UWCSEA IB Biology 2025 - B2.2 - Organelles and compartmentalisation

B2.2.1—What are organelles? Which cell parts are considered organelles? Which are not?

Organelles as discrete (separate) subunits of cells that are adapted to perform specific functions;

nuclei, vesicles, ribosomes and the plasma membrane are considered organelles;

cell wall, cytoskeleton and cytoplasm are not considered organelles

B2.2.2—Why are the the nucleus and cytoplasm separated into different compartments? What is the advantage of this?

Separation allows for distinct processes in different compartments of Eukaryotic cells;

Gene Transcription: Occurs in the nucleus.

Translation: Happens in the cytoplasm.

Post-transcriptional modification of mRNA can occur before it reaches ribosomes in the cytoplasm;

in contrast with prokaryotes there is No separation, leading to immediate interaction between mRNA and ribosomes.

B2.2.3—What are the advantages of compartmentalisation in the cytoplasm of cells? What structures are compartmentalised?

the advantage of compartmentalisation is that the concentration of specific metabolites and enzymes in particular areas can be increased;

biochemical processes can be separated where they would negatively affect each other; enhancing efficiency and safety.

examples Lysosomes: Digestive enzymes are kept separate from other cell parts

Phagocytic Vacuoles: Contain enzymes for engulfing and digesting foreign substances; which are kept separate;

AHL Only - B2.2.4 What are the adaptations of the mitochondrion for production of ATP by aerobic cell respiration?

Studied in Unit 4

Double Membrane: Allows optimum concentrations of enzymes used in respiration eg. Krebs cycle

Small Intermembrane Space: Allows build up of high concentration of protons (H+) ions

Cristae: Large surface area and increases space for electron transport and ATP synthesis;

Matrix Compartmentalisation: Houses enzymes and substrates for the Krebs cycle, optimising conditions for ATP production;

AHL Only - B2.2.5—What are the adaptations of the chloroplast for photosynthesis?

Studied in Unit 4

Thylakoid Membranes: Large surface area with embedded photosystems for light absorption.

Thylakoid Fluid: Small volume allows for build up of high concentration of protons (H+ ions)

Stroma Compartmentalization: Contains enzymes and substrates for the Calvin cycle, facilitating glucose synthesis.

AHL Only - B2.2.6—What are the functional benefits of the double membrane of the nucleus?

Nuclear Pores: Allow selective exchange of materials between nucleus and cytoplasm;

allows mRNA to leave and for DNA to be protected within nucleus;

Allows environment to be different than in the cytoplasm, with a different concentration of nucleotides;

and for DNA to be protected;

Vesicle Formation: Membrane breaks into vesicles during mitosis and meiosis for chromosome segregation.

AHL Only - B2.2.7—What is the structure and function of free ribosomes?

Free ribosomes are made of a small and large subunit;

they are protein and RNA;

they synthesise proteins which are kept within the cell; not for export;

AHL Only - B2.2.7—What is the structure and function of the rough endoplasmic reticulum?

made of a series of flattened membranes; with ribosomes bound to it;

Responsible for processing and secretion of proteins;

Modifies proteins received from the rough endoplasmic reticulum and prepares them for export;

creates vesicles containing proteins which bud off;

AHL Only - B2.2.8—What is the structure and function of the Golgi apparatus?

Made up of stacked, flat pouches; made of membrane;

Modifies and sorts proteins and lipids;

Packages materials for different cell parts or for release from the cell; through exocytosis;

AHL Only - B2.2.9—What is the structure and function of vesicles in cells? What is clathrin?

Vesicles are small, single membrane-bound sacs within the cytoplasm;

they transport materials within the cell and to the cell membrane;

they are involved in processes like endocytosis and exocytosis;

they help store and move nutrients, enzymes, and waste;

Clathrin is a protein used in vesicle formation in cells.

Clathrin aids in vesicle budding;

it forms a Cage-like Structure around membrane areas destined to become vesicles;

creating a Clathrin-Coated Pit;

allowing the shaping and pinching off of the membrane to create a vesicle.;

after, the clathrin coat disassembles through hydrolysis into individual molecules.