Biology Chapter on Cell Membranes

Plasma Membrane: Structure and Function

• Plasma membranes are crucial for homeostasis and cell survival, maintaining intracellular contents and coordination with other cells
• Mechanical barrier- Prevents leakage, and keeps internal contents inside and external ones out
• Adhesion- Holds cells together to form tissues
• Exchange- Facilitates nutrients, wastes, and secretions
• Response- Reacts to environmental changes and signals
• Ionic gradient maintenance- Important for electrical activity

Components of Cell Membrane: Lipids

• Lipids (phospholipids and cholesterol) form a barrier to water-soluble substances.
• Provide fluidity and stability to the membrane.
• Phospholipids- Have a hydrophilic head and hydrophobic tails.
• Cholesterol- A steroid that stiffens the membrane.

Components of Cell Membrane: Proteins

• Proteins are transmembrane or one surface-bound.
• Integral (intrinsic) proteins- Embedded in the lipid bilayer.
• Peripheral (extrinsic) proteins- Attached to the surface of the lipid bilayer.
• Variety of roles in specific cell functions.
• Includes aquaporins, ion channels, carrier molecules, membrane receptors, docking-marker acceptors, membrane-bound enzymes, and cell adhesion molecules (CAMs).

Components of Cell Membrane: Carbohydrates

• Carbohydrates are present only on the outer surface.
• Glycocalyx- The carbohydrate coat, involved in cell-cell recognition.
• Glycolipids- Carbohydrates linked to lipids.
• Glycoproteins- Carbohydrates linked to proteins.
• Function as self-identity markers.

Structure of Cell Membrane: The Fluid Mosaic Model

• The fluid mosaic model describes the trilaminar structure of the plasma membrane.
• Composed of a lipid bilayer with proteins embedded and carbohydrates on the outer surface.

Cell-Cell Adhesions

• Extracellular matrix: A biological glue secreted by cells that holds cells together.
• Specialized cell junctions:
  • Desmosomes - "Spot rivets" for strong cell adhesion.
  • Tight junctions - Tightly seals cells together.
  • Gap junctions - Channels between cells, allow passage of small substances.
• Cell adhesion molecules (CAMs) - The "velcro" proteins.

Extracellular Matrix

• AKA interstitial fluid, present in the space between cells.
• A gel-like meshwork of fibrous proteins.
• Collagen- A fibrous protein providing structural strength.
• Elastin- A fibrous protein providing elasticity.
• Fibronectin- A glycoprotein that acts as an important signaling factor.
• Important for cell function and signaling pathways.

Desmosomes/Adhering Junctions

• Desmosomes are strong cell junctions, also called "spot rivets".
• Plaque forms a thickening of the cytoplasm.
• Glycoprotein and keratin filaments connect cells.
• Found in tissues subjected to stress (e.g., skin, heart, uterus).

Tight/Impermeable Junctions

• Tightly seals adjacent cells together.
• Prevents leakage between cells.
• Found in epithelial tissues (e.g., intestines, kidneys).
• Materials pass through the cells, not between them (as in channels or carriers)

Gap/Communicating Junctions

• Channels between cells, allowing for ion movement.
• Connexons are the protein subunits that make up the channel.
• Involved in electrical signaling between cardiac and smooth muscle cells.
• Enables synchronized action and metabolic/communication links.

Membrane Transport

• Crucial for homeostasis.
• Selectively permeable- Allows some substances to pass through, but not others.
• Factors affecting permeability:
  • Lipid solubility
  • Particle size
• Forces required for transport:
  • Passive transport (no energy required) - Includes diffusion & osmosis
  • Active transport (energy required)
• Types of transport:
  • Unassisted (diffusion, osmosis): Substances move down their concentration gradient.
  • Assisted (Carrier-mediated, Vesicular): Substances require carriers or vesicles.
  • Carrier-mediated transport- Proteins that carry substances across the membrane.
    • Uniport- One substance moved across
    • Symport- Two substances moved in the same direction`
    • Antiport- Two substances moved in opposite directions.
  • Vesicular transport- Materials are enclosed in a vesicle for transport.
    • Endocytosis- Into the cell
    • Exocytosis- Out of the cell
• Specific examples: Glucose transport (facilitated diffusion), Na+-K+ pump (primary active transport), and SGLT (secondary active transport).

Diffusion

• Movement of substances from high to low concentration.
• Passive process- No energy required.
• Factors influencing the rate of diffusion (Fick's Law):
  • Concentration gradient
  • Surface area
  • Lipid solubility
  • Molecular weight
  • Distance

Osmosis

• Net diffusion of water across a semipermeable membrane.
• From high to low water concentration.
• Affects cell volume.
• Tonicity - The ability of a solution to cause a cell to gain or lose water.
• Isotonic- Concentration of solutes inside and outside the cell are equal
• Hypotonic- Concentration of solutes outside the cell is less than inside
• Hypertonic- Concentration of solutes outside the cell is more than inside

Electrical Gradient

• Charge difference between adjacent areas.
• Promotes movement towards the opposite charge, influenced by concentration gradient. Electrochemical gradient - electrical and concentration gradient together.

Carrier-mediated Transport

• Proteins (carriers) facilitating the movement of water-soluble substances across the membrane.
• Specific for a particular substance.
• Important for transport of many nutrients.
• Types of carrier-mediated transport:
  • Uniport
  • Symport
  • Antiport
• Characteristics: Specificity, Saturation, Competition, Tm- Transport maximum (rate-limiting factor)
• Facilitated diffusion- Passive transport that uses carriers
• Active transport - Requires energy (ATP) for movement of substances against their gradient

Facilitated Diffusion

• Passive transport using carrier proteins.
• Movement of substance from high to low concentration.

Active Transport

• Movement of substances against their concentration gradient.
• Requires energy (ATP).
• Primary active transport-ATP directly involved in the transport.
  • Na-K ATPase pump
• Secondary active transport- Energy harnessed from the concentration gradient of a different substance to move another. - SGLT (sodium-glucose cotransporter)

Vesicular Transport

• Large molecules and materials are transported in vesicles.
• Two types
  • Endocytosis- into the cell
  • Exocytosis- Out of the cell
• Includes pinocytosis, receptor-mediated endocytosis, and phagocytosis.

Cell-Cell Communication

• Mechanisms for cells to communicate with each other.
• Direct: (Gap junctions, Tunneling nanotubes, Juxtacrine)
• Indirect: (Paracrine, Autocrine, Endocrine, Neuronal)

Signal Transduction

• The process by which a signal from outside the cell changes a signal inside the cell.
• Extracellular messengers- Lipid and water soluble, can be molecules that trigger this transduction..
• Signal transduction process- "Transduction"- converting in one form of energy to another (e.g., radio waves to sound).
• Pathways: First messengers-bind to receptors (e.g., hormones). Second messengers- Activate intracellular pathways.
• Lipid soluble and water soluble messengers bind in different ways and initiate different processes.

Ion Channels

• Proteins forming channels in the plasma membrane allowing regulated passage of ions, water soluble substances.
  • Leak channels - Always open
  • Gated channels- Open or close in response to stimuli (Ligand-gated, voltage-gated).
• Ion movement across these channels initiates a response in the cell.

Second Messenger Pathways

• Relaying of messages within the cell.
• Main mechanisms- Cyclic AMP, Ca2+/DAG.
• Disturbances can lead to disease.

Membrane Potential

• Electrical difference across the plasma membrane.
• Separation of charges (ions).
• Generated by differences in ion concentrations and permeability.
• Resting membrane potential (RMP): The charge difference when the cell is at rest.
• Types of potentials-Graded potentials, and action potentials.

Changes in RMP

• Depolarization, Repolarization, Hyperpolarization.
• Important for nerve and muscle cell function.

Test your understanding of cell membranes with this quiz. Explore topics such as the fluid mosaic model, cell junctions, and the extracellular matrix. Challenge yourself with questions about the structure and functions of the cell membrane.