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1. How might multicellularity have arisen from single celled organisms?
Answer
Cooperation between single cells increases the chances of survival by offering a greater degree of protection and allowing specialisation of different cells within the multicellular organism. Hence, there is an environmental pressure to form colonies.
2. Which key invention enabled the development of cell theory?
Answer
The microscope, in particular the light microscope, although electron microscopes really enhanced our understanding of cell structure and confirmed the idea behind the cell theory.
3. What advantages might be gained by prokaryotic organisms becoming integrated into a single cellular unit?
Answer
As with multicellularity, communities of prokaryotes will have derived protection and specialisation from joining together into one unit.
4. What are the principal types of organelle found in eukaryotic cells?
Answer
- Membranous organelles (eg mitochondria, Golgi apparatus, endoplasmic reticulum)
- Filamentous organelles (eg microtubules, actin filaments, and intermediate filaments)
- Granular organelles (eg ribosomes, glycogen granules)
5. What component of membranes forms the bilayer?
Answer
A double layer of lipid molecules arranged with their polar (hydrophilic) heads on the outside and their hydrophobic tails on the inside.
6. How do instructions encoded in the DNA in chromosomes get out of the nucleus?
Answer
The DNA is transcribed into messenger RNA (mRNA) which is tagged by export proteins that assist it to exit through the nuclear pore.
7. Give three key ways in which the ER differs from the Golgi apparatus.
Answer
Structurally, ER forms membrane stacks that can spread throughout the cell in extensive arrays, whereas Golgi forms more discrete bodies. ER can have ribosomes attached on its surface. The synthesis of proteins starts in ER whereas the Golgi functions in protein sorting
8. What is the main difference between exocytosis and endocytosis?
Answer
Exocytosis is the process of secreting materials from the cell, whereas endocytosis is the process of importing them into the cell.
9. Can you explain what is meant by semi-autonomous replication of mitochondria?
Answer
Semi-autonomous replication is where part of the DNA necessary for encoding the mitochondrial structure is found in the mitochondrion itself, and part is in the cell nucleus. Hence, to make a complete mitochondrion, both need to work together: the mitochondrion is semi-autonomous rather than autonomous because it cannot replicate completely independently.
10. Microtubules are found in virtually every eukaryotic cell, but are they also found in prokaryotes?
Answer
In certain prokaryotes, tubular organelles resembling microtubules have been found.
11. How do intermediate filaments differ from microtubules?
Answer
- Microtubules are larger than intermediate filaments
- Microtubules form hollow tubes whereas intermediate filaments form stiff rods
- Microtubules are composed primarily of α– and β-tubulin dimers, whereas intermediate filaments are more varied in their composition
12. In which type of cell is actin found in the greatest quantity in the human body?
Answer
Muscle cells – actin and myosin are the basis of their contractility.
13. Give two main ways in which apoptosis differs from necrosis.
Answer
Apoptosis is a controlled event leading to cell death, whereas necrosis is a response to injury and causes release of dangerous enzymes that can kill cells; other factors are also released that trigger inflammatory responses in the body. Apoptosis can ‘fail’ so that cells continue to live when not required; for example, forming tumours
14. If a cell contains a large amount of RER, what do you think its main function is likely to be?
Answer
It is likely to be a cell involved in a large amount of protein synthesis; for example, in producing materials like hormones or mucus for secretion.