Lecture 5: Membrane Structure, Part I Flashcards
Prokaryotic Cell
Small, unicellular
DNA in cytoplasm
One circular chromosome
No organelles
Oldest cell type
Eukaryotic Cell
Large cells, diverse shapes and functions
Unicellular or multicellular
DNA is in nucleus
Multiple linear chromosomes
Several organelles
Evolved from prokaryotes
What is an organelle?
membrane bound structures in cell that performs specialized functions
Similarities between prokaryotic and eukaryotic cells
Both have DNA, ribosomes, cytoplasm, and plasma membrane
Do prokaryotic and eukaryotic cells have organelles?
Only eukaryotic
Endoplasmic Reticulum (ER)
membrane of enclosed sacs that produces components of plasma membrane (proteins and lipids) and material secreted by the cell
Nuclear envelope
Double membrane surrounding nucleus which works with nuclear pores to regulates entry of only allowing specialized proteins and molecules
Nucleus
Prominent organelle in a eukaryotic cell and contains linear chromosomes
Can be observed by light and electron microscopes
Enclosed by nuclear envelope
Surrounded by lipid bilayer
2 Membranes of Nuclear Envelope
Outer membrane: in contact with cytoplasm
Inner membrane: in contact with chromosomes
Nuclear pores
Small channel embedded within nuclear envelope which acts a getaway for molecules to move between nucleus and cytoplasm of a cell
Allows selective transport of materials
What is the association between the endoplasmic reticulum (ER) and the nuclear envelope?
Membranes of ER are continuous with outer membrane of nuclear envelope
Rough ER
has ribosomes for protein synthesis → near nucleus
Smooth ER
no ribosomes and important for lipid synthesis → far away from nucleus and close to plasma membrane since made out of lipids
Describe the association between the ER and the Golgi apparatus.
Works together
Proteins synthesized in ER and other molecules from ER are packaged into transport vesicles and enter golgi apparatus
Golgi Apparatus
stacks of membrane enclosed sacs that receive molecules in ER and and directs them to various locations within the cell
What happens when proteins traveling from cis to trans face of Golgi apparatus?
Proteins undergo processing and protein modification in stacks (cisternae)
Cis face of Golgi apparatus
Proteins enter from ER
Side nearest ER and facing nucleus
Trans face
exit side, where modified molecules are packaged into vesicles and sent to final destination within the cell
What is common destination of when proteins leave the Golgi Apparatus
Lysosomes
Lysosomes
irregularly shaped organelles that contain enzymes used to digest macromolecules
Have various hydrolytic enzymes (hydrolysases) that can degrade different macromolecules
Molecules that are degraded by lysosomes are often
Recycled
All living cells must have
plasma membrane
Plasma membrane
Defines boundary of cell and separates cytosol from extracellular environment
Has selectively permeability
Internal membrane
Form organelles
Provide cell with intracellular compartments to perform specific functions
Selective permeability
selective passage of molecules and ions into and out of the cell
What types of molecules can rapidly diffuse across the plasma membrane?
Small, nonpolar molecules
What types of molecules diffuse slowly across the plasma membrane?
Large polar, charged molecules
How do molecules that diffuse slowly (or not at all) cross the plasma membrane without diffusion?
Facilitated diffusion as molecules are assisted by specialized transport proteins in the membrane
Ex. glucose
Both ___ and ____ are composed of lipid bilayer
plasma and internal membrane
Lipid bilayer composition
Consists of nonpolar (hydrophobic) membrane composed of 2 layers of lipid molecules, cholesterol, and various membrane proteins
Fatty acid
building blocks of membrane lipids
Chemical structure of fatty acid
Amphipathic
Hydrophilic polar head
Hydrophobic nonpolar tail
Membrane lipids
composed of modified fatty acids in plasma or internal membrane
1 fatty acid hydrophilic head
2 fatty acid hydrophobic tails
Why are fatty acids ideal component for cell membrane
can interact with both aqueous and nonaqueous environments
What are common type of membrane lipids in cell membrane
Phospholipids
Phospholipid composition
Have hydrophilic phosphate group in the head and two hydrophobic tails
Why are phospholipids energetically favorable?
Energetically favorable for hydrophilic heads to contact water and hydrophobic tails to avoid contact with water and aggregate with each other
What is issue when phospholipids form sheets?
edges are exposed, exposure of hydrophobic molecules to hydrophilic environment
What does the sheet spontaneously form a sphere?
sealed compartment so hydrophobic tails at edge do not contact water
What is on both sides of lipid bilayer?
Aqueous environment
Lateral diffusion
common movement of lipids move within plane of lipid bilayer
Flip flop movement
phospholipids move from one monolayer to another without help of proteins and is energetically unfavorable
Membrane fluidity
Lateral diffusion of membrane lipids and depends on the composition of lipid bilayer
Do all membranes must be fluid?
Yes; always occuring
Membrane fluidity is determined by
Packing of hydrocarbon tails in lipid bilayer
Length of hydrocarbon tails
Number of Double Bonds
Cholesterol
Temperature
Packing of hydrocarbon tails in lipid bilayer on membrane fluidity
More fluid: loose packing of tails as trying to minimize all hydrophobic interactions
Less fluid: close packing of tails
Length of hydrocarbon tails on membrane fluidity
More fluid: short tail for less interaction between tails
Less fluid: long tail for more interaction between tails, more carbons = more hydrophobic interactions for less fluidity
Number of double bonds on membrane fluidity
More fluid: Unsaturated: one or more double bonds as there is less interaction between tails and bend to have more movement (liquid at room temp)
Less fluid: Saturated: no double bonds as more interaction between tails and very straight structure to engage in more hydrophobic interactions (solid at room temp)
At room temperature, how does cholesterol influence membrane fluidity
Less cholesterol at room temp increases fluidity
More cholesterol at room temp decreases fluidity
Melting temperature (Tm)
temperature where membrane transitions
from crystal like state to fluid like state
At low temp, how does cholesterol impact fluidity
Increase cholesterol, increases fluidity
Prevents phospholipids tails from packing too tightly
How do phospholipids affect the Tm?
through composition and length
At high temp, how does cholesterol impact fluidity
Increase cholesterol, decrease fluidity
Restrains movement
Why is maintaining a fluid membrane important?
Need fluid membrane for allowing proper functioning of the cell and movement of essential molecules across the membrane
Transmembrane Proteins
Extend through lipid bilayer
Have hydrophilic and hydrophobic regions (amphipathic)
Made of alpha helices
Can be single pas or multi pass
What are the 2 challenges for Transmembrane proteins?
- Many amino acids are hydrophilic (50%)
Cause issues as they do not want to be in hydrophobic interior
- Peptide bonds that connect amino acids in a protein have polar bonds causing partial charges
Partial charges and hydrophobic environments do not get along
How do transmembrane proteins overcome the 2 challenges?
- Use lots of nonpolar hydrophobic amino acids within transmembrane segments
- Put nonpolar amino acids in alpha helix
Provides stable structure being stabilized by H bonds and neutralizes partial charges
Single Pass transmembrane proteins
Only require 1 alpha helix to pass through lipid bilayer
Why are the strategies important for transmembrane proteins?
Create stable structure in nonpolar environment
To help protein channels pass important molecules through, they can use
polar amino acids on multipass proteins
What does the number of transmembrane segments say about a protein?
Number provides information about function of protein
Multipass transmembrane segments alpha helices use amino acids to create
hydrophilic pore
In one turn of alpha helix, 3 are nonpolar and 1 are polar
Multipass transmembrane proteins
Require multiple alpha helices to pass through lipid bilayer
Transmembrane Segment
20 nonpolar amino acids in protein used to pass through lipid bilayer
Can have polar uncharged, but NO POLAR CHARGED
+ score in hydrophobicity plot =
hydrophobic amino acid
negative - score in hydrophobicity plot =
hydrophilic amino acid
Kyte and Doolittle numerical scale of hydrophobicity plot
Transmembrane segments have value of more than 1.6 and are more than 20 amino acids long
Glycosylation
sugars attached to another molecule (lipid or protein)
Occurs in ER and Golgi
Occurs on non cytosolic side
Never exposed to cytoplasm
Sugars are made outside the cell
Glycocalyx or cell coat
carbohydrate layer on plasma membrane from glycosylated proteins and lipids; protect cells against mechanical and chemical damage
Glycolipid
Lipid + sugar
Acquire sugar groups in golgi and are always on non cytosolic face of membrane
Once lipids get glycosylated → cause symmetry
Never touch cytosol
Where does glycosylation occur?
ER and Golgi apparatus
How is glycosylation is established and maintained
made in golgi and ER –> enter transport vesicle –> goes to extracellular environment
Never touches cytosol
