Free Cell Structure and Function MCQs with Answers

21. 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.

22. 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.

23. 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.

24. 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.

25. The flagellum of a eukaryotic cell differs from that of a bacterium in that it

• A. is made of the protein flagellin
• B. rotates like a propeller at its base
• C. is shorter and never used for movement
• D. has a 9 plus 2 arrangement of microtubules enclosed by the plasma membrane

Explanation: A eukaryotic flagellum, as in a sperm, is an extension of the cell containing nine outer pairs of microtubules around a central pair, and it beats by the sliding of these microtubules using ATP. A bacterial flagellum is a much simpler solid filament of flagellin outside the cell that rotates like a screw. The two structures are not homologous despite sharing a name.

26. Active transport differs from facilitated diffusion in that active transport

• A. does not require a carrier protein
• B. requires ATP and can move a substance against its concentration gradient
• C. occurs only in plant cells
• D. moves only water molecules

Explanation: Both use carrier proteins, but facilitated diffusion is passive and can only run down a gradient, whereas active transport spends ATP and can therefore concentrate a substance on one side of the membrane. This is how glucose is fully reabsorbed in the kidney tubule and how root hair cells take in mineral ions from very dilute soil water. Cells performing much active transport are rich in mitochondria.

27. Osmosis is best defined as the movement of water molecules

• A. from a region of low water potential to one of high water potential
• B. against a concentration gradient using ATP
• C. from a region of higher water potential to one of lower water potential through a partially permeable membrane
• D. along with dissolved solutes through a partially permeable membrane

Explanation: Water always moves down its own water potential gradient, that is from the more dilute solution to the more concentrated one, and the membrane is what stops the solute simply moving the other way. Osmosis is a special case of diffusion, so it is passive and needs no ATP. Defining it in terms of water potential rather than concentration avoids the confusion of talking about the concentration of water itself.

28. A plant cell placed in pure water does not burst because

• A. the rigid cellulose wall resists further expansion once the cell is turgid
• B. water cannot enter the cell at all
• C. the vacuole absorbs the excess water permanently
• D. the plasma membrane is impermeable to water

Explanation: Water enters by osmosis until the wall pushes back with a pressure equal to the inward osmotic pull, and the cell stops taking in more water and is described as turgid. Turgor is what supports herbaceous plants, so wilting follows when it is lost. An animal cell has no wall and therefore does burst in the same conditions.

29. A red blood cell placed in distilled water will

• A. shrink and become crenated
• B. remain unchanged
• C. become turgid but survive
• D. swell and burst

Explanation: Water enters the cell by osmosis because the cytoplasm has the lower water potential, and with no cell wall to resist the pressure the membrane stretches until it ruptures, a process called haemolysis. Placed in a concentrated salt solution the same cell loses water and becomes crenated instead. This is why fluids given intravenously must be isotonic with blood plasma.

30. Plasmolysis is said to occur when

• A. a plant cell takes in water and becomes turgid
• B. the protoplast of a plant cell shrinks away from the cell wall in a concentrated solution
• C. the cell wall dissolves completely
• D. the nucleus divides into two

Explanation: In a solution of lower water potential than the cell sap, water leaves the vacuole, the protoplast contracts and pulls away from the wall, and the cell becomes flaccid and then plasmolysed. Because the wall is fully permeable, the external solution simply fills the space between it and the shrunken protoplast. The process can be reversed if the cell is returned to water soon enough.