Cell Structure and Types (4.1 and 4.2)

Question 1

Light Microscope (LM)

Answer 1

An optical instrument with lenses that refract (bend) visible light to magnify images and project them into a viewer’s eye or onto photographic film.

Question 2

Resolution

Answer 2

An important factor in microscopy is resolution, a measure of the clarity of an image. Resolution is the ability to distinguish two nearby objects as separate.

Question 3

Cell Theory

Answer 3

The theory that all living beings are composed of cells and that all cells come from other cells.

Question 4

Electron Microscope (EM)

Answer 4

A microscope that uses magnets to focus an electron beam through, or onto the surface of, a specimen.
Note —-» An electron microscope achieves a hundredfold greater resolution than a light microscope.

Question 5

Electron Microscope (Note)

Answer 5

Electron microscopes can distinguish biological structures as small as about 2 nanometers (nm), a 100-fold improvement over the light microscope. This high resolution has enabled biologists to explore cell ultrastructure, the complex internal anatomy of a cell.

Question 6

Scanning Electron Microscope (SEM)

Answer 6

A microscope that uses an electron beam to study the surface details of a cell or other specimens.

Question 7

Scanning Electron Microscope

Answer 7

The beam excites electrons on the surface, and these electrons are then detected by a device that translates their pattern into an image projected onto a video screen.

Question 8

Transmission Electron Microscope (TEM)

Answer 8

A microscope that uses an electron beam to study the internal structure of thinly sectioned specimens.

Question 9

Note 1 —-»

Answer 9

The TEM aims an electron beam through a very thin section of a specimen, just as a light microscope aims a beam of light through a specimen. The section is stained with atoms of heavy metals, which attach to certain cellular structures more than others. Electrons are scattered by these more dense parts, and the image is created by the pattern of transmitted electrons. Instead of using glass lenses, both the SEM and TEM use electromagnets as lenses to bend the paths of the electrons, magnifying and focusing the image onto a monitor. SEMs and TEMs are initially black and white but are often artificially colorized, as they are here, to highlight or clarify structural features.

Question 10

Note 2 —-»

Answer 10

Electron microscopes have truly revolutionized the study of cells and their structures. Nonetheless, they have not replaced the light microscope: Electron microscopes cannot be used to study living specimens because of the methods used to prepare the specimen to kill the cells.

Question 11

Note 3 —-»

Answer 11

Certain bacteria are as small as 0.2 μm and can barely be seen with a light microscope, whereas chicken eggs are large enough to be seen with the unaided eye. A single nerve cell running from the base of your spinal cord to your big toe may be 1 m in length, although it is so thin you would still need a microscope to see it.

Question 12

Which type of microscope would you use to study:

a. the changes in shape of a living human white blood cell
b. the finest details of surface texture of a human hair
c. the detailed structure of an organelle in a liver cell

Answer 12

a. Light Microscope
b. Scanning Electron Microscope
c. Transmission Electron Microscope

Question 13

4.2

Answer 13

Chapter 4.2

Question 14

Are there advantages to being so small?

But why aren’t most cells as large as chicken eggs?

Answer 14

The logistics of carrying out a cell’s functions appear to set both lower and upper limits on cell size. At a minimum, a cell must be large enough to house enough DNA, protein molecules, and structures to survive and reproduce.
The maximum size of a cell is influenced by geometry—the need to have a surface area large enough to service the volume of a cell. Active cells have a huge amount of traffic across their outer surface. A chicken egg cell isn’t very active, but once a chick embryo starts to develop, the egg is divided into many microscopic cells, each bounded by a membrane that allows the essential flow of oxygen, nutrients, and wastes across its surface.

Question 15

Note 4 —-»

Answer 15

Large cells have more surface area than small cells, but they have a much smaller surface area relative to their volume than small cells.

Question 16

Volume of a cube

Answer 16

Volume of a cube = Height x Width x Length

Question 17

Surface area of a cube

Answer 17

# of sides = 6
Surface area = 6 times (height and width)
∴ Surface area = 6(Height x Width)

Question 18

Plasma Membrane —–>

Answer 18

The membrane at the boundary of every cell that acts as a selective barrier to the passage of ions and molecules into and out of the cell.

Question 19

What is a cell’s surface like? And how does it control the traffic of molecules across it?

Answer 19

The plasma membrane, also referred to as the cell membrane, forms a flexible boundary between the living cell and its surroundings. For a structure that separates life from nonlife, this membrane is amazingly thin. It would take a stack of more than 8,000 plasma membranes to equal the thickness of a page of paper. And, as you have come to expect with all things biological, the structure of the plasma membrane correlates with its function.

Question 20

Note 5 —-»

Answer 20

Phospholipid molecules are well suited to their role as a major constituent of biological membranes. Each phospholipid is composed of two distinct regions—a head with a negatively charged phosphate group and two nonpolar fatty acid tails. Phospholipids group together to form a two-layer sheet called a phospholipid bilayer.

Question 21

Note 6 —-»

Answer 21

The phospholipids’ hydrophilic (water-loving) heads face outward, exposed to the aqueous solutions on both sides of a membrane. Their hydrophobic (water-fearing) tails point inward, mingling together and shielded from water. Embedded in this lipid bilayer are diverse proteins, floating like icebergs in a phospholipid sea. The regions of the proteins within the center of the membrane are hydrophobic; the exterior sections exposed to water are hydrophilic.

Question 22

Note 7 —-»

Answer 22

Nonpolar molecules, such as O2 and CO2, can easily move across the membrane’s hydrophobic interior. Some of the membrane’s proteins form channels (tunnels) that shield ions and polar molecules as they pass through the hydrophobic center of the membrane. Still, other proteins serve as pumps, using energy to actively transport molecules into or out of the cell.

Question 23

To convince yourself that a small cell has a greater surface area relative to volume than a large cell, compare the surface-to-volume ratios of the large cube and one of the small cubes in Figure 4.2A. —->

Answer 23

Large cube: 54/27 = 2
Small cube: 6/1 = 6 (surface area is 1 × 1 × 6 sides = 6 units2)
Volume is 1 × 1 × 1 (unit3)