🔬 Non-Cellular Life & Symbiotic Associations 🧬
🦠Viruses: The Acellular Enigmas ðŸ¦
Viruses are unique biological entities that blur the line between living and non-living. They are acellular organisms, meaning they do not have a cellular structure.
Assertion: Viruses are considered obligate parasites.
Reason: They can only replicate inside the living cells of host organisms, lacking their own cellular machinery for metabolism and reproduction.
- Acellular Nature: Characterized by having an inert crystalline structure outside the living cell.
- Obligate Parasites: They are obligate parasites, meaning they can only reproduce inside a host cell.
- Genetic Material: Possess either DNA or RNA as their genetic material, but never both. The genetic material is infectious.
- Protein Coat: Encased in a protein coat called a capsid, which is made up of smaller subunits called capsomeres.
- Capsomere Arrangement: Capsomeres are arranged in helical or polyhedral geometric forms.
- Discovery & Naming:
- D.J. Ivanowsky (1892): Recognized certain microbes causing tobacco mosaic disease as being smaller than bacteria.
- M.W. Beijerinck (1898): Called the extract of infected plants 'Contagium vivum fluidum' (infectious living fluid).
- W.M. Stanley (1935): Showed that viruses could be crystallized, and these crystals consist largely of proteins.
- Diseases Caused: Mumps, smallpox, herpes, influenza, AIDS. In plants, they cause mosaic formation, leaf rolling/curling, yellowing, vein clearing, dwarfing, and stunted growth.
🧠Test Your Knowledge: Viruses! ðŸ§
Click on a card to reveal the answer.
What is the primary characteristic of viruses that makes them acellular?
They have an inert crystalline structure outside the living cell.
Can a virus have both DNA and RNA as its genetic material?
No, they possess either DNA or RNA, but never both.
What is the protein coat surrounding the genetic material of a virus called?
It is called a capsid.
Who first recognized microbes smaller than bacteria causing tobacco mosaic disease?
D.J. Ivanowsky (1892).
What term did M.W. Beijerinck use to describe the infectious fluid from diseased plants?
'Contagium vivum fluidum' (infectious living fluid).
What significant discovery about viruses did W.M. Stanley make?
He showed that viruses could be crystallized and consist largely of proteins.
Name two human diseases caused by viruses.
Mumps, smallpox, herpes, influenza, or AIDS.
What are the smaller subunits of a capsid called?
Capsomeres.
Is the genetic material of a virus infectious?
Yes, the genetic material is infectious.
Describe two symptoms of viral diseases in plants.
Mosaic formation, leaf rolling/curling, yellowing, vein clearing, dwarfing, or stunted growth.
Are viruses considered living organisms?
They are often considered on the borderline between living and non-living due to their obligate parasitic nature and crystalline structure outside a host.
What types of geometric forms can capsomeres be arranged in?
Helical or polyhedral geometric forms.
What is the significance of viruses being able to be crystallized?
It suggests they are non-living outside a host cell, as living organisms generally cannot be crystallized.
What is the common name for the disease caused by the HIV virus?
AIDS.
Do viruses have their own cellular machinery for reproduction?
No, they lack their own cellular machinery and rely on host cells.
What type of nucleic acid is found in tobacco mosaic virus (TMV)?
TMV has RNA as its genetic material.
What is the primary component of viral crystals?
They consist largely of proteins.
Can viruses grow and divide like typical cells?
No, they replicate by hijacking host cell machinery, not by growth and division.
What is the general term for a disease that affects many plants by causing discoloration and distortion of leaves?
Mosaic formation.
Why are viruses considered non-cellular?
Because they lack cellular organization, including cytoplasm, organelles, and a cell membrane.
🧬 Viroids & Prions: Even Simpler Infectious Agents 🧬
1. Viroids
Assertion: Viroids lack the protein coat found in viruses.
Reason: They consist solely of a free RNA molecule, making them simpler infectious agents than viruses.
- Discovery: Discovered by T.O. Diener (1971).
- Composition: They are infectious agents consisting of free RNA.
- Lack Protein Coat: They lack the protein coat that is found in viruses.
- Molecular Weight: The RNA of viroids has a low molecular weight.
- Disease Caused: Causes potato spindle tuber disease.
2. Prions
Assertion: Prions cause neurological diseases.
Reason: They are abnormally folded proteins that can induce normal proteins to misfold, leading to neurodegenerative conditions.
- Composition: Consist of abnormally folded proteins.
- Size: Similar in size to viruses.
- Diseases Caused: Cause certain neurological diseases.
- Bovine Spongiform Encephalopathy (BSE): Commonly known as Mad cow disease in cattle.
- Creutzfeldt-Jakob disease (CJD): An analogous variant disease in humans.
🌿 Lichens: Nature's Pollution Indicators 🌿
Lichens represent a remarkable example of mutualistic symbiosis in nature.
Assertion: Lichens are good pollution indicators.
Reason: They are highly sensitive to air pollution and do not grow in polluted areas, making their presence or absence an indicator of air quality.
- Symbiotic Association: A mutually useful association between algae and fungi.
- Algal Component (Phycobiont): This part is autotrophic (prepares food through photosynthesis).
- Fungal Component (Mycobiont): This part is heterotrophic.
- Mutual Benefits:
- Algae: Prepare food for fungi.
- Fungi: Provide shelter and absorb mineral nutrients and water for the algal partner.
- Appearance: Their association is so close that they appear as a single organism in nature.
- Ecological Role: Lichens are considered good pollution indicators because they do not grow in polluted areas.
🧠Test Your Knowledge: Lichens! ðŸ§
Click on a card to reveal the answer.
What two organisms form a symbiotic association in lichens?
Algae and fungi.
What is the autotrophic component of a lichen called?
The phycobiont (algal component).
What role does the fungal component play in a lichen?
It provides shelter and absorbs mineral nutrients and water.
Why are lichens considered good pollution indicators?
Because they do not grow in polluted areas.
What is the heterotrophic component of a lichen called?
The mycobiont (fungal component).
Do lichens appear as two separate organisms or a single one?
They appear as a single organism due to their close association.
What do algae provide to the fungi in a lichen?
They prepare food for the fungi.
Is the relationship between algae and fungi in a lichen parasitic or mutualistic?
It is a mutualistic (mutually useful) association.
Can lichens grow in areas with high levels of sulfur dioxide?
No, they are sensitive to air pollution and will not grow there.
What do fungi absorb for the algal partner in a lichen?
Mineral nutrients and water.
Are lichens considered plants, fungi, or something else?
They are a composite organism formed by the symbiosis of algae and fungi, not strictly one or the other.
What is the primary mode of nutrition for the phycobiont?
Autotrophic (photosynthesis).
What is the primary mode of nutrition for the mycobiont?
Heterotrophic.
Can lichens survive in harsh environments where other organisms struggle?
Yes, their symbiotic nature allows them to colonize bare rocks and extreme environments.
Do lichens have roots, stems, or leaves?
No, they have a thallus body plan, lacking true roots, stems, or leaves.
What is the main benefit for the fungal partner in a lichen?
Receiving food (carbohydrates) from the algal partner.
What is the main benefit for the algal partner in a lichen?
Receiving shelter, water, and minerals from the fungal partner.
Are lichens fast-growing organisms?
No, they are generally very slow-growing.
What is the ecological significance of lichens as pioneer species?
They can colonize bare rock and contribute to soil formation.
Do lichens reproduce sexually or asexually?
They can reproduce both asexually (e.g., by fragmentation) and sexually (fungal component).
🌳 Mycorrhiza: The Fungal-Root Alliance 🌳
Mycorrhiza represents a vital symbiotic association between fungi and plant roots, crucial for plant health and ecosystem function.
Assertion: Mycorrhizal associations significantly increase the plant's ability to absorb water and minerals.
Reason: Fungal hyphae extend far beyond the plant roots, greatly increasing the surface area for absorption, especially for nutrients like phosphorus and nitrogen.
- Definition: A close association where fungal filaments (hyphae) either form a network around young roots (ectomycorrhiza) or penetrate root cells (endomycorrhiza).
- Benefit to Plant: This interaction significantly increases the surface area available for the plant to absorb water and minerals from the soil.
Types of Mycorrhiza
Mycorrhizal associations are broadly categorized into two main types based on how the fungal hyphae interact with the plant root cells:
Ectomycorrhiza (Ectotrophic Mycorrhiza):
- Interaction: Fungal hyphae form a dense sheath or mantle around the outside of the root.
- Penetration: They also grow between the root cortical cells, forming a network called the Hartig net.
- Cell Wall Interaction: The fungal hyphae do not penetrate the plant cell walls.
- Commonly Found: In trees like pines, oaks, and birches.
Endomycorrhiza (Endotrophic Mycorrhiza):
- Interaction: The fungal hyphae penetrate the cortical cells of the plant root.
- Most Common Type: The most common type is Arbuscular Mycorrhiza (AM), previously known as Vesicular-Arbuscular Mycorrhiza (VAM).
- Specialized Structures (within plant cells):
- Arbuscules: Highly branched, tree-like structures that facilitate nutrient exchange between the fungus and the plant.
- Vesicles: Swollen, oval-shaped structures that serve as storage organs for lipids.
- Prevalence: Found in a vast majority (around 80%) of plant species, including many agricultural crops.
🧠Test Your Knowledge: Mycorrhiza! ðŸ§
Click on a card to reveal the answer.
What is mycorrhiza?
A symbiotic association between fungi and plant roots.
What is the primary benefit for plants in a mycorrhizal association?
Enhanced nutrient absorption (especially phosphorus and nitrogen) and water.
In Ectomycorrhiza, do fungal hyphae penetrate plant cell walls?
No, they do not penetrate plant cell walls.
What is the network of fungal hyphae between root cortical cells in Ectomycorrhiza called?
The Hartig net.
What is the most common type of Endomycorrhiza?
Arbuscular Mycorrhiza (AM).
What are arbuscules, and what is their function?
Highly branched, tree-like structures that facilitate nutrient exchange.
What are vesicles, and what do they store?
Swollen, oval-shaped structures that serve as storage organs for lipids.
Name a plant for which mycorrhizal association is obligate.
Pinus.
What do fungi receive from the plant in a mycorrhizal association?
Sugars and nitrogen-containing compounds (photosynthates).
Approximately what percentage of land plant species form mycorrhizal interactions?
Approximately 80%.
What specific nutrient are fungi particularly efficient at absorbing for the plant?
Phosphorus and nitrogen.
How do mycorrhizal plants show increased resistance?
They show increased resistance to root-borne pathogens.
Name two environmental stresses that mycorrhizal plants show improved tolerance to.
Salinity and drought.
What is the general effect of mycorrhiza on overall plant growth?
It leads to enhanced plant growth and productivity.
What is the common name for Arbuscular Mycorrhiza (AM) previously?
Vesicular-Arbuscular Mycorrhiza (VAM).
When did arbuscular mycorrhizal interactions evolve?
400 to 450 million years ago.
What role did mycorrhiza play in the evolution of plants?
They played a critical role in the colonization of land by plants.
Give an example of a fungal genus known to form mycorrhiza.
Glomus.
How do fungal hyphae interact with root cells in Endomycorrhiza?
They penetrate the cortical cells of the plant root.
What is the significance of mycorrhiza being found in many agricultural crops?
It highlights their importance in enhancing crop productivity and nutrient uptake in agriculture.
📊 Comparative Table: Viruses, Viroids, and Prions 📊
Here's a quick comparison of these non-cellular infectious agents:
| Feature | Viruses | Viroids | Prions |
|---|---|---|---|
| Composition | Protein coat (capsid) + Genetic material (DNA or RNA) | Free RNA | Abnormally folded proteins |
| Protein Coat | Present | Absent | Absent (are protein themselves) |
| Genetic Material | DNA or RNA (never both) | RNA (low molecular weight) | None (protein only) |
| Infectious Agent | Yes | Yes | Yes |
| Size (relative) | Larger than viroids/prions | Smaller than viruses | Similar to viruses |
| Example Disease | AIDS, Mumps, Tobacco Mosaic Disease | Potato Spindle Tuber Disease | Mad Cow Disease (BSE), CJD |
💡 Did You Know? Click to Reveal! 💡
While viruses are often associated with disease, bacteriophages (viruses that infect bacteria) are being explored as a potential alternative to antibiotics in combating antibiotic-resistant bacterial infections!