3 Cellular Level of Organization

© 2011 Pearson Education, Inc.

PowerPoint® Lecture Presentations prepared byAlexander G. CheroskeMesa Community College at Red Mountain

3Cellular Level of Organization


Section 1: An Introduction to Cells

• Learning Outcomes

• 3.1 Describe the cell and its organelles, including the composition and function

of each.

• 3.2 Describe the chief structural features of the plasma membrane.

• 3.3 Differentiate among the structures and functions of the cytoskeleton.

• 3.4 Describe the ribosome and indicate its specific functions.




• 3.5 Describe the Golgi apparatus and indicate its specific functions.

• 3.6 Describe mitochondria, indicate their functions, and explain their

significance to cellular function.



• Typical cell

• Smallest living unit in the body

• ~0.1 mm in diameter

• Could not be examined until invention of microscope in 17th century

Animation: Your Cells



• Cell theory

1. Cells are building blocks of all plants and animals

2. All new cells come from division of preexisting cells

3. Cells are smallest unit that perform all vital physiological functions

© 2011 Pearson Education, Inc.Figure 3 Section 1

Cells are the building blocks of allplants and animals.

All new cells come from the divisionof pre-existing cells.

Cells are the smallest units that performall vital physiological functions.

Cell

Division

GrowthNewcells

Nutrients

Wastes

O2

CO2

The cell theory

The differentiation of the four tissue types from a single cell: the fertilized ovum

Epithelial tissue

Connective tissue

Muscle tissue

Neural tissue



• Each cell maintains homeostasis

• Coordinated activities of cells allow homeostasis at higher organizational levels



• Cells vary in structure and function but all descend from a single fertilized ovum

• Fertilized ovum contains genetic potential to become any cell

• Cell divisions occur creating smaller, different parcels of cytoplasm

• Cytoplasmic differences turn off/on specific genes in DNA and daughter cells become specialized

• = Differentiation

• Differentiated cells are responsible for all body functions



• Extracellular fluid

• Watery medium surrounding cells

• Called interstitial fluid (interstitium, something standing between) in most tissues


Module 3.1: Smallest living units of life

• Cell components

• Plasma membrane (cell membrane)

• Separates cell contents from extracellular fluid

• Cytoplasm

• Material between cell membrane and nuclear membrane

• Colloid containing many proteins

• Two subdivisions

1. Cytosol

• Intracellular fluid

2. Organelles (“little organs”)

• Intracellular structures with specific functions



• Organelles

• Nonmembranous

• Not completely enclosed by membranes

• In direct contact with cytosol

• Examples:

• Cytoskeleton

• Microvilli

• Centrioles

• Cilia

• Ribosomes



• Organelles

• Membranous

• Enclosed in a phospholipid membrane

• Isolated from cytosol

• Examples:

• Mitochondria

• Nucleus

• Endoplasmic reticulum

• Golgi apparatus

• Lysosomes

• Peroxisomes



• Organelles• Microvilli

• STRUCTURE: membrane extensions containing microfilaments

• FUNCTION: increase surface area for absorption

• Cytoskeleton• STRUCTURE: fine protein filaments or tubes

• Centrosome • Organizing center containing pair of centrioles

• FUNCTION: • Strength and support• Intracellular movement of structures and materials



• Organelles

• Ribosomes

• STRUCTURE: RNA and proteins

• Fixed: attached to endoplasmic reticulum

• Free: scattered in cytoplasm



• Organelles

• Peroxisome

• STRUCTURE: vesicles containing degradative enzymes

• FUNCTION:

• Catabolism of fats/other organic compounds

• Neutralization of toxic compounds

• Lysosome

• STRUCTURE: vesicles containing digestive enzymes

• FUNCTION:

• Removal of damaged organelles or pathogens



• Organelles

• Golgi apparatus

• STRUCTURE: stacks of flattened membranes (cisternae) containing chambers

• FUNCTION: storage, alteration, and packaging of synthesized products

• Mitochondria

• STRUCTURE:

• Double membrane

• Inner membrane contains metabolic enzymes

• FUNCTION: production of 95% of cellular ATP



• Organelles

• Nucleus

• STRUCTURE:• Fluid nucleoplasm containing enzymes, proteins,

DNA, and nucleotides

• Surrounded by double membrane

• FUNCTION:• Control of metabolism

• Storage/processing of genetic information

• Control of protein synthesis

Animation: Nucleus



• Organelles

• Endoplasmic reticulum (ER)

• STRUCTURE: membranous sheets and channels

• FUNCTION: synthesis of secretory products, storage, and transport

• Smooth ER

• No attached ribosomes

• Synthesizes lipids and carbohydrates

• Rough ER

• Attached ribosomes

• Modifies/packages newly synthesized proteins


Module 3.1 Review

a. Distinguish between the cytoplasm and cytosol.

b. Describe the functions of the cytoskeleton.

c. Identify the membranous organelles and describe their functions.


Module 3.2: Plasma membrane

• Plasma membrane

• Selectively permeable membrane that controls:

• Entry of ions and nutrients

• Elimination of wastes

• Release of secretions



• Plasma membrane components

• Glycocalyx

• Superficial membrane carbohydrates

• Components of complex molecules

• Proteoglycans (carbohydrates with protein attached)

• Glycoproteins (protein with carbohydrates attached)

• Glycolipids (lipids with carbohydrates attached)

• Functions

• Cell recognition

• Binding to extracellular structures

• Lubrication of cell surface



• Plasma membrane components (continued)• Integral proteins

• Part of cell membrane

• Cannot be removed without damaging cell

• Often span entire cell membrane• = Transmembrane proteins

• Can transport water or solutes

• Peripheral proteins• Attached to cell membrane surface

• Removable

• Fewer than integral proteins

© 2011 Pearson Education, Inc.Figure 3.2 1

Structure of the plasma membrane

EXTRACELLULAR FLUID

Glycolipid

Glycocalyx(extracellular

carbohydrates)Integral protein

with channel

CYTOPLASM

Integralglycoproteins

Peripheral proteins Cytoskeleton(microfilaments)

= 2 nm

Integral (transmembrane) proteins



• Plasma membrane structure

• Thin (6–10 nm) and delicate

• Phospholipid bilayer

• Mostly comprised of phospholipid molecules in two layers

• Hydrophilic heads at membrane surface

• Hydrophobic tails on the inside

• Isolates cytoplasm from extracellular fluid

Animation: Cell Membrane Barrier


Cholesterol

Hydrophilicheads

Hydrophobictails

The phospholipid bilayer that forms theplasma membrane



• Plasma membrane functions

• Physical isolation

• Regulation of exchange with external environment

• Sensitivity to environment

• Structural support

• Lipid bilayer provides isolation

• Proteins perform most other functions


Module 3.2 Review

a. List the general functions of the plasma membrane.

b. Which structural component of the plasma membrane is mostly responsible for its ability to isolate a cell from its external environment?

c. Which type of integral protein allows water and small ions to pass through the plasma membrane?


Module 3.3: Cytoskeleton

• Cytoskeleton (cellular framework) components

1. Microfilaments

• <6 nm in diameter

• Typically composed of actin

• Commonly at periphery of cell

• Microvilli

• Finger-shaped extensions of cell membrane

• Has core of microfilaments to stiffen and anchor

• Enhance surface area of cell for absorption

• Terminal web (layer inside plasma membrane in cells forming a layer or lining)



• Cytoskeleton (cellular framework) components (continued)

2. Intermediate filaments

• 7–11 nm in diameter

• Strongest and most durable cytoskeletal elements

3. Microtubules

• ~25 nm in diameter

• Largest components of cytoskeleton

• Extend outward from centrosome (near nucleus)


Structures of the cytoskeleton

Microvilli

Microfilaments

Plasma membrane

Terminal web

Intermediate filaments

Microtubule

Secretory vesicle

Endoplasmicreticulum

Mitochondrion

Microvilli SEM X 30,000



• Centrioles

• Cylindrical structures

• Composed of microtubules (9 groups of triplets)

• Two in each centrosome

• Control movement of DNA strands during cell division

• Cells without centrioles cannot divide

• Red blood cells

• Skeletal muscle cells


The structure of centrioles

Microtubulesin centriole



• Cilia

• Long, slender plasma membrane extensions

• Common in respiratory and reproductive tracts

• Also composed of microtubules

• Nine groups of pairs surrounding a central pair

• Anchored to cell surface with basal body

• Beat rhythmically to move fluids or secretions across cell


The structure of cilium

Basalbody

Plasmamembrane

Microtubules


The action of a beating cilium

Power stroke

Return stroke


Module 3.3 Review

a. List the three basic components of the cytoskeleton.

b. Which cytoskeletal component is common to both centrioles and cilia?

c. What is the function of cilia?


Module 3.4: Ribosomes

• Ribosomes

• Protein synthesis

• Two subunits (1 large, 1 small) containing special proteins and ribosomal RNA (rRNA)

• Must join together before synthesis begins

• Free ribosomes

• Throughout cytoplasm

• Manufactured proteins enter cytosol


The two subunits of a functional ribosome

Small ribosomalsubunit

Large ribosomalsubunit




• Network of intracellular membranes attached to nucleus

• Forms hollow tubes, sheets, and chambers (cisternae, singular, cisterna, reservoir for water)




• Two types

1. Smooth (SER)

• Lacks ribosomes

• Tubular cisternae

2. Rough (RER)

• Has attached (fixed) ribosomes

• Modification of newly synthesized proteins

• Export to Golgi apparatus

• Proportion of SER to RER depends on the cell and its functions

© 2011 Pearson Education, Inc.Figure 3.4 2 – 3

The structure of the endoplasmic reticulum (ER)

Nuclearenvelope

Cisternae

Tubularcisternae

Smoothendoplasticreticulum (SER)



• Functions of SER

• Synthesis of phospholipids and cholesterol

• Synthesis of steroid hormones

• Synthesis and storage of glycerides in liver and fat cells

• Synthesis and storage of glycogen in skeletal and liver cells


mRNA strand Ribosome Transport vesicles

Fixedribosomes

Growingpolypeptide

Enzyme

Protein

Glycoprotein

As a polypeptide issynthesized on aribosome, the growingchain enters thecisterna of the RER.

The polypeptideassumes itssecondary andtertiary structure.

The complete proteinmay become anenzyme or aglycoprotein.

Transport vesiclesdeliver proteins,enzymes, andglycoproteins to the Golgi apparatus.

The structure and functionof rough endoplasmicreticulum (RER)

Glycoproteins,proteins, and enzymesare packaged intransport vesicles.



• Function of RER

• Polypeptide synthesized on attached ribosome

• Growing chain enters cisterna

• Polypeptide assumes secondary/tertiary structures

• Completed protein may become enzyme or glycoprotein

• Products not destined for RER are packaged into transport vesicles

• Deliver products to Golgi apparatus


Module 3.4 Review

a. Describe the immediate cellular destinations of newly synthesized proteins from free ribosomes and fixed ribosomes.

b. Describe the structure of smooth endoplasmic reticulum.

c. Why do certain cells in the ovaries and testes contain large amounts of smooth endoplasmic reticulum (SER)?


Module 3.5: Golgi apparatus

• Golgi apparatus• Functions

1. Renews or modifies plasma membrane

2. Modifies or packages secretions for release from cell (exocytosis)

3. Packages special enzymes within vesicles for use in cytosol

• Typically consist of 5–6 flattened discs (cisternae)

• May be more than one in a cell

• Situated near nucleus

Animation: Golgi Apparatus



• Golgi apparatus

• Steps of function

1. Products from RER arrive at the forming face in transport vesicles

2. Transport vesicles fuse with Golgi apparatus and empty contents into cisternae

• Enzymes modify products

3. New vesicles move material between cisternae

4. Product arrives at maturing face



• Golgi apparatus• Products

• Membrane renewal vesicles• Add to plasma membrane

• Secretory vesicles• Contain products to be discharged from the cell

• Fuse with plasma membrane and release contents into extracellular environment

• Enzymes for cytosol• Contained within lysosomes (lyso-, a loosening

+ soma, body)

• Isolate damaging chemical reactions



• Lysosomes

• Isolated intracellular location for toxic chemicals involved in breakdown and recycling of large organic molecules

• Three basic functions

1. May fuse with another organelle to activate digestive enzymes

2. May fuse with another vesicle containing fluid or solid extracellular materials

3. May break down with cell injury or death causing autolysis (enzymes destroy cytoplasm)

• “Suicide packets”


The three basic functions of lysosomes

Waste products and debris are ejected from the cellwhen the vesicle fuses with the plasma membrane.

Vesicles containing fluids or solids mayform at the surface of the cell.

Extracellularsolid or fluid

Lysosomal enzymesare activated by

fusion withanother vesicle

or organelle

As digestionoccurs, nutrientsare reabsorbedfor recycling.

Lysosomesinitially contain

inactiveenzymes.

As the materialsor pathogens arebroken down by

lysosomal enzymes,released nutrients

are absorbed.

Golgiapparatus

Function 1: A lysosomemay fuse with the mem-brane of anotherorganelle, such as amitochondrion. Thisactivates the enzymesand begins thedigestion of thelysosomal contents.

Function 2: A lysosomemay also fuse with avesicle containing fluid orsolid materials fromoutside the cell.

Function 3: The lysosomal membranemay break down following injury to, ordeath of, the cell. The digestiveenzymes become active and thenattack the cytoplasm in a destructiveprocess known as autolysis. For thisreason, lysosomes are sometimescalled “suicide packets.”



• Membrane flow

• Continuous movement and exchange of materials between organelles using vesicles

• Can replace parts of cell membrane to allow cell to grow, mature, or respond to changing environment


Module 3.5 Review

a. List the three major functions of the Golgi apparatus.

b. The Golgi apparatus produces lysosomes. What do these lysosomes contain?

c. Describe three functions of lysosomes.


Module 3.6: Mitochondria

• Mitochondria (mitos, thread + chondrion, granule)

• Produce energy (ATP) for cells through the breakdown of carbohydrates (glucose)

• Vary widely in shape and number

• Red blood cells have none

• Cardiac muscle cells are 30% mitochondria by volume

Animation: Mitochondria


The role of mitochondria in the production of the high-energy compound ATP

Although most ATP productionoccurs inside mitochodria,the first steps take place in thecytosol. In this reactionsequence, called glycolysis(glycos, sugar + -lysis, aloosening), each glucosemolecule is broken down intotwo molecules of pyruvate. Thepyruvate molecules are thenabsorbed by mitochondria.

In the mitochondrial matrix, aCO2 molecule is removedfrom each absorbed pyruvatemolecule; the remainderenters the citric acid cycle,of TCA (tricarboxylic acid)cycle, an enzymatic pathwaythat systematically breaksdown the absorbed pyruvateremnant into carbon dioxideand hydrogen atoms.

The hydrogen atoms aredelivered to enzymes andcoenzymes of the cristaewhich catalyze thesynthesis of ATP fromADP and phosphate. Atthe end of this process,oxygen combines with thehydrogen atoms to formwater molecules.

The energy releasedduring a series of stepsperforms the enzymaticconversion of ADP toATP, which leaves themitochondrion.

CYTOPLASMGlucose

2 Pyruvate

Citric acidcycle

Enzymes andcoenzymesof cristae

MITOCHONDRION

MATRIX

ADP +phosphate



• Mitochondria

• Double membrane

• Outer (surrounds organelle)

• Inner (contains numerous folds called cristae)

• Encloses liquid (matrix)

• Cristae increase surface area for energetic reactions


Matrix CristaeCytoplasm

of cell

A colorized TEM of a mitochondrion TEM x 50,000



• Steps of ATP production

1. Glycolysis (glycos, sugar + -lysis, a loosening)

• Occurs in cytosol

• 1 glucose 2 pyruvate

• Pyruvate absorbed into mitochondria

2. In mitochondrial matrix

• CO2 removed from pyruvate

• Enters citric acid (or TCA, tricarboxylic acid) cycle

1.Systematically removes CO2 and hydrogen atoms



• Steps of ATP production (continued)

3. Enzymes and coenzymes use hydrogen atoms to catalyze ATP from ADP

• Also forms H2O

4. ATP leaves mitochondrion


Module 3.6 Review

a. Describe the structure of a mitochondrion.

b. Most of a cell’s ATP is produced within its mitochondria. What gas do mitochondria require to produce ATP?

c. What does the presence of many mitochondria imply about a cell’s energy requirements?


Section 2: Nucleus


• 3.7 Explain the functions of the cell nucleus, and discuss the nature and

importance of the genetic code.

• 3.8 Summarize the process of protein synthesis.

• 3.9 Summarize the process of transcription.

• 3.10 Summarize the process of translation.


Section 2: Nucleus

• Nucleus

• Usually largest cellular structure

• Control center for cellular operations

• Can direct synthesis of >100,000 different proteins

• Coded in sequence of nucleotides

• Determines cell structure/function

• Usually only one per cell

• Exceptions:

• Multiple: skeletal muscle cell

• None: mature red blood cells


Section 2: Nucleus

• The nucleus directs cellular responses to environmental (ECF) changes

• Short-term adjustments

• Enzyme activity changes

• Long-term adjustments

• Changes in enzymes produced

• Changes in cell structure from changes in structural proteins

• Often occur as part of growth, development, and aging

© 2011 Pearson Education, Inc.Figure 3 Section 2

The role of the nucleus in preservinghomeostasis at the cellular level

Changesin the

compositionof theECF

Binding tomembranereceptors

Diffusionthrough

membranechannels

EXTRACELLULAR FLUID (ECF)

Plasma membrane

SHORT-TERMADJUSTMENTS

LONG-TERMADJUSTMENTS

CYTOPLASM

Binding to nuclearreceptors that alter

genetic activity

DNA innucleus

Enzymeactivation orinactivation

Changes in the bio-chemical processesunder way in the cellresulting from thesynthesis of additionalenzymes, fewerenzymes, or differentenzymes

Changes in thephysical structure ofthe cell due to altera-tions in the rates ortypes of structuralproteins synthesized


Module 3.7: Nuclear structure

• Nuclear structures and functions

• Nuclear envelope

• Separates nucleus from cytoplasm

• Double membrane

• Perinuclear space (peri-, around)

• Space between layers

• Nuclear pores

• Allow passage of small molecules and ions



• Nuclear structures and functions (continued)

• Nucleoplasm

• Fluid contents of nucleus

• Fine filaments

• Ions

• Enzymes

• RNA and DNA nucleotides

• Small amounts of RNA

• DNA



• Nuclear structures and functions (continued)

• Nucleoli (singular, nucleolus)

• Transient, clear nuclear organelles

• Composed of:

• RNA

• Enzymes

• Proteins (histones)

• Form around DNA instructions for forming proteins/RNA

• Assemble RNA subunits

• Many found in large, protein-producing cells

• Liver

• Nerve

• Muscle


The structure of the nucleus

Nuclear envelope

Perinuclearspace

Nuclear pores

Nucleoplasm

Nucleolus



• DNA

• Instructions for protein synthesis

• Strands coiled

• Wrap around histone molecules forming nucleosomes

• Loosely coiled (chromatin) in nondividing cells

• Tightly coiled (chromosomes) in dividing cells

• To begin, two copies of each chromosome held together at centromere

• 23 paired chromosomes in somatic (general body) cells

• One each from mother/father

• Carry instructions for proteins and RNA

• Also some regulatory and unknown functions


The coiled structure of DNA in the nucleus of a nondividing cell

Nucleus of nondividing cell

Chromatin

Nucelosome

Histones DNA doublehelix


The tighter coiling of DNA to formchromosomes in dividing cells

Dividing cell

Centromere

Visible chromosome

Supercoiledregion


Module 3.7 Review

a. What molecule in the nucleus contains instructions for making proteins?

b. Describe the contents and the structure of the nucleus.

c. How many chromosomes are contained within a typical somatic cell?


Module 3.8: Protein synthesis

• DNA

• Long parallel chains of nucleotides

• Chains held by hydrogen bonds

• Four nitrogenous bases

1. Adenine (A)

2. Thymine (T)

3. Cytosine (C)

4. Guanine (G)

• Genetic code (sequence of nucleotides)

• Triplet code (three nucleotides specify single amino acid)



• DNA (continued)

• Gene

• Functional unit of heredity

• All the DNA nucleotides needed to produce a specific protein

• Size varies (~3003000 nucleotides)



• Gene activation

• Removal of histones and DNA uncoiling

• Messenger RNA (mRNA)

• Assembled by enzymes

• Connecting complementary RNA nucleotides

• (A, G, C, U)

• Contains information in triplets (codons)

• Leaves nucleus through pores

• Transfer RNA (tRNA)

• Contains triplets (anticodons) that bind to mRNA codons

• Each type carries a specific amino acid linked to form a polypeptide



Animation: Protein Synthesis: RNA Polymerase

Animation: Protein Synthesis: Transcription and Translation


The key events of protein synthesis

Uncoiling of the portion of DNA moleculecontaining an activated gene

Assembly of an mRNA strand by enzymes

Binding of transfer RNA (tRNA) moleculescarrying a specific amino acid

Linking of amino acids to forma polypeptide Polypeptide

methionine-proline-serine-leucine

mRNA strand

tRNA attachesto mRNA

Amino acid

Anticodon

Codon

Codon on mRNA

The mRNA strand containingthe complementary codonspasses through a nuclear poreand enters the cytoplasm.

Enzyme

Paired DNA strands

DNA triplets are exposed to the nucleoplasms


A summary of how DNA codes for a protein

The DNAtriplets

determine thesequence of

mRNA codons.

The mRNAcodons

determinethe sequence

of tRNAs.

The sequence of tRNAsdetermines the

sequence of aminoacids in the polypeptide

or protein.


Module 3.8 Review

a. List the three types of RNA involved in protein synthesis.

b. What is a gene?

c. Why is the genetic code described as a triplet code?


Module 3.9: Transcription

• Transcription (“to copy” or “rewrite”)

• Production of RNA from DNA template

• All three types of RNA are formed

• Example:

• mRNA (information for synthesizing proteins)



• Steps of transcription

1. Gene activation

• Occurs at control segment (1st segment of gene)

• Template strand (One DNA strand used to synthesize RNA)

2. RNA polymerase (enzyme)

• Binds to promoter

• Assembles mRNA strand

• Complementary to DNA

• Example: (DNA triplet TAC = mRNA AUG)

• Hydrogen bonds between nucleotides


Events in the process of transcription of mRNA

Gene

DNA

Gene activation, which results in temporarydisruption of the hydrogen bonds between thenitrogenous bases of the two DNA strands

The templatestrand is the

DNA strand thatwill be used to

synthesize RNA.

The segmentat the start of thegene is known as

the controlsegment.

Triplet 1

Triplet 2

Triplet 3

Triplet 4

Co

mp

lem

enta

rytr

iple

ts

1

2

3

4

1

2

3

4

Adenine

Guanine

Cytosine

Uracil (RNA)

Thymine (DNA)

The enzymeRNA polymerase

binds to theexposed control

segment and, using the triplets as a

guide, assembles astrand of mRNA.



• Steps of transcription (continued)

3. Transcription ends

• Stop codon reached

• mRNA detaches

• Complementary DNA strands reassociate (hydrogen bonding between complementary base pairs)


Hydrogen bonding betweenthe nitrogenous bases ofthe template strand andcomplementary nucleotidesin the nucleoplasm

Conclusion of transcriptionwhen stop codon is reached

RNApolymerase

RNAnucleotide

Codon 1

RNA polymerase works onlyon RNA nucleotides—it can

attach adenine, guanine,cytosine, or uracil, but neverthymine. If the DNA triplet is

TAC, the correspondingmRNA codon will be AUG.

mRNA strand

Codon 1

Codon 2

Codon 3

Codon 4(stop codon)

RNApolymerase

The production of functional mRNAfrom immature mRNA

Immature mRNA

Intronsremoved

Exons spliced togetherto from mature mRNA

Adenine

Guanine

Cytosine

Uracil (RNA)

Thymine (DNA)

Events in the process of transcription of mRNA (continued)



• Immature RNA

• Contains triplets not needed for protein synthesis

• “Edited” before leaving nucleus through pores

• Introns (removed nonsense regions)

• Exons (remaining coding segments)

• Creates shorter, functional mRNA

• Changing “edits” can produce mRNAs for different proteins


The production of functional mRNAfrom immature mRNA

Immature mRNA

Intronsremoved

Exons spliced togetherto form mature mRNA


Module 3.9 Review

a. Define DNA template strand.

b. What is transcription?

c. What process would be affected if a cell could not synthesize the enzyme RNA polymerase?


Module 3.10: Translation

• Translation (translate nucleic acids to proteins)

• Uses mRNA created in nucleus

• Leaves via nuclear pores

• Occurs in cytoplasm

Animation: Protein Synthesis: Translation Initiation



• Steps of translation

1. mRNA binds to small ribosomal subunit

• Binding between mRNA and tRNA

• mRNA codons with tRNA anticodons

2. Small and large ribosomal subunits assemble around mRNA strand

• Additional tRNAs arrive

• More than 20 kinds

• At least one for each amino acid


The process of translation

NUCLEUS

Entry of mRNA into cytoplasm

Adenine

Guanine

Cytosine

Uracil

Binding of mRNA strand to a smallribosomal subunit and arrival of thefirst tRNA

Smallribosomal

subunit

Start codon mRNA strand

tRNA binding sites

Anticodon

tRNA

Amino acid

Joining of small and largeribosomal subunits around themRNA strand and arrival ofadditional tRNAs

Largeribosomal

subunit



• Steps of translation (continued)

3. Ribosome attaches to next complementary tRNA

4. Ribosome links amino acids forming dipeptide

• More tRNAs arrive and continue forming polypeptide

5. Stops once stop codon is reached on mRNA

• Ribosomal subunits detach

• Leaves intact mRNA and new polypeptide

Animation: Protein Synthesis: Sequence of Amino Acids in the Newly Synthesized Polypeptide


Stopcodon

Attachment of tRNA withanticodon that is complementaryto codon on RNA strand

Formation of a depeptide,release of first tRNA, andarrival of another tRNA

Peptidebond

Completion of polypeptide anddetachment of ribosomalsubunits

Completedpolypeptide

Largeribosomal

subunit

Small ribosomalsubunit

Adenine

Guanine

Cytosine

Uracil

The process of translation (continued)



• Translation

• Produces a typical protein in ~20 seconds

• mRNA can interact with other ribosomes and produce more proteins

• Multiple ribosomes can attached to a single mRNA strand to quickly produce many proteins

Animation: Transcription Translation


Module 3.10 Review

a. What is translation?

b. The nucleotide sequence of three mRNA codons is AUU-GCA-CUA. What is the complementary anticodon sequence for the second codon?

c. During the process of transcription, a nucleotide was deleted from an mRNA sequence that coded for a protein. What effect would this deletion have on the amino acid sequence of the protein?


Section 3: Membrane Transport


• 3.11 Explain the process of diffusion, and identify its significance to the body.

• 3.12 Explain the process of osmosis, and identify its significance to the body.

• 3.13 Describe carrier-mediated transport and its role in the absorption and removal

of specific substances.

• 3.14 Describe vesicular transport as a mechanism for facilitating the

absorption or removal of specific substances from cells.



• Plasma membrane

• Acts as a barrier separating cytosol and ECF

• Must still coordinate cellular activity with extracellular environment

• Permeability (determines which substances can cross membrane)

• Freely permeable (any substances)

• Selectively permeable (some substances cross)

• Impermeable (none can pass)

• No living cell is impermeable

© 2011 Pearson Education, Inc.Figure 3 Section 3 1

Permeability characteristics of membranes

Freely permeable membranes Selectively permeable membranes Impermeable membranes

Freely permeable membranesallow any substance to pass withoutdifficulty.

Selectively permeable membranes,such as plasma membranes, permit thepassage of some materials and preventthe passage of others.

Nothing can pass through impermeablemembranes. Cells may be impermeableto specific substances, but no living cellhas an impermeable membrane.

Protein Protein Protein

Lipids Lipids Lipids

Ions Ions Ions

— — —Water Water Water

Carbohydrates Carbohydrates Carbohydrates



• Selectively permeable membranes

• Selective based on:

1. Characteristics of material to pass

• Size

• Electrical charge

• Molecular shape

• Lipid solubility

• Other factors

2. Characteristics of membrane

• What lipids and proteins present

• How components are arranged



• Selectively permeable membranes

• Types of membrane transport

1. Passive (do not require ATP)

• Diffusion

• Carrier-mediated transport

2. Active (require ATP)

• Vesicular transport



Characteristics of selectivelypermeable membranes EXTRACELLULAR

FLUID

CYTOPLASM

Materials may crossthe plasma membrane

through active orpassive mechanisms.

Passive mechanismsdo not require ATP.

Active mechanismsrequire ATP.

Diffusion ismovement drivenby concentration

differences.

Carrier-mediatedtransport involves

carrier proteins, andthe movement may

be passive or active.

Vesicular transportinvolves theformation ofintracellular

vesicles; this is anactive process.

Plasmamembrane


Module 3.11: Diffusion

• Diffusion

• Continuous random movement of ions or molecules in a liquid or gas resulting in even distribution

• Gradient

• Concentration difference or when molecules are not evenly distributed

• At an even distribution, molecular motion continues but no net movement

• Slow in air and water but important over small distances

Animation: Membrane Transport: Diffusion



• In ECF

• Water and solutes diffuse freely

• Across plasma membrane

• Selectively restricted diffusion

• Movement across lipid portion of membrane

• Examples: lipids, lipid-soluble molecules, soluble gases

• Movement through membrane channel

• Examples: water, small water-soluble molecules, ions

• Movement using carrier molecules

• Example: large molecules


Large molecules that cannot fitthrough the membrane channelsand cannot diffuse through themembrane lipids can only crossthe plasma membrane whentransported by a carrier mechanism.

Lipids, lipid-soluble molecules,and soluble gases (O2 and CO2)can diffuse across the lipid bilayerof the plasma membrane.

The effects of the plasma membrane, a selectively permeable membrane, onthe diffusion of various substances

Plasma membrane

Channelprotein

EXTRACELLULAR FLUID

CYTOPLASM

Water, small water-soluble molecules,and ions diffuse through membrane channelsthat vary in shape, size, and specificity.



• Factors that influence diffusion rates:

• Distance (inversely related)

• Molecule size (inversely related)

• Temperature (directly related)

• Gradient size (directly related)

• Electrical forces

• Attraction of opposite charges (+,–)

• Repulsion of like charges (+,+ or –,–)


Module 3.11 Review

a. Define diffusion.

b. Identify factors that influence diffusion rates.

c. How would a decrease in the oxygen concentration in the lungs affect the diffusion of oxygen into the blood?


Module 3.12: Osmosis

• Osmosis (osmos, a push)

• Net diffusion of water across a membrane

• Maintains similar overall solute concentrations between the cytosol and extracellular fluid

• Osmotic flow

• Movement of water driven by osmosis

• Osmotic pressure

• Indication of force of pure water moving into a solution with higher solute concentration

• Hydrostatic pressure

• Fluid force

• Can be estimate of osmotic pressure when applied to stop osmotic flow


Osmotic flow, the movement of water driven by osmosis

Water molecules

Solute molecules

Selectively permeable membrane

A selectively permeable membraneseparates these two solutions,which have different soluteconcentrations. Water molecules(small blue dots) begin to crossthe membrane toward solution B,the solution with the higherconcentration of solutes (largerpink circles).

At equilibrium, the soluteconcentrations on the twosides of the membrane areequal. Note that the volumeof solution B has increasedat the expense of that ofsolution A.

Pushing against a fluid generateshydrostatic pressure. Theosmotic pressure of solution Bis equal to the amount ofhydrostatic pressure, indicatedby the weight, required to stopthe osmotic flow.

Appliedforce

Volumesequal

Volumeincreased

Volumedecreased

Originallevel



• Osmolarity (osmotic concentration)

• Total solute concentration in an aqueous solution

• Tonicity

• Effect of osmotic solutions on cell volume

• Three effects

1. Isotonic (iso-, same + tonos, tension)

• Solution that does not cause osmotic flow across membrane



• Tonicity

• Three effects (continued)

2. Hypotonic

• Causes osmotic flow into cell

• Example: hemolysis (hemo-, blood + lysis, loosening)

3. Hypertonic

• Causes osmotic flow out of cell

• Example: crenation of RBCs



• Importance of tonicity vs. osmolarity: Example

• Administering large fluid volumes to patients with blood loss or dehydration

• Administered solution has same osmolarity as ICF but higher concentrations of individual ions/molecules

• Diffusion of solutes may occur across cell membrane

• Water will follow through osmosis

• Cell volume increases

• Normal saline

• 0.9 percent or 0.9 g/dL of NaCl

• Isotonic with blood


Module 3.12 Review

a. Describe osmosis.

b. Contrast the effects of a hypotonic solution and a hypertonic solution on a red blood cell.

c. Some pediatricians recommend using a 10 percent salt solution to relieve nasal congestion in infants. Explain the effects this treatment would have on the cells lining the nasal cavity. Would it be effective?


Module 3.13: Carrier-mediated transport


• Hydrophilic or large molecules transported across cell membrane by carrier proteins

• Many move specific molecules through the plasma membrane in only one direction

• Cotransport (>1 substance same direction)

• Countertransport (2 substances in opposite directions)

• Carrier called exchange pump



• Carrier-mediated transport• Three types

1. Facilitated diffusion

• Requires no ATP (= passive)

• Movement limited by number of available carrier proteins (= can become saturated)

2. Active transport

• Requires energy molecule or ATP (= active)

• Independent of concentration gradient

• Examples:

1.Ion pumps (Na+, K+, Ca2+, and Mg2+)

2.Sodium–potassium ATPase



Animation: Membrane Transport: Facilitated Diffusion

Animation: Membrane Transport: Active Transport


Facilitated diffusion

EXTRACELLULARFLUID

Glucosemolecule

Carrierprotein

Receptor site

CYTOPLASM

Facilitated diffusion begins when a specificmolecule, such as glucose, binds to a receptorsite on the integral protein.

The shape of the protein then changes, movingthe molecule across the plasma membrane. Thecarrier protein then releases the transportedmolecule into the cytoplasm. Note that this wasaccomplished without ever creating a continuousopen channel between the extracellular fluid andthe cytoplasm.

Glucose releasedinto cytoplasm


On average, for each ATP molecule consumed, three sodium ions are ejected and thecell reclaims two potassium ions. The energy demands are impressive: Sodium-potassium ATPase may use up to 40 percent of the ATP produced by a resting cell!

Sodium ion concentrations arehigh in the extracellular fluids,but low in the cytoplasm. Thedistribution of potassium in thebody is just the opposite: low inthe extracellular fluids and highin the cytoplasm. As a result,sodium ions slowly diffuse intothe cell, and potassium ionsdiffuse out through leakchannels. Homeostasis withinthe cell depends on the ejectionof sodium ions and the recaptureof lost potassium ions. Thesodium–potassium exchangepump is a carrier protein calledsodium–potassium ATPase. Itexchanges intracellular sodiumfor extracellular potassium.

EXTRACELLULARFLUID

Sodium–potassiumexchange

pump

CYTOPLASM

Active transport



• Carrier-mediated transport (continued)

• Three types (continued)

3. Secondary active transport

• Transport mechanism does not require ATP

• Cell often needs ATP to maintain homeostasis associated with transport


A sodium ion and a glucosemolecule bind to receptorsites on the carrier protein.

CYTOPLASM

Glucosemolecule

Sodiumion

Secondary active transport

The carrier protein then changes shape,opening a path to the cytoplasm andreleasing the transported materials. Itthen reassumes its original shape and isready to repeat the process.

To preserve homeostasis, thecell must then expend ATP topump the arriving sodium ionsout of the cell by using thesodium–potassium exchangepump. It thus “costs” the cellone ATP for every threeglucose molecules ittransports into the cell.

Na+–K+

pump

+

+


Module 3.13 Review

a. Describe the process of carrier-mediated transport.

b. What do the transport processes of facilitated diffusion and active transport have in common?

c. During digestion, the concentration of hydrogen ions (H+) in the stomach contents increases to many times that in cells lining the stomach. Which transport process could be responsible?


Module 3.14: Vesicular transport

• Vesicular transport

• Materials move across cell membrane in small membranous sacs

• Sacs form at or fuse with plasma membrane

• Two major types (both require ATP)

1. Endocytosis

2. Exocytosis



• Vesicular transport (continued)• Two major types (both require ATP)

1. Endocytosis (into cell using endosomes)

a. Receptor-mediated endocytosis

1) Ligand binds to receptor

2) Plasma membrane folds around receptors bound to ligands

3) Coated vesicle forms

4) Vesicle fuses with lysosomes

5) Ligands freed and enter cytosol

6) Lysosome detaches from vesicle

7) Vesicle fuses with plasma membrane again


Receptor-mediated endocytosis

Receptors bound toligands cluster together.Once an area of theplasma membrane hasbecome covered withligands, it formsgrooves or pockets thatmove to one area of thecell and then pinch offto form an endosome.

The endosomesproduced in this wayare called coatedvesicles, becausethey are “coated” by aprotein-fiber networkon the inner membranesurface.

The coated vesiclesfuse with lysosomesfilled with digestiveenzymes.

The lysosomalenzymes thenfree the ligandsfrom theirreceptors, andthe ligands enterthe cytosol bydiffusion oractive transport.

The vesiclemembranedetaches fromthe secondarylysosome.

After the vesiclemembranedetaches, itreturns to thecell surface,where its recep-tors becomeavailable to bindmore ligands.

EXTRACELLULAR FLUID Ligands

Ligandreceptors

Ligands bindingto receptors

Exocytosis Endocytosis

CYTOPLASM

Coatedvesicle

Ligandsremoved

Lysosome

FusionDetachment

Receptor-mediated endocytosis begins when materials in theextracellular fluid bind to receptors on the membrane surface. Mostreceptor molecules are glycoproteins, and each binds to a specifictarget molecule, or ligand, such as a transport protein or a hormone.



• Vesicular transport (continued)• Two major types (both require ATP)

1. Endocytosis (into cell using endosomes) (continued)

b. Pinocytosis (“cell drinking”)

• Formation of endosomes with ECF

• No receptor proteins involved

c. Phagocytosis (“cell eating”)

• Produces phagosomes containing solids

• Phagocytes or macrophages perform phagocytosis

2. Exocytosis

1.Vesicle discharges materials into ECF


Endosome

Plasma membrane

Pinocytosis begins with theformation of deep groovesor pockets that then pinchoff and enter the cytoplasm.The steps are similar tothose of receptor-mediatedendocytosis, but they occurin the absence of ligandbinding.

Pinocytosis TEM 20,000

Cytoplasm

Surrounding tissue

Bloodstream


Bacterium

Pseudopodium

Phagocytosis

Lysosome

Golgiapparatus

Exocytosis

Phagocytosis begins when cytoplas-mic extensions called pseudopodia(soo-dō-PŌ-dē-ah; pseduo-, falsepodon, foot; singular pseudopodium)surround the object.

The pseudopodia then fuse at theirtips to form a phagosome containingthe targeted material.

This vesicle then fuses with manylysosomes, whereupon its contentsare digested by lysosomal enzymes.

Released nutrients diffuse into thesurrounding cytoplasm.

The residue is then ejected from thecell through exocytosis.

The vesicular events linkingphagocytosis and exocytosis


Module 3.14 Review

a. Describe endocytosis.

b. Describe exocytosis.

c. When they encounter bacteria, certain types of white blood cells engulf the bacteria and bring them into the cell. What is this process called?


Section 4: Cell Life Cycle


• 3.15 Describe interphase, and explain its significance.

• 3.16 Describe the process of mitosis, and the cell life cycle.

• 3.17 Discuss the relationship between cell division and cancer.



• Cell division

• Production of daughter cells from single cell

• Important in organism development and survival

• Cells have varying life spans and abilities to divide

• Often genetically controlled death occurs (apoptosis)

• Two types

1. Mitosis (2 daughter cells, each with 46 chromosomes)

2. Meiosis (sex cells, each with only 23 chromosomes)

Animation: Cell Life Cycle



• Mitosis

• Pair of daughter cells half the size of parent cell

• Grow to size of original cell before dividing

• Identical copies of chromosomes in each

• Ends at complete cell separation (= cytokinesis)

• Followed by nondividing period (= interphase)

• Cell performs normal activities OR

• Prepares to divide again

• Chromosomes duplicated

• Associated proteins synthesized


The production of a pair of daughtercells from a single cell division

Originalcell

Celldivision

Daughtercells


Module 3.15: Interphase

• Phases

• G0 (performing normal cell functions)

• Examples:

• Skeletal muscle cells (stay in this phase forever)

• Stem cells (never enter G0; divide repeatedly)

• G1 (normal cell function plus growth and duplication of organelles)

• S (duplication of chromosomes)

• G2 (last minute protein synthesis and centriole replication)


Module 3.15: Interphase

• DNA replication

• Strands unwind

• DNA polymerase binds

• Assembles new DNA strand covalently linking nucleotides

• Works only in one direction

• One polymerase works continuously along one strand toward “zipper”

• One polymerase works away from “zipper”

• As “unzipping” occurs, another polymerase binds closer point of unzipping

• Two new DNA segments bound with ligases

• Two identical DNA strands formed


The events in DNAreplication, whichoccurs during theS phase of interphase

DNA replication beings when enzymes unwind thestrands and disrupt the hydrogen bonds between thebases. As the strands unwind, molecules of DNApolymerase bind to the exposed nitrogenous bases. Thisenzyme (1) promotes bonding between the nitrogenousbases of the DNA strand and complementary DNAnucleotides dissolved in the nucleoplasm and (2) links thenucleotides by covalent bonds.

As the two original stands graduallyseparate, DNA polymerase binds to thestrands. DNA polymerase can work inonly one direction along a strand of DNA,but the two strands in a DNA moleculeare oriented in opposite directions. TheDNA polymerase bound to the upperstrand shown here adds nucleotides tomake a single, continuous complementarycopy that grows toward the “zipper.”

DNA polymerase on the lower strand canwork only away from the zipper. So thefirst DNA polymerase to bind to thisstrand must add nucleotides and build acomplementary DNA strand moving fromleft to right. As the two original strandscontinue to unzip, additional nucleotidesare continuously being exposed to thenucleoplasm. The first DNA polymeraseon this strand cannot go into reverse; itcan only continue to elongate the strandit already started.

Thus, a second DNA polymerase must bind closer to the point of unzipping and assemble a comple-mentary copy (segment 2) that grows until it “bumps into” segment 1 created by the first DNA polymerase. The two segments are then spliced together by enzymes called ligases (LĪ-gās-ez; liga, to tie).

Adenine

Guanine

Cytosine

Thymine

Segment 2

Segment 1

DNA nucleotide


Duplicated DNA double helices


Module 3.15 Review

a. Describe interphase, and identify its stages.

b. What enzymes must be present for DNA replication to proceed normally?

c. A cell is actively manufacturing enough organelles to serve two functional cells. This cell is probably in what phase of interphase?


Module 3.16: Mitosis

• Mitosis

• Division and duplication of cell’s nucleus

• Phases

1. Prophase (pro-, before)

• Paired chromosomes tightly coiled

• Chromatid (each copy)

• Connected at centromere with raised area (kinetochore)

• Replicated centrioles move to poles

• Astral rays (extend from centrioles)

• Spindle fibers (interconnect centriole pairs)


The events in mitosis

Centrioles incentrosome

Nucleus

Interphase, whichprecedes mitosis

Prophase, the first phase of mitosis

The nuclearmembranedisintegratesduring this period.

The centrioleshave replicated,and the pairsnow move toopposite sidesof the nucleus.

Microtubules extend outwardfrom each pair of centrioles:astral rays extend into thecytoplasm, whereas spindlefibers interconnect thecentriole pairs.

Chromatids

Kinetochore

The kinetochore ofeach chromatidbecomes attachedto a spindle fiber.



• Mitosis (continued)• Phases (continued)

2. Metaphase (meta, after)

• Chromosomes align at metaphase plate

3. Anaphase (ana-, apart)

• Chromatids separate

• Drawn along spindle apparatus

4. Telophase (telo-, end)

• Cells prepare to enter interphase

• Cytoplasm constricts along metaphase plate (= cleavage furrow)

• Nuclear membranes re-form

• Chromosomes uncoil


Metaphase Anaphase

Metaphase plate

The two chromatids are now pulledapart and drawn to opposite ends ofthe cell along the spindleapparatus (the complex of spindlefibers). Anaphase ends when thechromatids arrive near the centriolesat opposite ends of the cell.

As the chromatids approachthe ends of the spindleapparatus, the cytoplasmconstricts along the plane ofthe metaphase plate, forminga cleavage furrow.

DaughtercellsCYTOKINESIS

Telophase, the finalphase of mitosis

Cytokinesis

The events in mitosis (continued)



• Cytokinesis (cyto-, cell + kinesis, motion)

• Begins with formation of cleavage furrow

• Continues through telophase

• Completion marks end of cell division


Module 3.16 Review

a. Define mitosis, and list its four stages.

b. What is a chromatid, and how many would be present during normal mitosis in a human cell?

c. What would happen if spindle fibers failed to form in a cell during mitosis?


Module 3.17: Tumors and cancer

• Cancer

• Illness that disrupts normal rates of cell division

• Characterized by permanent DNA sequence changes (= mutations)

• Most common in tissues with actively dividing cells

• Examples: skin, intestinal lining

• Compete with normal cells for resources


Module 3.17: Tumors and cancer

• Cancerous tumor (neoplasm; mass of cells) types

1. Benign

• Remain in original tissue

2. Malignant

• Accelerated growth due to blood vessel growth and supply to the area

• Invasion (cells migrating into surrounding tissues)

• Metastasis (formation of secondary tumors)


Module 3.17 Review

a. Define metastasis.

b. What is a benign tumor?

c. Define cancer.

3 Cellular Level of Organization

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