#13332 - Structure And Function Of Cell Organelles - Organisation of the Body

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 -50nm (perinulear cisterna) - TEM

-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

-each nucleosome- 166bp of DNA wound around histone, octamer of H2A,H2B,H3,H4 and H1 is bound to DNA

-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.

-Chromatin is studied using phytohaemogluttin which stimulates mitosis and colchine which inhibits mitosis

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.

Endoplasmic reticulum

Interconnected tubular membrane bound sacs known as cisternae.

RER

-Prominent in cells specialised for protein synthesis

- parallel stacks of flattened cisternae whose membrane is continuous with the outer membrane of the nuclear envelope

-there is polyribosomes on the cystolic surface of the RER’s membrane- presence of these ribsomes gives the RER basophilic properties

Functions:

-specialised for synthesis and secretion of protein -segregate proteins not destined for the cytosol- manufactures lysosomal enzymes, secreted proteins and integral proteins

-lumen of the RER, proteins are folded such as the haemoglobin to produce specific shapes. Also lumen is in control for protein quality checks.

-initial glycosylation of glycoproteins

-synthesis of phospholipids

-post translational modifications of protein

SER

-The SER is continuous with the RER

-SER lacks the associated polyribosomes- appears smooth rather than granular

-the cisternae are more tubular and are distributed within the cytoplasm

Functions:

-In cells that produce steroid hormones (adrenal cortex)- SER has enzymes for steroid synthesis

-SER abundant in the liver where it oxidises, conjugates and methylates hormones and toxic substances in order to degrade them- contains cytochrome P450 – has a role in detoxification of drugs and toxic compounds especially in the liver.

-synthesises phospholipids for all cell membranes-

-in the muscle it has an important role in releasing calcium ions which is needed for muscle contraction

-involved in packing and delivering proteins to the golgi apparatus

-site of membrane lipid synthesis, in liver the glycogen stored as granules on surface of smooth ER to b broken down to glucose

Ribosomes

Structure

-20-30 nm in size

-sites in cytoplasm rich in ribosomes stain intensely with basic dyes such as methylene and hematoxylin

-made up of complexes of four types RNA and proteins- form 2 different sized subunits

-2 types of ribsomes- one class found in prokaryotes/mitochondria/chloroplasts- 70S, other class is found in eukaryotic 80S

-eukaryotic cells- RNA of ribosomes are synthesised within the nucleus, proteins are synthesised in the cytoplasm then enter the nucleus where they associate with rRNAs. Subunits then leave the nucleus via the nuclear pores

-In eukaryotic cells has 60S and 40S. The smaller subunit reads the mRNA whereas the larger subunit binds to the amino acids together to form a polypeptide

-found attached to the Rough Endoplasmic Reticulum or in the cytoplasm

-responsible in manufacturing proteins. The mrna that leaves nucleus binds to the two subunits and starts assembling the protein.

Golgi Apparatus

Made up of stack of smooth flattened membrane bound sacs- cisterane.

The structure displays polarity- Golgi cisternae nearest to the RER is called the cis face, opposite end called trans face

-cytochemical methods and electron microscopy has shown that the cis and trans faces have different enzymes present-.

Function

-posttranslational modifications, packages and sends molecules to right destinations

- proteins delivered from RER and transports them through golgi apparatus and modified by ‘glycosylation’ before entering trans-golgi apparatus for sorting and delivering

- transports lipids around cells, and creates lysosomes.

-Enzymes within cisternae modify proteins by adding carbohydrates and...