Notes for Unit 1-2: Cell Structure and Function (Plasma Membrane, ECM, and Cytoskeleton)
Plasma Membrane: Structure and Function
Context: Unit 1-2 focus on Cell Structure and Function, with emphasis on the plasma membrane, extracellular matrix (ECM), and cytoskeleton.
Daily learning objectives (Unit 1-2):
List and describe the roles of the plasma membrane.
Define the structure, organization, and function of each component of the animal cell plasma membrane.
Define the function and general structure of the extracellular matrix and the two ECM subtypes: fibronectin ECM and laminin ECM.
Analyze the attachment of plasma membrane components with ECM components, identifying all participants in these connections.
The Animal Cell: Organelle Overview
All organelles of a cell are essential for eukaryotic animal cell function.
Key organelles listed (as in Figure 1):
Plasma membrane
Nucleus, nucleolus, nuclear envelope, chromatin, nuclear pores
Rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER)
Ribosomes
Golgi apparatus
Lysosome
Peroxisome
Mitochondria
Microfilaments, microtubules
Centrioles
Cilia
Core Membrane Concepts
1) Two major themes repeatedly encountered in developmental biology
The plasma membrane, the extracellular matrix (ECM), and the cytoskeleton are the membrane/cell interfaces most central to development.
2) Roles of the plasma membrane (overview)
Protection: Acts as a protective border for the cell.
Fluidity: Provides a flexible border allowing the cell to change shape.
Gateway: Regulates what can enter and exit the cell.
Identity: Surface proteins and sugars convey cell identity to neighboring cells and the environment.
Plasma Membrane Composition and Architecture
1) Lipid bilayer structure and amphipathicity
Animal cell membrane is composed of two layers of phospholipids and cholesterol.
Phospholipids are amphipathic: hydrophobic (nonpolar) tails and hydrophilic (polar) heads.
Heads face aqueous environments (outside and inside of the cell); tails face inward, forming the hydrophobic core.
In the diagrammatic sense:
Hydrophobic tails: nonpolar inner region
Hydrophilic heads: polar surfaces facing the cytosol and extracellular fluid
2) Cholesterol as membrane regulator
Cholesterol acts as a buffer to maintain appropriate membrane fluidity:
In cold temperatures, it prevents membranes from becoming too rigid.
In hot temperatures, it prevents membranes from becoming too fluid.
Role: modulates membrane stiffness and permeability in response to temperature.
3) Membrane proteins: integral vs peripheral
Integral membrane proteins: embedded through one or both phospholipid layers; span the bilayer.
Peripheral membrane proteins: sit on the membrane surface, attached to the membrane but not embedded.
Functions: transport of substances across the membrane and communication/signaling across the membrane.
4) Carbohydrate decoration on membrane components
Some proteins and lipids are decorated with sugar chains (glycosylation).
Prefixes:
Glycoprotein: protein with carbohydrate attached.
Glycolipid: lipid with carbohydrate attached.
Glycans are typically found on the extracellular surface and convey cell identity.
5) Major membrane components (summary)
Glycoprotein: protein + carbohydrate
Glycolipid: lipid + carbohydrate
Integral membrane protein
Peripheral membrane protein
Phospholipid bilayer
Cholesterol
Protein channel (transport pathways)
Detailed Membrane Components and Functions
Integral and peripheral proteins are primarily involved in transport and intercellular communication across the membrane.
Glycoproteins and glycolipids contribute to extracellular identity and cell recognition.
The membrane components (lipids, proteins, sugars) together create dynamic platforms for signaling, adhesion, and transport.
Labeling Exercise (Memory Cue)
A slide showed: labels 1–6 (with labels removed) and asked to recall all labels.
Concept: memorize the arrangement and components of the membrane (phospholipid bilayer, cholesterol, integral and peripheral proteins, glycoproteins, glycolipids).
Cholesterol and Plant Question (Thought Question)
Thought question prompt: cholesterol acts as a temperature buffer for animal cells. Plants live in very cold and hot environments, so why don’t plants have cholesterol? Provide at least two hypotheses.
Two plausible hypotheses:
Hypothesis 1: Plants utilize phytosterols (e.g., sitosterol, stigmasterol) instead of cholesterol to modulate membrane fluidity and rigidity, performing a similar buffering role.
Hypothesis 2: Plants rely more heavily on adjustments to fatty acid saturation (e.g., higher unsaturated fatty acids) and structural barriers (cell wall, waxy cuticle) to manage membrane fluidity across temperatures, reducing the need for cholesterol.
Extracellular Matrix (ECM): Overview
An ECM is a meshwork of proteins and carbohydrates secreted by cells, located on the extracellular side.
ECM provides external support for cells and tissues, acting like scaffolding around a house beyond the plasma membrane.
ECM Components and Core Functions
ECM components include:
Proteoglycan complex
Collagen fibers
Carbohydrates
Protein
Polysaccharide
Fibronectin
Integrin (cell surface receptor that binds ECM)
Microfilaments of the cytoskeleton (through connections to integrins)
Primary ECM functions:
Regulate cell adhesion (how cells stick to each other and to ECM)
Regulate cell migration (how cells move around within tissues)
Provide structural support for tissues
Types of ECM: Fibronectin ECM and Laminin ECM
Two ECM subtypes introduced: Fibronectin ECM and Laminin ECM. Collagen is present in both.
Fibronectin ECM:
Fibronectin fibers interact with collagen to form the meshwork.
Other components involved: plasma membrane, proteoglycan complex, integrins, and cytoskeletal connections via microfilaments.
Cells in a fibronectin ECM tend to be more mobile and less constrained by tissue boundaries; easier intercellular connections.
Fibronectin fibrils connect to integrins, which in turn connect to actin filaments inside the cytoplasm, linking extracellular interactions to the intracellular cytoskeleton.
Laminin ECM:
Forms a tight meshwork that supports epithelial tissues, known as the basal lamina.
Basal lamina composition includes collagen and laminin.
Fibronectin ECM: Details and Connections
Fibronectin–collagen interactions form the meshwork that supports cell adhesion and migration.
Fibronectin interacts with integrin receptors on the cell surface.
Integrins connect to actin filaments inside the cytoskeleton, creating a physical and signaling link between ECM and intracellular machinery.
Visual example: Fibronectin fibrils in a developing Xenopus (frog) embryo illustrate pathways for migrating cells, showing how ECM guides cell movement during development.
Laminin ECM and Basal Lamina
Laminin ECM forms a tight network known as the basal lamina, supporting epithelial tissues.
Basal lamina is a specialized ECM underlying epithelial layers (e.g., skin) and interacts with collagen and laminin networks to provide filtration, support, and signaling cues for epithelial cells.
Epithelial Tissues and Basal Lamina Integrity
Epithelial tissues form a tight array of cells with specialized functions (e.g., skin).
These tissues remain in a coherent array unless gene expression changes or basal lamina breaks down, which can lead to altered tissue organization and function.
A scanning electron micrograph (SEM) shows basal lamina beneath epithelial tissue, illustrating its structural role in tissue architecture.
Connections to Foundational Principles and Real-World Relevance
Membrane composition and fluidity are fundamental to how cells sense and respond to their environment, which is critical during development and tissue morphogenesis.
ECM composition and organization guide cell adhesion, migration, and differentiation, shaping tissue architecture and organ formation.
Integrin-mediated links between ECM and the cytoskeleton are central to mechanotransduction, translating physical cues into cellular responses.
Alterations in ECM integrity (e.g., basal lamina breakdown) are linked to developmental processes and diseases affecting epithelial tissues.
Ethical, Philosophical, or Practical Implications (not explicitly covered in the transcript but pertinent)
Understanding cell–ECM interactions informs tissue engineering and regenerative medicine, raising questions about how to emulate natural ECM to guide cell behavior safely.
Insights into ECM breakdown and tissue integrity have implications for cancer biology and metastasis, where cell migration and invasion are key factors.
The study of membrane components and ECM underscores the importance of ethical considerations in biomedical research, especially when translating basic science into therapies.
Notation and Equations (If Relevant)
This content is largely descriptive and conceptual; there are no explicit mathematical formulas presented in the transcript.
Where helpful, the relationship between extracellular and intracellular linkages can be summarized as:
Extracellular: Fibronectin/Laminin + Collagen interactions with cell surface receptors (e.g., Integrins).
Intracellular: Receptors connect to cytoskeletal elements (e.g., Actin via Integrins).
Overall linkage: ECM ⇄ Integrins ⇄ Actin cytoskeleton, facilitating adhesion, signaling, and migration.
Quick Reference: Key Terms
Plasma membrane
Phospholipid bilayer
Hydrophobic (nonpolar) and hydrophilic (polar) regions
Cholesterol (membrane fluidity buffer)
Integral membrane protein
Peripheral membrane protein
Glycoprotein
Glycolipid
ECM (Extracellular matrix)
Proteoglycan complex
Collagen
Laminin
Fibronectin
Integrin
Basal lamina
Xenopus embryonic development (as an example of cell migration in ECM
Summary of Figures and Visual Cues Mentioned
Figure 1: Anatomy of the animal cell with organelles
Figure 17: ECM conceptual diagram showing proteoglycan complex, collagen fiber, carbohydrates, protein, polysaccharide, fibronectin, plasma membrane, integrin, microfilaments, cytoskeleton
Figure 21: Fibronectin ECM with Fibronectin–Collagen interaction and connection to plasma membrane via Integrins
Figure 24: Fibronectin fibrils in Xenopus embryo illustrating migrating cell pathways
Figure 25: Laminin ECM forming basal lamina (tight meshwork of collagen and laminin)
Figure 27: SEM image of basal lamina under epithelial tissue