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Cell membrane is
semi-permeable, forms selective barriers, and allows cells to maintain their own aqueous environment
Do both eukaryotes and prokaryotes have cell membranes?
YESSSS
Eukaryotes have INTERNAL membranes as well though
What is the cell membrane made of?
Phospholipid bilayer and proteins
Phosphatidylcholine
common phospholipid in membrane; amiphatic
Unsaturated phospholipids
KINKY = more fluid (less packed)
Saturated phospholipids
Straight = less fluid (packed/rigid af)
Purpose of phospholipid bilayers in the membrane
form stable compartments (polar on outside, nonpolar on inside)
Allows for membrane fluidity
Forms a CLOSED system of the cell
Cholesterol
major component of membranes; rigid ring REDUCES membrane flexibility/fluidity
Organelle membranes have 2 sides:
cytosolic (outside) and internal side (lumen)
Transporter proteins
move phospholipids around in membranes
Membrane assembly in ER
Phospholipid synthesis adds to cytosolic side of bilayer (UNEVEN)
Scramblase transfers phospholipids RANDOMLY to make growth even
Bilayer growth is SYMMETRIC
Membrane addition in golgi
NOT random
Phospholipid compositions are different on lumen side vs cytosolic side (distinct sides)
Flippase causes bilayer to be ASYMMETRIC
Functions of membrane proteins
receive information
Import and export small molecules
Movement and expansion
Types of membrane proteins
Transporters/channels
Anchors (adhere cell)
Receptors
Enzymes
Membrane proteins associate differently with membrane due to
amino acid side chains
Types of membrane protein associations
Transmembrane (span membrane)
Monolayer-associated (attach to 1 side of membrane)
Lipid Linked (linked to membrane via lipids)
Protein attached (proteins anchor proteins to membrane)
Transmembrane proteins
typically form a pore (hydrophobic on outside facing bilayer and hydrophilic on inside to make a tunnel for molecules to come into cell)
protein stabilization in lipid bilayer is due to
protein-protein interactions:
-Cell cortex barriers
-cell to cell adhesion
-extracellular matrix
-diffusion (tight junctions)
proteins can be regionally restricted by
tight junctions
tight junctions
pinch and hold cell to restrict protein movement
eukaryotic cells have membrane modifications (carbohydrate) that aid in
cell signaling
cell recognition
cell to cell association
Types of membranes in cells
Bilayer (cell membrane) or intracellular membranes
Solutes cross membrane based on
size, charge, and solubility
Na+ and Cl- are higher (inside/outside) cell?
OUTSIDE
K+ and anions are higher (inside/outside) cell?
INSIDE
Small nonpolar molecules (hormones, O2, steroids) pass through membrane
easily
Small uncharged polar molecules (H2O) pass through membrane
fairly easily
Larger uncharged polar molecules (amino acids, nucleotides) pass through membrane
not easily
Ions pass through membrane
not easily- usually require help
Simple diffusion
movement of solutes from high conc → low conc (no energy)
channels
movement of solutes from high conc → low conc (no energy- passive trasnport)
transporters
movement of solutes from high conc → low conc (no energy-passive transport )
Pumps
movement of solutes from low conc → high conc (energy- ACTIVE transport)
Inside of cell is (neg/pos)?
NEGATIVE
Outside of cell is (neg/pos)?
POSITIVE
Sodium/Potassium pump
requires energy; pumps 3 Na+ out of cell and 2 K+ in to cell
Ouabain
inhibits Na/K pump by preventing K+ from binding
Symport
moves molecules in same direction (glucose-Na)
antiport
moves molecules in different directions (Na/K pump)
Uniport
moves 1 solute down its concentration gradient
Glucose-Na+ symport
moves Na+ and glucose into cell
Purpose of membrane bound organelles in eukaryotes
-establish compartments within cell
-compartments allow for specialized functions
Double membrane organelles in eukaryotes
nucleus, mitochondria,
How might nucleus double membrane have formed?
Protein sorting purpose
proteins are transported to their appropriate location(s)
Types of protein sorting
nuclear, across membranes, vesicles
Nuclear sorting
requires energy; protein remains folded
Across membrane sorting
requires energy; protein is unfolded
vesicles sorting
requires energy; protein remains folded
-pass through nuclear pore
Purpose of signal sequences
determine where protein is transported to
no signal sequence
protein stays in cytosol
is signal sequence removed at end?
yes- once protein reaches final destination
Protein translocators
transport proteins across membranes
Nuclear pore structure
have disordered regions that form a mesh
-large molecules cannot pass easily, but small molecules can
-large molecules require receptors and energy (GTP) to cross
Nuclear localization signal
directs protein from cytosol to nucleus (signal sequence)
How receptors work in nuclear pore
receptors bind target protein
protein links to nuclear pore fibrils
protein crosses pore
How do proteins enter mitochondria?
series of recognition interactions:
-signal sequence on protein binds to receptor on mitochondria surface
-protein translocator helps protein cross outer membrane (protein is unfolded)
-protein translocator helps protein cross inner membrane (unfolded)
-cleave signal sequence
-chaperone proteins help protein refold
How do proteins enter ER?
-Signal recognition proteins (SRP) bind to ribosome/ER complex
-SRP receptor recognizes SRPs
-translocator proteins bring protein(s) into ER
-Translation occurs in ER
-Transfer polypeptide across bilayer
How do vesicles work?
They have phospholipid bilayers, so they can FUSE with plasma membranes on either side
Endocytosis
bring things in
Exocytosis
take things out
How can proteins be modified in ER?
Sugar monomers add to amino acid of protein
-aid in recognition, protection, and/or holding
What happens if a protein is misfolded?
try to fix it or degrade it
Purpose of golgi?
final modifications to proteins and ship proteins off to their location(s)
Two main exocytosis pathways in golgi
constitutive secretion and regulated secretion
constitutive secretion
UNREGULATED exocytosis; always happening
regulated secretion
REGULATED exocytosis; needs a signal in order for exocytosis to begin
Endocytic
plasma membrane pinches inside
Endomembrane system
package, label, and ship proteins; ER, Golgi, peroxisomes, lysosomes, endosomes
Endosomes have 3 pathways
recycling, degradation, and transcytosis
recycling
early endosomes return to membrane
degradation
endosomes travel to lysosome to be degraded
transcytosis
endosomes move to other area of membrane
Endocytic pathways (do/do not) require receptor mediation
DO
Purpose of lysosomes
degrade material via enzymatic digestion
-acidic (high H conc) due to H+ pump
What happens to degraded materials?
transported out to cytosol
3 ways lysosomes receive content
phagocytosis, endocytosis, and autophagy
phagocytosis
eating
autophagy
old organelles are sent to lysosome and recycled back to cell (recycles parts of self)
Ways that genetic variation is generated?
Mutation within gene
Mutation in regulatory DNA
Gene duplication and divergence
Exon Shuffling
Transposition
Exon shuffling
moving exons around within 2 different genes `
transposition
move portion of gene and insert it into new gene
mutations can be
neutral
beneficial
harmful
horizontal transfer
lateral gene transfer between organisms
germ line cell
gametes
heritable mutations are exchanged via
germ-line cells
transposon
chromosomal segment that can undergo transposition
transposase enzyme
enzyme that binds to transposon to catalyze its movement to another part of genome
~50% of human genome
mobile gene elements
effects of exon rearrangement during transposition?
impacts coding for proteins and gene expression
two major retrotransposon groups in humans
L1 elements and Alu sequences
Similarities in retrotransposon groups
have RNA intermediate
reverse transcriptase
RNA → DNA
what do transposons and retrotransposons have in common with viruses?
Overtake cell machinery to make copies
Can have DNA or RNA genomes
Can impact eukaryotes and prokaryotes
Retrovirus
RNA virus that utilizes reverse transcriptase
Exons
stay
Introns
spliced out
Receptor for SARS-CoV-2
ACE2
Primary entry points for COVID19
endosomal or cell surface
Glycocalyx
physical barrier around cell; protects cell by preventing viruses from binding to it
Note 
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