The Cell (22-31) Flashcards
What is the function of a membrane?
Semi permeable barrier
To detect and interpret changes in extracellular environment
Provide anchorage sites for extracellular proteins and cytoskeleton
Provide an alternative environment to the cytoplasm
What are membranes made up of?
Protein
Lipids
Carbohydrates
the composition varies between different membranes
What are the key functions of lipids?
- Storage: fuel for metabolism - triglycerides
- Membranes - phospholipids, glycolipids, cholesterol
- Signalling - steroid hormones, eicosanoids
- Vitamins - A, D, E and K
Are lipids soluble in water?
No
(soluble in organic solvents like chloroform)
What are phosphoglycerides made up of?
2 fatty acids, glycerol, phosphate (ester bond)
Why are phospholipids amphiphilic?
They have a polar head group which is hydrophilic attached to 2 fatty acid chains which are hydrophobic
→ has both hydrophilic and hydrophobic properties
What are saturated fatty acids?
Have no C=C double bonds
→ melting point increases with length
Why do unsaturated fatty acid tails effect fluidity?
The C=C double bonds (usually cis) create kinks in the chains causing irregular packing
→ lower melting point/increased fluidity
(saturated chains can pack closely together)
What is arachidonic acid?
Synthesised from linoleic acid, precursor for eicosanoids and prostaglandin
→ functions as part of phospholipids in membranes
→ plays important role in inflammation
What are eicosanoids?
Signalling molecules
→ important in pain and inflammation
What is phosphatidylcholine?
A phospholipid with a modified phosphate group - choline head group
→ head group can be cleaved, choline is an important signalling molecule
What are sphingolipids?
Phospholipids with a sphingosine back bone with a hydrocarbon chain and one fatty acid attached
What is the function of sphingomyelin?
Sphingolipid with a choline head group
→ major component in myelin sheath - increases speed of electrical impulses
→ important in signal transduction and apoptosis
What are glycolipids?
Sugar containing lipids - sugar instead of the phosphate group
→ in animal cells derived from sphingosine
→ functions: immune responses, cell-cell recognition and attachment
What are sterols?
Modified steroids
→ common steroid structure: 4 hydrocarbon rings - planar
→ cholesterol is the only steroid in membranes
How can you measure the rate of lateral diffusion in a membrane?
Bleaching fluorophores with intense light then measuring their recovery
How can lipids move in a membrane?
- Rotation
- Flexion
- Lateral diffusion
- Transverse diffusion: flip-flop (rare - once every 3 days)
What happens when the temperature of a membrane is increased?
Lots of movement - too fluid
→ membrane disorders, can’t pack, increased permeability
What happens to membranes when they have unsaturated lipids?
Increased fluidity
→ unsaturated lipids gives kinks
What decreases the fluidity of a membrane?
- saturated lipids
- long chains
- low temperature
How does cholesterol affect different parts of phospholipids?
Middle region - stiffened by cholesterol’s rigid ring structure
End of the tails become more fluid
How does cholesterol affect fluidity?
High temperatures - decreases fluidity
→ interactions with phospholipids and rigid ring structure stiffen membrane and interfere with phospholipid mobility
Low temperatures - increases fluidity
→ flexible non-polar tail of cholesterol interfere with the tight packing of phospholipid chains
What does ethanol do to membrane fluidity?
Ethanol increases membrane fluidity
What is the function of phospholipid translators?
To catalyse the flip-flop event (transverse diffusion) to maintain phospholipid in the correct monolayer
What are the types of membrane proteins?
- integral membrane proteins
- peripheral membrane proteins
- proteins that bind to surface of integral membrane proteins
What is membrane topology?
The arrangements of proteins relative to the membrane - doesn’t change
→ maintained by hydrophobic and electrostatic interactions - +ve aa interact with -ve lipid head groups
What is the structure and function of ICAM?
Single transmembrane spanning helix, short cytoplasmic tail, 5 extracellular immunoglobulin domains
→ involved in cell adhesion, expressed on cells of the immune system and endothelial cells
What are porins?
B barrel proteins with a pore in the centre, hydrophobic exterior hydrophilic interior
→ act as channels that are specific to different types molecules
Where are peripheral membrane proteins?
Adhere to the cytoplasmic or ectoplasmic side of a membrane
→ don’t interact with the hydrophobic membrane core, interact with lipid head groups and integral membrane proteins via non-covalent bonds: H-bond, van der Waals
What is spectrin?
A peripheral cytoskeleton protein that creates a scaffold on the intra-cellular side of membranes
→ maintains plasma integrity with ankyrin via the spectrin-actin based skeletal structure
What is the role of carbohydrates on membranes?
- physical barrier
- mechanosensing
- possible roles in cell shape
→ only found on the exoplasmic side, attach to lipids and proteins
Why are membrane carbohydrates important for immune responses?
Due to their role in cell-cell recognition, communication and adhesion
→ distinguishing self and non-self cells
Why are transporters needed in membranes?
All compartments of the cell are unique and require a unique set of molecules, some need different molecules on different sides.
→ membranes must contain transport proteins for the import/export of metabolites
What can cross lipid bilayers?
- Small hydrophobic molecules - O2, N2, CO2
- Small uncharged polar molecules - ethanol, glycerol
- Water
What can’t cross lipid bilayers?
- Large uncharged polar molecules - glucose, sucrose
- Charged ions - K+, Na+, Cl-
- Charged polar molecules - amino acids, glucose-6-phosphate
How can O2 be transported across lipid bilayers?
Simple diffusion
→ its hydrophobic so can dissolve in the membrane hydrophobic core
→ moves from [high] to [low] until a dynamic equilibrium
What are protein channels?
Channels create a pore in the membrane for molecules to pass through
→ always passive transport
→ are specific/selective
What are protein carriers?
Carriers move molecules across the membrane, can co-transport ions
→ can be active or passive
→ are specific/selective
What are the 2 types of transport?
- Passive: facilitated diffusion - transported and channels
- Active transport - transporters and pumps
What are the differences between facilitated and simple diffusion?
Facilitated diffusion is;
1. faster
2. saturable
3. specific
How do active transporters work?
Use energy to transport solutes
→ can transport against a conc gradient
→ can establish conc gradients
→ many use ATP hydrolysis
What are the 3 main methods active transporters use to move solutes against gradients?
- ATP-driven pumps - uses ATP hydrolysis
- Light-driven pumps - uses light energy
- Coupled transporters - coupled to the potential energy of downhill conc gradient
What are the 3 classes of primary active transporters?
- P-type pumps: phosphylatethemslves during transportation cycle - ion gradients Na+, K+, H+, Ca+
- F-type pumps: use proton gradients to synthesise ATP from ADP and Pi - synthases
- Abc transporters: pump small molecules as opposed to ions
What did Waclaw Mayzel say about cell division?
He carefully described it, showing that salamander embryo cells took up aniline dyes that stained condensed structure in the nucleus
What happens at interphase (G2)?
Two centrosomes are visible
→ comprised of a pair of centrioles and associated microtubules
The nucleus is in tact and chromosomes aren’t visual by light microscopy (only by FISH)
Why do interphase chromosomes occupy their own distinct territories?
Their dispersed structure allows access of transcription factors to the DNA
→ interphase cells aren’t resting: they are actively making RNA and proteins
What happens during prophase?
- The 2 centrosomes move to opposite poles
- The chromosomes condense into sister chromatids held together at their centromeres
- The nuclear membrane disassembles
What happens during metaphase?
Sister chromatids attached to opposite spindle poles align along the equator
→ forming the metaphase plate
What is multiple myeloma?
A plasma cell cancer disorder characterised by complex karyotyping with frequent numerical and structural aberrations
What happens during anaphase?
- The sister chromatids are pulled apart by spindle fibres attached to their centromeres
→ chromosomes are at opposite poles - The cell and spindle elongates
- Cytokinesis starts - a cleavage furrow begins to form
What happens during telophase?
- Chromosomes uncoil and become less distinct
- New nuclear membranes form around the daughter nuclei + nucleoli reform
- Spindle fibres become less distinct
- Cytokinesis is almost complete
What happens during S phase?
DNA and centriole replication
What is the order of the cell cycle?
G1 → S phase → G2 → M
How did Sir Tim Hunt identify the first cell cycle controller?
He noticed that unfertilised (sea urchin) eggs and growing cells have a different pattern of protein expression
In eggs drug stimulated into growth new proteins (A,B,C) are made but others aren’t
→ in fertilised eggs one of the new proteins (A) accumulates and falls, named this cyclin as it cycles with the cell cycle
What is cyclin?
A controller
→ cyclin conc rise G1-G2
→ high cyclin conc stimulates mitosis and cyclin levels fall
What is cyclin-dependant kinase (CDK)?
CDK and regulatory cyclin form an inactive complex to control cell cycle progression
→ this is prepared for activity, then activated (or inhibited)
→ the activated complex phosphylates the targets and is then destroyed
What do G1 cyclin-CDK complexes do?
Prepare the cell for S phase
→ there are multiple G1 complexes which act as G1 controllers
→ the complexes phosphorylate their targets which prepares the cell for S phase and promotes the expression of S phase cyclin
What does the S phase complex do?
Governs chromosome replication
→ there is one S phase cyclin-CDK complex that phosphorylates its target which controls chromosome replication
What do the G2/M complexes do?
Activate the mitotic apparatus
→ G2/M cyclin-CDK complexes phosphate their targets which activate spindle formation
How does the fission yeast Schizosaccharomyces probe have much simpler control of the cell cycle then mammalian cells?
It uses one CDK which is adapted by different cyclins (we have 4)
What is the quiescent phase?
A temporary cell cycle stage where populations of cells rest and do not replicate before they are activated to re-enter the cell cycle
What is the G0 phase?
Non-dividing mammalian cells leave the cell cycle in G1 to enter G0
→ can remain there for days, weeks even cells lifetime
Why can cancer cells in G0 be resistant to a number of therapies?
Radiotherapy: targets rapidly growing cells - cells in G0 are relatively resistant
Chemotherapy: targets cells that are actively reproducing - G0 cells don’t cycle so are resistant
→ if these G0 cells re-enter the cell cycle a tutor may reform
What happens when mammalian cells pass through the restriction point?
They are committed to entering S phase (DNA rep) - point of no return
How do mitogenic signals stimulate a cell to leave G0
Through a highly conserved signal transduction pathway
1. Stimulation of growth factor receptors (at cell membrane) results is Ras recruitment
2. Signal transduction from Raf to MAPK
3. Expression of G1 cyclins and CDKs return G0 cells to G1
How are mitotic signals shut off?
Lysosomal targeting to activated growth factor receptors - Ras not recruited ultimately no expression of G1 cyclins and CDKs
→ important as inappropriate signals can cause unchecked proliferation - cancerous
What happens if there is mutational activation of receptors?
Unregulated proliferation
→ e.g. growth factors that are self-activating, auto-phosphorylating
→ e.g mutated Ras that is permanently activated
Why does the cell cycle need breaks?
To provide opportunity for repair
→ the cell cycle runs at full speed and the cell population becomes increasingly damaged
Why is DNA damaged repaired in G1?
During mitosis DNA repair mechanisms are turned off
→DNA damages that occurs during mitosis is repaired after mitosis
What is the cell cycle brake in G1 phase?
The restriction point - slows down entry into S phase
→ gives time for DNA repair following mitosis
→ key regulator - pRb (retinoblastoma protein)
→ checks if environment is favourable
What happens in the absence if functional pRb protein?
Retinal cells proliferate uncontrollably giving rise to retinoblastoma
→ pRb is a tumour-suppressor gene that functions by regulating the restriction point
How does Rb protein work to regulate the restriction point?
Slows down the entry into S phase until the G1 cyclin/CDK complexes accumulate enough to phosphorylate Rb protein and override its control
→ allows S phase proteins to be made
What does the cell brake at G2/M do?
Regulates entry into mitosis
→ checks if all DNA is replicated and damage repaired
What is the function of p53?
Promotes genome stability
→ activated p53 can lead to cell cycle arrest, DNA repair, cell cycle restart and apoptosis
→ cellular stability - genetic stability
How can p53 arrest cells in G1?
p53 is an unstable protein: if damaged DNA is detected p53 becomes stabilised in the nucleus
→ activated gene expression - some encode proteins that inhibit all mammalian cyclin/CDK complexes
What is the difference between apoptosis and necrosis?
Apoptosis - no intracellular contents released into environment, inflammation not stimulated
Necrosis - dying cells swell and burst, intracellular content related causing inflammation and damage to neighbouring cells
What are caspases?
Effectors of apoptosis
→ cleave at c terminal side of aspartic acid
→ kept inactive by trophic signals
→ apoptosis is our default state - we require congeal signals to stop dying
Why do proteins require sorting signals?
In the cytosol
→ if a protein doesn’t reside in the cytosol it needs to carry a sorting signal to reach the correct destination
What are the 3 types of protein sorting signals?
- Short peptides at the N or C termini - can be removed or kept, nuclear transport
- 3D domains - features of secondary/tertiary structure, transport of lysosomes
- Other molecules attached to the protein - post translational modifications, transports sugars lipids
What happens to sorting signals?
They are recognised by specific receptors - trigger transfer of the client protein to the correct destination
→ every organelle uses different receptors and different sorting processes
What are the main modes of protein transport?
- Gates transport - e.g. import into and export out of the nucleus
- Transmembrane transport - e.g. protein import into ER and mitochondria
- Vesicular transport - e.g. secretion along the organelles of the secretory pathway
How are proteins transported into the nucleus?
Proteins and other macromolecules move between the cytoplasm and the nucleus via large aqueous nuclear pore complexes (NPC)
→ gated transport
How is the passive diffusion from cytosol to the nucleus of large molecules blocked?
The diffusion barrier caused by unstructured regions of NPC proteins form a tangled network blocking passive diffusion
What are NLSs?
Nuclear localisation signals - tags a protein for import into the cell nucleus by nuclear transport
→ rich in lysine and proline
→ can be in any position of the passenger protein as long as they are exposed to the surface of the protein - need to be seen by receptors
What is the receptor for the NLS?
Importins - a family of cytosolic nuclear import receptors
→ each member being responsible for a set of cargo molecules
→ recognise specific NLS on nuclear proteins
How does the importing know when to let go of its cargo?
Importins bind to a GTPase ‘switch’ protein called Ran
→ Ran can either bind GTP (bound-inactive) or GDP (bound-active)
→ can hydrolyse GTP to GDP
→ assumes different conformation depending on which nucleotide it binds
→ different conformation = different activity
How does the conformation of Ran differ in the cytosol and nucleus?
Cytosol - Ran-GDP bound and has conformation A
Nucleus - Ran-GTP bound and has conformation B
→ the two conformation states drive the net import of cargo
What are NPCs in nuclear transport?
NPCs - nuclear pore complexes
→ span the nuclear envelope
What is the function of Ran-GTP?
Displace the cargo protein from import receptors on the NPC
→ binds to importing changing its conformation
Why is Ran-GDP abundant in the cytosol?
The GAP (GTPase activating protein, promotes hydrolysis of GTP to GDP) specific for Ran is always n the cytosol
→ so any Ran-GTP arriving in the cytosol, bound to the importin, is immediately converted to Ran-GDP
Why is Ran-GTP abundant in the nucleus?
The GEF (guanine nucleotide exchange factor, exchanges GDP with GTP) specific for Ran is always in the nucleus (binds to chromatin)
→ so any incoming Ran-GDP is immediately converted to Ran-GTP
What is the function of the ER?
- Lipid synthesis
- Protein translocation - through translocation pore, acquire their native structure
- quality control - misfolded proteins degraded
How do proteins enter the secretory pathway?
Secretory proteins carry an N-terminal signal sequence that targets proteins to the surface of the ER
→ leads to the docking of a ribosomes-nascent chain complex onto the ER membrane (creates ‘rough’ ER)
What is the golgi complex?
A stack of flattened membrane-enclosed sacs called cisternae (trans-medial-cis)
→ there are many vesicles around the rims of each cisterna
What is the function of the golgi complex?
- Protein and lipid modification
- Protein packaging and sorting
→ to the outside, plasma membrane and lysosomes
How do the ER, Golgi, endosomes, lysosomes and plasma membrane communicate?
Forward and backward moving transport vesicles
→ once in the ER proteins don’t need to cross anymore membranes
How do transport vesicles move cargo?
Transport vesicles bud from a donor compartment and fuse with an accept compartment (e.g. ER to Golgi)
→ allowing for transport of luminal and membrane cargo
Do proteins going to the plasma membrane from the ER need signals for their sorting?
No - they are secreted by default
→ proteins heading for intracellular destinations (e.g. lysosomes) require sorting signals to separate them
What is the extracellular matrix?
The material that surround animal cells
→ produces bone/teeth, tendons …
→ structural and regulatory roles
What is the proteoglycan gel made up of?
Hydrated polysaccharide glycosaminoglycans often linked to proteins
→ fills spaces around cells
→ collagen, fibronectin and elastic proteins are secreted into the gel
What is the function of the extracellular matrix?
Support cells and influence their: survival, development, migration, shape, proliferation and function
→ provides strength, elasticity and turgor (shock absorber, attracts water)
What do glycosaminoglycan (GAG) chains on the ECM do?
The glycosaminoglycan chains are negatively charged disaccharides that form linear chains
→ attract cations (Na+) causing larger amount of water to be sucked into the matrix creating turgor
Where are glycosaminoglycans (GAGs) and proteoglycans made?
GAGs and proteoglycans are synthesised in and secreted by cells within the ECM (e.g. fibroblasts)
What are the 4 main groups of glycosaminoglycans (GAGs)?
- Hyaluronan (hyaluronic acid)
- Dermatan sulfate
- Heparan sulfate
- Keratan sulfate
What happens to aggrecan in cartilage?
Aggrecan aggregates in cartilage
→ aggrecan aggregate is composed of aggrecan bound to hyaluronan
→ huge molecule, 100+ GAG chains
What does the varied nature of GAGs and proteoglycans mean?
Pore sizes in the gel and charge densities varies
→ influencing turgor and what cells may pass through the ECM
What does a pre-collagen molecule contain that a pro-collagen molecule doesn’t?
A signal peptide
→ directs the nascent protein into the ER and is then removed
What is the structure of fibrillar collagen?
N-terminal propeptide cysteine rich
C-terminal propeptide cysteine rich
Repetitive structure - every 3rd aa glycine
→ promotes trimerisation
What do the modifications of pro-collagen in the ER do?
Improve the solubility of the pro-collagen
What happens to the C terminal domains of collagen monomers in the ER?
They trimerise via disulphide bonds
→ winding produces a stiff triple stranded helical structure
→ C and N terminal highly soluble
What are the functions of the N and C terminal propeptides on collagen?
Keep the trimers apart so they don’t form fibrils
→ until late in secretory pathway, propeptides are removed forming tropocollagen
What are fibripositors?
Tubular extensions at the plasma membrane that accept GPCs (Golgi to PM carriers) containing collagen
→ opens by fusing with the PM and the collagen fibril is extruded out of the cell
How is collagen arranged in bone?
The collagen fibrils have a ‘plywood’ arrangement that doesn’t allow for stretching
→ makes bone stiff, strong, tough and lightweight
How is collagen arranged in tendons?
Collagen fibrils aggregate (cluster) to form a fibre
→ can stretch in one direction without breaking
→ tendons: tough band of dense fibrous tissue that connects muscle to bone
What happens to collagen if there isn’t enough VitC?
Collagen hydroxylation requires VitC
→ reduced hydroxylation collagen is misformed, connective tissue problems arise
→ causes scurvy
What is elastin?
Hydrophobic elastic protein with extensive crosslinks
→ highly prevalent in the ECM of arteries
→ gives tissues their elasticity
What is the function of fibronectin in the ECM?
Adhesive dimeric protein that has multiple sites for binding - to itself, other ECM molecules, receptors
→ the cytoskeleton can contract and pull on fibronectin in the matrix to create tension - organises ECM
When do cells degrade ECM?
When cells migrate through connective tissue localised segregation of ECM is required
