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PY1.1-7 | General Physiology — Part 1
CLINICAL SCENARIO
Every drug you'll prescribe crosses a cell membrane. Every blood test you'll order measures substances dissolved in body fluids. Every vital sign you'll check reflects homeostasis in action. The cell is where all of medicine begins — and the cell membrane is the gatekeeper that decides what enters and what stays out. By the end of this section, you'll understand why a red blood cell bursts in pure water, why glucose needs a protein escort to enter your cells, and why your body spends nearly 30% of its energy budget just pumping sodium and potassium.
WHY THIS MATTERS
As a doctor, cell physiology is not abstract science — it's the operating system of every clinical problem you'll face. When you give a patient an intravenous drip, you must know whether to use normal saline or 5% dextrose — the wrong choice can make cells swell and burst or shrink and die. When you treat diabetic ketoacidosis, you're fighting a pH disaster caused by failing buffer systems. When you interpret a blood report showing low calcium, you need to understand calcium homeostasis to decide whether it's a parathyroid problem or a vitamin D deficiency. General physiology isn't a separate chapter — it's the foundation beneath every chapter.
RECALL
From NCERT Class 11 Biology, you already know the basic parts of a cell: the nucleus (contains DNA), the cell membrane (boundary), mitochondria (powerhouse), endoplasmic reticulum (protein/lipid factory), and ribosomes (protein synthesis). You also know that osmosis is the movement of water through a membrane and that diffusion is the movement of molecules from high to low concentration. We'll now build on these foundations with the precision a medical student needs.
The Cell — Fundamental Unit of Life (PY1.1)
The human body contains approximately 37 trillion cells, and while they vary enormously in shape and function — from a disc-shaped red blood cell to a neuron a metre long — they share a common structural blueprint.
Figure: The Cell — Fundamental Unit of Life (PY1.1)
Every cell has three essential components:
- Cell membrane (= plasma membrane) — the outer boundary. It decides what enters and leaves. We'll study this in detail next.
- Cytoplasm — the gel-like interior, containing organelles suspended in cytosol (the fluid portion, about 70% water).
- Nucleus — the control centre, containing DNA organised into 46 chromosomes. The nucleus directs protein synthesis through messenger RNA (mRNA).
Key organelles and their functions:
- Mitochondria — generate ATP (the energy currency) through oxidative phosphorylation. A liver cell has ~2,000 mitochondria; a red blood cell has none (it relies on glycolysis alone). Think of mitochondria as power plants — more active cells have more power plants.
- Rough endoplasmic reticulum (RER) — studded with ribosomes, it synthesises proteins destined for export (secreted proteins, membrane proteins). Cells that secrete a lot of protein (e.g., plasma cells making antibodies) have abundant RER.
- Smooth endoplasmic reticulum (SER) — synthesises lipids and steroids, detoxifies drugs (hepatocytes have extensive SER), and stores calcium (especially in muscle cells, where it's called the sarcoplasmic reticulum).
- Golgi apparatus — the post office. It receives proteins from the RER, modifies them (glycosylation, phosphorylation), packages them into vesicles, and sends them to their destination — either the cell membrane for secretion or lysosomes.
- Lysosomes — contain digestive enzymes (acid hydrolases) that break down worn-out organelles, ingested bacteria, and cellular debris. Lysosomal storage diseases (e.g., Gaucher disease, Tay-Sachs disease) occur when a specific lysosomal enzyme is deficient.
- Peroxisomes — contain oxidase enzymes that break down very-long-chain fatty acids and neutralise reactive oxygen species. They produce hydrogen peroxide (hence the name) and then break it down with catalase.
In Biochemistry right now, you're studying the proteins and lipids that make up these organelles. The collagen, enzymes, and structural proteins you're learning about in BI are manufactured by the RER and Golgi right here.
The Cell Membrane — Gatekeeper of the Cell (PY1.1)
The cell membrane is not a simple wall — it's a dynamic, fluid structure that controls every interaction between the cell and its environment.
Figure: The Cell Membrane — Gatekeeper of the Cell (PY1.1)
The Fluid Mosaic Model (Singer & Nicolson, 1972) describes the membrane as a phospholipid bilayer with proteins embedded in it, like icebergs floating in a sea of lipid.
Phospholipids — the main structural component:
• Each phospholipid has a hydrophilic head (loves water, faces outward) and two hydrophobic tails (fear water, face inward).
• They spontaneously arrange into a bilayer — heads out, tails in — creating a barrier that water-soluble molecules cannot easily cross.
• Think of it as a sandwich: the bread (hydrophilic heads) faces the watery environment on both sides, and the filling (hydrophobic tails) forms the oily interior.
Cholesterol — inserted between phospholipid tails:
• At body temperature, cholesterol makes the membrane less fluid (prevents too much movement).
• At lower temperatures, it prevents solidification (keeps the membrane flexible).
• Think of cholesterol as a thermostat — it keeps membrane fluidity in a usable range.
Membrane proteins — the functional components:
• Integral (transmembrane) proteins — span the entire bilayer. They serve as channels (pores for ions), carriers (transport molecules across), receptors (detect signals), and enzymes.
• Peripheral proteins — attached to the inner or outer surface. They serve as structural anchors (connecting the membrane to the cytoskeleton) and signalling molecules.
Glycocalyx — a carbohydrate coat on the outer surface:
• Formed by glycoproteins and glycolipids
• Functions: cell recognition (blood group antigens are glycolipids), protection, cell adhesion
• This is why your immune system can distinguish 'self' from 'non-self' — it reads the glycocalyx.
The membrane is selectively permeable: Small, nonpolar molecules (O₂, CO₂, N₂) pass freely. Small, uncharged polar molecules (water, urea) pass slowly. Large, uncharged polar molecules (glucose) and all ions (Na⁺, K⁺, Cl⁻) cannot cross unaided — they need transport proteins. This selectivity is the basis of EVERYTHING in transport physiology.
SELF-CHECK
A cell is actively secreting large amounts of protein (e.g., a plasma cell producing antibodies). Which organelle would you expect to be most abundant in this cell?
A. Smooth endoplasmic reticulum
B. Rough endoplasmic reticulum
C. Peroxisomes
D. Lysosomes
Reveal Answer
Answer: B. Rough endoplasmic reticulum
The rough endoplasmic reticulum (RER) is the site of synthesis for secretory and membrane proteins. Cells that produce large quantities of protein for export (plasma cells, pancreatic acinar cells) have abundant RER. The smooth ER handles lipid synthesis and drug detoxification. Peroxisomes handle fatty acid oxidation. Lysosomes handle digestion of cellular waste.