All Free Biology MCQs with Answers

321. The statement that all cells arise from pre-existing cells was added to the cell theory by

• A. Robert Hooke
• B. Schleiden and Schwann
• C. Rudolf Virchow
• D. Anton van Leeuwenhoek

Explanation: Schleiden and Schwann established that plants and animals are made of cells, but it was Virchow who added in 1855 that every cell comes from the division of another, which ruled out spontaneous generation. Hooke first used the word cell after looking at cork, and Leeuwenhoek first saw living microorganisms. The three statements together form the modern cell theory.

322. The large central vacuole of a mature plant cell is bounded by a membrane called the

• A. tonoplast
• B. plasmalemma
• C. nuclear envelope
• D. cell wall

Explanation: The tonoplast controls what enters and leaves the cell sap, and the vacuole's contents create the osmotic pressure that keeps the cell turgid and so supports the non woody parts of a plant. It also stores sugars, salts, pigments and waste products. The plasmalemma is the plasma membrane at the cell surface, and the cell wall outside it is not a membrane at all.

323. Proteins made on the rough endoplasmic reticulum are modified, sorted and packaged for export by the

• A. lysosome
• B. nucleolus
• C. peroxisome
• D. Golgi apparatus

Explanation: Vesicles carry newly made protein from the endoplasmic reticulum to the Golgi, where sugars are added, the product is sorted and it is packed into vesicles for secretion or for use inside the cell. Cells that secrete heavily, such as those of the pancreas, have a large and active Golgi. Lysosomes are one of the products of the Golgi rather than an alternative to it.

324. The nucleolus is the site of

• A. DNA replication
• B. synthesis of ribosomal RNA and assembly of ribosomal subunits
• C. protein synthesis for export
• D. storage of the cell's lipid reserve

Explanation: The nucleolus is a dense region of the nucleus with no membrane around it, and it transcribes ribosomal RNA and combines it with proteins to form the subunits that are then exported through the nuclear pores. Cells making large quantities of protein have prominent nucleoli, which is a useful clue under the microscope. Protein synthesis itself takes place on the ribosomes in the cytoplasm.

325. The nuclear envelope differs from the plasma membrane in that it

• A. is made of protein only
• B. has no pores of any kind
• C. consists of two membranes perforated by pores
• D. is freely permeable to all molecules

Explanation: Two membranes enclose the nucleus, and the pores that pierce them at intervals let mRNA and ribosomal subunits out and let nucleotides, enzymes and regulatory proteins in, while keeping the chromosomes separate from the cytoplasm. The outer membrane is continuous with the rough endoplasmic reticulum. Free permeability would defeat the whole purpose of enclosing the genetic material.

326. Which structure is normally absent from the cells of higher plants?

• A. Centrioles
• B. Mitochondria
• C. Golgi apparatus
• D. Ribosomes

Explanation: Higher plant cells organise their spindle without centrioles, which is why the presence of a pair of centrioles beside the nucleus is taken as an animal cell feature in a diagram question. Centrioles are present in animal cells and in the motile cells of lower plants. Mitochondria, Golgi and ribosomes are common to both kingdoms.

327. Plasmodesmata are

• A. pores in the nuclear envelope
• B. the enzymes that digest the cell wall
• C. protein channels in the plasma membrane of animal cells
• D. cytoplasmic strands passing through the walls and connecting adjacent plant cells

Explanation: These fine channels of cytoplasm run through pits in the cell walls and join neighbouring plant cells into one continuous system, so water, ions and small molecules can move from cell to cell without crossing the wall. That route is called the symplast pathway. The equivalent connections in animal tissues are gap junctions.

328. An electron microscope can reveal detail that a light microscope cannot because

• A. it magnifies the specimen many more times
• B. the wavelength of an electron beam is far shorter, giving much greater resolving power
• C. it can be used on living specimens
• D. it uses ultraviolet light instead of visible light

Explanation: Resolution is limited by wavelength, and the very short wavelength of an electron beam allows two points less than a nanometre apart to be seen as separate, against about 200 nanometres for a light microscope. Magnification without resolution simply enlarges a blur, which is the distinction the question is testing. The specimen must be dead and in a vacuum, so living material cannot be examined.

329. The cell wall of a fungus is made mainly of

• A. cellulose
• B. peptidoglycan
• C. chitin
• D. lignin

Explanation: Fungal walls are built from chitin, the same nitrogen containing polysaccharide found in the exoskeleton of insects, which is one reason fungi are placed in their own kingdom rather than with plants. Plant walls are cellulose and bacterial walls are peptidoglycan. Lignin is an additional strengthening material deposited in the walls of woody plant tissue.

330. The cell wall of a bacterium is composed of

• A. peptidoglycan
• B. cellulose
• C. chitin
• D. phospholipid

Explanation: Peptidoglycan, also called murein, is a mesh of sugar chains cross linked by short peptides, and it is unique to bacteria, which is why penicillin can block its synthesis and kill a growing bacterium without harming human cells. The Gram stain works by distinguishing a thick peptidoglycan layer from a thin one. A phospholipid bilayer is a membrane, not a wall.