A2.2 Cell structure

In a cell, what is the effect of a large surface area to volume ratio?

A. Slower rate of exchange of waste materials.

B. Faster heat loss.

C. Faster rate of mitosis.

D. Slower intake of food.

Answer: B

A. Incorrect: A large surface area to volume ratio (SA:V) is a prerequisite for maximizing metabolic exchange rates. If this ratio is too small, waste products will accumulate because they are produced faster than they can be removed.

B. Correct. Heat transfer between a cell and its environment is determined by its surface area.

• The rate of metabolism (which generates heat) is proportional to the volume of the cell.
• If the SA:V ratio is too small, the cell may overheat because metabolic processes generate heat faster than it can be lost through the cell surface.
• Therefore, a large SA:V ratio (a large surface area relative to volume) allows the cell to lose heat more rapidly, maintaining a stable temperature, an important factor in body temperature regulation.

C. Incorrect. A cell stops growing and may divide once it reaches its maximum size (when the SA:V ratio falls below a critical limit). Cell division produces smaller daughter cells, thereby restoring a higher SA:V ratio.

D. Incorrect. A large SA:V ratio facilitates faster nutrient uptake.

What is a difference between eukaryotic and prokaryotic cells?

A. Cell walls are found only in prokaryotes.

B. Naked DNA with histones is found only in prokaryotes.

C. Compartmentalization is found only in eukaryotes.

D. Cilia and flagella are found only in prokaryotes.

Answer: C

A. Incorrect. Although prokaryotic cells (such as bacteria) have a cell wall, cell walls are also found in some eukaryotic cells, such as plant cells (which contain cellulose) and fungal cells (which contain chitin).

B. Incorrect. The term “naked DNA” is contradictory if applied to eukaryotes. Naked DNA refers to DNA that is not associated with histone proteins, which is a characteristic of prokaryotic DNA.

C. Correct. Compartmentalization is an important structural feature found only in eukaryotic (nucleated) cells.

• Eukaryotic cells exhibit compartmentalization because they contain membrane-bound organelles embedded within the cytoplasm.
• This provides the advantage of protecting DNA by enclosing chromosomes within the nucleus.
• Prokaryotic cells (non-nucleated) lack such membrane-bound organelles; therefore, they do not have compartmentalization in this sense.
• The Endosymbiotic Theory is a biological hypothesis proposed to explain the origin of key double-membraned organelles in eukaryotic cells, such as mitochondria. 

D. Incorrect. Flagella are found in both prokaryotes and eukaryotes (e.g., sperm cells). Cilia are mainly found in eukaryotes (e.g., Paramecium or cells lining the respiratory tract). Although their structures differ, neither of these structures is exclusive to prokaryotes.

A red blood cell is 8 µm in diameter. If drawn 100 times larger than its actual size, what diameter will the drawing be in mm?

A. 0.08 mm

B. 0.8 mm

C. 8 mm

D. 80 mm

Answer: B

`"Magnification" (M)= frac{"Size of image (I)"}{"Actual size of specimen (A)"}`

→ Equivalently: I =A×M

Actual size (A) = 8 μm; Required unit: milimeters (mm)
Calculate the image size (I) in micrometers: 
I =8 μm × 100 =800 μm

Convert the unit from micrometers (μm) to millimeters (mm):
I (mm) =`frac{800 μm}1000` = 0.8  

How do cells in multicellular organisms differentiate?

A. Some cell types divide by mitosis more often than others.

B. They express some of their genes but not others.

C. Some of their proteins denature but not others.

D. Their DNA content changes with time.

Answer: B

A. Incorrect. Although some cell types (e.g., stem cells) divide at different rates, mitosis is simply the process used to produce genetically identical cells. The rate of division depends on the type of cell, not on the main mechanism that causes the cell to become specialized.

B. Correct.

• This is the core mechanism of cell differentiation which occurs when cells become specialized to perform specific functions.
• Cells express certain sets of genes while preventing the expression of others.
• This regulation of gene expression determines which proteins are produced, and therefore defines the specialized structure and function of each cell.

C. Incorrect. The denaturation is a structural change in a protein that causes it to lose its biological properties, which usually results from increased temperature or changes in pH. And it is not a regulatory mechanism for cell specialization.

D. Incorrect.  All cells in a multicellular organism contain the same genome, since they all originate from a single original cell. Differentiated cells use that genetic information selectively, rather than changing the DNA composition itself.

What is evidence for the endosymbiotic theory in eukaryotic cells?.

A. Mitochondrion with DNA

B. Golgi complex in cytoplasm

C. Single nuclear membrane

D. Ribosomes in cytoplasm

Answer: A

A. Correct. This is an important piece of evidence for the Endosymbiotic Theory. Both mitochondria and chloroplasts have their own DNA, which is usually circular and naked, similar to that of prokaryotic cells. Moreover, they contain 70S ribosomes, also resembling those found in prokaryotes.

B. Incorrect: The Golgi apparatus is a characteristic organelle of eukaryotic cells. Its presence is explained by the process of compartmentalization, likely resulting from the infolding of the plasma membrane, rather than being direct evidence for the endosymbiotic origin. 

C. Incorrect. The nucleus of a eukaryotic cell is surrounded by a double membrane. The Endosymbiotic Theory suggests that the nucleus might have formed through the infolding of the plasma membrane of an ancient prokaryotic cell.

D. Incorrect. Ribosomes are present in the cytoplasm of both prokaryotic (70S) and eukaryotic (80S) cells. The evidence for the Endosymbiotic Theory lies in the presence of 70S ribosomes inside mitochondria and chloroplasts, not in the 80S ribosomes of the general cytoplasm.

What route is used to export proteins from the cell?

A. Golgi apparatus → rough endoplasmic reticulum → plasma membrane

B. Rough endoplasmic reticulum → Golgi apparatus → plasma membrane

C. Golgi apparatus → lysosome → rough endoplasmic reticulum

D. Rough endoplasmic reticulum → lysosome → Golgi apparatus

Answer: B

A. Incorrect. The initial order is reversed. Proteins are first synthesized in the rough endoplasmic reticulum (RER) and then transferred to the Golgi apparatus.

B. Correct. This is the correct pathway for protein synthesis and secretion. Proteins are synthesized by ribosomes on the RER, transferred to the Golgi apparatus for modification and packaging, and then transported in vesicles that fuse with the plasma membrane to release the proteins outside the cell (exocytosis/secretion).

C. Incorrect. Lysosomes contain digestive enzymes and are the destination for certain proteins, but this is not the pathway for protein export out of the cell.

D. Incorrect. Proteins may go from the RER to lysosomes, but the final step for export should involve the plasma membrane, not the Golgi apparatus.

 Which structure is found in animal cells?

A. Cell wall

B. Chloroplast

C. Pili 

D. Mitochondria

Answer: D

A. Incorrect. The cell wall is not present in animal cells. It is found in plant cells (usually made of cellulose), fungi (made of chitin), and prokaryotic cells.

B. Incorrect. The chloroplast is an organelle containing chlorophyll and is the site of photosynthesis. It is found in plant cells (and some protists), but not in animal cells.

C. Incorrect. Pili are structures found in prokaryotic cells. They help bacteria attach to surfaces or other bacteria during conjugation. Animal cells do not have pili.

D. Correct. Mitochondria are double-membraned organelles found in eukaryotic cells (including animal cells). They are the sites of aerobic cellular respiration, where ATP (the main energy currency of the cell) is produced.

How does the endosymbiotic theory explain the origin of mitochondria in eukaryotes?

A. Autotrophic eukaryotes fused with photosynthetic bacteria.

B. Small aerobic bacteria survived inside anaerobic prokaryotes

C. Anaerobic prokaryotes were engulfed by small aerobic bacteria.

D. Invaginations occurred in large prokaryotes to increase surface area for gas exchange.

Answer: B

A. Incorrect. The theory describes engulfment, not fusion. Moreover, mitochondria originated from aerobic bacteria, not photosynthetic bacteria (which later gave rise to chloroplasts).

B. Correct. The theory suggests that a large anaerobic prokaryotic cell engulfed a smaller aerobic bacterium. The aerobic bacterium survived inside the host cell and evolved into mitochondria, providing more efficient energy (ATP) production for the host. The presence of mitochondria derived from aerobic bacteria supports this theory.

C. Incorrect. This reverses the roles of the host and the symbiont. The symbiont was the small aerobic bacterium, while the host was the larger prokaryotic cell that engulfed it.

D. Incorrect. The formation of invaginations (membrane infoldings) in large prokaryotes is thought to have led to the nuclear membrane and the endoplasmic reticulum, not mitochondria. Mitochondria originated through the engulfment of a free-living bacterium.

State two differences in structure between plant and animal cells. [2]

Any two of the following: 

Answers do not need to be shown in a table format.

Sample answer:

- Plant cells have a cell wall (mainly composed of cellulose), whereas animal cells do not have a cell wall [1].

- Plant cells contain plastids, such as chloroplasts, while animal cells lack chloroplasts or any type of plastids [1].

Compare the structure of prokaryotic and eukaryotic cells. [5]

Any four of the following, information from both boxes on the same line, is needed for 1 mark.

Differences [4 max]:

Similarities: Award 1 mark for any combination of two different items [2 max].

cytoplasm/plasma membrane/contains DNA/contains ribosomes;
(Plus up to [2] for quality)

Sample answer: 

All living cells, including prokaryotic and eukaryotic cells, have a plasma membrane that surrounds the cell, controlling the movement of substances in and out, and cytoplasm where cellular activities occur [1]. However, prokaryotic cells have no nucleus, while eukaryotic cells have a distinct nucleus that contains their genetic material [1]. In prokaryotic cells, the DNA is naked and arranged in a loop, whereas in eukaryotic cells, the DNA is associated with histone proteins and forms linear strands [2]. Prokaryotic cells also have 70S ribosomes, which are smaller than the 80S ribosomes found in eukaryotic cells [1]. Additionally, prokaryotic cells lack membrane-bound organelles such as mitochondria, which are present in eukaryotic cells [1].