Sections 5.1-5.5
Section 5.1
Structure of Cell Membranes
 Lipids are the main components of membranes and proteins are embedded in the
membrane, and carbs are usually attached to the glycolipids and proteins.
 Phospholipids are the major type of lipids found in the membrane and contain
hydrophilic (phosphate head group bc polar so it forms hydrogen bonds w/water)
and hydrophobic regions (2 fatty acid tails bc nonpolar so doesn’t form hydrogen
bonds).
 Amphipathic is a molecule that has hydrophobic and hydrophilic regions.
 In an aqueous environment, the polar head groups interact with the water and the
nonpolar tail groups are on the inside away from the water.
 There are lipids with bulky heads and a single hydrophobic fatty acid tail that are
wedge-shaped and packed into spherical structures called micelles.
 Lipids that have less bulky head groups and two hydrophobic tails form a bilayer.
 A bilayer is a two-layered structure that has the hydrophilic portion facing out
towards the aqueous environment and the hydrophobic facing in.
 These bilayers form closed structures with inner space since free edges would
expose hydrophobic chains to the aqueous environment.
1. This is why bilayers are good cell membranes and how they’re selfhealing (sealed by rapid rearrangement of the lipids around damaged are
bc of water excluding nonpolar molecules.
 Bilayer forms as long as the concentration of free phospholipids is high enough
and pH is similar to the cell’s.
2. pH makes sure that the head groups are in ionized form and hydrophilic.
3. When phospholipids are added to test tube of water (pH 7 neutral) then
they form bilayer structures called liposomes.
4. Enzyme doesn’t catalyze the chem. Reaction, and membrane forms by
itself and depends on the chem. properities of lipids, and the liposomes
may capture macromolecules present in solution.
 Liposomes can have proteins (nucleic acids) in their interiors and proteins guide
lipid synthesis within the cell.
 Cell membranes are dynamic bc lipids freely associate w/one another bc of
extensive van der waals forces between fatty acid tails
 Weak interactions are easily broken and reformed so lipid molecules can move
within the plane of the membrane.
 Because membrane lipids can move in the plane of the membrane, it is a fluid
membrane.
 The more tightly the membrane is packed, the lower the lipid mobility is, and
strength of interactions depends on length of the fatty acid tails and presence of
double bonds between neighboring carbon atoms.
 Saturated fatty acid tails (no double bonds, straight tightly packed, reduce
mobility).
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Double bonds in unsaturated fatty acids produce kinks (reduce tightness of
packing and enhance lipid mobility)
Cholesterol is a major component of animal cell membranes (30% of membrane
lipid by mass).
Phospholipids and cholesterol is amphipathic (hydrophilic and hydrophobic)
In cholesterol, the hydrophilic region is a OH (hydroxyl group) & hydrophobic
region consists of 4 interconnected carbon rings with an attached hydrocarbon
chain.
The hydrophilic head group of cholesterol interacts w/head group of
phospholipids while ring structure participates in van der waals interactions with
fatty acid chains.
At normal temps the interaction of rigid ring structure of cholesterol
w/phospholipid fatty acid tails reduces mobility of the phospholipids & fluidity of
membrane.
At low temps, cholesterol prevents phospholipids from packing tightly w/ other
phospholipids and increases membrane fluidity, which means cholesterol helps
maintain consistent state of membrane fluidity by preventing dramatic transitions
from a fluid to solid.
Lipids assemble into defined patches (lipid rafts) and cholesterol and proteins
accumulate in these regions and membranes aren’t always uniform fluid bilayer
(actually it contains regions w/discrete components).
Liquid flip-flop (the spontaneous transfer of a lipid between layers of the bilayer)
is very rare bc it requires the hydrophilic head to go through the hydrophobic
interior of the membrane.
This shows how the 2 layers differ in composition.
Proteins represent the 50% of the cell membrane in mass (RBC).
Some proteins act as transporters or they move ions across membrane
Transporters include channels which allow movement of molecules thru them
and carriers which facilitate movement.
Other proteins act as receptors that allow cell to receive signals or they are
enzymes that catalyze chemical reactions or anchors that attach to the other
proteins that help maintain cell shape &structure.
Integral membrane proteins are permanently w/cell membranes and can’t be
separated from membrane
1. Transmembrane proteins are the most common and span the entire lipid
bilayer.
2. Composed of 3 regions: 2 hydrophilic regions and a connecting
hydrophobic region
3. Hydrophilic on inside interacts w/cytoplasm and the one on outside
interacts w/signaling molecules
Peripheral membrane proteins are associated w/lipid bilayer or w/integral
membrane proteins thru weak noncovalent interactions (easily separated from
membrane
1. These interact w/polar heads of lipids or w/integral membrane proteins via
weak noncovalent interactions (H-bonds).
2. Transiently associated w/membrane and helps transmit info received from
external signals
3. Limit ability of transmembrane proteins to move within membrane and
assist proteins to clump into lipid rafts.
 FRAP (fluorescence recovery after photobleaching) in which proteins in CM are
labeled w/ fluorescent dye molecules &can be visualized using fluorescent
microscope. Laser bleaches fluor. Dye molecules in small area of CM and if
proteins couldn’t move then they wouldn’t gain color back.
 Jonathan Singer and Garth Nicolson (fluid mosaic model) allows molecules to
move laterally within membrane.
Section 5.2
 Cells enclosed by plasma membrane (CM). internal environment is determined
by pH or salt concentration. PM is ACTIVE
 Cell wall is outside PM and maintains the cell shape
 PM maintains homeostasis (constant environment) and acts as a selective
barrier (PM lets some molecules in & out freely; lets others in &out only under
certain conditions; &prevents other molecules from passing at all.
 Many macromolecules (proteins and polysaccharides) are too large to pass thru
PM.
 Gases, small polar molecules pass freely
 Protein transporters in membrane allow export &import of molecules that can’t
cross the CM on their own.
 Each cell has own specific function due to proteins in CM
 Passive transport (simplest form of movement in&out of cells) works by
diffusion (random movement of molecules).
 Concentration gradient (difference in concentration), diffusion results in net
movement of molecule from hi conc to lo conc
 Diffusion occurs in absence of concentration gradient bc of random motion of
molecules
 Hydrophobic molecules (triacylglycerols) are able to diffuse thru CM bc the lipid
bilayer is hydrophobic as well
 Facilitated diffusion is when a molecule moves by diffusion thru a membrane
protein &bypasses the lipid bilayer
 Facilitated and diffusion can happen bc of random motion of molecules &net
movement bc of concentration gradient.
 Proteins exist in 2 shapes; one that’s open to 1 side of the cell and the other that’s
open to the other side.
 Water moves in and out of cells by passive transport (the molecules are small
enough to move passively thru membrane by simple diffusion)
 Aquaporins are specific protein channels (allow water to flow thru the PM more
readily by facilitated diffusion).
 Osmosis is diffusion of water; water conc drops as solute conc rises
 Primary active transport uses the energy of ATP
1. Passive transport only works if the higher conc is on the outside and lower
on the inside.
2. However, many molecules that cells require are in low conc in
environment (some molecules can be synthesized tho)
 Uphill movement of substances against conc gradient is called active transport
(requiring energy).
1. During active transport, cells move substances thru transport proteins
embedded in cell membrane (some serve as pumps)
2. Energy comes from ATP and uses it by primary active transport
 Many cells use transport protein to build up concentration of small ion on one side
of membrane and resulting conc gradient stores energy that can drive the
movement of other substances across the membrane against their conc gradient.
 Charge and chemical gradients are known as electrochemical gradient.
 Movement of protons is hi lo and its coupled molecule is lo hi. (secondary
active transport bc driven by movement of protons &not ATP directly).
 Cells maintain size and composition using active transport and use it to maintain
intracellular fluid isotonic as the extracellular fluid.
 contractile vacuoles are compartments that take up excess water from inside the
cell and expel it into external environment by contraction.
 Cell wall is rigid and resists expansion, allows pressure to build up in a cell when
it absorbs water.
 Force exerted by water pressing against an object results in hydrostatic pressure
(turgor pressure)
 Plant cells have vacuoles (absorbs water and contributes to turgor pressure)
 Cell wall is composed of proteins and carbs; polysaccharides (best known is
cellulose-polymer of sugar glucose)
 Fungi have cell walls of chitin and polyer based sugars and bacteria has cell walls
of peptidoglycan (amino acids and sugars)
5.3 The Internal Organization of Cells
 Prokaryotes don’t have a nucleus (bacteria and archaea)
 Eukaryotes (plants, animals, fungi, and protists) have a nucleus.
 In eukaryotes, diverse sterols are synthesized and present in cell membranes and
prokaryotes don’t synthesize sterols but synthesize hopanoids (five-ringed
structures have similar function as cholesterol in mammalian CMs)
 Prokaryotes have their dna in the nucleoid and bacteria carry DNA in circular
molecules (plasmids) which carry some genes. Plasmids are transferred between
bacteria thru pili (hollow structures) that extend from one cell to the other (genes
for antibiotic resistance are transferred like this).
 Prokaryotes are small (1 micrometer, high ratio of surface area:volume, or large
amount of membrane SA for absorption relative to the V of the cell, and also lack
internal organization) and eukaryotes are 10x larger in diameter and 1000x larger
in volume.
 Eukaryotic-DNA in nucleus, nuclear membrane allows for complex regulation of
gene expression, DNA is transcribed to RNA inside the nucleus, RNA molecules
carry genetic message from inside to outside nucleus (synthesis of proteins).
 Organelles are in eukaryotes and have specific functions.
1. ER- synthesis of proteins and lipids
2. Golgi apparatus- modifies proteins and lipids produced by ER and acts as
sorting station
3. Lysosomes- contain enzymes that break down macromolecules (proteins,
nucleic acids, lipids, and complex carbs)
4. Mitochondria- energy for the cell
5. Cytoskeleton- protein scaffold that helps cells to maintain shape and
serves as network of tracks for movement of substances in cells
6. Chloroplasts- in plant cells only. Convert energy of sunlight into
chemical energy.
7. Cytoplasm- everything but the nucleus
8. Cytosol- region of cell inside PM but outside organelles (jelly-like
environment).
5.4 The Endomembrane System
 Membranes of organelles are physically connected by membrane “bridges” or
communicate by budding and fusing of vesicles (small membrane-enclosed sacs
hat transport substances)
 The endomembrane system includes nuclear envelope, endoplasmic reticulum,
Golgi apparatus, lysosomes, plasma membrane, and vesicles that move between
them.
 Endomembrane divides interior of cell into two distinct areas bc of the selective
permeability of cell membranes (one is inside of the space defined by membranes
and one outside space).
 Molecule in interior space of ER can stay in the ER or end up in the golgi
apparatus or outside cell by fusing of a vesicle between organelles
 Molecule associated w/ER can move to golgi membrane or plasma membrane by
vesicle transport
 Molecules in cytosol are separated by membranes of the endomembrane system
which allow specific functions to take place in the spaces defined by membranes
and in the membrane itself.
 Vesicles can fuse w/PM (exocytosis) and provides a way for a vesicle to empty its
contents to the extracellular space/to deliver proteins embedded in vesicle
membrane to PM.
 Vesicles can also bud off from the PM which brings material from outside the cell
into a vesicle then can fuse w/other organelles (endocytosis).
 Nucleus houses genome and is where RNA synthesis occurs. Nuclear envelope is
the boundary of the nucleus and has 2 membranes (inner and outer- each lipid
bilayer w/associated proteins)
 The two membranes are continuous w/each other @protein openings (nuclear
pores) which act as gateways that allow molecules to move in&out of nucleus
and essential to communicate w/rest of cell
 Transfer of info encoded by DNA depends on movement of RNA molecules out
of nucleus, and control of how/when the info is expressed depends on movement
of proteins into the nucleus.
 ER is involved in protein &lipid synthesis, the outer membrane of the nuclear
envelope is physically continuous w/the ER.
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It’s a noticeable feature of eukaryotic cells (could be ½ of total membrane) and
ER produces/transports lipids/proteins used inside &outside cell.
ER is where lipids that make up membranes are produced and transmembrane
proteins and proteins for the golgi apparatus, lysosomes, or export out of cell are
synthesized in ER.
ER consists of a complex network of interconnected tubules &flattened sacs.
Interior of ER is continuous throughout (lumen). ER is mazelike and allows a
large amount of membrane SA to fit within the cell (convoluted).
Cells that don’t secrete large amounts of protein, ER is small and vice versa.
Rough endoplasmic reticulum have small, rounded particles called ribosomes (site
of protein synthesis, where amino acids are assembled into polypeptides guided
by info stored in mRNA).
Smooth ER doesn’t have ribosomes and actively bud off to produce vesicles that
are free to move in cytosol. Each vesicle is formed from a patch of ER membrane
and encloses a portion of ER lumen, so vesicles are an effective means of moving
proteins that are embedded in ER membrane or free floating inside
SER is site of fatty acid and phospholipid biosynthesis
Cells that synthesize steroid hormones have well-developed SER that produces
big quantities of cholesterol &contains enzymes that convert cholesterol to steroid
hormones.
Golgi apparatus modifies and sorts proteins and lipids, primary roles:
1. Further modifies proteins and lipids produced by the ER.
2. Acts as sorting station as they move to their final destination
3. The site where most of the cell’s carbs are synthesized
Looks like series of flattened membrane sacs (cisternae) which are stacked and
surrounded by many small vesicles and transport proteins from ER to the Golgi
and the PM.
Enzymes w/in the golgi chemically modify proteins&lipids as they pass thru,
golgi contains different set of enzymes that catalyze specific reactions; general
movement of vesicles from ER to golgi to final destinations.
Chemical modification in golgi ex: glycosylation (sugars covalently linked to
lipids or specific amino acids of proteins, sugars are added when lipids/proteins
move thru golgi.
Glycoproteins important bc of eukaryotic cell surface bc of attached sugars that
can protect protein from enzyme digestion by blocking access to the peptide
chain.
Glycoproteins form flexible and protective coating over PM and distinctive
shapes that sugars contribute to glycoproteins &glycolipids also allow cell surface
components to be recogn. By other cells in external environ.
Lysosomes are derived from golgi that degrade damaged or unneeded
macromolecules and contain variety of enzymes that break down proteins, nucleic
acids, lipids, and complex carbs (macromolecules).
Macromolecules destined for degradation are packaged by golgi into vesicles,
then they fuse with lysosomes
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Golgi can sort key proteins which is why lysosomes form and enzymes inside are
synthesized in RER sorted in golgi and packaged into lysosomes.
 Golgi sorts and delivers specialized proteins that become embedded in lysosome
cell membranes
 Proteins in lysosomal membranes transport breakdown products of
macromolecules (amino acids and sugars) across the membrane to cytosol for the
cell to use.
 Normal cellular environment has a pH of 7.
 Protein sorting directs proteins to their proper location in or out of the cell to
perform their function.
 Proteins produced on free ribosomes start off in cytosol and sorted to their final
destination after translation and these proteins are often directed to their proper
cellular compartments by means of their amino acid sequences (signal
sequences).
 Proteins with no signal sequence remain in cytosol, and most proteins with signal
sequence at amino ends are targeted to mitochondria or chloroplasts
 Signal sequence in nucleus is called nuclear localization signal that enables
proteins to move thru pores in nuclear envelope.
 Proteins by ribosomes on RER end up in lumen of endomembrane system or
embedded in membrane or secreted out of cell
 Polypeptide chains of proteins can move into lumen or membrane bc of aminoterminal signal sequence and its recognized after synthesis by an RNA-protein
complex (signal-recognition [SRP])
 The SRP binds to the signal sequence and the ribosome and makes the translation
pause, then it binds w/receptor on RER, the SRP dissociated and translation
continues.
 SRP receptor brings the ribosome to channel thru the hydrophobic membrane of
RER and each polypeptide chain goes thru channel and a specific protease cleaves
the signal sequence as it emerges in the lumen of the ER, and the finished protein
ends up in lumen of RER.
 Proteins for lumen of ER, golgi, lysosome or exterior of cell don’t have signalanchor sequence and are fed thru channel to lumen and chaperone proteins assist
w/protein folding. Some proteins retained in ER but some transport in vesicles to
golgi, OR secreted by exocytosis.
 When there’s transmembrane proteins, the signal-anchor sequence is hydrophobic
and can become intimately associated w/membrane and exits channel and diffuses
laterally in lipid bilayer.
 Ribosome then dissociates from channel and translation continues and when it’s
completed, polypeptide is released, carboxyl end of chain remains on cytosolic
side of ER membrane, the amino end is in the ER lumen and membrane-bound
region between them anchors both sides to membrane
 Transmembrane proteins can stay in ER membrane or in other internal
membranes or PM (channels, pumps, receptors, enzymes).
5.5 Mitochondria and Chloroplasts
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Mitochondria and chloroplasts aren’t in the endomembrane system but are
specialized to provide energy for the cell.
Both semi-autonomous (grow and multiply independently and have own
genomes)
Mitochondria provide eukaryotic cells w/most of its usable energy (converts
sugars to ATP which drives chemical reations)
1. Rod-shaped w/outer membrane
2. Inner mitochondrial membrane (proton electrochemical gradient is
generated and energy stored in gradient is used to synthesized ATP for use
by cell)
3. When breaking down sugar and synthesizing ATP, oxygen is consumed
and CO2 is released. (respiration)
4. Site of cellular respiration
Chloroplasts capture energy from the sunlight and photosynthesis (release of
oxygen-waste) occurs there.
1. Internal membrane-bound compartment (thylakoid). It contains
specialized light-collecting pigments (chlorophyll).
2. Chlorophyll is green and helps chloroplasts capture energy from sunlight.
3. Enzymes in cytoplasm use light energy from pigments and CO2 to
produce carbs.