B.2.2 - ORGANELLES AND COMPARTMENTALISATION Flashcards
Define organelles
Organelles are discrete subunits of cells that are adapted to perform specific functions.
They can be solid structures or membrane-bound sacs enclosing specific combinations of dissolved solutes
List organelles:
- not enclosed be a membrane
- enclosed by a single membrane
- enclosed by a double membrane
- NOT organelles – discrete structures within the cell
NOT ENCLOSED BY A MEMBRANE
1. ribsomes
2. centrioles
3. nucleolus
ENCLOSED BY A SINGLE MEMBRANE
1. plasma membrane
2. vacuole
3. vesicle
4. endoplasmic reticulum (smooth, rough)
5. golgi apparatus
6. lysosomes
ENCLOSED BY A DOUBLE MEMBRANE
1. nucleus
2. mitochondria
4. chloroplast
NOT ORGANELLES – DISCRETE STRUCTURES
1. cell wall (extracellular, outside of the cell)
2. cytoplasm (many metabolic reactions, no specific function)
3. cytoskeleton (not a discrete structure)
When did the study of organelle function become possible?
The invention of ultracentrifuges for cell fractionation allowed for the study of specific individual organelle functions:
- Cells are disrupted in a homogeniser (blender) and the resulting mixture, homogenate, is centrifuged
- Centrifugation separates the organelles
- Heavier organelles fall to the bottom to form a pellet
- The fluid/supernatant is removed and centrigued at higher speeds to separate remaining organelles
Outline the advantage of the separation of the nucleus
- The activities of gene transcription, translation, and post-translational modification of mRNA can happen before the mRNA meets the ribosome in the cytoplasm
- The cytoplasm contains enzymes that degrade mRNA
- Prokaryotes lack a nuclear membrane and mRNA produced by transcription has no post-transcriptional modification and immediately is translated by the cytoplasmic ribosomes
List the separate compartments and function as a result of membrane-bound organelles
- Mitochondrion/chloroplast:
- enzymes (ATPase) is concentrated due to folding
- separation of pH decreased (acidic) due to H+ ions accumulating
- localised reactions - Vescile
- transports proteins - Nucleus
- protects DNA
Outline the functions of membranes
- Biological membranes act as barriers between aqueous solutions
- semi-permeable
- membrane around an organelle creates a compartment with controlled conditions
Outline the advantages of compartmentalisation
- Concentration of metabolites: enzymes and substrates can be localised and much more concentrated
- Separation of incompatible biochemical processes
- Localised conditions: pH (H+ ions) and other such factors can be kept at optimal level - the optimal pH level for one process in one part of the cell
- Toxic/damaging substances can be isolated: like digestive enzymes that are stored in lysosomes
- Transport: materials can be transported within the cell
Ex: phagocytic vacuole/phagosome: digestions of the contents in a food vacuole
Outline the benefit of the double membrane of the nucleus and the functions of the nucleus
BENEFIT:
1. Pores allow movement of mRNA, tRNA, proteins, and enzymes (helicase) out of the nucleus
- Outer membrane of the nucleus is continuous with a rough endoplasmic reticulum
FUNCTION:
1. Contains genetic/herditary information
- DNA within nucleus found is chromatin: less condensed from of cell’s DNA that organised to form chromosomes during phosphse of mitosis/meiosis
- Contains 1+ nucleoli, an organelle that synthesised ribsoomes
State the structure of ribsomes
Ribosomes consist of rRNA and proteins. It has 2 subunits: one large, and one small.
Large subunit:
- peptidyl transferase enzyme
- 3 binding sites of tRNA:
E site (exit): tRNA molecule exists
P site (peptidyl): growing polypeptide chain
A site (aminoacyl): new tRNA molecules bind
Small subunit:
- binding site for mRNA
State the function of free and bound ribosomes
FREE: ribosomes floating in the cytoplasm - produce proteins for use within the cell
BOUND: round to rough ER - produces proteins for export out of the cell/use in lysosomes
Outline the structure and function of the golgi apparatus
STRUCTURE:
1. flattened membrane-enclosed sacs called cisternae
2. consist of cis side (faces nucleus) where vesicles are received
3. Fuse and trans side (faces cytoplasm) where vesicles are released and formed
FUNCTION:
1. Processes, modifies, and sorts proteins for export or lysosome use:
- adds a carbohydrate to make a glycoprotien
- OR combines with other polypeptides to form a quaternary structure protein
- Lysosomes or vacuoles for digestive enzymes in the cell
- Exports proteins out of the cell by exocytosis like protein hormones, insulin, glucagon
- Integral proteins within the plasma membrane like channels, pumps, adhesion proteins, receptor proteins
State the structure and function of vesicles
STRUCTURE:
1. formed from infolding of membrane
2. protein clathrin causes invagination, pinching of the membrane:
- creates a coat that helps phospholipid create a rounded shape as vesicle is forming
- once vesicle is formed, the clathrin coat is removed
FUNCTION:
1. Transport vesicle: Transports proteins and lipids from one location to another WITHIN CELL (ex. from er to golgi apparatus)
- Secretory vesicle: Transports proteins OUT of cell (ex. from Golgi to lysosomes/vacuoles for digestive enzymes)
- Storage of digestive enzymes in lysosomes
- Takes materials into cell by endocytosis (receptor-mediated)
- Vesicle fusion: merging of vesicle with another organelle/part of cell membrane - adds phospholipids to make the target structure larger, adding membrane-bound proteins to plasma membrane/passing contents to the Golgi apparatus
Outline the process and linkages between structure/function of the organelles
- NUCLEUS: genes instruct the cell what proteins to make -> required proteins is transcribed from DNA to mRNA -> mRNA leaves nucleus through nuclear pores and delivers information to ribosomes
- RIBOSOMES: use mRNA information as a template to translate to make the correct polypeptide/protein
- VESICLES: contains newly-synthesised proteins, budded-off from rough endoplasmic reticulum
- VESICLES: from RER fuse to form golgi cisterna at the CIS face of the golgi
- GOLGI: sorts proteins and may add carbohydrate chains to form glycoproteins
6a. LYSOSOMES bud-off from the Golgi containing hydrolytic enzymes
6b: SECRETORY VESICLES containing (glyco)proteins bud off from the TRANS-face of Golgi
- EXOCYTOSIS: vesicles fuse with cell membrane and release proteins outside the cell
