Lec 6

Chapter Overview

Focus on biomembranes in biochemistry as outlined by Prof. Yury Polikanov at the University of Illinois at Chicago.

What are Biological Membranes?

Biological membranes are crucial structures found in all living cells, serving to compartmentalize and protect cellular processes. Each cell is enclosed by a cell membrane, a selective barrier that separates its internal components from the external environment. In eukaryotic cells, additional internal membranes segment cellular functions further, creating various organelles, each with specialized tasks.

These membranes are composed of complex lipid-based structures organized into pliable sheets(flexible), typically a lipid bilayer, that are dynamic and fluidic in nature. They consist of a variety of lipids and proteins, some of which are glycosylated, meaning they have carbohydrate molecules attached that can impact functions such as cell signaling and recognition.

sugar molecule attatched to a protien or lipid.

Electron Micrograph of Biological Membranes

Visual representation of a range of biological membranes includes:

  • Endoplasmic Reticulum: Involved in protein and lipid synthesis.

  • Nuclear Membrane: Encases DNA and regulates gene expression.

  • Mitochondria: The powerhouse, involved in energy production through ATP synthesis.

  • Secretory Granules: Storage and release of substances like hormones and neurotransmitters.

Functions of Biomembranes

  • Define Cell Boundaries: Maintain structural integrity while facilitating communication and interaction with the external environment.

  • Selective Import and Export: Nutrients such as glucose and lactose are selectively imported, while metabolic wastes, including certain antibiotics, are actively transported out of the cell. membrane has to allow for this.

  • Retain Cellular Metabolites: Membranes are selectively permeable; they are impermeable to large polar solutes but allow small polar and nonpolar solutes to pass through, balancing the internal environment. some diffusion, controlled by dif protiens

  • Signal Transmission: Membranes sense and transmit external signals (e.g., hormones), allowing cells to respond to their changes. through plasma membrane

  • Compartmentalization: This segregates different reactions (e.g., energy production from consumption) for efficiency and protection from harmful enzymes.certain reactions in certain regions.

  • Nerve Signal Production and Transmission: Essential for the conduction of electrical impulses in neurons.

  • Energy Storage: Biomembranes enable the formation of proton gradients, harnessed for ATP synthesis during cellular respiration.

Common Features of Biomembranes

  • Structure: Generally sheet-like and flexible, ranging from 30 to 100 Å in thickness, primarily composed of lipid bilayers with embedded proteins. 2 leaflet of lipids. outer is often more positively charged, inner leadlet is often more negatively charged.

  • Formation: Spontaneous assembly in aqueous solutions occurs due to hydrophobic and hydrophilic interactions among lipids.

  • Protein Interactions: Membrane proteins contribute to the asymmetrical organization and dynamic nature of membranes, affecting fluidity and functionality. carbohydrate moieties always outside of the cell

  • electrically polarized inside negative ~60mV

Amphipathic Lipid Structures in Water

agrigate into structures in water

The formation of structures like micelles, bilayers, and liposomes is influenced by:

  • Type of Lipid: Variations in fatty acid saturation and length affect fluidity and permeability.

  • Lipid Concentration: Higher concentrations can lead to distinct structures.

  • vesicles forming

Lipid Micelles

Formed in solutions of amphipathic molecules with larger hydrophilic heads than tails (like fatty acids or detergents(sds)). Micelles aggregate above a specific concentration known as critical micelle concentration (CMC), essential for drug delivery and other applications. - individual units are wedgeshaped (cross section of head greater than taht of side chain.

Lipid Bilayers

Created by amphipathic molecules with heads and tails of equal size (e.g., phospholipids, sphingolipids). The bilayer structure consists of hydrophilic heads interacting with the aqueous environment, with hydrophobic tails packed within, forming two distinct leaflets essential for membrane integrity. 2 leaflets of lipid monolayers. 1 leaflet fakces cytoplasm other faces extrecellular space or the inside of membrane-enclosed organelle.

Lipid Vesicles – Liposomes

These are small bilayers that can spontaneously seal into vesicle structures, playing a key role in drug delivery by encapsulating therapeutic agents and fusing with target cell membranes.

Membrane Dynamics: Lateral and Transverse Diffusion

  • Lateral Diffusion (Uncatalyzed) : Describes the rapid movement of lipids within the same leaflet, approximately 1 μm/s, crucial for the membrane’s fluid nature.

  • Transverse Diffusion (uncatalyzed transbilayer flipflop): Refers to the rare spontaneous flipping of lipids between bilayer leaflets, which usually necessitates specialized enzymes (catalizer) (like flippases and floppases- use atp to move lipids against the concentration gradient) to regulate lipid distribution. rare bc the charge head group must transverse the hydrophobic zone of the membrane. much slower for same reason.

Fluid Mosaic Model of Biomembranes

Proposed by Singer and Nicholson in 1972, this model depicts membranes as dynamic structures composed of various lipids and proteins that float in or on the fluid lipid bilayer, contributing to the membrane's functionality. Key components include:

  • Integral Proteins: Firmly anchored within the membrane structure, some may span the bilayer, playing critical roles in transport and signaling.

  • Peripheral Proteins: Loosely associated with the membrane surface, can be easily removed by changes in salt concentration or pH. noncovalently attached some are linked to membrane lipids.

Membrane Budding and Fusion

Biomembranes are dynamic entities that continuously gain and lose segments, through processes regulated by specific proteins. Notable examples include:

  • Vesicle Budding: From the Golgi apparatus.

  • Exocytosis: Secretion of substances from cells.

  • Endocytosis: Uptake of external materials.

  • Neurotransmitter Release: Synaptic response in nerve signaling.

  • Entry of influenza virus into the host cell.

  • Release of neurotransmitters at nerve synapses.

  • Separation of two plasma membranes during mitosis.

EXAMPLE FROM SLIDE : Neurotransmitter Vesicle Fusion

Composition of Biomembranes

The composition of membranes varies across different organisms, tissues, and organelles: ratio of lipid to protein varies different biomembranes

type of phispholipid varies in dif biomembranes

  • Differences in lipid-to-protein ratios, types of phospholipids, and sterol abundance affect membrane properties.

  • Cholesterol is a common feature in plasma membranes but is relatively absent in mitochondria; galactolipids are abundant in plant membranes, playing roles in membrane stability and fluidity. absent in animals

Sterols and Hopanols in Biomembranes

  • Eukaryotic Cell Membranes: Often contain sterols such as cholesterol in animal cells and phytosterols in plant cells, which enhance membrane integrity and fluidity. ergosterol in fungi

  • Aerobic Prokaryotes: Utilize hopanols that contribute to the rigidity and permeability of membranes under varying environmental conditions.

Membrane Composition in Archaea (CHECK SLIDEs FOR MORE INFO)

Archaeal membranes exhibit distinct phospholipid structures, featuring ether linkages unlike the ester linkages found in bacteria and eukaryotes, allowing them to thrive in extreme environments.

Organisms Adapt to Membrane Composition (CHECK SLIDE FOR MORE INFO)

Cells adapt to environmental changes through modifications in membrane fluidity, which is influenced by the ratio of unsaturated to saturated fatty acids, allowing for optimal function across temperature variations.

Functions of Proteins in Biomembranes

Membrane proteins have integral roles in various biological processes:

  • Reception: Proteins act as receptors for hormones and neurotransmitters, enabling communication and response to external signals.

    • hormones insulin receptor

    • nuerotransmitters acetycholine receptor

    • pheromones taste and smell receptiors

    • light opsin

  • Transport: Proteins function as channels and pumps, facilitating the movement of ions, nutrients, and other molecules across the membrane.

    • (channels gates and pumps)

    • nutrients glucose transporter, maltoporin

    • ions potassium channel sodium channel

    • nuerotransmitters serotonin reuptake protein

  • Enzymatic Actions: Many proteins serve as enzymes, like ATP synthase, critical for energy production.

    • lipid biosynthesis some acyltransferases

    • atp synthesis f0f1 atpace/ atp synthase

Types of Membrane Proteins 3 topological classes

  • Peripheral Proteins: Loosely attached, easily removed with changes in ionic conditions (high salt wash or change in pH). associate with the polar head groups of membrane lipids.

  • Integral Proteins: Span the entire membrane, requiring detergents for extraction due to their embedded nature. asymemetric. stretches of hydrophobic amino acids in the protein interact with the hydrophobic regions of the membrane. (6 types) NEED TO KNOW FOR EXAM SO GOOGLE IT> a lot pf slides

  • Anchored (amphitropic) Proteins has its own slide too: Have lipid modifications that facilitate reversible attachment to membranes, allowing flexibility in function and localization.

CHECK SUMMARY SLIDE

The lipid and protein distribution exhibits asymmetry between the inner and outer leaflets of a bilayer. For example, phosphatidylserine externalization during cell activation signals pathways such as blood coagulation.

Transport Across Biomembranes

  • Diffusion: Small nonpolar molecules can passively diffuse across membranes, while polar solutes require assistance from membrane proteins for transportation. There are two main categories:

    • Transporters: Specific and often ATP-dependent, controlling the movement of solutes across the membrane.

    • Channels: Facilitate rapid passive transport of ions, characterized by fast movement and no saturation.

  • passive transport mechanisms

Active Transport Mechanisms

  • Primary Active Transport: Driven directly by ATP hydrolysis, moving substances against their concentration gradient.

  • Secondary Active Transport: Utilizes the energy from the movement of one species down its gradient to energize the transport of a different species against its gradient.

Summary of Key Concepts CHECK SUMMARY SLIDE

Biomembranes consist of asymmetric lipid bilayers interspersed with proteins that carry out various functions critical to cellular processes such as transport, signaling, and maintaining homeostasis. The fluid mosaic model reinforces the dynamic nature of membranes, emphasizing the integral roles of membrane proteins in cell biology.