Basis of Life- Cells, Protoplasm and Tissues
Basis of Life- Cells, Protoplasm and Tissues MCQ Questions
7.
The cell theory was formulated by:
B.
Robert Hooke and Robert Brown
C.
Leeuwenhoek and Flemming
ANSWER :
A. Schleiden and Schwann
Approach
The question asks about the scientists responsible for formulating the cell theory. We need to identify the correct pair of scientists from the given options who are credited with this fundamental biological theory.
Step-by-step
- Recall the history of cell biology and the development of the cell theory.
- The cell theory is a foundational concept in biology that states that all living organisms are composed of cells, and cells are the basic unit of life.
- Matthias Schleiden, a botanist, proposed in 1838 that all plant tissues are composed of cells.
- Theodor Schwann, a zoologist, extended this idea in 1839, stating that all animal tissues are also composed of cells and that cells are the fundamental units of life.
- Together, Schleiden and Schwann are credited with formulating the initial version of the cell theory.
- Later, Rudolf Virchow added the third tenet, "Omnis cellula e cellula," meaning "all cells arise from pre-existing cells."
Correct Option: A) Schleiden and Schwann are universally recognized for formulating the initial cell theory.
Incorrect Options
- B) Robert Hooke and Robert Brown: Robert Hooke observed and named "cells" in cork tissue in 1665, but he did not formulate the cell theory. Robert Brown discovered the nucleus in plant cells in 1831. They were important contributors to early cell biology but not the formulators of the cell theory.
- C) Leeuwenhoek and Flemming: Anton van Leeuwenhoek made significant improvements to the microscope and was the first to observe living cells (like bacteria and protozoa) in the late 17th century. Walther Flemming described chromosome behavior during mitosis in the 1880s. Neither formulated the cell theory.
- D) Virchow and Pasteur: Rudolf Virchow contributed the crucial third tenet to the cell theory (that cells arise from pre-existing cells) in 1855. Louis Pasteur disproved spontaneous generation and made significant contributions to microbiology and vaccination, but he was not involved in formulating the cell theory itself.
8.
The smallest known free-living cells are:
Approach
The question asks to identify the smallest known free-living cells. This requires knowledge of the relative sizes and characteristics of different types of microorganisms.
Step-by-step
- Understand "free-living cells": This refers to organisms that can survive and reproduce independently without relying on a host for all their metabolic needs.
- Evaluate each option based on size and free-living nature:
- Mycoplasmas: These are a genus of bacteria that lack a cell wall. They are known to be the smallest prokaryotes and the smallest free-living organisms, typically ranging from \(0.2\) to \(0.3\) micrometers (\(\mu\text{m}\)) in diameter.
- Bacteria: While bacteria are prokaryotes and generally small, mycoplasmas are a specific type of bacteria and are smaller than most other bacteria. The average bacterium is about \(1-10\) \(\mu\text{m}\).
- PPLO (Pleuropneumonia-like organisms): This is an older term used to refer to mycoplasmas, especially those first isolated from cattle with pleuropneumonia. So, PPLO essentially refers to mycoplasmas.
- Viruses: Viruses are much smaller than bacteria and mycoplasmas (typically \(20-400\) nanometers, \(\text{nm}\)). However, viruses are not considered free-living cells because they are obligate intracellular parasites, meaning they cannot reproduce or carry out metabolic functions outside a host cell. They lack cellular machinery.
- Compare the options: Mycoplasmas are the smallest known free-living cells. PPLO is an older name for mycoplasmas. Bacteria, in general, are larger than mycoplasmas. Viruses are smaller but not free-living cells.
Correct Option:
C) Mycoplasmas are indeed the smallest known free-living cells. They are prokaryotic organisms that lack a cell wall and can survive and reproduce independently.
Incorrect Options
- A) PPLO — PPLO stands for Pleuropneumonia-like organisms, which is an older term for Mycoplasmas. While technically referring to the same group, Mycoplasmas is the more current and precise scientific term. However, if Mycoplasmas were not an option, PPLO would be the correct choice. Since Mycoplasmas is explicitly listed, it is the better answer.
- B) Bacteria — While bacteria are prokaryotes and generally small, Mycoplasmas are a specific type of bacteria and are significantly smaller than most other bacterial species. Therefore, "Bacteria" as a general category is not the most precise answer for the *smallest* free-living cells.
- D) Viruses — Viruses are much smaller than Mycoplasmas, but they are not considered free-living cells. They are obligate intracellular parasites, meaning they require a host cell to replicate and carry out metabolic processes. They lack the cellular machinery to sustain themselves independently.
9.
The largest isolated single cell known is:
Approach
The question asks to identify the largest isolated single cell among the given options. This requires knowledge of cell biology and general biological facts about cell sizes.
Step-by-step
- Understand the term "single cell": A single cell refers to an organism that consists of only one cell, or a cell that can exist independently and is not part of a larger tissue or organism in the context of its measurement.
- Evaluate the options for size and cellular nature:
- Amoeba proteus: This is a large single-celled protozoan, visible to the naked eye under ideal conditions, but not the largest.
- Human nerve cell: While very long (up to a meter), it is a part of a multicellular organism (human) and its volume is not as large as the largest single cells. Its length is due to its specialized function, not overall cellular mass.
- RBC of elephant: Red blood cells are small, specialized cells for oxygen transport, and are part of a multicellular organism. They are microscopic.
- Ostrich egg: An ostrich egg, before fertilization and development, is essentially a single, enormous cell (the ovum or zygote). It is the largest single cell known, easily visible and weighing over a kilogram.
- Compare the sizes: The ostrich egg is by far the largest among the options and is considered the largest single cell known to science.
Correct Option: D) Ostrich egg — An unfertilized ostrich egg is a single cell (ovum) that can weigh up to 1.5 kg and measure about 15 cm in length, making it the largest known single cell.
Incorrect Options
- A) Amoeba proteus — While a large single-celled organism, its size is typically in the range of 500-1000 micrometers (\(0.5\) to \(1\) mm), which is significantly smaller than an ostrich egg.
- B) Human nerve cell — A human nerve cell (neuron) can be very long, sometimes over a meter, but its diameter is microscopic (tens of micrometers). It is a specialized cell within a multicellular organism and its overall volume is tiny compared to an ostrich egg.
- C) RBC of elephant — Red blood cells (RBCs) are very small, microscopic cells (typically 6-8 micrometers in diameter in mammals) specialized for oxygen transport. Even an elephant's RBCs are microscopic and not considered large in the context of single cells.
10.
Prokaryotic cells are represented by all EXCEPT:
Approach
The question asks to identify the organism that is NOT a prokaryotic cell from the given options. To answer this, we need to recall the characteristics of prokaryotic and eukaryotic cells and classify each option accordingly.
Step-by-step
- Understand Prokaryotic Cells: Prokaryotic cells are single-celled organisms that lack a membrane-bound nucleus and other membrane-bound organelles (like mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus). Their genetic material (DNA) is typically found in a nucleoid region. Examples include bacteria and archaea.
- Understand Eukaryotic Cells: Eukaryotic cells are generally larger and more complex, possessing a true nucleus enclosed in a nuclear membrane and various membrane-bound organelles. Examples include animals, plants, fungi, and protists.
- Evaluate each option:
- Amoeba: Amoeba is a single-celled eukaryotic organism belonging to the kingdom Protista. It has a well-defined nucleus and other organelles.
- Blue-green algae (Cyanobacteria): These are photosynthetic prokaryotes. Despite the name "algae," they are bacteria and lack a true nucleus and membrane-bound organelles.
- Bacteria: Bacteria are the classic examples of prokaryotic cells. They lack a membrane-bound nucleus and other complex organelles.
- Mycoplasma: Mycoplasma are the smallest known bacteria and are prokaryotic. They lack a cell wall but possess all other prokaryotic features.
- Identify the exception: Based on the evaluation, Amoeba is a eukaryotic organism, while blue-green algae, bacteria, and mycoplasma are all prokaryotic. Therefore, Amoeba is the one that does not represent a prokaryotic cell.
Correct Option: A) Amoeba is a eukaryotic organism, characterized by the presence of a true nucleus and membrane-bound organelles. All other options (Blue-green algae, Bacteria, Mycoplasma) are prokaryotic organisms.
Incorrect Options
- B) Blue-green algae (Cyanobacteria) — These are photosynthetic bacteria, which are prokaryotic cells. They lack a membrane-bound nucleus and other organelles.
- C) Bacteria — Bacteria are the most well-known examples of prokaryotic cells, characterized by the absence of a true nucleus and membrane-bound organelles.
- D) Mycoplasma — Mycoplasma are a genus of bacteria and are among the smallest known prokaryotic cells. They lack a cell wall but are fundamentally prokaryotic in their cellular organization.
11.
Which of the following is absent in prokaryotic cells?
B.
Membrane-bound nucleus
ANSWER :
B. Membrane-bound nucleus
Approach
The question asks to identify the cellular component that is absent in prokaryotic cells. We need to recall the fundamental differences between prokaryotic and eukaryotic cells, particularly regarding their internal organization and organelles.
Step-by-step
- Understand Prokaryotic Cells: Prokaryotic cells are simple, single-celled organisms that lack a membrane-bound nucleus and other membrane-bound organelles. Their genetic material (DNA) is located in a region called the nucleoid, which is not enclosed by a membrane.
- Evaluate Option A (Cell membrane): Both prokaryotic and eukaryotic cells have a cell membrane, which encloses the cytoplasm and regulates the passage of substances into and out of the cell. Therefore, the cell membrane is present in prokaryotic cells.
- Evaluate Option B (Membrane-bound nucleus): This is a defining characteristic that distinguishes prokaryotic cells from eukaryotic cells. Prokaryotic cells do not have a true nucleus enclosed by a nuclear membrane. Their genetic material is free in the cytoplasm.
- Evaluate Option C (Ribosome): Ribosomes are essential for protein synthesis and are present in both prokaryotic and eukaryotic cells. Prokaryotic ribosomes are typically 70S, while eukaryotic ribosomes are 80S, but ribosomes themselves are present.
- Evaluate Option D (Cell wall): Most prokaryotic cells (like bacteria) have a cell wall outside the cell membrane, which provides structural support and protection. Plant cells and fungal cells (eukaryotes) also have cell walls, but animal cells do not. Since many prokaryotes have a cell wall, it is not absent in them.
Correct Option: B) Membrane-bound nucleus
Prokaryotic cells are characterized by the absence of a membrane-bound nucleus. Their genetic material is located in a region called the nucleoid, which is not enclosed by a membrane.
Incorrect Options
- A) Cell membrane — The cell membrane is a universal feature of all cells, both prokaryotic and eukaryotic, forming the outer boundary of the cytoplasm.
- C) Ribosome — Ribosomes are present in both prokaryotic and eukaryotic cells, as they are essential for protein synthesis. Prokaryotic ribosomes are 70S.
- D) Cell wall — Most prokaryotic cells (e.g., bacteria) possess a cell wall, which provides structural integrity and protection.
12.
The genetic material in prokaryotic cells is:
A.
Circular DNA naked in nucleoid region
B.
Histone-wrapped chromosomal DNA
D.
Linear DNA enclosed in nuclear envelope
ANSWER :
A. Circular DNA naked in nucleoid region
Approach
The question asks about the nature of genetic material in prokaryotic cells. To answer this, we need to recall the fundamental differences between prokaryotic and eukaryotic cell structures, specifically regarding their genetic organization.
Step-by-step
- Understand Prokaryotic Cells: Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and other membrane-bound organelles. Their genetic material is not enclosed within a nuclear envelope.
- Identify the form of DNA in Prokaryotes: The main genetic material in prokaryotic cells (like bacteria) is typically a single, circular chromosome.
- Consider the association with proteins: Unlike eukaryotic DNA, prokaryotic DNA is not associated with histone proteins to form chromatin. It is often referred to as "naked" DNA, though it is associated with some non-histone proteins that help in its packaging and supercoiling.
- Locate the genetic material: Since there's no nucleus, the genetic material is found in a region of the cytoplasm called the nucleoid.
- Evaluate the options:
- A) "Circular DNA naked in nucleoid region" accurately describes the genetic material of prokaryotes.
- B) "Histone-wrapped chromosomal DNA" describes eukaryotic DNA.
- C) "RNA only" is incorrect; DNA is the primary genetic material.
- D) "Linear DNA enclosed in nuclear envelope" describes eukaryotic DNA.
Correct Option: A) Circular DNA naked in nucleoid region
This option correctly describes the key characteristics of prokaryotic genetic material: it is circular, not associated with histones (hence "naked" in this context), and located in the nucleoid region because there is no true nucleus.
Incorrect Options
- B) Histone-wrapped chromosomal DNA — This describes the genetic material in eukaryotic cells, where DNA is complexed with histone proteins to form chromatin, which then condenses into chromosomes. Prokaryotes do not have histones for DNA packaging.
- C) RNA only — While RNA plays crucial roles in prokaryotic cells (e.g., mRNA, tRNA, rRNA), it is not the primary genetic material that carries all hereditary information. DNA serves this function.
- D) Linear DNA enclosed in nuclear envelope — This describes the genetic material in eukaryotic cells. Eukaryotic DNA is linear, and it is contained within a membrane-bound nucleus. Prokaryotes have circular DNA and lack a nuclear envelope.
