7.1 Life Is Cellular - Weebly

Lesson OverviewCell StructureLysosomesOne function of lysosomes is the breakdown of lipids, carbohydrates,and proteins into small molecules that can be used by the rest of thecell.

Lesson OverviewCell StructureLysosomesLysosomes are also involved in breaking down organelles that haveoutlived their usefulness.Biologists once thought that lysosomes were only found in animal cells, but itis now clear that lysosomes are also found in a few specialized types ofplant cells as well.

Lesson OverviewCell StructureThe CytoskeletonEukaryotic cells are given their shape and internal organization by anetwork of protein filaments known as the cytoskeleton.Certain parts of the cytoskeleton also help to transport materialsbetween different parts of the cell, much like conveyer belts that carrymaterials from one part of a factory to another.Microfilaments and microtubules are two of the principal proteinfilaments that make up the cytoskeleton.

Lesson OverviewCell StructureMicrofilamentsMicrofilaments are threadlike structures made up of a protein calledactin.They form extensive networks in some cells and produce a tough,flexible framework that supports the cell.Microfilaments also help cells move.Microfilament assembly and disassembly is responsible for thecytoplasmic movements that allow cells, such as amoebas, to crawlalong surfaces.

Lesson OverviewCell StructureMicrotubulesMicrotubules are hollow structures made up of proteins known astubulins.They play critical roles in maintaining cell shape.Microtubules are also important in cell division, where they form astructure known as the mitotic spindle, which helps to separatechromosomes.

Lesson OverviewCell StructureMicrotubulesIn animal cells, structures known as centrioles are also formed fromtubulins.Centrioles are located near the nucleus and help to organize celldivision.Centrioles are not found in plant cells.

Lesson OverviewCell StructureMicrotubulesMicrotubules help to build projections from the cell surface,which are known as cilia and flagella, that enable cells to swimrapidly through liquids.Microtubules are arranged in a “9 2” pattern.Small cross-bridges between the microtubules in theseorganelles use chemical energy to pull on, or slide along, themicrotubules, allowing cells to produce controlled movements.

Lesson OverviewCell StructureOrganelles That Build ProteinsWhat organelles help make and transport proteins?Proteins are assembled on ribosomes.Proteins made on the rough endoplasmic reticulum include thosethat will be released, or secreted, from the cell as well as manymembrane proteins and proteins destined for lysosomes and otherspecialized locations within the cell.The Golgi apparatus modifies, sorts, and packages proteins andother materials from the endoplasmic reticulum for storage in thecell or release outside the cell.

Lesson OverviewCell StructureOrganelles That Build ProteinsCells need to build new molecules all the time, especially proteins, whichcatalyze chemical reactions and make up important structures in the cell.Because proteins carry out so many of the essential functions of livingthings, a big part of the cell is devoted to their production and distribution.Proteins are synthesized on ribosomes, sometimes in associationwith the rough endoplasmic reticulum in eukaryotes.

Lesson OverviewCell StructureRibosomesRibosomes are small particles of RNA and protein found throughoutthe cytoplasm in all cells.Ribosomes produce proteins by following coded instructions thatcome from DNA.Each ribosome is like a small machine in a factory, turning out proteinson orders that come from its DNA “boss.”

Lesson OverviewCell StructureEndoplasmic ReticulumEukaryotic cells contain an internal membrane system known as theendoplasmic reticulum, or ER.The endoplasmic reticulum is where lipid components of the cellmembrane are assembled, along with proteins and other materials thatare exported from the cell.

Lesson OverviewCell StructureEndoplasmic ReticulumThe portion of the ER involved in the synthesis of proteins is calledrough endoplasmic reticulum, or rough ER. It is given this namebecause of the ribosomes found on its surface.Newly made proteins leave these ribosomes and are inserted into therough ER, where they may be chemically modified.

Lesson OverviewCell StructureEndoplasmic ReticulumThe other portion of the ER is known as smooth endoplasmic reticulum(smooth ER) because ribosomes are not found on its surface.In many cells, the smooth ER contains collections of enzymes thatperform specialized tasks, including the synthesis of membrane lipidsand the detoxification of drugs.

Lesson OverviewCell StructureGolgi ApparatusProteins produced in the rough ER move next into the Golgi apparatus,which appears as a stack of flattened membranes.The proteins are bundled into tiny vesicles that bud from the ER andcarry them to the Golgi apparatus.

Lesson OverviewCell StructureGolgi ApparatusThe Golgi apparatus modifies, sorts, and packages proteins and othermaterials from the ER for storage in the cell or release outside the cell.It is somewhat like a customization shop, where the finishing touchesare put on proteins before they are ready to leave the “factory.”

Lesson OverviewCell StructureGolgi ApparatusFrom the Golgi apparatus, proteins are “shipped” to their finaldestination inside or outside the cell.

Lesson OverviewCell StructureOrganelles That Capture and ReleaseEnergyWhat are the functions of chloroplasts and mitochondria?Chloroplasts capture the energy from sunlight and convert it intofood that contains chemical energy in a process calledphotosynthesis.Mitochondria convert the chemical energy stored in food intocompounds that are more convenient for the cells to use.

Lesson OverviewCell StructureOrganelles That Capture and ReleaseEnergyAll living things require a source of energy. Most cells are powered byfood molecules that are built using energy from the sun.Chloroplasts and mitochondria are both involved in energyconversion processes within the cell.

Lesson OverviewCell StructureChloroplastsPlants and some otherorganisms contain chloroplasts.Chloroplasts are the biologicalequivalents of solar powerplants. They capture the energyfrom sunlight and convert it intofood that contains chemicalenergy in a process calledphotosynthesis.

Lesson OverviewCell StructureChloroplastsTwo membranes surround chloroplasts.Inside the organelle are large stacks of other membranes, whichcontain the green pigment chlorophyll.

Lesson OverviewCell StructureMitochondriaNearly all eukaryotic cells, including plants, contain mitochondria.Mitochondria are the power plants of the cell. They convert thechemical energy stored in food into compounds that are moreconvenient for the cell to use.

Lesson OverviewCell StructureMitochondriaTwo membranes—an outer membrane and an inner membrane—enclose mitochondria. The inner membrane is folded up inside theorganelle.

Lesson OverviewCell StructureMitochondriaOne of the most interesting aspects of mitochondria is the way in which theyare inherited.In humans, all or nearly all of our mitochondria come from thecytoplasm of the ovum, or egg cell. You get your mitochondria fromMom!

Lesson OverviewCell StructureMitochondriaChloroplasts and mitochondria contain their own genetic informationin the form of small DNA molecules.The endosymbiotic theory suggests that chloroplasts andmitochondria may have descended from independent microorganisms.

Lesson OverviewCell StructureCellular BoundariesWhat is the function of the cell membrane?The cell membrane regulates what enters and leaves the cell andalso protects and supports the cell.

Lesson OverviewCell StructureCellular BoundariesA working factory has walls and a roof to protect it from the environmentoutside, and also to serve as a barrier that keeps its products safe andsecure until they are ready to be shipped out.

Lesson OverviewCell StructureCellular BoundariesSimilarly, cells are surrounded by a barrier known as the cellmembrane.Many cells, including most prokaryotes, also produce a strongsupporting layer around the membrane known as a cell wall.

Lesson OverviewCell StructureCell WallsThe main function of the cell wall is to provide support and protectionfor the cell.Prokaryotes, plants, algae, fungi, and many prokaryotes have cellwalls. Animal cells do not have cell walls.Cell walls lie outside the cell membrane and most are porous enoughto allow water, oxygen, carbon dioxide, and certain other substancesto pass through easily.

Lesson OverviewCell StructureCell MembranesAll cells contain a cell membrane that regulates what enters and leavesthe cell and also protects and supports the cell.

Lesson OverviewCell StructureCell MembranesThe composition of nearly all cell membranes is a double-layered sheetcalled a lipid bilayer, which gives cell membranes a flexible structureand forms a strong barrier between the cell and its surroundings.

Lesson OverviewCell StructureThe Properties of LipidsMany lipids have oily fatty acid chains attached to chemical groupsthat interact strongly with water.The fatty acid portions of such a lipid arehydrophobic, or “water-hating,” while theopposite end of the molecule is hydrophilic, or“water-loving.”

Lesson OverviewCell StructureThe Properties of LipidsWhen such lipids are mixed with water, theirhydrophobic fatty acid “tails” cluster togetherwhile their hydrophilic “heads” are attracted towater. A lipid bilayer is the result.

Lesson OverviewCell StructureThe Properties of LipidsThe head groups of lipids in a bilayer are exposedto water, while the fatty acid tails form an oilylayer inside the membrane from which water isexcluded.

Lesson OverviewCell StructureThe Fluid Mosaic ModelMost cell membranes contain protein moleculesthat are embedded in the lipid bilayer.Carbohydrate molecules are attached to many ofthese proteins.

Lesson OverviewCell StructureThe Fluid Mosaic ModelBecause the proteins embedded in the lipidbilayer can move around and “float” among thelipids, and because so many different kinds ofmolecules make up the cell membrane, scientistsdescribe the cell membrane as a “fluid mosaic.”

Lesson OverviewCell StructureThe Fluid Mosaic ModelSome of the proteins form channels and pumpsthat help to move material across the cellmembrane.Many of the carbohydrate molecules act likechemical identification cards, allowing individualcells to identify one another.

Lesson OverviewCell StructureThe Fluid Mosaic ModelAlthough many substances can cross biologicalmembranes, some are too large or too strongly chargedto cross the lipid bilayer.If a substance is able to cross a membrane, themembrane is said to be permeable to it.A membrane is impermeable to substances that cannotpass across it.Most biological membranes are selectively permeable,meaning that some substances can pass across themand others cannot. Selectively permeable membranesare also called semipermeable membranes.

Lesson Overview7.3 Cell Transport

Lesson OverviewCell TransportPassive TransportWhat is passive transport?The movement of materials across the cellmembrane without using cellular energy is calledpassive transport.

Lesson OverviewCell TransportPassive TransportEvery living cell exists in a liquid environment.One of the most important functions of the cellmembrane is to keep the cell’s internal conditionsrelatively constant. It does this by regulating themovement of molecules from one side of themembrane to the other side.

Lesson OverviewCell TransportDiffusionThe cytoplasm of a cell is a solution of many differentsubstances dissolved in water.In any solution, solute particles tend to move from an areawhere they are more concentrated to an area where they areless concentrated.The process by which particles move from an area of highconcentration to an area of lower concentration is known asdiffusion.Diffusion is the driving force behind the movement of manysubstances across the cell membrane.

Lesson OverviewCell TransportDiffusionSuppose a substance is present in unequalconcentrations on either side of a cell membrane.

Lesson OverviewCell TransportDiffusionIf the substance can cross the cell membrane, itsparticles will tend to move toward the area where it isless concentrated until it is evenly distributed.

Lesson OverviewCell TransportDiffusionAt that point, the concentration of the substance on both sides of thecell membrane is the same, and equilibrium is reached.

Lesson OverviewCell TransportDiffusionEven when equilibrium is reached, particles of a solution willcontinue to move across the membrane in both directions.Because almost equal numbers of particles move in eachdirection, there is no net change in the concentration on eitherside.

Lesson OverviewCell TransportDiffusionDiffusion depends upon random particle movements.Substances diffuse across membranes without requiring thecell to use additional energy.The movement of materials across the cell membrane withoutusing cellular energy is called passive transport.

Lesson OverviewCell TransportFacilitated DiffusionCell membranes have proteins that act as carriers, or channels,making it easy for certain molecules to cross.Molecules that cannot directly diffuse across the membranepass through special protein channels in a process known asfacilitated diffusion.Hundreds of different proteins have been found that allow particularsubstances to cross cell membranes.The movement of molecules by facilitated diffusion does notrequire any additional use of the cell’s energy.

Lesson OverviewCell TransportOsmosis: An Example of FacilitatedDiffusionThe inside of a cell’s lipid bilayer ishydrophobic—or “water-hating.” Becauseof this, water molecules have a toughtime passing through the cell membrane.Many cells contain water channelproteins, known as aquaporins,that allow water to pass rightthrough them. Withoutaquaporins, water would diffuse inand out of cells very slowly.The movement of water throughcell membranes by facilitateddiffusion is an extremelyimportant biological process—theprocess of osmosis.

Lesson OverviewCell TransportOsmosis: An Example of FacilitatedDiffusionOsmosis is the diffusion of water through a selectivelypermeable membrane.Osmosis involves the movement of water molecules from anarea of higher concentration to an area of lower concentration.

Lesson OverviewCell TransportHow Osmosis WorksIn the experimental setup below, the barrier is permeable to water but not tosugar. This means that water molecules can pass through the barrier, but thesolute, sugar, cannot.

Lesson OverviewCell TransportHow Osmosis WorksThere are more sugar molecules on the right side of the barrier than on theleft side. Therefore, the concentration of water is lower on the right, wheremore of the solution is made of sugar.

Lesson OverviewCell TransportHow Osmosis WorksThere is a net movement of water into the compartment containing theconcentrated sugar solution.Water will tend to move across the barrier until equilibrium is reached. At thatpoint, the concentrations of water and sugar will be the same on both sides.

Lesson OverviewCell TransportHow Osmosis WorksWhen the concentration is the same on both sides of themembrane, the two solutions will be isotonic, which means“same strength.”

Lesson OverviewCell TransportHow Osmosis WorksThe more concentrated sugar solution at the start of theexperiment was hypertonic, or “above strength,” compared tothe dilute sugar solution.The dilute sugar solution was hypotonic, or “below strength.”

Lesson OverviewCell TransportOsmotic PressureFor organisms to survive, they must have a way to balance theintake and loss of water.The net movement of water out of or into a cell exerts a forceknown as osmotic pressure.

Lesson OverviewCell TransportOsmotic PressureBecause the cell is filled with salts, sugars, proteins, and othermolecules, it is almost always hypertonic to fresh water.As a result, water tends to move quickly into a cell surroundedby fresh water, causing it to swell. Eventually, the cell mayburst.

Lesson OverviewCell TransportOsmotic PressureIn plants, the movement of water into the cell causes the centralvacuole to swell, pushing cell contents out against the cell wall.Since most cells in large organisms do not come in contact with fresh water,they are not in danger of bursting.

Lesson OverviewCell TransportOsmotic PressureInstead, the cells are bathed in fluids, such as blood, that are isotonic andhave concentrations of dissolved materials roughly equal to those in thecells.Cells placed in an isotonic solution neither gain nor lose water.

Lesson OverviewCell TransportOsmotic PressureIn a hypertonic solution, water rushes out of the cell, causinganimal cells to shrink and plant cell vacuoles to collapse.

Lesson OverviewCell TransportOsmotic PressureSome cells, such as the eggs laid by fish and frogs, must come into contactwith fresh water. These types of cells tend to lack water channels.As a result, water moves into them so slowly that osmotic pressure does notbecome a problem.

Lesson OverviewCell TransportOsmotic PressureOther cells, including those of plants and bacteria, that comeinto contact with fresh water are surrounded by tough cell wallsthat prevent the cells from expanding, even under tremendousosmotic pressure.

Lesson OverviewCell TransportOsmotic PressureNotice how the plant cell holds its shape in hypotonic solution, while theanimal red blood cell does not.However, the increased osmotic pressure makes such cells extremelyvulnerable to injuries to their cell walls.

Lesson OverviewCell TransportActive TransportWhat is active transport?The movement of materials against a concentration differenceis known as active transport. Active t

Lesson Overview Life Is Cellular Electron Microscopes Light microscopes can be used to see cells and cell structures as small as 1 millionth of a meter. To study something smaller than that, scientists need to use electron microscopes. Electron microscopes use beams of e