Physiology
General Physiology
General physiology for MBBS and NEET-PG: cell membrane structure, membrane transport, body fluid compartments, resting membrane potential and homeostasis, mapped to NMC codes PY1.
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General Physiology
General physiology for MBBS and NEET-PG: cell membrane structure, membrane transport, body fluid compartments, resting membrane potential and homeostasis, mapped to NMC codes PY1.
General physiology sets the foundation for the whole subject. It covers the structure of the cell membrane, the mechanisms of transport across it, body fluid compartments and their composition, the origin of the resting membrane potential, and the principle of homeostasis through feedback control.
High-yield: General Physiology
- The cell membrane is a fluid mosaic of a phospholipid bilayer with embedded proteins, and it is selectively permeable.
- Simple diffusion moves lipid-soluble substances and gases down their concentration gradient without a carrier.
- Facilitated diffusion uses a carrier or channel protein, is saturable, and needs no energy.
- Primary active transport uses ATP directly, as in the sodium-potassium pump that moves three sodium out for every two potassium in.
- Secondary active transport couples the movement of one solute to the electrochemical gradient of another, as in the sodium-glucose cotransporter.
- Osmosis is the movement of water across a semipermeable membrane from lower to higher solute concentration.
- Tonicity describes the effect of a solution on cell volume; isotonic saline is about 0.9 percent sodium chloride.
- The resting membrane potential of most cells is near minus 70 millivolts and depends mainly on potassium permeability.
- Body water is about 60 percent of body weight, split into two-thirds intracellular and one-third extracellular fluid.
- The extracellular fluid is high in sodium and chloride, while the intracellular fluid is high in potassium.
- Homeostasis is maintained by negative feedback loops that oppose the initial change.
- Endocytosis brings material into the cell, while exocytosis releases it, and both need energy.
- The Gibbs-Donnan effect explains the unequal distribution of diffusible ions when a non-diffusible charged protein is present.
- Aquaporins are water channels that allow rapid water movement across cell membranes.
Membrane transport at a glance
- **Simple diffusion:** Down the gradient, no carrier, no energy. Gases and lipid-soluble solutes.
- **Facilitated diffusion:** Carrier or channel, down the gradient, saturable, no energy.
- **Primary active transport:** Uses ATP directly. Sodium-potassium pump: 3 Na out, 2 K in.
- **Secondary active transport:** Driven by the sodium gradient. Sodium-glucose cotransport.
NMC competencies in this chapter
- **PY1.1:** Structure and functions of the cell membrane
- **PY1.2:** Transport mechanisms across the cell membrane
- **PY1.3:** Body fluid compartments and their composition
- **PY1.4:** Homeostasis and the concept of feedback control
- **PY1.5:** Membrane potential and its ionic basis
- **PY1.6:** Intercellular communication and signalling
- **PY1.7:** Apoptosis and cell injury
- **PY1.8:** Principles of receptors and second messengers
Frequently Asked Questions
How much of body weight is water and how is it distributed?
Total body water is about 60 percent of body weight. Two-thirds of it is inside cells as intracellular fluid and one-third is outside cells as extracellular fluid.
What is the difference between active and passive transport?
Passive transport moves solutes down their gradient without energy, while active transport uses energy, either ATP directly or the sodium gradient, to move solutes against their gradient.
Why is the resting membrane potential negative?
At rest the membrane is far more permeable to potassium than to sodium, so potassium leaves the cell and leaves the interior negative, giving a value near minus 70 millivolts.
What does the sodium-potassium pump do?
It uses ATP to pump three sodium ions out of the cell for every two potassium ions in, which maintains the ionic gradients and cell volume.
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