Photosynthesis

Autotrophs vs. Heterotrophs

Organisms obtain energy and carbon in different ways, which are fundamental to understanding ecological roles and metabolic diversity.

• Autotrophs: Organisms that produce their own organic molecules from inorganic sources (e.g., plants, algae, some bacteria). Most use photosynthesis to convert light energy into chemical energy.

• Heterotrophs: Organisms that obtain organic molecules by consuming other organisms or their byproducts (e.g., animals, fungi, many bacteria).

• Example: Arabidopsis thaliana (a plant) is an autotroph; humans are heterotrophs.

Chloroplast Structure and Photosynthesis

Chloroplasts are the organelles where photosynthesis occurs in plants and algae. Their structure is closely related to their function.

• Thylakoids: Flattened sacs where light reactions take place.

• Grana: Stacks of thylakoids that increase surface area for light absorption.

• Stroma: Fluid surrounding thylakoids; site of the Calvin cycle.

• Double membrane: Outer and inner membranes enclose the chloroplast.

• Example: The arrangement of thylakoids maximizes light capture for photosynthesis.

Light Wavelength and Photosynthesis

Photosynthetic pigments absorb specific wavelengths of light, which drives the process of photosynthesis.

• Chlorophyll a: Main pigment, absorbs blue-violet and red light.

• Accessory pigments: (e.g., chlorophyll b, carotenoids) broaden the spectrum of light used.

• Action spectrum: Shows the relative effectiveness of different wavelengths in driving photosynthesis.

• Example: Green light is least effective because it is reflected, not absorbed.

Light Reactions

The light reactions convert solar energy into chemical energy (ATP and NADPH) in the thylakoid membranes.

• Photolysis: Splitting of water to release electrons, protons, and oxygen.

• Electron transport chain: Transfers electrons, generating a proton gradient for ATP synthesis.

• Products: ATP, NADPH, and O2.

• Equation:

Photosystems in Light Reactions

Photosystems are complexes of pigments and proteins that capture light energy.

• Photosystem II (PSII): Absorbs light, initiates electron transport, and splits water.

• Photosystem I (PSI): Absorbs light, transfers electrons to NADP+ to form NADPH.

• Example: The Z-scheme describes the flow of electrons from water to NADP+ via PSII and PSI.

The Calvin Cycle

The Calvin cycle uses ATP and NADPH to fix CO2 into organic molecules in the stroma.

• Three phases: Carbon fixation, reduction, regeneration of RuBP.

• Key enzyme: Rubisco catalyzes CO2 fixation.

• Equation:

C3, C4, and CAM Plants

Plants have evolved different strategies to fix carbon, adapting to various environments.

• C3 plants: Use the Calvin cycle directly; most common; susceptible to photorespiration.

• C4 plants: Fix CO2 into a four-carbon compound; spatial separation of steps; adapted to hot, dry climates (e.g., maize).

• CAM plants: Fix CO2 at night; temporal separation of steps; adapted to arid environments (e.g., cacti).

• Comparison Table:

Viruses and Prions

Definitions

Viruses and prions are infectious agents with unique structures and replication strategies.

• Virus: Non-cellular infectious agent composed of genetic material (DNA or RNA) and a protein coat (capsid); may have an envelope.

• Prion: Infectious protein that causes neurodegenerative diseases by inducing misfolding of normal proteins.

• Example: Influenza virus (virus); prion causing Creutzfeldt-Jakob disease.

Structure of Viruses and Prions

• Virus structure: Genetic material (DNA or RNA), capsid (protein coat), sometimes a lipid envelope derived from host cell membrane.

• Prion structure: Abnormal, misfolded protein; lacks nucleic acids.

Life Cycle of Viruses

Viruses replicate by infecting host cells and using their machinery to produce new viral particles.

• Attachment: Virus binds to specific receptors on host cell.

• Entry: Viral genome enters host cell.

• Replication and synthesis: Host machinery produces viral components.

• Assembly: New viral particles are assembled.

• Release: Viruses exit the cell, often destroying it.

Host Range of a Virus

• Host range: The spectrum of host species and cell types a virus can infect, determined by viral surface proteins and host cell receptors.

• Example: Rabies virus infects many mammals; HIV infects specific human immune cells.

Virus-Host Cell Interactions

• Viruses hijack host cell machinery for replication.

• Some viruses integrate their genome into the host's DNA (e.g., retroviruses).

• Host cells may be lysed or persistently infected.

Lytic vs. Lysogenic Cycles

• Lytic cycle: Virus replicates rapidly, lyses host cell, and releases new virions.

• Lysogenic cycle: Viral genome integrates into host DNA (prophage), replicates with host, can later enter lytic cycle.

• Example: Bacteriophage lambda can switch between cycles.

Prokaryotic Organisms: Bacteria and Archaea

Definitions

• Bacteria: Domain of prokaryotic, unicellular organisms with peptidoglycan cell walls.

• Archaea: Domain of prokaryotic, unicellular organisms with unique membrane lipids and cell walls (no peptidoglycan); often found in extreme environments.

Structure of Prokaryotic Cells

• Cell wall: Provides shape and protection; composition differs between Bacteria and Archaea.

• Plasma membrane: Controls entry and exit of substances.

• Nucleoid: Region containing circular DNA.

• Ribosomes: Sites of protein synthesis.

• Additional structures: Flagella (movement), pili (attachment), capsules (protection).

Gram-Positive vs. Gram-Negative Bacteria

• Gram-positive: Thick peptidoglycan layer; stains purple with Gram stain.

• Gram-negative: Thin peptidoglycan layer plus outer membrane; stains pink/red.

• Table:

DNA Structure in Prokaryotes

• Single, circular chromosome located in the nucleoid.

• May contain plasmids—small, circular DNA molecules with additional genes.

Genetic Exchange in Prokaryotes

• Transformation: Uptake of free DNA from the environment.

• Conjugation: Direct transfer of DNA between cells via a pilus.

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

• Example: Antibiotic resistance genes can spread via these processes.

Metabolism of Prokaryotic Cells

• Prokaryotes display diverse metabolic pathways:

  • Phototrophs: Obtain energy from light.

  • Chemotrophs: Obtain energy from chemicals.

  • Autotrophs: Use CO2 as carbon source.

  • Heterotrophs: Use organic compounds as carbon source.

• Example: Cyanobacteria are photoautotrophs; Escherichia coli is a chemoheterotroph.

Protists

Note: While not detailed in the guide, protists are covered in the relevant textbook chapters and are eukaryotic organisms that are not plants, animals, or fungi. They display diverse forms and modes of nutrition.

Additional info: Academic context and examples have been added to expand on the brief points in the original study guide, ensuring the notes are self-contained and suitable for exam preparation.