cell structure exam 2

plasma membrane is selectively permeable

hydrophobic and small polar yes
ions and large polar no

osmosis

the net flow or solvent molecules (water) through a semipermeable membrane through which solute molecules cannot pass

which of the following statements is incorrect about the difference and similarities between diffusion and osmosis?

a. the overall effect of diffusion is to equalize concentration throughout the medium, while osmosis is the movement of solvent particles across a semipermeable membrane to dilute the concentrated solution and equalize the concentration on both sides of the membrane.

b. both osmosis and diffusion act so as to equalize the concentration of two solutions.

c. both diffusion and osmosis are passive transport processes, which eans they do not require any input of extra energy to occur

d. In both diffusion and osmosis, particles move from an area of higher concentration to one of lower concentration.

e. One big difference between osmosis and diffusion is that both solvent and solute particles are free to move in diffusion, but when we talk about osmosis, only the solute molecules cross the membrane.

e. One big difference between osmosis and diffusion is that both solvent and solute particles are free to move in diffusion, but when we talk about osmosis, only the solute molecules cross the membrane.

isotonic solution

has the same concentration of solutes both inside and outside the cell (net osmosis = 0)

hypotonic solution

higher concentration of solutes inside the cell (net osmosis = inside the cell)

hypertonic solution

higher concentration of solute outside the cell (net osmosis = outside the cell)

osmotic pressure

the pressure that needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane

• required to achieve osmotic equilibrium

in a hypotonic solution, animal cells…

will fill with too much water and lyse, isotonic is ideal

in a hypotonic solution, plant cells…

are ideal due to their cell walls

in hypertonic solution, both plant and animal cells…

will shrivel (plants — plasmolyzation)

cell lysis

the breaking down of the cell membrane, releasing cell contents

why are transmembrane proteins the perfect molecule to build channels in the cell membrane?

they span the entire membrane due to the hydrophobic/philic domains

within membrane proteins

composed of nonpolar amino acids, are hydrophobic which anchors the protein into the membrane

integral membrane protein functions

transport
receptor proteins
enzymatic activity
cell adhesion
structural support
cell recognition
anchoring cytoskeleton

peripheral membrane protein functions

enzymatic activity
cell signaling
structural support
regulation
cytoskeleton attachment

on outer surface of membrane proteins

composed of polar amino acids, are hydrophilic and extend into extracellular fluid and cytosol

glycoprotein function

cell recognition
cell adhesion
receptor function
protection
facilitated transport

only transmembrane proteins…

aquaporins

moves water rapidly into and out of cells

aquaporins structure

transmembrane protein that forms pores in membrane

narrow enough to physically restrict molecules larger than water

ion pumps

actively transport ions against a concentration gradient

ion channels

allow ions to passively flow down a concentration gradient

sodium-potassium pump

sodium–potassium ATPase) is an enzyme (an electrogenic transmembrane ATPase) found in the membrane of all animal cells

classification of ion channels

chloride channels
potassium channels
sodium channels
calcium channels
proton channels
non-selective cation channels

voltage-gated ion channels

class of transmembrane proteins that form ion channels that are activated by changes in the electrical membrane potential near the channel. The membrane potential alters the conformation of the channel proteins, regulating their opening and closing.

what causes membrane potential to depolarize?

initiated by the opening of sodium ion channels within the plasma membrane

endocytosis

large particles moving into a cell by enclosing them in a vesicle made out of cell membrane

endocytosis process

cell membrane folds inward, forming pocket around particles

the pocket pinches off, creating a new vesicle

phagocytosis

type of endocytosis

cells ingesting large particles through vesicles called phagosomes which are then targeted to lysosomes for enzymatic degradation

phagocytosis mechanism

initiated by binding of specialized receptors on phagocyte cell membrane to distinct molecular patterns on target particles

binding followed by actin polymerization, which causes membrane to form pseudopodia

pseudopodia surrounds particle and forms vesicle called phagosome

immune system cells

neutrophils
macrophages
dendritic cells
B lymphocytes

neutrophils

abundant in blood, quickly enter tissues, phagocytize pathogens in acute inflammation

macrophages

longer lived cells

invasion mechanisms

bacteria induce phagocytosis via zipper mechanism

• once phagocytized, bacteria escape phagosome and live within host cytosol

some induce physical change (trigger mechanism) in membrane inducing phagocytosis

salmonella endocytosis

salmonella destroys the microvilli of enterocytes and then invaginate into the cell where they exist within a salmonella containing vacuole (SCV)

trigger model

bacteria inject effector proteins

pinocytosis

type of endocytosis

cell takes in small amounts of extracellular fluid, cells grabs whatever is in the area (nonspecific)

pinocytosed material is held in small vesicles, smaller than the food vacuoles

spontaneous endocytosis

bulk-phase endocytosis and pinocytosis

happen without need for specific signals or stimuli, pinocytosis is nonspecific

receptor mediated endocytosis

receptor proteins (coated pits) on the cell surface are used to capture a specific target molecule

the receptors and attached molecules are taken into cell via vesicle

clathrin coat

have heavy chain and light chain

self-organizes

clathrin coated vesicles

need clathrin, adaptor protein, cargo complex: membrane + cargo receptor + cargo

dynamin

pinches off clathrin-coated vesicles from membrane

contains a binding domain that tethers protein to membrane

lipoproteins (cholesterol)

LDL (low density lipoprotein) — causes buildup of plaque in blood vessels, “bad” cholesterol

HDL (high density lipoprotein) — absorbs cholesterol and carries it back to liver to get flushed from body, “good” cholesterol

exocytosis

form of bulk transport in which materials are transported outside of cell in vesicles

cells release signaling proteins and waste products

process of exocytosis

travel — vesicles move to cell membrane

anchor — vesicle anchors to membrane

fusion — vesicle fuses with membrane to release contents

exocytosis functions

• removes waste

• important for chemical signaling and cell-cell communication

• rebuilds the cell membrane

exocytotic vesicles

come from the golgi apparatus (proteins/lipids synthesized in membrane, go to golgi for sorting)

some come from endosomes (cell repurposes early endosomes)

constitutive exocytosis

delivers membrane proteins and lipids to cell’s surface and expels substances to the cell’s exterior

regulated exocytosis

commonly occurs in secretory cells

relies on presence of extracellular signals

lysosomal exocytosis

lysosomes carry waste material to cell membrane

constitutive exocytosis steps

1. vesicle trafficking

2. tethering

3. docking

4. fusion

regulated exocytosis steps

1. vesicle trafficking

2. tethering

3. docking

4. priming

5. fusion

trafficking

transports vesicles to membrane using motor proteins (requires ATP)

tethering

vesicle links to cell membrane

docking

attachment of vesicle to membrane

priming

signal is received by the cell that permits fusion

fusion

the two membranes fuse, a pore opens, and vesicle releases contents

what are possible outcomes of exocytosis disregulation?

ultimately cell death

exocytosis in pancreas

insulin and glucagon release

dysregulation results in diabetes

exocytosis in neurons

neurosignals

how many modes of fusion exist in exocytosis?

three:

• total fusion

• “kiss and run”

  • vesicle doesn’t fuse, merely releases contents and leaves

• compound exocytosis

  • multiple vesicles join together and release contents together

constitutive secretory pathway

directly delivers fresh membrane lipids and proteins to cell membrane

unregulated

regulated pathway

used for specific cargo (hormones, neurotransitters)

priming function

SNARE proteins (soluble N-ethylmaleimide-sensitive factor attachment protein REceptor) drive membrane fusion

SNARE complex pulls 

calcium

binds to synaptotagmin inducing a conformational change that allows the SNARE complex to form more fully

synaptic vesicle cycle

1. synaptic vesicles are loaded with neurotransmitter by active transport

2. newly filled synaptic vesicles are then transported to the active zones of synapses (tethering)

3. synaptic vesicles are docked to presynaptic membrane

chemical signals in endocytosis

receptor-ligand interaction

signaling molecules

pathogen entry

mechanical signals in endocytosis

membrane tension

shear stress

filopodia and lamellipodia

extracellular matrix interaction

spontaneous endocytosis

cell makes vesicle without signal

will occur when it lowers free energy of the system

initiation of spontaneous endocytosis

membrane tension

• when membrane experiences decreases tension due to cell movement or mechanical stress; lower tension promotes the formation of membrane invaginations and vesicle budding

lipid composition

• lipid rafts rich in cholesterol and sphingolipids create microdomains in membrane that promote formation of vesicles

actin dynamics

• anctin filaments push membrane inwards

dynamin-independent endocytosis

• Some forms of endocytosis occur independently of dynamin, a GTPase essential for vesicle fission.

How Membrane Bending and Invagination Uses Energy

Actin polymerization is an ATP-driven process where ATP is hydrolyzed to provide the energy for the assembly and disassembly of actin filaments.

How Dynamin-Mediated Fission Uses Energy

Dynamin, a GTPase, is essential for pinching off vesicles from the plasma membrane

How Vesicle Movement Uses Energy

Motor proteins, such as dynein and kinesin, facilitate this transport. These motor proteins require ATP hydrolysis to generate the mechanical force necessary for vesicle movement

How Membrane Fusion Uses Energy

fusion itself requires energy

however SNARE proteins, essential for membrane fusion, undergo conformational changes that are ATP-dependent

nuclear membrane

has two phospholipid bilayer which are held together by the nuclear pore complex, nuclear lamin, sun-linc complex

nuclear lamin

meshwork of intermediate filaments (type A and B), provides a skeleton

chromatin are anchored to lamin

sun-linc complex

holds membrane together through anchors

connects nucleus to cytoplasm

required for intracellular force transmission

• is utilized for mechanotransduction — translates physical forces (compression, tension, shear stress) into biochemical signals

What are possible outcomes for an organism that has a weak or no connection between the nucleoskeleton and the cytoskeleton?

dysregulation of muscle function

dysregulation of muscle function

dilated cardiomyopathy (impaired pumping of left ventricle which dilates and impairs blood flow)

arrhythmogenic cardiomyopathy (heart fails to contract properly)

emery-dreifuss muscular dystrophy (muscle wasting, cardiac weakness, joint stiffness from improper muscle formation and function)

Emery-Dreyifuss Muscular Dystrophy

caused by mutation in emerin

disrupts connection between emerin and lamin

satellite cells

specialized progenitor cells in muscle tissue

mechanical stress

causes chromatin remodeling via the linc complex

chromatin remodeling exposes MRF (muscle regulating factors) promoter accessibility and causes production of MRF

MRFs produced in response to growth factors

nucleocytoplasmic transport

gated transport between cytoplasm and nucleus

Imported Traffic through Nuclear Pores

RNA polymerase
snRNPs
DNA polymerases
ribosomal proteins
histones
transcription factors

Exported Traffic through Nuclear Pores

40S ribosomal subunits
60S ribosomal subunits
tRNAs
mRNAs
snRNAs

MINFLUX super-resolution fluorescent microscopy

3D image at 2nm resolution

demonstrates octamer organization of Nup96 in the nuclear pore complex

proteomics

determines how many types of proteins and their interactions in the nuclear pore

via protein purification (SDS PAGE) or protein-protein interactions (tagging proteins)

nucleoporins in NPC

1/3 are disordered due to highly hydrophobic amino acid regions (FG motifs) — causes NPC structure to constantly change

proteins in NPC

nucleoporins (structure)

karypherins (transport)

How will hydrophobic repeats within intrinsically disordered proteins impact their structure and organization within the central channel of the NPC?

the FG repeats will interact with each other and form a selective barrier (does not cause extreme saturation)

HeLa

immortal cell line that is durable and prolific

NPC selectivity

sets up a size-dependent permeability barrier for passive diffusion of macromolecules

cannot take it molecules >60kDa

Small molecules through NPC

passive diffusion of small molecules (<40-60kDa) through NCP

signal independent and ATP independent diffuson 

large cargoes in NPC

require karyopharins (importins or exportins)

importins/exportins

very hydrophobic, utilize heat repeats, 

Crm1 (export)

Importin-beta 1 (import)

nuclear localization signal (NLS)

amino acid sequence that “tags” a protein for import into the cell nucleus by nuclear transport

usually consists of positively charges lysines or arginines

is recognized and bound by importins

nuclear export signal (NES)

short amino acid sequence that targets a protein for export from nucleus to cytoplasm through NPC using nuclear transport

consists of hydrophobic residues rich is lysine

is recognized and bound by exportins

Ran GTP

RAs-related nuclear protein

associates with GTP to power NTRs for import and export

powers exportins

Ran (RAs-related nuclear protein)

GTP binding nuclear protein that is involved in transport into and out of the cell nucleus during interphase and is involved in mitosis

RanGAP

protein involved in transport of other proteins from cytosol to nucleus

acts as GTPase-activating protein, catalyzing the conversion of cytosolically-bound RanGTP to RanGDP