AP Biology Notes Outline Enduring Understanding 2.D

L. Carnes

Big Idea 2: Biological systems utilize free energy and molecular building blocks

to grow, to reproduce and to maintain dynamic homeostasis.

Enduring Understanding 2.D: Growth and dynamic homeostasis of a biological system are influenced by changes in the system’s environment.

Learning Objectives: Essential Knowledge 2.D.1: All biological systems from cells and organisms to populations, communities and ecosystems are affected by complex biotic and abiotic interactions involving exchange of matter and free energy.

– (2.22) The student is able to refine scientific models and questions about the effect of complex biotic and abiotic interactions on all biological systems, from cells and organisms to populations, communities and ecosystems.

– (2.23) The student is able to design a plan for collecting data to show that all biological systems (cells, organisms, populations, communities and ecosystems) are affected by complex biotic and abiotic interactions.

– (2.24) The student is able to analyze data to identify possible patterns and relationships between a biotic or abiotic factor and a biological system (cells, organisms, populations, communities and ecosystems).

Essential Knowledge 2.D.2: Homeostatic mechanisms reflect both common ancestry and divergence due to adaptation in different environments.

– (2.25) The student can construct explanations based on scientific evidence that homeostatic mechanisms reflect continuity due to common ancestry and/or divergence due to adaptation in different environments.

– (2.26) The student is able to analyze data to identify phylogenetic patterns or relationships, showing that homeostatic mechanisms reflect both continuity due to common ancestry and change due to evolution in different environments.

– (2.27) The student is able to connect differences in the environment with the evolution of homeostatic mechanisms. Essential Knowledge 2.D.3: Biological systems are affected by disruptions to their dynamic homeostasis.

– (2.28) The student is able to use representations or models to analyze quantitatively and qualitatively the effects of disruptions to dynamic homeostasis in biological systems.

Essential Knowledge 2.D.4: Plants and animals have a variety of chemical defenses against infections that affect dynamic homeostasis. – (2.29) The student can create representations and models to describe immune responses. – (2.30) The student can create representations or models to describe nonspecific immune defenses in plants and animals.

Required Readings: Textbook Ch. 12 (pp. 242); Ch. 11 (pp. 207); Ch. 24 (pp. 565) Textbook Ch. 54 (pp. 1202-1203); Ch. 53 (pp. 1187-1188) Textbook Ch. 54 (pp. 1204-1210); Ch. 55 (pp. 1226-1227) Textbook Ch. 36 (pp. 776-778); Ch. 44 (pp. 954-969; Ch. 42 (pp. 898-903); Ch. 40 (pp. 862-868) Textbook Ch. 56 (pp. 1249-1250); Ch. 55 (pp. 1236-1242) Textbook Ch. 39 (pp. 845-847); Ch. 43 (pp. 930-948) Article: Biodiversity and Ecosystem Stability

Practicing Biology Homework Questions: Questions #33-46

Essential Knowledge 2.D.1: All biological systems from cells and organisms to populations, communities and ecosystems are affected by complex biotic and abiotic interactions involving exchange of matter and free energy.

All biological systems, from cells to ecosystems, are influenced by complex biotic and abiotic interactions. The availability of resources influences activities in cells and organisms; examples include responses to cell density, biofilm(s) formation, temperature responses, and responses to nutrient and water availability. The availability of resources affects a population’s stability in size and its genetic composition; examples include birth rates versus death rates from bacteria to mammals and global distribution of food for humans.

Cell Activities: Cell Density Cell activities are affected by interactions with biotic and abiotic factors. In addition to many internal chemical factors, studies using animal cells in culture have led to the identification of many external factors, both chemical and physical, that can influence cell division and therefore control cell density. Contact cell cycle control mechanisms include density-dependent inhibition and anchorage dependence.

• In density-dependent inhibition, crowded cells stop dividing. • Anchorage dependence is a phenomenon in which cells must be attached to a substratum (i.e. extracellular matrix of a

tissue) in order to divide. (1) Cells anchor to dish surface and divide (anchorage

dependence). (2) When cells have formed a complete single layer, they stop

dividing (density-dependent inhibition). (3) If some cells are scraped away, the remaining cells divide to

fill the gap and then stop once the contact each other (density-dependent inhibition).

(4) Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells.

Cell Activities: Biofilms Metabolic cooperation between different prokaryotic species often occurs in surface-coating colonies known as biofilms. Cells in a biofilm secrete signaling molecules that recruit nearby cells, causing the colonies to grow. The cells also produce proteins that stick the cells to the substrate and to one another. Channels in the biofilm allow nutrients to reach cells in the interior and wastes to be expelled.

Soil-dwelling bacteria use chemical signals to share information about nutrient availability.

When food is scarce, starving cells secrete a molecule that reaches neighboring cells and stimulates them to aggregate.

The cells form a structure that produces thick-walled spores capable of surviving until the environment improves.

Organism Activities: Symbiosis Organism activities are affected by interactions with biotic and abiotic factors. Symbiosis is a relationship where two or more species live in direct and intimate contact with one another.

In parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process.

Mutualistic symbiosis, or mutualism (+/+ interaction), is an interspecific interaction that benefits both species. A mutualism can be obligate, where one species cannot survive without the other; or facultative, where both species can survive alone.

In commensalism (+/0 interaction), one species benefits and the other is apparently unaffected. Organism Activities: The Predator-Prey Relationship Predation is a type of density-dependent population control whereby there is a biological interaction where a predator (an animal that is hunting) feeds on its prey (the animal that is attacked). As a prey population builds up, predators may feed preferentially on that species, consuming a higher percentage of individuals. This is an important means of population control in nature.

Ecosystem Activities: Trophic Structure The stability of populations, communities and ecosystems is affected by interactions with biotic and abiotic factors. Trophic structure is the feeding relationships between organisms in a community – it is a key factor in community dynamics. Food chains link trophic levels from producers to top carnivores. A food web is a branching food chain with complex trophic interactions. Each food chain in a food web is usually only a few links long. Two hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesis:

• The energetic hypothesis suggests that length is limited by inefficient energy transfer

• The dynamic stability hypothesis proposes that long food chains are less stable than short ones

Energetic: 10% of energy stored in the organic matter of each trophic level is converted to organic matter at the next trophic level…this indicates that food chains should be relatively longer in habitats of higher photosynthetic production, since the starting amount of energy is greater than in habitats with lower photosynthetic production. Dynamic Stability: Population fluctuations at lower trophic levels are magnified at higher levels – potentially causing the local extinction of top predators. In a variable environment, top predators must be able to recover from environmental shocks that can reduce the food supply all the way up the food chain. The longer the chain, the more slowly top predators recover from environmental setbacks. So…food chains should be shorter in unpredictable environments. Ecosystem Activities: Species Diversity Species diversity of a community is the variety of organisms that make up the community. It has two components: species richness and relative abundance. Species richness is the total number of different species in the community. Relative abundance is the proportion each species represents of the total individuals in the community.

• There is growing evidence that the functioning of ecosystems is linked to biodiversity. Species play essential roles in ecosystems, so local and global species losses could threaten the stability of the ecosystem.

• There is currently great concern about the stability of both natural and human-managed ecosystems, particularly given the myriad global changes already occurring.

• Stability can be defined in several ways, but the most intuitive definition of a stable system is one having low variability (i.e., little deviation from its average state) despite shifting environmental conditions.

• Resilience is a somewhat different aspect of stability indicating the ability of an ecosystem to return to its original state following a disturbance or other perturbation.

• Diverse ecosystems are more likely to return to a state of stability following a disturbance than less diverse systems. For example, plant species harness the energy of the sun to fix carbon through photosynthesis, and this essential biological process provides the base of the food chain for myriad animal consumers. At the ecosystem level, the total growth of all plant species is termed primary production, and communities composed of different numbers and combinations of plant species can have very different rates of primary production.

Ecosystem Activities: Algal Blooms An algal bloom is a rapid increase or accumulation in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Nutrient limitation is very common in freshwater lakes and streams. Many studies have documented that sewage and fertilizer runoff from farms and yards adds large amounts of “missing” nutrients to lakes. Cyanobacteria and algae grow rapidly in response to these added nutrients, ultimately reducing the oxygen concentration and clarity of the water (known as eutrophication). Eutrophication has many ecological impacts, including the eventual loss of all but the most tolerant fish species from the lakes.

Essential Knowledge 2.D.2: Homeostatic mechanisms reflect both common ancestry and divergence due to adaptation in different environments.

Homeostatic mechanisms reflect both continuity due to common ancestry and change due to evolution in different environments. Supporting evidence includes a sampling of homeostatic control systems that are conserved across biological domains. Organisms have evolved various mechanisms for obtaining nutrients and getting rid of wastes, including gas exchange, osmoregulation and nitrogenous waste production. Structural and functional evidence supports the relatedness of all domains. This evidence can be seen in the following mechanisms:

• Organisms have various mechanisms for obtaining nutrients and eliminating wastes. • Homeostatic control systems in species of microbes, plants and animals support common ancestry.

Mechanisms for Obtaining Nutrients and Eliminating Wastes Organisms that live in water are often able to exchange carbon dioxide and oxygen gases through their surfaces. These exchange surfaces are moist, thin layers across which diffusion can occur.

Organismal response to the challenge of drying out tends to make these surfaces thicker, waterproof, and to retard gas exchange. Consequently, another method of gas exchange must be modified or developed.

The plant solution to gas exchange is a new structure, the guard cells that flank openings (stomata) in the above ground parts of the plant. By opening these guard cells the plant is able to allow gas exchange by diffusion through the open stomata.

The physiological systems of animals operate in a fluid environment. The relative concentrations of water and solutes in this environment must be maintained within fairly narrow limits in order to maintain dynamic homeostasis. Freshwater animals show adaptations that reduce water uptake and conserve solutes. Desert and marine animals face desiccating environments with the potential to quickly deplete the body water:

• Osmoregulation is the process by which animals control solute concentrations and balance water gain and loss. It is based largely on controlled movement of solutes between internal fluids and the external environment.

• Excretion gets rid of metabolic wastes. Most metabolic wastes must be excreted from the body. One of the most important types is nitrogenous wastes from the breakdown of proteins and nucleic acids.

An animal’s nitrogenous wastes reflect its phylogeny and habitat. The type and quantity of an animal’s waste products may have a large impact on its water balance. Among the most important wastes are the nitrogenous breakdown products of proteins and nucleic acids.

Ammonia is very water soluble and very toxic and therefore can only be tolerated in low concentrations. Animals that excrete it as nitrogenous wastes need access to lots of water. It is generally produced only by aquatic animals where water loss is not a competing problem. Advantage – can be excreted directly to water (diffusion) without being diluted. NOT EXPENSIVE TO PRODUCE – BUT HIGHLY TOXIC AND REQUIRES DIRECT ACCESS TO LOTS OF WATER! Most aquatic animals, including fishes produce ammonia.

Terrestrial animals simply do not have access to enough water to excrete ammonia – too much water is required to dilute it safely. Urea is produced by the liver of most vertebrates, where ammonia is combined with carbon dioxide in an energy expensive process. Because urea is less toxic, water is conserved during this process and therefore urea is excreted with a minimal loss of water. Its low toxicity also means that it can also be transported in the circulatory system and stored safely in high concentrations. The only disadvantage here is the cost to produce the urea from ammonia. EXPENSIVE TO PRODUCE – BUT ALLOWS FOR WATER CONSERVATION – LOW TOXICITY! Mammals, most amphibians, and sharks produce urea.

Uric Acid is non-toxic and does not readily dissolve in water. Because of this, it can be excreted as a paste or crystal with very little water loss. This is of great advantage to animals with little access to water – but there is a cost…it is energetically expensive to produce/requires a considerable amount of ATP. MORE EXPENSIVE TO PRODUCE THAN UREA – BUT ALLOWS FOR EVERN MORE WATER CONSERVATION – EXTREMELY LOW TOXICITY. Many reptiles, including birds and even insects produce uric acid!

Homeostatic Control Systems in Various Species: Variation in Excretory Systems Diverse excretory systems are variations on a tubular theme. Excretory systems regulate solute movement between internal fluids and the external environment. Most excretory systems produce urine by refining a filtrate derived from body fluids. Although they vary widely among animal groups, they are all generally built on a tubular theme that provides a large surface area for the exchange of water and solutes. Key functions of the excretory system include:

1) Filtration. The excretory tubule collects a filtrate from the blood. Water and solutes are forced by blood pressure across the selectively permeable membranes of a cluster of capillaries and into the excretory tubule.

2) Reabsorption. The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids.

3) Secretion. Other substances, such as toxins and excess ions, are extracted from body fluids and added to the contents of the excretory tubule.

4) Excretion. The filtrate leaves the system and the body.

The protonephridia of flatworms form a network of dead-end tubules connected to external openings. These tubules branch throughout the body.

• Specialized cells called flame bulbs containing cilia cap the branches of each protonephridium. • During filtration, beating of the cilia draws water and solutes from the interstitial fluid through the flame bulb, releasing

filtrate into the tubule network. • The urine excreted by freshwater flatworms has a low solute concentration, helping to balance osmotic uptake of water

from the environment. Most annelids have metanephridia, excretory organs that open internally to the coelom and are enveloped by a capillary network.

• As the cilia beat, fluid is drawn into a collecting tubule, which includes a storage bladder that opens to the outside. • The metanephridia of an earthworm have both excretory and osmoregulatory functions. • Earthworms inhabit damp soil and usually experience a net uptake of water by osmosis through their skin. • Their metanephridia balance the water influx by producing urine that is dilute (hypoosmotic to body fluids).

Kidneys are the excretory organs of vertebrates – they function in both excretion and osmoregulation.

• Nephrons and associated blood vessels are the functional unit of the mammalian kidney. • The mammalian excretory system centers on paired kidneys which are also the principal site of water balance and salt

regulation. • Filtrate becomes urine as it flows through the mammalian nephron and collecting duct – there are 5 main steps in the

transformation of blood filtrate to urine. In the PROXIMAL TUBULE, secretion and reabsorption substantially alter the volume and composition of filtrate. The pH of body fluids is controlled, and bicarbonates, salt, and water are absorbed. In the DESCENDING LOOP OF HENLE, reabsorption of water continues. In the ASCENDING LOOP OF HENLE, the filtrate loses salt without giving up water and becomes more dilute. In the DISTAL TUBULE, K+ and NaCl levels are regulated, as is filtrate pH. The COLLECTING DUCT carries the filtrate though the medulla to the renal pelvis, and the filtrate becomes more concentrated by the reabsorption of salt.

Vertebrate animals occupy a wide variety of habitats, and variations in nephron structure and function equip the kidneys of different vertebrates for osmoregulation in their various habitats.

• Desert Mammals: nephron structure allows them to rid the body of slats and nitrogenous wastes without squandering water (secrete hyperosmotic/highly concentrated urine).

• Aquatic Mammals: have a much lower ability to concentrate urine because dehydration is not a challenge. • Birds & Reptiles – nephrons specialized for conserving water so urine is generally highly concentrated. • Freshwater Fishes/Amphibians – secrete large amounts of very dilute urine to excrete excess water continuously. • Marine Fishes – filtration rates are low and very little urine is excreted.

Homeostatic Control Systems in Various Species: Variation in Thermoregulatory Systems Thermoregulation is the process by which animals maintain an internal temperature within a tolerable range. Each animal species has an optimal temperature range, and thermoregulation helps keep body temperature within that optimal range.

• Endotherms: warmed mostly by heat generated by metabolism. Birds and mammals are endothermic, their bodies are warmed mostly by heat generated by metabolism. They typically have higher metabolic rates than ectothermic animals.

• Ectotherms: gain most of their heat from external sources. Amphibians and reptiles other than birds are ectothermic, they gain their heat mostly from external sources. They generally have lower metabolic rates than endotherms.

Animals can have either a variable or a constant body temperature. Poikilotherm: an animal whose body temperature varies with its environment. Homeotherm: an animal with a relatively constant body temperature. Thermoregulation depends on an animal’s ability to control the exchange of heat with its environment. All organisms exchange heat by four physical processes: 1) Conduction: the direct transfer of heat when objects of different temperatures come into contact with each other. 2) Convection: heat is lost by convection when a stream of air (wind) is cooler than body surface temperature. 3) Radiation: objects exchange radiation with each other and with the atmosphere – warmer objects lose heat to cooler ones. 4) Evaporation: evaporation of water from body surfaces or breathing passages cools the body. Mechanisms to Balance Heat Loss and Gain:

• Integumentary System: skin, hair, and nails all provide mechanisms to prevent heat loss and/or gain.

• Insulation: reduces flow of heat between animals and their environment. • Evaporative Heat Loss: water absorbs considerable heat when it evaporates; this

heat is carried away from the body surface with the water vapor. • Behavioral Responses: sun basking, huddling, fanning wings, etc. • A countercurrent exchange system traps heat in the body core, thus reducing

heat loss from extremeties, particularly when they are in contact with ice or snow. Heat in the arterial blood emerging from the body core is transferred directly to the returning venous blood instead of being lost to the environment. This occurs because arteries carrying warm blood down the legs of the goose or flippers of the dolphin are in close contact with veins conveying cool blood in the opposite direction. This arrangement facilitates heat transfer from arteries to veins (black arrows). FORM FITS FUNCTION!

• The regulation of body temperature in humans and other mammals is brought about by a complex system based on feedback mechanisms. The sensors that control thermoregulation are concentration in the hypothalamus, a region of the brain near the ears.

Essential Knowledge 2.D.3: Biological systems are affected by disruptions to their dynamic homeostasis.

Disturbances to dynamic homeostasis effect biological processes. Disruptions at the molecular and cellular levels affect the health of the organism; examples include physiological responses to toxic substances and dehydration/desiccation. Disruptions to ecosystems impact the dynamic homeostasis or balance of the ecosystem, including the interactions between specific organisms therein; examples include invasive species, natural disasters, and human activities. As the human population has grown, our activities have disrupted the trophic structure, energy flow, and chemical cycling of many ecosystems. Current global concerns include disruptions to nutrient cycles, acid precipitation, toxins and biological magnification, greenhouse gases and global warming, and a thinning ozone layer. Disruptions at the molecular and cellular levels affect the health of the organism. Dehydration is the excessive loss of body water, with an accompanying disruption of metabolic processes. Extreme dehydration, or desiccation, is fatal for most animals.

– Symptoms may include headaches, decreased blood pressure, and dizziness or fainting when standing up due to orthostatic hypotension. Untreated dehydration generally results in delirium, unconsciousness, swelling of the tongue and, in extreme cases, death.

– The symptoms become increasingly severe with greater water loss. One's heart and respiration rates begin to increase to compensate for decreased plasma volume and blood pressure, while body temperature may rise because of decreased sweating.

– At around 5% to 6% water loss, one may become groggy or sleepy, experience headaches or nausea, and may feel tingling in one's limbs.

– With 10% to 15% fluid loss, muscles may become spastic, skin may shrivel and wrinkle (decreased skin turgor), vision may dim, urination will be greatly reduced and may become painful, and delirium may begin. Losses greater than 15% are usually fatal.

Disruptions to ecosystems impact the dynamic homeostasis or balance of the ecosystem. Introduced species are those that humans move from native locations to new geographic regions

Without their native predators, parasites, and pathogens, introduced species may spread rapidly

Introduced species that gain a foothold in a new habitat usually disrupt their adopted community

Sometimes humans introduce species by accident, as in case of the brown tree snake arriving in Guam as a cargo ship “stowaway”

Humans have deliberately introduced some species with good intentions but disastrous effects. An example is the introduction of kudzu in the southern United States

As the human population has grown, our activities have disrupted the trophic structure, energy flow, and chemical cycling of many ecosystems. In addition to transporting nutrients from one location to another, humans have added new materials, some of them toxins, to ecosystems. Disruptions that deplete nutrients in one area and increase them in other areas can be detrimental to ecosystem dynamics.

The quality of soil varies with the amount of organic material it contains, and agriculture removes from ecosystems nutrients that would ordinarily be cycled back into the soil.

Nitrogen is the main nutrient lost through agriculture; thus, agriculture greatly affects the nitrogen cycle:

Plowing: mixes soil and speeds up decomposition of organic matter – applying fertilizer makes up for lost nitrogen when crops are harvested….harvesting of plants causes nitrogen from fertilizers to be leached from the ecosystem.

The key problem with excess nutrients is the critical load - the amount of added nutrients that plants can absorb without damaging the ecosystem.

When excess nutrients are added to an ecosystem, the critical load is exceeded. Remaining nutrients can contaminate groundwater as well as freshwater and marine ecosystems.

Sewage runoff causes extensive dead zones and eutrophication, excessive phytoplankton & algal growth that can greatly harm freshwater and marine ecosystems.

The burning of wood and of fossil fuels releases oxides of sulfur and nitrogen that react with water in the atmosphere, forming sulfuric and nitric acid – which eventually fall to Earth’s surface as acid rain. Sulfur and nitrogen pollutants often drift hundreds of kilometers before falling as acid precipitation.

This lowers the pH of streams and lakes and affects soil chemistry and nutrient availability. In terrestrial ecosystems, the change in soil pH causes calcium and other nutrients to leach from the soil – these nutrient deficiencies affect the health of plants directly, mainly by leaching nutrients from the leaves.

In freshwater ecosystems, acid precipitation can cause acid-tolerant fish to replace other species, which dramatically changes the dynamics of food webs.

Humans release many toxic chemicals, including synthetics previously unknown to nature. In some cases, harmful substances persist for long periods in an ecosystem. One reason toxins are harmful is that they become more concentrated in successive trophic levels. Biological magnification concentrates toxins at higher trophic levels, where biomass is lower.

Organisms acquire toxic substances from the environment, and some of the poisons accumulate in specific tissues like fat.

Magnification occurs because the biomass at any given trophic level is produced from a much larger biomass ingested from the level below.

Top level carnivores tend to be the organisms most severely affected by toxic compounds in the environment. Be sure you understand and can explain the effect of DDT on the pelicans, ospreys and eagles described by Rachel Carson in her book “Silent Spring”.

One pressing problem caused by human activities is the rising level of atmospheric carbon dioxide. Due to the burning of fossil fuels and other human activities, the concentration of atmospheric CO2 has been steadily increasing.

CO2, water vapor, and other greenhouse gases reflect infrared radiation back toward Earth; this is the greenhouse effect. This effect is important for keeping Earth’s surface at a habitable temperature. Increased levels of atmospheric CO2 are magnifying the greenhouse effect, which could cause global warming and climatic change.

Increasing concentration of atmospheric CO2 is linked to increasing global temperature, which can affect the dynamics of ecosystems worldwide:

o spread of C3 plants into terrestrial environments that currently favor C4 plants

o decrease in ice coverage in northern coniferous forests and tundra (loss of habitat for species adapted to these areas)

o increase in likelihood of forest wildfires o altering the geographic distribution of

precipitation – leading to much drier areas that are currently used for agriculture

o Global warming can be slowed by reducing energy needs and converting to renewable sources of energy. Stabilizing CO2 emissions will require an international effort.

Life on Earth is protected from damaging effects of UV radiation by a protective layer of ozone molecules in the atmosphere. Satellite studies suggest that the ozone layer has been gradually thinning since 1975. Destruction of atmospheric ozone probably results from chlorine-releasing pollutants such as CFCs produced by human activity.

1) Chlorine from CFCs interacts with ozone (O3) forming chlorine monoxide (ClO) and Oxygen (O2).

2) Two chlorine monoxide (ClO) molecules react, forming chlorine peroxide (Cl2O2).

3) Sunlight causes chlorine peroxide (Cl2O2) to break down into O2 and free chlorine atoms.

4) The chlorine atoms can begin the cycle again – further destroying ozone.

Scientists first described an “ozone hole” over Antarctica in 1985; it has increased in size as ozone depletion has increased Decreased ozone levels in the stratosphere increase the intensity of UV rays reaching Earth’s surface.

• Ozone depletion causes DNA damage in plants and poorer phytoplankton growth (responsible for a significant amount of Earth’s overall productivity).

• An international agreement signed in 1987 has resulted in a decrease in ozone depletion.

Essential Knowledge 2.D.4: Plants and animals have a variety of chemical defenses against infections that affect dynamic homeostasis.

Plants and animals have evolved a variety of defenses against infections and other disruptions to homeostasis including both non-specific and specific immune responses. In innate/nonspecific immunity, recognition and response rely on shared traits of pathogens. In acquired/specific immunity, lymphocyte receptors provide pathogen-specific recognition. Nonspecific/Innate Immune Responses Plants, invertebrates and vertebrates have multiple, nonspecific immune responses. Plant defenses against pathogens include molecular recognition systems with systemic responses; infection triggers chemical responses that destroy infected and adjacent cells, thus localizing the effects. Invertebrate immune systems have nonspecific response mechanisms, but the lack pathogen-specific defense responses. Vertebrate immune systems have nonspecific and nonheritable defense mechanisms against pathogens. Chemical Defenses in Plants 1) Resistance in plants is based on the binding of molecules from the

pathogen to receptors in plant cells. 2) Identification triggers a signal transduction pathway. 3) Plant cells produce antimicrobial molecules; seal off infected areas,

then destroy themselves. 4) Before they die, infected plant cells release a signaling molecule

that is distributed to the rest of the plant. 5) This molecule initiates signal transduction pathway in nearby plant

cells. 6) Resistance is activated – plant is protected for several days.

Plants prevent excess herbivory by using both physical defenses (thorns) and chemical defenses (toxic compounds). Some plants even “recruit” predatory animals that help defend the plant against specific herbivores. Parasitoid wasps inject their eggs into their prey (caterpillars). Eggs hatch within caterpillars – and wasp larvae eat them from inside out. Plant leaf damaged by caterpillars releases volatile compounds that attract parasitoid wasps Sometimes, this type of behavior can provide an “early warning system” for nearby plants of the same species. As a result of chemical activation, nearby plants activate specific genes that make them less susceptible to infestation.

Nonspecific/Innate Immunity in Invertebrates In insects, an exoskeleton made of chitin forms the first barrier to pathogens. The digestive system is protected by low pH and lysozyme, an enzyme that digests microbial cell walls. Hemocytes circulate within hemolymph and carry out phagocytosis, the ingestion and digestion of foreign substances including bacteria. Phagocytosis by Invertebrate:

1) Pseudopodia surround microbe. 2) Microbes are engulfed into cell. 3) Vacuole containing microbes forms. 4) Vacuole and lysosome fuse. 5) Toxic compounds and lysosomal enzymes destroy microbes. 6) Microbial debris is released by exocytosis.

Animals are constantly under attack by pathogens, infectious agents that cause disease. Dedicated immune cells patrol the body fluids, searching out and destroying foreign cells. These defenses make up an immune system. Most animal immune systems use receptors that specifically bind molecules from foreign cells or viruses (cellular communication). There are two general strategies for such molecular recognition: innate immunity and acquired immunity. Innate immunity is present before any exposure to pathogens and is effective from the time of birth. It involves nonspecific responses to pathogens. Innate immunity consists of external barriers plus internal cellular and chemical defenses. Acquired immunity, or adaptive immunity, develops after exposure to agents such as microbes, toxins, or other foreign substances. It involves a very specific response to pathogens.

Nonspecific/Innate Immunity in Vertebrates Barrier defenses in vertebrates include the skin and mucous membranes of the respiratory, urinary, and reproductive tracts:

• Mucus traps and allows for the removal of microbes • Many body fluids including saliva, mucus, and tears are hostile to microbes • The low pH of skin and the digestive system prevents growth of microbes

White blood cells (leukocytes) engulf pathogens in the body. Groups of pathogens are recognized by TLR, Toll-like receptors found on leukocytes. During an immune response, a white blood cell engulfs a microbe, then fuses with a lysosome to destroy the microbe. There are different types of phagocytic cells:

• Neutrophils engulf and destroy microbes • Macrophages are part of the lymphatic system and

are found throughout the body • Eosinophils discharge destructive enzymes • Dendritic cells stimulate development of acquired

immunity Peptides and proteins function in innate defense by attacking microbes directly or impeding their reproduction. Interferon proteins provide innate defense against viruses and help activate macrophages. About 30 proteins make up the complement system, which causes lysis of invading cells and helps trigger inflammation. In local inflammation, histamine and other chemicals released from injured cells promote changes in blood vessels that allow more fluid, more phagocytes, and antimicrobial proteins to enter the tissues.

All cells in the body (except red blood cells) have a class 1 MHC protein on their surface. MHC molecules are normal cell-surface proteins that are encoded by a family of genes called the major histocompatibility complex. These are a group of genes that differ from one individual to another – A MAJOR COMPONENT OF “SELF”. Cancerous or infected cells no longer express this protein; and natural killer (NK) cells attack these damaged cells:

• NKC’s often trigger apoptosis in the cells they attack. • Apoptosis is programmed cell death brought about by signals that trigger

the activation of a cascade of “suicide” proteins in the cells destined to die. Specific/Acquired Immunity in Vertebrates Mammals use specific immune responses triggered by natural or artificial agents that disrupt dynamic homeostasis. The mammalian

immune system includes two types of specific responses: cell mediated and humoral:

• In the cell-mediated response, cytotoxic T cells, a type of lymphocytic white blood cell, “target” intracellular pathogens when antigens are displayed on the outside of the cells.

• In the humoral response, B cells, a type of lymphocytic white blood cell, produce antibodies against specific extracellular antigens.

http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter22/animation__the_immune_response.html

http://bcs.whfreeman.com/thelifewire/content/chp18/1802004.html (humoral response) http://bcs.whfreeman.com/thelifewire/content/chp18/1802003.html (cell-mediated response)

The humoral immune response involves the activation of B cells, resulting in the production of secreted antibodies that circulate the blood and lymph. THIS IS ANTIBODY-MEDIATED IMMUNITY – extracellular protection (B cells produce antibodies against specific antigens). The cell-mediated immune response involves the activation of cytotoxic T cells which directly destroy certain target cells that are infected with intracellular pathogens. THIS IS CELL-MEDIATED IMMUNITY – intracellular protection (cytotoxic T cells target intracellular pathogens when antigens are presented on the outside of cells).

Humoral and cell-mediated immunity defend against different types of threats (extracellular v. intracellular protection). In both humoral and cell-mediated immunity, cells communicate with each other through direct contact. B cells are stimulated when they come in contact with an antigen on the surface of an invading cell. Macrophages display antigens to their surface where T cells bind and become active. Cancerous or infected cells lacking class I MHCs are attacked by natural killer cells – which triggers apoptosis of the damaged cell.

Helper T Cells: A Response to Nearly All Antigens

Cytotoxic T Cells: A Response to Infected and Cancerous Cells

B Cells: A Response to Extracellular Pathogens

The Antigen/Antibody Relationship An antigen is any foreign molecule that is specifically recognized by lymphocytes and elicits a response from them. Lymphocytes provide the specificity and diversity of the immune system. A lymphocyte actually recognizes and binds to just a small, accessible portion of the antigen called an epitope.

An epitope is a localized region on the surface of an antigen that is chemically recognized by antibodies; it is also called an antigenic determinant.

Antibodies are proteins produced by B cells, and each antibody is specific to a particular antigen.

Antigens in the body are recognized by antibodies to the antigen.

B cell receptors bind to specific, intact antigens – whether that antigen is free or on the surface of a pathogen. T cells bind to small fragments of antigens presented on the surface of macrophages.

The Primary and Secondary Immune Response The selective proliferation and differentiation of lymphocytes that occur the first time the body is exposed to a particular antigen represents the primary immune response: It peaks about 10-17 days after the initial exposure to the antigen – during this time B cells and T cells are being activated and a stricken individual may feel ill. In the secondary immune response memory cells facilitate a faster, more efficient response. If an individual is exposed to the same antigen again, the response is faster, of greater magnitude, and more prolonged – this is the secondary immune response. It typically 2- 7 days to respond. Long-lived memory cells generated in the primary response to antigen (A) give rise to a heightened secondary response to the same antigen, but do not affect the primary response to a different antigen (B). 1° Immune Response

1. Following the first exposure to a foreign antigen, a lag phase occurs in which no antibody is produced, but activated B cells are differentiating into plasma cells. The lag phase can be as short as 2-3 days, but often is longer, sometimes as long as weeks or months.

2. The amount of antibody produced is usually relatively low. 3. Over time, antibody level declines to the point where it may be

undetectable. 2° Immune Response

1. If a second dose of the same antigen is given days or even years later, an accelerated 2° or anamnestic immune response (IR) occurs. This lag phase is usually very short (e.g. 3 or 4 days) due to the presence of memory cells.

2. The amount of antibody produced rises to a high level. 3. Antibody level tends to remain high for longer.

Immune responses to antigens may be categorized as primary or secondary responses. The primary immune response of the body to antigen occurs on the first occasion it is encountered. Depending on the nature of the antigen and the site of entry this response can take up to 14 days to resolve and leads to the generation of memory cells with a high specificity for the inducing antigen. The humoral response, mediated by B cells with the help of T cells, produces high-affinity and antigen-specific antibodies. This is in contrast with the CD8 T-cell response which leads to the generation of large numbers of antigen-specific cells that are capable of directly killing infected cells. Antigen-specific CD4 T cells, which provide help to B cells in the form of cytokines and other stimulatory factors, can also be expanded upon antigenic stimulation. The secondary response of both B- and T cells is observed following subsequent encounter with the same antigen and is more rapid leading to the activation of previously generated memory cells. This has some quantitative and qualitative differences from the primary response. Key Concepts:

The innate immune system is the first line of defense against infectious agents. When this is breached, the adaptive immune system provides a more efficient response to clearing pathogens.

The adaptive immune system has the capacity to ‘remember’ previous antigens, a process termed immunological memory.

Antigen-specific T cells are selected during a primary immune response and expand to produce clones of T cells with high specificity for the activating antigen.

In a B cell primary response to a thymus-dependent antigen, the immune system selects B cells with a high affinity and specificity for the antigen and these become memory cells.

In a secondary response to the same antigen, memory cells are rapidly activated. This process is quicker and more effective than the primary response.