midterm 2

Solute and Diffusion

  • Diffusion: The movement of a solute from an area of high concentration to an area of low concentration.

    • Solutes move down their concentration gradient, which tells us that "they spread out."

    • High Concentration: A crowded area with many solute particles.

    • Low Concentration: A less crowded area of solute particles.

    • Solutes spread until they are evenly distributed and no longer concentrated in a specific area.

    • Movement is always directed from high concentration to low concentration areas.

  • Cell Membrane:

    • Functions as a barrier that blocks most solutes from diffusing through its phospholipid bilayer.

    • Contains both hydrophobic and hydrophilic properties:

      • Hydrophobic: "Water fearing", does not mix well with water.

      • Hydrophilic: "Water loving", mixes well with water.

    • Most solutes are hydrophilic; however, they cannot diffuse through the hydrophobic cell membrane.

    • Key Concept: Hydrophilic solutes cannot enter the cell directly due to the hydrophobic nature of the membrane.

What Can Pass Through Cell Membrane?

  • Water Molecules: Can pass through phospholipids despite being hydrophilic.

  • Hydrophobic Solutes: Can diffuse directly through the membrane without assistance.

Mechanisms of Solute Passage

  1. Simple Diffusion:

    • The diffusion of a molecule directly through the phospholipid bilayer.

    • Only hydrophobic solute molecules can undergo simple diffusion (e.g., steroid hormones, O₂, CO₂).

  2. Channel Proteins:

    • Proteins embedded in the cell membrane that form tunnels allowing specific solutes to pass through.

    • Each channel protein has a specific binding site for its targeted solute molecule.

    • Facilitated Diffusion (Passive Transport): Movement of solutes from high to low concentration through a channel protein without requiring energy.

    • Cotransport: Involves channel proteins transporting two different solute types simultaneously, with two binding sites.

  3. Endocytosis and Exocytosis:

    • Endocytosis: Bringing solute molecules into a cell by bending the membrane inward to form a vesicle.

    • Exocytosis: The process of expelling solute molecules from a cell using a vesicle that fuses with the cell membrane.

Osmosis and Tonicity

  • Osmosis: The movement of water across a cell membrane toward the side with the highest solute concentration.

    • Key Quote: "Water always moves towards solutes."

  • Tonicity:

    • Isotonic: A solution with equal solute concentration to that of the cell. The cell maintains its size.

    • Hypertonic: A solution with higher solute concentration than the cell. The cell shrinks (crenates) due to loss of water.

    • Hypotonic: A solution with lower solute concentration than the cell. The cell swells and may lyse (burst).

    • All solutes attract water, influencing the movement of water during osmosis.

Common Isotonic Solutions in Hospitals

  • 0.9% saline, Ringer's lactate, 5% glucose.

Osmolarity (OSM)

  • Tips for calculating osmolarity:

    1. Compare total solute concentration inside and outside the cell, excluding penetrating solutes that cannot cause osmosis.

    2. Water always moves towards the area of higher total solute concentration. Example:

    • 0.6 OSM Solute A + 1.0 OSM Solute C = 1.6 OSM; water moves outside the cell.

Nervous System Overview

Two Regions of the Nervous System

  1. Central Nervous System (CNS): Composed of the Brain and Spinal Cord.

  2. Peripheral Nervous System (PNS): Consists of all nervous tissue outside the CNS, including sensory and motor neurons.

Functions of the Nervous System

  • 1) Sense Stimuli: Detects stimuli like sight, touch, smell, etc.

  • 2) Formulate a Response: Involves perception and reflex actions occurring in the CNS.

  • 3) Transmit Signals Rapidly: Signals rapid transport between body parts, e.g., from sense organs to the CNS, and from CNS to muscles.

Neurons and Neuroglial Cells

  • Neurons: The primary cells responsible for sensing stimuli, formulating responses, and carrying electrical signals.

  • Neuroglial Cells: Cells that support and assist neurons but do not carry out sensory or signaling functions.

Types of Neurons

  1. Sensory Neurons (Afferent): Detect stimuli and send signals to the CNS.

  2. Interneurons: Located within the CNS; they process signals from sensory neurons and communicate with other interneurons to generate elaborative responses.

  3. Motor Neurons (Efferent): Convey signals from the CNS to muscles or glands.

Structure of Neurons

  • Cell Body: Contains the nucleus and organelles.

  • Dendrites: Extensions that receive signals.

  • Axon: Conducts electrical signals away from the cell body.

  • Axon Terminals: Release neurotransmitters to pass signals to target cells.

Neurotransmitters

  • Chemical messengers released from axon terminals that transmit signals across synapses.

  • Types of Neurotransmitters:

    • Excitatory Neurotransmitters: Promote action potentials (e.g., acetylcholine, norepinephrine).

    • Inhibitory Neurotransmitters: Prevent action potentials (e.g., GABA, glycine).

Action Potentials

  • Electrical Signal: Travels along the axon, starting at -70 mV (polarized) and changes to +30 mV (depolarized) due to sodium ions entering through voltage-gated sodium channels.

  • Repolarization: Following an action potential, the membrane returns to -70 mV due to potassium ions exiting through voltage-gated potassium channels. The Refractory Period is when a region cannot initiate a new action potential until it repolarizes.

Summary of Reflex Arcs

  • Reflexes are rapid, involuntary responses to stimuli comprising sensory and motor neurons, some including interneurons.

Sensory Receptors

  • Definition: Cells that detect stimuli (light, sound, taste, etc.) and convert stimuli into nerve signals, sending these to the brain.

  • Types:

    • Cutaneous Receptors: In skin for touch; includes pressure, temperature, and pain receptors.

    • Proprioceptors: Sense body part positions in muscles and joints.