Cell Structure: Organelles and Proteins

This assignment will cover the structure of a cell, the role of the organelles within the cell and the production of proteins within a cell. The cell which has been chosen for this assignment is the Beta Cell which produces the protein Insulin.

“One of the greatest medical events was the discovery of insulin. The importance of insulin is juxtaposed with that of glucose, our body’s basic unit of fuel. Both are required for life. In order to regulate glucose metabolism, insulin circulates through blood vessels to deliver its message by means of a “handshake” with its cognate cell surface receptor. For those with diabetes mellitus, the body is either unable to produce insulin, or unable to produce it in sufficient amounts, and is therefore powerless in maintaining proper levels of blood glucose. Even though insulin is a life saver, it does not cure the disease. Insulin injections themselves are not without risk. Improvements in the treatment of diabetes will come from a better understanding of how insulin is made in the pancreas and released into the bloodstream, and how it promotes uptake of circulating glucose by tissues including muscle and fat.”

Insulin is a hormone which is made in the pancreatic beta cells. These Beta cells are found in the pancreas in islets of Langerhans.

“When the protien insulin is made, the messenger RNA transcript is translated into an inactive protein called preproinsulin (see Image 1.1). Preproinsulin contains an amino-terminal signal sequence that is required in order for the precursor hormone to pass through the membrane of the endoplasmic reticulum (ER) for post-translational processing. The post-translational processing clips away those portions not needed for the bioactive hormone. Upon entering the ER, the preproinsulin signal sequence, now useless, is proteolytically removed to form proinsulin. Once the post-translational formation of three vital disulfide bonds occurs, specific peptidases cleave proinsulin. The final product of the biosynthesis is mature and active insulin. Finally, insulin is packaged and stored in secretory granules, which accumulate in the cytoplasm, until release is triggered.”

“The process by which insulin is released from beta cells, in response to changes in blood glucose concentration, is a complex and interesting mechanism that illustrates the intricate nature of insulin regulation. Type 2 glucose transporters (GLUT2) mediate the entry of glucose into beta cells (see panel 2). As the raw fuel for glycolysis, the universal energy-producing pathway, glucose is phosphorylated by the rate-limiting enzyme glucokinase. This modified glucose becomes effectively trapped within the beta cells and is further metabolized to create ATP, the central energy molecule. The increased ATP:ADP ratio causes the ATP-gated potassium channels in the cellular membrane to close up, preventing potassium ions from being shunted across the cell membrane. The ensuing rise in positive charge inside the cell, due to the increased concentration of potassium ions, leads to depolarization of the cell. The net effect is the activation of voltage-gated calcium channels, which transport calcium ions into the cell. The brisk increase in intracellular calcium concentrations triggers export of the insulin-storing granules by a process known as exocytosis. The ultimate result is the export of insulin from beta cells and its diffusion into nearby blood vessels. Extensive vascular capacity of surrounding pancreatic islets ensures the prompt diffusion of insulin (and glucose) between beta cells and blood vessels.”

“Insulin release is a biphasic process. The initial amount of insulin released upon glucose absorption is dependent on the amounts available in storage. Once depleted, a second phase of insulin release is initiated. This latter release is prolonged since insulin has to be synthesized, processed, and secreted for the duration of the increase of blood glucose. Furthermore, beta cells also have to regenerate the stores of insulin initially depleted in the fast response phase.”

So what is a cell and how is it structured?

Firstly, there are two types of cell

Prokaryotic cells

• DNA= You can see from the image that the DNA is spread through much of the cell and is not restricted to the Nucleus.
• SIZE= A Prokaryotic cell has a smaller infrastructure therefore is much smaller than an Eukaryotic cell
• ORGANISATION= Prokaryotic cells like bacteria are always single-celled and generally do their job independently of other cells.
• ORGANELLES= Unlike Eukaryotic cells these cells only hold one type of organelle and are structured in a much simpler form.

Prokaryotic cells are either Bacteria or Archaea

• Bacteria= “cell membrane contains ester bonds; cell wall made of peptidoglycan; have only one RNA polymerase; react to antibiotics in a different way than archaea do.”
• Archaea= “cell membrane contains ether linkages; cell wall lacks peptidoglycan; genes and enzymes behave more like Eukaryotes; have three RNA polymerases like eukaryotes; and extremophiles.”

Eukaryotic cells

• DNA= is within the membrane bound nucleus
• SIZE= is much larger than an Prokaryotic cell
• ORGANISATION= they are often multicellular
• ORGANELLES= they have many types of organelles

Eukaryotic cells are everything else other than Bacteria or Archaea for example; plants, humans, animals, sea life, vegetation etc. These proteins are secreted into their surrounding extracellular fluid. These are synthesized in ribosomes. Ribosome’s are the protein assemblers of a eukaryotic cell. They have two subunits; a large and a small subunit.

Carbohydrates

“Carbohydrates consist of the elements carbon (C), hydrogen (H) and oxygen (O) with a ratio of hydrogen twice that of carbon and oxygen. Carbohydrates include sugars, starches, cellulose and many other compounds found in living organisms. In their basic form, carbohydrates are simple sugars or monosaccharides. These simple sugars can combine with each other to form more complex carbohydrates. The combination of two simple sugars is a disaccharide. Carbohydrates consisting of two to ten simple sugars are called oligosaccharides, and those with a larger number are called polysaccharides.”

Now the definition of carbohydrates has been discussed, it is time to talk about the role of a carbohydrate in the body. Carbohydrates provide energy to the body and these are taken from the monosaccharide glucose. ‘Monosaccharides are the simplest carbohydrates and are classified according to whether they are aldehyde or ketone derivatives, as well as the number of atoms contained in the molecule. Single hexoses, glucose and galactose require no digestion and can be absorbed directly into the bloodstream. Hexoses contain six carbon atoms, and are found in foods, while pentoses, ribose and deoxyribose contain five carbon atoms and are produced during the metabolism of foodstuffs.’

Glucose can be made from Glycogen which is found in the liver and muscles; however it can also be found in small amounts in other organs and tissues around the body. Glucose molecules split into smaller molecules and these oxidise to form water which in turn provides a large amount of energy.

Carbohydrates are important as they provide fuel to the central nervous system and the muscle system which is extremely important due to the sheer number and mass of nerves and muscles in our bodies which is needed to operate.

Proteins’

“Proteins consist of amino acids which are characterized by the -CH(NH2)COOH substructure. Nitrogen and two hydrogen’s comprise the amino group, -NH2, and the acid entity is the carboxyl group, -COOH. Amino acids link to each when the carboxyl group of one molecule reacts with the amino group of another molecule, creating a peptide bond -C(= O)NH- and releasing a molecule of water (H2O). Amino acids are the basic building blocks of enzymes, hormones, proteins, and body tissues. A peptide is a compound consisting of 2 or more amino acids. Oligopeptides have 10 or fewer amino acids. Polypeptides and proteins are chains of 10 or more amino acids, but peptides consisting of more than 50 amino acids are classified as proteins.”

Lipids

A lipid is a fat-soluble molecule. To put it another way, lipids are insoluble in water but soluble in at least one organic solvent. The other major classes of organic compounds (nucleic acids, proteins, and carbohydrates) are much more soluble in water than in an organic solvent. Lipids do not share a common molecule structure.

Lipids cannot be dissolved in water, they are said to be hydrophobic, however ether, acetone and other lipids can dissolve a lipid.

Lipids

Lipids can be broken down into groups, the three major groups are

• Fats; which are made up from three fatty acids and glycerol. These are called triglycerides and they can either be liquid or solid when they are at room temperature. When they are in their liquid stage, they are called oils and those that are in their solid stage are called fats. These can be saturated or unsaturated depending on the lipids structure.
• Phospholipids; are made up from two fatty acids and one glycerol unit with a polar molecule and phosphate group. The polar head section of the molecule is hydrophilic which mans that it is attracted to water while the tail is hydrophobic which means that it is repelled by water. The non-polar tails organise themselves so that they are all facing each other and the polar heads face the opposite direction to interact with the water. Phospholipids are important and a vital component of cell membranes. These enclose the cytoplasm and all the other contents of a cell.
• Steroids and Waxes; steroids are made up from cholesterol, sex hormones such as; estrogens’, progesterone, testosterone and also include cortisone. Waxes which many plants, fruits and even animal fur or feathers have to protect them from water penetration are made up from an ester of fatty acid and alcohol.

Nucleic Acids

Nucleic acids allow organisms to transfer genetic information from one generation to the next. There are two types of nucleic acids: deoxyribonucleic acid, better known as DNA and ribonucleic acid, better known as RNA.

When a cell divides, its DNA is copied and passed from one cell generation to the next generation. DNA contains the “programmatic instructions” for cellular activities. When organisms produce offspring, these instructions, in the form of DNA, are passed down. RNA is involved in the synthesis of proteins. “Information” is typically passed from DNA to RNA to the resulting proteins.

Organelles

A cell has different kinds of organelles which are vital to the operation of a cell, otherwise they would not be able to produce the proteins such as Insulin which are needed for the body. A cell is the smallest unit which has all the properties of life and cells arise only from cells.

Cell wall gives rigidity and support in plants.  Animals do NOT have cell walls, but do have an extracellular matrix to provide support.

Mitochondria are an organelle involved in cellular respiration, producing ATP, the molecule cells use for energy.

Chloroplast is an organelle plants use to convert light energy into chemical energy (make food) in a process called photosynthesis.

A ribosome attaches to the mRNA and begins to make the protein insulin which moves into the RER (rough endoplasmic reticulum) 

Insulin is transported from RER to Golgi via a transport vesicle

Insulin is modified in the Golgi with part of the protein being cut out.  Often carbohydrate tags are added in the Golgi though not in the case of insulin.   The insulin is stored in secretory vesicles in the cell and released into the blood by exocytosis past the plasma membrane.

Protein receptors on other cells bind to insulin causing a response

Right at the heart of a cell is the Nucleosus which is uspended in the nucleus. This is what effectively the workstation of the cell is.

Cells that have similar functions form tissues. Tissues that have similar functions form organs. Organs that have similar functions form organ systems. After cell division, the new cell change in structure so as to perform a particular function. This is called cell specialisation. Cells with similar changes are found together and form a tissue. Different tissues are bound and work together to form an organ. All living things are made of cells. Cells are organized together to make tissues. Tissues are organized together to make organs. Organs are organized together to make systems.

The more complex a living organism is, the higher this organization is.

Now, there are living organisms made of only 1 cell, so the cell is the thing all organisms have in common. However, most living things have organs and organ systems, even if they are simple ones.