Cell Structure and Types (4.3 and 4.4)

Question 1

Types of cells

Answer 1

Cells are of two distinct types: prokaryotic and eukaryotic.

Question 2

Prokaryotic cells

Answer 2

A type of cell lacking a membrane-enclosed nucleus and other membrane-enclosed organelles. They are found only in the domains Bacteria and Archaea.

Question 3

Note 1 —-»

Answer 3

Prokaryotic cells were the first to evolve and were Earth’s sole inhabitants for more than 1.5 billion years. Evidence indicates that eukaryotic cells evolved from some of these ancestral cells about 1.8 billion years ago. Biologists recognize three domains or major groups of organisms. The microorganisms placed in domains Bacteria and Archaea consist of prokaryotic cells. These organisms are known as prokaryotes. All other forms of life are placed in the domain Eukarya. They are composed of eukaryotic cells and are referred to as eukaryotes.

Question 4

Eukaryotic cells

Answer 4

A type of cell that has a membrane-enclosed nucleus and membrane-enclose organelles. All organisms, except for bacteria and archaea are composed of eukaryotic cells.

Question 5

Note 2 —-»

Answer 5

Eukaryotic cells are distinguished by having a membrane-enclosed nucleus, which houses most of their DNA, and many membrane-enclosed organelles that perform specific functions. Prokaryotic cells are smaller and simpler in structure. Both types of cells, however, share certain basic features. In addition to being bounded by a plasma membrane, the interior of all cells is filled with a thick, jelly-like fluid called cytosol, in which cellular components are suspended. All cells have one or more chromosomes, which carry genes made of DNA. They also contain ribosomes, tiny structures that make proteins according to instructions from the genes. The inside of both types of cells is called the cytoplasm. However, in eukaryotic cells, this term refers only to the region between the nucleus and the plasma membrane.

Question 6

Cytosol

Answer 6

The semi-fluid portion of the cytoplasm.

Question 7

Ribosome

Answer 7

A cell organelle consisting of RNA and protein organized into two subunits and functions as the site of protein synthesis in the cytoplasm.
Note —-» In eukaryotic cells, the ribosomal subunits are constructed in the nucleolus.

Question 8

Cytoplasm

Answer 8

The contents of a eukaryotic cell between the plasma membrane and the nucleus that consists of a semifluid medium and organelles.

Question 9

Note 3 —-»

Answer 9

The DNA is coiled into a region called the nucleoid (“nucleus-like”), but no membrane surrounds the DNA.

Question 10

Nucleoid

Answer 10

A non-membrane bounded region in the prokaryotic cell where the DNA is concentrated.

Question 11

Note 4 —-»

Answer 11

The ribosomes of prokaryotes are smaller and differ somewhat from those of eukaryotes. These molecular differences are the basis for the action of some antibiotics, which specifically target prokaryotic ribosomes. Thus, protein synthesis can be blocked for the bacterium that’s invaded you, but not for you, the eukaryote who is taking the drug.

Question 12

Note 5 —-»

Answer 12

Outside the plasma membrane of most prokaryotes is a fairly rigid, chemically complex cell wall. The wall protects the cell and helps maintain its shape. Some antibiotics, such as penicillin, prevent the formation of these protective walls. Again, because your cells don’t have such walls, these antibiotics can kill invading bacteria without harming your cells. Certain prokaryotes have a sticky outer coat called a capsule around the cell wall, helping to glue the cells to surfaces or to other cells in a colony. In addition to capsules, some prokaryotes have surface projections. Short projections help attach prokaryotes to each other or their substrate. Longer projections called flagella (singular, flagellum) propel a cell through its liquid environment.

Question 13

Flagella

Answer 13

A long cellular appendage specialized for locomotion.

Note —-» The flagella of prokaryotes and eukaryotes differ in both structure and function.

Question 14

List three features that are common to prokaryotic and eukaryotic cells.
List three features that differ.

Answer 14

Both types of cells have plasma membranes, chromosomes containing DNA, and ribosomes.
Prokaryotic cells are smaller, do not have a nucleus or other membrane-enclosed organelles, and have somewhat different ribosomes.

Question 15

Note 6 —-»

Answer 15

All eukaryotic cells—whether from protists (a diverse group of mostly unicellular organisms), fungi, animals, or plants—are fundamentally similar to one another and profoundly different from prokaryotic cells. Let’s look at an animal cell and a plant cell as representatives of the eukaryotes.

Question 16

Note 7 —-»

Answer 16

Cells have multiple copies of all of these structures (except for the nucleus). Your cells have hundreds of mitochondria and millions of ribosomes. A plant cell may have 30 chloroplasts packed inside. Cells also have different shapes and relative proportions of cell parts, depending on their specialized functions.

Question 17

Note 8 —-»

Answer 17

The most obvious hallmark of a eukaryotic cell is its nucleus. But it also contains various other organelles (“little organs”), which perform specific tasks. Just as the cell itself is wrapped in a membrane made of phospholipids and proteins that perform various functions, each organelle is bounded by a membrane with a lipid and protein composition that suits its function.

Question 18

Organelle

Answer 18

A membrane-enclosed structure, with a specialized function within a cell.

Question 19

Note 9 —-»

Answer 19

The organelles and other structures of eukaryotic cells can be organized into four basic functional groups:
1. The nucleus and ribosomes carry out the genetic control of the cell.
2. Organelles involved in the manufacture, distribution, and breakdown of molecules include the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and peroxisomes.
3. Mitochondria in all cells and chloroplasts in plant cells function in energy processing.
4. Structural support, movement, and communication between cells are the functions of the cytoskeleton, plasma membrane, and plant cell wall.
The cellular components identified in these two figures will be examined in detail in the modules that follow.

Question 20

Cellular Metabolism

Answer 20

All the chemical activities of a cell.

Question 21

Note 10 —-»

Answer 21

In essence, the internal membranes of a eukaryotic cell partition it into functional compartments in which many of its chemical activities—collectively called cellular metabolism—take place. In fact, various enzymes essential for metabolic processes are built into the membranes of organelles. The fluid-filled spaces within such compartments are locations where specific chemical conditions are maintained. These conditions vary among organelles and favor the metabolic processes occurring in each. For example, while a part of the endoplasmic reticulum is engaged in making hormones, neighboring peroxisomes may be detoxifying harmful compounds and making hydrogen peroxide (H2O2) as a poisonous by-product of their activities. But because the H2O2 is confined within the peroxisomes, where it is converted to H2O by resident enzymes, the rest of the cell is protected.

Question 22

Note 11 —-»

Answer 22

Except for lysosomes and centrosomes, the organelles and other structures of animal cells are found in plant cells. Also, although some animal cells have flagella or cilia, among plants, only the sperm cells of a few species have flagella.

Question 23

Not1 12 —-»

Answer 23

A plant cell also has some structures that an animal cell lacks. For example, a plant cell has a rigid, rather thick cell wall. Chemically different from prokaryotic cell walls, plant cell walls contain the polysaccharide cellulose. Plasmodesmata (singular, plasmodesma) are cytoplasmic channels through cell walls that connect adjacent cells. An important organelle found in plant cells is the chloroplast, where photosynthesis occurs. Unique to plant cells is a large central vacuole, a compartment that stores water and a variety of chemicals.

Question 24

Note 13 —-»

Answer 24

Eukaryotic cells contain nonmembranous structures as well. The cytoskeleton, which you were introduced to in the chapter introduction, is composed of different types of protein fibers that extend throughout the cell. And ribosomes are found in the cytosol as well as attached to certain membranes.

Question 25

Identify the structures in the plant cell that are not present in the animal cell.

Answer 25

Chloroplasts, central vacuole, cell wall, and plasmodesmata