Cell Structure Flashcards
Describe the nucleus and outline its function.
Nucleus
-Prominent, spherical organelle in eukaryotic cell
-Surrounded by a nuclear envelope (a double membrane) which is perforated with nuclear pores and continuous with RER
-contains the nucleolus and chromatin
Function:
-to contain the hereditary material (DNA)
-to control cell activities by regulating protein synthesis
-contains enzymes for DNA replication, transcription and RNA processing
Which substances pass between cytoplasm and nucleus via nuclear pores?
- Free nucleotides and enzymes (for DNA replication and transcription), proteins (to make up ribosomal units) which enter the nucleus
- mRNA, tRNA and large and small ribosomal subunits which leave the nucleus
Describe the nucleolus and outline its function.
Non-membranous, sphere/s within nucleus.
Contains large amounts of DNA, rRNA and protein.
Function:
-to synthesise rRNA and proteins into large and small ribosomal subunits using rRNA synthesised in nucleolus and proteins exported from cytoplasm
Describe chromatin in the nucleus and outline its function.
-Hereditary material of the cell
-Thin, elongated threads of DNA coiled around histone proteins.
Euchromatin: lightly stained, transcriptionally active, exists in a diffused, extended state
Heterochromatin: darkly stained, transcriptionally inactive, found along the edge of the nucleus
Function:
DNA which contains genes, transcribed, translated to form polypeptides
Describe RER and outline its function.
-A network of membranous flattened sacs called cisternae
-Has ribosomes bound to the outer surface
-Continuous with the outer membrane of the nuclear envelope
Function:
-to transport proteins which are synthesised by the ribosomes on its surface to the GA; transport vesicles bud off from the RER
-To allow proteins to fold into their native 3D-conformation in the cisternal space and glycosylate them to form glycoproteins, catalysed by enzymes found within membranes of RER
Describe SER and outline its function.
-A network of membranous tubular sacs called cisternae
-Lacks ribosomes on outer surface
Function:
-Contains many embedded enzymes that catalyse the synthesis of a variety of lipids and carbohydrates
-To detoxify drugs and poisions (thus SER abundant in liver)
-SER called sarcoplasmic reticulum stores Ca2+ ions
Describe golgi apparatus.
-Membrane-bound flatenned sacs called cisternae and associated Golgi vesicles
-Consists of a ‘cis’ face where new cisternae are being formed by fusion of transport vesicles and a ‘trans’ face from which Golgi vesicles continuously bud off
Outline function of golgi apparatus.
-Glycosolate proteins and lipids to form glycoproteins and glycolipids
-Modify existing glycoproteins and glycolipids by modifying/cleaving the existing sugar chains
-Sort and package proteins into different vesicles and target the proteis to different parts of the cell or for secretion out of cell
-Form lysosomes
-Synthesise polysaccharides such as pectin which is transported in vesicles that eventually fuse with the cell surface membrane
Describe lysosome and outline its function.
-Membranous sac containing hydrolytic enzymes (e.g. proteases, lipases, nucleases used to digest macromolecules)
-Contents are acidic. Enzymes optimum pH
Function:
-Digest material taken in by the cell by endocytosis (phagocytosis)
-Release enzymes from cells by exocytosis for extracellular digestion
-Digest unwanted or worn-out organelles (autophagy)
-Self-destruct a cell after its death (autolysis)
Describe mitochondrion and outline its function.
Structure:
-Outer membrane is smooth
-Inner membrane is highly infolded to form numerous cristae
>provides a large surface area for attachment of proteins (e.g. electron carriers)
and enzymes (e.g. ATP synthase) that function in oxidative phosphorylation
-The inner membrane divides the mitochondria into two internal compartments
the intermembrane space between the outer and inner membrane
-the matrix enclosed by the inner membrane
-The fluid matrix, containing 70S ribosomes, circular DNA and various enzymes
involved in the Krebs cycle
Function:
Site of aerobic respiration. They carry out metabolic processes that generate ATP
through oxidation of sugars, fats and other fuels with the uptake of oxygen.
The Krebs cycle occurs in the matrix whereas oxidative phosphorylation occurs on the
cristae
Describe structure of chloroplast
-Lens-shaped structure surrounded by a
double membrane
-Within the chloroplast is an internal
membrane system which consists of
flattened sacs called thylakoids (a
stack of thylakoids = granum) &
intergranal lamella
-Fluid within chloroplast surrounding the
grana is called stroma (contains
circular DNA, 70S ribosomes,
enzymes & starch grains)
-Chlorophyll molecules are located on
the thylakoid membrane.
-ATP synthase complex on thylakoid
membrane project into stroma
Describe ribosome and outline its function.
-Consists of a small and large subunit
-Each subunit is made up of protein and rRNA which are assembled in the nucleolus
-May be found either freely floating in cytosol or bound to RER
-Small and large subunit only come together during translation
Act as the site for protein synthesis
Walk through the process of how the endomembrane system manufactures proteins.
DNA is used as a template to synthesize mRNA in the nucleus (transcription) -> mRNA leaves the nucleus via the nuclear pores-> mRNA is used as template to synthesize polypeptides (translation) on the ribosomes of the RER-> polypeptides enter the lumen of the cisternae of the RER where it
undergoes folds into its tertiary conformation and may undergo glycosylation-> transport vesicle buds off from the RER and carries the proteins to the GA > vesicle fuses with the ‘cis’ face of the GA and the proteins undergo further modification, sorting and packing ->a secretory vesicle containing the protein will bud off from the ‘trans’ face of the GA and be transported to and fuse with the cell surface membrane, releasing the protein content of the vesicle by exocytosis. Microtubules direct the movement of the transport vesicle to the GA and the secretory vesicle to the cell surface membrane. and energy from ATP hydrolysis is needed for exocytosis.
