HL Bio - Cellular Bio (II): B2.2: Organelles and Compartmentalisation

Define organelles

discrete structures that are adapted to perform one or more vital functions

Suggest two reasons why eukaryotic cells have more organelles than prokaryotic cells

1. Smaller - concentrate on limited range of functions

2. To allow functions to be integrated - rapid function e.g. simultaneous transcription and translation

Explain why 3 structures may not be considered organelles

1. Cell wall: extracellular

2. Cytoskeleton: narrow protein filaments are spread through much of the cell; not discrete structure

3. Cytoplasm: includes many different structures and functions

Describe two methods to separate organelles

1. Kitchen blender - bursts cells and releases organelles to homogenise tissues

2. Differential centrifugation - separate organelles based on density and size

Nucleus - mitochondria and chloroplast - membrane - ribosomes

Explain why prokaryotic cells can carry out simultaneous transcription and translation while eukaryotic cells cannot

Eukaryotic: nuclear envelope double membrane prevents mRNA from leaving nucleus via nuclear pores for translation, so that it can carry out post transcriptional modification

Prokaryotic: no membrane bound organelles - no nuclear envelope - allows mRNA to be translated once synthesised

Describe the advantages of compartmentalisation [4]

1. Efficiency of metabolism - enzymes concentrate over smaller volume = higher enzymatic activity

2. Localised regions - maintain different internal environments e.g. pH, ionic concentrations for optimal enzymatic activity for metabolism

3. Isolation of toxic substances - prevents organelles e.g. lysosomes with hydrolytic enzymes from damage to cell

4. Flexibility and stabilization - allows cell to perform multiple complex functions at once, promoting cell differentiation and cell specialisation

Outline the adaptations of the mitochondrion [6]

1. Double membrane - outer controls movement of substances, inner provides space for ETC

2. Cristae - increase SA:V for ATP synthase and ETC - higher rate of ATP synthesise

3. Small volume in IM Space - rapid accumulation of protons

4. Compartmentalisation of mitochondrial matrix - enzymes and DNA for Kreb's Cycle

5. Mitochondrial DNA - allows mitochondrion to synthesise enzymes and key proteins without relying on nuclear DNA

6. ATP synthase - catalyse synthesise of ATP from ADP + Pi

Describe the adaptations of the chloroplasts [8]

1. Double membrane - controls movement of substances

2. Thykaloid membranes - large area for absorption of light by chlorophyll

3. Granum - larger SA:V for absorption of light

4. Small volume in thykaloid space - rapid accumulation of protons

5. 70S ribosomes and DNA in stroma - synthesise of enzymes and proteins for photosynthesis without relying on nuclear DNA

6. Compartmentalisation of stroma - pH and enzymes required for Calvin's Cycle

Describe the adaptations of the nuclear envelope [5]

1. Compartmentalisation of nucleus - protects and isolates DNA; controlled gene expression

2. Inner membrane - maintains shape of nucleus; supports the organisation of chromatin

3. Nuclear pores - controls the rate of transcriptions and translation

4. Continuous with RER - facilitates transport of proteins

5. Breaks down during cell devision - allows chromosomes to move to poles of the cell

Describe the adaptations of the Golgi apparatus [5]

1. Membrane flattened sacs called cisternae - higher SA:V for protein processing and enzymatic activity

2. Stacked cisternae - processing and modification of molecules

3. Enzyme specific compartments - stepwise modification of molecules

4. Faces - cis face faces RER, trans face faces plasma membrane

5. Associated vesicles - transports material to and from Golgi apparatus

Outline the uses of RER ribosomes and free ribosomes:

RER

1. secretion out of the cell

2. protein membranes

3. lysosomes

Free

- for use within the cell

Describe the how clathrin forms vesicles in receptor mediated endocytosis

Provides mechanical support and energy to deform membrane - pull inwards - plasma membrane curves until vesicle is formed