Prokaryotes – Bacteria and Archaea

Overview of Prokaryotes

Prokaryotes are unicellular organisms that lack a membrane-bound nucleus and organelles. They are classified into two domains: Bacteria and Archaea. Prokaryotes are among the most abundant and diverse life forms on Earth.

• Bacteria: One of the two main groups of prokaryotes, characterized by the presence of peptidoglycan in their cell walls.

• Archaea: Prokaryotes that often live in extreme environments and have unique membrane lipids and cell wall components.

Cell Structure and Components

• Peptidoglycan: A polymer that forms a mesh-like layer outside the plasma membrane of most bacteria, providing structural support.

• Outer membrane: Found in Gram-negative bacteria, this membrane provides an additional barrier to the environment.

• Capsule: A sticky, protective layer outside the cell wall that helps bacteria adhere to surfaces and evade the immune system.

• Fimbriae/Pili: Hair-like appendages that allow prokaryotes to attach to surfaces or other cells.

• Flagella: Long, whip-like structures used for movement.

• Nucleoid: Region in the cytoplasm where the prokaryotic chromosome is located.

• Plasmid: Small, circular DNA molecules that replicate independently of the chromosome.

Gram Stain and Cell Wall Types

• Gram-positive bacteria: Have thick peptidoglycan cell walls and stain purple with Gram stain.

• Gram-negative bacteria: Have thin peptidoglycan layers and an outer membrane; stain pink/red.

Reproduction and Genetic Variation

• Binary fission: Asexual reproduction where a cell divides into two identical cells.

• Genetic recombination: Increases genetic diversity through mechanisms such as:

  • Transformation: Uptake of foreign DNA from the environment.

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

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

Adaptations and Survival

• Endospore: A resistant, dormant structure formed by some bacteria to survive harsh conditions.

• Extremophiles: Archaea that thrive in extreme environments.

  • Thermophiles: Live in very hot environments.

  • Halophiles: Thrive in highly saline environments.

  • Methanogens: Produce methane as a metabolic byproduct.

Classification

• Three-domain system: Classification of life into Bacteria, Archaea, and Eukarya.

Example:

Escherichia coli is a Gram-negative bacterium commonly found in the intestines of animals.

Protists

Introduction to Protists

Protists are a diverse group of mostly unicellular eukaryotic organisms. They can be photoautotrophs, heterotrophs, or mixotrophs, and play key roles in ecological communities.

• Photoautotroph: Organisms that use light energy to synthesize organic compounds.

• Heterotroph: Organisms that obtain nutrients by consuming other organisms.

• Mixotroph: Organisms that can use both autotrophic and heterotrophic modes of nutrition.

Endosymbiosis and Evolution

• Endosymbiosis: A symbiotic relationship where one organism lives inside another. Key to the evolution of mitochondria and plastids (e.g., chloroplasts) in eukaryotes.

• Primary endosymbiosis: The engulfment of a prokaryote by a eukaryotic cell, leading to organelles like mitochondria and chloroplasts.

• Secondary endosymbiosis: A eukaryote engulfs another eukaryotic cell that already contains endosymbionts.

Major Protist Groups

• Brown algae, Red algae, Green algae: Photosynthetic protists important in aquatic ecosystems.

• Diatoms: Unicellular algae with silica cell walls.

• Dinoflagellates: Protists with two flagella, some cause harmful algal blooms.

• Slime molds: Fungus-like protists that decompose organic matter.

Example:

The evolution of mitochondria from Alphaproteobacteria via endosymbiosis is a key event in eukaryotic history.

Fungi

Structure and Function

Fungi are heterotrophic eukaryotes that absorb nutrients from their environment. They play essential roles as decomposers, parasites, and mutualists.

• Hyphae: Thread-like filaments that make up the body of a fungus.

• Mycelium: A network of hyphae that forms the main body of the fungus.

• Fruiting body: The reproductive structure that produces spores (e.g., mushrooms).

• Molds: Rapidly growing, asexually reproducing fungi.

• Yeasts: Unicellular fungi that reproduce by budding.

Fungal Life Cycles

• Plasmogamy: Fusion of cytoplasm from two parent mycelia.

• Heterokaryon: A fungal cell with two or more genetically distinct nuclei.

• Karyogamy: Fusion of nuclei to form a diploid zygote.

• Meiosis: Produces haploid spores.

• Mitosis: Used for asexual reproduction (e.g., budding in yeasts).

Symbiotic Relationships

• Mycorrhizae: Mutualistic associations between fungi and plant roots.

  • Arbuscular mycorrhizal fungi: Penetrate plant root cells.

  • Ectomycorrhizal fungi: Surround root cells but do not penetrate.

• Lichens: Symbiotic associations between fungi and photosynthetic organisms (algae or cyanobacteria).

• Soredia: Small clusters of fungal hyphae and algal cells that serve as reproductive units in lichens.

Example:

Baker's yeast (Saccharomyces cerevisiae) reproduces asexually by budding.

Animal Form and Function and Homeostasis

Levels of Organization

Animals are organized into hierarchical levels: cells, tissues, organs, and organ systems. Each level contributes to the structure and function of the organism.

• Anatomy: Study of the structure of organisms.

• Physiology: Study of the functions of organisms and their parts.

• Adaptation: Evolutionary process that increases an organism's fitness in its environment.

• Acclimatization: Physiological adjustment to environmental changes.

Animal Tissues

• Epithelial tissue: Covers body surfaces and lines cavities; functions in protection, absorption, and secretion.

• Connective tissue: Supports and binds other tissues (e.g., bone, blood, cartilage).

• Muscle tissue: Responsible for movement; includes skeletal, cardiac, and smooth muscle.

• Nervous tissue: Conducts electrical impulses; found in the brain, spinal cord, and nerves.

Homeostasis and Regulation

• Homeostasis: Maintenance of a stable internal environment.

• Regulator: Organism that uses internal mechanisms to control internal change.

• Conformer: Organism that allows internal conditions to change with external conditions.

• Negative feedback: A control mechanism that reduces the stimulus (e.g., body temperature regulation).

• Homeostatic system components: Sensor (detects change), Integrator (compares to set point), Effector (responds to restore balance).

Thermoregulation

• Endothermic: Generate heat by metabolism (e.g., mammals, birds).

• Ectothermic: Gain heat from external sources (e.g., reptiles, amphibians).

• Vasodilation: Widening of blood vessels to increase heat loss.

• Vasoconstriction: Narrowing of blood vessels to reduce heat loss.

• Countercurrent exchange: Transfer of heat between fluids flowing in opposite directions.

• Physical processes of heat exchange: Radiation, evaporation, convection, conduction.

Example:

Humans maintain body temperature through sweating (evaporation) and shivering (muscle activity).

Nervous System

Structure and Function of Neurons

The nervous system coordinates responses to stimuli using specialized cells called neurons. Neurons transmit electrical and chemical signals throughout the body.

• Neuron: Basic unit of the nervous system.

• Dendrite: Receives signals from other neurons.

• Axon: Conducts impulses away from the cell body.

• Synapse: Junction between two neurons.

• Neurotransmitter: Chemical messenger released at synapses.

• Glial cells: Support and protect neurons.

Electrical Properties of Neurons

• Resting potential: The membrane potential of a neuron at rest, typically around -70 mV.

• Sodium-potassium pump (Na+/K+ pump): Maintains resting potential by pumping 3 Na+ out and 2 K+ in.

• Action potential: Rapid change in membrane potential that travels along the axon.

• Threshold: Minimum depolarization needed to trigger an action potential.

• Depolarization: Membrane potential becomes less negative.

• Repolarization: Return to resting potential.

• Hyperpolarization: Membrane potential becomes more negative than resting.

• Voltage-gated ion channels: Open or close in response to changes in membrane potential.

• Refractory period: Time during which a neuron cannot fire another action potential.

• Saltatory conduction: Action potentials jump between nodes of Ranvier in myelinated axons, increasing speed.

Synaptic Transmission

• Chemical synapse: Neurotransmitters cross the synaptic cleft to transmit signals.

• Electrical synapse: Direct passage of ions through gap junctions.

• EPSP (Excitatory postsynaptic potential): Depolarizes the postsynaptic membrane, increasing likelihood of firing.

• IPSP (Inhibitory postsynaptic potential): Hyperpolarizes the postsynaptic membrane, decreasing likelihood of firing.

• Summation: Integration of multiple EPSPs and IPSPs (spatial and temporal).

Organization of the Nervous System

• Central nervous system (CNS): Brain and spinal cord.

• Peripheral nervous system (PNS): Nerves outside the CNS.

• Sensory neurons: Transmit sensory information to the CNS.

• Interneurons: Connect neurons within the CNS.

• Motor neurons: Transmit signals from the CNS to effectors (muscles/glands).

Brain Structure and Function

• Forebrain: Includes cerebrum, thalamus, hypothalamus; involved in sensory processing, reasoning, and homeostasis.

• Midbrain: Involved in vision, hearing, and motor control.

• Hindbrain: Includes cerebellum, medulla oblongata, and pons; controls vital functions and coordination.

• Cerebral cortex: Outer layer of the cerebrum; involved in perception, thought, and voluntary movement.

• Limbic system: Involved in emotion and memory (includes amygdala and hippocampus).

Autonomic Nervous System

• Sympathetic division: Prepares the body for "fight or flight" responses.

• Parasympathetic division: Promotes "rest and digest" activities.

• Enteric division: Regulates digestive tract activity.

Example:

The sodium-potassium pump maintains the resting membrane potential by moving Na+ out and K+ into the neuron, using ATP:

Muscles

Types of Muscle Tissue

Muscle tissue is specialized for contraction and movement. There are three main types:

• Skeletal muscle: Voluntary, striated muscle attached to bones.

• Cardiac muscle: Involuntary, striated muscle found in the heart.

• Smooth muscle: Involuntary, non-striated muscle found in walls of organs.

Muscle Structure

• Muscle fiber: A single muscle cell.

• Myofibril: Bundles of contractile proteins within muscle fibers.

• Sarcomere: The functional unit of muscle contraction, defined by Z lines.

• Actin (thin filament) and Myosin (thick filament): Proteins responsible for contraction.

• Z line: Defines the boundary of a sarcomere.

• M line: Center of the sarcomere.

Sliding Filament Model

• Muscle contraction occurs when myosin heads bind to actin and pull the thin filaments toward the center of the sarcomere.

• The sarcomere shortens, but the filaments themselves do not change length.

Regulation of Contraction

• Tropomyosin and Troponin complex: Regulatory proteins that control access of myosin to actin.

• Calcium ions (Ca2+): Bind to troponin, causing tropomyosin to move and expose binding sites on actin.

• Sarcoplasmic reticulum (SR): Stores and releases Ca2+ in response to action potentials.

• Transverse tubules (T-tubules): Conduct action potentials into the muscle fiber.

Neuromuscular Junction and Muscle Activation

• Motor neuron: Stimulates muscle fiber at the neuromuscular junction.

• Acetylcholine (ACh): Neurotransmitter that triggers muscle action potential.

• ATP: Provides energy for myosin head movement and detachment from actin.

Muscle Fiber Types

• Slow-twitch fibers: Contract slowly, resist fatigue, rely on aerobic metabolism.

• Fast-twitch fibers: Contract quickly, fatigue rapidly, rely on anaerobic metabolism.

Comparison of Muscle Types

Example:

During muscle contraction, ATP binds to myosin, allowing it to detach from actin and "re-cock" for another power stroke.

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