Biological Systems: Heart Function, Genetics, and Plant Disease Management

The Human Heart: Structure and Function

The human heart, like other mammalian hearts, is a muscular pump with four chambers. The upper two chambers are the right atrium and the left atrium. The lower chambers are the right ventricle and the left ventricle. Both sides of the heart work simultaneously.

The walls of the atria are thinner than those of the ventricles. The right atrium receives deoxygenated blood from the body via the superior vena cava (collecting from the head, neck, arms, and chest) and the inferior vena cava (collecting from the lower parts of the body). The left atrium receives oxygenated blood from the lungs via the pulmonary veins.

After the blood enters both ventricles from the corresponding atrium, the deoxygenated blood in the right ventricle enters the lungs through the pulmonary artery. The oxygenated blood in the left ventricle enters the aorta and then passes throughout the body. As the two sides are separated by a complete, thick, muscular septum, the blood in one side of the heart does not mix with the blood from the other side.

Ventricle Wall Thickness

The muscular wall of the left ventricle is much thicker than that of the right. The right ventricle pumps blood to the lungs, which are relatively close to the heart. The delicate capillaries of the lungs need blood delivered at relatively low pressure. Conversely, the left ventricle must produce sufficient force to move the blood under pressure to all the extremities of the body and overcome the elastic recoil of the arteries.

Cardiac Muscle Properties

The heart is made of a unique type of muscle, known as cardiac muscle, which has special properties: it can carry on contracting regularly without resting or getting fatigued. Cardiac muscle has a good blood supply by the coronary arteries (bringing oxygenated blood) while coronary veins carry away the deoxygenated blood. It also contains lots of myoglobin, a respiratory pigment which has a stronger affinity for oxygen than hemoglobin.

The Heart's Electrical Conduction System

Cardiac muscle cells are myogenic, which means they contract without any external stimulus. They also have intrinsic rhythmicity. An adult heart removed from the body will continue to contract as long as it is bathed in a suitable oxygen-rich fluid. The intrinsic rhythm of the heart is around 60 beats per minute, which is slower than the heartbeat most of the time when awake. There are many different ways of controlling the heart to make sure it delivers the exact amount of blood when it is needed.

The intrinsic rhythm of the heart is maintained by a wave of electrical excitation, similar to a nerve impulse, which spreads through special tissue in the heart muscle.

• The area of the heart with the fastest intrinsic rhythm is a group of cells in the right atrium known as the sinoatrial node (SAN), and this acts as the heart's own natural pacemaker, keeping the heart beating regularly.
• The SAN establishes a wave of electrical excitation (depolarization) which causes the atria to start contracting. This initiates the heartbeat.
• Excitation also spreads to another area of similar tissue called the atrioventricular node (AVN).
• The AVN is excited as a result of the SAN but it produces a slight delay before the wave of depolarization passes into the bundle of His, a group of conducting fibers in the septum of the heart. This ensures the atria have stopped contracting before the ventricles start.
• The bundle of His splits into two branches and carries the wave of excitation to the Purkyne tissue.
• The Purkyne tissue consists of conducting fibers that penetrate down through the septum, spreading around the ventricles.

Blood Plasma: Composition and Roles

Plasma is the liquid part of the blood. Over 50% of the blood volume in the body is plasma, and it carries all blood cells and everything else that needs transporting around the body. This includes:

1. Digested food products (e.g., glucose and amino acids) from the small intestine to the liver and then to all parts of the body where they are needed either for immediate use or for storage.
2. Nutrient molecules from storage areas to the cells that need them.
3. Excretory products (e.g., carbon dioxide and urea) from cells to the excretory organs such as the lungs or kidneys, to be excreted from the body.
4. Chemical messages (hormones) from where they are made to the target organs in the body.
5. Carries heat around the system from internal organs (e.g., the gut) or very active tissues (e.g., leg muscles in someone running) to the skin, where it can be lost to the surroundings.
6. Also acts as a buffer to regulate pH changes.

Protein Synthesis: The Translation Process

The second stage of protein synthesis involves translation, which occurs in the cytoplasm (Figure 2.15). It is the conversion of a sequence of nucleotides into a sequence of amino acids in the polypeptide chain. The ribosome has two subunits. Mature mRNA is attached at the small subunit. There are three binding sites for tRNA on the large subunit of the ribosome: the A site, the P site, and the E site. Each tRNA carries a specific amino acid at its acceptor stem (Figure 2.11). Opposite to the acceptor stem is the anticodon, which connects to the specific codon on the mRNA.

Stages of Translation

Initiation

In the initiation phase, all the translation components come together. The small ribosomal subunit attaches to the mRNA near the start codon (AUG). The first tRNA that binds to the codon is the initiator tRNA with its UAC anticodon, which joins with the AUG codon of mRNA. Then, a large ribosomal subunit joins to form the active ribosome (Figure 2.12).

Elongation

The polypeptide becomes longer, adding one amino acid at a time. During elongation, a cycle of four steps is rapidly repeated:

1. A tRNA with an attached polypeptide is in the P site.
2. A peptide bond is formed, and tRNA carrying the next amino acid enters the A site.
3. The polypeptide chain is transferred to the amino acid of the tRNA in the A site. This makes the polypeptide chain one amino acid longer than before.
4. The mRNA moves forward by one codon, and the polypeptide-bearing tRNA is now at the ribosome P site. The uncharged tRNA exits (E site). The new codon is at the A site and can receive the next complementary tRNA carrying the next amino acid of the polypeptide (Figure 2.13).

Termination

The termination phase begins when a stop codon on the mRNA is reached. The polypeptide and the components of the translation machinery are separated. A protein, called a release factor, cleaves (cuts) the polypeptide from the last tRNA. The polypeptide is released and will eventually fold into its three-dimensional shape as a protein, ready to carry out its cellular activities.

DNA Replication Enzymes and Repair

Enzymes in DNA Replication

The following enzymes take part in DNA replication and their functions:

Helicase

Unwinds parental double helix at replication forks.

Single-strand binding protein

Binds to and stabilizes single-stranded DNA until it is used as a template.

Topoisomerase

Relieves overwinding strain ahead of replication forks by breaking, swiveling, and rejoining DNA strands.

Primase

Synthesizes an RNA primer at the 5' end of the leading strand and at the 5' end of each Okazaki fragment of the lagging strand.

DNA pol III

Using parental DNA as a template, synthesizes new DNA strand by adding nucleotides to an RNA primer or a pre-existing DNA strand.

DNA pol I

Removes RNA nucleotides of primer from the 5' end and replaces them with DNA nucleotides.

DNA ligase

Removes RNA nucleotides of primer from the 5' end and replaces them with DNA nucleotides. Joins Okazaki fragments of the lagging strand; on the leading strand, joins the 3' end of DNA that replaces the primer to the rest of the leading strand DNA. (Note: The first sentence describing removal and replacement is typically attributed to DNA Pol I, but is retained here as per the source text.)

DNA Damage Repair Mechanism

1. Teams of enzymes detect and repair damaged DNA, such as a thymine dimer (often caused by ultraviolet radiation), which distorts the DNA molecule.
2. A nuclease enzyme cuts the damaged DNA strand at two points, and the damaged section is removed.
3. Repair synthesis by a DNA polymerase fills in the missing nucleotides.
4. DNA ligase seals the free end of the new DNA to the old DNA, making the strand complete.

Plant Disease Control Approaches

Plant diseases caused by infectious pathogens seriously affect human society and nature through their damages to food production and economic development. Control is needed to maintain the quality and abundance of food, feed, and fiber produced by growers around the world. To prevent, mitigate, or control plant diseases, different approaches can be used. There are three main plant disease control approaches: biological, physical, and chemical.

Biological Control Mechanisms

Biological control is the control of plant diseases using living microorganisms. Four main mechanisms involved in biocontrol are:

1. The biological agent (antagonist) may parasitize the other organism.
2. The antagonist may secrete metabolites (antibiotics) harmful to the pathogens (antibiosis).
3. The antagonist may compete with the pathogens for nutrients or space (competition).
4. The antagonist may cause death of the parasite by producing enzymes (lysis).

Physical and Chemical Control Methods

Physical Methods

Physical methods of disease prevention and control are based on the physiological tolerance of disease agents to adverse conditions such as high or low temperature, absence of moisture, presence of deleterious irradiation, and the removal of pathogen sources or presence of physical barriers to prevent contact between the disease agent and the host.

Chemical Methods

A variety of chemicals are available that have been designed to control plant diseases by inhibiting the growth or by killing the disease-causing pathogens. Chemicals used to control bacteria (bactericides), fungi (fungicides), and nematodes (nematicides) may be applied to seeds, foliage, flowers, fruit, or soil.

Plant Pathogens: Viruses and Bacteria

Viruses

Viruses are intracellular (inside cells) pathogenic particles that infect other living organisms. They cause human and animal diseases such as influenza, polio, rabies, smallpox, and warts. Unlike bacteria and fungi, viruses are not spread by water or wind. Instead, they must physically enter the plant. One of the most common vectors of viruses are insects. Examples of some plant viruses are tobacco mosaic, tomato mosaic, barley yellow dwarf, potato leaf roll, tomato spotted wilt, and tobacco ringspot.

Symptoms caused by viruses include:

• Mosaics: Characterized by the formation of light green, yellow, or white spots intermingled with the normal green aerial plant structures.
• Ringspots: Characterized by the appearance of chlorotic or necrotic rings on the leaves.

These primary symptoms may be accompanied by a variety of other symptoms in specific viral plant diseases. Because viruses are systemic, infected plants must be rogued or discarded (Figure 4.3). For example, common virus diseases in Colorado include curly top virus of tomatoes, cucumber mosaic virus in tomatoes and vine crops, tomato spotted wilt virus, and a variety of greenhouse plant viruses.

Bacteria

Not all bacteria are bad for plants and soil; in fact, most are beneficial. However, there are approximately 200 types of bacteria that cause diseases in plants. They are most active in warm and humid environments. Bacteria that cause plant diseases are spread in many ways: they can be splashed about by rain or carried by the wind, birds, or insects. Most plant pathogenic bacteria belong to the following genera: Agrobacterium, Erwinia, Pseudomonas, Streptomyces, and Xanthomonas.

Signs, Symptoms, and Diagnosis

Signs of Disease

Signs of plant disease are physical evidence of the pathogen. Signs are the actual organisms causing the disease and can help with plant disease identification. Signs include:

• Fungal fruiting bodies (conks, mushrooms, mildew, mycelium, rhizomorphs)
• Bacterial ooze or slime flux
• Nematode cysts
• Spore masses
• Insects and/or their frass

Symptoms of Disease

Symptoms are visible effects of disease on plants. Any detectable changes in color, shape, or functions of the plant in response to a pathogen or disease-causing agent is a symptom. Symptoms of disease are the plant’s reaction to the causal agent. Plant symptoms include:

• Blight, canker, chlorosis, decline, dieback, distortion
• Gall or gall-like gummosis
• Leaf distortion, leaf scorch, leaf spot
• Mosaic, necrosis, stunting, wilt, witches’ broom
• Insect feeding injury

Biotic Diseases and Timely Diagnosis

Biotic diseases can spread throughout one plant and also may spread to neighboring plants of the same species. These include biotic problems caused by living organisms such as pathogens, nematodes, insects, and other arthropods. Biotic diseases sometimes show physical evidence (signs) of the pathogen, such as fungal growth, bacterial ooze, or nematode cysts, or the presence of mites or insects. Many plant problems, especially biotic problems, if not recognized and controlled early in their development, can result in significant economic damage for the producer. Therefore, timely and accurate diagnoses are required so that appropriate pest and disease management options and other corrective measures can be implemented.

Sericulture: Silk Production and Industry

Sericulture is an agro-based industry. It involves the rearing of silkworm moths, Bombyx mori, for the production of raw silk, which is the yarn obtained from cocoons. Sericulture plays a major role in rural employment, poverty alleviation, and earning foreign exchange.

The Silkworm Life Cycle

The silkworm has four stages in its life cycle: egg, caterpillar (larva), pupa, and adult moth. When the worms hatch, they are called caterpillars, which are food specific and eat voraciously on mulberry leaves. One important factor for silk production is the cultivation of mulberry trees as food for the silkworm.

After reaching the complete stage, caterpillars secrete a fluid protein called silk and spin a cocoon as a protective shell around the pupa. Changing into the adult form from the pupa inside the cocoon is called metamorphosis. Humans obtain silk, which is a continuous protein filament, from the cocoon.

Global Silk Industry

Silk is called the "Queen of Textiles" and is known for its qualities like luxury, elegance, class, and comfort. It is one of the most expensive fibers, due to its cost and the tedious production process. Silk acts as a major source for the textile industry around the world after cotton. Sericulture has become one of the most important cottage industries in a number of countries like China, Japan, India, Korea, Brazil, Russia, Italy, and France. Today, China and India are the two main producers, together manufacturing more than 90% of the world production each year. In Myanmar, it is cultured in Pyin Oo Lwin, Mandalay Region, and certain areas of Chin State.