PROKARYOTIC CELLS VS. EUKARYOTIC CELLS
There are several differences between prokaryotic and eukaryotic cells:
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Cells are usually small since it gets harder to sustain themselves the bigger they get. Increasing the diameter of a cell would increase the volume of the cell by a much greater factor than increasing the surface area. It's important for a cell to have a relatively large surface area, especially if that cell interacts with their surroundings by exchanging materials. A cell with a greater volume and a smaller surface area won't be able to acquire enough materials from their surroundings to sustain their large volume.
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ENDOMEMBRANE SYSTEM - EXCLUDING NUCLEUS AND RIBOSOMES
The nucleus contains the genetic information of the cells. It's made up of a nuclear envelope, which is a double membrane perforated by pores that allow things to go back and forth from the cytoplasm to the interior of the nucleus. Each pore is lined by an intricate protein structure called a pore complex, which regulate the entry and exit of RNA and protein molecules, or other macromolecules. The nuclear envelope is held up within by the nuclear lamina, which provides structure to the nucleus. Inside the nucleus, the chromosomes and the proteins that make them up is called chromatin. The nucleolus contains densely gathered chromatin, which is responsible for make rRNA, which then makes ribosomes after the large and small subunits leave the nucleus and join together during translation.
Free ribosomes are suspended wherever in the cytosol, but the bound ribosomes are bound to the rough endoplasmic reticulum, which then makes the necessary proteins based off the mRNA coming from the nucleus. The bound ribosomes make proteins that are usually used within the cell or to become part of the plasma membrane. These secretory proteins are covalently bonded with carbohydrates, forming glycoproteins, which separates them from the proteins formed by the free ribosomes in the rough ER. The glycoproteins then leave the rough ER through the transport of vesicles, which take them to the Golgi apparatus. The Golgi apparatus acts like the warehouse of the cell, receiving, packaging, manufacturing, and shipping the proteins to wherever they're needed in the cell. The proteins are modified, stored, and then sent to other locations, like a box being banded with a ZIP code to go to a certain address. The smooth endoplasmic reticulum functions in diverse metabolic processes. It helps detoxify drugs by adding hydroxyl groups to them, making them more soluble. The smooth ER is also responsible for the synthesis of lipids, such as phospholipids, steroids, and oils. Sex organs have a very elaborate smooth ER for the purpose of making sex hormones. Another function of this organelle is the metabolism of carbohydrates and storage of calcium ions. Lysosomes create an acidic environment for hydrolytic enzymes to break apart macromolecules. If one lysosome breaks, it won't harm the entire cell, but if several break, the cell can be destroyed by self-digestion. The enzymes and the lysosome membranes are made in the rough ER and sent to the Golgi apparatus for further processing. Lysosomes break down macromolecules in a process known as phagocytosis, which is when a food vacuole fuses with a lysosome, subsequently getting digested from the hydrolytic enzymes present. Lysosomes are necessary to break down substances that would otherwise cause a harmful build-up in the cell, such as the build-up of fats in the brains of people with Tay-Sachs because they lack the lysosomes to digest the fats. Vacuoles are large vesicles derived from the endoplasmic reticulum and Golgi apparatus. Vacuole membranes are selective in transporting solutes across it, leading to a difference in solution inside the vacuole and in the cytosol. Food vacuoles carry food into the cell. Contractile vacuoles, found in freshwater protists, pump out water if there is too much water entering through diffusion. If the contractile vacuole didn't exist, then through osmosis, the cell would've burst because of the freshwater environment. However, the contractile vacuole maintains a constant concentration difference, preventing such a thing from happening. The central vacuole in plants also help in the growth of plant cells, which grow as the cell acquire more water, which allows the cell itself to grow with minimal investment in the growth of the cytosol. |
ENDOSYMBIONT THEORY
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Mitochondria provide the cell with ATP by breaking down glucose in a process called cellular respiration. Chloroplasts, on the other hand, using light energy to create glucose, which is later stored so that it could be used later in a process called photosynthesis. Chloroplasts are found in plants and algae, not animals, though mitochondria can be found in both plants and animals.
Mitochondria and chloroplasts were thought to originate from prokaryotic cells who later developed a symbiotic relationship with eukaryotic cells who engulfed them. This theory, called the endosymbiont theory, comes from the significant similarities between chloroplasts, mitochondria, and prokaryotic cells. Mitochondria and chloroplasts contain their own circular DNA that is separate from the DNA from the eukaryotic cell they dwell in. They also produce their own ribosomes, showing a behavior autonomous of the host cell they occupy, reproducing and growing on their own accord. Mitochondria and chloroplasts are also double-membraned, similar to ancestral engulfed prokaryotes thought to have become mitochondria and chloroplasts.
Peroxisomes are a specialized metabolic compartment that, like the smooth ER, detoxifies drugs, and like the lysosome, breaks down macromolecules into smaller molecules, such as breaking down fatty acids into smaller molecules that are then transferred to the mitochondria to act as fuel for cellular respiration. Peroxisomes remove hydrogen atoms from various substrates and join it with oxygen (O2) to form hydrogen peroxide, which is used in the breaking down process. Hydrogen peroxide is potentially toxic itself, however, peroxisomes contain enzymes that break it down into water. Glyoxysomes in plants contain specialized enzymes that convert fatty acids to sugar for the seed to use as an energy source until it can start photosynthesizing and producing its own sugar. Peroxisomes can grow in size by incorporating proteins and lipids from the cytosol and ER. They increase in number by splitting in two after reaching a certain size.
Mitochondria and chloroplasts were thought to originate from prokaryotic cells who later developed a symbiotic relationship with eukaryotic cells who engulfed them. This theory, called the endosymbiont theory, comes from the significant similarities between chloroplasts, mitochondria, and prokaryotic cells. Mitochondria and chloroplasts contain their own circular DNA that is separate from the DNA from the eukaryotic cell they dwell in. They also produce their own ribosomes, showing a behavior autonomous of the host cell they occupy, reproducing and growing on their own accord. Mitochondria and chloroplasts are also double-membraned, similar to ancestral engulfed prokaryotes thought to have become mitochondria and chloroplasts.
Peroxisomes are a specialized metabolic compartment that, like the smooth ER, detoxifies drugs, and like the lysosome, breaks down macromolecules into smaller molecules, such as breaking down fatty acids into smaller molecules that are then transferred to the mitochondria to act as fuel for cellular respiration. Peroxisomes remove hydrogen atoms from various substrates and join it with oxygen (O2) to form hydrogen peroxide, which is used in the breaking down process. Hydrogen peroxide is potentially toxic itself, however, peroxisomes contain enzymes that break it down into water. Glyoxysomes in plants contain specialized enzymes that convert fatty acids to sugar for the seed to use as an energy source until it can start photosynthesizing and producing its own sugar. Peroxisomes can grow in size by incorporating proteins and lipids from the cytosol and ER. They increase in number by splitting in two after reaching a certain size.
CYTOSKELETON
There are three basic kinds of cytoskeleton structures:
- microtubules: made up of alpha and beta tubulin dimers and wrapped in the shape of a hollow rod. Microtubules are like the highway of the cell, allowing vesicles to move along them from one part of the cell to another, using motor proteins and ATP to power its "walk" through the microtubules spread along the cell. Microtubules also aid in cell motility, such as flagella and cilia movement. This is due to ATP reacting with the dynein ladders between microtubule doublets arranged in 9+2 formation in cilia or flagella (compared to the 9+0 formation at the basal body, with microtubule triplets instead of microtubule doublets), which then causes one of the microtubule doublets to shift upward. However, since the doublets can't shift due to being restrained by cross-linking proteins between the doublets, the force of the "upward" motion then causes the cilia or flagella to bend in a wavelike manner, which is what propels the motion of the cell. Microtubules are also important during mitosis and meiosis, since they form the spindle fibers that separate the chromosomes. In animals, the spindle fibers and centrioles are created by the centrosome. However, plants and algae don't have centrosomes, and instead are born with the amount of centrioles that they'll ever need in their lifespan.
- microfilaments: two intertwined strands of actin. This is the smallest cytoskeleton structure. Microfilaments help with cell motion, since the actin reacts with another protein, myosin, which then causes them both to contract. Microfilaments help support the cell's shape by giving the cortex, or the outer cytoplasmic layer of the cell, a semisolid consistency of gel, contrasting with the more fluid sol, or the interior of the cytoplasmic layer. When actin reacts with myosin, many important things can happen. For example, during cytokinesis in mitosis and meiosis, the pinching of the cell into two separate cells is made possible by this reaction. Amoebas are able to "walk" by extending their pseudopodia (fake feet) and then moving the rest of their bodies towards them. Another product of actin-myosin reactions is the conversion from the sol to gel and from gel to sol transformations in the cytoplasm of the cell, a process known as cytoplasmic streaming, which helps distribute materials throughout the cell.
- intermediate filaments: made up of fibrous proteins coiled together, such as keratin. Intermediate filaments are more permanent and sturdier than microtubules and microfilaments, who are always rearranging their shape and place within the cell. Intermediate filaments reinforce cell shape, fix its position and keep it anchored, and also make up the nuclear lamina within the nucleus.
EXTRACELLULAR MATRIX (ECM)
In plant cells, the cell is surrounded by a cell wall. The plant cells are stacked together like little boxes, however, the little boxes don't fill up completely. The spaces in between the cell walls of adjacent plant cells is called the middle lamella. Some plant cells add a secondary cell wall between the primary cell wall and the plasma membrane. All plant cells are connected by plasmodesmatas, which connects all the cells into one living thing. The plasmodesmatas serve to transfer substances from one cell to the other through diffusion or other means.
The ECM of animal cells is a bit different, since animals cells don't have a cell wall. The ECM is made up of glycoproteins (proteins covalently bonded with carbohydrates), such as collagen. Collagen is embedded within a network of proteoglycan complexes, which are made up of proteoglycans, a small core protein covalently bonded with several carbohydrates attached to it. A proteoglycan complex is created when a lot of these proteoglycan molecules noncovalently bond to a single large polysaccharide molecule. The plasma membrane of animals cells are studded with transmembrane proteins called integrins, and some of these proteins are connected to these proteoglycan complexes through fibronectins, or anal pores.
Animal cells also have cell to cell connections like plasmodesmatas:
The ECM of animal cells is a bit different, since animals cells don't have a cell wall. The ECM is made up of glycoproteins (proteins covalently bonded with carbohydrates), such as collagen. Collagen is embedded within a network of proteoglycan complexes, which are made up of proteoglycans, a small core protein covalently bonded with several carbohydrates attached to it. A proteoglycan complex is created when a lot of these proteoglycan molecules noncovalently bond to a single large polysaccharide molecule. The plasma membrane of animals cells are studded with transmembrane proteins called integrins, and some of these proteins are connected to these proteoglycan complexes through fibronectins, or anal pores.
Animal cells also have cell to cell connections like plasmodesmatas:
- tight junctions: the plasma membrane of neighboring cells are pressed tightly together, bound together by proteins like buttons holding together a button-down shirt. This prevents extracellular leakage of fluid, which is why so many tight junctions are within epithelial cells to make the skin as watertight as possible to prevent leakage
- desmosomes: anchor parts of the cell membrane together, attaching like rivets, or nails going through a wall. They're made up of sturdy intermediate filaments and are found in muscle cells. Muscle tears can sometimes be the result of ruptured desmosomes
- gap junctions: most similar to the plasmodesmatas found in plant cells. They provide channels for the substances to travel from one cell to another.