2 - cell structure & organelles Flashcards
(46 cards)
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.
