Biology Study Sheet - Viruses and Protists

BIOLOGY STUDY SHEET

1. VIRUSES

Are Viruses Alive?

  • Not Living Organisms

    • Reasons for classification as non-living:

    • Not Made of Cells: Viruses lack cellular structure, differentiating them from living organisms.

    • Cannot Perform Life Functions Independently: They rely on host cells to carry out metabolic processes.

    • Only Reproduce Inside a Host Cell: Outside a host, viruses are inert and consist of lifeless chemical components.

Characteristics of Life (Viruses FAIL most)

  • Cellular Organization: Viruses do not possess a cellular makeup.

  • Metabolism: They cannot metabolize nutrients independently.

  • Homeostasis: Lack mechanisms to maintain internal balance.

  • Reproduction: Can only replicate using host cells.

  • Heredity: They carry genetic material (DNA or RNA) but cannot pass on traits independently.

  • Response: Do not respond to internal or external stimuli like living organisms do.

  • Growth & Development: Viruses do not grow or develop; they assemble in host cells.

  • Evolution: While viruses can evolve, they do so within a host and are not autonomous.

Viral Structure

  • Core: Contains either DNA or RNA that holds the genetic information.

  • Capsid: A protective protein coat that encases the viral genome.

  • Receptor Sites: Protein structures that allow attachment to a host cell.

  • Envelope (in some viruses): A lipid layer derived from the host that surrounds the capsid; it determines the host range of the virus by facilitating entry into host cells.

Shapes of Viruses

  • Rod/Filament: Example - Ebola

  • Spherical: Examples - HIV, influenza virus

  • Geometric: Example - Bacteriophage

Size of Viruses

  • Measurements: Viruses are measured in nanometers (nm).

  • Size Range: Typical viral sizes range from 20–400 nm.

Classification of Viruses

Based on:

  1. Host Infected: Different viruses infect different types of organisms.

  2. Structure: Classification by shape, nucleic acid type, and presence/absence of an envelope.

2. VIRAL REPRODUCTION

Why Viruses Need a Host

  • No Ribosomes: Viruses lack the machinery for protein synthesis.

  • No Enzymes: They do not have the enzymes necessary for metabolic processes.

  • No Energy Source: Need a host cell to supply energy for replication.

LYTIC CYCLE (FAST, DESTROYS CELL)

  1. Attachment: Virus binds to the surface of a host cell.

  2. Synthesis: Viral DNA controls the host cell's machinery to produce new viral components.

  3. Assembly: New viral particles are assembled within the host cell.

  4. Release: The host cell bursts (lysis), releasing new viruses into the environment; this leads to a virulent infection.

LYSOGENIC CYCLE (HIDDEN/DORMANT)

  • Insertion: Viral DNA integrates into the host DNA, becoming a provirus.

  • Dormancy: The viral DNA can remain inactive within the host for years.

  • Replication: Each time the host cell divides, it replicates the viral DNA along with its own.

Key Terms

  • Retrovirus: A type of virus that transcribes its RNA into DNA using reverse transcriptase.

  • Reverse Transcriptase: An enzyme that facilitates the conversion of RNA to DNA.

    • Trigger: External factors can activate the retrovirus to enter the lytic cycle.

Effects of Lysogenic Cycle

  • No Symptoms During Dormancy: Infected individuals may show no signs initially.

  • Can Still Spread Disease: Though dormant, the virus can be transmitted to others.

  • Examples: Diseases such as AIDS and herpes are commonly linked to lysogenic cycles.

Origin Theories of Viruses

  1. Used to be Cells: Some theorize that viruses originated from once-independent cells.

  2. Pre-Cellular Life: Hypothesis posits viruses emerged before cellular life.

  3. Pieces of DNA/Proteins: The most widely accepted theory proposes that viruses originated as fragments of DNA or proteins that became capable of replication independently.

3. ENDOSYMBIOTIC THEORY

What it Explains

  • Eukaryotic Cells and Prokaryotes: The theory proposes that eukaryotic cells evolved from prokaryotic cells.

  • Mitochondria & Chloroplasts: Once free-living bacteria that became symbiotic within larger host cells.

How it Happened

  • Engulfment Process: Larger ancestral cells engulfed small bacteria, leading to a symbiotic relationship.

  • Mutual Benefits: There developed a mutualistic relationship where both parties benefit, eventually leading to permanent organelles within the host cell.

Evidence Supporting Endosymbiotic Theory

  • Mitochondria & Chloroplasts:

    • Have their own circular DNA distinct from nuclear DNA.

    • Possess their own ribosomes, resembling those found in bacteria.

    • Similar in size to bacteria.

    • Contain a double membrane structure, indicating engulfment.

    • Can reproduce by binary fission, similar to bacterial reproduction.

    • Show sensitivity to antibiotics, reinforcing their bacterial lineage.

Evolutionary Order

  • Prokaryotes lead to the formation of Protists (first eukaryotic organisms).

4. KINGDOM PROTISTA

General Characteristics

  • Eukaryotic: Members of this kingdom are made of eukaryotic cells.

  • Mostly Unicellular: Although some are multicellular, the majority are unicellular organisms.

  • Habitat: Primarily aquatic environments.

  • Reproduction: Mainly through asexual reproduction, particularly binary fission.

Groups within Kingdom Protista

  1. Plant-like Protists (Algae)

  2. Animal-like Protists

  3. Fungi-like Protists

4.1 Plant-like Protists (Algae)
Features

  • Autotrophs: Perform photosynthesis to create energy.

  • Contain Chlorophyll: Enables photosynthesis.

  • Cell Walls: Composed of cellulose in many species.

Types of Algae

  1. Unicellular Algae:

    • Phytoplankton: Crucial components of aquatic ecosystems; produce approximately 67% of the Earth's oxygen and serve as the base of the food chain.

  2. Multicellular Algae:

    • Seaweed: Utilized in various food products and as resources.

Algal Blooms

  • Excess Growth: Can lead to significant drops in oxygen levels in aquatic environments.

  • Consequences: Fish and other aquatic life may die; some blooms release toxins.

4.2 Animal-like Protists
Features

  • Heterotrophs: Obtain nutrition through consumption of other organisms.

  • Classification by Movement: Based on the method of locomotion.

Types of Animal-like Protists

  1. Pseudopods (Amoeba):

    • Move using extensions of their cytoplasm (false feet).

    • Feed through phagocytosis.

    • Reproduce via binary fission.

    • Contractile Vacuole: Structure to expel excess water.

  2. Flagellates (Euglena):

    • Move using one or more whip-like flagella.

    • Can photosynthesize or consume food.

    • Disease Example: African sleeping sickness.

  3. Ciliates (Paramecium):

    • Move using numerous small hair-like structures called cilia.

    • Characterized by having two nuclei.

    • Reproduce through:

      • Binary Fission (asexual)

      • Conjugation (sexual).

  4. Sporozoa (e.g., Plasmodium):

    • Lacks movement; considered parasites.

    • Reproduce through spores.

    • Disease Example: Malaria.

4.3 Fungi-like Protists

  • Multicellular: Often exist in forms such as plasmodium.

  • Heterotrophs: Feed on decaying organic material.

  • Reproduce by Spores: Similar to true fungi.

Quick Memorization Tips

  • Viruses: Not considered alive unless they are in the process of reproduction.

  • Lytic Cycle: Characterized by cell lysis upon viral replication.

  • Lysogenic Cycle: Involves a dormant state before potentially activating into the lytic cycle.

  • Endosymbiotic Theory: Explains the evolution of organelles from engulfed bacteria.

  • Protists: Display a high degree of diversity; their mode of movement is a critical factor in classification of animal-like protists.