Notes on Cell Structure, Plasma Membrane, and Cell Junctions

Cell Diversity and Core Concepts

• Cells vary greatly in appearance and function but share core features. Examples from the lecture:
  • Fibroblast: prefix fibro- means fiber; blast indicates a cell that builds something.
  • Osteoblast: prefix osteo- means bone; osteoblasts are bone-building cells.
  • Erythrocyte (red blood cell, RBC): prefix erythro- means red. RBCs lack a nucleus to maximize hemoglobin storage for oxygen transport, giving a biconcave disc shape.
  • Epithelial cells: introduced as a type to be studied in chapter 4; various epithelial cell types exist.
  • Skeletal muscle cells: multinucleated, with striations enabling strong contraction.
  • Smooth muscle cells: lack striations; different appearance and function.
  • Adipocytes: fat cells; nucleus is pressed to the side; large fat droplet stores energy.
  • Macrophages: prefix macro- means large; phage means to eat, so they are large phagocytic cells.
  • Nerve cells (neurons): cell body, dendrites (input structures), and a long axon (output signal transmission).
  • Sperm cell: smallest cell in the human body; contains a flagellum for motility.
• Question raised (for class): What is the largest cell in the human body? (Not answered in this slide.)
• Core takeaway: cell appearance reflects function; structure and organization underpin physiology.

Structure–Function Principles

• Homogeneity vs heterogeneity: tissues show diversity, but structure generally reflects function.
• Three universal features of cells:
  • Plasma membrane (cell boundary)
  • Cytoplasm
  • Nucleus (genetic control center)
• Emphasis of the course: focus on the plasma membrane and its role in mediating interactions with the extracellular environment.

Major Organelles and Boundaries

• Nucleus: stores genetic information (DNA). Genes are transcribed to RNA, which is used to synthesize proteins.
• Endoplasmic reticulum (ER): the cell’s factory for building proteins and other molecules.
  • Rough ER is studded with ribosomes and synthesizes proteins.
  • Smooth ER (not elaborated here) handles other synthetic and metabolic tasks.
• Golgi apparatus: the shipping and sorting center of the cell; proteins and lipids are modified, sorted, and sent to their destinations.
• Mitochondrion: powerhouse of the cell; site of cellular respiration; converts oxygen into usable energy (ATP).
• Plasma membrane (cell membrane): boundary separating intracellular fluid from extracellular fluid; regulates movement of substances in and out of the cell.
• Extracellular environment concepts:
  • Interstitial fluid: fluid surrounding cells; mostly water with dissolved salts and sugars.
  • Blood plasma: extracellular fluid within blood vessels.
  • Cerebrospinal fluid: extracellular fluid of brain and spinal cord (cerebro- = brain; spinal = spinal cord).
• Extracellular matrix (ECM): glue-like substance that holds cells together and interfaces with inorganic/organic structures.

The Plasma Membrane: Composition and Architecture

• Three biomolecule categories that make up the membrane (the major components):
  • Proteins
  • Lipids
  • Carbohydrates
• A fourth category (nucleic acids) is acknowledged as part of cellular material, but the three major constituents above are the primary membrane components.
• Lipids: two main types
  • Phospholipids: amphipathic molecules with a phosphate head (hydrophilic, polar) and fatty acid tails (hydrophobic, nonpolar).
  • Cholesterol: about $\frac{1}{5}$ of membrane lipids; provides rigidity and stability.
• Membrane organization: hydrophilic heads face water (extracellular and cytosolic sides); hydrophobic tails cluster away from water, forming a bilayer.
• The membrane is described as a fluid mosaic: lipids and proteins can move laterally within the layer; cholesterol contributes to fluidity and rigidity balance.
• Consequences of cholesterol levels:
  • Increasing cholesterol: membrane becomes thicker and more rigid, reducing permeability for some substances.
  • Decreasing cholesterol: membrane loses rigidity and can become more prone to damage or rupture.
• Glycocalyx: a carbohydrate-rich layer on the extracellular surface formed by:
  • Glycoproteins (protein with carbohydrate attached)
  • Glycolipids (lipid with carbohydrate attached)
  • Function: cell recognition, immune system signaling, and protection; it acts like a tag that helps cells identify self vs. non-self.
• Clinical note on glycocalyx: cancer cells can alter glycocalyx to evade immune detection, hindering immune clearance of abnormal cells.

Membrane Proteins: Types and Functions

• Two main classes:
  • Integral (in the membrane) proteins: often transmembrane, spanning the whole bilayer; have both hydrophobic and hydrophilic regions to interact with both sides of the membrane.
  • Peripheral proteins: loosely attached to the membrane surface (either intracellular or extracellular) and can detach for different tasks.
• The membrane proteins' functions (there are $6$ key roles):
  • Transport: move substances across the membrane; includes channel proteins, pumps, and carriers.
  • Receptors: bind signaling molecules (e.g., hormones, neurotransmitters) to elicit a cellular response.
  • Enzymes: catalyze chemical reactions at or near the membrane.
  • Cell-to-cell recognition: glycoproteins or glycoproteins/glycolipids enable the immune system to distinguish self from non-self.
  • Cell-to-cell junctions: help cells adhere to one another or to extracellular matrix.
  • Attachment to extracellular matrix or cytoskeleton: anchor cells and assist movement or structural integrity.
• Transport proteins details:
  • Channel proteins: create a hydrophilic core that allows polar/charged substances to pass through the membrane by diffusion; passive transport.
  • Pumps: require energy (ATP) to move substances against their gradient; substrate binding changes protein conformation to move substances across.
  • Carriers: bind to substances on one side and shuttle them across without direct energy consumption (facilitated diffusion).
• Receptors and signaling: receptors bind specific ligands to trigger intracellular responses; critical for nervous and endocrine signaling.
• Enzymes: active inside or outside the cell, accelerating specific chemical reactions.
• Cell-to-cell recognition and adhesion: glycoproteins and other membrane components help immune surveillance and tissue organization; some proteins anchor cells to each other or to ECM components.
• Attachment points: proteins connect to cytoskeleton or ECM (e.g., collagen fibers) to stabilize position or enable movement.

The Glycocalyx: Sugar Coats and Immune Recognition

• Glycocalyx is formed by:
  • Glycoproteins (protein with carbohydrate) and/or
  • Glycolipids (lipid with carbohydrate)
• Function: extracellular recognition and immune system communication; helps cells identify self vs. non-self.

Plasma Membrane Functions and Permeability

• Primary function: selective barrier — plasma membrane is permeable to some substances and impermeable to others.
• Communication: glycoproteins and glycolipids, along with receptors, enable intercellular signaling.
• Cell-to-cell communication and recognition: glycocalyx plays a key role in recognition and immune response.

Cell Junctions: How Cells Tie Together

• Three main types of cell junctions:
  • Tight junctions: integral proteins form an impermeable seal between adjacent cells; prevent leakage of substances between cells.
  • Common location: digestive tract (GI tract), where enzymes and acids should remain inside the lumen rather than entering underlying tissues.
  • Desmosomes: button-like plaques that provide mechanical strength, resisting tension and pulling forces; important in tissues subjected to stretching (e.g., skin and muscle).
  • Gap junctions: channels that directly connect neighboring cells to allow passage of ions and small molecules; crucial for synchronized activities (e.g., cardiac muscle and some smooth muscles).
• Implications: junction types determine tissue integrity, barrier function, and coordinated physiological responses.

Quick Review and Exam-Oriented Points

• Distinct cell types reflect specialized functions, but all share core components: $ext{plasma membrane}$, $ext{cytoplasm}$, and $ext{nucleus}$.
• Key organelles and their roles: nucleus (DNA storage and transcription), ER (protein synthesis), Golgi (processing and shipping), mitochondria (energy production).
• Extracellular space types and importance: interstitial fluid, plasma, cerebrospinal fluid, and ECM significance for tissue organization.
• Plasma membrane composition: proteins, lipids (phospholipids and cholesterol), carbohydrates; glycocalyx as a recognition tag; lipid bilayer with hydrophilic heads and hydrophobic tails.
• Lipid mechanics: phospholipid heads are hydrophilic (polar) and tails are hydrophobic (nonpolar); cholesterol (~$\frac{1}{5}$ of lipids) supports membrane rigidity.
• Membrane proteins: integral (often transmembrane) vs peripheral; functions include transport, receptors, enzymes, cell recognition, adhesion, and cytoskeletal/ECM attachments; about $6$ major functional roles.
• Glycocalyx and clinical relevance: alterations can affect immune recognition and cancer surveillance.
• Cell junctions: tight (barrier), desmosomes (tension resistance), gap junctions (intercellular communication) – all essential for tissue integrity and function.

Example Connections to Real-World Contexts

• RBCs lack nucleus to maximize oxygen-carrying capacity via hemoglobin; this is a design trade-off between energy metabolism and gas transport.
• Cholesterol’s dual role as a membrane stabilizer and a modulator of fluidity mirrors how cell membranes adapt to physiological conditions (e.g., temperature changes, tissue types).
• Glycocalyx alterations in cancer illustrate how cell surface chemistry governs immune recognition and how tumors can evade immune detection.
• Desmosomes and tight junctions explain why certain tissues are resistant to mechanical damage and how barriers like the intestinal epithelium prevent leakage of digestive enzymes into underlying tissues.