Chapter 3- Part 3

February 4, 2019 | Author: Hana | Category: Messenger Rna, Rna, Dna, Nucleic Acids, Cell Membrane
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C H A P T E R 

2 Chemistry, Biochemistry, and Cell Physiology Part 3

PowerPoint® Lecture Slides prepared by Stephen Gehnrich, Salisbury University

Cellular Membranes Two main roles 

Isolate cells from the environment 

Control of intracellular conditions

Organize intracellular pathways into subcellular compartments

Cellular Membranes Two main roles 

Isolate cells from the environment 

Control of intracellular conditions

Organize intracellular pathways into subcellular compartments

Membrane Structure

Lipid Profile 

Lipid bi-layer  

Phospholipids 

Primarily phosphoglycerides

Other lipids 

Sphingolipids 

Glycolipids 

Alter electrical properties Communication between cells

Cholesterol 

Increase fluidity while decreasing permeability

Membrane Properties of Cholesterol

Membrane Heterogeneity More PE and PS phosphoglycerides in inner leaflet More PC phosphoglycerides in the outer leaflet Glycolipids only in the outer leaflet Lipid rafts 

Enriched in glycolipids and cholesterol

More rigid and thicker 

Membrane Heterogeneity

Membrane Fluidity 

Environmental conditions affect membrane fluidity 

For example, low temperature increases van der Waals forces between lipids and restricts movement

Homeoviscous adaptation 

Cell keeps membrane fluidity constant by altering the lipid profile

Temperature and Membrane Fluidity

Membrane Proteins 

Can be more than half of the membrane mass

Structural and regulatory functions

Two main types 

Integral membrane proteins 

Tightly bound to the membrane

Embedded in bilayer or spanning the entire membrane

Peripheral membrane proteins 

Weaker association with the lipid bilayer 

Membrane Proteins

Membrane Transport 

Cells must transport molecules across membranes

Three main types of transport: 

Passive diffusion

Facilitated diffusion

Active transport

Distinguished by direction of transport, nature of the carriers, and the role of energy

Membrane Transport

Passive Diffusion 

Lipid-soluble molecules

  No 

Molecules cross lipid bilayer 

  No 

specific transporters are needed energy is needed

Depends on concentration gradient 

High concentration

Steeper gradient results in faster rates

low concentration

Facilitated Diffusion 

Hydrophilic molecules

Protein transporter is needed

  No 

energy is needed

Depends on concentration gradient 

High concentration

Steeper gradient results in faster rates

low concentration

Facilitated Diffusion Three main types of protein carriers: 

Ion channels 

Small pores for specific ions

Open and close in response to cellular conditions  “Gated” channels

Porins 

Like ion channels, but for larger molecules

Permeases 

Function more like an enzyme

Carries molecule across membrane

Ion Channels

Active Transport 

Protein transporter is needed

Energy is required

Molecules can be moved from low to high concentration

Active Transport Two main types of active transport 

Primary active transport  

Secondary active transport  

Direct use of an exergonic reaction Couples the movement of one molecule to the movement of a second molecule

Distinguished by the source of energy

Primary Active Transport 

Hydrolysis of ATP provides energy 

Transporters are ATPases

Three types  P-type 

Pump specific ions (e.g., Na +, K +, Ca2+)

 F-type

and V-type

Pump H+

ABC type

Carry large organic molecules (e.g., toxins)

Secondary Active Transport 

Use energy in electrochemical gradient of one molecule to drive another molecule against its gradient  Antiport

or exchanger carrier: molecules move in opposite directions

 Symport

or cotransporter carrier: molecules move in the same direction

Electrical Gradients   

All transport processes affect chemical gradients Some transport processes affect electrical gradients Electroneutral carriers 

Transport uncharged molecules or exchange an equal number of particles with the same charge

  Electrogenic 


Transfer a charge  For example, Na+/K +ATPase  exchanges 3Na+ for 2K +

Membrane Potential (Vm) 

Difference in charge inside and outside the cell membrane 

Concentration gradients formed by active transport

Two main functions 

Provide energy for membrane transport

Changes in membrane potential used by cells in cellto-cell signaling

Equilibrium Potential (E ion) Each ion has its own equilibrium potential 

Ion concentration gradient

Ion diffuses down its concentration gradient

Eion is the Vm at which the ion is at electrochemical equilibrium

Depends upon the size of the concentration gradient

Eion can be calculated using the Nernst equation 

Assumes electrochemical equilibrium

Equilibrium Potential (E ion)

Membrane Potential (Vm) 

Cell membranes are not at equilibrium 

Varying permeability

Multiple ion gradients

Goldman equation 

Accounts for permeability and multiple ions

Vm is most dependent upon Na+, K +, and Cl – 

  Na+/K + ATPase

maintains Na+ and K + gradients across membrane

Changes in Membrane Potential (Vm) Changes in membrane permeability cause changes in membrane potential 

Depolarization 

Cell becomes more positive on the inside

For example, if Na+ ions enter 

Hyperpolarization 

Cell becomes more negative on the inside

For example, if K + ions leave

Depolarization and Hyperpolarization

Cellular Structures 

Eukaryotic cells share many common cellular compartments

Compartmentalization allows for regulation of specific processes

Mitochondria  Produce most of the cell’s ATP 

Intricate network of internal membranes 

Large surface area

Mitochondrial reticulum   Network

of interconnected mitochondria

Mitochondrial DNA (mtDNA) 

Some mitochondrial proteins

Required for mitochondrial biogenesis

Most genes for mitochondrial proteins are in the nucleus


Cytoskeleton  Network of protein-based fibers   Microfilaments 

Flexible chains of actin

  Microtubules 

Tubes of tubulin

   Intermediate 


Composed of many types of monomers

Maintains cell structure  

External cell shape Organization of intracellular membranes

Cellular processes For example, movement, signal transmission

Functions of the Cytoskeleton 

Maintains cell structure 

External cell shape

Organization of intracellular membranes

Cellular processes 

Movement 

Motor proteins

Signal transduction


Endoplasmic Reticulum and Golgi Apparatus 

Membranous organelles

Proteins are made on the ER 

Proteins are modified and packaged into vesicles by the Golgi apparatus 

Vesicles carry proteins between compartments

Vesicles are carried throughout the cell by motor proteins moving on cytoskeletal tracks

Contents of vesicles can be released from the cell via exocytosis

Extracellular substances can be taken into the cell via endocytosis

Intracellular Traffic

Extracellular Matrix 

Gel-like “cement” between cells

Cell membranes are bonded to the matrix 

Insect exoskeleton, vertebrate skeleton, and mollusc shells are modified extracellular matrices

Molecules of the matrix are synthesized within the cells and secreted by exocytosis

Extracellular Matrix Molecules of the extracellular matrix 





Extracellular Matrix

Extracellular Matrix

Extracellular Matrix Cells can break down the extracellular matrix with matrix metalloproteinases Cells can move through tissues by controlling the  production and breakdown of the matrix 

For example, blood vessel growth and penetration

Physiological Genetics and Genomics Physiological diversity resides in genes  

How genes differ between species How genes are regulated in individual cells

Homeostatic regulation depends upon having    

the right protein, in the proper place, at the proper time, with the appropriate activity

 Nucleic Acids Two types: 

DNA – deoxyribonucleic acid 

Genetic blueprint

Genes in nucleus

RNA – ribonucleic acid 

Read and interpret DNA to make protein

Three main forms 

Transfer RNA (tRNA)

Ribosomal RNA (rRNA)

Messenger RNA (mRNA)

 Nucleic Acids 

DNA and RNA are polymers of nucleotides 

linked by phosphodiester bonds

  Nucleotide  Nitrogenous 

Cytosine, Adenine, Guanine,Thymine (DNA only), Uracil (RNA only)

Sugar 


Deoxyribose (DNA), ribose (RNA)



Double-stranded a-helix 

Two strands of nucleotides linked by hydrogen bonds 

Complementary strands


 Nucleotides

can form bonds with only one other

nucleotide 

A + T: two hydrogen bonds

G + C: three hydrogen bonds

Structure of DNA

Histones  

Mammalian DNA is several meters long DNA is compressed by DNA-binding proteins (histones) 

DNA in this form is referred to as chromatin

Advantages of compression by histones 

Large amounts of DNA fit into small volumes  Reduces damage caused by radiation and chemicals 

Must be uncompressed for DNA and RNA synthesis

DNA Organization 

Genome 

Chromosome 

DNA sequence within a chromosome Used to produce RNA

Exons 

Separate segments of DNA

Genes 

Entire collection of DNA within a cell

Segments of DNA that encode RNA

Introns 

Interspersed DNA sections between exons

DNA Organization

Genome Size

Transcription Synthesis of messenger RNA (mRNA) 

DNA is wrapped by histones

Must be unwrapped to allow transcription

Transcription regulators form regulatory complexes at promoter  

Region of the gene where transcription begins

mRNA synthesis begins


Mature mRNA 

Primary mRNA transcript   Exons – sequences

that will code for the protein

  Introns – noncoding


Introns are removed and exons are spliced together 

mRNA is polyadenylated  

200+ adenosines are added to the 3´ end

  poly A+


mRNA is exported from the nucleus to the cytoplasm

mRNA is ultimately degraded by nucleases (RNases)

Protein Synthesis (Translation)   Ribosomes 

Made of rRNA and proteins

Bound to endoplasmic reticulum

Catalyze the formation of peptide bonds between amino acids

  Transfer 


Carry the amino acids that bind to a codon (three nucleotides on mRNA)

Protein Degradation  

Proteins may have structural changes that result in dysfunction Structural changes recruit enzymes that mark the  protein with a small protein called ubiquitin 

Ubitquitin-labeled protein is then bound by a large enzyme complex called a proteasome 

Enzymes degrade the protein to amino acids

Protein Isoforms 

Variations in protein structure 

Genetic rearrangements 

Alternative splicing of exons


Gene duplications 

Subsequent mutation of some copies

Origins of Protein Isoforms

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