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Lecture 1: Cell Bio
Different Cell Types
• Humans have at least 200 different cell types. • Yet, all cells share some basic components…
Cellular Similarities: Basic Cell Anatomy • Plasma Membrane - lipid bilayer • Cytoplasm • Cytoskeleton - actin
microfilaments, microtubules, IFs • Nucleus - nuclear envelope,DNA • Ribosomes - protein synthesis • Rough ER - protein synthesis,
glycosylation • Smooth ER - lipid and steroid
synthesis/metabolism, receptor attachment,enzyme storage/prod, carb metab, Ca regulation, G6P (gluconeogenesis)
• Golgi Apparatus - sorting, modifying & packaging proteins, ↓ apoptosis
• Lysosomes - digests proteins with acidic enzymes
• Mitochondria - makes ATP, FA oxidation/synth, cholesterol→ steroids, ↑ apoptosis, Ca reg
• Centrosome with Centrioles - anchor for microtubules esp during mitosis
THE PLASMA MEMBRANE Composition, Permeability & Movement of substances across
The Plasma Membrane
1. The lipid bilayer is made of these lipids:
1. phospholipids (75%) 2. cholesterol (20%) 3. glycolipids (5%)
2. Proteins are embedded in the lipids
The Plasma Membrane
Phospholipids, cholesterol, glycolipids, membrane proteins
Permeability of the Lipid Bilayer • Freely Permeable to
small, or uncharged, non-polar, molecules: – Steroids, O2, CO2, N2,
Fat-soluble vit: A,D,E,K
• Slightly Permeable to small and/or polar molecules eg: – Urea, Water, Ethanol
• Impermeable to large or charged molecules such as – sugar (Glucose), amino
acids, nucleotides, proteins, Ions (Na+, Cl-)
Green arrows can simply diffuse across the membrane. Red arrows need facilitation to get across
FYI: Sizes of some molecules
• Charged ions (Cl-, K+) and Larger molecules (glucose..) cannot simply diffuse through the lipid bilayer.
• They cannot get into or out of a cell without facilitation.
What drives the movement of the small uncharged molecules
across a cell membrane?
Concentration Gradient
• Substances or Solutes passively diffuse through a semipermeable membrane from high concentration to low concentration, seeking equilibrium
Phospholipids are an AMPHIPATHIC (ie both polar and non-polar) substance The Phosphate Heads are polar, or slightly charged, so it can form hydrogen
bonds with water, thus, are attracted to water, or, Hydrophilic. The Fatty Acid Tails are non-polar, or uncharged. They are attracted to other
lipids. They are repelled by water or Hydrophobic.
75% (Amphipathic) Phospholipids
MEMBRANE PROTEINS Channels, Carriers, Transporters, Active & Passive transport
Functions of Membrane Proteins
• Receptors • Ion channels • Transporters
• Enzymes • Cell Identity Markers
• Linkers
Integral & Peripheral Membrane Proteins
1. Integral (Transmembrane) Proteins: extend through lipid bilayer 2. Peripheral Proteins: attached to inside or outside surface of
plasma membrane
1. Passive Transport • Its called “PASSIVE”
because NO ENERGY IS REQUIRED for substances to diffuse down their gradients.
A. In Simple DIFFUSION,
substances pass directly through the lipid bilayer – no protein is required
B. Facilitated DIFFUSION the substance is either too big or charged and thus requires a channel protein or a carrier protein to enter the cell
B. Proteins that Facilitate Diffusion
• Channel Proteins eg ion channel
• DO NOT CHANGE SHAPE • But, may be GATED
– ligand-gated – voltage-gated – stretch-gated
• Carrier = Transport Proteins eg glucose transporter
• Transport molecules down concentration gradient by BINDING & CHANGING SHAPE
Passive vs. Active Transport: Is energy required to
cross the cell’s membrane?
1. Passive Transport – NO ENERGY IS
REQUIRED – Molecules flow from high
concentration to low concentration or down their electrochemical gradients
2. Active Transport
– REQUIRES ENERGY (usually ATP)
– Molecules are pushed AGAINST their gradients
In which direction does a charged solute want to move? Electrochemical Gradient
• Electrical Gradient - charged substances are drawn to areas of opposite charge
+ • Concentration
gradient - substances tend to go from high concentration to low concentration
= • Electrochemical
gradient - net gradient for a substance depends on both concentration & charge
ç è
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2. Active Transport (Requires ENERGY)
ENERGY is needed to transport substances AGAINST their concentration or electrochemical gradients
• 1°Active Transport requires chemical energy (eg hydrolysis of ATP) to
move a substance against its gradient
• 2°Active Transport uses electrochemical gradient of one substance (usually Na) to transport a second substance against its gradient – Symport - 2 substances move in same direction – Antiport - 2 substances move in opposite directions
The Classic example of Active Transport: Na /
K / ATPase pump
• Pumping both Na & K, AGAINST their ELECTROCHEMICAL GRADIENTS requires energy.
• Binding of Na, triggers hydrolysis of ATP. Breaking the ATP bonds releases Energy + ADP +Pi
• The energy changes the shape of the transporter to pump out the 3 Na – 3 Na+ out , 2 K+ in – Creates a Net + charge outside cell, keeps Na+ out of cells
VESICULAR TRANSPORT Exocytosis & Endocytosis
Vesicular Transport • Vesicles (like bubbles) transport
large or polar substances across the cell membrane. ALL REQUIRE ATP
1. ENDOCYTOSIS - large substances (eg dead bacteria) enter the cell by invagination & pinching off a little bit of the plasma membrane – Phagocytosis, pinocytosis, – Receptor-mediated endocytosis
2. EXOCYTOSIS - large substances (eg neurotransmitters) exit the cell via vesicles that fuse with the plasma membrane.
3. Transcytosis - cross cell. large substances enter one side of cell and leave the other side
OSMOSIS Movement of SOLVENT across membranes
A special case of Diffusion OSMOSIS: Movement of WATER
• Movement of SOLVENT ie WATER through a semipermeable membrane • Water moves towards areas with more solute (or higher osmolarity) • Water moves by simple diffusion or through special membrane proteins,
called aquaporins
Osmosis & Tonicity Since water follows solute: • if there is more solute is inside
the cell, ie the interstitial fluid is hypotonic to the cell, water will enter the cell and it will swell or burst: hemolysis
• If there is more solute outside the cell, in the interstitial fluid ie interstitial fluid is hypertonic to the cell, water exits the cell and it crenates
• Under normal conditions, the osmotic pressure of the interstitial fluid = osmotic pressure of the cell’s cytosol So…water moves into and out of a cell at the same rate.
Summary of Movement across the Cell Membrane
• Movement of solutes across a cell membrane can happen by: – Passive Transport
• Simple Diffusion – directly across the lipid bilayer • Facilitated Diffusion – needs channel & carrier but no energy
– Active Transport • 1° (requires ATP) • 2° - symport & antiport (uses another substance’s gradient)
– Vesicular Transport • Endocytosis • Exocytosis • Transcytosis
• Movement of the solvent (not the solute) – Osmosis (movement of water)
PROTEIN SYNTHESIS How a cell makes its proteins
Membrane Proteins & the Fluid Mosaic
• Protein positions and amounts are not static in the membrane but are constantly moving, changing places, adding or eliminating pieces like a mosaic that is fluid
2 Steps of Protein Synthesis • Step 1: in Nucleus
TRANSCRIPTION – DNA is transcribed
into mRNA
• Step 2: in Ribosome TRANSLATION – mRNA is translated
into a protein – Ribosomes can be
• free in the cytoplasm • or attached onto the
Rough Endoplasmic Reticulum (RER)
The Nucleus contains Chromatin
• In a non-dividing cell nucleus, the chromatin is loose and diffuse • Chromatin is a strand of nucleosomes with linker DNA in between. • A nucleosome is DNA wrapped around 8 histone proteins • Only when a cell divides, the chromatin folds and packs tightly into
Chromosomes
A Nucleotide consists of: 1. a Phosphate group, 2. a Deoxyribose Sugar, 3. And a Nitrogenous Base
– Adenine (A) – Guanine (G) – Thymine (T) – Cytosine (C)
The Base Pairs are: – Adenine – Thymine – Cytosine – Guanine
DNA consists of Nucleotides
TRANSCRIPTION: DNA codes for RNA
• A gene is a section of DNA that codes for a protein.
• RNA polymerase binds to a gene’s promoter region on the DNA.
• RNA pol. unwinds the double helix & pairs RNA nucleotides to DNA nucleotides from one strand
• When the RNA pol. reaches a
terminator region, it falls off & transcription stops
RNA exits the Nucleus
• m/t/rRNA exits the nucleus through a nuclear pore in the nuclear envelope – Nuclear envelope is a double membrane of 2 lipid bilayers – Nuclear envelope is continuous with rough endoplasmic reticulum
• The nucleus contains chromatin, RNA, proteins • The Nucleolus is the part of DNA that codes for ribosomes
• Cytoplasm = Everything between the nucleus & the plasma membrane: Cytosol, Organelles & Cytoskeleton
mRNA leaves the Nucleus & enters Cytoplasm
The Cytoplasm consists of:
Cytosol • 75-90% water • & dissolved substances:
– ions – glucose – amino acids – fatty acids – proteins – lipids – ATP – waste products
Organelles • Centrosome • Cilia & flagella • Ribosomes • Rough Endoplasmic Reticulum • Smooth E R • Golgi complex • Lysosome • Peroxisome • Proteosome
• Mitochondria
Cytoskeleton • Microfilaments • Microtubules • Intermediate Filaments
mRNA enters a Ribosome
Ribosomes can be: 1. FREE - In the cytosol 2. Attached onto the Rough Endoplasmic Reticulum giving it its“rough”
look 3. or in the mitochondria
TRANSLATION: RNA codes for PROTEIN
• mRNA binds to a ribosome • One amino acid is attached to each tRNA (transfer RNA) • Each tRNA anticodon matches an mRNA codon • Amino acids bind together to form an elongating peptide chain
mRNA Codons & tRNA Anticodons
• A Codon is a set of 3 nucleotides on the mRNA. • The An“t”icodon is a set of 3 nucleotides on “t”RNA. • Each codon sequence correlates to a specific amino acid.
Review: from DNA to Cytosolic protein
Transmembrane vs Secretory proteins • a signal sequence on the amino acid
chain complexes with Signal Recognition Particle (SRP).
• Together, they guide the ribosome towards the ER.
• SRP Receptors on the ER, bind the SRP & remove it so that the signal sequence can enter a Translocation channel.
• Secretory proteins will cross the RER membrane completely to enter the lumen of the RER
• Transmembrane proteins will
cross the RER membrane partially and remain membrane-bound **
The Golgi Apparatus, or “Post Office”
• After the RER, proteins move to the golgi apparatus via vesicles.
• The Golgi apparatus modifies the proteins (eg by adding a sugar for glycoproteins)
• The Golgi then sorts the proteins headed for the same destination into vesicles. Eg All lysosomal proteins will all be in 1 vesicle.
The Secretory Pathway • The golgi apparatus
sorts proteins into vesicles.
• Some vesicles will fuse with the cell membrane.
• If the proteins were
completely surrounded within the lumen of the vesicle, they will be secreted out of the cell when the vesicle fuses.
Protein moves from RER to Golgi Apparatus
• Protein modification & sorting • 3 possible places for a RER / Golgi protein to end up:
1. Membrane Vesicle: As a plasma transmembrane protein 2. Secretory Vesicle: Secreted out of the cell 3. Transport Vesicle: Intracellular destination eg lysosome
OTHER ORGANELLES
Smooth ER – Fats & Ca • The Smooth Endoplasmic
Reticulum has no ribosomes
• Synthesizes fatty acids & steroids (like estrogen, testosterone)
• Detoxifies lipid-soluble drugs, pesticides, alcohol,
• Helps release glucose into blood (cleaves P from G6P)
• In muscle cells, it stores & releases Ca2+
Lysosomes digest
• Contains the cell’s digestive enzymes – Autophagy - breaks down its own damaged organelles – Phagocytosis - digests external substances like bacteria – Autolysis - destroys the entire cell that contains the lysosome
• Enzymes only work at extremely acidic pH (5) – Proton transporter imports H+ to keep pH low inside
Proteosomes
• Barrel-shaped organelles that degrade cytosolic proteins, coated with ubiquitin, into peptides.
• Proteins targeted for degradation may be non-functional, misfolded or…
• Contain oxidases - enzymes that oxidize organic substances like fatty acids, amino acids & toxic substances like alcohol
• Numerous in liver cells where toxic substances accumulate for detoxification.
• By-product of oxidation is hydrogen peroxide, H2O2 (toxic) • Contain catalase enzyme to degrade H2O2
Peroxisome
THE CYTOSKELETON
A Cell’s CYTOSKELETON • The cytoskeleton
organizes and moves components of the cell.
• It is made up of: 1. ACTIN
MICROFILAMENTS 2. INTERMEDIATE
FILAMENTS 3. MICROTUBULES
ACTIN MICROFILAMENTS • Made of Actin & Myosin Proteins
1. Responsible for Cell SHAPE – Forms nonmotile Microvilli – Creates cell polarity ie Apical vs
basolateral surfaces
2. Enables Cell MOVEMENT – Muscle contraction – Cell locomotion
3. Requires ATP to assemble
4. The associated motor molecule is Myosin
Microfilaments & cell movement • Myosin is the motor
molecule that moves the actin
• Cell movement is a result of actin microfilaments extending forward & pushing the cell in that direction
MICROTUBULES & the Centrosome
• Emanate from centrioles within the centrosome – Minus (-) end is anchored at centriole – Plus (+) end grows
• Requires GTP to assemble • Motor proteins: Kinesin & Dynein MOVE VESICLES along the microtubule in
opposite directions (VESICULAR TRANSPORT) • Align & pull apart chromosomes during Mitosis • Make up cilia & flagella
Cytoskeleton Assembly & Disassembly
• Microfilaments are made of ACTIN – Myosin is the
motor molecule
• MICROTUBULES – Kinesin moves
things towards (+) end
– Dynein moves things towards (-) end (at the centriole)
INTERMEDIATE FILAMENTS
• Scaffolding/Stability – Eg Nuclear lamina
• Not Dynamic • Cell-type specific
– Eg keratin
THE MITOCHONDRIA Krebs Cycle, Electron Transport Chain and ATP production
Mitochondrial Energy Production
• Fatty acids, amino acids and sugars enter the Citric Acid Cycle within the mitochondria, to produce NADH, a source of electrons and H+ ions. and The electrons enter the Electron Transport Chain while the H+ ions power oxidative phosphorylation by ATP synthase complex to make ATP.
The Electron Transport Chain
• H+ ions from NADH are pumped into, and collect in the mitochondrial intermembrane space.
• The H+ concentration gradient is used as 2° active transport energy source to turn ATP synthase which makes ATP
• H+ entering the mitochondrial matrix attaches to O2 to make H2O
Other mitochondrial functions
1. Steroid synthesis (pregnenolone, cortisol, aldosterone) 2. Triggers Apoptosis (cell death)
Back to the Beginning • Nucleus - Trasncription of DNA to
mRNA which goes out nuclear pores of nuclear envelope
• Free ribosome - mRNA translation for cytosolic protein synthesis
• RER - studded with ribosomes. mRNA Translation & Translocation for secretory or transmembrane proteins.
• Golgi Apparatus - modifies proteins & tags for destination, ↓apoptosis
• Lysosomes - acidic enzymes • Centriole - microtubules attach,
mitosis (cell division), vesicle shuttle
• Cytoskeleton- actin microfilaments, microtubules, Intermediate filament
• Mitochondria - ATP generation from FA oxidation,protein, carb. cholesterol→ steroids, ↑apoptosis
• SER - lipid & steroid synthesis/metab, receptor attachment, G6P, enzyme storage/prod, carb metab, Ca regulation
• Peroxisome - oxidative enzymes, detoxes,
THE CELL CYCLE, APOPTOSIS, AND CANCER
Cell division & Cell death
Cells have 3 possible states of existence:
1. Remain alive and functional without dividing (G0)
2. Grow & Divide = The Cell Cycle • Interphase (G1, S, G2) • Mitosis
• Nuclear division: Prophase, Metaphase, Anaphase, Telophase • Cytokinesis: cytoplasmic division
3. Die (Apoptosis)
The Cell Cycle
• Consists of 2 major periods: 1. Interphase: the cell is replicating itself but not dividing 2. Mitotic phase: the nucleus is dividing
• Is the period from cell reproduction to the next reproduction
Interphase: (growth &
replication) G1, S, G2
The majority of a cell’s lifespan is spent in Interphase: • G1 - normal cell functions, duplication of organelles & cytosol components • S - DNA replicates • G2 (Gap2) - centrosome duplication completes.
S phase of Interphase:
DNA replication
• DNA replicates during S (synthesis) phase.
• DNA strands separate and new DNA is synthesized along each strand
Mitotic Phase (Division)
• Mitotic phase: a cell’s nucleus and cytoplasm are dividing.
• Includes: – Prophase – Metaphase – Anaphase – Telophase
23 Pairs of Chromosomes
• Everyone has 23 homologous pairs of chromosomes with with unique banding patterns.
• One member in each pair is inherited from each parent
Mitosis • Prophase:
chromosomes are visible, nucleus disappears
• Metaphase: chromosomes align
• Anaphase: chromosomes split
• Telophase: chromosomes uncoil, 2 nuclei form
Cell Cycle – Interphase & Mitotic Cell Division
Cell division: Mitosis vs Meiosis • There are 2 major kinds of
cells - somatic cells and germ cells
• Somatic Cell Division ie Mitosis produces 2 identical diploid daughter cells for: – Healing: replaces dead or
injured cells – Growth: adds new cells
• Reproductive Cell Division ie Meiosis produces haploid gametes: – sperm or oocytes
Turning on cell division
• CDK’s – Cyclin Dependent protein Kinases will turn on G1, then turn it off, and continue an orderly progression through S, G2, etc
• Cyclins - turn CDKs on & off
Apoptosis vs. Necrosis
• Apoptosis - Regulated, genetically programmed cell death (suicide) that does not produce inflammation. Cell is destroyed from within, shrinks & is engulfed by phagocytes.
• Necrosis - pathological cell death resulting in cell swelling, bursting, spilling contents & inflammation
APOPTOSIS
CANCER
• Disease state characterized by uncontrolled, or abnormal cell division = CANCER
• excess tissue resulting from uncontrolled cellular division = TUMOR or NEOPLASM
• A cancerous neoplasm is called a malignant tumor. Most malignant tumors spread to other parts of the body, or, metastasize.
Proto-oncogenes & Oncogenes
• Genes are sections of DNA that code for proteins
• Proto-oncogenes are normal genes that code for proteins that tend to deal with growth or cell division, such as growth factors
• If these proto-oncogenes should mutate, they become oncogenes and cell division becomes uncontrolled
• Tumor suppressor genes are genes that code for proteins that stop cell division, or induce apoptosis eg BRCA.
• Loss of these genes lead to uncontrolled cell division.
Tumor Suppressor
Genes