3.1 Summary Questions & Exam Style Questions Flashcards
Chapter 3 - Biological Molecules
Explain how atoms join together to form molecules. [2]
Atoms form bonds with each other when pairs of electrons are shared (1) according to the bonding
rules (1).
Explain the difference between a cation and an anion. [4]
A cation is an ion with a net positive charge (1), i.e. it has lost one or more electrons (1). An
anion is an ion with a net negative charge (1), i.e. it has gained one or more electrons (1).
Explain how the bonds between the atoms in both water and carbon dioxide molecules fulfill the ‘bonding rules’. [4]
Water – one oxygen atom binds to two hydrogen atoms (1), oxygen can form two bonds, each
hydrogen can only form one bond (1). Carbon dioxide – one carbon atom and two oxygen atoms
(1), carbon can form four bonds, each oxygen atom can form two bonds, therefore carbon forms a
double bond with each oxygen atom (1).
The image below, obtained in 1953, helped confirm the recently proposed structure of DNA. The equipment that was used to obtain the image is also shown, X-ray diffraction photograph of DNA (deoxyribonucleic acid). This image was obtained in 1953 and results from a beam of X-rays being scattered onto a photographic plate by the DNA. Various features about the structure of the DNA can be determined from the pattern of spots and bands. The cross of the bands indicates the helical nature of DNA. (Image from OCR A textbook pg.43)
a) Suggest why the X-ray diffraction technique used to produce this image was not considered a form of microscopy but the use of electrons to produce the images is called electron microscopy. [3]
b) Explain why cells are visible with light microscopes but electron microscopes are needed to see ribosomes. [3]
a) X ray diffraction does not involves lenses (1) / electron microscope uses electromagnetic lenses
(1) / beams focused in electron microscopy to produce image (1).
b) Cells are larger than ribosomes (1) / cells are larger than, half the wavelength / resolution limit, of
light (1) / electron microscopes have greater resolution (than light microscopes)
(1) / idea that molecules are smaller than resolution limit of light and larger than resolution limit of
electron beam (1).
