Organelles & Compartmentalisation

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Cell Organelles

Eukaryotic cells have a more complex ultrastructure than prokaryotic cells, with the cytoplasm divided into membrane-bound compartments called organelles.
Organelles are bound by either a single or double membrane.
Structures without a membrane are not considered organelles: cell wall, cytoskeleton, and cytoplasm.
Compartmentalised organelles include the nucleus, vesicles, ribosomes, and the plasma membrane.
Compartmentalisation allows enzymes and substrates to be localised at higher concentrations, and keeps damaging substances separated (e.g. digestive enzymes stored in lysosomes).
It also maintains optimal conditions for processes (e.g. optimal pH for digestive enzymes) and allows the number and location of organelles to be altered based on cell requirements.
Organelles can become specialised for specific functions, with structures adapted to help them carry out their roles.
The separation of organelles by membranes allows each organelle to carry out its own chemical reactions without interference from the rest of the cell.

The nucleus

Cell Fractionation

Progress in understanding organelle function followed the invention of the ultracentrifuge and the development of cell fractionation.
Cell fractionation separates cell compartments so they can be studied individually, and requires a pure sample containing only the specific organelle.
The three stages are: homogenisation (breaking up the sample with a homogeniser), filtration (through gauze), and ultracentrifugation (spinning the filtrate in a centrifuge).
A centrifuge separates materials by spinning; the speed is altered to separate components based on their molecular weight.
Before the ultracentrifuge was invented, research into separate organelles was limited.

Cell Compartmentalisation

The nucleus is a key organelle distinguishing eukaryotic cells from prokaryotic cells, allowing processes to occur more efficiently.
In prokaryotes, transcription and translation occur simultaneously, allowing rapid responses to environmental stimuli.
In eukaryotes, these processes occur separately due to the compartmentalisation of the nucleus.
mRNA is modified in isolation within the nucleus before it contacts a ribosome, reducing the chance of errors in the mRNA code and the resulting protein.
The cytoplasm is not an organelle, but its separation from organelles via membranes is advantageous.
Organisation into discrete membrane-bound organelles allows separation of incompatible biochemical processes, ensuring pathways run smoothly without interference.
For example, nitrogenase (used for nitrogen fixation in plant cells) is sensitive to oxygen and is positioned in an anaerobic part of the cytoplasm away from aerobic reactions.
Lysosomes contain lytic enzymes that could harm the cell if not contained by the lysosome membrane; during endocytosis a phagocytic vacuole forms around harmful substances, keeping them separate until a lysosome digests them.

Adaptations of Mitochondria

Mitochondria are rod-shaped organelles 0.5–1.0 μm in diameter; they are the site of aerobic respiration and synthesise ATP.
ATP synthesis occurs during oxidative phosphorylation, relying on the electron transport chain and ATP synthase.
Mitochondria have two phospholipid membranes: the outer membrane is smooth and permeable to small molecules; the inner membrane is folded into cristae, less permeable, and is the site of the electron transport chain and ATP synthase.
The intermembrane space has a low pH due to a high concentration of protons; the concentration gradient across the inner membrane is essential for ATP synthesis.
The matrix is an aqueous solution containing ribosomes, enzymes, and circular mitochondrial DNA.
The double membrane and small volume of intermembrane space allow concentration build-up of hydrogen ions required for respiration.
The large surface area of cristae holds many electron transport chain proteins and ATP synthase enzymes, enabling more ATP production.
More active cell types can have larger mitochondria with longer, more tightly packed cristae; muscle cells have more mitochondria per cell than fat cells.
Compartmentalisation of enzymes and substrates in the matrix ensures reactions like the Krebs cycle happen more efficiently.

Structure of a mitochondrion

Adaptations of Chloroplasts

Chloroplasts are the organelles in plant cells where photosynthesis occurs; they are roughly 2–10 μm in diameter (larger than mitochondria).
Each chloroplast is surrounded by a double-membrane envelope; the outer membrane is permeable to ions and small molecules, while the inner membrane contains selective transport proteins.
The stroma is a cytosol-like fluid containing CO₂, sugars, enzymes, and other molecules; starch grains or lipid droplets may be present after photosynthesis.
A separate membrane system in the stroma consists of flattened fluid-filled sacs called thylakoids, which stack to form grana (singular: granum), connected by stroma lamellae.
Thylakoid membranes contain pigments, enzymes, and electron carriers; pigments are arranged in light-harvesting clusters called photosystems.
In a photosystem, pigment molecules are arranged in funnel-like structures; energy passes down to the primary pigment reaction centre.
Stroma adaptations: gel-like fluid contains enzymes for the light-independent stage; enzymes and substrates of the Calvin cycle are compartmentalised; the stroma surrounds the grana for rapid transport of products.
Grana adaptations: large surface area for many photosystems, maximising light absorption and providing membrane space for electron carriers and ATP synthase.
Other adaptations: chloroplast DNA codes for some proteins and enzymes used in photosynthesis; ribosomes allow translation of these proteins; the inner membrane controls flow of molecules; the thylakoid space has a very small volume so a proton gradient develops quickly.

Chlorophyll within chloroplasts

Organelles in Protein Synthesis

The nucleus is separated from the cytoplasm by a double membrane called the nuclear envelope, studded with nuclear pores.
Nuclear pores allow mRNA and ribosomes to travel out, and enzymes (e.g. DNA polymerases) and signalling molecules to travel in.
The outer membrane of the nucleus is continuous with the endoplasmic reticulum; ribosomes attached to sections form the rough endoplasmic reticulum, while sections without ribosomes form the smooth endoplasmic reticulum.
During mitosis and meiosis, the nuclear membrane breaks into vesicles during prophase and reforms at telophase.
Ribosomes are found freely in the cytoplasm of all cells, or bound to the ER to form rough ER (only in eukaryotic cells); they are the site of protein synthesis.
Ribosomes consist of a large and a small subunit composed of protein and ribosomal RNA (rRNA); protein provides structure, and rRNA facilitates binding of mRNA and tRNA and catalyses peptide bond formation.
Ribosomes have three tRNA binding sites and one mRNA binding site; mRNA sits in a groove between the subunits and the ribosome moves along, forming a polypeptide.
Free ribosomes synthesise proteins for use within the cell (cytosol, mitochondria, chloroplasts); membrane-bound ribosomes synthesise proteins destined for lysosomes or secretion.
Signal sequences in the growing polypeptide chain direct the free ribosome to the ER; signal recognition proteins bind and pause translation, the ribosome binds to an ER receptor, translation re-initiates, and the polypeptide moves inside the ER.
The synthesised protein can be carried via a vesicle to the Golgi apparatus before being secreted.
The Golgi apparatus consists of flattened sacs called cisternae; it modifies proteins and lipids before packaging them into Golgi vesicles.
The cis side faces the ER and receives vesicles; the trans side faces the plasma membrane and sends out modified products.
Proteins passing through the Golgi are usually exported (e.g. insulin), put into lysosomes, or delivered to membrane-bound organelles.

Vesicle Formation

Vesicles are membrane-bound sacs used for transport and storage.
Types include peroxisomes (digest fatty acids), lysosomes (digest cellular waste or harmful substances), transport vesicles (move molecules within the cell), and secretory vesicles (transport substances out via exocytosis).
Clathrins are proteins that help with vesicle formation; they line vesicles transporting molecules between membrane-bound compartments.
Order of events: (1) a clathrin-coated pit forms on the cell membrane; (2) receptor proteins bind target molecules; (3) cytoskeleton proteins help the pit deepen and seal off, trapping target molecules; (4) a vesicle is formed.
Vesicles are not the same as vacuoles: vacuoles are larger and their membrane cannot fuse with other cellular membranes, unlike vesicle membranes.

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1.Which of the following are not considered organelles?
Easy
- AVesicles
- BPlasma membrane
- CCell wall
- DRibosomes
2.Which of the following describe how the mitochondria are adapted for the production of ATP by aerobic cell respiration? I. Large surface area of cristae. II. Mitochondria have a double membrane with a large volume of intermembrane space. III. Compartmentalisation of enzymes and substrates of the Krebs cycle in the matrix.
Medium
- AI and II only
- BII and III only
- CI and III only
- DI only
3.What is an advantage of the separation of the nucleus and cytoplasm into separate compartments in eukaryotes?
Medium
- APost-transcriptional modification of mRNA can happen before the mRNA meets ribosomes in the cytoplasm.
- BPost-transcriptional modification of mRNA can happen after the mRNA meets ribosomes in the cytoplasm.
- CPost-transcriptional modification of mRNA happens immediately upon arrival at ribosomes.
- DPost-transcriptional modification of mRNA can occur in the cytoplasm.
4.Which of the following is true of ribosomes?
Medium
- AProteins for transport are synthesised by free ribosomes.
- BRibosomes consist of a large and a small subunit composed of protein and transfer RNA (tRNA).
- CRibosomes exist in the cells of all living organisms.
- DRibosomes on the rough endoplasmic reticulum synthesise proteins for retention within the cell.
5.Vesicles are membrane-bound sacs used for transport and storage. Clathrins are proteins that help with the formation of vesicles. What is the correct order of events for vesicle formation?
Medium
- A clathrin coated pit is formed on the surface of the cell membrane
- Receptor proteins on the cell surface bind to the target molecules
- Cytoskeleton proteins help the clathrin pit to deepen and eventually seal off, trapping the target molecules inside
- A vesicle is now formed
6.The chloroplast is adapted for photosynthesis. Which of the following is not an adaptation of the chloroplast for photosynthesis?
Medium
- AA large surface area of thylakoid membranes with photosystems.
- BA large volume of fluid inside thylakoids.
- CCompartmentalisation of enzymes and substrates of the Calvin cycle in the stroma.
- DChloroplast DNA that contains genes which code for some of the proteins and enzymes used in photosynthesis.
7.The cell wall is considered an organelle.
Easy
True or false?
8.Which of the following are advantages of compartmentalisation in eukaryotic cells? (Select all that apply.)
Medium
- AEnzymes and substrates can be localised at higher concentrations.
- BDamaging substances such as digestive enzymes can be kept separated from the rest of the cell.
- COptimal conditions such as pH can be maintained for specific processes.
- DAll cellular reactions occur in a single compartment.
- EThe number and location of organelles cannot be altered.

Mehr Fragen

9.What is the function of the mitochondria?
Easy
- ATo synthesize ATP
- BTo synthesize proteins
- CTo digest cellular waste
- DTo modify proteins for secretion
10.Match each organelle with its correct description.
Medium
- Nucleus
- Mitochondria
- Chloroplast
- Golgi apparatus
- Surrounded by a double membrane and contains the cell's DNA
- Rod-shaped organelle 0.5–1.0 μm in diameter, site of aerobic respiration
- Has a thylakoid membrane with photosystems and compartmentalised enzymes
- Made of flattened sacs but does not have a double membrane
11.Which of the following correctly describes the outer membrane of a mitochondrion?
Medium
- ASmooth and permeable to several small molecules
- BFolded into cristae and less permeable
- CThe site of the electron transport chain
- DThe location of ATP synthase
12.Which of the following is a role of the Golgi apparatus?
Medium
- AModify proteins and lipids before packaging them into vesicles
- BSynthesise proteins from mRNA
- CProduce ATP by oxidative phosphorylation
- DAbsorb light energy for photosynthesis
13.Which of the following are functions of the nuclear pores? (Select all that apply.)
Medium
- AAllow mRNA to travel out of the nucleus
- BAllow ribosomes to travel out of the nucleus
- CAllow DNA polymerases to travel into the nucleus
- DAllow signalling molecules to travel into the nucleus
- EAllow ATP synthase to enter the nucleus
14.Which of the following statements about the stroma of a chloroplast are correct? (Select all that apply.)
Medium
- AIt is a gel-like fluid containing enzymes that catalyse the light-independent reactions
- BIt contains enzymes and substrates of the Calvin cycle
- CIt surrounds the grana, making transport of products from the light-dependent stage rapid
- DIt contains the electron transport chain of the light-dependent stage
- EIt is the site of the proton gradient used to generate ATP
15.In a chloroplast, the thylakoid space has a very small volume, allowing a proton gradient to develop very quickly.
Easy
True or false?
16.The inner membrane of the chloroplast envelope is freely permeable to all ions and molecules.
Medium
True or false?
17.Match each organelle with its correct function.
Medium
- Lysosome
- Peroxisome
- Secretory vesicle
- Golgi apparatus
- Contains lytic enzymes that digest cellular waste or harmful substances
- Contains enzymes that digest fatty acids
- Transports substances out of the cell via exocytosis
- Modifies proteins and lipids before packaging them into vesicles
18.Place the stages of cell fractionation in the correct order.
Medium
- Homogenisation
- Filtration
- Ultracentrifugation
19.A scientist homogenises liver tissue and then centrifuges the homogenate at increasing speeds. After the first low-speed spin, the pellet is discarded and the supernatant is spun again at a higher speed. Which of the following best explains why the supernatant is kept rather than the pellet after the first spin?
Medium
- AThe pellet contains the smallest organelles, which are needed for the next step.
- BThe supernatant contains the organelles that have not yet been pelleted and can be separated at higher speeds.
- CThe pellet contains soluble enzymes that must be removed before further centrifugation.
- DThe supernatant contains only broken cell debris that must be discarded.
20.A cell biologist treats a sample of cells with a drug that prevents the formation of clathrin-coated pits. Which process is most directly affected?
Medium
- AThe synthesis of proteins on free ribosomes.
- BThe uptake of target molecules by receptor-mediated endocytosis.
- CThe movement of mRNA out of the nucleus through nuclear pores.
- DThe translation of proteins on the rough endoplasmic reticulum.
21.A researcher is studying a plant cell and observes a membrane-bound compartment with the following features: it is surrounded by a double membrane, contains stacked flattened sacs, and has its own DNA and ribosomes. Which of the following statements about this compartment are correct? (Select all that apply.)
Medium
- AIt is the site of the light-independent reactions of photosynthesis.
- BIt contains photosystems in its thylakoid membranes.
- CIt is the main site of ATP synthesis by aerobic respiration.
- DIt can synthesise some of its own proteins using its ribosomes.
- EIts inner membrane is freely permeable to all ions and molecules.
22.In a chloroplast, the thylakoid space has a very small volume, which allows a proton gradient to develop quickly.
Medium
True or false?
23.Place the steps of cell fractionation in the correct order.
Medium
- Homogenisation of tissue using a homogeniser
- Filtration of the homogenate through gauze
- Ultracentrifugation at increasing speeds
- Collection of the purified organelle sample
24.A pancreatic cell secretes large amounts of digestive enzymes. Compared with a fat cell, which of the following would you expect to observe in the pancreatic cell?
Hard
- AFewer mitochondria and less rough endoplasmic reticulum.
- BMore mitochondria and more rough endoplasmic reticulum.
- CMore free ribosomes and fewer Golgi apparatuses.
- DFewer vesicles and a smaller Golgi apparatus.
25.A student homogenises a sample of liver tissue and filters it through gauze. The filtrate is then centrifuged. What is the purpose of filtering the homogenate through gauze before ultracentrifugation?
Medium
- ATo remove unbroken cells and large debris that would contaminate the organelle pellets
- BTo break open the cells so that the organelles are released into the filtrate
- CTo separate the organelles according to their density before centrifugation
- DTo dissolve the membranes of the organelles so that they can be isolated individually
26.During cell fractionation, a tissue homogenate is centrifuged at low speed, the pellet is removed, and the supernatant is then centrifuged at a higher speed. Which organelle would be expected in the pellet obtained after the first low-speed centrifugation?
Medium
- ANucleus
- BMitochondrion
- CRibosome
- DChloroplast
27.A researcher wishes to obtain a pure sample of mitochondria from plant tissue. Which of the following steps would be appropriate? (Select all that apply.)
Hard
- AHomogenise the tissue in a blender to break open the cells
- BFilter the homogenate through gauze to remove unbroken cells and debris
- CCentrifuge the filtrate at increasing speeds and collect the pellet containing mitochondria at the appropriate speed
- DUse a centrifuge that spins at a single low speed only
- EAdd a detergent to dissolve all organelle membranes before centrifugation
28.Place the steps of cell fractionation in the correct order, starting with the intact tissue.
Medium
- Homogenise the tissue in a blender
- Filter the homogenate through gauze
- Centrifuge the filtrate at increasing speeds
- Collect the pellet containing the desired organelle
29.A scientist studying a rare mitochondrial disease finds that the mitochondria in affected muscle cells have cristae that are much less folded than normal. Which of the following best explains the effect of this abnormality on the cells?
Medium
- ALess ATP is synthesised because there is a smaller surface area for electron transport chain proteins and ATP synthase enzymes
- BMore ATP is synthesised because the intermembrane space is larger
- CThe mitochondria can no longer replicate their DNA
- DThe mitochondria can no longer carry out the Krebs cycle because the matrix is missing
30.A cell biologist treats cells with a drug that prevents the formation of a proton gradient across the inner mitochondrial membrane. Which of the following processes will be most directly affected?
Hard
- AATP synthesis by ATP synthase during oxidative phosphorylation
- BThe Krebs cycle in the mitochondrial matrix
- CThe breakdown of fatty acids in peroxisomes
- DThe translation of proteins on mitochondrial ribosomes
31.A centrifuge separates materials by spinning them, and the speed can be altered to separate different components of the cell based on their molecular weight.
Easy
True or false?
32.In cell fractionation, the pellet obtained after the first low-speed centrifugation contains the smallest organelles such as ribosomes.
Medium
True or false?
33.Match each organelle with the speed at which it is typically pelleted during cell fractionation, from lowest to highest speed.
Medium
- Nucleus
- Mitochondrion
- Ribosome
- Lowest speed
- Medium speed
- Highest speed
34.Which of the following are adaptations of the chloroplast for photosynthesis? (Select all that apply.)
Medium
- AA large surface area of thylakoid membranes with photosystems
- BCompartmentalisation of enzymes and substrates of the Calvin cycle in the stroma
- CChloroplast DNA that contains genes coding for some proteins and enzymes used in photosynthesis
- DA large volume of fluid inside the thylakoid space
- EA single membrane envelope that is freely permeable to all ions
35.A student is asked to explain how the structure of a mitochondrion helps it carry out its function. Which of the following is the most complete explanation?
Hard
- AThe cristae increase the surface area of the inner membrane, providing more electron transport chain proteins and ATP synthase enzymes, so more ATP is produced.
- BThe cristae increase the surface area of the inner membrane, so more ATP is produced.
- CThe double membrane increases the surface area for the Krebs cycle.
- DThe matrix contains ribosomes and DNA, which directly synthesise ATP.
36.Which of the following correctly describes the role of the inner membrane of the chloroplast envelope?
Medium
- AIt contains transport proteins that control the flow of molecules between the stroma and cytosol.
- BIt is freely permeable to all ions and molecules.
- CIt is the site of the Calvin cycle.
- DIt contains the photosystems that absorb light.
37.Which of the following statements about the thylakoid space in a chloroplast are correct? (Select all that apply.)
Hard
- AIt has a very small volume.
- BA proton gradient develops across the thylakoid membrane.
- CThe small volume allows the proton gradient to develop very quickly.
- DIt contains the enzymes of the Calvin cycle.
- EIt is the site of protein synthesis by chloroplast ribosomes.
38.A cell biologist adds a drug that blocks the function of signal recognition proteins. Which of the following processes will be most directly affected?
Medium
- AThe movement of a ribosome to the endoplasmic reticulum to synthesise a secretory protein
- BThe transcription of mRNA in the nucleus
- CThe replication of mitochondrial DNA
- DThe formation of a proton gradient in the thylakoid space
39.Which of the following is not considered an organelle?
Medium
- ACell wall
- BVesicle
- CRibosome
- DPlasma membrane
40.Post-translational modifications occur more slowly in eukaryotic cells as proteins are processed by other organelles. Which organelle is involved in post-translational modifications, and what is its role?
Hard
- AGolgi apparatus — it modifies proteins and lipids before packaging them into vesicles.
- BNucleus — it modifies proteins before they are secreted.
- CMitochondrion — it modifies proteins by adding carbohydrate chains.
- DRibosome — it modifies proteins by folding them into their final shape.
41.State the name of organelle X.
Easy
42.In addition to the mitochondria, identify two other compartmentalised structures with double membranes characteristic of a eukaryotic cell.
Easy
43.Alpers disease affects approximately 1 in 100 000 individuals. There are 68.35 million people in the UK. Calculate the number of individuals in the UK with Alpers disease.
Medium
44.Complete the sentence about vesicle formation.
Easy
In vesicle formation, a ____ coated pit forms on the cell membrane, and ____ proteins help the pit to deepen and seal off.
45.A mitochondrion has a diameter of 0.8 μm. What is this diameter in nanometres?
Medium
46.Complete the sentence about the inner membrane of the chloroplast envelope.
Medium
The inner membrane of the chloroplast envelope contains ____ proteins that control the flow of molecules between the stroma and cytosol.
47.Name the protein that lines vesicles during their formation.
Medium
48.Complete the sentences about the nucleus and ribosomes.
Easy
The nucleus is surrounded by a double membrane called the ____. The outer membrane of the nucleus is continuous with the ____, and ribosomes attached to it form the rough endoplasmic reticulum.
49.A chloroplast has a diameter of 5 μm. What is this diameter in nanometres?
Medium
50.State the name of the organelle to which a ribosome must bind in order to produce a protein that will be secreted from the cell.
Medium
51.Complete the sentence about the Golgi apparatus.
Easy
The Golgi apparatus consists of flattened sacs of membrane called ____, and its role is to modify proteins and lipids before packaging them into vesicles.
52.Name the structures that allow mRNA and ribosomes to travel out of the nucleus.
Medium
53.Distinguish between the destination of proteins produced by free ribosomes and those produced by ribosomes attached to organelle X.
Medium
54.Describe the path of the protein after it is produced, until it is secreted out of the cell.
Medium
55.Describe the structure of ribosomes.
Medium
56.Define compartmentalisation.
Easy
57.Describe and explain the importance of the double membrane in mitochondria.
Medium
58.Alpers disease affects approximately 1 in 100 000 individuals. There are 68.35 million people in the UK. Calculate the number of individuals in the UK with Alpers disease.
Medium
- A752 individuals
- B820 individuals
- C684 individuals
- D616 individuals
59.A mitochondrion has a diameter of 0.8 μm. What is this diameter in nanometres?
Medium
- A960 nm
- B720 nm
- C880 nm
- D800 nm
60.A chloroplast has a diameter of 5 μm. What is this diameter in nanometres?
Medium
- A6000 nm
- B4500 nm
- C5500 nm
- D5000 nm

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