Chapter 4 Answers
Cells
Visual Understanding
Fig 4.3
The first microscope was used in about 1590. Electron microscopes came into common
use about 70 years ago. Just over 100 years ago most physicians did not wash up
between patients, even when someone had just died, or was very sick. Explain why it
took so long to convince doctors to wash their hands.
Light microscopes were not sufficiently powerful to be able to see bacteria. There was no
evidence that physicians carried germs on their hands from patient to patient. Ignaz
Philipp Semmelweiss (1818-1865), worked at Vienna Medical School. At the time, the
Vienna Lying-In hospital had two maternity wards, one staffed by midwives, and the
other by medical students. The mortality rate among women attended by midwives was
approximately 2-3%; however, the students’ ward had a rate of 10% or more.
Semmelweiss believed that the students, who received much of their medical training in
the autopsy room, were carrying infections from cadavers they dissected to the women in
the ward. His work was ignored by others, and it wasn’t until Lister and Pasteur in the
1860’s and 1870s experimented with bacteria (still invisible as individuals, but whose
presence could be detected) that handwashing in hospitals began to be more common,
although it is still a problem today.
Fig. 4.6
A hormone molecule is a messenger, circulating through the bloodstream. It needs to
find only a particular subset of cells to deliver its message. How does the hormone find
the correct cells?
The protein molecules embedded in each cell’s membrane do many things. Some of the
proteins are there specifically to serve as recognition signals and docking bays, rather
like a neon sign and a parking lot. The hormone molecule sees the correct “sign” and
pulls into the correct parking place.
Challenge Questions
World of Cells
You are designing a new single-celled organism. Discuss the problems of size, getting
molecules such as nutrients and wastes in and out, temperature, and energy.
If you design a very small cell, it will lose a lot of heat to its surroundings because of its
low surface to volume ratio. The larger it becomes, the greater the problems with
obtaining enough nutrients, moving nutrients and wastes in, out, and around the cell, and
making enough energy to keep the cell healthy and active. Since the larger the cell, the
higher the surface to volume ratio, it may also have some trouble just getting all of the
nutrients and wastes through the cell membrane. Imagine everyone at a football game
all having to enter and leave the stadium through one or two small doors.
Kinds of Cells
Antibiotics are medicines that target bacterial infections in vertebrates. How can an
antibiotic kill all the bacterial cells and not harm vertebrate cells; what part of the
bacterial cell must antibiotics be targeting and why?
One major difference between bacterial cells and vertebrate cells is that bacteria have a
cell wall outside of the cell membrane. Therefore most antibiotics target something
having to do with making the components for the cell wall, assembling the cell wall, or
keeping the cell wall intact. This means that they don’t harm vertebrate cells at all, since
they don’t have cell walls.
Tour of a Eukaryotic Cell
Compare the organelles in a typical animal cell to the parts of a city; e.g. the nucleus is
city hall, and the DNA is all the city’s laws and instructions, and the endoplasmic
reticulum is the city streets.
Examples may vary: mitochondria as the power plant / generating station, ribosomes as
the factories, Golgi complex as the packaging plants, peroxisomes as the factories that
use hazardous materials, lysosomes as the garbage collectors and recycling system,
plasma membrane as the city limits
Transport Across Cell Membranes
Compare the mechanisms required for a cell to obtain all the different kinds of molecules
that it needs.
The smallest, simplest molecules can slip directly through the plasma membrane in the
processes of diffusion and osmosis. Some larger molecules, such as nutrients and
amounts of some liquids, are taken into the cell by forming a hollow in the cell
membrane, filling the hollow with the desired substance, then surrounding the substance
with the cell membrane and “pinching off” the loop of membrane to form a packet, now
inside the cell. This can also be reversed. If it is a single particle coming into the cell it
is called endocytosis; leaving the cell is exocytosis. If it is several small particles coming
into the cell it is phagocytosis, if it is a liquid it is pinocytosis.
Certain molecules that the cell needs will attach to receptors on the membrane surface
and be drawn in through receptor-mediated endocytosis.
There are places on the cell membrane with small channels that allow only certain
molecules to pass; this is selective diffusion. In other places the desired molecule
attaches to a special carrier protein that guides it through the channel like a tugboat
bringing a large ship into a harbor; this is facilitated diffusion – facilitated means made
easier.
Some molecules are so important that the cell will expend energy bringing them across
the cell membrane; this is active transport, such as the sodium-potassium pump.