Review Topic 2
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Transcript of Review Topic 2
Review Topic 2
Cell membrane defines cell
• Cell membrane separates living cell from aqueous environment– thin barrier = 8nm thick
• Controls traffic in & out of the cell– allows some substances to cross more easily
than others• hydrophobic (nonpolar) vs. hydrophilic (polar)
Many Functions of Membrane ProteinsOutside
Plasmamembrane
InsideTransporter Cell surface
receptorEnzymeactivity
Cell surface identity marker
Attachment to thecytoskeleton
Cell adhesion
“Antigen”
“Channel”
Membrane carbohydrates
• Play a key role in cell-cell recognition– ability of a cell to distinguish one cell from
another• antigens
– important in organ & tissue development
– basis for rejection of foreign cells by immune system
Simple Diffusion
• Move from HIGH to LOW concentration– “passive transport”– no energy needed
diffusion osmosis
movement of water
Facilitated Diffusion
• Diffusion through protein channels– channels move specific molecules across
cell membrane– no energy needed
“The Bouncer”“The Bouncer”
open channel = fast transport
facilitated = with help
HIGH
LOW
Concentration of water
• Direction of osmosis is determined by comparing total solute concentrations
– Hypertonic - more solute, less water, low water potential
– Hypotonic - less solute, more water, high water potential
– Isotonic - equal solute, equal water
hypotonic hypertonic
water
net movement of water
freshwater balanced saltwater
Managing water balance
• Cell survival depends on balancing water uptake & loss
Aquaporins
• Water moves rapidly into & out of cells– evidence that there were water channels
• protein channels allowing flow of water across cell membrane
1991 | 2003
Peter AgreJohn Hopkins
Roderick MacKinnonRockefeller
Cell (compared to beaker) hypertonic or hypotonic
Beaker (compared to cell) hypertonic or hypotonic
Which way does the water flow? in or out of cell
.05 M .03 M
Do you understand Osmosis…
Active Transport
“The Doorman”“The Doorman”
conformational change
• Cells may need to move molecules against concentration gradient– conformational shape change transports solute from
one side of membrane to other – protein “pump”– “costs” energy = ATP
ATP
LOW
HIGH
Endocytosis
phagocytosis
pinocytosis
receptor-mediated endocytosis
fuse with lysosome for digestion
non-specificprocess
triggered bymolecular signal
Enzymes vocabularysubstrate
• reactant which binds to enzyme• enzyme-substrate complex: temporary association
product • end result of reaction
active site • enzyme’s catalytic site; substrate fits into active site
substrate
enzyme
productsactive site
Properties of enzymes
• Reaction specific– each enzyme works with a specific substrate
• chemical fit between active site & substrate– H bonds & ionic bonds
• Not consumed in reaction– single enzyme molecule can catalyze thousands
or more reactions per second• enzymes unaffected by the reaction
• Affected by cellular conditions– any condition that affects protein structure
• temperature, pH, salinity
Induced fit model
• More accurate model of enzyme action– 3-D structure of enzyme fits substrate– substrate binding cause enzyme to change
shape leading to a tighter fit • “conformational change”• bring chemical groups in position to catalyze
reaction
Protein denaturation
• Unfolding a protein– conditions that disrupt H bonds, ionic bonds,
disulfide bridges• temperature• pH• salinity
– alter 2° & 3° structure• alter 3-D shape
– destroys functionality• some proteins can return to their functional shape
after denaturation, many cannot
Enzyme concentration
enzyme concentration
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Substrate concentration
substrate concentration
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37°
Temperature
temperature
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7
pH
pH
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20 1 3 4 5 6 8 9 10
pepsin trypsin
What’shappening here?!
11 12 13 14
pepsin
trypsin
Salinity
salt concentration
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Compounds which help enzymes• Activators
– cofactors • non-protein, small inorganic
compounds & ions– Mg, K, Ca, Zn, Fe, Cu– bound within enzyme molecule
– coenzymes• non-protein, organic molecules
– bind temporarily or permanently toenzyme near active site
• many vitamins– NAD (niacin; B3)– FAD (riboflavin; B2)– Coenzyme A
Mg inchlorophyll
Fe inhemoglobin
Compounds which regulate enzymes• Inhibitors
– molecules that reduce enzyme activity– competitive inhibition– noncompetitive inhibition– irreversible inhibition– feedback inhibition
Allosteric regulation
• Conformational changes by regulatory molecules – inhibitors
• keeps enzyme in inactive form
– activators • keeps enzyme in active form
Conformational changes Allosteric regulation
Cooperativity
• Substrate acts as an activator– substrate causes conformational
change in enzyme• induced fit
– favors binding of substrate at 2nd site– makes enzyme more active & effective
• hemoglobinHemoglobin 4 polypeptide chains can bind 4 O2; 1st O2 binds now easier for other
3 O2 to bind
Feedback inhibition
• Example– synthesis of amino
acid, isoleucine from amino acid, threonine
– isoleucine becomes the allosteric inhibitor of the first step in the pathway
• as product accumulates it collides with enzyme more often than substrate does
threonine
isoleucine
Interphase
• 90% of cell life cycle– cell doing its “everyday job”
• produce RNA, synthesize proteins/enzymes
– prepares for duplication if triggered
I’m working here!
Time to divide& multiply!
Prophase
• Chromatin condenses – visible chromosomes
• chromatids
• Centrioles move to opposite poles of cell – animal cell
• Protein fibers cross cell to form mitotic spindle– microtubules
• actin, myosin
– coordinates movement of chromosomes
• Nucleolus disappears• Nuclear membrane breaks down
green = key features
Transition to Metaphase
• Prometaphase– spindle fibers attach to
centromeres • creating kinetochores
– microtubules attach at kinetochores
• connect centromeres to centrioles
– chromosomes begin moving
green = key features
Metaphase
• Chromosomes align along middle of cell– metaphase plate
• meta = middle
– spindle fibers coordinate movement
– helps to ensure chromosomes separate properly
• so each new nucleus receives only 1 copy of each chromosome
green = key features
Anaphase
• Sister chromatids separate at kinetochores – move to opposite poles
– pulled at centromeres
– pulled by motor proteins “walking”along microtubules
• actin, myosin• increased production of
ATP by mitochondria
• Poles move farther apart– polar microtubules lengthen
green = key features
Telophase
• Chromosomes arrive at opposite poles– daughter nuclei form – nucleoli form– chromosomes disperse
• no longer visible under light microscope
• Spindle fibers disperse• Cytokinesis begins
– cell division
green = key features
Cytokinesis
• Animals– constriction belt of actin
microfilaments around equator of cell
• cleavage furrow forms• splits cell in two• like tightening a draw
string
Cytokinesis in Plants
• Plants– cell plate forms
• vesicles line up at equator
– derived from Golgi
• vesicles fuse to form 2 cell membranes
– new cell wall laid down between membranes
• new cell wall fuses with existing cell wall