cell biology Flashcards
what is the role of a plasma membrane
provides a cell boundary and regulates movement of materials in and out of the cell
what are 3 key features of plasma membrane
flexible to prevent rupture
self-repairing to prevent cell damage
selectively-permeable to allow certain molecules to move in and out of the cell
what are the 3 components of all membrane
lipids
proteins
carbohydrates which are made up of oligosaccharide chains that can covalently bond to proteins to form glycoproteins
how are phospholipids in plasma membranes arranged
phospholipids are amphipathic so form a bilayer
the two layers in a bilayer are called leaflets
how can phospholipids move in the plasma membrane
they can move laterally, rotate or flip flop between leaflets
why are membranes with unsaturated fatty acids more fluid
unsaturated fatty acids have kinks so can’t pack together as closely
which membrane phospholipid is negatively charged
phosphatidyl serine
how do micelles form
in an aqueous solution the hydrophobic fatty acid tails face inwards and the hydrophilic phosphate heads face outwards
how do plasma membranes self-repair
if the edges of a phospholipid bilayer come into contact with water they form a sealed compartment for repair
what is the function of cholesterol in plasma membrane
they pack between phospholipids to make the membrane surface more rigid
what is the function of proteins in the plasma membrane
they are embedded in the bilayer and can act as transporters, receptors or enzymes
what is a microdomain and how is it formed
transmembrane domains that bring together proteins to perform one function
what is the function of tight junctions in plasma membranes
act as diffusion barriers between proteins in each domain and in the extracellular leaflet of the bilayer
what is the function of the cytoskeleton
it is anchored to the plasma membrane by proteins and helps maintain the structure of cells such as red blood cells
why are membranes described as being asymmetric
the protein composition on one side of the lipid bilayer is different from the other side
what is protein topology and how is it maintained
the orientation of proteins in the membrane
maintained by anchoring the proteins to domains in the membrane
what is the function of phosphatidyl serine
it is negatively charged and drives sodium uptake into the cell
provides the nucleation site for the coagulation cascade that causes blood clotting on platelets
it also moves to the outer leaflet to initiate apoptosis
how do carrier proteins work
the solute binds to the binding site and the carrier moves across the bilayer to release to solute inside the cell
how do channel proteins work
the solute doesn’t bind directly there is only a weak interaction
it is faster because there is no binding
what are uniporter and symporter carrier proteins
uniporter carriers move one solute and symporters move two
what allowed unicellular organisms to develop into multicellular
the separation of transcription and translation by the nucleus
how did the nucleus evolve and what is the evidence for this
a cell engulfed another cell and the DNA of one cell became the main DNA and the other cell formed the nucleus
this is supported by the nucleus having a double membrane
how are chromosomes formed
DNA is very long so it is wrapped around histones to form chromatin which is packaged into chromosomes
what are the 2 forms of chromatin
heterochromatin: found around the edge of the nucleus and nucleolus and makes up genes that are less actively transcribed
euchromatin: found in the middle of the nucleus and stains less densely with EM staining, it makes up genes that are more actively transcribed
how is gene transcription controlled in the nucleus
chromosomes occupy specific discrete territories that are inherited and can change during cell differentiation which can alter gene transcription
what is the role of the nucleolus in the nucleus
processes ribosomal RNA to produce ribosomes
also processes mRNA and tRNA
what is the role of speckles in the nucleus
carry out pre-mRNA processing
what is the role of cajal bodies in the nucleus
carry out splicing
what is the role of PML bodies in the nucleus
store materials
what is the lamina in the nucleus
a fibrous meshwork that supports the nuclear membrane and keeps it asymmetric so the protein composition is different on each side
how do mutations affect the lamina
causes the structure to change which affects the genome and causes global problems across the body
how can proteins move through pores in the nuclear membrane
their molecular weight has to be less than 50,000 and they are moved by activate signal-dependent transport
the signal is a specific, positively charged peptide sequence within the protein
how are new phospholipids inserted into the plasma membrane
they are made in the endoplasmic reticulum and are added to the cytosolic half of the membrane by exocytosis
what is the role of the enzyme scramblase in plasma membranes
it mixes up the phospholipids for symmetrical growth on each side of the bilayer
moves non-specific phospholipids between the leaflets
what is the role of the enzyme flippase in plasma membranes
catalyses flipping of specific phospholipids between leaflets to maintain asymmetry
what is the structure of the endoplasmic reticulum
it is connected to the nuclear envelope and forms hollow tubes and flattened sacs
the chambers are called cisternae and they are lined with lumen
what are the 2 domains of the endoplasmic reticulum
rough ER: associated with ribosomes
smooth ER: no ribosomes
what is the role of the endoplasmic reticulum
ensures proteins are made and folded correctly
stores and detoxifies molecules
rough: synthesises quaternary proteins
smooth: synthesises lipids
how are ogliomeric proteins made in the rough endoplasmic reticulum
- mRNA is translated into a linear polypeptide chain by a ribosome on the rough ER
- the polypeptide is fed through a pore into the RER lumen
- for translocation of the polypeptide it needs a certain signal that targets it to the pore and is chewed off by peptidase
what is the role of the smooth endoplasmic reticulum
synthesises phospholipids and cholesterol
contains sensors that detect low cholesterol levels
releases calcium ions through cholesterol acting as calcium straws
produces steroid hormones
synthesises and stores glycerides, glycogen and calcium
how do vesicles transport molecules
the ER is known as the donor compartment and it sends vesicles to the target compartment (golgi)
1. cargo inside the donor compartment is gathered together into an exit site
2. the exit site buds off to form a vesicle that fuses with the golgi
3. the vesicle and golgi membrane fuses and cargo is delivered
what is the vesicle coating
a coating made up of proteins that surrounds vesicles providing structure and acting as a barrier to fusion
it is discarded after the vesicle buds off
what are SNARE’s
protein receptors that help vesicles fuse with the correct target
what are the 2 types of SNARE’s
v-SNAREs: found in the vesicle membrane
t-SNAREs: found in the target membrane
how do SNARE’s work
after the vesicle coating is removed the v-SNARE interacts with the t-SNARE to promote fusion by bringing the membranes close together
what is the structure of the golgi apparatus
made up of flattened discs called cisternae that communicate with the ER and membrane using vesicles and tubules
what are the 3 functions of the golgi apparatus
modify and package secreted proteins
modification of the plasma membrane
delivery of material to other organelles
what is the trans-golgi network
a major sorting station for newly-made proteins to deliver them to different places in the cell
how are proteins transported to the plasma membrane
proteins are stored in the endoplasmic reticulum and move by exocytosis to the golgi
proteins are sorted into vesicles at the trans-golgi network and transported to the plasma membrane in vesicles
what is endocytosis and when is it used
when a cell takes up substances by invagination of its membrane to form vesicles
used to take up nutrients, for signalling, in antibodies, enzymes, viruses and bacteria
what can happen to material that has be endocytosed
recycled and reused in the cell
transported across the cell (transcytosis)
degraded
what is phagocytosis and when is it used
the uptake of large particles
used to destroy bacteria and carry out apoptosis
what is the process of phagocytosis
- surface receptors on the phagocyte recognise and bind to antibodies on the coating around bacterial cells
- actin extends from the cells forming pseudopods that engulf the bacterium
- the bacteria is hydrolysed by lysozymes
what is frustrated phagocytosis
when two phagocytes try to engulf the same bacterium
how is new membrane made
making new membrane from scratch requires large amounts of energy so membranes are recycled
what is macropinocytosis and when is it used
take up of fluid from the external environment
used in cancer cells to take up nutrients
what is clathrin
a protein that coats vesicles an example is LDL which is used to carry cholesterol
how does LDL take up cholesterol
- LDL receptors are synthesised and expressed on the cell
- LDL binds to receptors and clusters into areas of the membrane called coated pits
- the coated vesicles bud off and the coat is dissociated so v-SNAREs can interact
- v-SNAREs interact with the early endosome allowing the vesicle to fuse with it
- LDL is released from the receptors and broken down into cholesterol
what is dynamin
a protein that assembles around the neck of budding vesicles and hydrolyses GTP causing the membrane to constrict so the vesicle can bud off
what are the conditions needed for endocytic pathways
a pH gradient that gets more acidic across the pathway
what is the multivesicular body (MVB) and intraluminal vesicles (IVLs)
the MVB is also known as the late endosome and it contains lots of IVL that degrade cargo using lysosomes
what is the function of the double membrane in mitochondria
inner membrane: folded to increase surface area for energy production, folds are called cisternae
outer: prevents leakage of ions such as hydrogen to maintain proton gradient, contains porins for molecules to enter
what is the role of the mitochondria
synthesise ATP
produce tRNA and mRNA which are encoded for by the mitochondrial genome
how is ATP synthesised in mitochondria
- glycolysis: glucose is broken down into pyruvate in the cytoplasm
- pyruvate enters the mitochondria and carbon dioxide is removed
- krebs cycle
- proton gradient formed which drives ATPase
what are peroxisomes
simple organelles that carry out oxidative reactions
they only have a single membrane and don’t contain DNA or ribosomes
what happens in oxidative reactions in peroxisomes
- enzymes combine with the compound and oxygen
- hydrogen is removed to for hydrogen peroxide
- peroxidases use hydrogen peroxide to produce water and remove oxygen from the compound
what is the life cycle of a mitochondria
- if a mitochondria is damaged and accumulates debris it will segregate the debris to one end
- it will undergo a quality control reaction where is splits in two (fission) so the half without the debris can keep functioning normally
- the half containing the debris will undergo mitophagy to destroy it
- new mitochondria are made by joining two normal halves together (fusion)
what are the complexes embedded in the outer membrane of mitochondria
translocator of the outer membrane (TOM)
sorting and assembly machinery (SAM)
TIM 23 spans both membranes
what are the complexes embedded in the inner membrane of mitochondria
translocator of the inner membrane (TIM) 23
cytochrome oxidase activity (OXA)
TIM 23 spans both membranes
how are proteins transported into the mitochondrial matrix
- the N-terminal sequence of the protein is recognised and binds to import receptors on the TOM complex using chaperone proteins
- the protein is translocated into the matrix using TOM and TIM23 complexes
- the signal is cleaved off
what is the role of chaperone proteins in the mitochondria
they coat polypeptide chains to help them fold correctly and keep them in the correct orientation to dock with TOM
how are proteins transported into the outer mitochondrial membrane
- proteins in the intermembrane space are kept unfolded by chaperone proteins
- they are folded and inserted into the outer membrane using the SAM complex
- they move through porins which are beta-barrel proteins
what are the 2 routes used to transport proteins into the inner mitochondrial membrane
- through the TOM and TIM23 complexes
- protein enters the matrix and the signal is cleaved off causing it to be inserted into the inner membrane by the OXA complex
how do proteins enter the peroxisomal membrane
- Pex5 recognises the signal sequence
- peroxins use ATP to transport folded proteins into the membrane
