Structure of plasma membrane

Structure of phospholipid molecule

• Polar head

  • containing negatively charged phosphate group

  • hydrophilic (water-loving) → interact with water molecule

• 2 nonpolar fatty acid tails

  • hydrophobic (water-fearing) → will not mix with water

• Hydrophobic tail bury themselves in the center

• Hydrophilic heads line up both sides, in contact with water

Components of plasma membrane

Functions of membrane proteins

• structural proteins

• transporters

  • channel proteins

  • carrier proteins

• enzymes

• membrane receptor proteins

Functions 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 recognise other molecules

• Structural support

  • cell shape maintained by cytoskeletal proteins attached to membrane proteins

Membrane transport

• Different types of membrane transport

  1. Simple diffusion

     1. Movement from high concentration to low concentration

     2. Does not require energy

     3. E.g. gas exchange between cells and ECF in lungs

  2. Osmosis

     1. Diffusion of water molecules across plasma membrane down its concentration gradient

     2. Presence of solutes reduce water concentration

     3. Hypertonic - solution with greater solute concentration

     4. Water travel by osmosis from hypotonic environment to hypertonic environment

     5. Animal cells = crenation (too little water in cells) or hemolysis (too much water, causing cell to burst)

        Plant cells = plasmolysis (too little water) or turgid (too much water, but plant cell doesn’t burst because there is cell wall to protect it)

  3. Facilitated diffusion

     1. Similar to simple diffusion, but requires a carrier

     2. Facilitate carrier-mediated transport

     3. Example:

        • ion channels & aquaporin (water) channels

        • transport of glucose -- glucose transporter (GLUT)

  4. Active transport

     1. Primary active transport

        • involves carrier protein

        • utilises ATP to drive transport of molecules against concentration gradient

     2. Second active transport

        • utilises potential energy stored in electrochemical gradients of ions to drive transport of another molecule (contransport)

        • gradients established and maintained by carrier proteins that utilises ATP

  5. Vesicular transport

     1. Transport of large molecules across membrane

     2. Formation of membrane-enclosed vesicles

     3. Active method of vesicular transport

        • Endocytosis

        • Exocytosis

Primary active transport - Na+ - K+ ATPase pump

1. Cytoplasmic Na binds to pump proteins

2. Na binding promotes hydrolysis of ATP, energy release 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

Co-transport molecules across the plasma membrane

• Same direction (symport) - glucose and amino acids

• Opposite directions (antiport) - Na and H ions

Endocytosis

• Process by which substances move into cell

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

• Pinocytosis - nonselective uptake of ECF fluid

• 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

• Physiological signals: Electrical and chemical signals

• Basic communications systems:

  • Direct contact (gap junctions and contact-dependent signals)

  • Paracrine and autocrine signalling

  • Endocrine signalling

  • Synaptic signalling

Direct contact communication

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

Immune cells & Growth/Development

Chemical signalling between cells

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

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

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 accros small gap to target cell and has a rapid effect

Neurohormone

• chemicals released by neurons into blood for action at distant targets

Chemical Messengers

Signalling transduction pathway

• Describe the mechanisms of signal transduction pathway

  • lipophilic vs lipophobic messengers

• Identify the different functional classes of membrane-bound receptors

Signalling pathway: Receptor protein

• Lipophilic (hydrophobic) signal molecules

  • bind to cytosolic (inside the cell) receptors or nuclear receptors (e.g. transcription in nucleus)

• Lipophobic (hydrophilic) signal molecules

  • stay in the ECF and bind receptor proteins

Membrane-bound receptors

Extracellular signals activates membrane receptors

→ alters intracellular molecules to create cellular response

Three major types:

• Channel-linked receptors

  similar to sodium-potassium pump, but a pump is about TRANSFERRING materials. A channel-linked receptor has to bind with something first to be able to allow materials to enter/exit.

  • Change of membrane potential

    Acetylcholine, for example, binds to a ligand-gated channel in muscle cells. The binding increases the permeability of the membrane to Na+ and causes it to rush in and depolarize the membrane. The response is brief and does not last very long.

• Enzyme linked receptors

  • Example: Tyrosine kinase

    Ligand binds to receptor, which triggers the enzyme, in this case it is tyrosine kinase. Tyrosine kinase adds phosphate onto a particular protein.

    

• G protein-linked receptors

  Very specific.

  • Respond to ligands by activating G proteins

  • Three subunits: a- (binds to GDP), B- and y subunits

  • Cytoplasmic tail of receptor linked to G protein

    • changes from GDP to GTP to become activated

  • G proteins are activated:

    • open ion channels in the membrane

    • alter enzyme activity on cytoplasmic side of membrane - linked to amplifier enzymes

Modulation of signalling pathways

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

Multiple ligands for one receptor - agonist/antagonist

• primary ligand activates a receptor

• agonist will also activate the receptor

• an antagonist will block receptor activity

Multiple receptor for single ligands

Example: blood vessels

• Epinephrine + a-receptor → intestinal blood vessel constricts

• Epinephrine + B-receptor → skeletal muscle blood vessel dilates

Termination of signalling pathways

Receptor activity can be stopped in different ways:

1. Extracellular ligands degraded by enzymes (e.g. protease)

   • e.g. breakdown of neurotransmitter acetylcholine

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

   • e.g. serotonin re-uptake by transporter protein

3. Endocytosis of receptor-ligand complex

   • Ligand removed, receptors returned to membrane as endocytosis (lysis occurs and removes ligand + receptors)

Summary

• Describe the structure of plasma membrane with relation to their functions

• Identify the different types of membrane transport

• Describe the different types of cell-to-cell communication systems

• Describe the membrane-bound receptors and mechanisms of signal transduction pathways