Diving Into How a Cell Works
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Explore Chapter 2 of Dr. Macosko's lectures.
the connection of cell membranes
Even though we tend to view organelles as existing and functioning in the cell independent from each other, this is not the case. For some processes, multiple organelles are involved. For instance, in protein synthesis, the DNA in the nucleus copies its instructions for that protein into an mRNA molecule, which leaves the nucleus and goes to a ribosome outside the nucleus to be translated into a polypeptide. But often modifications are required, so the protein will enter the lumen (inside space) of the rough ER then go to the Golgi Complex for final modification, and sorting and shipping. Clearly the functions of these compartments are related, the way organs in the body work together to form an organ system (the cardiovascular system, etc.). But it's not just the functions that tie these organelles together. Some of these organelles physically connect (the nuclear envelope is continuous with the rough ER) and membrane from one organelle can end up in another as vesicles break off of one organelle and transport cargo to another. (Proteins modified by the rough ER travel to the Golgi Complex in vesicles that bud off from the rough ER then merge with the Golgi Complex.) Therefore it is often helpful to consider a number of the organelles as a system, called the endomembrane system. Endo refers to inside, so it means "membranes inside the cell". However, even the plasma membrane can be considered to be part of this system, since vesicles delivering proteins to be secreted from the cell become part of the plasma membrane (and during the process of endocytosis, materials brought into the cell become entrapped in a vesicle that was formed from the plasma membrane).
the cpu of the cell
The nucleus is an organelle found only in eukaryotic cells. Sometimes called the "control center" of the cell, it is where most of the nucleic acids, such as DNA and RNA, are located. The scientist who discovered that the nucleus is what holds the genetic information of cells was Danish biologist Joachim Hammerling who in an important experiment in 1943 discovered the role of the nucleus in controlling the shape and features of the cell. In discovering this, Hammerling also demonstrated that the nucleus is the organelle that contains a cell's genetic material. Deoxyribonucleic acid, DNA, one of the important molecules that is found in the nucleus, functions as the physical carrier of inheritance. Though mostly found in the nucleus, a small amount is located in two other organelles, chloroplasts and mitochondria, due to their bacterial origins. Ribonucleic acid, RNA, the second important molecule often found in the nucleus, is made in the nucleus using the DNA base sequence as a template. After it is transcribed in the nucleus, RNA moves out into the cytoplasm where it functions in the assembly of proteins.
the powerhouse of the cell
Mitochondria are small organelles, similar to the size of bacteria, that are present in eukaryotic cells. A typical cell contains a few hundred to a few thousand mitochondria. Muscle cells, which have high energy demands, have more mitochondria than other cells and research has shown that regular exercise increases the number and size of mitochondria in muscle cells to meet the energy demands. A mitochondrion has two membranes separated by a region called the intermembrane space. The inner membrane, the site of ATP synthesis, has many folds with projections called cristae, which greatly increase the surface area to enhance ATP production. The region enclosed by the inner membrane is called the mitochondrial matrix.
The main function of the mitochondria is to make ATP. However, mitochondria do not make ATP directly, instead they convert chemical energy stored in the covalent bonds of organic molecule into ATP, a useable form of energy for cells. Sugars, fats, and amino acids store a large amount of energy, and it is the breakdown of these molecules into simpler ones that releases the energy to make ATP. ATP is utilized by cells to carry out their functions such as muscle contraction, nutrient uptake, and cell division, just to name a few. Mitochondria possess their own DNA, referred to as the mitochondrial genome. Similar to bacteria, the mitochondrial genome consists of a single circular chromosome and is very small in comparison to the genome found in the nucleus of the cell. The amount of DNA contained in the nucleus is about 200,000 times greater than the mitochondrial genome. Mitochondria are also capable of replication via binary fission, which is literally splitting in two.
The mitochondrial chromosome is duplicated and the organelle splits into two separate organelles with each of them containing the genetic material. Mitochondria division occurs to assure that after cell division, each cell has an adequate number of mitochondria, which enables cells to produce enough energy for cell growth.
Diving into the building blocks of cells: atoms
Most of our universe consists of matter and energy. Matter is defined as anything that has mass and occupies space, while energy is the capacity to do work. All matter is composed of elements that consist of only one type of atom. Atoms cannot be broken down into substances with different chemical or physical properties; they are the smallest particles into which an element can be divided by chemical processes.
The proton is located in the center (or nucleus) of an atom. Every atom has at least one proton. Protons have a charge of +1. Elements differ from each other in the number of protons they contain.
The neutron is another elemental particle also located in the nucleus of atoms (except in the case of hydrogen). Neutrons have no charge and have a mass similar to protons. The electron is a very small particle located outside the nucleus that has a charge of -1. Its mass is negligible, as it takes approximately 1800 electrons to equal the mass of just one proton. Electrons orbit the nucleus at nearly the speed of light. The high speed at which they travel makes it almost impossible to pinpoint their precise location. So electrons are said to occupy orbitals or areas where they have a high statistical probability of being located.
So What about isotopes?
The number of protons in an atom determines what element it is. However, the atoms of an element can have differing numbers of neutrons; these forms of an element are called isotopes. Some isotopes are unstable and therefore tend to lose energy (decay) through a process called The process of radioactive decay is valuable for many reasons. When isotopes of heavy elements like uranium or plutonium decay, they release large amounts of energy, which can be used to generate power. In biology, a fraction of the carbon, hydrogen, nitrogen, and other atoms found in living cells will be radioactive isotopes. Also, since isotopes decay at a steady rate into other elements, we can use the ratio of isotopes to estimate the age of fossils and other artifacts. Isotopes are referred to by their mass number, which is the number of protons plus the number of neutrons. Most of the carbon on Earth is carbon-12, meaning it has 6 protons and 6 neutrons (=12 total) in the nucleus. Carbon-14 is a radioactive isotope with 2 more neutrons in the nucleus than carbon-12. Carbon-14 decays slowly to produce Nitrogen-14. The ratio of C-14 to N-14 in a sample can help determine the age of wood and other biological materials that are up to 60,000 years old.
the name is bond. chemical bond.
In covalent bonds, atoms share electrons rather than giving or taking electrons. This is possible because electrons move so fast that they are able to be shared. In a covalent bond, the electron clouds surrounding the atomic nuclei overlap, as shown in Figure 1.
Covalent chemical bonds are formed by pairs of shared electrons. Each line in a bond represents one shared pair. Covalent chemical bonds between atoms are usually quite stable; often it takes a considerable amount of energy to break them. Also, depending on how the shared electrons are arranged, covalent bonds can store a lot of chemical energy.
Unlike carbon, which tends to share electrons, some atoms give up or take on extra electrons. If an atom loses an electron, it becomes positively charged due to the loss of the electron's negative charge and is now called a cation. If an atom gains an electron it becomes negatively charged because it an has an extra negative charge and is now called an anion. The attraction between the oppositely charged anion and cation is called an ionic bond.
Even though two atoms share electrons in a covalent bond, some atoms attract the electrons more strongly than others. This results in the electrons spending more time with one atom in the bond than with the other, creating a polar covalent bond.
Overall, the slight charges produced on the oxygen and hydrogen atoms in water means that each water molecule is polar, containing both slightly positive and negative regions. Weak electrical attractions between the slightly positive area on the hydrogen of one molecule and the negative area of another molecule create hydrogen bonds, shown in Figure 5. Individual hydrogen bonds are very weak, but many hydrogen bonds (abbreviated H-bonds) together are strong enough to hold molecules together in groups, and force molecules to fold into specific three-dimensional shapes. Hydrogen bonding between water molecules also accounts for many of the unique properties of water.