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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What’shappening here?!

Substrate concentration

substrate concentration

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What’shappening here?!

37°

Temperature

temperature

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What’shappening here?!

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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What’shappening here?!

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