Plasma Membrane and Concentration Gradient

The Plasma Membrane

• Functions as a barrier that encloses the cell's inner environment
• Thin, flexible barrier that surrounds and separates the cell's interior from its external environment
• Composed of a phospholipid bilayer, a double layer of phospholipid molecules
• Phospholipid molecules arranged with hydrophobic tails facing inward and hydrophilic heads facing outward
• Described by the fluid mosaic model, suggesting a dynamic structure composed of phospholipids, proteins, and carbohydrates
• Proteins act as gatekeepers, facilitating molecule passage and cell communication

The Concentration Gradient

• Describes the difference in a substance's concentration between two regions
• Drives the movement of substances across the cell membrane
• Substances move from high to low concentration areas to reach equilibrium
• Essential for physiological processes like action potential generation in neurons/muscle cells
• Critical for establishing the resting potential of cells needed for electrical signal transmission
• Aids understanding of different channels' roles in the plasma membrane
• Channels are specialized proteins facilitating substance movement
• Types of substances passing through channels determined by channel properties and concentration gradient
• Ion channels enable ion movement across the membrane, responding to ion concentration changes
• Gated channels open/close based on stimuli such as voltage changes or molecule binding
• Pore-forming channels form from multiple channel protein oligomerization and can be selective/non-selective

Membrane Proteins

• Integral and peripheral proteins are two protein types in the plasma membrane

Integral Proteins

• Embedded within the phospholipid bilayer as an integral part of the membrane structure
• Known as trans-membrane proteins, spanning the membrane thickness
• Integral proteins have hydrophobic regions interacting with phospholipid tails, integrating them into the structure
• Channel proteins are types of integral proteins, creating channels for ion and small molecule passage

Channel Proteins

• Integral membrane proteins that allow ions and small molecules to pass through the membrane by creating a channel for them
• Crucial in substance movement across the membrane for maintaining cellular and tissue homeostasis

Ion Channels

• Proteins allowing ions to pass through the membrane
• Can be selective (allowing specific ions) or non-selective (allowing any ion)
• Integral membrane proteins composed of subunits forming a channel
• Selective types are specific to ions like sodium, potassium, calcium, or chloride
• Classified based on activation properties (always open or gated)
• Some are activated by voltage changes or the binding of a specific molecule
• Sodium-potassium pump maintains the resting membrane potential
• Calcium channels aid muscle contraction and neurotransmitter release
• Chloride channels regulate salt and water balance in the kidneys
• Found in nerve, muscle, and epithelial cells
• They are crucial for action potential generation/transmission, muscle contraction, and ion/water transport

Gated Channels

• Proteins controlling ion flow across the cell membrane in response to specific stimuli
• Function as tiny gates that open/close for regulated ion passage
• Voltage-gated channels are activated by voltage changes across the membrane
• They are important in action potential generation/propagation
• Ligand-gated channels are activated by a specific molecule (ligand) binding to the channel protein
• Mechanically-gated channels are activated by physical stimuli like pressure/stretch and are important in blood pressure regulation and sensory stimuli detection

Leak Channels

• Present in neurons, muscle, and epithelial cells contributing to the resting potential
• Pore is lined that is lined with amino acids which interact with ions as they pass through the the channel
• Non-selective but exhibit some preference based on size and charge and regulate cell volume
• An analogy is a leaky faucet
• Always open and allow ions to pass through the membrane continuously to maintain resting potential and regulate cell volume

Pore-Forming Channels

• Channel proteins formed by oligomerization of multiple channel proteins subunits
• Can be either selective or non-selective depending on the proteins that form the channel
• Plays an essential role in many physiological processes Examples:
• voltage-gated potassium channels: activated by changes in voltage and allow the movement of potassium ions that help in the regulation of the resting potential of cells and in the repolarization phase of action potential
• connexons: formed by the oligomerization of six connecxin proteins and allow for the movements of ions and small molecules between cells; important in cell communication and regulation of enzyme activity
• aquaporins: are selective for the movements of water molecules and allow the rapid movement of water across the membrane; important in the regulation of water balance in the body
• porins: formed by the oligomerization of multiple porin proteins and allow the movement of small molecules through the outer membrane of bacteria; important for the uptake of nutrients and the elimination of waste products in bacteria

Aquaporins

• Selective channels allowing only water passage, preventing other molecules/ions from passing
• Selectivity is enabled by structural features, including a selectivity filter formed by arranged amino acids
• Play a crucial role in the kidney's water reabsorption
• The absence of aquaporins would lead to water loss and dehydration
• Types of aquaporins:
• Aquaporin 0 (AQP0): found in the lens of the eye and maintains transparency
• Aquaporin 1 (AQP1): found in the endothelial cells and important in regulating blood pressure.
• Aquaporin 2 (AQP2): found in the Kidneys and regulate water balance
• Aquaporin 4 (AQP4): found in the brain and regulates water balance- main problem with brain edema
• Important in proper hydration/electrolyte balance in cells, with dysfunction leading to diseases like diabetes insipidus which has excessive thirst and excretion
• Subtypes classified based on their specific functions

Carrier Proteins

• Intergral membrane proteins that bind to specific molecules
• Proteins undergo conformational changes upon binding of the specific molecule, which allows the molecule to be transported across the membrane
• Uniporters: carrier proteins that transport a single type of substances across the membrane; can be passive or active; an example is glucose transporter
• Symporters: carrier proteins that transport two types of substances across the membrane in the same direction; ex. sodium-glucose co-transporter
• Antiporters: proteins that transport two types of substances across the membrane in opposite directions; ex. sodium-potassium pump

Receptor Proteins

• Diverse group of integral membrane proteins important in the communication and response between cells as well as external stimuli
• G protein-coupled receptors (GPCRs) is the largest family of receptor proteins, which consist of a single polypeptide chain that transverses the cell membrane seven times, forming a helical structure
• Ionotropic receptors: another type of receptor protein that directly gate ion channels; consist of subunits that come together to form a transmembrane pore
• Enzyme-linked receptors: a type of receptor protein that have enzyme an extracellular ligand binding domain and an intracellular enzymatic domain; important in the regulation of enzyme activity and in the transmission

Peripheral Proteins

• Proteins are that are not embedded: are attached to the surface of the membrane and held by noncovalent bonds; can be easily removed from the membrane; do not span entire membrane
• Types:
• enzymes: catalyze chemical reactions
• signaling transduction: involved in the signal of transmission of signals between cells and within cells; ex., G protein receptor
• cell adhesion proteins: ex., integrins are involved in a cell adhesion and the extracellular matrix
• structural surface proteins: spectrin is a peripheral protein that provides structural support to the cytoskeleton

Signal Cascades

• A series of events that occurs inside a cell in response to a signal from the outside
• Important because they allow for cells to communicate with each other and for them to respond to changes in their environment
• Plays an important role in important processes in the body

Cellular Communication

Cell communication comes in several forms:

• Direct: occurs when cells directly contact others that allows them to exchange signals through gap junctions
• Paracrine: a signaling molecule is released where they can bind to receptor and trigger a response- important in many physiological processes/functions and found in Immune Systems; gap junctions and tight junctions can be involved
• Autocrine: hormone that lets cells bind on the same cell, but regulating its own behavior
• Endocrine: secreted into bloodstream and travels to distant cells -ex. Hormones

Ligands

• Molecules that bind to specific proteins that are called receptors on the surface of cells, or inside cell
• Can be classified into several different types based on their chemical properties and the types of the receptors they bind to
• Are essential as they allow cells to respond signals from their environment and communicate with other cells

Amplificiation of the Signal

One Important aspect:

• Ligand of- receptor that activates the pathway
• These molecules can send other do signaling molecules that has a cascade that amplifies the origins
• Essential for sells to respond appropriately

Cell Membrane Fluidity

• Defined as the measure of ease which lipid molecules in the membrane can move and change position
• Not Constant
• Influenced by temperature, lipid composition, cholesterol, membrane proteins

Factors Influencing Cell Membrane Fluidity

• Temperature: at low temperatures, lipids are rigid and hinder function; at high temperatures, over fluid affects function
• Lipid Composition: unsaturated membranes with a high proportion of unsaturated fatty acids tend to be fluid
• Cholesterol: with low concentrations, an be increased membrane fluidity by disrupting packing of lipid molecules
• Membrane Proteins:. The presence of membrane proteins can also affect the fluidity of the membrane. These proteins can act as anchors, holding the lipid molecules in place and decreasing the fluidity of the membrane.

Passive Transport

• Movement of molecules across the membrane without energy along concentration gradient
• Types:
  • diffusion (small non polar molecules like oxygen and carbon dioxide
  • osmosis (diffusion of water)
  • facilitated diffusion (larger polar with transport proteins) where transport proteins act as channels or carriers that allow molecules to move down their concentration gradient

Active Transport

• Requires energy to move molecules against concentration gradient:
  • types 1-primary) - directly uses energy with ATP and ex. sodium-potassium pump
  • 2-secondary) secondary active, uses of energy generated from the movement of one molecule down it's concentrate to transport other molecule
• In summary requires energy move molecules against the concentration gradient

Sodium-Glucose Transporter

• Protein responsible for the transport of glucose and sodium ions across the cell membrane
• Vital regulating blood sugar levels and electrolyte balance in the body
• Glucose is supported by energy of active transport and ions are pumps against their concentration gradient for ATPase

Final Review

• Diffusion: is the movement of molecules until the concentration is across the membrane
• Facilitated is the movement of small molecules but uses transports proteins helps that does not require energy.
• active transport Requires energy and movies molecules against concentration

Bulk Transport

• Cells by which large molecules and particles are move into and out of the cell (proteins and organelles). There are four main types:- Endocytosis- Phagocytosis- Pinocytosis- Exocytosis-
• Endyocytosis: cells take in molecules and paricles w/vescle; can be 3 types (phago, pino and recept mediated)
• Phagocytosis: specialized cells callphagocytes found in the immune cellengulfsolid particles by the formation of aphagosome fuse w/lisosome -
• Pinocytosis is the process by which cells takes in small amounts of fluid
• Receptor mediated endocytosis is the way the cells takes is specific molecules on the cell
• Exocytosis is process through which the cell recreate or secrete molecules particles by fusing to release it outside the cell
• Exocytosis is the movement of materials out of the cell (waste or product released), while endocytosis is the movement of molecules/particles to the cell