Physiology Introduction

What Does Physiology Study?

Physiology is the branch of biology that explores the functions and mechanisms occurring within living organisms. It focuses on how biological systems carry out processes essential for life.

• Definition: Physiology studies the normal functions of living organisms and their parts.

• Levels of Biological Organization: Physiology can be studied at various levels, including molecular, cellular, tissue, organ, and system levels.

• Example: Investigating how the heart pumps blood or how neurons transmit signals.

Homeostasis

Homeostasis refers to the maintenance of a stable internal environment within an organism, despite changes in external conditions.

• Definition: Homeostasis is the process by which biological systems maintain stability while adjusting to conditions that are optimal for survival.

• Dynamic Steady State: The term "dynamic steady state" means that physiological variables are kept within narrow ranges, but they are not static; they fluctuate around a set point.

• Example: Regulation of body temperature, blood glucose levels, and pH.

• Key Point: Homeostasis is achieved through feedback mechanisms, often involving sensors, control centers, and effectors.

Fluid Compartments of the Body

The body is divided into fluid-containing compartments, each with distinct compositions and functions.

• Major Compartments: Intracellular fluid (ICF) and extracellular fluid (ECF).

• ICF: Fluid within cells; makes up about two-thirds of total body water.

• ECF: Fluid outside cells; includes interstitial fluid and plasma.

• Example: Identifying compartments on a diagram of a cell and labeling ICF and ECF.

• Key Point: The ECF surrounds cells and provides the medium for exchange of substances.

Membrane Transport

Osmotic Equilibrium and Chemical Disequilibrium

The human body maintains osmotic equilibrium (equal water concentration across membranes) but chemical disequilibrium (unequal distribution of solutes).

• Osmotic Equilibrium: Water moves freely across cell membranes, balancing concentrations.

• Chemical Disequilibrium: Solutes such as ions and proteins are distributed unevenly between compartments.

• Example: Sodium is more concentrated in ECF, potassium in ICF.

Plasma Membrane Properties

The plasma membrane controls the movement of substances into and out of cells, using selective permeability.

• Permeable: Substances that can cross the membrane freely.

• Impermeable: Substances that cannot cross the membrane.

• Semi-permeable: Membranes that allow some substances to pass while blocking others.

• Penetrating vs. Nonpenetrating Solutes: Penetrating solutes can cross the membrane; nonpenetrating solutes cannot.

• Example: Oxygen (penetrating), sodium ions (nonpenetrating without channels).

Passive vs. Active Transport

Transport across membranes can be passive (no energy required) or active (energy required).

• Passive Transport: Movement of substances down their concentration gradient without energy input.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

• Example: Diffusion (passive), sodium-potassium pump (active).

Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration.

• Definition: Diffusion is a passive process driven by concentration gradients.

• Equation:

• Where J is the flux, D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.

• Super Important Concept: Substances always diffuse down their concentration gradient unless energy is used to move them otherwise.

Types of Membrane Transport

There are several forms of membrane transport, each with distinct mechanisms and requirements.

• Simple Diffusion: Direct movement of molecules through the membrane.

• Facilitated Diffusion: Movement of molecules via membrane proteins (channels or carriers).

• Primary Active Transport: Uses energy directly to move substances against their gradient (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses energy stored in gradients created by primary active transport.

• Example: Glucose transport into cells via sodium-glucose cotransporter (secondary active transport).

Channels and Carriers

Membrane proteins facilitate transport by forming channels or acting as carriers.

• Channel: Forms a pore for substances to pass through; can be gated (voltage-gated, ligand-gated, mechanically-gated).

• Carrier: Binds to substances and changes shape to move them across the membrane.

• Example: Sodium channel (channel), glucose transporter (carrier).

Classification Table: Types of Membrane Transport

The following table summarizes the main types of membrane transport, their energy requirements, and examples.

Identifying Transport Types

To determine the type of membrane transport in a given scenario, analyze the direction of movement, energy requirement, and involvement of membrane proteins.

• Key Point: Passive transport moves substances down gradients; active transport moves them against gradients.

• Example: If molecules move from high to low concentration without energy, it is passive transport.

Gated Channels

Channels can be classified based on their gating mechanisms.

• Voltage-Gated: Opens in response to changes in membrane potential.

• Ligand-Gated: Opens when a specific molecule binds.

• Mechanically-Gated: Opens in response to physical deformation.

• Example: Sodium channels in neurons are voltage-gated.

Additional info: Some explanations and examples were expanded for clarity and completeness.