Chapter 1-7: Cellular Organelles and Membrane Structure (Vocabulary Flashcards)
Semi-autonomous organelles: mitochondria and chloroplasts
These two organelles are described as semi-autonomous: partly independent, functioning a bit like their own units within the cell.
They each contain their own distinct genetic information: a mitochondrial genome (distinct chromosome) and a chloroplast genome (distinct chromosome).
They cannot survive in isolation outside the cell; they rely on the rest of the cell for materials, but they operate somewhat independently in providing energy-related functions.
The space between their membranes is called the intermembrane space.
Chloroplasts feature an inner membrane folded into thylakoid membranes; stacked thylakoids form grana, and the space inside the thylakoids is the thylakoid lumen. These membrane structures optimize energy-transforming reactions in photosynthesis.
Mitochondria also have extensive inner membrane folding (cristae) to increase membrane surface area for energy transformation.
In both organelles, the elaborate membrane architecture increases workspace for energy transformation processes, enabling efficient production and storage of energy.
Photosynthesis and energy transformation in chloroplasts vs mitochondria
Chloroplasts: use light energy to build nutrient molecules (photosynthesis). The process captures light energy and converts it into chemical energy stored in organic molecules.
Mitochondria: break down organic nutrients to release energy stored as ATP to power cellular work (cellular respiration).
Plant cells contain chloroplasts as well as mitochondria; animal cells contain mitochondria but not chloroplasts.
The chloroplast builds nutrients; the mitochondrion provides usable energy by converting nutrients to ATP for cellular work.
Structure-focused overview: chloroplasts and mitochondria
Chloroplast structure:
Outer membrane
Intermembrane space
Inner membrane
Internal membranes: thylakoid membranes, which stack into grana; thylakoid lumen is the space inside the thylakoids
Mitochondria structure:
Outer membrane
Intermembrane space
Inner membrane (folded into cristae) to increase surface area and house energy-transforming machinery
In both organelles, the elaborate internal membrane geometry increases surface area to host enzymes and complexes required for energy transformations.
Endosymbiosis theory and organelle origins
Endosymbiosis theory describes how mitochondria and chloroplasts originated:
Mitochondria arose from ancestral proteobacteria.
Chloroplasts arose from ancestral cyanobacteria.
An ancestral eukaryotic cell engulfed a prokaryotic cell; rather than digesting it, a mutualistic relationship formed and evolved into modern organelles.
Key evidence for endosymbiosis:
Organelles have their own circular genomes resembling prokaryotic chromosomes.
Organelles possess ribosomes similar to prokaryotic ribosomes.
They replicate by a process similar to bacterial binary fission.
Ancestors have modern relatives: proteobacteria (mitochondria) and cyanobacteria (chloroplasts).
The genetic content of organelle genomes is small but essential for energy-transformation functions.
Genome and replication features in mitochondria and chloroplasts
Both organelles contain their own circular DNA molecules, resembling bacterial chromosomes, rather than the linear multiple chromosomes found in the eukaryotic nucleus.
Nuclear chromosomes in eukaryotes are linear and multiple; mitochondrial and chloroplast genomes are typically circular and singular in each organelle.
Both organelles have their own ribosomes, which resemble prokaryotic ribosomes rather than eukaryotic cytosolic ribosomes.
They grow and divide independently, using binary fission-like processes similar to bacteria.
The presence of their own genetic material and ribosomes supports their semi-autonomous status.
Protein sorting and targeting: co-translational vs post-translational
Protein sorting is essential to deliver proteins to the correct cellular location.
Co-translational sorting:
Sorting occurs concurrently with translation.
Signal sequences, typically at the N-terminus, direct ribosome–nascent chain complexes to the rough endoplasmic reticulum (ER).
Proteins entering the ER are packaged into vesicles, transported to the Golgi, processed, sorted, and then directed to lysosomes/vacuoles, plasma membrane, or for secretion.
Post-translational sorting:
Proteins are fully synthesized in the cytosol and then imported into target organelles (nucleus, mitochondria, chloroplasts, peroxisomes).
Targeting signals on the fully folded protein are recognized and facilitate import.
If a protein lacks any targeting signal, it remains in the cytosol.
This dual sorting system ensures proteins reach their proper cellular destinations.
Extracellular matrix (ECM) and cell interactions in animals
ECM components fall into two main protein categories:
Adhesive proteins: sticky, responsible for binding and anchoring ECM components to each other and to cells; they help hold the matrix together and connect the matrix to cells.
Structural proteins: provide strength and resilience to tissues, helping absorb and distribute mechanical forces.
ECM also contains carbohydrates, especially polysaccharides attached to proteins:
Glycosaminoglycans (GAGs): long, unbranched polysaccharides with strong negative charges.
Proteoglycans: core protein with covalently attached GAG chains.
The assembly of GAGs and proteoglycans creates a hydrated, water-rich cushion that resists compression due to the negative charges attracting water molecules, forming a protective gel-like ECM.
Visual: the ECM encloses a sheet of cells; proteins (yellow with brown stripes) and carbohydrate-rich proteoglycans (green chains with a purple core) form a hydrated matrix around cells.
Plant cell walls vs animal ECM: structural differences
Plant cells surround themselves with a rigid cell wall, in contrast to the more flexible ECM of animal cells.
Cell wall layers:
Primary cell wall: flexible to allow growth and expansion.
Secondary cell wall: thicker, sturdier, and more fixed once growth ceases.
Construction sequence:
The plant cell first builds the primary cell wall (cellulose-rich and flexible).
As growth slows, it deposits the secondary cell wall in between the primary wall and the plasma membrane, resulting in layered cellulose fibers.
Cellulose fibers are oriented in different directions across layers to maximize strength in multiple directions.
Review: big-picture study tool and cellular systems
A suggested study technique is to create big comparison tables (e.g., bacteria vs plant vs animal cells) to organize features, presence/absence, and functions.
A conceptual view: cells function as integrated systems consisting of major components that coordinate to maintain life processes.
The discussion sets the stage for continuing into Chapter 5, focusing on the cell membrane in more detail.
The cell membrane: structure, model, and function
Core structural foundations:
Phospholipid bilayer: essential building block; without it, membranes cannot form.
Proteins: numerous and essential for membrane function; they provide most membrane-specific activities.
Cholesterol: present in animal membranes, a steroid lipid with a four-ring structure; modulates membrane properties.
Carbohydrates: present as glycolipids and glycoproteins on the outer surface, contributing to cell recognition and signaling.
Major membrane components:
Phospholipids: amphipathic molecules with hydrophobic tails and a hydrophilic head; they form bilayers with two leaflets.
Proteins: embedded or associated with the bilayer; provide transport, signaling, enzymes, and structural roles.
Cholesterol: intercalates within the bilayer, influencing fluidity and rigidity.
Carbohydrates: attached to lipids (glycolipids) or proteins (glycoproteins) on the extracellular surface.
Phospholipids and leaflet asymmetry:
Two leaflets (inner cytosolic face and outer extracellular face) are not identical in composition.
Specific phospholipids predominate on the inner vs. outer leaflet, contributing to membrane organization and function.
The Singer–Nicolson fluid mosaic model:
The membrane is a mosaic of components that move laterally within the plane of the membrane.
The membrane is semi-fluid: lipids and proteins diffuse laterally but do not typically flip-flop across the bilayer without enzymatic help.
The model explains dynamic organization and function of membranes.
Membrane proteins: three major structural categories
Transmembrane proteins: span the entire bilayer, with hydrophobic regions in the membrane core and hydrophilic regions exposed to aqueous environments; stability requires hydrophobic residues in membrane-spanning segments.
Lipid-anchored proteins: proteins covalently linked to lipid molecules anchored in the membrane.
Peripheral proteins: non-covalently attached to the membrane surface or to other membrane proteins.
All transmembrane and lipid-anchored proteins are often considered integral membrane proteins because they have a portion embedded in the lipid bilayer.
The concept of membrane organization and protein attachment: clustering, anchoring, and functional localization.
Membrane fluidity and how cells regulate it
Membranes are semi-fluid and two-dimensional in-plane movement occurs, but molecules do not readily flip-flop between leaflets without enzymes (flipases).
Lipid rafts: microdomains—clusters of lipids that move together within the membrane plane, contributing to local organization and protein recruitment.
Fluidity is essential for membrane function (transport, signaling, and interactions with the environment). It must be balanced for structural integrity.
Factors influencing membrane fluidity:
Temperature: cold temperatures decrease fluidity (membrane becomes more rigid); warm temperatures increase fluidity (membrane becomes more fluid).
Phospholipid tail length: shorter tails increase fluidity; longer tails decrease fluidity due to more hydrophobic interactions and tighter packing. Short tails promote more movement; long tails restrict it.
Tail length range discussed: from about 14 to 24 carbons.
Saturation vs unsaturation of fatty acids: unsaturated tails (kinks due to double bonds) prevent tight packing and increase fluidity; saturated tails (straight chains) pack tightly and decrease fluidity.
Cholesterol: generally stabilizes the membrane; its effect depends on temperature and lipid composition, contributing to maintaining appropriate fluidity across temperatures.
Cells actively regulate membrane composition to maintain homeostasis and adapt to environmental changes.
The membrane is a dynamic interface with the environment; its fluidity and organization support nutrient uptake, signaling, and cell integrity.
Experimental evidence for membrane fluidity
A classic 1970 experiment demonstrated membrane protein movement in fused cells at different temperatures:
Setup: mouse cell membranes were fused and incubated at two temperatures: 0^{\circ}\text{C} (freezing) and 37^{\circ}\text{C} (body temperature).
At 0^{\circ}\text{C}: the membrane proteins remained largely immobile, staying on their original half of the membrane.
At 37^{\circ}\text{C}: the membrane proteins moved and redistributed across the shared membrane space.
This experiment supported the concept of membrane fluidity and the ability of proteins to diffuse within the membrane plane under physiological temperatures.
Structural constraints and membrane organization
In addition to the base fluidity, structural constraints are imposed in some membranes to organize function:
Proteins can be anchored to the cytoskeleton inside the cell to restrict movement or localization.
Proteins can be anchored to extracellular matrix components outside the cell.
These anchors help create functional zones in the membrane where specific activities occur, while the overall membrane remains fluid.
Practice prompt (summarized):
A common study exercise is to compare bacteria, plant cells, and animal cells across multiple features to understand similarities and differences.
Build a table listing features present or absent and explain how these features contribute to cell function.
Summary takeaways
Mitochondria and chloroplasts are semi-autonomous organelles with their own circular genomes and prokaryotic-like ribosomes, evidence for the endosymbiosis origin of these organelles.
They perform complementary energy-related roles in cells: chloroplasts capture light energy to build nutrients; mitochondria convert nutrients into ATP.
Protein sorting systems (co-translational and post-translational) ensure proteins reach their correct destinations.
ECM (in animals) and cell walls (in plants) provide structural support and protection through adhesive/structural proteins and glycosaminoglycans or cellulose, respectively.
The cell membrane is a dynamic, semi-fluid mosaic of phospholipids, proteins, cholesterol, and carbohydrates, with asymmetry between leaflets and various protein categories (transmembrane, lipid-anchored, peripheral).
Membrane fluidity is tightly regulated by temperature, lipid tail length, saturation, and cholesterol, and can be modulated by cellular architecture such as lipid rafts and protein anchors.
Experiments from the 1970s demonstrated real-time membrane dynamics, reinforcing the concept of a fluid mosaic model and the functional importance of membrane organization.