L2 Human Physiology

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🧠 Concepts Introduced

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Plasma membrane

Recap Learning outcome #1:

  • Identify the structure and components of plasma membrane - Lipid Bilayer - CPCP

  • Describe the functions of plasma membrane and membrane proteins - PECS (L2-HP)-Plasma-membrane

(L2-HP)-Components-of-plasma-membrane

Function of plasma membrane

  • Physical barrier

    • separate intracellular fluid and extracellular fluid (ECF)

  • Exchange of materials with the environment

    • entry of ions and nutrients into cell

    • elimination of cellular waste and release of products

  • Communication between cell and environment

    • surface proteins respond and recognize other molecules

  • Structural support

    • cell shape maintained by cytoskeletal proteins attached to membrane proteins

Phospholipid molecule

  • Polar head

    • hydrophilic
    • negatively charged phosphate group

  • 2 nonpolar fatty acid tails

    • hydrophobic

Membrane proteins

  • structural proteins
  • transporters
    • channel proteins
    • carrier proteins
  • enzymes
  • membrane receptor proteins

Membrane transport

Recap Learning outcome #2:

  • Explain the different types of membrane transport - SOFAE
  • Differentiate between passive transport, primary and secondary active transport - Energy & Co-transport

Simple Diffusion

  • movement from down its concentration high concentration to low concentration
  • does not require energy
  • eg. gas exchange (O2, CO2) between cells and ECF in lungs

Osmosis

  • water molecules down its concentration (from high concentration to low cincentration)
  • travel from hyponic to hypertonic encironment
(L2-HP)-Osmosis

Facilitated Diffusion (depend on protein)

  • similar to simple diffusion but require a carrier
  • facilitate transfer from high to low concentration
  • passive barrier - mediated transport
  1. ion channels & aquaporin (water) channels
  2. transport of glucose - glucose transporter (GLUT)

Active transport (ATP + depend on protein)

Primary active transport Na+ - K+ - ATPase pump:

  • involves carrier protein
  • utilizes ATP to drive transport of molecules against concentration gradient

Phosphorylated protein

  1. Cytoplasmic Na+ binds to pump proteins

  2. Na+ binding promotes hydrolysis of ATP, energy released during reaction phosphorylates the pump

  3. Phosphorylation causes the pump to change shape, expel Na+ to the outside

  4. Two extracellular K+ binds to pump

  5. K+ binding triggers release of the phosphate, the dephosphorylated pump resumes its original confirmation

  6. Pump protein binds ATP releases K+ to the inside, and Na+ site are ready to bind Na+ again. Cycle repeats

Secondary active transport :

  • utilizes potential energy stored in electrochemical gradients of ions to drive transport of another molecule (cotransport)
  • gradients established and maintained by carrier proteins that utilizes ATP

Co-transport molecules across the plasma membrane - Same direction (symport) - glucose and amino acids - Opposite direction (antiport) - Na & H ions

(L2-HP)-Primary-vs-Secondary-Active-Transport

Endocytosis & Exocytosis (vesicles)

  • Transport of large molecules across membrane

  • Formation of membrane-enclosed vesicles

  • Active method of membrane transport

  • Two types of vesicular transport: Endocytosis Exocytosis

Endocytosis Process by which substances move into cell

  1. Phagocytosis - selective uptake of multimolecular particles (e.g., bacteria and cellular debris)

  2. Pinocytosis - nonselective uptake of ECF fluid

  3. Receptor-mediated endocytosis - selective uptake of large molecule (e.g., protein)

Exocytosis

  • Process by which substance is transported out of cell

  • Provides mechanism for secreting large polar molecules

  • Enables cell to add specific components to membrane

Cell to cell communication

Recap Learning outcome #3

  • Describe the different types of cell-to-cell communication systems - SPED
  • Identify the different groups of chemical messengers - PNH

Physiological signals: Electrical and chemical signals

Basic communications systems:

  • Direct contact (gap junctions and contact-dependent signals)
  • Paracrine and autocrine signaling
  • Endocrine signaling
  • Synaptic signaling

Direct Contact Communication

Direct contact communication of cells

  • Gap junctions Heart, smooth muscle cells, neuron Protein channels - transfer between adjacent cells Membrane proteins: Connexins 6 Connexins forms a channel - Connexon Gap Junction: Connexons from 2 cells
  • Contact - dependent signals will release signals to bound on the target

Chemical signaling between cellls

Paracrine: Autocrine signal - chemical signals act on cells that secreted it

Paracrine signal - chemicals act on cells in the immediate vicinity of cell secreting the signal

Some molecules may act as both an autocrine and paracrine signal

Signals reach target cell by diffusing through interstitial fluid - distance is the limiting factor for diffusion

Neurotransmitter: Nervous system - combination of chemical and electrical signals to communicate over long distance

Nerve cells can extend long processes called axons to very near the target cells

Electric signals travels along neuron until it reaches very end of cell, where it is translated into a chemical signal secreted by neuron

Neurotransmitter - chemical signals secreted by neurons diffuse across small gap to target cell and has a rapid effect

Neurohormone - chemicals released by neurons into the blood for action at distant targets

Hormones: Endocrine signals - cells of endocrine glands secrete hormone into ECF. hormones enter blood and carried by blood in cells in body, target cells respond to hormone

Signaling tranduction pathway

Recap Learning outcome #4

  • Identify the receptors for lipophilic and lipophobic chemical messengers -Cytosol/Nuclear Vs Plasma Membrane
  • Name the three categories of membrane receptors - ECG

Receptor protein

Lipophilic (hydrophobic) signal molecules

  • bind to cytosolic receptors or nuclear receptors

Lipophobic (hydrophilic) signal molecules

  • stay in the ECF and bind receptor proteins.
(L2-HP)-Lipophilic-and-Lipophobic-receptor-signaling

Membrane - bound receptors

Extracellular signals activates membrane receptor → alters intracellular molecules to create cellular response

Three major types:

  • Channel-linked receptor
  • Enzyme-linked receptors
  • G-proteins - linked receptors

Channel - linked receptors

  • Acetylcholine, for example, binds to a ligand- gated channel in muscle cells

  • Binding increases permeability of membrane to Nat and causes it to rush in and depolarize the membrane

  • Response is brief and does not last very long

Enzyme - linked receptors

Phosphorylated protein (L2-HP)-Enzyme-linked-receptors

G protein - linked receptors (GPCR)

G-proteins

G-Protein-amplification

Modulation of signaling pathways

Target cell response - determined by receptor or its associated intracellular pathways

Termination of signaling pathways

Receptor activity can be stopped in different ways:

  1. Extracellular ligands degraded by enzymes

    • e.g., breakdown of neurotransmitter acetylcholine

  2. Chemical messengers transported back into pre-synaptic cells for recycling

    • e.g., serotonin reuptake by transporter protein

  3. Endocytosis of receptor-ligand complex

    • Ligand removed, receptors returned to membrane as endocytosis

Links to this page:

  1. Connexon
  2. Direct contact communication of cells
  3. G-proteins
  4. Phosphorylated protein
  5. Pinocytosis
  6. Receptor-mediated endocytosis
  7. Secondary active transport
  8. L5 Cell Biology
  9. L7 Clinical Chemistry
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