How do cells solve the problem of protecting and compartmentalizing cellular components?
Lipid bilayers
Fluid-Mosaic Model
Within the âseaâ of membrane lipids, integral membrane proteins are floating around
All cellular membranes have lots of proteins embedded in them
What does it mean that lipid bilayers are selectively permeable?
Certain substances can readily diffuse across the membrane, certain substances can cross the membrane (but not easily), and certain substances cannot pass the membrane at all
What factors affect the rate of diffusion across a membrane?
Size
Charge
Structure of a phospholipid
Hydrophilic head
Polar group
Phosphate group
Glycerol
Hydrophobic tails
2 fatty acid chains
Amphipathic
Having a hydrophilic part and hydrophobic part (ex. phospholipid)
What is the most chemically variable portion of a lipid?
The polar group/head group
Can confer unique functions to the lipid
How can fatty acid tails in a lipid vary?
Length
Micelles
Spherical structure spontaneously formed by single layer of lipids
Formed by cone-shaped phospholipids (1 fatty tail)
Liposome
Spherical structure spontaneously formed by double layer of lipids
Formed by cylinder-shaped phospholipids (2 fatty tails)
Cone-shaped phospholipid
1 fatty tail
Forms micelles
Cylinder-shaped phospholipid
2 fatty tails
Form liposomes/bilayers
Saturated hydrocarbon chains
Hydrocarbons have maximum hydrogen bonding and no C=C double bonds
Unsaturated hydrocarbon chains
Missing some hydrogens, so have C=C bonds
Causes a bend or kink in the chain
Prevents close packing of hydrocarbon tails and reduces hydrophobic interactions
Why are hydrocarbon chains called (fatty) acids?
Because they have a hydroxyl group that can act as an acid and give up H+
Cis-double bonds in fatty acid tails
Occur naturally
Two H on the same side
Two non-H on the same side
Cis = same
How do saturated vs. unsaturated fatty acids affect membrane permeability?
No unsaturated fatty acids = low permeability
Many unsaturated fatty acids = high permeability
More porous
Cholesterol
Makes membranes more rigid, less permeable, a stabilized to fluidity changes
Steroid molecule
Present in most eukaryotic membranes
Amphipathic structure
How does temperature affect membrane fluidity and permeability?
Higher temperature = higher fluidity = higher permeability
Lower temperature = lower fluidity = less permeability
How does the length of fatty acid tails affect membrane permeability?
Longer tails = less permeable
Shorter tails = more permeable
Diffusion
Ions and molecules diffuse spontaneously from regions of higher concentration to regions of lower concentration
Driven by entropy (tendency of the universe to become more disordered)
What types of molecules can diffuse through a membrane?
Small mostly non-polar molecules
Very small polar molecules (ex. water)
Osmosis
The movement of water across a semipermeable membrane
Water is drawn toward higher solute concentration
What happens to a cell in a hypertonic solution?
Cell shrinks
Water moves out of the cell to area of higher solute concentration
What happens to a cell in a hypotonic solution?
Cell swells/lyses
Water moves into the cell to an area of higher solute concentration
Hypertonic solution
Compared to cell, solution has higher solute concentration
Hypotonic solution
Compared to cell, solution has lower solute concentration
Transmembrane/integral membrane proteins (defân, characteristic, shape)
Transverse the entire membrane
Tend to contain hydrophobic amino acid residues (that interact with the hydrophobic core of the membrane)
Usually form alpha helix
Peripheral membrane proteins
Reversibly associated with either the internal or external side of the membrane
Could be bound to a lipid or another protein embedded in the membrane
How do ion channels and transporters influence membrane permeability?
By allowing specific polar solutes to pass that are normally impermeable
Ion channels
Open to allow a specific solute to pass (usually ions)
Either open or closed
Open/closed status is regulated: âgatedâ
Transporters/carrier proteins
Undergo conformational (shape) change
Bind solutes on one side of the membrane and release them on the other
Passive transport
Occurs when a solute moves down its concentration gradient (high to low concentration)
Occurs spontaneously (no energy input needed) due to diffusion
Can occur through the membrane directly or through a channel/transporter protein
Active transport/pumping
Occurs when a cell expends energy (usually ATP) to move a solute against its concentration gradient (low to high concentration)
Requires a protein pump
Simple diffusion
Solute passes through the membrane along its concentration gradient by itself (no channel/transporter protein or energy required)
Passive transport
Facilitated diffusion
Solute passes through membrane via a transporter or channel protein along its concentration gradient
Passive transport
Sodium-potassium pump (+ where does it get energy)
Uses energy of ATP hydrolysis to create electrochemical gradient
Animal cells
Pumps Na+ ions out of the cell
As a result, Na+ higher outside the cell
Pumps K+ ions into the cell
As a result, K+ is higher inside the cell
Gradients are a form of ________ energy
Stored (potential)
Coupled transport
One molecules moves down its concentration gradient, releasing energy
Potential energy of electrochemical gradient (generated by active transport)
Another molecule is driven with this energy to move against its concentration gradient
What types of things can pass through cell surface transporters? Why?
Amino acids and nucleotides can pass
Small monomers
Nucleic acids and proteins cannot pass
Larger
Reason: there is a size limit to what can enter cells through transporters/channels; transporters/channels are selective
Selective
Binds/recognizes or allows passage of some things but not others
What do all cells have?
Plasma membrane (separates cell from its environment)
DNA (genome = entire chromosomal DNA seq. of cell)
Cytosol (aqueous interior of cell)
Ribosomes (âmachinesâ for protein synthesis)
Cytoskeleton (structural support)
What are things that only some cells have?
Nucleus (encloses genome)
Organelles (membrane-enclosed sub compartments)
Cell wall (protective outer layer)
Flagellum/cilium (projection for movement)
Plasma membrane
Acts as a selective barrier against the extracellular environment
What is characteristic of eukaryotic cells?
Nucleus
Bigger
Organelles (many intracellular membranes)
Motile
Many bacteria are motile (can move) due to the presence of flagella
Plasmid
Prokaryotes may contain a small, circular piece of extra-chromosomal DNA called a plasmid
Easily shared
Often where bacteria have genes conferring antibacterial resistance
Nucleoid
Circular chromosome is supercoiled into nucleoid (one big packaged chromosome)
Cell wall (bacteria)
Almost all bacteria have a semi-rigid but permeable cell wall composed of peptidoglycan that prevents osmotic lysis
Mitochondria
Harness energy from chemical compounds (e.g. glucose, fats)
Convert energy into ATP
Present in virtually all eukaryotes (including plant cells)
Double membrane with aqueous compartment in between
Chloroplasts
Capture the sunâs energy
Synthesize simple sugars via photosynthesis
Double membrane with additional third interior membrane (thylakoid membrane)
Life originated as _____________
Prokaryotic (no nucleus nor intracellular membranes)
Endomembrane system
Evolved from inner folds of the plasma membrane
Pinched off and enclosed DNA (nucleus)
Created eukaryotes
Refers to the set of intracellular organelles that are interconnected via vesicular traffic
Membranes of which organelles are not part of the endomembrane system?
Mitochondria
Chloroplast
These were engulfed: endosymbiotic theory
They grow and multiply independently of other membrane compartments
Endosymbiotic theory
Mitochondria and chloroplasts evolved from engulfed prokaryotes that gave an advantage to the host eukaryotic cell
Zip code hypothesis
Proteins are directed to the correct cellular location by signals within their amino acid sequence
Usually encoded by primary amino acid sequence
Zip codes are most prevalent in eukaryotes
What is the default location for a protein in the absence of a specific signal/zip code?
Cytoplasm/cytosol
Nuclear envelope
Double-membrane structure that contains nucleus
Continuous with ER
Supported by nuclear lamina (cytoskeletal structure)
Perforated by nuclear pore complexes that act as gates
Nuclear pore complexes (NPCs)
Allow selective passage of molecules between nucleus and cytoplasm
Vesicular traffic
Refers to when membrane bound structures are budding off and/or fusing with another membrane bound structure
Members of endomembrane system
ER
Golgi
Lysosome
Endosomes
Plasma membrane
Exocytosis
A vesicle that has budded off from the endomembrane system can fuse with the plasma membrane and deliver its contents into the extracellular space
Things are sent out of the cell
Endocytosis
Material from outside the cell is brought into a vesicle that can then fuse with other organelles
Bringing things into the cell
Vesicular transport
A vesicle can pinch off of one membrane and fuse with another
What characteristic of cells allows vesicular transport to occur?
Lipid bilayers are fluid and can fuse and intermix with each other
Rough ER vs. smooth ER
Rough ER: integral membrane and lumenal protein synthesis
Has ribosomes bound to it (in the process of translating something)
Smooth ER: primary site of lipid synthesis
No ribosomes
Where are integral membrane proteins and proteins inside the endomembrane system synthesized?
ER
Secretion/exocytosis
Vesicular transport out of the cell
What is the secretary pathway through the cell?
ER
Golgi
Plasma membrane
Note: some proteins that are needed to function in the ER or Golgi can be retained there by mechanisms similar to the zip code concept
Glycosylation
Proteins that go through the ER and Golgi are glycosylated = covalently attached to carbohydrate chains (modified by carbohydrates)
Occurs in the lumen of the Golgi
This is why cell-surface and extracellular proteins are glycoproteins
For integral plasma membrane proteins, the lumenal, glycosylated portions end up in the extracellular space
Endoplasmic reticulum structure
Forms a continuous network of interconnected membrane tubules (some rough, some smooth)
Cisternae
The Golgi apparatus consists of a series of flattened membrane sacs called cisternae
Endocytic pathway
Stuff outside the cell or in the plasma membrane can be brought into the cell by endocytosis
Endocytic cargo can be recycled (sent back to the plasma membrane) or directed to a lysosome
This is not done because the cell wants the protein it is bringing in (if that were the case, the cell would just make the protein itself); it is usually about bringing something in to be degraded
Lysosomes
Acidic and degradative organelles
They break down any biological molecule brought to them into building blocks for reuse
Cellular compost bin
How do lysosomes maintain an acidic pH?
Proton pumps
Lysosomal/acidic hydrolases
Degrading enzymes that only work at low pH
Signal/signal sequence
Signal within protein that is currently being translated directed a ribosome to the surface of the ER
At the ER, the ribosome associates with a translocation channel through which the proteinâs synthesis is completed, resulting in a lumenal protein or integral membrane protein
Series of events leading to synthesis of lumenal/membrane protein
Signal recognition particle (SRP) binds to a signal sequence in the amino end of the growing polypeptide, halting translation
SRP binds to SRP receptor on ER membrane
SRP receptor brings ribosome to transmembrane channel (SRP dissociates)
Protein synthesis resumes; growing polypeptide chain is threaded through the channel
Protein ends up in the lumen of the ER where it will remain, be transported to the lumen of another organelle, or be secreted out of the cell
Signal anchor sequences
Signal anchor sequences become transmembrane domains in transmembrane proteins
Hydrophobic portion
They are released from the translocation channel into the ER membrane and diffuse laterally into its lipid bilayer
General cytoskeletal functions
Structure and support
Shape, strength
Shape could include controlling membrane projections
Intracellular transport
Contractility and motility
Spatial organization
What types of subunits does the cytoskeleton consist of?
Noncovalent polymers of protein
Microtubules (smallest)
Microfilaments (biggest)
Intermediate filaments (middle)
Microtubule
Smallest
Hollow tube formed from tubulin dimers
More rigid than actin or intermediate filaments and rupture when stretched
Globular shaped
Consists of alpha and beta tubulin
Microfilaments
Largest
Double helix of actin monomers
Actin = 3Âș protein structure that binds to itself
Flexible
Most concentrated just beneath the plasma membrane
Intermediate filaments
Strong fiber composed of intermediate filament proteins
Fibrous (stringy) subunits; rope-like
Doesnât fold into globular shape
Provide cell mechanical strength
Very flexible