Term Paper on Cells | Biology

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Term Paper on Cells



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1. Term Paper on the Meaning of Cells:

All living organisms on earth are made up of cells. Cells are small compartments that hold all of the biological equipment necessary to keep an organism alive and successful on earth. The main purpose of a cell is to organize all the functions of the body.

The two major parts of a cell are the nucleus and the cytoplasm. The cytoplasm is made of cytosol and organelles. Cytosol is the fluid that fills the cytoplasm. Cell organelles are suspended in it. Organelles are highly organized physical structures. The nucleus is separated from the cytoplasm by a nuclear membrane and the cytoplasm is separated from the surrounding fluids by a cell membrane.


2. Term Paper on the Cell Membrane:

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Structure:

Cell membrane is like a big plastic bag with some tiny holes. Cell membrane surrounds the cell and is made of lipids and proteins. It is semi permeable, allowing some substances to pass through it and excluding others. The selective permeability of the cell membrane is due to presence of regulated ion channels and other transport proteins embedded in it. The structure of a cell membrane varies from one place to other depending on the function, but they share some common features.

Cell membrane is about 7.5 nm thick and is made up of proteins and phospholipids. The phospholipids make the plastic bag and the proteins are found around the holes and help movement in and out the cell.

Phospholipids Component of the Cell Membrane:

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Phospholipids make up the lipid bilayer and the major phospholipids are phosphatidylcholine and phosphatidylethanolamine. The head end of each phospholipids molecule is made of phosphate and is soluble in water and is called as the hydrophilic end. The tail end is the fatty acid portion which is insoluble in water and is called as the hydrophobic end. The hydrophobic ends of the bilayer are repelled by water of the ECF and ICF but are attracted to each other and line up in the center. The hydrophilic end covers the side which is in contact with water.

The lipid bilayer is a fluid and fluidity depends on cholesterol molecule, and therefore, portions of the membrane can flow from one point to another.

Protein Component of the Cell Membrane:

There are three types of Proteins in the cell membrane namely:

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1. Peripheral Proteins:

They are not bonded as strongly to the membrane but just sit on the surface of the membrane either inside or outside anchored with a few hydrogen (H) bonds.

2. Integral Proteins:

They are embedded in the hydrophobic (middle) layer of the membrane.

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3. Trans-Membrane Proteins:

They cross the membrane through and through and act as pathways for ions and molecules by either functioning as pump which actively transports ions, as carrier proteins and still some as ion channels.

Functions of the Cell Membrane:

i. The cell membrane protects the cytoplasm and the organelles. It acts as a barrier permitting only some substances to pass through it.

ii. Integral proteins give stability to the cell membrane.

iii. Peripheral proteins seated on the outer surface of the cell membrane act as receptors for neuro­transmitters and hormones. They also function as antigens.

iv. Some proteins function as cell adhesion molecules that anchor cells to their neighbours or to basal lamina.

v. Trans-membrane proteins act as carrier proteins and channels for the transport of ions, glucose and other water soluble substances.

vi. The lipid bilayer helps in the transport of lipid soluble substances like oxygen and carbon dioxide which is vital for cell metabolism.


3. Term Paper on the Cell Organelles:

1. Mitochondria:

Structure:

It is a sausage-shaped structure. It is made up of outer and inner membranes and the latter is folded to form selves called cristae onto which oxidative enzymes are attached. The inner cavity of the mitochondria is filled with matrix that contains large quantities of dissolved enzymes that are necessary for extracting energy from nutrients. Both these enzymes work in harmony to cause oxidation of nutrients and release of energy. Mitochondria are self-replicate.

Function:

It synthesizes a high energy ATP (adenosine triphosphate) and the same is transported out of the mitochondria to the other areas of cell to be utilized for performing cellular functions.

Applied Physiology:

Sperm contributes no mitochondria to zygote; hence any disease related to mitochondria is purely maternal:

i. Mitochondrial diseases comprise those disorders that in one way or another affect the function of the mitochondria or are due to mitochondrial DNA. Mitochondrial diseases take on unique characteristics both because of the way the diseases are often inherited and because mitochondria are so critical to cell function. The subclass of these diseases that have neuromuscular disease symptoms are often referred to as a mitochondrial myopathy.

ii. Leber’s hereditary optic neuropathy causes multiple sclerosis and visual loss.

2. Endoplasmic Reticulum (ER):

Structure:

It is a network of sacs and the outer limb of the sac is continuous with the nuclear membrane. It is of two types namely Rough endoplasmic reticulum with ribosomes on it which gives it the rough appearance and the smooth endoplasmic reticulum. The rough ER is abundant in cells which synthesizes protein. A modification of this in the skeletal and cardiac muscle is called as sarcoplasmic reticulum.

Functions:

i. The rough endoplasmic reticulum is the site of protein synthesis.

ii. Smooth endoplasmic reticulum is the site of steroid synthesis wherever necessary.

iii. Smooth ER helps in detoxification of toxic substances and neutralization of hormones and noxious substances.

3. Golgi Apparatus:

Structure:

Each Golgi apparatus consists of 5-7 membranous sacs which are flattened sacs. It has two ends. The vesicle pinched off from the ER fuses with one end and exit via the other end after processing.

Functions:

i. It is the distribution and shipping departments for the cell’s chemical products. It modifies proteins and fats built in ER and prepare them for digestion. ER vesicles pinch off from ER and fuse with Golgi apparatus to be processed there. Then they are released as lysosomes and secretory vesicles from Golgi apparatus.

ii. Packaging of secretory products into secretory granules.

iii. Incorporation of carbohydrates into the newly synthesized proteins to form glycoproteins.

4. Lysosomes:

Structure:

They are membrane bound vesicles pinched off from Golgi apparatus. It contains proteases, lipases and amylases.

Function:

Lysosomes provide an intracellular digestive system that allows the cell to digest damaged cellular structures, unwanted matter and food particles with the help of these digestive enzymes.

Applied Physiology:

Like other genetic diseases, individuals inherit lysosomal storage diseases from their parents. Although each disorder results from different gene mutations that translate into a deficiency in enzyme activity, they all share a common biochemical characteristic, i.e. all lysosomal disorders originate from an abnormal accumulation of substances inside the lysosome. Lysosomal storage disorders are caused by lysosomal dysfunction, usually as a consequence of deficiency of a single enzyme required for the metabolism of lipids, glycoproteins (sugar containing proteins) or so-called mucopolysaccharides.

They are:

i. Fabry’s disease

ii. Tay-Sachs disease

iii. Gaucher’s disease, etc.

5. Peroxisomes:

Structure:

They are membrane bound vesicles formed by budding off from smooth ER. They contain oxidative enzymes such as oxidases and catalases.

Functions:

i. They function mainly to detoxify poisonous substances.

ii. They breakdown the excess fatty acid.

6. Nucleus:

Structure:

It is the information and administrative center of the cell. The nucleus is made up of chromosomes that are made up of DNA molecules (deoxyribonucleic acid). Each DNA molecule is made up of genes that carry complete blueprint of all the heritable species. The unit of heredity is the genes which are present on the chromosomes that form the largest part of nucleus.

Nucleus is surrounded by nuclear membrane which is a double layer membrane, the outer layer of which is continuous with the membrane of rough ER, and therefore, there is continuous space with ER. The nucleus of most cells contains nucleolus that is rich in RNA which is the site of synthesis of ribosomes. The nucleoli are prominent in growing cells.

Functions:

i. Nucleoli synthesize ribosome that is needed for protein synthesis.

ii. Nucleus controls cell division.

iii. Messenger RNA from the nucleus has codon for the synthesis of protein.


4. Term Paper on the Cytoskeleton:

It is unique to eukaryotic cells. It is a dynamic three- dimensional structure that fills the cytoplasm. This structure acts like muscle and skeleton for movement and stability of a cell.

The primary types are:

i. Micro­filaments,

ii. Microtubules and

iii. Intermediate filaments.

i. Microfilaments:

These are fine thread like protein fibers, 3-6 nm in diameter. They are composed predominan­tly of a contractile protein called actin. They carry out cellular movement including gliding, contraction and cytokinesis.

ii. Microtubules:

They are cylindrical tubes, 20-25 nm in diameter. They are composed of protein tubulin and act as a scaffold to determine cell shape and provide “tracks” for cell organelles and vesicles to move on. They also form spindle fibers for separating chromo­somes during mitosis. When arranged in geometric patterns inside flagella and cilia, they are used for locomotion.

Applied Physiology:

Since microtubules help in organelle movement, any drug that binds with microtubule and makes them stable can prevent organelle movement which can help in cancer treatment.

iii. Intermediate Filaments:

They are about 10 nm diameter and provide tensile strength for the cell.


5. Term Paper on the Molecular Motors of Living Organisms:

They are biological molecular machines that are essential for movement in living organisms, i.e. to move proteins, organelles from one to other part of cell. Protein based molecular motors use the chemical free energy released by the hydrolysis of ATP in order to perform mechanical work.

Some examples of bio­logically important molecular motors are as follows:

1. Cytoskeleton Motors:

They are classified as:

a. Microtubule Based:

Kinesin and Dynein:

i. Kinesin moves cargo inside the cell along microtubules.

ii. Dynein produces the axonemal beating of cilia and flagella. It also transports materials along proton gradient inside microtubules towards the cell nucleus.

b. Actin Based:

Myosin I-V:

Myosin forms cross-bridges to actin filaments and the myosin heads move to generate force. This produces movement ranging from contraction of villi to skeletal muscle contraction.

2. Nucleic Acid Motors:

RNA polymerase transcribes RNA from a DNA template. DNA polymerase turns single-standard DNA into double-standard DNA. Topoisomerase reduces super coiling of DNA.

3. Rotary Motors:

ATP synthase generates ATP using the mitochondria.


6. Term Paper on Protein Synthesis:

Protein synthesis is the process in which cells build proteins. It is a multi-step process.

Step 1:

Transcription:

The first step in protein synthesis is the transcription of a messenger RNA (mRNA) from a nuclear DNA gene in the nucleus. Here the double helix nuclear DNA is unzipped by the enzyme helicase, leaving single nucleotide chain opens to be copied. RNA polymerase reads the DNA strand and synthesizes a single strand of messenger RNA. The mRNA then leaves the nucleus through nuclear pores and migrates into cytoplasm and functions as codons.

Codon is a unit of three adjacent nucleotides along a DNA or messenger RNA molecule that designates a specific amino acid to be incorporated into a polypeptide. The order of the codons along the DNA or messenger RNA determines the sequence of the amino acids in the polypeptide. There is an initiation codon which always initiates amino acid sequencing and a stop codon which stops the polypeptide chain with that amino acid when a ribosome scans through it.

Step 2:

Translation:

This takes place in rough ER. It is the process of converting the mRNA codon sequences into an amino acid polypeptide chain.

This involves sub steps namely:

(a) Amino acid activation,

(b) Initiation,

(c) Elongation and

(d) Termination.

(a) Amino Acid Activation:

Each type of amino acid combines with specific RNA called transfer RNA (tRNA) which has anticodon, which is a sequence of three adjacent nucleotides in tRNA designating a specific amino acid that binds to a corresponding codon in mRNA during protein synthesis. Thus it forms tRNA-amino acid complex which recognizes a particular codon on the mRNA and can deliver the appropriate amino acid to appropriate place in the chain of new protein.

(b) Initiation:

A ribosome attaches to the mRNA and starts to read the codons of the mRNA.

(c) Elongation:

tRNA-amino acid complex brings the corresponding amino acid, in contact with the mRNA molecule in the ribosome where the anticodon of the tRNA attaches temporarily to its specific codon of the mRNA as the ribosome reads mRNA thus lining the amino acid in sequence.

(d) Termination:

Reading of final mRNA codon stops at stop codon which ends the synthesis of peptide chain. The polypeptide chain buds off from rough ER and fuses with Golgi apparatus.

Step 3:

Post-Translational Modification and Protein Folding:

This takes place in Golgi apparatus. Post-translation modification includes the formation of disulfide bridges (or) attachment of functional groups such as acetate, phosphate, various lipids and carbohydrates.

Finally during and after synthesis, polypeptide chains often fold to assume secondary and tertiary structures. This is known as protein folding.

Applied Physiology:

The regulation of protein synthesis plays an important role in transcription, in the control of gene expression. Once thought solely to act globally, translational control has now been shown to be able to control the expression of most genes specifically. Deregulation of this process is associated with a range of pathological conditions, notably cancer and several neurological disorders, and can occur in many ways.

These include alterations in the expression of initiation factors and mutations in regulatory mRNA sequence. Translational control is increasingly open for study in both fresh and fixed tissue, and this rapidly developing field is yielding useful diagnostic and prognostic tools that will hopefully provide new targets for effective treatments.


7. Term Paper on the Intercellular Connection and Communication in Cells:

Each cell is connected to adjacent cell and basal lamina by intercellular connection. There are various types of connections involved.

They are:

1. Gap Junctions:

Gap junctions or nexus is a specialized intercellular connection which is an opening from one cell to another. It is large enough for cytoplasm to move from one cell to another and help for the movement of molecules. The diameter of channel is about 2 nm. Special trans-membrane protein known as, connexins join to form aqueous channel known as connexon. Connexon of one cell join with the connexon of the other cell to form the pore or gap. For example myocardial cell and visceral smooth muscle.

Function:

They permit rapid propagation of electrical activity from cell-to-cell.

Applied Physiology:

Mutation in the gene for connexon causes X-linked form of Charcot-Marie-Tooth disease which causes peripheral neuropathy.

2. Desmosomes:

They are also known as macula adherens. It is a cell structure specialized for cell to cell adhesion. They are spot like adhesions randomly arranged on the lateral sides of plasma membrane.

Function:

They help to resist shearing forces and are found in simple and stratified squamous epithelium. They act as linking proteins that attach the cell surface adhesion protein to keratin cytoskeleton filament.

Applied Physiology:

If there is a genetic defect in the desmosomal protein, the skin can pull apart and allow abnormal movements of fluid within the skin, resulting in blisters called blistering disease such as pemphigus vulgaris. Blistering is due to abnormality in desmosome—keratin filament complex leading to breakdown in cell adhesion.

3. Hemidesmosomes:

They appear similar to desmosome but rather than linking two cells, they attach cell to the extracellular matrix. They are asymmetrical and are found in epithelial cells, generally connecting basal surface of keratinocytes in the dermis of skin. For example, in teeth they attach junctional epithelium to the enamel.

4. Tight Junction:

They are otherwise called zonula occludens. They are tight areas between two cells whose membrane join together forming a virtually impermeable barrier to fluid. Tight junctions join together the cytoskeletons of adjacent cells. They are composed of a branching network of sealing strands, each strand acting independently from others. Therefore, the efficiency of the junctions in preventing ion passage increases exponentially with the number of strands.

The main function is to:

1. Hold cells together

2. Maintain polarity

3. Prevent passage of molecules and ions through the space between cells. For example, blood brain barrier in the brain, walls of renal tubules.

Applied Physiology:

Mutation in the gene for tight junction leads to hereditary deafness.

Intercellular Communication of the Cells:

Cells communicate with each other via chemical messengers. These messengers either bind with receptors on the surface of cell or cytoplasm or nucleus or trigger sequence of changes to bring about physiological effects.

There are three basic types namely:

1. Endocrine:

In which hormones and growth factors reach cells via blood circulation.

2. Paracrine:

Here the products of cell diffuse to the neighboring cells.

3. Autocrine:

The chemicals released from cell bind to receptors on the same cell and bring about effect.

There is another variant apart from basic type which is present in the central nervous system. It is the neural communication in which neuro-transmitters are released at the synaptic junctions from nerve cells and act on the postsynaptic cell. An additional type called juxtacrine communication is identified which is a type of intercellular communication that is transmitted via oligosaccharide, lipid, or protein components of a cell membrane, and may affect either the emitting cell or the immediately-adjacent cells.

It occurs between adjacent cells that possess broad patches of closely-opposed plasma membrane linked by trans-membrane channels known as connexon. The gap between the cells can usually be between only 2 and 4 nm. Unlike other types of cell signaling (such as paracrine and endocrine), juxtacrine signaling requires physical contact between the two cells involved. Juxtacrine signaling has been observed for some growth factors, cytokine cellular signals.


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