2 - cell structure & organelles Flashcards
what are organelles?
organelles are membrane-bound, subcellular structures that perform specialised functions.
what are caveats?
Caveat: some large assemblies of macromolecules that carry out specialised functions but are not membrane-bound are typically referred to as organelles e.g. ribosomes and centrosomes.
what is the cytosol?
the fluid portion of the cell enclosed by the plasma membrane that is not part of any membrane-bound organelle.
what does the cytosol contain?
- dissolved molecules: small molecules (e.g. salts) and macromolecules (e.g. proteins)
- cytoskeletal filaments
- small structures such as ribosomes
what is the cytoplasm?
everything within the plasma membrane of the cell except the nucleus
- the cytosol
- the organelles suspended with the cytosol, aside from the large nucleus.
what is the nucleus?
the nucleus is the largest cell organelle.
it is enclosed by the nuclear membrane or nuclear envelope.
the nuclear membrane consists of two lipid bilayers - the inner and outer membrane. the space between them (20-40 nm wide) is the perinuclear space.
what is the outer nuclear membrane?
the outer nuclear membrane us continuous with the endoplasmic reticulum (ER), though with far higher protein concentrations.
the perinuclear space is directly connected with the lumen of the ER.
what is the inner nuclear membrane?
enclosing the nucleoplasm, the inner nuclear membrane:
- consists a number of proteins involved in structural organisation of the nucleus
- it is connected to the outer membrane by nuclear pores which penetrate the membranes
- is covered by the nuclear lamia, a mesh of protein filaments that stabilise the nuclear membrane and provides an anchor point of chromatin and nuclear pores.
what are nuclear pores?
nuclear membrane contains 1000s of nuclear pore complexes (NPCs):
- protein-lined channels that pass through the inner and outer nuclear membranes and regulate molecular traffic to and from the nucleus.
the NPC is a large and complex structure, it vertebrates:
~125-150 nm diameter, 80 nm length, constructed from 50-100 different proteins.
what do the nuclear pores do?
- allows small molecules and ions to freely diffuse into/out of nucleus
- allows necessary proteins to enter the nucleus if they have special sequences that indicate they belong there.
- RNA destined for the cytoplasm has nuclear export sequences that tag them for release through the nuclear pores.
what is chromatin?
the nucleus contains the genetic material in the form of chromatin.
a complex of DNA and proteins that forms chromosomes.
during cell division the chromatin condenses and chromosomes become visible in their familiar form
what are the two forms chromatin exists in?
euchromatin: less condensed and is “active”
heterochromatin: highly condensed and “inactive” (typically not transcribed)
what is the nucleolus?
the largest structure within the nucleus (clearly visible by light microscopy). NOT membrane bound
primary function - responsible for synthesising ribosomes, which then move to the cytosol to synthesise proteins.
nucleoli are made of proteins, DNA and RNA and form around specific chromosomal regions.
what is the endomembrane system?
network of interrelated membranes:
- the organelles involved are not continuous with one another
- but they exchange membrane material via a special type of transport
membrane lipids and proteins are synthesised in the ER and are transported through the network in membrane-bound vesicles.
what are the organelles of the endomembrane system
- the endoplasmic reticulum
- the Golgi apparatus
- lysosomes
- transport vesicles
what are the two types of ER
smooth and rough.
these are continuous with each other.
what is the ER?
the ER is a 3D network of flattened membrane compartments called cisternae.
what is the rough ER?
the RER is covered in bead like structures called ribosomes.
the primary function of the RER is protein synthesis
what is the smooth Er
the smooth ER has no ribosomes.
SER synthesis ad secreted lipids and steroids; depending on cell type it may have other important specialised functions.
e.g. sarcoplasmic reticulum
what are ribosomes?
ribosomes synthesise proteins from amino acid building blocks - “a micro-machine for making proteins”
ribosomes can join up amino acids at up to 200 a.a. per minute.
in a mammalian cell there can be as many as 10 million ribosomes.
what two subunits do ribosomes consist of?
proteins and rRNA.
molecular mass of ~4 MDa (about two thirds ribosomal RNA, one third protein)
where can ribosomes be found?
they can either be attached to the RER or nuclear membrane or can float free in the cytosol.
RER: plasma membrane, endomembrane system
free floating: cytosol, nucleus, mitochondria or peroxisomes.
what is the Golgi apparatus?
the golgi is a stack of cisternae.
functions as a processing and packaging site and part of the secretory pathway.
proteins can be modified and sorted, concentrated and packed into outgoing vesicles.
enables secretion of proteins and other signalling molecules via secretory pathways.
what are lysosomes the site of?
intracellular digestion:
- break down cellular components (macromolecules, organelles) and pathogens.
where do lysosomes bud off from?
they bud off from Golgi and are a branch off the secretory pathway.
what do lysosomes contain that are capable of breaking down all biological macromolecules?
They contain numerous different (>50) hydrolytic enzymes capable of breaking down all biological macromolecules.
these enzymes are optimally active in an acidic environment: lysosomal pH is 4.5-5.0
what is endocytosis?
form of active transport in which a cell internalises external molecules or particles via a vesicle that originates from the plasma membrane.
what is an endosome?
vesicle in which extracellular molecules which have been internalised.
what is phagocytosis?
form of endocytosis involving uptake of large particles e.g. pathogens.
what is a phagosome?
vesicle that has formed around a foreign particle.
what is an autophagy?
regulated degradation system for cytoplasmic contents (e.g. abnormal intracellular proteins, excess or dysfunctional organelles.)
what are autophagosomes?
double-membraned vesicles containing intracellular material for degradation.
what are peroxisomes?
small round organelles containing at least 50 enzymes, with very high concentration of oxidative enzymes.
they are not part of the secretory pathway.
what is the function of peroxisomes?
variety of functions including the metabolism of organic molecules - oxidation of excess long chain fatty acids.
what is the main chemical produced by oxidation in peroxisomes?
the cytotoxic hydrogen peroxide.
contain large amounts of the enzyme catalase that converts hydrogen peroxide to water and oxygen.
can peroxisomes undergo fission?
Peroxisomes can divide (undergo fission).
what do the mitochondria do?
mitochondria are often described as the powerhouse of the cell.
they generate the majority of cellular adenosine triphosphate (ATP), the principal currency of chemical energy.
involved in the regulation of metabolism.
they have many other roles, including in intracellular signalling e.g. in control of cell proliferation.
what makes mitochondria unique organelles?
mitochondria have their own genome, making mitochondria unique organelles.
mitochondria DNA is maternally inherited (maternal lineage can be traced far back in time)
…
although mitochondria synthesise a few of their own proteins, the vast majority are encoded in the nuclear genome.
~ 3000 different proteins in a mitochondrion but only about 13 are encoded by mitochondrial DNA (mtDNA)
mutations in mtDNA can cause some forms of mitochondrial disease: motivation for the development of “3 person IVF”
mitochondria fission and fusion
mitochondria are independently replacing organelles. mitochondrial replication is not coupled to cell division.
a mitochondrion divides by fission; two mitochondria can merge by fusion.
what is the structure of a mitochondria?
they are medium sized organelles that are typically ~1-3 µm long:
HOWEVER size and shape can vary.
what are the two membranes of the mitochondria?
outer mitochondrial membrane (OMM): contains porins that allow the passage of small molecules.
inner mitochondrial membrane (IMM): protein rich membrane and the site of ATP production.
what is in between the two mitochondrial membranes?
the inter-membrane space
inner membrane is deeply folded producing cristae.
what does the shape of a mitochondria depend on?
depending on cell type. mitochondria can adopt a variety of shaped and be static or highly dynamic.
in some cells mitochondria are static organelles e.g. myocytes (muscle cells)
in other cell types they are highly motile e.g. neurons, cultured cells.
what similarities do mitochondria share with bacteria?
- size
- circular genomes
- certain enzymes
- replication machinery (prokaryotic-like ribosomes)
- double membrane
- the lipid cardiolipin is exclusively found in the IMM and bacterial cell membranes
what did the similarities in mitochondria and bacteria lead to?
these observations and other evidence led to endosymbiotic theory.
mitochondria evolved from an ancient prokaryote that was engulfed, but not destroyed, but another type of prokaryote.
- the engulfed bacteria was aerobic (used oxygen for respiration)
- the host was anaerobic (could not use oxygen for respiration) - it benefited from energy production by engulfed bacterium.
- the two developed a symbiotic relationship with the aerobic bacteria eventually becoming mitochondria.
host = eukaryotic cell
engulfed bacteria = organelle
