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Chapter Seven- Cells MicroscopesMicroscopes have enabled scientists to look at cells. The first types of microscopes used were light microscopes, and these are the kind we use in labs. Two values are important when dealing with microscopes : Magnification and Resolving power.Resolving Power is a measure of the clarity of the image. Magnification is how much larger the object appears than its real size.Electron microscopes have helped scientists discover the parts of cells. Electron microscopes focus a beam of electrons through a specimen. They are a hundred times better than a light microscope.There are two different types of electron microscopes: The transmission electron microscope and the scanning electron microscope. The transmission electron microscope is mainly used to study the inside structure of cells.The scanning electron microscope is used to see the surface of the specimen. It creates a three 3-D image.One bad thing about electron microscopes is that they kill the specimens they are looking at.Isolating OrganellesSometimes, scientists want to look at certain organelles of a cell. Cell fractionation takes cells apart, allowing scientists to look at certain organelles.Machines that fractionate cells are called centrifuges. It is a machine that is like a merry-go-round of test tubes that spins so fast that cells break apart. The most powerful centrifuges are ultracentrifuges.Fractionation begins when the centrifuge spins, and the cells become this soupy mixture called homogenate. The heavier organelles sink to the bottom of the homogenate and become this mass called the pellet. The lighter organelles stay in the liquid part of the homogenate and is called the supernatant.Scientists can repeat this process until they obtain the organelle they want. Centrifuges allow scientists to gather large quantities of a certain organelle to study!Prokaryotic and Eukaryotic Cell DifferencesMost importantly, a prokaryotic cell does not have a nucleus!! Its DNA is concentrated in a region called the nucleoid, but there is no membrane that holds the DNA. It only has one circular chromosome.Prokaryotes are .1-10 micrometers. Eukaryotes are 10-100 micrometers.Eukaryotic cells do have a nucleus. The nucleus holds DNA within a membrane called the “nuclear envelope”. The area between the nucleus and the cell membrane is called the cytoplasm. It contains a semifluid substance called the cytosol and organelles within the cytosol.Prokaryotic cells are smaller than eukaryotic cells. They also have cell walls, while eukaryotic cells do not.No compartmentalization in prokaryotes, only in eukaryotes.Cells cannot be too big, because then they do not have a large enough surface area for metabolism. When a cell grows in size, its volume increases, but its surface area decreases. This is why cells are so small, and why bigger organisms have more cells, not BIGGER cells.The plasma membrane surrounds the cell, and allows stuff like oxygen, nutrients, and wastes to pass through it.Usually, biological membranes consist of a double layer of phospholipids and proteins embedded in it.The Nucleus The nucleus contains most of the DNA that control the eukaryotic cell. (The other DNA is found in chloroplasts and mitochondria). The nucleus is usually the most visible organelle.The nucleus is surrounded by the nuclear envelope. The nuclear envelope is a double membrane. Each membrane is a lipid bilayer with proteins. Each membrane has pores that allow macromolecules and particles to pass through.One side of the nuclear envelope is lined by nuclear lamina, which is a lot of protein filaments that maintain the shape of the nucleus.Inside the nucleus, DNA is organized with proteins into a mass called chromatin. When the cell is going to divide, the chromatin divided and coils up to become chromosomes. Each species has its own number of chromosomes. We have 46 chromosomes.Another important structure in the nucleus is the nucleolus, where ribosome parts are created. Then these ribosome parts exit the nucleus through the nuclear pores and assemble in the cytoplasm.The nucleus gives instructions for protein synthesis. Messenger RNA created in the nucleus leaves through the nuclear pores and attaches to ribosomes, providing instructions of what proteins to make.RibosomesRibosomes are the sites where cells make proteins. If a cell makes a large amount of proteins, it will have lots of ribosomes, and a large nucleolus.There are two different types of ribosomes: free ribosomes and bound ribosomes. Free ribosomes hang out in the cytosol, and bound ribosomes are ribosomes that are bound to membranes of organelles. Proteins made by free ribosomes work in the cytosol, while proteins made by bound ribosomes are either going to be included in membranes, exported from the cell, or used by certain organelles. For example, organs that secrete proteins, like the pancreas, have cells with more bound ribosomes.Bound and free ribosomes have the same structure. The Endomembrane System The endomembrane system includes the different membranes of the eukaryotic cell. The nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vacuoles, perixosomes, and the plasma membrane are all part of the endomembrane system.1st part of Endomembrane system: The Endoplasmic Reticulum (ER)-The endoplasmic reticulum (ER) is a really big organelle. The ER consists of the smooth endoplasmic reticulum and the rough endoplasmic reticulum.The smooth ER creates lipids such as steroids and phospholipids. Cells that secrete sex steroids have large quantities of smooth ER. The smooth ER also helps with metabolism and with detoxifying poisons. The rough ER is rough because ribosomes are attached to it. It looks “rough” through a microscope because ribosomes are attached. The rough ER synthesizes secretory proteins. Secretory proteins are proteins that glands will secrete throughout the body. Glycoproteins, which are proteins bonded to carbohydrates, are usually secretory proteins.Transport vesicles are little membranes that the ER makes that surround a secretory protein. They move the protein from one part of the cell to another.The rough ER also makes membrane proteins.The Golgi ApparatusTransport vesicles from the ER usually travel to the Golgi apparatus. The Golgi Apparatus is a center of manufacturing, warehousing, sorting, and shipping. Products from the ER are modified and sent to other places by the Golgi Apparatus. The Golgi apparatus is a bunch of flattened sacs on top of each other. They look like a stack of pita bread. Each sac has a cis face, which is the side that a molecule enters to be altering and shipped. The trans face is the side where a molecule leaves the Golgi apparatus and is sent to another part of the cell. Molecular tags are added to products leaving the Golgi apparatus and the vesicles that contain them, so that they will reach their destination point.LysosomesLysosomes are membrane bound sacks of enzymes. The cell uses lysosomes to help digest macromolecules.Lysosome enzymes only work within the lysosome, because it has an acidic pH. Lysosomes digest macromolecules within their membranes, because if they didn’t, the enzymes would damage the cells!-Certain single celled organisms eat by phagocytosis, which is when they engulf a piece of food and the lysosomes break it down.-Sometimes, lysosomes will digest old parts of the cell, and recycle its molecules for the cell. VacuolesVacuoles are membrane bound sacs in a cell. They have lots of different jobs.Food vacuoles are vacuoles that contain food until a lysosome comes and digests it.Contractile vacuoles are vacuoles that pump water out of a cell. This is common in protists that live in freshwater.Most plant cells have a central vacuole. It can store proteins, ions, and wastes produced by metabolic processes. Sometimes vacuoles will have pigments that make the plant look colored. Also, it may hold poison to ward off animals. The central vacuoles is surrounded by a membrane called the tonoplast.Mitochondria and ChloroplastsMitochondria are the sites of cellular respiration, and get energy from fats and sugars. They have their own DNA.They are found in almost all eukaryotic cells. They have two membranes, and the first membrane is smooth. However, the second membrane is wiggly looking. The folds of the inner membrane are called cristae. The cristae give lots of space for cellular respiration to happen.The region between the two membranes is called the intermembrane space. The space inside the second membrance is called the mitochondrial matrix.A chloroplast is one type of plastid. Plastids are plant organelles.The chloroplast has two membranes. Inside the 2nd membrane are these flattened sacs called Thylakoids. They are stacked on top of each other, like a stack of checkers. These stacks are called grana. There is fluid surrounding the grana, and it is called the stroma. PeroxisomesPeroxisomes are membrane bound organelles that contain enzymes. These enzymes create hydrogen peroxide by adding hydrogen ions to oxygen. It creates hydrogen peroxide as a result, and it is able to be flushed from the cell.They can break down fats and detoxify alcohol and other harmful things. Ex. The liver, which detoxifies alcohol when you drink, has a large amount of peroxisomes.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~The CytoskeletonThe cytoskeleton is a network of fibers in the cell.It has many jobs: It provides support for the cell and helps it maintain its shape. It also helps keep organelles in their place. It can be changed to suit the changing cell.The cytoskeleton is also involved with cell motion. It can cause cells to move, to engulf a food particle, or cause cytoplasmic streaming(when the cytoplasm can circulate materials).There are three types of fibers that make up the cytoskeleton.1. Microtubules-The thickest kind. They are made of a protein called tubulin. Microtubules support the cell and can be a path for organelles to move along. -Microtubules grow out from a centrosome, which is a region located near the nucleus. Within the centrosome of an animal cell are two centrioles, which are a ring of microtubules. -Sometimes, eukaryotes have cilia or flagella, which are arrangements of microtubules that allow the cell to move. A flagellum is a tail like thing that causes the cell to move. Cilia are little hairlike structures that move the cell. They are made of microtubules.Cilia and flagella have the same type of structure. Nine pairs of microtubules form a ring, and then in the middle of the ring is another pair of microtubules. This is called “9+2 pattern” .The cilia or flagella are connected to a structure on the cell called the basal body.-A protein called dynein allows the flagella or cilia to move.-Microfilaments- The thinnest kind of fiber. It is made from the protein actin. - In combination with other proteins, they form a three-dimensional network just ??????????? inside cell membrane that helps support the cell shape.-Microfilaments help with cell motility, especially in muscle cells. -Actin and Myosin, another protein, are linked together and contract to create movement. Pseudopodia are extensions of the cell created by actin and myosin that causes the cell to move. If you’ve ever seen amoebas move, this is pseudopodia.Microfilaments cause the cleavage furrow in telophase and cytoplasmic streaming.Intermediate filaments- The middle size fiber. They are made from keratin proteins. They also support cell structure. They are more permanent than the other two fibers.Cell Surfaces and JunctionsPlant cells have cell walls, while animal cells do not. The cell wall protects the plant cell, maintains its shape, and prevents too much absorption of water. Made of cellulose. (which is indigestible)A young plant cell has a thin and flexible cell wall called the primary cell wall. Between different cells is a substance called the middle lamella. The middle lamella acts like glue that sticks different cell walls together.When a plant cell stops growing, it strengthens its wall. Sometimes they will add a secondary cell wall between the plasma membrane and the primary cell wall.Though animal cells do not have cell walls, we do have an extracellular matrix. The main part of the extracellular matrix are glycoproteins (remember, these are proteins bonded to carbohydrates)The most common glycoprotein in the matrix is collagen. Collagen is connected to a network of other proteins called proteoglycans. Integrins are proteins imbedded in the cell membrane that communicates with the extracellular matrix. Cells are often organized into organs or tissues, so they must be joined together. Plasmodesmata are junctions between plant cells. Plasmodesmata are holes in the cell wall that allow plant cells to connect and exchange their cytoplasm. In animal cells, there are three different types of junctions: Tight junctions, desmosomes, and gap junctions. Tight junctions: Intercellular junctions that hold cells together very tightly so that there is no leakage of substances through the intercellular space. This is like cells of the small intestine. This makes sure that nutrients from food do not leak out but are absorbed.Desmosomes: These are junctions that rivet cells together, but still allow substances to flow in the spaces between cells.Gap Junctions: These are junctions between cells that are specialized to allow exchanges between cells. ................
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