Structure And Function Of Cell Organelles Notes

Ways to study cells

-light microscopy

-immuno cytochemistry –labelled antibodies are used to identify proteins and glycoproteins

-insitu hybridisation – localisation of a strand of DNA or RNA using a labelled complementary probe

-fluorescence microscopy

-electron microscopy-resolution of 0.1nm- show structures within organelles, membrane, viruses, macromolecules- dna and protein

Structure and function of cell organelles

The ultrastructure of organelles can be seen through electron microscopy- the proteins of organelles can be labelled using immunocytochemistry where the antibodies can be labelled with fluorescent markers so they can be seen

Nucleus

Functions: gene replication and repair

-gene transcription- production of mRNA for protein synthesis in the cytoplasm

-ribosome production by the nucleolus

Nuclear envelope

-Electron microscopy- double membrane separated by narrow space (perinulear cisterna)

-Internal membrane of nucleus has protein structure called ‘fibrous lamina’- made up of three lamina proteins

-Function of the nuclear lamina: spatially organises the nuclear pore, this stabilizes the nuclear envelope. Non dividing cells, chromosomes are associated with fibrous lamina

-Poly ribosomes attached to the outermembrane, nuclear envelope part of RER.

-sites when inner and outermembrane of nclear envelope fuse, there are gaps ‘nuclear pores’- this controlled pathway between nucleus and cytoplasm- nuclear pores are composed of an octagonal pore complex, which consists of 8 large proteins organised around a hole

-ions and molecules with diameter up to 9nm pass freely through nuclear pore- mRNA, proteins and steroid hormones, molecular complexes more than 9nm are transported actively

Chromatin

-chromosomes in non dividing cells are attached to the inner membrane of the nuclear envelope and are found in different states of uncoiling- degree of coiling varies with cell activity, more transcriptionally active the cell the less coiled the chromatin which allows more DNA surface available for transcription

-identified by basic stains- hematoxylin and methylene blue

-Two types of chromatin- Heterochromatin and euchromatin

-Heterochromatin- chromatin is densely coiled and appears as coarse granules known as chromocentres- two types of heterochromatin- constitutative- sections of DNA in eukaryotes that consist of highly repetivie sequences of DNA bases, found in the centromere and telomere- these sections are transcriptionally inactive- facultative chromatin- under specific signalling cues the chromatin looses its condensed structure and becomes transcriptionally active – one X chromosome in females is selected early in development

-euchromatine- chromatin is less densely coiled, widely dispered and fills the majority of the nucleus-transcriptionally active

-coiled strands of DNA bound to histones (protein)- structural unit is nucleosome (DNA wrapped around histones), nucleosome is coiled on its axis to form solenoid and hoops.

Nucleolus

-electron dense structure as seen by the electron microscope as it is rich in rRNA

-spherical structure, rich in rRNA and protein.

-basophilic when stained with hematoxylin

-lightly stained sections consist of nuclear organiser DNA- code for rRNA, darker stained sections are either pars fibrosa (densely packed ribonucleoprotein, contains a large amount of primary transcripts of Rrna genes), pars granulosa- densely staining maturing ribosomes

-during the early stages of embryonic development when proliferation is taking place, the nucleolus is very large as cells are actively producing a large amount of proteins

Nuclear matrix

Fills space between chromatin and nucleoli in nucleus. Mostly has proteins, metabolites, ions

Cytoplasm

-compoased of matrix (cytosol) which are embedded with organelles, cytoskeleton and deposits of carbohydrates, lipids and pigments

Plasma membrane

-membrane made of phospholipids bilayer-hydrophobic tails and hydrophilic head, cholesterol breaks up structure and makes it more fluid, proteins

-7.5 to 10nm thickness, visible only in electron microscope

-has intergral and peripheral proteins

-function

selective barrier that regulates passage of certain materials into and out of cell

plays important role in cell recognition

mass transfer occurs at the lipids, endocytosis and exocytosis

Mitachondria

-spherical organelles, 0.5 micrometers wide that can be up to 10 micrometer long

-under electron microscope- outer and inner mitochondrial membrane--mitochondrial membranes have a large number of protein molecules

- inner mitochondrial membrane folds to have cristae. -cristae are flat and shelf like, they increase the surface area- contains enzymes and other components of oxidative phosphorylation and electron transport systems. The number of cristae is releted to the energy activity of the cell

-Between the outer and inner membrane in the intermembrane space

-inner mitochondrial membrane enclose the matrix, rich in protein and contain circular DNA- double stranded DNA synthesised within the mitochondria and duplication is independent of nuclear DNA, also contains 3 types of RNA- mRNA, tRNA and Rrna

-mitachondrial ribosomes, smaller than cystolic ribosomes enables for protein synthesis to occur in the matrix- but only a small proportion of mitochondrial proteins is produced locally .

-matrix also has Enzyme for the Krebs cycle and for fatty acid beta oxidation found in the matrix

-in the process of mitosis each daughter cell receives half the mitochondria from parent cell- new mitochondria originate from pre existing mitochondria by growth and fission of the organelle

Function

-accumalate in parts of the cell where energy utilisation is high such as the apical surface of ciliated cells/ middle of a spermatozoa or the base of ion transferring cells

-transform chemical energy of metabolites present in the cytoplasm into energy- 50% into ATP, rest released as heat used to maintain body temp, krebs cycle and electron transport chain occurs.

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