continuously by colliding of electrons.

As a result of this loss of energy by radiation the electrons should eventually fall into the nucleus and be retained there.

These drawbacks were explained by Bohr’s atomic model which I will explain in detail in the forthcoming posts.

Properties of cells

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  1. Cells are highly complex and organized
  2. Every cell has genetic information that is encoded in its genes (genes have ability to construct cellular structures; ability to perform cellular functions and genes also have the capability to make copies of cells by the division of mother cells into daughter cells)
  3. Cells require a constant input of energy for the development and maintenance of complexity.
  4. Every cell has the ability to perform chemical transformations in the presence of enzymes.
  5. Cells not only perform activities like respiration, movement , digestion, assimilation, transport etc. but also has specific responses to hormones, growth factors, extracellular materials and other substances present in cells
  6. Every cell has the capability of self regulation (e.g. if a cell is unable to repair its damaged DNA then such a cell can transform into cancer cell that can destroy the entire organism)
  7. Cells have a finite lifetime that is they maintain the homeostasis between cell division and cell death.
  8. Cells store their hereditary information in a linear DNA molecular code.
  9. Some cells can fix nitrogen whereas some cells fix carbon dioxide for its metabolism
  10. Cells are enclosed by an amphipathic lipid plasma membrane

The Cell

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History

Cell is the basic structural and functional unit of life. In 1665 Robert Hooke observed think slices of cork tissue using a compound microscope and saw a honeycomb like arrangement in the texture of the think section which is surrounded by the cell wall. He coined the term “cell” for these individual units

In 1831 Robert Brown observed a spherical body in each cell which he called the “nucleus”

In 1839 M.J.Shleiden and Theodore Schwann proposed the cell theory.

The cell theory

  1. All living organisms are made up of one or more cells.
  2. The cell is the structural and functional unit of life.
  3. All cells arise from pre-existing cells.

Shape of cells

The shape of cell is variable since it depends mostly on the functional requirement of the organism and varies according to the environmental condition.

It may be irregular like in the case of amoeba or fixed as in case of acetabularia.

When the shape is fixed, different types of shapes may exist like spherical,  cuboidal, spindle,  rectangular, flattened, oval, columnar, flagellated, disc shaped etc. are found.

Size of cells

The size of the cells also varies considerable. Some organisms are observed under the compound microscope since they are not visible to naked eyes, whereas some are macroscopic.

  The largest cell is ostrich egg (20cm)

                       The smallest cells are mycoplasmas (.1µ)

Factors affecting the size of the cells

  1. The ratio between the volume of nucleus to the cytoplasm of the cell.
  2. The ratio of cell surface to volume of cell
  3. The rate of metabolism

Average sizes of some living cells

NAME OF CELL AVERAGE SIZE
Giraffe nerve cell 2m
Ostrich egg 190 x 140 mm
Chick egg 60 x 40 mm
Human ovum .1mm
Amoeba 100µ
E. coli 1.5 x .7

 

Different forms of cells

Plant cells

NAME OF CELL TYPICAL SHAPE EXAMPLE
Epidermal Cuboidal or brick Epidermis
Vascular Elongated Phloem
Supporting Elongated with heavy walls Bast cells
Parenchyma Spheroidal Mesophyll
Algae Diverse Chlorella
Unicellular fungi Diverse Fungi
Bacteria Diverse E. coli
Filamentous fungi Elongated Neurospora

 

Animal cells

NAME OF CELL TYPICAL SHAPE EXAMPLE
Protozoa Slipper shaped Amoeba, paramecium
Spermatozoa Flagellated Sea urchin sperm
Eggs Spherical Frog egg
Blood cells Disc shaped Leucocytes
Epithelial Cuboidal Epidermis
Nerve Cell body with long axon Sensory and motor
Muscular Spindle Smooth
Connective Spheroidal Bone cells

 

Thomson’s Cathode Ray Experiment

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The cathode ray tube

It consists of a glass tube with wires inserted at both ends and a vacuum created inside by removing the air. An electric charge passed across the tube through the wires creates a fluorescent glow. This cathode ray became known as electron gun. Certain types of glass produced a fluorescent glow at the positive end of the tube. William Crookes discovered that a tube coated in fluorescing material at the positive end would produce a focused dot when the rays from the electron gun hit it.

Cathode Rays

The cathode rays could not move around solid objects. It travelled in straight lines which is the property of waves. The focused nature of the beam indicated to particle nature.

Now the question was whether

a) The ray and the charge could be separated?

b) If the ray was a wave or it was particulate?

Thomson conducted 3 experiments to find a definitive and comprehensive answer about the nature of the cathode rays.

Experiment 1

Hypotheses: Rays emitted from the electron gun are inseparable from the latent charge

Procedure:  Thomson constructed a cathode ray tube with a metal cylinder on the end with 2 slits in it which lead to electrometers (measures small charges). By applying magnetic field across the tube there was no activity in the electrometers.

Inference: The charge had been bent away by the magnet. And thus the negative charge and the ray were inseparable and intertwined.

Experiment 2

Hypotheses: The rays carried a negative charge.

Procedure: For this Thomson used a cathode ray tube with a fluorescent coating at one end and a near perfect vacuum. In the middle of the tube there were two electric plates placed opposite to each other separated by a small distance. One a positive anode and the other a negative cathode.

Inference: The rays were deflected by the electric charge that proved that the rays were made of charged particles carrying a negative charge.

Experiment 3

Aim: to deduce the nature of particles.

Procedure: He used the data from the previous experiment and tried to calculate the charge to mass ratio by comparing how much the particles were bent by electrical currents of varying strengths.

Inference: He found that the charge to mass ratio was so large that the particles we a thousand times smaller than a hydrogen ion.

These 3 experiments and the results it presented were of utmost value to the field of atomic structure in chemistry.

Thus Thomson discovered the electron in the year 1897 which lead to the theory that atoms were made of tiny particles. He proved that atoms were divisible and that they were not the most elementary particles.

These experiments set the base for the future scientists like Bohr and Rutherford and lead to the discovery of the other subatomic particles.

I will be putting up posts regarding the scientists and their discoveries in detail shortly.

More On The Structure Of Atom Focus: J.J.Thomson

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Discovery of the electron

Electron was discovered by the means of cathode ray experiment.

In this experiment when current is passed through a cathode ray tube the surface of the tube directly opposite to the cathode glowed. The hypothesis was that the glow was caused by a stream of particles which was then called the cathode rays. The rays travelled from the cathode to the anode when there was a passage of current.

J.J.Thomson’s Atomic Model

According to this model an atom consists of a uniform sphere of positive charge in which electrons are embedded to confer the electrical neutrality.

The number of the positively charged particles must be equal to the number of electrons.

This model justifies the electrostatic force of repulsion among the electrons which is balanced by the positively charged protons by creating an electro-statically stable system.

But this model failed to explain the ionization as well as scattering experiments.

Thomson’s Experiments

Thomson conducted a series of 3 important experiments using the cathode ray tube and came up with the following conclusions

  1. He proved that the negative charge and the ray were inseparable and intertwined
  2. The rays were made up of charged particles carrying a negative charge.
  3. He deduced that the charge to mass ratio was so large that the particles either carried a huge charge or were a thousand times smaller than a hydrogen ion. He decided upon the second conclusion and came up the idea that the cathode rays were made of particles that emanated from within the atoms themselves.

 

Structure of Atom: An Introduction

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When we start talking about structure of atom the first thing we must ask ourselves is: What is an atom?

An atom is the most basic smallest stable entity of which all matter is composed of.

An atom consists of protons and neutrons (which make up the dense central nucleus) which is positively charged and the electrons (which revolve around the nucleus in discrete orbits) and are negatively charged.

This definition has been arrived from Rutherford and Bohr’s atomic theories.

Rutherford’s Model – Planetary Model

J J Thompson’s plum pudding model was disproved by his former student Ernst Rutherford who discovered that the major mass as well as the positive charge of the atom was concentrated in the centre. He arrived at this conclusion after performing the gold foil experiment with his colleagues Hans Geiger and Ernst Marsden. He concluded that in an atom a cloud of electrons surrounded a very small compact nucleus of positive charge.

Points about the atom and its constituent particles

  1. In a neutral atom the number of electrons is equal to number of protons
  2. The electrons are negatively charged and the protons positively charged making the net charge on the atom zero
  3. The electrons move in circular or elliptical orbits around the nucleus
  4. The electronic structure of an atom refers to the way in which the electrons are arranged around the nucleus and the energy levels they occupy
  5. Each electron is characterized by a set of quantum numbers
  6. The shells arranged around the nucleus are said to be K, L, M, N and so on with sub shells s, p, d and f.

Note: points 5 and 6 refers to the quantum theory of atom about which I will be posting in a detailed manner

 Electronic Theory or Dalton’s atomic theory

It is a theory of chemical combination proposed by the British chemist John Dalton (1766-1844) in the year 1803. It involves the following postulates

  1. Elements consists of indivisible small particles that is atoms
  2. All atoms of the same element are identical
  3. Different elements have different types of atoms
  4.  Atoms can neither be created not be destroyed
  5. Compound elements are formed when atoms of different element join in simple ratios to for compound atoms ( molecules)

Why Watson and Crick? Destiny or Chance?

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When people talk DNA we think Watson and Crick. But the fact is it really wouldn’t have been them at all. Here is why.

During the time that people were trying to find out what the genetic material was Watson was a Physicist working behind the lines for the war and Crick an ornithologist. What brought them together and got them interested in this hunt for the genetic material was their common interest in X-RAY Crystallography.  X-RAY Crystallography gives an idea of the structure of the material you are studying by giving a picture of the electron densities of the material. Pauling had just used this wonderful technique to unravel the alpha helix structure of proteins. Using techniques of biochemistry scientists had determined that DNA was basically composed of 4 bases which are Adenine, Guanine, Thymine and Cytosine. Using this information Watson and Crick could not make much progress, so Watson hired a mathematician to arrive at a conclusion. After much study the mathematician concluded and presented the result that Adenine and Thymine and Cytosine and Guanine occurred at a 1to1 ration. But still Watson and crick could not make the connection. In 1951 Watson after attending a lecture on Rosalind Franklin’s work up to date and could have discovered the structure of DNA. But he did not. Why? Well for the simple reason that Watson did not take notes!!! So Watson after returning from the lecture along with Crick designed a model for DNA structure which was ultimately termed as a failure.

It is said that Franklin while working alone in her lab had obtained an X-Ray Crystallographic image which suggested that DNA was actually made of 2 strands. And while she was working on getting more evidence to prove this, Watson and Crick accidently stumbled upon this image which made them to correlate Chargaff’s pairing rule and the result submitted by their mathematician which led them to conclude that DNA was actually a helically coiled molecule made of 2 strands running anti-parallel to each other.

Since their findings were consistent to that of Chargaff’s and explained how the molecule maintained its structure during cell division it was immediately accepted by the scientific world. This discovery has been called the most important discovery in the field of biology for the last 100 years. By the time the Nobel Prize was awarded for this discovery in 1962 Franklin had died. And the prize was shared among Watson, Crick and Wilkins. So the question remains if Franklin had been alive who would have got the prize?

The first genetic link to ADHD

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The research

Cardiff university’s new research suggests that Attention Deficit Hyperactivity Disorder a common psychological disorder among children and teens could actually have a genetic basis.

According to this research small segments of DNA are duplicated or missing in patients with ADHD. These segments are known as copy number variants (CNV’s). CNV’s were found to be common with the ones indicated to be responsible for autism and schizophrenia which suggests that ADHD is in fact a neurodevelopmental disorder. Genes involved with the brain chemical dopamine are also being investigated as the dopamine levels in affected patients are very low. It is also found that children with ADHD have a thinner brain tissue in the areas of brain associated with attention. This could actually help discard a lot of myths about ADHD such as it being a result of bad parenting or social background etc only. Being genetic ADHD  is capable of being inherited which is consistent with the observation that children with relatives affected with ADHD were more likely to be affected by it. It was found that rare CNV’s are almost twice as common among children with ADHD and also CNV’s can be seen in almost everyone with brain disorders. Overlaps are particularly common in the 16th chromosome especially one known to play a role in brain development’s it can be concluded that CNV’s actually indicate manifestations of some common disorders of the brain. The occurrence of common CNV’s among autism schizophrenia and ADHD could indicate shared biological origins.

Implications of this research

According to research by Michigan State University, nearly one million children in the US are potentially misdiagnosed with ADHD simply because they are immature and naughty. This diagnosis leads to the children put on behavior modifying stimulants like Ritalin which have several side effects ranging from mild nausea to sometimes life threatening depression. This new finding could prove to be a dependable way of diagnosing ADHD and prevent the thousands of misdiagnosis.

Myths and Misconceptions

It should be noted that ADHD is not cause by a single genetic change but due to a combination of many. Also this research does not imply that ADHD is purely genetic but rather shows how both genetic and environmental factors interact to create a psychological disorder.  There are in fact a long list of environmental factors which can cause ADHD like prenatal stress, abuse during childhood, smoking during pregnancy and many others. Parents should not rely totally on drugs like Ritalin. In fact drugs must be prescribed only in severe cases since the chemistry behind ADHD is not clearly known

ADHD like many other disorders is still not fully understood and its causes are yet to be identified. The latest finding should help in relieving the social stigma that the affected children face. ADHD can be treated only with the help of both psychological counseling, support from parents and friends and in extreme cases drugs. So parents of children with ADHD should seek professional help rather than just blaming it on genes or the environment.