Chapter 4

In cell theory, all organisms are composed of?

4.1 Cellular Level of Organization

• Detailed study of the cell in the 1830s  

• A unifying concept in biology

• Originated from the work of biologists in 1838–1839 • Cell Theory:

 All organisms are composed of cells.

 All cells come only from preexisting cells because cells  are self-reproducing.

 Cells are the basic units of structure and function in  organisms.

Cell size

• Cells range in size from one millimeter (mm) down to one  micrometer (μm) in diameter.

• Cells need a large surface area of plasma membrane to  adequately exchange materials.

• The surface-area-to-volume ratio requires that cells be  small.

What are the two taxonomic domains that prokaryotic cells are placed in?

 Large cells – surface area relative to volume decreases  which also decreases the efficiency of transporting  materials in and out of the cell

 Small cells – larger surface-area-to-volume ratio is  advantageous for exchanging molecules  

Microscopy Today: Compound Light Microscope • Light is passed through the specimen.

• Then it is focused by a series of glass lenses.

• It forms an image on the human retina.

• The maximum magnification is about 1000X.

• The compound light microscope resolves objects separated  by 0.2 mm, 500X better than human eye.

 Assuming the resolving power of the human eye is 1.0

Microscopy Today: Transmission Electron Microscope

What is the function of endoplasmic reticulum?

• Abbreviated T.E.M.

• Electrons are passed through specimen and

then they are focused by a set of magnetic lenses.

• An image is formed on a fluorescent screen similar to a TV  screen. We also discuss several other topics like Is soil a natural entity of unconsolidated organic?

 The image is then photographed.

• TEM provides greater magnification than a compound light  microscope.

• Resolves objects separated by 0.0002 mm, 100,000X better  than human eye.

Microscopy Today: Scanning Electron Microscope

• Abbreviated S.E.M.

• The specimen is sprayed with a thin coat of metal.  Then an electron beam is scanned across the surface of the specimen.

 The surface metal emits secondary electrons.

• The emitted electrons are detected and focused by magnetic lenses.

• A 3-dimensional image is formed on a fluorescent screen  similar to a TV screen.

 Image is then photographed

Magnification, Resolution, and Contrast

• Magnification is the ratio between the size of an image and  its actual size.

 An electron microscope magnifies objects hundreds of  times more than a compound light microscope.

• Resolution is the minimum distance between two objects  that allows them to be seen as two separate objects.  A microscope with poor resolution would let a student  see one cellular granule compared with two granules  with greater resolution.

• Contrast is the difference in shading of an object compared  to its background. Don't forget about the age old question of How many atoms of silver could fit across the diameter of a dime?

 Fluorescently tagged antibody molecules can help  visualize subcellular components like specific proteins.

Illumination, Viewing, and Recording

• Light rays can be bent (refracted) and focused as they pass  through glass, but electrons can’t.

• Electrons have a charge and can be focused by  electromagnetic lenses. If you want to learn more check out What are the essential polysaccharides in the living world?
Don't forget about the age old question of Hippocampus is a neural center located in?

 The electrons leaving the specimen are then directed to a screen or photographic plate, which is sensitive to  electrons which can be viewed by humans.

Confocal Microscopy—A Major Advance in Illumination

A narrow laser beam is scanned across a transparent specimen.  The beam is focused on one very thin plane in the cell. Don't forget about the age old question of In psychology, graded potential used to describe what?
If you want to learn more check out What are the contemporary concepts of a family?

 A microscopist can “optically section” a specimen by focusing up and down.

• Sections are made at different levels.

• This allows assembly of a 3-dimensional electronic  image on a computer screen that can be displayed and  rotated on the screen.

Video­Enhanced Contrast Microscopy

The microscope image can be recorded by a TV camera.

In video-enhanced contrast microscopy, a TV camera converts the light image into an electronic image, which is entered into  computer.

 The computer makes the darkest areas of the image darker and  the lightest areas lighter.

The result is a high-contrast image which can be manipulated  further for greater contrast.

4.2 Prokaryotic Cells

• Lack a membrane-bound nucleus

• Structurally smaller and simpler than eukaryotic cells (which have a nucleus)

• Prokaryotic cells are placed in two taxonomic domains:  Bacteria  

• Cause diseases but are also environmentally  

important as decomposers

• Can be useful in manufacturing products and  

drugs

 Archaea

• Live in extreme habitats

 Two prokaryotic domains are structurally similar but  biochemically different

The Structure of Prokaryotes

• Extremely small: 1–1.5 μm wide and 2–6 μm long  • Occur in three basic shapes:  

 Spherical coccus

 Rod-shaped bacillus

 Spiral spirillum (if rigid) or spirochete (if flexible) • Cell Envelope includes:

 Plasma membrane – lipid bilayer with embedded and  peripheral proteins

• Can form internal pouches (mesosomes), which  increase surface area

 Cell wall – maintains the shape of the cell and is  strengthened by peptidoglycan  

 Glycocalyx – layer of polysaccharides on the outside of the cell wall  

• Well organized and resistant to removal (capsule)

Prokaryotic Cytoplasm and External Structures

• Cytoplasm

 Semifluid solution

• Encased by plasma membrane

• Contains water, inorganic and organic molecules,  and enzymes  

 Nucleoid is a region that contains the single, circular  DNA molecule.

 Plasmids are small accessory (extrachromosomal)  rings of DNA.

 Ribosomes are tiny structures in the cytoplasm that  synthesize proteins.

• External Structures

 Flagella – provide motility

 Fimbriae – small, bristle-like fibers that sprout from the cell surface

 Conjugation pili – rigid tubular structures used to pass DNA from cell to cell (singular pilius)

4.3Introduction to Eukaryotic Cells

- Eukaryotic cells contain:

 Membrane-bound nucleus that houses DNA

 Specialized organelles

 Plasma membrane which

• separates cell contents from environment

• regulates passage of materials in and out

• is composed of a phospholipid bilayer with  

embedded proteins

 The first two distinguish eukaryotic from prokaryotic  cells

• Eukaryotic cells are also much larger than prokaryotic cells. • Eukaryotic cells are compartmentalized.

 They contain small structures called organelles which • perform specific functions  

• isolate reactions from other reactions

Origin of the Eukaryotic Cell

• The fossil record suggest that the first cells were  prokaryotes.

• Biochemical data shows eukaryotes are more closely related  to archaea than bacteria.  

• The nucleus is believed to have evolved by invagination of  the plasma membrane.

 The invagination process also explains origins of  endoplasmic reticulum and Golgi.

 Energy organelles, mitochondria and chloroplasts, may  have originated when eukaryotic cell engulfed smaller  prokaryotic cells.

• Eukaryotic cell would have benefitted the form  ability to utilize oxygen or synthesize organic food. • Endosymbiotic theory is the name of the  

hypothesis.

Introduction to Eukaryotic Cells

- There are two classes of organelles.

 Endomembrane system

• Organelles that communicate with one another – Via membrane channels

– Via small vesicles

 Energy-related organelles

• Mitochondria and chloroplasts

• Independent and self-sufficient

Structure of a Eukaryotic Cell

• Plant and animal cell diagrams are generalized for study purposes. • Specialized cells may have more or fewer copies of organelles,  depending on their functions.

• Example: Liver cells, which detoxify drugs, have more  smooth endoplasmic reticulum than other cells.

• Example: Nerve cells, which carry electrical impulses,  have more plasma membrane.

• The cell is a system of interconnected organelles that work together.  Example: Nucleus is a compartment that houses genetic  material.

• It communicates with ribosomes in the cytoplasm.

Structure of a Eukaryotic Cell

• Production of specific molecules takes place in or on organelles by  enzymes in membranes.

• Products are transported around cell by vesicles.

 Sacs made of membrane material

• Vesicles move around using cytoskeletal network.

 Protein fibers are like railroad tracks

• Plant cells, fungi, and many protists have cell walls.

 Plant cell walls contain cellulose, a structural polysaccharide

4.4

The Nucleus and Ribosomes

• The Nucleus

 Command center of cell, usually near center

 Separated from cytoplasm by nuclear envelope

• Consists of double layer of membrane

• Nuclear pores permit exchange between nucleoplasm  and cytoplasm

 Contains chromatin in semifluid nucleoplasm

• Chromatin contains nucleic acids and proteins.

– condenses to form chromosomes

» Chromosomes are formed during cell division.

» Chromosomes are carriers of genetic 

information.

 Dark nucleolus composed of rRNA

• produces subunits of ribosomes

Ribosomes

• Composed of rRNA

 Consist of a large subunit and a small subunit.

• Subunits are made in nucleolus.

• May be located:

 on the endoplasmic reticulum (thereby making it “rough”), or

free in the cytoplasm, either singly or in groups, called 

polyribosomes.

• Site of protein synthesis in the cell

• In the process of transcription and translation:

 information for the gene is copied into mRNA, which is exported into the cytoplasm.

 ribosomes receive the mRNA with a coded message from DNA  with the correct sequence of amino acids to make a protein.  proteins synthesized by cytoplasmic ribosomes stay in  cytoplasm; those by attached ribosomes end up in ER.

 The central dogma of molecular biology is 

• the DNA­mRNA­protein sequence of events

4.5 The Endomembrane System

• Series of intracellular membranes that compartmentalize the cell  • Restrict enzymatic reactions to specific compartments within cell • Consists of:

 Nuclear envelope

 Membranes of endoplasmic reticulum

 Golgi apparatus

 Vesicles

• Several types

• Transport materials between organelles of system

Endoplasmic Reticulum

• A system of  membrane channels and saccules (flattened vesicles)  continuous with the outer membrane of the nuclear envelope  • Rough ER

 Studded with ribosomes on cytoplasmic side

 Protein anabolism

• Synthesizes proteins

• Modifies and processes proteins

– Adds sugar to protein

– Results in glycoproteins

» Important in cell functions

 Forms transport vesicles

• Substances can move to Golgi apparatus.

• Smooth ER

 No ribosomes

 Synthesis of lipids

• In testes, testosterone is produced by smooth ER.

 Site of various synthetic processes, detoxification, and storage • The liver, with abundant smooth ER, detoxifies drugs.  Forms transport vesicles

• Substances can move to Golgi apparatus.

The Golgi Apparatus

• Golgi Apparatus

• Named for Camillo Golgi

 Consists of flattened, curved saccules

 Resembles stack of hollow pancakes

 Modifies proteins and lipids with “signal” sequences

• Receives vesicles from ER on cis (or inner) face

• After modification, prepares for “shipment” and packages  proteins and lipids in vesicles that leave Golgi from trans  (or outer) face

– Some transported to locations within cell

– Some exported from cell (secretion, exocytosis)

– Others returned to ER or merged with plasma 

membrane