Cell membranes


Overview of Cell Membranes

  • Definition: The cell membrane, also known as the plasma membrane, is a selective barrier that regulates the exchange of materials between a cell and its environment.

  • Composition: Mainly made up of phospholipids, proteins, carbohydrates, and cholesterol, creating a dynamic structure.

Structure of Cell Membranes

Extracellular Matrix (ECM) Components

  • Fibers: Composed of various proteins including glycoproteins and carbohydrates.

  • Glycoproteins: Assist in cell recognition and signaling.

  • Glycolipids: Lipids with carbohydrate chains attached, contributing to cell membrane stability and recognition.

  • Cholesterol: Intercalates within the phospholipid bilayer to affect fluidity.

Components within the Membrane

  • Phospholipid Bilayer: Structure made of hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

  • Proteins:

    • Integral Proteins: Firmly embedded in the membrane, functioning in transport and communication.

    • Peripheral Proteins: Loosely associated with the membrane surface, involved in signaling and maintaining the cell’s shape.

Recap of Cell Theory

  • Origins: Formulated by Schwann in 1839, it encompasses critical principles about cellular life.

    • Key Tenets:

    • All organisms are composed of cells.

    • Cells are the fundamental unit of life.

    • All cells arise from pre-existing cells.

    • Cells exhibit diversity and specialization.

Visualization of Cells

  • Microscopy:

    • Electron Microscopy: Required to visualize cellular organelles due to their small size.

    • Light/Fluorescent Microscopy: Useful for larger structures, capable of resolving down to 0.2 μm.

  • Cell Size:

    • Prokaryotic cells: 1-10 μm

    • Eukaryotic cells: 10-100 μm

  • Surface Area to Volume Ratio: As cells grow, this ratio decreases, necessitating a higher number of cells as organisms increase in size.

Types of Cells

Prokaryotic Cells

  • Size: Typically smaller with a few non-membrane-bound organelles.

    • Components include:

    • Plasma membrane (lipid bilayer)

    • Chromosomes (DNA)

    • Cytosol (semifluid gel)

    • Ribosomes

    • Cell wall, cilia, flagella, and pili.

  • Examples: Bacteria and Archaea.

Eukaryotic Cells

  • Size: Generally larger, containing numerous organelles.

    • Components include:

    • Membrane-bound organelles (e.g., endoplasmic reticulum, Golgi apparatus, nucleus).

    • Non-membrane-bound organelles (e.g., ribosomes).

  • Examples: Animals, plants, fungi, and protists.

The Endomembrane System

  • Definition: Network of internal membranes involved in the transport of materials within a cell.

  • Components:

    • Plasma Membrane: Selectively permeable, allowing nutrient uptake and waste expulsion.

    • Golgi Apparatus: Acts as a shipping center for proteins, involved in glycosylation (adding sugar chains to proteins).

    • Comprised of cisternae with two distinct sides (cis and trans).

    • Endoplasmic Reticulum (ER):

    • Rough ER: Engaged in protein synthesis with ribosomes on its surface.

    • Smooth ER: Free of ribosomes, involved in lipid synthesis, detoxification, and calcium storage.

    • Lysosome: Contains hydrolytic enzymes for digestion and recycling of macromolecules.

    • Nucleus: Surrounded by a double lipid bilayer (nuclear envelope) with large pores for entry/exit.

    • Vacuoles: Membrane-bound sacs with diverse functions including storage and maintenance of turgor pressure in plant cells.

Endosymbiotic Theory

  • Concept: Proposes mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotes, leading to a symbiotic relationship.

  • Evidence:

    • Both organelles possess their own DNA, capable of gene expression and protein synthesis.

    • Ribosome structure resembles that of bacteria.

    • Membrane structures are similar to bacterial membranes.

Cytoskeleton

  • Function: Provides structural support, aids in communication, and facilitates movement within the cell.

  • Components:

    • Microtubules: Hollow tubes composed of tubulin, offering strength and motility.

    • Microfilaments: Actin polymers that assist in cell movement and attachment.

    • Intermediate Filaments: Fibrous proteins providing mechanical support.

  • Cell Motility: Uses flagella and cilia (structured in a 9+2 arrangement) for movement.

Membrane Composition and Properties

Fluid-Mosaic Model

  • Membrane Fluidity: The phospholipid bilayer allows lateral movement of components, influenced by the degree of saturation in hydrocarbon tails.

  • Role of Cholesterol: Stabilizes membrane structure and fluidity, especially in varied temperature conditions.

Heterokaryon Experiment

  • Definition: Fusion of two different cells demonstrated protein mobility within membrane structures, further confirming the fluid nature.

Membrane Protein Classification

Types of Membrane Proteins

  1. Integral (Trans-) Membrane Proteins: Embedded across the membrane, amphipathic.

  2. Peripheral Membrane Proteins: Associated with one side of the membrane, not fully embedded.

Functions of Membrane Proteins

  • Transport: Involved in the movement of ions and molecules.

  • Enzymatic Activity: Facilitate biochemical reactions.

  • Signal Transduction: Act as receptors for signals.

  • Cell Attachment: Connect to the extracellular matrix.

  • Cell Recognition: Help identify cells.

Membrane Carbohydrates

  • Role: Bound to proteins (forming glycoproteins) or lipids (forming glycolipids), involved in cell recognition and identification.

  • Clinical Relevance: Important for defining blood types and initiating immune responses, which is crucial in organ transplantation.

Membrane Synthesis

  • Process:

    1. Endoplasmic Reticulum: Synthesis of membrane lipids and proteins.

    2. Golgi Apparatus: Modifies and prepares for transport.

    3. Vesicle Transport: Transports components to the plasma membrane.

Membrane Permeability

Selectivity of the Membrane

  • Permeable Compounds: Small non-polar molecules easily pass through.

  • Non-Permeable Compounds: Polar molecules and ions need transport proteins.

Types of Transport Proteins

  • Channels: Facilitate passive transport.

  • Pumps: Involved in active transport using energy.

Passive Transport Mechanisms

Diffusion

  • Definition: Movement of molecules from high concentration to low concentration without energy expenditure.

    • Types:

    • Simple Diffusion: Nonpolar molecules pass directly through the lipid bilayer.

    • Facilitated Diffusion: Polar and charged molecules use specific channels for movement.

Active Transport Mechanisms

  • Definition: Movement of ions or molecules against the concentration gradient, requiring energy.

  • Importance: Essential for maintaining biological gradients crucial for various cellular processes (e.g., Na+/K+ pump).

Sodium-Potassium Pump (Na+/K+ Pump)

  • Function: Transports Na+ out of the cell and K+ into the cell, crucial for maintaining cells’ electrochemical gradient.

  • Mechanism: Requires ATP to move ions against their gradients.

Osmoregulation

  • Definition: The control of water balance and osmotic pressure within cells and organisms.

  • Types of Solutions:

    • Hypotonic: Lower solute concentration, water rushes in.

    • Isotonic: Equal solute concentration, ideal for animal cells.

    • Hypertonic: Higher solute concentration, water rushes out.

  • Importance: Plant cells thrive in hypotonic environments for turgor pressure; animal cells in isotonic environments to retain shape and function.

Large Molecule Transport

Mechanisms for Large Molecules

  • Exocytosis: The process of secreting large molecules via vesicle fusion with the plasma membrane.

  • Endocytosis: Includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis (specific uptake).