Reading Essentials for Biology Glencoe (794133), страница 19
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It is oneof the fundamental ideas of the science of biology. The threemain parts of the cell theory are summarized below:1. All living things are made of one or more cells.2. Cells are the basic units of structure and function inliving things.1. What are the three mainideas of cell theory?________________________________________________________________________3. All cells come from other cells.How do microscopes help scientists learnabout cells?Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.In the 1930s and 1940s, microscopes were improved.
Electronmicroscopes allowed scientists to magnify an object up to 500 000times using a beam of electrons instead of a beam of light. A scanning electron microscope (SEM) lets scientists see a cell’s threedimensional shape. A transmission electron microscope (TEM)lets scientists see the structures inside a cell.Microscopes are continually being improved so scientists cangather more information about cells.Two Basic Cell TypesUsing microscopes, scientists saw that all cells contain smallstructures called organelles.
Each organelle has a specific function in the cell. Some cell organelles are held together by a membrane, but others are not.Scientists group cells into two categories—cells that havemembrane-bound organelles and cells that do not. Cells thatdo not contain membrane-bound organelles are called prokaryotes (pro kar ee AWTS). Unicellular organisms, such as bacteria,are prokaryotes.If the cell has organelles that are held together by a membrane,the cell is called a eukaryote (yew kar ee AWT).
Most cells youcan think of are eukaryotic. These include most of the multicellular organisms you know. Having membrane-bound organelles isan advantage for eukaryotic cells because chemical reactions indifferent parts of the cell can happen at the same time.Eukaryotic cells have a central organelle called a nucleus thatcontrols all of the cell’s activities. Prokaryotes do not have anorganized nucleus.
Instead, they have loose strands of DNA.READING ESSENTIALS2. Compare Which cells aremore complex? (Circleyour choice.)a. prokaryoticb. eukaryotic3. What does a nucleus do?________________________________________________________________________Chapter 769NameDateClassSection7.1The Discovery of Cells, continued◗ After You ReadMini Glossarycell: the basic unit of all living thingscell theory: theory that states all organisms aremade of one or more cells; the cell is thebasic unit of organisms; and all cells comefrom preexisting cellscompound light microscope: microscope using aseries of light and lenses to magnify objectselectron microscope: microscope using a beamof electrons instead of lenses to magnifyobjectseukaryote (yew kar ee AWT): unicellular or multicellular organisms (like yeast, plants, andanimals) that contain a nucleus and membrane-bound organellesnucleus (NEW klee us): the cell organelle thatcontrols the cell’s activities and contains DNAorganelle: membrane-bound structures withparticular functions within some cellsprokaryote (pro kar ee AWT): unicellular organisms (like bacteria) that lack membranebound organelles1.
Circle two terms from the Mini Glossary above that are related to each other. On the linesbelow, tell how these terms are related.________________________________________________________________________________________________________________________________________________________________________ProkaryoticCellsBoth Typesof CellsCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.2. Use the Venn diagram below to help you review what you have read. List what makes prokaryotic and eukaryotic cells different.
Then list their common characteristics in the middle.EukaryoticCells3. Write the three main ideas of the cell theory in the spaces below.The Cell TheoryVisit the Glencoe Science Web site at science.glencoe.com to findyour biology book and learn more about the discovery of cells.70Chapter 7READING ESSENTIALSNameDateClassSection7.2 The Plasma Membrane◗ Before You ReadA window screen in your home allows air to pass through while keeping insects out.
In this sectionyou will learn about a cell structure that has the same basic function: allowing some things to passthrough while keeping other out. On the lines below, list some things that you think would beallowed to pass into a cell, some that would be allowed to pass out of the cell, and some that wouldbe kept out of the cell.______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Maintaining a BalanceThe plasma membrane is the flexible boundary of a cell thatseparates a cell from its surroundings.
It allows nutrients to enterthe cell and waste to be removed. Keeping this healthy balancewithin a cell is called homeostasis.To maintain homeostasis, the plasma membrane allows somemolecules into the cell and keeps others out. This is called selective permeability. Some molecules are allowed in at any time.Other molecules are only admitted at certain times and in limitedamounts. Others are not allowed in at all.1. What is the function ofthe plasma membrane?________________________________________________________________________Structure of the Plasma MembraneEarlier you learned that lipids are large molecules made up ofglycerol and three fatty acids. A phospholipid (fahs foh LIH pid)is made up of glycerol, two fatty acids, and a phosphate group.The plasma membrane is made up of two layers of phospholipidsarranged back to back in what is called a phospholipid bilayer.The phosphate group is an important part of the plasma membrane.
The group is found in the head of the phospholipid molecule. The head is polar. The tails of fatty acid chains hang fromthe head. The tails are nonpolar. The two phospholipid layers arearranged so the polar heads are facing out, and the nonpolar tailsare facing in.READING ESSENTIALS2. Which part of a phospholipid molecule containsthe phosphate group?Which is nonpolar?________________________________Chapter 771NameDateClassSection7.2The Plasma Membrane, continuedWhat is the fluid mosaic model?Plasma membranePhospholipidsSurfaceproteinPhospholipidsTransportprotein3. Name three elementsthat float in the plasmamembrane.________________________________________________Water, which all living things need, is also polar. The polar phosphate heads on the surface of the membrane allow the polar waterto interact with the membrane.
Because they are nonpolar, fattyacid tails avoid water. This makes a barrier that is water-soluble onthe outside of the membrane, but water-insoluble inside themembrane. This prevents water-soluble molecules from easilymoving through the plasma membrane.This organization of the plasma membrane is called the fluidmosaic model. It is fluid because the phospholipids are not fixedin one place, but float in the membrane. Protein molecules alsofloat with the phospholipids.
The model is called a mosaic becauseof the patterns the proteins create on the membrane’s surface.Specific proteins called transport proteins work to regulatewhich molecules are allowed to enter and which are allowed toleave the cell. Other proteins help cells identify chemical signals.Proteins on the inner surface of the membrane help support thecell structure.Cholesterol also floats on the surface of the membrane. It helpskeep the fatty acid tails from sticking together. Cholesterol is animportant part of a healthy diet partly because of the role it playsin the plasma membrane.◗ After You ReadMini Glossaryfluid mosaic model: structural model of theplasma membrane where phospholipids andproteins float within the surface of themembranephospholipid (fahs foh LIH pid): a large moleculewith a glycerol backbone, two fatty acidchains, and a phosphate groupselective permeability: a process in which amembrane allows some molecules to passthrough while keeping others outtransport proteins: proteins that move neededsubstances or waste materials through theplasma membrane into or out of the cellplasma membrane: the flexible boundary of a cell1.
Read the key terms and definitions in the Mini Glossary above. Circle one key term. Then,in the space provided, write the definition of the term in your own words.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________72Chapter 7READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.________________________NameDateClassSection7.2The Plasma Membrane, continued2.
Use the partially completed outline below to help you review what you have read. Fill in theblanks where missing information is needed.I. Parts of a phospholipid moleculeA. 2 fatty acidsB. 1C. 1II. Fluid mosaic model of plasma membraneA. Called fluid becauseB. Called mosaic because3.
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