20 April 2005

Lecture 40

Reading, Chapter 13 -16 and 19

A. Prokaryotes (and viruses)

4. Human pathogens

b. Viral diseases

Therapies for HIV infection

Antiviral drugs

Most antibiotics don't work against viral infections because the virus is using your cells to multiply. Most drugs that would stop this would kill your cells too. A few drugs specifically inhibit viral proteins and slow or stop viral growth. For HIV, two drugs are in common use. AZT inhibits the reverse transcriptase that copies the viral RNA into DNA that can integrate into the host cell chromosome. Protease inhibitors inhibit the viral protease that is needed to cut HIV proteins to the proper length for assembly into new viruses.

Antiviral drugs are used in combination to treat HIV infection. This delays the evolution of drug resistance in the HIV of the infection. HIV evolves rapidly. The reverse transcriptase that copies viral RNA genes into DNA makes a lot of errors. This leads to small changes in HIV proteins such that anti-viral drugs don't work for very long. Within an infected individual, HIV strains with AZT-resistant reverse transcriptases and inhibitor-resistant proteases appear after a time. The resistant versions of the two proteins are unchanged enough that they can still do their normal jobs but changed enough that AZT and protease inhibitors no longer bind to them. Combining both AZT and protease inhibitors slows the appearance of resistance, since the chances of random mutations conferring resistance to two different drugs at once are small.

Vaccines

Vaccines work well against many viruses but an effective HIV vaccine has not yet been developed. There are several reasons for this, including the following:

1. Vaccines work by stimulating the immune system to recognize and attack a specific invader. HIV specifically attacks the immune system and suppresses its activity. By its nature, HIV renders any vaccine less effective.

2. HIV evolves rapidly. The reverse transcriptase that copies viral RNA genes into DNA makes a lot of errors. This leads to small changes in HIV proteins such that they are no longer recognized by a vaccinated immune system.

3. While latent, the virus is relatively inaccessible to the immune system.

HIV immunity

A very few people appear to be immune to HIV infection. They have been exposed to HIV many times but never infected. These people have a mutation in the gene that codes for the HIV receptor protein on their T cells. The changes in this protein caused by the mutation prevent HIV from recognizing their T cells and infection does not occur.

 

B. Eukaryotes

The eukaryotes are believed to have arisen from endosymbioses of prokaryotes over a billion years ago. A diagram illustrating the necessary endosymbioses are shown on pages 288 and 319 of your text. The eukaryotes are divided into four kingdoms.

1. Kingdom Protista

The protistans are a very diverse group of organisms. Many are single cells. Others have many cells and can be quite large. Those that are single cells are amazing in that their cells can perform all the functions needed for life: acquiring food, producing all the biomolecules that they need from it, sensing their environment and responding to it. In contrast, no one of your cells can do all of these things.

The protistans are eukaryotes. They have nuclei and mitochondria. Some have chloroplasts. They are thought to be comparable to the first eukaryotic cells, which resulted from endosymbiosis of mitochondria and choloroplasts. Those that have mitochondria are thought to be similar to the ancestors of animals and fungi, both of which lack chloroplasts. Those that have both mitochondria and chloroplasts are thought to be similar to the ancestors of plants.

We will divide the protistans into two general groups based on whether they are photosynthetic or not.

a. Heterotrophs

These are organisms that get their food from other living things by eating them or absorbing the biomolecules released as their bodies decay. You are a heterotroph. Heterotrophic protistans include the following:

Amoebas - Large blob-like single cells that ooze about engulfing bits of debris and other organisms for food.

Paramecium - A cigar-shaped single cell that is covered with small hairs by which it swims.

slime molds - Smears of protoplasm that move across the soil and absorb biomolecules from it. They are often found as small shiny patches on decaying logs.

Parasites - Protistan parasites of humans include the malaria parasite, the Giardia parasite, and the parasite that causes sleeping sickness.

The heterotrophic protistans are sometimes called "protozoa", meaning "first animals". It is thought that the ancestor of Kingdom Animalia was similar to a modern heterotrophic protistan.

b. "Autotrophs"

These are organisms that have chloroplasts and are photosynthetic. They make sugars and other biomolecules from carbon dioxide using sunlight. Thus they are "autotrophs", meaning "self-feeders". Autotrophic protistans and include the following:

Phytoplankton - This word means tiny floating plants. They are algae that are single cells or a few cells. They are found suspended in lakes, ponds, and oceans. Two of these are of note. Diatoms have shells made of a glass-like material that can be quite beautiful. Diatoms in the ocean perform roughly a third of the world's photosynthesis and thus produce a third of the oxygen we breathe. Dinoflagellates are small swimming cells. About half of the different kinds of dinoflagellates have chloroplasts and are autotrophs. The other half are heterotrophs. Dinoflagellates cause "red tides". These are population explosions of dinoflagellates in enclosed bays that can kill fish and make clams and mussels toxic to eat.

Seaweeds - These are the large algae you see at the seashore. Some seaweeds, such as the giant kelps, can be hundreds of feet in length and form underwater forests. They are still protists, though.

The autotrophic protistans are also known as the "algae". Algae perform roughly half of the Earth's photosynthesis and thus make a big contribution to all iving things.

A further distnction of the algae is that the ancestor of land plants is thought to have been a member of the green algae, based on similar biochemistry and gene sequences.

2. Kingdom Fungi

The fungi include molds, yeasts, and mushrooms. They have the following general characteristics:

1) Fungi are composed of eukaryotic cells. The yeasts are mostly single cells. Molds and mushrooms are usually multicellular.

2) Fungi have a filamentous construction. The body of a fungus is typically a tangle of threads, each composed of many cells in a row.

3) Fungi are heterotrophs. Fungi eat by secreting digestive enzymes and digesting the materials around them. They then absorb the resulting "soup" of biomolecules. Their filamentous structure makes fungi very effective at absorbing food in this way. Every cell of the thousands or millions in a fungal filament is in contact with the outside environment from which food is being absorbed. This increases the surface area for absorption and shortens the distance over which food molecules must diffuse.

4) Fungi are non-motile. They have no flagella at any stage in their life history and generally do not exhibit dramatic "crawling" like amoebas. They get around in two ways: by growing to where they need to go (being a filament is good for this) or by dispersing themselves as spores, which are tiny cells that act like seeds and can travel long distances on the wind.

5. Fungal cells have cell walls (as do plant cells). Each cells is encased in a "shell" that is made of a carbohydrate called chitin. Fungal cell walls limit the flexibility of fungal cells, which is why fungi don't crawl much. Also, they protect fungal cells from digesting themselves or being digested by bacteria. Fungal digestive enzymes break down the walls of plant cells, which is usually what is available to eat, but not the chitin walls of the fungal cells. Plants, on the other hand, can secrete digestive enzymes that break down chitin as a way to defend themselves against fungi.

Some examples of fungi include the following:

Bread mold These fungi are always filaments. They do not produce a large structure (mushroom) for dispersing spores. They do produce small "sporangia" for making spores. These are the small black dots you sometime see among the white fuzz on moldy bread.

 

Yeasts These fungi are mostly single cells. Some of them are important in brewing and baking. Brewing yeasts add alcohol to wine and beer, producing it as a byproduct of their anaerobic respiration (fermentation). Baking yeasts add carbon dioxide gas to bread, causing it to rise. The carbon dioxide is also a byproduct of anaerobic respiration.

 

Mushrooms are filamentous fungi in soils. Periodically, they aggregate to form mushrooms, which are short-lived structures that act to disperse spores on the wind.

 

Mycorrhizae (root fungi). These are filamentous fungi that associate with plant roots and benefit the plants by increasing their absorption of water and nutrients. 80% of all plant species have root fungi and some plant species will not grow without them. The first land plants had few or no roots and it is thought that they survived only by the help of root fungi, co-pioneers of the land.

 

Fungi are ecologically significant because they do much of the decomposition of dead plant and animal bodies. For people, they are significant in several ways. 30% of fungi are parasitic and cause fungal diseases, mostly in plants. Human fungal pathogens cause conditions like athlete's foot, thrush, and yeast infections. Fungi are also sources of useful drugs and chemicals, including penicillin and the drug cyclosporin, which is required for organ transplants to prevent rejection of the transplanted tissue by the patient's immune system.

The drugs from fungi benefit the fungi by deterring animals and bacteria from eating them. They are toxic or render an animal susceptible to being eaten itself. Both fungi and plants use a variety of "chemical weapons" to protect themselves from pathogens and predators.

3. Kingdom Plantae

Kingdom Plantae consists of the land plants. They have the following characteristics:

1. Plants have eukaryotic cells organized into multicellular organisms that have tissues and organs. All plants are multicellular.

2. Plants have the ability to do photosynthesis, which is the production of sugars from carbon dioxide, powered by sunlight and water. Oxygen is also produced as a byproduct. Plants perform roughly half of global photosynthesis and thus half the oxygen in our atmosphere. Plant cells have chloroplasts that do the photosynthesis. The green pigments in the chloroplast are chlorophyll a and chlorophyll b. Cyanobacteria, the photosynthetic protists (algae), and land plants all have chlorophyll a. Only the green algae and land plants have chlorophyll b, however. This is one reason that the green algae are thought to be the ancestors of the land plants.

3. Plant cells have a semi-rigid casing called the cell wall. Plant cell walls are composed of the carbohydrate cellulose, which is the material of which wood, paper, and cotton fiber are constructed. This differs from the Fungi, which have a cell wall of the nitrogen-bearing carbohydrate chitin. The algae also have cell walls but made of other materials. Only the green algae have cell walls made of cellulose the same as plants do.

4. Plant cells often have a large "bag" of salt water in the middle. This structure is called the "central vacuole". It allows plant cells to get big without expending a lot of energy on construction materials. They just fill up their vacuoles with water and salts to get bigger.

 

Unlike animals, plants don't move or eat things. They have no muscles or nerves. This lifestyle is a consequence their cell structure. Plant cell walls are semi-rigid, preventing the speed and range of movement exhibited by animal cells. Cell walls also make it impossible for plant cells to ingest chunks of food. Fortunately, plant cells have chloroplasts and feed themselves rather than capture and ingest other living things, which makes movement and eating unnecessary in the first place. The central vacuole of plant cells helps them grow cheaply toward what they need with a minimum of materials, also compensating for the inability to move or eat.

Plants are tremendously important to people. They produce half of the oxygen we breathe, all of our food, fibers for clothing, wood for building, and many useful drugs and chemicals.

 Three concepts help explain the plant lifestyle:

a. Plants are inflatable. They must grow quickly and efficiently in two directions at once to get the water and sunlight they need. To do this, plant cells fill up their central vacuoles with water and salts from the soil. Their cell walls allow them to draw enough water into their vacuoles that they develop pressure. Animal cells, like yours, explode if they are pressurized. The cell wall of plant cells prevents them from exploding. The pressure in plant cells can be greater than that in an automobile tire. This pressure helps plants to stand upright with a minimum of structural material, using internal water pressure to make them rigid. As an example, plants deprived of water lose their water pressure and wilt, drooping to the ground like soggy balloons.

 

b. Plants are linear. Their shoots (the above-ground part) must grow up to get light for photosynthesis and to disperse pollen and seeds. Plant roots must grow down to get water from the soil, which is needed to replace the water lost by shoots to the air during photosynthesis. Because plant bodies have to grow up and down at once, they tend to be long and extended rather than short and compact.

 

c. Plant evolution consists largely of adaptations to living on land. Plants are thought to have invaded the land from aquatic habitats before animals did. The story of land plants is one of adaptation to terrestrial life. some of the major adaptations to land are:

A water-proof "skin" on the body surface and enclosed reproductive structures. All land plants have these features but no green algae do.

Vascular tissue. Vascular tissue transports water from roots to leaves and sugars made by photosynthesis from leaves to roots. Vascular tissue allows plants to get tall, improving light absorption and the dispersal of offspring.

Seeds. Seeds contain a baby plant plus stored food for its first weeks of growth. Seeds can dry out and live for years, waiting for favorable conditions for growth. They can also be transported long distances by wind, water, or animals. Seeds work so well that some seed plant populations die out completely every winter. Their continued survival depends on their seeds.

Flowers. Plants with flowers are the most prevalent in modern times. They represent the plant lifestyle that is best suited to life on land.

Flowers are used to attract insects and other animals by means of scent, a sugar reward, or mimicry of an attractive insect of the opposite sex. The visiting animal gets dusted with pollen and can then transport the pollen for miles to other flowers of the same species. This allows immobile plants to breed with individuals that are far away and to which they are not closely related. This tends to increase the diversity of genes in offspring, improving their health and adaptability.

Flowers also produce fruits, which contain seeds. The color, smell, and nutritional value of many fruits cause animals to eat them. The seeds in these fruits are not digested, however. They pass out of the animal in its waste. By the time this happens, the animal has moved on and thus animals act to distribute seeds into new territory, complete with a portion of helpful fertilizer.