Chapter 4

Biology and Society: Antibiotics: Drugs that Target Bacterial Cells

           Antibiotics were first isolated from mold in 1928.

           The widespread use of antibiotics drastically decreased deaths from bacterial infections.

 

           Most antibiotics kill bacteria while minimally harming the human host by binding to structures found only on bacterial cells.

           Some antibiotics bind to the bacterial ribosome, leaving human ribosomes unaffected.

           Other antibiotics target enzymes found only in the bacterial cells.

 

THE MICROSCOPIC WORLD OF CELLS

           Organisms are either

          single-celled, such as most prokaryotes and protists, or

          multicelled, such as

            plants,
            animals, and
            most fungi.

 

Microscopes as Windows on the World of Cells

           Light microscopes can be used to explore the structures and functions of cells.

           When scientists examine a specimen on a microscope slide,

          light passes through the specimen and

          lenses enlarge, or magnify, the image.

           The most powerful electron microscopes can

          magnify up to 100,000 times and

          distinguish between objects 0.2 nanometers apart.

           Light microscopes are still very useful for studying living cells.

 

The Two Major Categories of Cells

           The countless cells on Earth fall into two basic categories:

           Prokaryotic cells — Bacteria and Archaea and

           Eukaryotic cellsprotists, plants, fungi, and                    animals.

           All cells have several basic features.

          They are all bounded by a thin plasma membrane.

          Inside all cells is a thick, jelly-like fluid called
the cytosol, in which cellular components are suspended.

          All cells have one or more chromosomes carrying genes made of DNA.

          All cells have ribosomes, tiny structures that build proteins according to the instructions from the DNA.

           Prokaryotic cells are older than eukaryotic cells.

          Prokaryotes appeared about 3.5 billion years ago.

          Eukaryotes appeared about 2.1 billion years ago.

           Prokaryotic cells are

          usually smaller than eukaryotic cells and

          simpler in structure.

           Eukaryotes

          Only eukaryotic cells have organelles, membrane-enclosed structures that perform specific functions.

          The most important organelle is the nucleus, which

            houses most of a eukaryotic cells DNA and
            is surrounded by a double membrane.

           A prokaryotic cell lacks a nucleus. Its DNA is coiled into a nucleus-like region called the nucleoid, which is not partitioned from the rest of the cell by membranes.

 

An Overview of Eukaryotic Cells

           Eukaryotic cells are fundamentally similar.

           The region between the nucleus and plasma membrane is the cytoplasm.

           The cytoplasm consists of various organelles suspended in the liquid cytosol.

           Unlike animal cells, plant cells have

          chloroplasts, which convert light energy to the chemical energy of food in the process of photosynthesis, and

          protective cell walls.

           Only animal cells have lysosomes, bubbles of digestive enzymes surrounded by membranes.

 

MEMBRANE STRUCTURE

           The plasma membrane separates the living cell from its nonliving surroundings.

 

           The remarkably thin membranes of cells are composed mostly of

          lipids and

          proteins.

           The lipids belong to a special category called phospholipids.

           Phospholipids form a two-layered membrane, the phospholipid bilayer.

 

 

The Process of Science:
What Makes a Superbug?

           Particularly dangerous strains of bacteria, known as MRSA, are unaffected by several common antibiotics.

           Observation: Some bacteria use a protein called PSM to disable human immune cells by forming holes that rip apart the plasma membrane.

           Question: Does PSM play a role in MRSA infections?

           Hypothesis: MRSA bacteria lacking the ability to produce PSM would be less deadly than normal MRSA strains.

           Experiment: Researchers infected

          seven mice with normal MRSA and

          eight mice with MRSA that does not produce PSM.

           Results:

          All seven mice infected with normal MRSA died.

          Five of the eight mice infected with MRSA that does not produce PSM survived.

           Conclusions:

          MRSA strains appear to use the membrane-destroying PSM protein, but

          factors other than PSM protein contributed to the death of mice (possibly other membrane-destroying proteins).

 

Cell Surfaces

           Plant cells have rigid cell walls surrounding the membrane.

           Plant cell walls

          are made of cellulose,

          protect the cells,

          maintain cell shape, and

          keep cells from absorbing too much water.

           Animal cells

          lack cell walls and

          typically have an extracellular matrix, which

            helps hold cells together in tissues and
            protects and supports them.

           The surfaces of most animal cells contain cell junctions, structures that connect cells together into tissues, allowing them to function in a coordinated way.

 

THE NUCLEUS AND RIBOSOMES: GENETIC CONTROL OF THE CELL

           The nucleus is the chief executive of the cell.

          Genes in the nucleus store information necessary to produce proteins.

          Proteins do most of the work of the cell.

 

Structure and Function of the Nucleus

           The nucleus is separated from the cytoplasm by a double membrane called the nuclear envelope.

           Pores in the envelope allow materials to move between the nucleus and cytoplasm.

           The nucleus contains a nucleolus where ribosomes are made.

 

           Stored in the nucleus are long DNA molecules and associated proteins that form fibers called chromatin.

           Each long chromatin fiber constitutes one chromosome.

           The number of chromosomes in a cell depends on the species.

 

Ribosomes

           Ribosomes are responsible for protein synthesis.

           Ribosome components are made in the nucleolus but assembled in the cytoplasm.

 

           Ribosomes may assemble proteins while the ribosomes are

          suspended in the fluid of the cytoplasm or

          attached to the outside of the nucleus or an organelle called the endoplasmic reticulum.

 

How DNA Directs Protein Production

           DNA programs protein production in the cytoplasm by transferring its coded information into messenger RNA (mRNA).

           Messenger RNA exits the nucleus through pores in the nuclear envelope.

           A ribosome moves along the mRNA, translating the genetic message into a protein with a specific amino acid sequence.

 

THE ENDOMEMBRANE SYSTEM: MANUFACTURING AND DISTRIBUTING CELLULAR PRODUCTS

           Many membranous organelles forming the endomembrane system in a cell are interconnected either

            directly by their membranes or

            by transfer of membrane segments between them.

 

The Endoplasmic Reticulum

           The endoplasmic reticulum (ER) is one of the main manufacturing facilities in a cell.

           The ER

          produces an enormous variety of molecules,

          is connected to the nuclear envelope, and

          is composed of smooth and rough ER.

Rough ER

           The rough in rough ER refers to ribosomes that stud the outside of this portion of the ER membrane.

           These ribosomes produce membrane proteins and secretory proteins.

           Some products manufactured by rough ER are dispatched to other locations in the cell by transport vesicles, sacs made of membrane that bud off from the rough ER.

Smooth ER

           The smooth ER

          lacks surface ribosomes,

          produces lipids, including steroids, and

          helps liver cells detoxify circulating drugs.

 

The Golgi Apparatus

           The Golgi apparatus

          works in partnership with the ER and

          receives, refines, stores, and distributes chemical products of the cell.

 

Lysosomes

           A lysosome is a membrane-bound sac of digestive enzymes found in animal cells.

           Lysosomes are absent from most plant cells.

           Enzymes in a lysosome can break down large molecules such as

          proteins,

          polysaccharides,

          fats, and

          nucleic acids.

 

           Lysosomes have several types of digestive functions.

          Many cells engulf nutrients in tiny cytoplasmic sacs called food vacuoles.

          These food vacuoles fuse with lysosomes, exposing food to enzymes to digest the food.

          Small molecules from digestion leave the lysosome and nourish the cell.

           Lysosomes can also

          destroy harmful bacteria,

          break down damaged organelles, and

          sculpt tissues during embryonic development, helping to form structures such as fingers.

 

Vacuoles

           Vacuoles are large sacs of membrane that bud from the

          ER,

          Golgi apparatus, or

          plasma membrane.

           Contractile vacuoles of protists pump out excess water in the cell.

           Central vacuoles of plants

          store organic nutrients,

          absorb water, and

          may contain pigments or poisons.

 

CHLOROPLASTS AND MITOCHONDRIA: ENERGY CONVERSION

           Cells require a continuous energy supply to perform the work of life.

           Two organelles act as cellular power stations:

            chloroplasts and

            mitochondria.

 

Chloroplasts

           Most of the living world runs on the energy provided by photosynthesis.

           Photosynthesis is the conversion of light energy from the sun to the chemical energy of sugar and other organic molecules.

           Chloroplasts are

          unique to the photosynthetic cells of plants and algae and

          the organelles that perform photosynthesis.

           Chloroplasts are divided into three major compartments by internal membranes:

             the space between the two membranes,

             the stroma, a thick fluid within the chloroplast, and

             the space within grana, membrane-enclosed discs and tubes that trap light energy and convert it to chemical energy.

 

Mitochondria

           Mitochondria

          are the organelles of cellular respiration,

          are found in almost all eukaryotic cells, and

          produce ATP from the energy of food molecules.

           An envelope of two membranes encloses the mitochondrion:

            an outer smooth membrane and

            an inner membrane that

            has numerous infoldings called cristae and
            encloses a thick fluid called the matrix.

           Mitochondria and chloroplasts contain their own DNA, which encodes some of their proteins.

           This DNA is evidence that mitochondria and chloroplasts evolved from free-living prokaryotes in the distant past.

 

THE CYTOSKELETON: CELL SHAPE AND MOVEMENT

           The cytoskeleton is a network of fibers extending throughout the cytoplasm.

 

Maintaining Cell Shape

           The cytoskeleton

          provides mechanical support to the cell and

          helps a cell maintain its shape.

           The cytoskeleton contains several types of fibers made from different proteins:

          Microtubules are straight and hollow tubes that guide the movement of organelles and chromosomes.

          Intermediate filaments and microfilaments are thinner and solid.

           The cytoskeleton provides anchorage and reinforcement for many organelles.

           The cytoskeleton is dynamic.

           Changes in the cytoskeleton contribute to the amoeboid (crawling) movements of

          the protist Amoeba and

          some of our white blood cells.

 

Cilia and Flagella

           Cilia and flagella are motile appendages that aid in movement.

          Flagella propel the cell through their undulating, whiplike motion.

          Cilia move in a coordinated back-and-forth motion.

          Cilia and flagella have the same basic architecture, but cilia are generally shorter and more numerous than flagella.

 

Evolution Connection:
The Evolution of Antibiotic Resistance

           Many antibiotics disrupt cellular structures of invading microorganisms.

           Introduced in the 1940s, penicillin worked well against such infections.

           But over time, bacteria that were resistant to antibiotics, such as the MRSA strain, were favored.

           The widespread use and abuse of antibiotics continue to favor bacteria that resist antibiotics.