A Tour of the Cell


  • All organisms are made of cells.
    • Many organisms are single-celled.
    • Even in multicellular organisms, the cell is the basic unit of structure and function.
  • The cell is the simplest collection of matter that can live.
  • All cells are related by their descent from earlier cells.

Concept 6.1 To study cells, biologists use microscopes and the tools of biochemistry


  • The discovery and early study of cells progressed with the invention of microscopes in 1590 and their improvement in the 17th century.
  • In a light microscope (LM), visible light passes through the specimen and then through glass lenses.
    • The lenses refract light such that the image is magnified into the eye or onto a video screen.
  • Microscopes vary in magnification and resolving power.
    • Magnification is the ratio of an object’s image to its real size.
    • Resolving power is a measure of image clarity.
      • It is the minimum distance two points can be separated and still be distinguished as two separate points.
      • Resolution is limited by the shortest wavelength of the radiation used for imaging.
  • The minimum resolution of a light microscope is about 200 nanometers (nm), the size of a small bacterium.
  • Light microscopes can magnify effectively to about 1,000 times the size of the actual specimen.
    • At higher magnifications, the image blurs.
  • Techniques developed in the 20th century have enhanced contrast and enabled particular cell components to be stained or labeled so they stand out.
  • While a light microscope can resolve individual cells, it cannot resolve much of the internal anatomy, especially the organelles.
  • To resolve smaller structures, we use an electron microscope (EM), which focuses a beam of electrons through the specimen or onto its surface.
    • Because resolution is inversely related to wavelength used, electron microscopes (whose electron beams have shorter wavelengths than visible light) have finer resolution.
    • Theoretically, the resolution of a modern EM could reach 0.002 nanometer (nm), but the practical limit is closer to about 2 nm.
  • Transmission electron microscopes (TEMs) are used mainly to study the internal ultrastructure of cells.
    • A TEM aims an electron beam through a thin section of the specimen.
    • The image is focused and magnified by electromagnets.
    • To enhance contrast, the thin sections are stained with atoms of heavy metals.
  • Scanning electron microscopes (SEMs) are useful for studying surface structures.
    • The sample surface is covered with a thin film of gold.
    • The beam excites electrons on the surface of the sample.
    • These secondary electrons are collected and focused on a screen.
    • The result is an image of the topography of the specimen.
    • The SEM has great depth of field, resulting in an image that seems three-dimensional.
  • Electron microscopes reveal organelles that are impossible to resolve with the light microscope.
    • However, electron microscopes can only be used on dead cells.
  • Light microscopes do not have as high a resolution, but they can be used to study live cells.
  • Microscopes are major tools in cytology, the study of cell structures.
  • Cytology combined with biochemistry, the study of molecules and chemical processes in metabolism, to produce modern cell biology.

Isolating Organelles by Cell Fractionation.
Cell biologists can isolate organelles to study their functions.

  • The goal of cell fractionation is to separate the major organelles of the cells so their individual functions can be studied.
  • This process is driven by an ultracentrifuge, a machine that can spin at up to 130,000 revolutions per minute and apply forces of more than 1 million times gravity (1,000,000 g).
  • Fractionation begins with homogenization, gently disrupting the cell.
  • The homogenate is spun in a centrifuge to separate heavier pieces into the pellet while lighter particles remain in the supernatant.
    • As the process is repeated at higher speeds and for longer durations, smaller and smaller organelles can be collected in subsequent pellets.
  • Cell fractionation prepares isolates of specific cell components.
  • This enables the functions of these organelles to be determined, especially by the reactions or processes catalyzed by their proteins.
    • For example, one cellular fraction was enriched in enzymes that function in cellular respiration.
    • Electron microscopy revealed that this fraction is rich in mitochondria.
    • This evidence helped cell biologists determine that mitochondria are the site of cellular respiration.
  • Cytology and biochemistry complement each other in correlating cellular structure and function.

Concept 6.2 Eukaryotic cells have internal membranes that compartmentalize their functions

• Basic unit of of organism is one of two types of cells, prokaryotic, or eukaryotic.
• Domian Bacteria and Archaea consist of prokaryotic cells
• Domain Eukarya consists of eukaryotic cells.

Comparing Prokaryotic and Eukaryotic Cells
• All cells have:

o Plasma membrane, in the membrane is cytosol, where organelles are found.
 Allows passage of oxygen, nutrients, and wastes to cell.
o Chromosomes, carrying genes (DNA)
o Ribosomes, tiny organelles that make proteins

• In prokaryotic cells, DNA is in the nucleoid, but nothing seperates it from the rest of the cell.
• In eukaryotic cells, DNA is in the nucleus
o Region between is cytoplasm

• Eukaryotic cells are bigger than than prokaryotic cells.
• Prokaryotic cells are much simpler.

A Panoramic View of the Cell
• Each membrane has a composition of lipids and proteins for specific functions

Animal cell
• Most of cells metabloic activities occur in the cytoplasm, which contains many organelles in the cytosol.

Plant Cell
• Plastids- membrane enclosed organelles
o Most important is chloroplast, which carries photosynthesis.

Concept 6.3 The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes

Two organelles involved in the genetic control of the cell.
The Nucleus: Genetic Library of the Cell

  • Contains most of the genes in the Eukaryotic cell
  • The Nuclear Envelope, also known as a double membrane, protects the nucleus' contents from the cytoplasm.
  • The Pore Complex, made of proteins, regulate entries and exits of macromolecules save for the pores.
  • Nuclear 'Matrix' - Framework around the cell nucleus.
  • Nuclear Lamina - Net like array of protein filaments that maintains the shape of the nucleus.
  • DNA
  • Ordered in small units called chromosomes ( Structures that carry genetic information )
  • Each Chromosone is made up of chromatin, a mix of Proteins and DNA
  • 46 chromosomes in a Human cell, sex cells only have 23
  • Nucleolus
  • RNA
  • Proteins are imported from the cytoplasm and are assembled into small ribosomal subunits in the nucleolus.

Ribosomes: Protein Factories in the Cell

  • Made of rRNA ( ribosomal RNA ) and protein
  • Carries out protein synthesis
  • Not enclosed in a membrane
  • Process
  • Ribosomes build proteins in two main cytoplasmic locations.
  • Free ribosomes are floating about in the cytoplasm while Bound ribosomes are attached to the endoplasmic reticulum ( Or nuclear envelope )
  • Free ribosomes create enzymes that catalyze the first steps of sugar breakdown
  • Bound ribosomes make proteins that are destined for insertion to membranes, packaging within certain organelles ( Like lysosomes ), or for exportation from the cell. (Secretion)
  • Both Free ribosomes and Bound ribosomes can switch places and alternate jobs freely.

Concept 6.4 The endomembrane system regulates protein traffic and performs metabolic functions in the cell

The Endoplasmic Reticulum: Biosynthetic Factory
Functions of Smooth ER
Functions of Rough ER
The Golgi Apparatus: Shipping and Receiving Center
Lysosomes: Digestive Compartments
Vacuoles: Diverse Maintenance Compartments
The Endomembrane System: A Review

Concept 6.5 Mitochondria and chloroplasts change energy from one form to another

*one form to another/ eukaryotic cells, mitochondria and chloroplasts are organells used to convert energy that cells use for work.
*mitochondra- sites of cellular respiration
*chloroplats-only in plants and algae,site for photosynthesis.
*peroxisome-an oxidative organell that is not part of the endomembrane system.
Mitochondria: Chemical Energy Conversion
*nearly all eukarotic cells, including those of plants, animals, fungi, and protists.
*mitochondria are enclosed by two membranes.
*outer membrane is smooth, but the inner membrane is covered with foldings called __Cristae.
Chloroplasts: Capture of Light Energy

  • specialized member of a family related to plant organells called plastids.

*amyloplasts-colorless plastids that stor starch.
*chromoplasts-pigments that givefruit and flowerstheir orange and yellow hues.
*ckloroplasts-continue the green pigments chlorophyll
along with enzymes and other molecules that function in the photosynthetic production of sugar.
*inside the chloroplats is membranous systems in the form of flattend, interconnected such called thylukioids.
*thylakoids are like stacks of poker chips, each stack is called granam.
*fluid outside the thylakoids is the stroma.
Peroxisomes: Oxidation
*is specialized metabolic conpartment bounded by a single membrane.
*also contain enzymes that transfer hydrogen from vareous substrutes to oxygen, producing hydrogen peroxide as a by-product, is where it gets its name.
*glysomes-founed in the fat-storing tissues of plant sacks.

Concept 6.6 The cytoskeleton is a network of fibers that organize structures and activities in the cell

*Scientist used to think that the organelles of a eukaryotic cell could float feely in the cytosol

a network of fibers extending throughout the cytoplasm

*Three types of cytoskeleton molecular structure types microtubules, microfilaments, and intermediate fillaments
Roles of the Cytoskeleton: Support, Motility, and Regulation
*Gives support to cells and maintains its shape
*forms a geodesic dome
*stabilized by a balance by the opposing forces of the cell
*very important to animal cells because they have no cell walls
*Allows cell movement through the help of motor proteins

Motor proteins
proteins that create the movement of the cilia or flagella by gripping microtubules in the organelles and pushing them past each other

Components of the Cytoskeleton
Microfilaments (Actin Filaments)
Intermediate Filaments

Concept 6.7 Extracellular components and connections between cells help coordinate cellular activities

Cell Walls of Plants
The Extracellular Matrix (ECM) of Animal Cells
Intercellular Junctions
Plants: Plasmodesmata
Animals: Tight Junctions, Desmosomes, and Gap Junctions
The Cell: A Living Unit Greater Than the Sum of Its Parts
1.cell walls of plants
-cellwall-an extracellular structure of plant cells that distinguishes them from animal cells.
-uses: protection, maintain shape,and prevents excessive water intake
-helps to keep the plant upright.
-cell walls are thicker than plasma membranes
-primary cellwall-a relatively and flexible wall created by a young plant cell
-middle lamella- what's inbetween primary walls of adjacent cells
-the middle lamella acts as a glue for adjacent cells
-secondary cellwall- deposited in several laminated layers for plant cell protection and support
2.The extra cellular matrix (EMC) of animal cells
-extra cellular matrix- the substance in which animal tissue cells are embedded consisting of protein and polysacchuriodes
-collagen- forms strong fibers outside the cells.
-collagen makes up about half the total protin in the human body
-proteoglycons- consists of a small core protein with many carbohydate chains covalently attached,so that it may be up to 95% carbohydrate.
-fibronectin and other protins called integrins
-integrins- built into the plasma membrane
-integrins transmit changes between the ECM and cytoskeleton

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