Biology - The Biology of Cells | Biology

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StudyGuideBiology–The Biology of Cells1. Cells DefinedOne of the most important ideas in biology is thatall living things are made of cells. Cells are thesmallest units of life, and every organism depends on them to survive.Some living things are very simple and haveonly one cell. These are calledunicellular organisms.Other organisms, like plants and animals, are much more complex. They are made ofmany cellsthatwork together. In these organisms, similar cells formtissues, tissues combine to formorgans, andorgans work together asorgan systems.Even though these organisms may seem complicated, everything they do—growing, moving, andstaying alive—depends on what happens inside their cells. Cells are truly at the center of all life.1.1 The Discovery of CellsThe study of cells began with the invention of the microscope.In the mid-1600s, an English scientist namedRobert Hookelooked at a thin slice of cork using one ofthe first microscopes. What he saw surprised him. The cork was made up of many tiny, box-likespaces. These spaces reminded him of the small rooms in a monastery, which were calledcells.Because of this similarity, Hooke gave them the namecells—a name we still use today.Around the same time, a Dutch merchant namedAnton van Leeuwenhoekmade even moredetailed observations. Using powerful microscopes that he designed himself, he studiedplant cells,animal cells, and tiny living organisms. His work helped scientists realize that cells were not justfound in cork but were part of many forms of life.1.2 The Development of Cell TheoryAs scientists continued studying cells, they began to notice important patterns.In 1838, a German botanist namedMatthias Schleidenconcluded thatall plants aremade of cells.One year later, his colleagueTheodor Schwann, an anatomist, expanded this idea by stating thatallanimals are also made of cells.

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StudyGuideA few years later, in 1858, another scientist namedRudolf Virchowadded a crucial idea. Heproposed thatnew cells do not appear on their own. Instead,all cells come from preexistingcells.1.3 The Cell TheoryTogether, these discoveries formed what we now call thecell theory, which includes three mainideas:•All living things are made of one or more cells.•The cell is the basic unit of structure and function in living organisms.•All cells come from preexisting cells.This theory is a foundation of modern biology and helps us understand how life is organized and howliving things grow and reproduce.2. Movement Through the Plasma MembraneFor a cell to stay alive, it must constantly exchange materials with its surroundings. Nutrients need toenter the cell, and wastes must leave. This exchange happens through theplasma membrane,which acts like aselective gate. Materials move across this membrane in several different ways,depending on their size, type, and direction of movement.2.1 Diffusion: Moving from Crowded to Less CrowdedOne of the simplest ways materials move across the membrane is throughdiffusion.Diffusion is the movement of molecules from an area where they aremore concentratedto an areawhere they areless concentrated. This happens naturally because molecules are always movingand bumping into one another.As a result, there is anet movementof molecules away from crowded areas and toward lesscrowded areas. This movement follows what is called aconcentration gradient, which simply meansthe difference in concentration between two areas.A good example of diffusion is adding a drop of dye to water. At first, the dye is very concentrated inone spot. Over time, it spreads out evenly throughout the water without any stirring. That spreading isdiffusion.

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StudyGuide2.2 Osmosis: Diffusion of WaterAnother important type of movement isosmosis.Osmosis is the movement ofwater moleculesfrom an area of higher water concentration to an areaof lower water concentration. Like diffusion, it happens naturally and does not require energy.Osmosis occurs across asemipermeable membrane. This type of membrane allows somemolecules, such as water, to pass through while blocking others. Because osmosis involves onlywater, it is considered a special type of diffusion.Osmosis helps cells maintain the right balance of water, which is essential for normal cell function.2.3 Facilitated Diffusion: Help from ProteinsSome molecules are too large or too specific to pass through the plasma membrane on their own.This is wherefacilitated diffusioncomes in.In facilitated diffusion,special proteinsin the membrane help certain molecules cross themembrane. These proteins act like doors or channels, allowing only specific substances to passthrough.Even though proteins help with this process, the movement still follows the rules of diffusion.Molecules move from an area of higher concentration to an area of lower concentration. No energy isneeded.2.4 Active Transport: Moving Against the GradientSometimes, cells need to move materialsagainstthe concentration gradient. This means movingsubstances from an area of lower concentration to an area of higher concentration. This process iscalledactive transport.Because this movement goes against the natural flow, the cell must useenergy. That energy usuallycomes from a molecule calledATP (adenosine triphosphate).Active transport uses membrane proteins to move substances where they are needed. A goodexample occurs inhuman nerve cells, where sodium ions are constantly pumped out of the cell—

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StudyGuideeven though there is already a high concentration of sodium outside. This process prepares the nervecell to send signals later.2.5 Endocytosis and Exocytosis: Large-Scale TransportSome materials are too large to pass through the membrane using the methods above. For thesesituations, cells useendocytosisandexocytosis.2.5.1Endocytosis: Bringing Materials InEndocytosisis a process in which the plasma membrane wraps around particles or fluid near the cellsurface. The membrane then pinches off and forms a small sac called avesicle, which moves intothe cytoplasm.There are two main types of endocytosis:•Phagocytosis: the vesicle contains solid particles•Pinocytosis: the vesicle contains liquid dropletsThese processes allow cells to take in large materials that cannot cross the membrane on their own.2.5.2Exocytosis: Sending Materials OutExocytosisis the opposite of endocytosis. In this process, materials made inside the cell arepackaged into vesicles. These vesicles move to the plasma membrane, fuse with it, and release theircontents outside the cell.2.6 Why These Processes MatterAll of these transport methods work together to keep the cell’s internal environment stable. Bycarefully controlling what enters and leaves, the cell can grow, respond to its environment, andcontinue functioning properly.

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StudyGuide3. Quiz Movement through the Plasma Membrane1.QuestionFill in the blank:What do active transport and facilitated diffusion have in common?Answer Choices•They both move substances up their concentration gradient.•They both require energy.•They both require membrane proteins.Correct AnswerThey both require membrane proteins.Why This Is CorrectBothactive transportandfacilitated diffusionusemembrane proteinsto help substances moveacross the cell membrane. These proteins act as channels or carriers that allow specific molecules orions to pass through.The other choices are incorrect because:•Onlyactive transportmoves substancesagainsttheir concentration gradient.•Onlyactive transportrequires energy, whilefacilitated diffusion does not.2. QuestionFill in the blank:The diffusion of water is called:Answer Choices•endocytosis•facilitated diffusion•osmosis

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StudyGuideCorrect AnswerosmosisWhyThis Is CorrectOsmosisis the movement (diffusion) ofwater moleculesacross a selectively permeable membranefrom an area of higher water concentration to an area of lower water concentration.The other options are incorrect because:•Endocytosisinvolves the cell taking material in using vesicles.•Facilitated diffusionmoves substances other than water with the help of membraneproteins.3. QuestionFill in the blank:When a white blood cell engulfs a bacterium by folding itinto its cell membrane andcreating a vesicle around it, this is called:Answer Choices•facilitated diffusion•phagocytosis•pinocytosisCorrect AnswerphagocytosisWhy This Is CorrectPhagocytosisis a type of endocytosis in which a cellengulfs large particles, such as bacteria, bysurrounding them with its cell membrane and enclosing them in a vesicle. White blood cells usephagocytosis as a defense mechanism to destroy pathogens.

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