General Biology Study Notes: Digestion, Circulation, Excretion, Nerves, and Muscles

Digestion & Nutrition

Macromolecules & Diets

Macromolecules are essential nutrients that provide energy and building blocks for growth and maintenance in living organisms.

• Carbohydrates (Sugars, Starch): Primary energy source. Found in high amounts in herbivore fruits, grains, and omnivore diets.

• Proteins: Required for growth, repair, and enzymes. "Essential amino acids" are those the body cannot synthesize and must be obtained from diet (e.g., meat, eggs, beans). A "complete" protein contains all essential amino acids.

• Fats/Lipids: Long-term energy storage, insulation, cell membranes.

• Chemical Energy (from food): Used for movement, including biosynthesis, active transport, and cell growth.

Digestive System Structures & Journey of Food

The digestive system breaks down food into absorbable nutrients through mechanical and chemical processes.

• Mouth: Mechanical digestion (chewing increases surface area). Chemical digestion of starch by salivary amylase.

• Stomach: Chemical digestion of proteins by pepsin (activated from pepsinogen by HCl). Food becomes chyme.

• Small Intestine: Primary site of digestion and absorption.

  • Duodenum: First part. Enzymes (from intestine & pancreas) digest fats, proteins, carbohydrates. Bile (from liver) emulsifies fats.

  • Large Intestine: Reabsorbs water and electrolytes. Houses symbiotic bacteria for vitamin synthesis.

Comparative Digestive Anatomy

Digestive systems vary among animal groups, reflecting dietary adaptations.

• Gastrovascular Cavity: A single opening for ingestion and egestion. Found in simple animals (e.g., cnidarians, flatworms).

• Alimentary Canal: A tube with a mouth and anus, allowing for efficient digestion and absorption. Found in annelids, arthropods, chordates.

• Adaptations: Carnivores have sharp teeth for tearing and shorter digestive tracts. Herbivores have flat teeth for grinding, longer tracts, and often a large cecum for fermenting plant material. Omnivores have a mix.

Circulation & Gas Exchange

Blood and Hemolymph

Circulatory systems transport nutrients, gases, and wastes throughout the body.

• Blood vs. Hemolymph: Blood is contained within a closed circulatory system (vessels). Hemolymph is the fluid in an open circulatory system that directly bathes tissues (e.g., in insects).

Blood Vessels

• Arteries: Carry blood away from the heart.

• Capillaries: Site of gas and nutrient exchange with tissues. Small diameter and thin walls.

• Veins: Carry blood back to the heart.

Gas Exchange Surfaces

• Lungs: Main organ for gas exchange in vertebrates.

• Counter-current Exchange: Blood and water flow in opposite directions, maximizing O2 uptake (e.g., fish gills).

Circulatory Pathways

• Fish (2-chambered heart): Atrium → Ventricle → Gills (O2 picked up) → Body → Atrium.

• Mammals/Birds (4-chambered heart): Double circuit. Oxygenated blood: Left Atrium → Left Ventricle → Body. Deoxygenated blood: Right Atrium → Right Ventricle → Lungs → Left Atrium.

Oxygen Transport

• Hemoglobin: Protein in red blood cells that carries O2.

• Oxygen Saturation Curve: Shows how readily hemoglobin binds/releases O2 based on partial pressure of O2 (pO2). Low pO2 in active tissues causes more O2 to be released.

• Bohr Shift: At low pH (from CO2 buildup in active tissues), hemoglobin releases more O2 at a given pO2. This is adaptive for supplying working muscles.

Osmoregulation and Excretion

Nitrogenous Waste

Excretory systems remove metabolic wastes and regulate water and salt balance.

• Ammonia (NH3): Very toxic; requires lots of water for dilution. Least energetically expensive to produce. Found in aquatic animals.

• Urea: Less toxic; requires less water. Energetically costly to produce. Found in mammals, amphibians.

• Uric Acid: Least toxic; requires minimal water, but is the most energetically expensive to produce. Found in birds, insects, reptiles.

Vertebrate Nephron (Kidney Functional Unit)

• Bowman's Capsule: Filtration of blood.

• Proximal Tubule: Reabsorption of water, nutrients.

• Loop of Henle: Concentrates urine.

  • Descending Limb: Permeable to water. Filtrate becomes more concentrated.

  • Ascending Limb: Impermeable to water. Filtrate becomes less concentrated as salts are reabsorbed.

• Distal Tubule: Fine-tuning of salt and water balance.

• Collecting Duct: Final concentration of urine; regulated by antidiuretic hormone (ADH).

Other Nephridia

• Protonephridium: Network of dead-end tubules with flame cells. Found in Platyhelminthes (flatworms).

• Metanephridium: Open, ciliated funnels (nephrostomes) that collect coelomic fluid. Found in annelids (earthworms).

Nerves

Neurons vs. Nerves

A neuron is a single nerve cell. A nerve is a bundle of many neurons.

Ion Gradients

• Resting cell: High K+ inside, High Na+ and Ca2+ outside.

Parts of a Neuron

• Dendrites

• Cell Body (Soma)

• Axon

Action Potential (AP) Propagation

• Resting Potential: ~ -70 mV. Established by Na+/K+ pump.

• Threshold: The membrane potential at which an AP is triggered. Voltage-gated Na+ channels open.

• Depolarization: Na+ enters the axon, making it more positive.

• Repolarization: K+ exits the axon, restoring negative charge.

• Hyperpolarization: Membrane potential drops below resting.

Synaptic Transmission

• AP reaches axon terminal, causing Ca2+ channels to open.

• Ca2+ triggers vesicles to release neurotransmitter into the synaptic cleft.

• Neurotransmitter binds to chemically-gated channels on postsynaptic cell.

Post-Synaptic Potentials

• EPSP (Excitatory Post-Synaptic Potential): Depolarizes the cell (e.g., Na+ channels open), making an AP more likely.

• IPSP (Inhibitory Post-Synaptic Potential): Hyperpolarizes the cell (e.g., Cl- channels open), making an AP less likely.

• The post-synaptic cell integrates all EPSPs and IPSPs at the axon hillock; if the sum reaches threshold, an AP fires.

• Temporal Summation: Multiple signals from one neuron in quick succession.

• Spatial Summation: Multiple signals from several neurons at the same time.

Schwann Cells & CNS

• Schwann Cells: Glial cells in the Peripheral Nervous System (PNS) that form myelin sheaths, which insulate axons and speed AP conduction.

• Central Nervous System (CNS): Brain and spinal cord.

Muscles

Three Muscle Types

Muscle tissue enables movement and force generation in animals.

• Skeletal: Striated, voluntary. Attached to bone.

• Cardiac: Striated, involuntary. Found only in the heart.

• Smooth: Non-striated, involuntary. Found in the gut, blood vessels, etc.

Sliding Filament Model of Contraction

Muscle contraction occurs when actin and myosin filaments slide past each other, shortening the muscle fiber.

1. Stimulation: Nerve releases ACh, triggering an AP in the sarcolemma (muscle cell membrane).

2. Ca2+ Release: AP travels via T-tubules, triggering the sarcoplasmic reticulum to release Ca2+.

3. Cross-Bridge Formation: Ca2+ binds to troponin, causing tropomyosin to move and expose actin binding sites. Myosin binds to actin.

4. Power Stroke: Myosin hydrolyzes ATP to ADP + Pi, re-energizing and returning to its original position.

5. Detachment: A new ATP binds to myosin, causing it to detach from actin.

6. Relaxation: When stimulation stops, Ca2+ is pumped back into the SR, tropomyosin re-covers the binding sites, and the muscle relaxes.

Key Equations

• Nernst Equation (for ion equilibrium potential): $E = \frac{RT}{zF} \ln \frac{[\text{ion outside}]}{[\text{ion inside}]}$

• ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Comparative Table: Nitrogenous Waste Types

Type	Toxicity	Water Requirement	Energy Cost	Examples
Ammonia	High	High	Low	Aquatic animals
Urea	Moderate	Moderate	Moderate	Mammals, amphibians
Uric Acid	Low	Low	High	Birds, insects, reptiles