Summary
Highlights
Physiology studies how the body functions. There are two approaches: teleological (why it happens, e.g., 'why breathe? so you don't die') and mechanical (how it happens, e.g., 'how breathe? creating concentration gradients for oxygen'). In this class, the focus is on the mechanical approach to understand the processes.
The first topic is cell membrane physiology, specifically how molecules are transported across the membrane. Molecules naturally move from high to low concentration following a concentration gradient. The cell membrane is semi-permeable, meaning some molecules can pass through while others cannot, complicating this natural movement.
The cell membrane is primarily composed of phospholipids (hydrophilic heads and hydrophobic tails), forming a double-layered structure. Other components include cholesterol, which strengthens the membrane, and various proteins with functions such as recognition (immune system), structural support (cytoskeleton connection), transportation (channels and carriers), and receptors (binding neurotransmitters/hormones).
The four basic functions of the cell membrane are: physical isolation (separating inside from outside), regulating exchange with the environment (allowing molecules to move in and out), communication (via receptors for chemical signals), and providing support and structure to the cell.
Molecules made of lipids, like steroid hormones (e.g., estrogen, progesterone), can easily pass through the cell membrane via simple diffusion, moving from high to low concentration. This is exemplified by birth control patches, where hormones diffuse through the skin into the body.
Cell membranes contain various proteins: transporter proteins (for ions and larger molecules like glucose), structural proteins (maintaining cell shape or enabling movement), receptor proteins (binding chemical messengers like neurotransmitters and hormones), and enzymes (facilitating chemical processes at lower temperatures).
The ease with which a molecule crosses the cell membrane depends on its lipid solubility (lipid-soluble molecules pass easily), size (smaller molecules move faster), and charge (uncharged, non-polar molecules pass more easily than charged ions like sodium or chloride, which require specific channels).
Small gas molecules and small, uncharged polar molecules (like urea), and steroid hormones (lipid-soluble) can pass through the cell membrane via simple diffusion. However, larger molecules (amino acids, glucose, nucleotides) and charged particles (ions) require assistance to cross the membrane.
All molecular movement requires some form of energy. In biology, transport is categorized into passive and active based on the requirement of biological energy (ATP). Passive transport does not require ATP and relies on concentration gradients (e.g., simple diffusion, facilitated diffusion). Active transport requires ATP to move molecules against their concentration gradient.