Cpt 4.1

CELL MEMBRANE

  • The cell membrane acts as a selectively permeable barrier that regulates the entry and exit of substances within the cell.

    • Function: Maintains homeostasis by controlling the internal environment of the cell.

  • It is primarily composed of phospholipids, which create a hydrophilic (water-attracting) exterior and a hydrophobic (water-repelling) interior.

CELL MEMBRANE STRUCTURE

  • Bilayer Formation: Cell membranes consist of a bilayer, comprised of two layers of phospholipids.

    • Leaflets: Each bilayer has an inner leaflet and an outer leaflet.

    • Phosphate Heads: The phosphate heads face either the intracellular or extracellular environment.

    • Lipid Tails: The lipid tails from each layer face each other, providing a barrier that allows for selective permeability due to forces of hydrophobicity.

MEMBRANE PROTEINS

Integral Proteins

  • Definition: Integral proteins are permanently embedded within the membrane, interacting with the hydrophobic core.

  • **Key Features: **

    • They typically have hydrophobic regions that interact with the membrane interior and hydrophilic regions that interact with the extracellular or cytoplasmic environment.

Functions of Integral Proteins:

  1. Transport Proteins:

    • Include channel proteins (e.g. glucose transporter, aquaporins for water transport).

    • Function: Move molecules across the membrane.

  2. Receptors:

    • Detect signals such as hormones and neurotransmitters.

    • Trigger cellular responses (e.g. insulin receptors).

  3. Enzymatic Activity:

    • Some integral proteins function as enzymes, catalyzing reactions (e.g. ATP synthase in mitochondria).

  4. Cell Adhesion:

    • Facilitate connections between cells in tissues (e.g. cadherins in tight junctions).

Peripheral Proteins

  • Definition: Peripheral proteins are temporarily associated with the membrane, often interacting with integral proteins or components of the cytoskeleton.

Functions of Peripheral Proteins:

  1. Structural Support:

    • Attach to the cytoskeleton to help maintain cell shape (e.g. spectrin in red blood cells).

  2. Signal Transduction:

    • Relay signals from receptors to intracellular pathways (e.g. proteins in cell signaling).

  3. Enzymatic Activity:

    • Some may function as enzymes to catalyze reactions on the membrane surface (e.g. phospholipase C in lipid signaling).

  4. Cell Recognition:

    • Some peripheral proteins are glycoproteins involved in immune responses (e.g. MHC proteins in immune cells).

MEMBRANE PROTEINS CATEGORIES

  • Membrane Receptors:

  1. Receptors that Bind to Chemical Messengers:

    • Bind to hormones and other chemical signals from external sources.

  2. Enzymes:

    • Enzymes that degrade chemical messengers to terminate their effects.

  3. Ion Channel Proteins:

    • Constantly open channels allowing ions to pass through (e.g., Na+, K+, Cl-).

  4. Gated Ion Channels:

    • Channels that open/close in response to certain stimuli.

  5. Cell-Identity Markers:

    • Glycoproteins helping distinguish self from non-self (e.g., in the immune system).

FLUID MOSAIC MODEL

  • Definition: The cell membrane is described as a dynamic, not rigid structure.

  • Key Features: Lipids and proteins in the membrane can move laterally, creating a fluid mosaic of components.

PERMEABILITY OF LIPID BILAYER

  • The cell membrane is selective in what it allows through:

    • Noncharged Molecules: Can pass freely (e.g., O2, NH3).

    • Large Uncharged Polar Molecules: Need assistance to pass through (e.g., glucose, glycerol).

    • Ions: Generally cannot pass freely (e.g., Na+, K+, Cl-).

  • The principle of "Like Dissolves Like": Molecules pass through based on their chemical properties and solubility.

  • Diffusion: Movement of substances occurs from areas of high concentration to low concentration without energy input (passive process).

PASSIVE TRANSPORT

Types of Passive Transport:

  1. Diffusion:

    • Involves movement from high concentration areas to low concentration areas, requiring no energy.

  2. Facilitated Diffusion:

    • Also does not require energy, but relies on a channel protein to assist the transport of larger or polar molecules (e.g. glucose, amino acids, ions).

ACTIVE TRANSPORT

  • Definition: Transport mechanism that moves substances from low concentration to high concentration.

  • Energy Requirement: Utilizes energy in the form of ATP.

  • Example: Na+/K+ ATPase (pump) creates gradients essential for cellular function.

SECONDARY ACTIVE TRANSPORT

  • Utilizes existing energy gradients: Utilizes gradients established by primary active transporters (e.g., Na+/K+ ATPase).

  • Example of Symport Carrier: SGLT2 symporter transports glucose into cells alongside sodium.

ENDOCYTOSIS

  • Definition: The process whereby a cell engulfs substances from its external environment using vesicles.

  • Steps:

    1. The plasma membrane folds inward around a substance.

    2. A vesicle forms, enclosing the substance.

    3. The vesicle moves into the cytoplasm for processing.

  • Types of Endocytosis:

    • Phagocytosis: Engulfing large particles (e.g., bacteria).

    • Pinocytosis: Engulfing fluids and small dissolved molecules.

    • Receptor-Mediated Endocytosis: Uses specific receptor proteins to target specific molecules.

EXOCYTOSIS

  • Definition: The process through which a cell expels materials using vesicles fused with the plasma membrane.

  • Steps:

    1. A vesicle forms inside the cell containing waste, hormones, or proteins.

    2. The vesicle migrates toward the plasma membrane.

    3. The vesicle fuses with the membrane, releasing its contents outside the cell.

  • Functions of Exocytosis:

    • Secretion of hormones (e.g., insulin from pancreatic cells).

    • Neurotransmitter release (e.g., dopamine from neurons).

    • Waste removal, including cellular debris.

CELL SIGNALING AND COMMUNICATION

  • Overview: Cell signaling is the process through which cells detect, interpret, and respond to signals from their environment. Membrane receptors are integral to this process.

  • Types of Receptors:

    1. Ion Channel-Linked Receptors

    2. G-Protein Coupled Receptors (GPCR)

    3. Enzyme-Linked Receptors (Tyrosine Kinase Receptors - RTKs)

    4. Intracellular Receptors (membrane-bound but function inside the cell).

NEURONAL SIGNALING

Neurotransmitters:

  1. Acetylcholine (ACh):

    • Released by neurons to bind to nicotinic receptors on muscle cells, inducing muscle contraction via sodium influx.

  2. Dopamine:

    • Binds to GPCRs in the brain, influencing mood, motivation, and motor control. Imbalances may lead to conditions such as Parkinson's disease or schizophrenia.

HORMONAL SIGNALING

Hormones:

  1. Insulin:

    • A peptide hormone secreted by pancreatic beta islet cells to reduce blood sugar levels.

    • Binds to insulin receptors on muscle and fat cells to facilitate glucose uptake.

    • Impacted by type 1 and type 2 diabetes.

  2. Cortisol:

    • A steroid hormone that regulates the stress response by acting on intracellular receptors, influencing gene expression.

    • Requires a transport protein to travel to its target cells (non-polar nature of cortisol).