Membrane Structure, Transport, and Endosymbiosis

Endosymbiotic Origin of Mitochondria and Chloroplasts

• Semi-autonomous organelles inside eukaryotic cells

  • Have their own DNA and reproduce on their own timeline, independent of the cell cycle

  • Contain ribosomes that resemble bacterial ribosomes more than cytosolic ribosomes

  • Have double membranes: an inner membrane and an outer membrane, with the hypothesis that one membrane came from a bacterial ancestor and the other from the host cell’s membrane

• Evolutionary history

  • Evidence supports that mitochondria and chloroplasts originated from free-living prokaryotes that entered a host cell via endosymbiosis and became integrated

  • The organelles could not live independently outside the cell today but function semi-autonomously inside the cell

• Evidence and reasoning discussed in class activity

  • Four potential lines of evidence were presented to evaluate this endosymbiotic origin theory

  • If organelles divide independently of the host cell, that is evidence of their historical independence (relic of their free-living past)

  • The options evaluated were used to identify which would not support endosymbiosis

• Specific evidence highlighted

  • 1) Mitochondria and chloroplasts contain their own circular DNA, resembling bacterial genomes, not nuclear DNA; supports independent, bacterial-like ancestry

  • 2) They can grow and reproduce on their own within the cell, independent of cell division timing; supports prior independent life

  • 3) Ribosomes in mitochondria and chloroplasts have structural differences from cytosolic ribosomes, being more similar to bacterial ribosomes; supports bacterial origin

  • 4) They acquire some necessary proteins from the cytosol of the host cell; this is true but does not support independent origin, rather it shows integration and dependence on the host

• Conclusion from the discussion

  • The statement that would not be evidence for independent endosymbiotic origin is item 4

• Functional roles of these organelles

  • Mitochondria: cellular respiration; convert food molecules (e.g., sugars) into usable energy; found in all eukaryotic cells

  • Chloroplasts: photosynthesis in plants and algae; convert light energy into chemical energy (sugars)

• Structural features emphasized

  • Both organelles have double membranes

  • Internal compartments: mitochondria have a matrix; chloroplasts contain thylakoid discs (often referred to as thylakoids, used for energy conversion processes)

• Outlook for later study

  • Will explore mitochondria and chloroplasts in more depth in the next unit as metabolic centers of energy conversion

Membrane Structure and the Fluid Mosaic Model

• Description of the general membrane structure

  • Membranes are fluid mosaics: flexible and dynamic with many components

  • Fluidity arises because phospholipids are not covalently bonded in a polymer; they can move laterally and rotate, allowing bending and reshaping of the membrane

  • Mosaic aspect comes from a variety of other molecules embedded in or associated with the lipid bilayer (proteins, cholesterol, glycolipids, glycoproteins)

• Phospholipid basics

  • Polar, hydrophilic heads interact with water; nonpolar, hydrophobic tails avoid water

  • In aqueous environments, phospholipids spontaneously form bilayers or micelles due to hydrophobic and hydrophilic interactions; not covalently bonded, but arranged to minimize free energy

• Lipid mobility and membrane dynamics

  • Lipids can switch places (lateral diffusion), spin (rotational movement), and occasionally flip from one leaflet to the other (flip-flop) with energy input

  • Flip-flop across the bilayer is energetically unfavorable because polar heads must traverse a hydrophobic interior

• The importance of fluidity for function

  • A completely rigid membrane would hinder transport and shape changes; fluidity enables trafficking, remodeling, and dynamic responses to the environment

• Historical experiment reference (conceptual)

  • A classic study showed that when two different cell membranes fuse, proteins and lipids rapidly intermix across the joined membrane within about an hour, illustrating membrane fluidity and dynamic organization

• Core components of the membrane mosaic

  • Lipids: phospholipids (with saturated or unsaturated tails) and cholesterol

  • Proteins: integral (embedded, some spanning the membrane) and peripheral (associated but not embedded)

  • Carbohydrate tags: glycosylation on proteins and lipids (glycoproteins and glycolipids) contribute to cell recognition and membrane identity

Lipids, Tail Properties, and Fluidity

• Tail structure determines packing and fluidity

  • Shorter hydrocarbon tails tend to pack less tightly, increasing fluidity and creating larger gaps between lipids

  • Unsaturated tails (double bonds causing kinks) also disrupt tight packing, increasing fluidity

  • Saturated tails are straight and pack tightly, increasing rigidity and decreasing fluidity

• Temperature effects

  • Higher temperatures increase molecular motion and fluidity; lower temperatures promote rigidity

• Practical implications

  • More saturated tails → stiffer, more solid-like membrane at a given temperature

  • More unsaturated tails → more fluid, more permeable membrane at the same temperature

• Real-world analogy

  • Unsaturated fats (e.g., olive oil) are liquid at room temperature; membranes with unsaturated tails behave similarly, remaining more fluid

  • Saturated fats (e.g., butter) are solid at room temperature; membranes enriched in saturated tails are more rigid

Cholesterol: A Buffer for Membrane Fluidity

• Cholesterol’s role in the membrane

  • Cholesterol is a sterol with a rigid ring structure; largely nonpolar and hydrophobic

  • It sits among the phospholipid tails and interacts with them, interrupting tight packing

  • Acts as a buffer, preventing extremes in membrane fluidity

• Effects at temperature extremes

  • High temperatures: cholesterol reduces excessive fluidity, helping maintain membrane integrity

  • Low temperatures: cholesterol prevents complete solidification by inserting between tails and creating irregular packing, maintaining some fluidity

• Conceptual takeaway

  • Cholesterol helps membranes adapt to environmental temperature by buffering against membranes becoming too fluid or too rigid

• Visual cue from the lecture

  • Cholesterol’s ring structure contributes to a stabilizing influence within the hydrophobic core of the bilayer

Membrane Proteins and the Glycocalyx

• Types of membrane proteins

  • Integral proteins: embedded in the lipid bilayer; some span the entire membrane (transmembrane) while others are anchored in one leaf of the bilayer

  • Peripheral proteins: not embedded; attach loosely to the membrane surface

• Transmembrane proteins and their properties

  • The middle (transmembrane) region is rich in nonpolar amino acids, compatible with the hydrophobic core

  • The ends exposed to water on either side may contain polar or charged amino acids

• Functions of membrane proteins

  • Transport: channels and carriers that move substances across the membrane

  • Enzymatic activity: catalyze reactions at or near the membrane

  • Signal transduction: receptor proteins that bind external signaling molecules and convey signals intracellularly

  • Cell-to-cell recognition: glycoproteins and glycolipids with carbohydrate tags act as identifiers

  • Linkage to cytoskeleton and extracellular matrix: help anchor the cell and coordinate shape changes

• Glycoproteins and glycosylation (the glycocalyx)

  • Carbohydrate tags (glycosylation) attach to proteins and lipids, forming a carbohydrate-rich layer on the extracellular surface

  • Carbohydrate groups provide cell-to-cell recognition cues and identity markers (e.g., distinguishing self from non-self or different cell types)

  • These tags are primarily exposed on the outer membrane surface and contribute to membrane asymmetry (sidedness)

• Gene expression note

  • A notable fraction of genes codes for membrane proteins; in humans, roughly 25–30% of genes code for transmembrane proteins, underscoring their importance across biology

• Visual and conceptual takeaways

  • Membrane proteins serve diverse roles and are integral to the membrane’s functionality, including shaping interactions with the environment and coordinating internal processes

Transport Across Membranes: Permeability, Gradients, and Mechanisms

• Selective permeability of membranes

  • Membranes do not allow all substances to cross with equal ease; some pass readily, others require assistance

• Permeability patterns and patterns to notice

  • Small, nonpolar molecules (e.g., O2, CO2, N2) cross easily

  • Polar molecules and charged species cross with more difficulty and often require assistance

  • Water is small but polar; it crosses more slowly and often via facilitated diffusion or osmosis

• Why permeability matters

  • The membrane’s hydrophobic core repels water and charged/ionized species, creating the need for specific transport mechanisms

  • Permeability differences enable gradients to exist across the membrane, which can store energy and drive processes

• Electrochemical gradients

  • Ions (e.g., Na+, Cl−) can create both a concentration gradient and a membrane potential (voltage), forming an electrochemical gradient

  • This gradient can store energy to do work when ions move across the membrane via channels or transporters

• Three broad transport categories (criteria used in the lecture)

  • Diffusion (passive) vs facilitated diffusion (passive with help) vs active transport (requires energy)

  • Criteria used to classify: energy requirement (yes/no) and whether a transport protein is involved (yes/no)

• Diffusion (passive, no transport protein)

  • Substances move down their concentration gradient directly through the bilayer if they are small and nonpolar

  • Process continues until equilibrium (equal concentrations on both sides), though random motion continues

• Facilitated diffusion (passive, requires a transporter)

  • Solutes with difficulty crossing the lipid bilayer (e.g., larger or polar molecules) use specific channels or carriers

  • Movement is still down the concentration gradient; no additional energy is required

• Active transport (requires energy and often a transporter)

  • Moves solutes against their concentration gradient (uphill), from low to high concentration

  • Requires energy input (e.g., from ATP) and a specific transporter protein to drive the process

• Practical takeaways

  • The presence or absence of energy and transport proteins helps determine how substances cross membranes

  • Transport mechanisms are essential for maintaining gradients, nutrient uptake, and homeostasis

Osmosis and Diffusion: Water Movement and Tonicity

• Diffusion basics (revisited)

  • Random motion of particles leads to net movement from regions of high concentration to low concentration

  • Equilibrium means equal concentrations on both sides, not that motion stops

• Osmosis: diffusion of water

  • Water moves down its own osmotic gradient, i.e., toward the side with more dissolved solutes (less free water)

  • Free water concentration drives the direction of water movement across a selectively permeable membrane

• Terms of tonicity and what they mean

  • Hypertonic: side with higher solute concentration (lower free water)

  • Hypotonic: side with lower solute concentration (higher free water)

  • Isotonic: equal solute concentrations on both sides (equivalent free water)

• Egg experiments (conceptual examples)

  • Egg in pure water swells and becomes bloated due to water uptake by osmosis (hypotonic external solution relative to the egg interior)

  • Egg in corn syrup shrivels as water exits the egg to dilute the hypertonic external solution (hypertonic external solution)

• Applications to animal and plant cells

  • Animal cells in hypotonic solutions tend to swell and may lyse without structural support

  • Animal cells in hypertonic solutions tend to shrink and become crenated

  • Plant cells resist lysis due to the rigid cell wall; in hypotonic solutions they become turgid (beneficial for maintaining structure), while in hypertonic solutions they may plasmolyze (membrane pulls away from cell wall)

• Plasmolysis as a plant cell response

  • Dehydration of cell causes the membrane to detach from the cell wall, which can lead to wilting in plants

• Relevance to physiology and medicine

  • Osmotic balance is critical in blood and body fluids; improper tonicity can cause cell swelling or shrinkage with severe consequences

Quick Review and Connections

• Key takeaways about membranes

  • Fluid mosaic model captures the dynamic, heterogeneous nature of the membrane

  • Lipid composition (tail length, saturation) and cholesterol modulate fluidity and stability

  • Membrane proteins provide transport, signaling, recognition, and structural functions, with a broad repertoire supported by a sizable portion of the genome

  • Endosymbiotic origin explains the presence of mitochondria and chloroplasts in eukaryotic cells, supported by circular DNA, ribosomal similarity to bacteria, and independent replication

• Real-world relevance

  • Understanding membrane composition helps explain how cells adapt to temperature and environment

  • Membrane transport underpins nutrient uptake, ion homeostasis, and signaling, all essential to physiology and health

• Foundational experiments referenced

  • The dynamic mixing of membrane components in fused cells demonstrated membrane fluidity and mobility

  • Classic studies on diffusion and osmosis underlie modern understanding of permeability and gradients

• Foundational equations and formulas not explicitly stated in lecture notes

  • The discussion centers on concepts like gradients, diffusion, and energy requirements rather than explicit mathematical formulations; key ideas include concentration gradients, electrochemical gradients, and the energy costs of active transport

Connections to the Rest of the Course

• Building blocks for metabolism and bioenergetics

  • Mitochondrial energy conversion and chloroplast energy capture tie directly to membrane-based processes and gradients

• Cellular organization and communication

  • Glycocalyx and membrane proteins underpin cell recognition and signaling crucial for tissue formation and immune responses

• Pathophysiology implications

  • Abnormal membrane transport or osmotic imbalances contribute to disease states; understanding these mechanisms informs medical approaches