Cell Organelles and Chromosomes
Nucleus
The Nucleus was first reported by Robert Brown in 1831. It controls the life and activities of the cell and is the repository of hereditary information. In animal cells, it generally occupies a central position, while in plant cells it is pushed towards the periphery by the large central Vacuole. A cell with one nucleus is called Mononucleate; cells with two nuclei are Binucleate, and those with more than two are Multinucleate. The nucleus is only visible when the cell is in the non-dividing stage, when it contains the Chromatin network and a soluble fluid called Nucleoplasm.
Central Control: The nucleus directs all cellular activities and houses the hereditary material
Position: Central in animal cells; peripheral in plant cells due to the large vacuole
Nucleate States: Mononucleate (one nucleus), binucleate (two nuclei), multinucleate (more than two)
The nucleus is enclosed by a Nuclear Membrane, which is actually a double-membrane structure called the nuclear envelope. The outer membrane is continuous with the Endoplasmic Reticulum at certain points. The outer and inner membranes are fused at intervals, creating openings called Nuclear Pores. These pores allow the exchange of materials between the nucleus and the cytoplasm. Undifferentiated cells (such as egg cells) possess about 30,000 nuclear pores per nucleus, while differentiated cells like Erythrocytes have only 3–4 pores per nucleus. Each pore has a definite structure that regulates the traffic of substances passing through it.
Double Membrane: The nuclear envelope consists of an outer membrane (continuous with ER) and an inner membrane enclosing nuclear contents
Nuclear Pores: Openings where the two membranes fuse, allowing controlled exchange of materials between nucleus and cytoplasm
Pore Number Variation: Undifferentiated cells (e.g., eggs) have ~30,000 pores; differentiated cells (e.g., red blood cells) have only 3–4
ER Continuity: The outer nuclear membrane connects directly with the endoplasmic reticulum
Functions of Nuclear Pores
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Allow mRNA to exit the nucleus to the cytoplasm for protein synthesis
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Permit ribosomal subunits assembled in the nucleolus to reach the cytoplasm
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Allow proteins (e.g., histones, transcription factors) to enter the nucleus
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Enable exchange of small molecules and ions between nucleoplasm and cytoplasm
The Nucleolus is a darkly stained body within the nucleus that lacks its own membrane boundary. It is the site of Ribosomal RNA (rRNA) synthesis and storage. The nucleolus consists of two regions: a peripheral granular area composed of precursors of ribosomal subunits, and a central fibrillar area containing large molecular weight RNA and rDNA. Ribosomes are assembled in the nucleolus and then exported to the cytoplasm through nuclear pores, making the nucleolus the 'factory of ribosomes'.
No Membrane: The nucleolus is a dense, non-membrane-bound structure inside the nucleus
rRNA Synthesis: Ribosomal RNA is transcribed and stored in the nucleolus
Peripheral Granular Area: Contains precursors of ribosomal subunits
Central Fibrillar Area: Contains high molecular weight RNA and ribosomal DNA (rDNA)
Ribosome Assembly: Complete ribosomes are assembled here and exported to the cytoplasm via nuclear pores
The Nucleoplasm is the soluble, semi-fluid material enclosed within the nuclear envelope. Along with the cytoplasm, it forms the Protoplasm. The nucleoplasm contains the chromatin network, the nucleolus, and various dissolved molecules including nucleotides, enzymes, and proteins necessary for DNA and RNA synthesis.
Composition: Semi-fluid matrix containing nucleotides, enzymes, proteins, and other molecules needed for nuclear functions
Protoplasm Component: Together with cytoplasm, the nucleoplasm constitutes the protoplasm
Chromatin Network: The diffuse, thread-like genetic material dispersed within the nucleoplasm
Endoplasmic Reticulum
The Endoplasmic Reticulum (ER) is a network of membrane-bound channels extending throughout the cytoplasm. Under the electron microscope, these channels appear continuous with the Plasma Membrane and also in contact with the Nuclear Membrane. The material inside the channels is separated from the cytoplasm by spherical or tubular membranes called Cisternae. ER exists in two morphological forms that differ in structure and function.
Network of Channels: Membrane-enclosed tubules and sacs that extend throughout the cytoplasm
Cisternae: Spherical or tubular membranes forming the internal spaces of ER, separating ER contents from the cytoplasm
Continuity with Other Membranes: ER channels are continuous with the plasma membrane and the nuclear membrane
The Rough Endoplasmic Reticulum (RER) is distinguished by the presence of Ribosomes attached to its outer surface. RER is primarily involved in the synthesis of proteins. After synthesis, these proteins are either stored in the cytoplasm or exported out of the cell through the ER channels, passed on to the Golgi Apparatus for modification, and then shipped to their final destination via vesicles that bud off from the Golgi.
Ribosome Attachment: The rough appearance is due to ribosomes studded on the cytoplasmic face of the ER membrane
Protein Synthesis: RER is the site of synthesis of proteins destined for secretion, membrane insertion, or lysosomal incorporation
Transport Role: Newly synthesised proteins enter the ER lumen and are transported to the Golgi apparatus for processing
The Smooth Endoplasmic Reticulum (SER) lacks ribosomes on its surface, giving it a smooth appearance. SER is involved in the metabolism of various types of molecules, particularly lipids. It helps detoxify harmful drugs and poisons. In muscle cells and nerve cells, SER plays a role in the transmission of impulses. SER is also important for transporting materials from one part of the cell to another.
No Ribosomes: The absence of ribosomes gives SER its smooth appearance under the electron microscope
Lipid Metabolism: SER synthesises lipids and lipoproteins
Detoxification: SER contains enzymes that break down harmful drugs and poisons — it is especially abundant in liver cells
Calcium Storage: In muscle cells, SER (called the sarcoplasmic reticulum) stores and releases calcium ions for muscle contraction
Impulse Transmission: SER assists in transmitting impulses in nerve cells
RER vs SER Comparison
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RER — has ribosomes on surface; SER — no ribosomes
•
RER — synthesises proteins; SER — synthesises lipids and metabolises drugs
•
RER — transports proteins to Golgi; SER — transports materials within the cell
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RER — abundant in secretory cells (pancreas, plasma cells); SER — abundant in liver cells and steroid-producing cells
•
Both — provide mechanical support and maintain cell shape
Golgi Apparatus
The Golgi Apparatus was discovered by Camillo Golgi in 1898. It is found in virtually all eukaryotic cells. The apparatus consists of stacks of flattened, membrane-bound sacs called Cisternae, along with associated vesicles — together these form the Golgi Complex. It is a complex system of interconnected tubules surrounding the central stacks. Vesicles derived from the budding of SER fuse to form new cisternae at the outer convex surface (the Forming Face). The inner concave surface is the Maturing Face, where cisternae break up into vesicles that carry products to their destinations.
Discovery: Camillo Golgi, 1898
Cisternae Stacks: Flattened, membrane-bound sacs arranged in parallel stacks
Forming Face (Cis Face): Outer convex surface where vesicles from SER fuse to form new cisternae
Maturing Face (Trans Face): Inner concave surface where cisternae break up into secretory vesicles
Directional Flow: Materials move from the forming face through the cisternae to the maturing face
The Golgi Apparatus is primarily concerned with cell secretions. Proteins synthesised on ribosomes are passed through the Endoplasmic Reticulum to the Golgi, where they are modified by the addition of carbohydrates and converted into Glycoproteins or Glycolipids. The finished products are packed inside membrane-bound vesicles before export. For example, in mammals, the pancreas secretes enzyme-containing granules that aid digestion — the Golgi apparatus plays a key role in forming these granules. The most important function of the Golgi is the modification, sorting, and packaging of proteins and lipids.
Protein Modification: Adds carbohydrate groups to proteins, converting them into glycoproteins
Lipid Modification: Converts lipids into glycolipids
Sorting and Packaging: Categorises modified molecules and packages them into vesicles for transport to their correct destinations
Secretion: Forms secretory granules (e.g., digestive enzymes in pancreatic cells, hormones in endocrine cells)
Products Processed by the Golgi Apparatus
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Secretory proteins — enzymes, hormones, extracellular matrix components
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Lysosomes — vesicles containing hydrolytic enzymes budded off from the Golgi
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Glycoproteins — proteins with added carbohydrate groups
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Glycolipids — lipids with added carbohydrate groups
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Cell wall components — in plant cells, the Golgi helps assemble cellulose and pectin for the cell wall
The Endoplasmic Reticulum and the Golgi Apparatus work together as an integrated secretory pathway. Proteins synthesised on ribosomes attached to the RER enter the ER lumen. Transport vesicles bud off from the ER and carry these proteins to the forming face of the Golgi. After modification within the Golgi cisternae, secretory vesicles pinch off from the maturing face and travel to the Plasma Membrane for export, or to other destinations within the cell. This coordinated pathway ensures that cellular products are properly processed before reaching their final destination.
RER to Golgi Transport: Vesicles budding from ER carry proteins to the forming face of the Golgi
Golgi Processing: Proteins are modified, sorted, and packaged as they pass through the Golgi cisternae
Golgi to Destination: Vesicles from the maturing face deliver products to the plasma membrane (for secretion), to lysosomes, or to other organelles
Mitochondria
Mitochondria are among the most important organelles in eukaryotic cells, responsible for manufacturing and supplying energy. Often called the 'powerhouse of the cell', they appear as vesicles, rods, or filaments under a compound microscope, and show complex morphology under the electron microscope. Each mitochondrion is bound by two membranes: a smooth outer membrane and an inner membrane that forms infoldings called Cristae into the inner chamber, which contains the Mitochondrial Matrix. The mitochondrial membranes share structural similarity with other cell membranes.
Double Membrane: Outer membrane is smooth; inner membrane is folded into cristae projecting into the matrix
Cristae: Infoldings of the inner membrane that increase the surface area for the chemical reactions of respiration
Mitochondrial Matrix: The fluid-filled inner compartment containing enzymes, coenzymes, organic salts, inorganic salts, circular DNA, and ribosomes
Variable Morphology: Number, shape, and size of mitochondria vary depending on the physiological activity of the cell
The inner mitochondrial membrane is the site of the electron transport chain and oxidative phosphorylation. Its surface is covered with small knob-like structures known as $F_1$ Particles (also called oxysomes or ATP synthase). These particles are embedded in the cristae membranes and are responsible for synthesising ATP from ADP and inorganic phosphate during aerobic respiration. The cristae greatly increase the surface area of the inner membrane, maximising the capacity for ATP production.
F₁ Particles: Small knob-like projections on the inner surface of cristae; these are ATP synthase enzymes that catalyse ATP formation
Electron Transport Chain: The series of protein complexes embedded in the inner membrane that transfer electrons and pump protons
Surface Area: Cristae folds maximise the inner membrane surface area for the reactions of aerobic respiration
Oxidative Phosphorylation: The process by which ATP is generated using the proton gradient established by the electron transport chain
Mitochondria extract energy from organic food molecules and convert it into ATP (adenosine triphosphate), the universal energy currency of the cell. The mitochondrial matrix contains a large number of enzymes and coenzymes that drive vital metabolic processes including Kreb's Cycle, aerobic respiration, and fatty acid metabolism. Spent energy in the form of ADP is regenerated into ATP by the mitochondria. The number of mitochondria in a cell correlates with its energy demands — highly active cells like muscle cells and liver cells contain many mitochondria.
Energy Extraction: Organic food molecules are broken down through Kreb's cycle and the electron transport chain to release energy
ATP Production: The released energy is captured and stored as ATP, which powers all cellular activities
Self-Replicating: Mitochondria contain their own circular DNA and ribosomes, allowing them to replicate independently
Energy Demand Correlation: Cells with high metabolic activity (muscle, liver, sperm cells) have more mitochondria than less active cells
Mitochondria contain their own circular DNA and ribosomes, which is evidence that they were once free-living prokaryotic organisms that entered into a symbiotic relationship with eukaryotic cells (endosymbiotic theory). The presence of DNA and ribosomes means mitochondria can synthesise some of their own proteins and replicate independently of the cell nucleus. This semi-autonomous nature is a key feature distinguishing mitochondria from other organelles.
Circular DNA: Mitochondria possess their own small, circular DNA molecule, distinct from nuclear DNA
Own Ribosomes: Mitochondrial ribosomes (55S) are different from cytoplasmic ribosomes (80S) and resemble prokaryotic ribosomes
Semi-Autonomous: Can replicate independently, but still depend on nuclear genes for most proteins
Chromosomes
The nucleus is deeply stained by basic dyes because of the Chromatin material within it. In non-dividing cells, chromatin appears as a diffuse, thread-like network dispersed in the Nucleoplasm. During cell division, this chromatin condenses and coils into distinct, darkly stained, thread-like structures called Chromosomes. This condensation makes the genetic material easier to distribute evenly between daughter cells.
Chromatin (Interphase): Diffuse, loosely coiled form of DNA and proteins visible in non-dividing cells
Chromosomes (Dividing): Highly condensed, tightly coiled structures formed from chromatin during cell division
Staining: Both chromatin and chromosomes stain deeply with basic dyes because DNA is acidic and binds basic stains
Each Chromosome consists of two identical copies called Chromatids, held together at a central constriction called the Centromere. A chromatid is an exact replica of the chromosome. The centromere is the site where Spindle Fibres attach during cell division to pull chromatids apart. Each chromosome is composed of DNA and proteins. All the information necessary to control cell activities is located on chromosomes in the form of Genes, which are transmitted from one generation to the next.
Chromatids: Two identical copies of a chromosome, joined at the centromere; they separate during cell division
Centromere: The constriction point where spindle fibres attach; holds sister chromatids together
DNA and Protein Composition: Chromosomes are made of DNA wrapped around histone proteins
Genes: Segments of DNA on chromosomes that carry the instructions for specific traits
Chromosome Structure Summary
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Chromatid — one of the two identical copies of a replicated chromosome
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Centromere — the constriction point joining sister chromatids; attachment site for spindle fibres
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Chromatin — loosely organised DNA + protein in non-dividing cells
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Gene — a segment of DNA on a chromosome that encodes a specific trait or function
The number of chromosomes is constant within a species across all individuals and generations. Normal body (somatic) cells contain a Diploid number of chromosomes ($2n$), while germ cells (Gametes) contain a Haploid number ($n$). In diploid cells, chromosomes exist in homologous pairs — one inherited from each parent.
$$2n \; \text{(diploid)} \longrightarrow n \; \text{(haploid)}$$
Reduction from diploid to haploid chromosome number during gamete formation (meiosis)
$2n$=diploid chromosome number in somatic cells(chromosomes per cell)
$n$=haploid chromosome number in gametes(chromosomes per cell)
$2n = 46$ (human somatic cells)
→$n = 23$ (human gametes)
Diploid ($2n$): The complete set of paired chromosomes in somatic (body) cells
Haploid ($n$): Half the diploid number; found in gametes (sperm and egg cells)
Species-Specific Constant: Every species has a fixed chromosome number maintained across generations
Chromosome Numbers in Selected Organisms
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Human — 46 (2n), 23 (n)
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Chimpanzee — 48 (2n), 24 (n)
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Frog — 26 (2n), 13 (n)
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Fruit fly (Drosophila melanogaster) — 8 (2n), 4 (n)
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Onion — 16 (2n), 8 (n)
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Potato — 48 (2n), 24 (n)
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Garden pea — 14 (2n), 7 (n)
Application Strategy
Worked Examples
Problem Statement
A cell produces large quantities of a protein hormone that is secreted into the bloodstream. Through which sequence of organelles does this protein pass before it leaves the cell?
protein synthesis pathwayER and Golgi relationship
Step 1
Protein synthesis on ribosomes
The hormone protein is first synthesised by ribosomes attached to the surface of the rough endoplasmic reticulum (RER), which is the site of protein synthesis for exported proteins.
Step 2
Entry into the ER lumen
The newly synthesised protein is pushed into the channels (cisternae) of the rough endoplasmic reticulum.
Step 3
Transport to the Golgi apparatus
Transport vesicles bud off from the ER and carry the protein to the forming face (cis face) of the Golgi apparatus.
Step 4
Modification and packaging in the Golgi
Inside the Golgi cisternae, the protein is modified (carbohydrate groups may be added to form a glycoprotein), sorted, and packaged into a secretory vesicle at the maturing face (trans face).
Step 5
Exocytosis at the plasma membrane
The secretory vesicle fuses with the plasma membrane, releasing the hormone outside the cell into the bloodstream.
Problem Statement
If a somatic cell of an organism has 48 chromosomes, what is the diploid number ($2n$), the haploid number ($n$), and how many chromosomes will be present in each gamete and in a cell during metaphase of mitosis?
diploidhaploidchromosome replication
Strategic Toolkit
$$2n \; \text{(diploid)} \longrightarrow n \; \text{(haploid)}$$
Step 1
Identify the diploid number
The chromosome number given for a somatic (body) cell is the diploid number, since somatic cells contain two complete sets of chromosomes.
$$2n = 48$$
Step 2
Calculate the haploid number
The haploid number is half the diploid number. This is the number of chromosomes in each gamete (sperm or egg).
$$n = \frac{2n}{2} = \frac{48}{2} = 24$$
Step 3
Determine gamete chromosome count
Each gamete carries the haploid number of chromosomes, which is 24 for this organism.
$$\text{Gametes: } n = 24 \text{ chromosomes}$$
Step 4
Determine metaphase chromosome count
During metaphase of mitosis, DNA has replicated but chromatids have not yet separated. Each of the 48 chromosomes consists of two chromatids, so the chromosome count is still 48.
$$\text{Metaphase of mitosis: } 48 \text{ chromosomes (each with 2 chromatids)}$$
Problem Statement
A hepatocyte (liver cell) contains abundant amounts of both rough and smooth endoplasmic reticulum. Explain the specific role of each type in this cell type.
RER functionSER functionliver cell specialisation
Step 1
RER in hepatocytes
The liver produces many blood plasma proteins (such as albumin and clotting factors). These proteins are synthesised by ribosomes on the RER, making hepatocytes rich in rough ER.
Step 2
SER in hepatocytes — detoxification
The liver detoxifies harmful substances such as drugs and alcohol. SER contains enzymes (e.g., cytochrome P450) that break down these toxins. This is why hepatocytes have extensive smooth ER.
Step 3
SER in hepatocytes — lipid metabolism
The liver also plays a central role in lipid metabolism, including synthesis of cholesterol and lipoproteins. These processes occur on the smooth endoplasmic reticulum.
Problem Statement
Explain how the structure of mitochondria is adapted to its function of ATP production. Which specific component is directly responsible for making ATP, and where is it located?
mitochondria structurecristaeF1 particlesATP synthesis
Step 1
Double membrane provides compartments
The outer membrane separates the mitochondrion from the cytoplasm, while the inner membrane creates a distinct compartment (the matrix) where Kreb's cycle enzymes are concentrated.
Step 2
Cristae maximise surface area
The infoldings of the inner membrane (cristae) greatly increase the surface area available for housing the electron transport chain protein complexes and F1 particles.
Step 3
F1 particles synthesise ATP
F1 particles (ATP synthase) are embedded on the inner surface of the cristae. They use the proton gradient established by the electron transport chain to catalyse the formation of ATP from ADP and inorganic phosphate.
Step 4
Matrix houses Kreb's cycle
The mitochondrial matrix contains the enzymes and coenzymes of Kreb's cycle, which produce NADH and FADH2 that feed electrons into the electron transport chain on the inner membrane.
Problem Statement
An egg cell has approximately 30,000 nuclear pores per nucleus, while a mature red blood cell has only 3–4. Explain this difference in terms of the metabolic activity and state of each cell type.
nuclear porescell differentiationmetabolic activity
Step 1
Egg cell is undifferentiated and metabolically active
Egg cells are undifferentiated and require a high rate of molecular exchange between the nucleus and cytoplasm. Large amounts of mRNA must exit the nucleus to support rapid protein synthesis during early development, requiring many nuclear pores.
Step 2
Mature red blood cell is highly differentiated
Mature erythrocytes are terminally differentiated cells that have lost their nucleus entirely in mammals. Even in organisms where the nucleus is retained, these cells have minimal metabolic and transcriptional activity, so very few nuclear pores are needed.
Step 3
Correlation between pore number and cellular activity
The number of nuclear pores reflects the level of nuclear-cytoplasmic exchange required. Highly active, undifferentiated cells need many pores; specialised, low-activity cells need few.
Memorize formulas
Diploid to Haploid Reduction
Somatic cells have $2n$ chromosomes (diploid); gametes have $n$ (haploid). The reduction occurs during meiosis. Example: human $2n=46$, $n=23$.
Formula
$$2n \longrightarrow n$$
Review
1
The nuclear membrane is composed of two lipid bilayers and contains pores. What is the primary function of nuclear pores?
Ans
To allow selective transport of molecules between the nucleoplasm and cytoplasm
Review
Tap to reveal
Identify the structure
The nuclear membrane (nuclear envelope) is a double lipid bilayer perforated by nuclear pores.
Nuclear pores are channels in the nuclear envelope
Determine function
Nuclear pores have a selective permeability barrier allowing regulated transport of large molecules like mRNA out and proteins into the nucleus.
Selective transport between nucleoplasm and cytoplasm
2
Which organelle is the site of ribosomal RNA (rRNA) synthesis in eukaryotic cells?
Ans
Nucleolus
Review
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Recall nucleolus function
The nucleolus is a dense body within the nucleus responsible for rRNA synthesis and ribosome assembly.
rRNA is produced in the nucleolus
3
Chromatin, the loosely organised form of DNA found within the nucleus during interphase, is composed of DNA wrapped around:
Ans
Histone proteins
Review
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Define chromatin
Chromatin is the complex of DNA and proteins found in the nucleus during interphase when chromosomes are not condensed.
Chromatin = DNA + proteins
Identify the protein component
DNA wraps around octamers of histone proteins to form nucleosomes, the basic repeating unit of chromatin.
Histone proteins package DNA into chromatin
4
Ribosomes are attached to the surface of rough endoplasmic reticulum. What distinguishes rough ER from smooth ER?
Ans
Rough ER contains cristae for ATP production
Review
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Compare RER and SER structure
Rough ER has ribosomes studding its outer surface; smooth ER has no ribosomes. Both consist of membrane-bound cisternae.
RER = ribosome-studded; SER = ribosome-free
Compare functions
RER synthesises and transports proteins. SER synthesises lipids and steroids, detoxifies drugs, and stores calcium ions.
RER → proteins; SER → lipids, detoxification, Ca²⁺ storage
5
A cell that produces large amounts of steroid hormones would be expected to have an abundance of which organelle?
Ans
Smooth endoplasmic reticulum
Review
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Identify the product type
Steroid hormones are lipid-derived molecules.
Steroids are lipids
Match to organelle
Smooth ER is specialised for lipid and steroid biosynthesis. Cells with high steroid output have extensive SER networks.
Abundant smooth ER in steroid-producing cells
6
The Golgi apparatus receives vesicles from the ER at its forming face and releases processed material at its maturing face. What is the primary role of the Golgi complex?
Ans
Modifying, sorting, and packaging proteins and lipids for secretion or delivery
Review
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Trace the pathway
Proteins and lipids are synthesised in the ER → transported in vesicles → received at the forming face of the Golgi → processed → released at the maturing face.
ER → forming face → Golgi cisternae → maturing face → vesicles
Identify modifications
Within the Golgi, molecules are modified by addition of carbohydrate groups, forming glycoproteins and glycolipids, then sorted and packaged.
Glycosylation, sorting, and packaging
7
Glycoproteins and glycolipids are molecules formed when carbohydrate groups are attached to proteins and lipids respectively. In which organelle does this glycosylation primarily occur?
Ans
Golgi apparatus
Review
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Define glycosylation
Glycosylation is the enzymatic addition of carbohydrate chains to proteins (forming glycoproteins) or lipids (forming glycolipids).
Proteins/lipids + carbohydrates = glycoproteins/glycolipids
Locate the site
The Golgi apparatus is the main site where final glycosylation and packaging of these modified molecules occurs before secretion.
Golgi apparatus → primary glycosylation site
8
The inner membrane of mitochondria is folded into projections called cristae. What is the primary significance of cristae?
Ans
They anchor the ribosomes used for mitochondrial protein synthesis
Review
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Identify cristae structure
Cristae are folds of the inner mitochondrial membrane projecting into the matrix.
Cristae = inner membrane folds
Determine functional significance
The increased surface area from folding allows more electron transport chain complexes and F₁ particles (ATP synthase) to be embedded in the membrane, maximising ATP production.
Greater surface area → more ETC complexes → more ATP
9
F₁ particles are found on the inner mitochondrial membrane, specifically on the cristae. What is their function?
Ans
Catalysing the synthesis of ATP from ADP and inorganic phosphate
Review
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Identify F₁ particles
F₁ particles are stalked structures on the inner mitochondrial membrane, also known as ATP synthase.
F₁ particles = ATP synthase
Determine function
They use the proton gradient across the inner membrane to catalyse: ADP + Pᵢ → ATP.
$$ADP + P_i \xrightarrow{F_1\text{ particles}} ATP$$
10
The Kreb's cycle takes place in which region of the mitochondrion?
Ans
Intermembrane space
Review
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Locate Kreb's cycle
The Kreb's cycle is a series of enzyme-catalysed reactions that oxidise acetyl-CoA to CO₂, generating NADH and FADH₂.
Kreb's cycle enzymes are soluble in the matrix
Distinguish from ETC
While the Kreb's cycle occurs in the matrix, the electron transport chain is embedded in the inner membrane (cristae).
Matrix = Kreb's cycle; Cristae = ETC
11
During cell division, a chromosome consists of two sister chromatids joined at a specific region. What is this region called?
Ans
Centromere
Review
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Identify chromosome structure
A duplicated chromosome consists of two identical sister chromatids connected at a constricted region.
Two chromatids joined at one point
Name the junction
The centromere is the specialised region where sister chromatids are held together and where spindle fibres attach via the kinetochore during mitosis.
Centromere joins sister chromatids
12
In humans, somatic cells contain 46 chromosomes. What is the chromosome number in human gametes (sperm and egg cells)?
Ans
44
Review
Tap to reveal
Apply diploid to haploid reduction
Somatic cells are diploid ($$2n$$). Meiosis produces haploid gametes ($$n$$).
$$2n \longrightarrow n$$
Calculate
For humans, $$2n = 46$$, so $$n = 46/2 = 23$$.
$$n = 23$$ chromosomes in gametes
13
Mature mammalian erythrocytes (red blood cells) are unusual because they lack which organelle?
Ans
Nucleus
Review
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Identify erythrocyte specialisation
Mammalian erythrocytes lose their nucleus and all organelles during maturation to maximise haemoglobin-carrying capacity.
Anucleate (no nucleus, no organelles)
Explain functional advantage
Without a nucleus and organelles, more haemoglobin molecules can be packed in, improving oxygen transport efficiency.
More haemoglobin → greater O₂ carrying capacity
14
A hepatocyte (liver cell) has an exceptionally high number of nuclear pores per unit area compared to an erythrocyte precursor. What is the most likely reason?
Ans
Hepatocytes have a high metabolic demand requiring rapid export of mRNA and import of proteins
Review
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Recall hepatocyte function
Hepatocytes are highly metabolically active, synthesising large amounts of proteins, lipids, and glycogen.
High protein synthesis demand
Link to nuclear pores
More nuclear pores allow faster export of mRNA transcripts and import of transcription factors, matching the cell's high metabolic activity.
More pores → faster nucleocytoplasmic transport
15
When multiple ribosomes simultaneously translate a single mRNA molecule, the structure formed is called a:
Ans
Signal recognition particle
Review
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Define polysome
Multiple ribosomes attached to and translating a single strand of mRNA form a polysome.
Multiple ribosomes on one mRNA = polysome
Explain significance
Polysomes allow efficient, simultaneous production of multiple copies of the same protein from one mRNA molecule.
Increased protein synthesis efficiency
16
Lysosomes contain hydrolytic enzymes capable of digesting cellular waste and foreign material. Which organelle is primarily responsible for the formation of lysosomes?
Ans
Mitochondria
Review
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Trace lysosome formation
Hydrolytic enzymes are made in the RER → transported to the Golgi → packaged into vesicles that become primary lysosomes.
RER (synthesis) → Golgi (packaging) → Lysosomes
Identify the key organelle
The Golgi apparatus is the site where lysosomal vesicles are formed and released.
Golgi apparatus → lysosome formation
17
ATP production through oxidative phosphorylation occurs at which specific location within the mitochondrion?
Ans
Inner mitochondrial membrane (cristae)
Review
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Identify oxidative phosphorylation components
The electron transport chain and ATP synthase are embedded in the inner mitochondrial membrane (cristae).
ETC + F₁ particles on cristae
Describe the process
Electrons from NADH/FADH₂ pass through ETC complexes, pumping H⁺ into the intermembrane space. The gradient drives H⁺ back through F₁ particles, producing ATP.
Inner membrane = site of oxidative phosphorylation → ATP
18
A gene is best described as:
Ans
A protein molecule that regulates cell division
Review
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Define gene
A gene is a discrete unit of heredity — a specific sequence of DNA that encodes a functional product (protein or RNA).
Gene = DNA segment → functional product
19
Cell fractionation is a technique used to separate organelles based on differences in their size and density. Which instrument is essential for this technique?
Ans
Ultracentrifuge
Review
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Define cell fractionation
Cells are homogenised and the homogenate is subjected to differential centrifugation to separate organelles.
Homogenise → differential centrifugation
Identify the instrument
An ultracentrifuge spins samples at extremely high speeds (up to 100,000 × g or more), causing organelles to separate into pellets based on mass and density.
Ultracentrifuge is essential
20
A researcher observes that a certain cell type secretes large amounts of glycoproteins. If the secretory pathway is disrupted at the Golgi apparatus, which of the following would most directly occur?
Ans
Lipid synthesis would cease entirely
Review
Tap to reveal
Identify the Golgi's role in secretion
The Golgi modifies proteins (glycosylation) and packages them into vesicles destined for secretion or delivery to other compartments.
Golgi → glycosylation + vesicle packaging
Predict the effect of disruption
Without Golgi function, proteins would remain unmodified and unpackaged — glycoproteins would not form and secretion would be blocked.
Glycosylation fails → secretion impaired
Question 1 of 20
0:00
1
The nuclear membrane is composed of two lipid bilayers and contains pores. What is the primary function of nuclear pores?