Electron Microscope
- Electron Microscope has higher resolution and higher magnification
- Magnification is the degree to which viewed image is larger than actual specimen
- Resolution is the minimum distance in which 2 points can be distinguished as separate
- Cell Size: Nucleus, Chloroplast, Mitochondrion, Lysosome, Centriole, Ribosome
Nuclear Envelope
- Double membrane perforated by nuclear pores
- Out: RNA, ribosomal subunits to form ribosomes in cytoplasm
- In: Nucleotides, ribosomal proteins, enzymes for replication/transcription
Chromatin in non-dividing nucleus is mostly heterochromatin, which is dark, to the sides, tightly coiled and not expressed.
DNA that is expressed is euchromatin, light, centre around nucleolus, loosely coiled
Nucleolus
- Site of transcription of genes from ribosomal DNA to rRNA
- Site of assembly of rRNA and ribosomal proteins into ribosomal subunits
Lysosomes
Single membrane vesicles containing hydrolytic enzymes, maintaining an acidic environment — usually seen as tiny white circles, not seen in plant cells
- Digestion of material taken in via endocytosis
- Fuses with endocytic vesicles containing food or phagosomes, useful products absorbed unwanted excreted via exocytosis - Autophagy
- Breakdown of unwanted cellular structures eg old organelles - Release of hydrolytic enzymes for breakdown of extracellular content
- Autolysis/apoptosis
- For worn cells after senescence when contents of many lysosomes released causing self digestion
Vacuole
Plants have large central vacuole surrounded by tonoplast membrane containing cell sap
1. Maintains turgor pressure of cell
2. Vacuole can enlarge with minimal cytoplasm growth to help in plant growth
3. Storage of waste products, food and digestive enzymes
Animals have small numerous food vacuoles; can also be used as contractile vacuoles
Microtubules
Hollow rods made of tubulin that grow by adding tubular subunits
1. Help maintain cellular shape by acting as cytoskeleton
2. Intracellular Transport by serving as tracks for organelles with motor proteins to move
3. Chromosome movement in cell division
4. Central component of centrioles, cilia and flagella
Centrioles
- Non-membrane bound organelle as pair of rod like structures perpendicular to each other made up of 9 triplets of microtubules
- Centrioles move to opposite poles and organise microtubules to form spindle fibres to ensure proper separation of chromosome, they thus serve as the microtubules organising centre
Ribosomes
- Site of translation of mRNA to protein with P site, A site and E site
- Consists of small subunit which has mRNA binding site and large subunit which has peptidyl transferase to catalyse peptide bond formation
- Free ribosomes float in cytosol to synthesis proteins for cytosol
- Bound ribosomes attached to RER synthesise proteins for
1. exocytosis out of cell
2. packaging within certain organelles (eg hydrolytic enzymes of lysosomes)
3. inserted into plasma membrane - whether or not ribosome is bound depends on whether mRNA has a signal peptide sequence
Rough Endoplasmic Reticulum
Single membrane bound, network of flat tubules and sacs called cisternae and studded with bound ribosomes
- Large polypeptides transported into lumen/cisternal space via protein membrane channels on RER surface (often tested in As fsr)
1. Channel protein functions as receptor to bind to signal peptide sequence on polypeptides
2. Channel protein also holds bound ribosomes in position on RER
- Protein folded into specific 3D conformation
Smooth Endoplasmic Reticulum
Single Membrane; no ribosomes and more tubular with high SA:V ratio
1. Synthesis of carbs and lipids
2. Detoxify drugs and poisons
3. Sarcoplasmic reticulum in muscle cells store Ca2+ ions
Golgi Apparatus
Convex cis face faces RER, while concave trans face faces plasma membrane
1. Glycosylation adds oligosaccharides to form glycoproteins
2. Modify existing glycoproteins/glycolipids by cleavage
3. Formation of lysosomes
4. Synthesise polysaccharides like pectin
5. Sorts and packages proteins into vesicles targeted for transport to other parts of cell/exocytosis
Overall Protein Synthesis Pathway
- Transcription of gene produces mRNA
- Translation of mRNA produces polypeptides by ribosomes that enter the lumen of cisternae of RER to be folded into their specific 3D conformation and modified (and maybe glycosylated)
- Transport vesicles bud off from RER and fuse with the cis face of Golgi Apparatus to undergo further modification, sorting and packaging
- A secretory vesicle buds off from the trans face to fuse with cell surface membrane for exocytosis/secretion or insertion of transmembrane proteins
- Alternatively, vesicle can be packaged into other organelles within cell
- Microtubules direct this movement of vesicles via cytoskeleton
