Cell Membranes and Transport

Membranes and membrane transport

• Phospholipids & amphipathic lipids form bilayers in water
• Acts as a dynamic structure to enable life
• Defines cell boundaries
• Creates specialized compartments within eukaryotic cells
• Basic structure is consistent across all biological membranes; a fluid bilayer of phospholipids
• Acts as a selective barrier
• Controls passage of substances/selectively permeable
• Crucial for maintaining cellular homeostasis
• Made of lipids, proteins & carbohydrates, constantly moving & interacting in a fluid environment
• Plays a vital role in cellular life through communication, transport & maintaining cellular homeostasis

Lipid bilayers as barriers

• Hydrophobic hydrocarbon chains have low permeability to large molecules & hydrophilic particles
• Effective barriers between aqueous solutions
• Phospholipids are amphipathic, containing hydrophilic/hydrophobic regions
• Hydrophilic phosphate heads interact with the aqueous environment
• Hydrophobic hydrocarbon tails cluster, forming the membrane's core
• Acts as a barrier to hydrophilic molecules like ions/glucose
• Smaller molecules pass through more easily than larger

Selective Barrier

• Restricts the passage of hydrophilic molecules, creating a controlled environment
• Hydrophobic core attracts aqueous molecules

Simple diffusion across membranes.

• Oxygen & carbon dioxide molecules move between phospholipids
• The driving force is diffusion
• Movement of particles from an area of higher concentration to lower concentration
• Requires no energy
• Driven by constant random motion of particles
• The hydrophobic core restricts the movement of charged ions & polar molecules
• Non-polar molecules like oxygen diffuse across, following the concentration gradient.
• Smaller polar molecules like urea/ethanol diffuse across at a slower rate

Oxygen diffusion in the cornea

• Relies on it to obtain oxygen from the air
• Oxygen diffuses from the air, across the tear film, through the cornea & into the corneal cells
• Crucial for molecule movement across cell membranes, allowing cells to obtain essential substances like oxygen & eliminates waste.

Integral and peripheral proteins in membranes

• They have diverse structures, locations & functions
• Integral are embedded in one/both lipid layers
• Peripheral are attached to one/other surface of the bilayer

Two major types of membrane proteins

• Embedded within hydrophobic core of the phospholipid bilayer
• Span/ partially embedded in one of the phospholipid layers
• Crucial for functions like transport & cell signaling
• Hydrophilic, attached to the membrane surface
• Not embedded in the lipid bilayer
• Interact with integral proteins/phospholipid heads

Membrane protein orientation

• Oriented so they can carry out their functions correctly
• Transport proteins in plasma membrane of root cells are oriented to pick up potassium ions from the soil & pump them into the root cell.

Protein content and membrane function

• Varies depending on their function
• High metabolic activity membranes like mitochondria & chloroplasts have a higher protein content (up to 75%)
• Less active function membranes like myelin sheath of nerve fibers have a lower protein content (~18%)
• Crucial for cell activities, from transport/signaling to energy production

Movement of water molecules across membranes by osmosis and the role of aquaporins

• Water molecules constantly move in & out of cells, no net movement when concentrations are equal
• Solute concentration differences create a water concentration gradient
• Moves from regions of lower solute concentration (higher water concentration) to regions of higher solute concentration
• A passive process, so requires no energy input
• Aquaporins facilitate water transport
• Significantly increase the rate of water movement across the membrane
• Abundant in kidney & root cells, which require high water functionality
• Highly selective, allow water molecules through while preventing protons (H+)
• Crucial for maintaining balance within cells & organisms
• Provide specialized channels for rapid & selective water movement

Channel proteins for facilitated diffusion

• Structure allows specific ions to diffuse through when channels are open
• Acts as transport with a helping hand
• Integral membrane proteins create pores for ions or polar molecules
• Highly selective, allows only one type of particle to pass through
• Maintains correct ion concentrations
• A passive process, relies on the concentration gradient
• Cells regulate substance entry & exit by controlling the channel proteins present
• Some channels open/close in response to cellular signals
• Allows membrane permeability dynamic control

Pump proteins for active transport

• Use energy from ATP to transfer particles across membranes
• Moves particles against a concentration gradient
• Move substances against their concentration gradient, from a low to high concentration region
• Requires energy (unlike passive transport)
• Use ATP to move substances, often transport substances only in one direction
• Go through conformational changes to transport substances
• Crucial for maintaining proper ion concentrations within cells.
• Enables processes like nerve impulses & muscle contraction

Selectivity in membrane permeability

• Facilitated diffusion and active transport allow for it
• Simple diffusion isn't selective, depends on size & hydrophilic/hydrophobic properties

Membrane Selectivity

• Allows certain small salutes to pass while being freely permeable to the solvent
• Simpler concept associated with artificial membranes- dialysis
• Exhibit a greater degree of control, allowing passing for specific particles and restricts others
• Characteristic of cell membranes

Mechanisms for Selective Permeability

• Uses channel proteins
• Selective transport of substances
• Selectivity of pump proteins & actively transporting specific substances
• Less Selective; Influenced by the size & polarity of molecules
• The selective barrier controls substance movement
• Homeostasis & specialized functions occur

Structure and function of glycoproteins and glycolipids

• Carbohydrate structures linked to proteins/lipids
• Location of carbohydrates on the extracellular side of membranes
• Assist in and enable cell adhesion and also cell recognition

The Sugar Coat of the Cell

• Proteins with carbohydrate chains
• Protein portion embedded in the membrane; carbohydrate chains project outwards
• Lipids with carbohydrate chains
• Lipid portion anchors in the membrane
• Glycoproteins & glycolipids play roles in cell-cell recognition
• Helps cells distinguish between self & non-self
• Essential for immune system function
• Form the glycocalyx; a carbohydrate-rich layer on the outer surface of plasma membrane
• Creates hydrated gel-like layer that protects cell & facilitates cell-cell interactions
• Adjacent layers contribute to tissue integrity

Blood–brain barrier

• Prevents the passage of large molecules & cells from the blood into tissue, protecting the brain
• Membrane plays a vital role in cell-cell recognition and maintains overall tissue integrity

Fluid mosaic model of membrane structure

• Includes peripheral & integral proteins, glycoproteins, phospholipids & cholesterol on a two-dimensional representation
• Shows both hydrophobic and hydrophilic regions

Dynamic assembly

• Not rigid; its phospholipid bilayer is fluid
• Fluidity for flexibility
• Proteins diffuse & interact
• Components include phospholipids, proteins & carbohydrates
• Mosaic components constantly moving & interacting
• They Span the entire membrane, parts protrude on both sides.

Relationships between fatty acid composition of lipid bilayers and their fluidity

• Unsaturated have lower melting points, making membranes flexible at lower temperatures
• Saturated make membranes stronger at higher temperatures

Fatty Acid Composition and Membrane Fluidity

• Saturated have straight chains, while unsaturated have “kinks” due to double bonds
• Pack tightly and increase membrane density & reduce its fluidity
• Kinks pack loosely, membrane more fluid & flexible
• Temperature Influence ratio depends on the temperature
• A higher amount is needed to maintain membrane fluidity at lower temperatures
• Antarctic fish have a higher amount to prevent becoming rigid

Cholesterol and membrane fluidity in animal cells.

• Modulator & stabilizer of membrane fluidity
• Amphipathic with hydrophilic & hydrophobic regions, interacts with phospholipid heads & tails
• Fluidity plays a role in regulating membrane fluidity
• Restricts the movement of phospholipids at high temperatures so it remain more stable
• Prevents phospholipids from packing too tightly & prevents the membrane from becoming rigid at low temperatures
• Stabilizing structure can occur
• Prevents the leakage of ions
• Ensures optimal cellular function

Membrane fluidity and the fusion and formation of vesicles

• Fluid nature of the lipid bilayer is crucial for vesicle formation and fusion
• Phospholipids can move laterally for it to bend & curve
• Small region pulled away from rest
• Requires energy & protein participation
• Brings materials in, this process is when cells acquire nutrients, fluids, & even large particles
• Releases cells, this is how cell secrete hormones & enzymes

Gated ion channels in neurons

• Open & close to electrical charge
• Change in voltage opens potassium to flow out of cell

Sodium-potassium pumps as an example of exchange transporters

• Crucial to producing membrane potentials
• Moves three Na+ ions out of the cell for every two K+ ions into the cell using ATP
• Generates the ionic gradients necessary for nerve impulse transmission and various cellular functions

Sodium-dependent glucose cotransporters as an example of indirect active transport

• Integral in absorbing GLucose by cells in the small intestine & reabsorption by cells in the nephron
• Facilitate it into cells, alongside sodium ions.
• Sodium ions that have been pumped out of c lead to a concentration gradient

Adhesion of cells to form tissues

• Cell-adhesion molecules (CAMs) are required for the different cell-cell junctions
• Tissues form when cells connect through specialized junctions
• There are many that all have unique properties
• Have embedded domains in the membrane
• Assist homophilic and heterophilic Interactions
• This enables cell tissue formation and enables their tissue to connect/communicate