B2.2 ORGANELLES & COMPARTMENTALISATION

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:

1. not enclosed be a membrane

2. enclosed by a single membrane

3. enclosed by a double membrane

4. 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

3. 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:

1. Cells are disrupted in a homogeniser (blender) and the resulting mixture, homogenate, is centrifuged

2. Centrifugation separates the organelles

3. Heavier organelles fall to the bottom to form a pellet

4. The fluid/supernatant is removed and centrigued at higher speeds to separate remaining organelles

Outline the advantage of the separation of the nucleus

1. The activities of gene transcription, translation, and post-translational modification of mRNA can happen before the mRNA meets the ribosome in the cytoplasm

2. The cytoplasm contains enzymes that degrade mRNA

3. 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

1. Mitochondrion/chloroplast:

• enzymes (ATPase) is concentrated due to folding

• separation of pH decreased (acidic) due to H+ ions accumulating

• localised reactions

2. Vesicle - transports proteins

3. Nucleus - protects DNA

Outline the functions of membranes

1. Biological membranes act as barriers between aqueous solutions 2. semi-permeable

3. membrane around an organelle creates a compartment with controlled conditions

Outline the advantages of compartmentalisation

1. Concentration of metabolites: enzymes and substrates can be localised and much more concentrated

2. Separation of incompatible biochemical processes

3. 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

4. Toxic/damaging substances can be isolated: like digestive enzymes that are stored in lysosomes

5. 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

2. Outer membrane of the nucleus is continuous with a rough endoplasmic reticulum

FUNCTION:

1. Contains genetic/herditary information

2. DNA within nucleus found is chromatin: less condensed from of cell's DNA that organised to form chromosomes during phosphse of mitosis/meiosis

3. 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 exits

• 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: bound 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

2. Lysosomes or vacuoles for digestive enzymes in the cell

3. Exports proteins out of the cell by exocytosis like protein hormones, insulin, glucagon

4. 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)

2. Secretory vesicle: Transports proteins OUT of cell (ex. from Golgi to lysosomes/vacuoles for digestive enzymes)

3. Storage of digestive enzymes in lysosomes

4. Takes materials into cell by endocytosis (receptor-mediated)

5. 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

1. 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

2. RIBOSOMES: use mRNA information as a template to translate to make the correct polypeptide/protein

3. VESICLES: contains newly-synthesised proteins, budded-off from rough endoplasmic reticulum

4. VESICLES: from RER fuse to form golgi cisterna at the CIS face of the golgi

5. 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

7. EXOCYTOSIS: vesicles fuse with cell membrane and release proteins outside the cell