cell and molecular biology exam 3

what types of molecules are able to diffuse across a simple lipid bilayer

small non polar molecules & steroid hormones

what affects do cholesterol have on fluidity of the membrane

cholesterol packs tightly between the phospholipids which causes the membrane to stiffen, making it less fluid

mechanisms for creating lipid asymmetry in membrane

flipase

types of proteins in cell membrane

transporters

channels

anchors

receptors

enzymes

anchors

helps with structure, keeps cell together

receptors

cell communication

amphipathic molecules

has 2 properties (lipids = hydrophobic tail & hydrophilic head)

principles of membrane transport

Lipid bilayers are impermeable to ions and uncharged polar molecules

Ion concentration is different on inside and outside

Differences in the concentration across membrane creates a membrane potential

The two classes of membrane transport proteins: transporters and channels

Solutes cross membranes by either passive or active transport

Both the concentration gradient and membrane potential influence the passive transport

Water moves across cell membrane down the gradient (osmosis)

active transport

goes against the gradient and requires energy

passive transport

goes along with gradient and doesn’t require energy

transporters

protein goes through a shape change

channel

size and charge of ion and shape of hole dictates what comes through

ion channels tend to be

ion-selective and gated

mechanisms that control ion channel gating

mechanical

voltage

ligand

cell tactics to avoid swelling

protozoan - contractile vacuole

plant cell -cell wall and vacuole

animal cell - control concentration of ions (pumps)

Na/K pump involves

2 K & 3 Na

Na/K pump

happens in animal cells, expels Na+ to bring in K+

• ATP is also needed since both go against their gradients

another example of pumps

Ca2+ pumps keep the cytosolic Ca2+ concentration low

known membrane enclosed organelles

cytosol

nucleus

ER

Golgi apparatus

lysosomes

endosomes

mitochondria

chloroplast

peroxisome

cytosol

contains metabolic pathways, protein synthesis, and cytoskeleton

nucleus

holds genetic material

ER

synthesis of most lipids, synthesis of proteins for distribution to many organelles and plasma membrane

golgi apparatus

modification, sorting, packaging of proteins and lipids for either secretion or delivery to another organelle

lysosome

intracellular degradation

endosome

sorting endocytosed material

mitochondria

ATP synthesis by oxidative phosphorylation

chloroplast

ATP synthesis and carbon fixation by photosynthesis

peroxisomes

oxidation of toxic molecules

organelles that evolved by invagination of the plasma membrane

nucleus

ER

golgi

endosome

lysosome

evolved by engulfment of primitive bacteria (endosymbiosis)

mitochondria

chloroplast

three mechanisms used to transport proteins into organelles

transport through nuclear pores (protein stays folded)

transport across membrane (protein is unfolded)

transport by vesicle (protein stays folded)

steps for transporting a protein into the nucleus

1. Prospective nuclear protein has a nuclear localization signal (NLS) attached to it

2. The nuclear import receptor (NLS receptor) recognizes the signal attached to the protein and grabs onto it

3. Cytosolic fibrils help bring the protein and NLS receptor into the cytosol where it is pulled through a gel-like meshwork of nuclear fibrils 

4. It then goes through the nuclear basket, the NLS receptor lets go and the protein has been delivered to the nucleus

Ran-GDP help NLS

into the nucleus

Ran-GTP help NLS

out of nucleus

Blobel and Sabatini experiment

1. Radioactive protein and ER membrane were mixed together and centrifuged

2. They has two different densities, making it possible to isolate them

3. But to know if the protein is really in the membrane, they added protease to the outside of the cell and the protein was protected since it was inside

4. To further test it they used a detergent to poke holes in the membrane, which allowed the protease in which would degrade the protein

ER sequence

val-ser-leu-leu-leu-val-gly-ile-leu-phe-trp-ala-thr-glu-ala-glu-gln-leu-glu-val

• A lot of hydrophobic amino acids and then negatively charged ones

mitochondria sequence

ser-leu-arg-gln-ser-ile-arg-phe-phe-lys-pro-ala-thr-arg-thr-leu-cys

• Arg and lys are positively charged

general mechanisms for ER translocation

1. ER signal sequence emerges from ribosome causing pause in translation

2. SRP recognizes and binds to signal sequence

3. SRP guides stalled ribosome to ER membrane where it binds with SRP receptor

4. Ribosome binds to translocator (Sec61p) and translation resumes

5. The elongation of polypeptide “pushes” the protein into the ER lumen

co-translational translocation

involves the sec61 complex and ribosome blocking the channel, which allows the protein that is being made to push into the membrane

post-translational translocation

involves the sec61 complex, but it happens after the protein has been made. Protein has to also be unfolded since it is already made.This translocation also involves sec62, 63, 71, 72, binding protein (BiP), and energy (ATP to ADP). The energy helps the BiP bind to the protein and help “pull” or  “push” it across the membrane

the use of start/stop sequences tell us

how many times a protein may cross the membrane

five basic steps to form vesicle

1. Budding (bending membrane)

2. Movement/Transport 

3. Uncoating 

4. Tether (bring them close together)

5. Fusion (SNARE proteins)

types of coat proteins

clathrin

COPI

COPII

clathrin act at

plasma membrane

clathrin and adaptin 1

acts at trans golgi network (going into membrane)

clathrin and adaptin 2

acts at plasma membrane (leaving membrane)

COPI goes from

golgi to ER

COPII goes from

ER to golgi

COPII involves

Sar1-GTP (hangs in cytoplasm) & Sar1-GDP (recruits)

constitutive (unregulated) exocytosis

happens all of the time

regulated exocytosis

only happens when needed and there is a specific signal (insulin)

receptor mediated endocytosis

• LDL particle that contains cholesterol attaches to LDL receptor

• Both pieces are sucked into the plasma membrane where it is closed into a vesicle

• The receptor then detaches from the LDL particle and there is a ph drop that results in a endosome

• Everything then fuses together which forms a lysosome and releases free cholesterol

what is FRAP used for

to prove that some proteins (diffuse) move in the membrane, some may not, and how fast they may be moving

• involves bleaching a certain area of a dyed cell