B2.2Theme B — Form and function
Organelles and compartmentalization — IB Biology SL Revision Notes & Topical Questions
5 min read 11 parts Lesson 5 of 13 2 quizzes
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
B2.2 Organelles and compartmentalization — IB BIOLOGY SL notes: Explain why eukaryotic cells are divided into organelles and how compartmentalisation improves cell function.Organelle = a discrete, specialised part of a cell with a characteristic structure suited to its roleThe cy…
- Membrane-bound examples
- Not organelles
- Higher concentrations
- Separation of harmful substances
- Optimal conditions
- Separate control
Part 1 of 9Organelles
Organelle
Organelle a discrete, specialised part of a cell with a characteristic structure suited to its role
- The cytoplasm of eukaryotic cells is divided into membrane-bound compartments.
- Each organelle is bound by a single or double membrane.
- Different organelles do different tasks, so the cell works as a coordinated whole.
Membrane-bound examples
- Membrane-bound examples nucleus, mitochondria, chloroplasts, rough and smooth ER, Golgi apparatus, lysosomes, vacuoles and vesicles
Ribosomes
- Ribosomes counted as organelles although they have no membrane, because they are distinct units
Not organelles
- Not organelles the cell wall, the cytoskeleton and the cytoplasm
- The cell wall lies outside the cell.
- The cytoskeleton is a diffuse network.
- The cytoplasm is the medium itself.
Part 2 of 9Which structures count as organelles
| Structure | Organelle? | Membrane | Main role |
|---|---|---|---|
| Nucleus | Yes | Double | Stores DNA and is the site of transcription |
| Mitochondrion | Yes | Double | Aerobic respiration and ATP production |
| Chloroplast | Yes | Double | Photosynthesis |
| Ribosome | Yes | None | Protein synthesis |
| Lysosome | Yes | Single | Digestion of materials |
| Vesicle | Yes | Single | Transport and storage |
| Golgi apparatus | Yes | Single | Modifies and packages proteins |
| Cell wall | No | None | Support and resists bursting |
| Cytoskeleton | No | None | Shape and movement |
| Cytoplasm | No | None | Site of many reactions |
Part 3 of 9Advantages of compartmentalisation
Advantages of compartmentalisation
Higher concentrations
- Higher concentrations enzymes and substrates are localised, so they react faster in a small volume
Separation of harmful substances
- Separation of harmful substances digestive enzymes are kept in lysosomes so they do not digest the cell
Optimal conditions
- Optimal conditions each compartment keeps its own pH or conditions
Separate control
- Separate control different metabolic pathways can be controlled separately
Specific membranes
- Specific membranes each membrane carries specific proteins such as pumps or electron carriers
Flexibility
- Flexibility the cell can change the number and location of organelles to suit its needs
- Active muscle cells have many mitochondria.
Structure fits function
- Structure fits function each organelle's shape reflects its job
- Double membranes surround the nucleus, mitochondria and chloroplasts.
- Inner membranes may be folded to add area.
- Vesicles carry proteins between the ER, Golgi and plasma membrane.
Part 4 of 9Nucleus and cytoplasm kept apart
Nucleus and cytoplasm kept apart
Nuclear envelope
- Nuclear envelope a double membrane whose pores regulate the exit of mRNA and the entry of proteins
Prokaryotes
- Prokaryotes no nucleus, so ribosomes attach to mRNA as it forms
- Transcription and translation occur together, allowing rapid responses to environmental stimuli.
Eukaryotes
- Eukaryotes transcription and translation occur in different compartments
- mRNA is modified inside the nucleus before it meets a ribosome.
- This reduces errors in the mRNA code and in the protein made.
- Gene expression can be regulated by controlling mRNA export.
- DNA is shielded from some cytoplasmic enzymes.
Part 5 of 9Gene expression in a eukaryotic cell
1
DNA unwinds in the nucleus.
2
RNA polymerase builds pre-mRNA.
3
Introns are removed from the pre-mRNA.
4
The mature mRNA leaves through a nuclear pore.
5
A ribosome binds the mRNA in the cytoplasm.
6
The polypeptide is assembled.
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | Absent, DNA in the nucleoid | Present and enclosed by an envelope |
| Timing of translation | Can start while mRNA is still being made | Only after the mRNA leaves the nucleus |
| mRNA processing | Little or none | Introns removed before export |
| Membrane-bound organelles | None | Many |
| Ribosomes | 70S | 80S in the cytoplasm |
Part 6 of 9Compartments within the cytoplasm
Compartments within the cytoplasm
Lysosome
- Lysosome a single membrane around hydrolytic enzymes such as proteases, lipases and nucleases
- Proton pumps keep the interior acidic at about pH 5.
- The enzymes work best there and are much less active in the cytosol at about pH 7.2.
- If a lysosome leaks, the higher pH of the cytosol reduces enzyme activity.
Autophagy
- Autophagy lysosomes digest worn-out organelles and the components are recycled
Nitrogenase
- Nitrogenase an enzyme for nitrogen fixation that oxygen damages
- It is kept in an oxygen-poor region away from aerobic reactions.
Incompatible reactions
- Incompatible reactions membranes let pathways with conflicting needs run side by side
- Specific metabolites can build up only where they are needed.
Part 7 of 9Fate of an engulfed bacterium
1
The membrane surrounds the bacterium by endocytosis.
2
A phagocytic vacuole forms inside the cytoplasm.
3
Lysosomes move to the vacuole and fuse with it.
4
Hydrolytic enzymes work at the acidic pH inside.
5
The bacterium is digested and useful products are absorbed.
6
Waste is released by exocytosis.
- Benefit
- the vacuole keeps harmful material away from the cytoplasm until a lysosome can digest it safely
Part 8 of 9Cell fractionation
Nature of science
- Nature of science new techniques often open new fields, and organelle function was little known before ultracentrifuges
Cell fractionation
Cell fractionation breaking cells open and spinning the mixture to obtain pure samples of each organelle
1
Blend the tissue in a cold, isotonic buffer so cells burst but organelles survive (homogenisation).
2
Pass the homogenate through gauze to remove unbroken cells and debris (filtration).
3
Spin the filtrate at rising speeds in an ultracentrifuge.
4
The heaviest parts form a pellet first: nuclei, then mitochondria and chloroplasts, then ER fragments, then ribosomes.
5
Test each pellet for its enzymes and reactions.
Part 9 of 9More from the specification
More from the specification
Complex
- Two or more molecules joined, such as an antibody and antigen.
Ultrastructure
- The internal structure of a cell seen with an electron microscope.
Key terms
| Term | Meaning |
|---|---|
| Organelle | A distinct subunit of a cell with its own structure and a specialised role. |
| Compartmentalisation | Division of a cell into membrane-bound regions so that different conditions and processes can exist side by side. |
| Nuclear pore | A protein-lined channel in the nuclear envelope that controls traffic between nucleus and cytoplasm. |
| Lysosome | A membrane-bound vesicle holding hydrolytic enzymes that work best at low pH. |
| Phagocytic vacuole | A vesicle formed when a cell engulfs a particle or microbe by endocytosis. |
| Cell fractionation | Breaking cells open and spinning the mixture to separate organelles. |
| Ultracentrifuge | A machine that spins samples very fast so that parts separate by mass and density. |
| Nitrogenase | An oxygen-sensitive enzyme used in nitrogen fixation. |
Avoid these mistakes
1
Counting the cell wall as an organelle — it lies outside the cell and is not an organelle
2
Saying ribosomes are not organelles because they lack a membrane — they are distinct units and count
3
Saying prokaryotes process mRNA before translation — translation begins while the mRNA is still being made
4
Saying lysosome enzymes work best at cytosol pH — they work best at about pH 5
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