Cell Membranes Overview

Cell Membranes (Biomembranes)

• Cell membranes (biomembranes) are vital components of all cells.
• Their discovery is linked to advancements in microscopic techniques, especially transmission electron microscopy.
• A typical trilaminar structure was observed.
• Cell membranes have similar structures, with slight differences in chemical composition relating to cell differentiation and specialization.

Functions of Cell Membrane

• Every cell is surrounded by a cytoplasmic membrane separating intracellular and extracellular space.
• Average membrane thickness is 60-100 nm.
• The membrane is selectively permeable, maintaining dynamic equilibrium between the cell and its environment.
• It includes enzymes, receptors and transport proteins, signaling systems, and antigens.
• The membrane performs functions such as substance intake, signal interactions, and recognition.
• It's a vital part of many cellular organelles.

Main Components of Cell Membranes

• Phospholipids are the major components.
• A phospholipid molecule consists of a polar (hydrophilic) head and two non-polar (hydrophobic) fatty acid tails.
• In an aqueous environment, the hydrophilic heads of phospholipids are oriented towards water, while the hydrophobic tails point inwards, creating a phospholipid bilayer.
• Phospholipids are chemically independent, enabling lateral movement.

Other Important Components of Biological Membranes

• Integral proteins: embedded within, or spanning the bilayer, affecting its hydrophobic sections; difficult to separate from the membrane.
• Peripheral proteins: situated outside the bilayer linked by ionic bonds; easily isolated from the membrane.
• Protein type and number vary dependent on cellular differentiation and cell cycle phase.
• Cell regulation controls the specific membrane protein composition.

Functions of Membrane Proteins

• Membrane proteins perform diverse functions.
• Some act as structural components (structural proteins).
• Others facilitate ion transport across the membrane (pumps and ion channels).
• Some facilitate substance movement along electrochemical gradients (facilitated diffusion).
• Many act as receptors binding hormones, neurotransmitters, and signals.
• Some proteins serve as enzymes.
• Membrane proteins and glycolipids contribute to the formation of antigens.

Membrane Receptors

• Membrane receptors are protein structures in the cell membrane.
• Their function is recognizing and binding signal molecules (e.g., hormones, neurotransmitters).
• The membrane interacts with its surroundings.
• Receptor types include those that are part of ion channels, receptors with enzymatic activity, and receptors coupled to G proteins (the largest family).

Transport of Substances Through the Membrane

• Transfer of substances into or out of cells is facilitated by two basic mechanisms:
• Passive transport
• Active transport

Passive Transport

• Passive transport involves substance movement along the concentration gradient without energy expenditure.
• The speed of transport is determined by the concentration gradient and the size of the substance.
• Substances with low molecular weight (e.g., water, oxygen, carbon dioxide, urea, methanol, ethanol) are transported this way.
• Diffusion is an unordered movement of dissolved molecules from high to low concentration across the membrane until equilibrium is reached.
• Osmosis exemplifies passive transport.

Osmosis

• Osmosis is the movement of water across a selectively permeable membrane from an area of high water concentration to one of low water concentration until equilibrium is reached.
• Hypertonic solutions have higher solute concentrations outside the cell than inside, causing water to exit and cell shrinkage.
• Hypotonic solutions have lower solute concentrations outside the cell than inside, causing water to enter and cell swelling.
• Isotonic solutions have equal solute concentrations inside and outside the cell, leading to no net water movement.

Facilitated Diffusion

• Facilitated diffusion is a form of passive transport employing plasma membrane proteins for substance movement across a concentration gradient without energy input.
• The transport protein binds the substance and undergoes conformational change, releasing it into the cytoplasm. Glucose transport is a good example.

Active Transport

• Active transport moves substances against a concentration or electrochemical gradient using cellular energy (ATP).
• Systems like channels and pumps transport proteins through the membrane.
• It's a selective and controlled process often regulated by receptors.
• Primary active transport: Utilizes direct ATP hydrolysis to change transport protein's conformation facilitating the movement of substances against their gradient (e.g. Na+-K+ pump, H+ pump).
• Secondary active transport: couples the movement of one substance along its gradient to the movement of another substance against its gradient (e.g. cotransport, antiport).

Endocytosis and Exocytosis

• Endocytosis and exocytosis facilitate transport of large molecules.
• Endocytosis: Transports molecules from outside the cell into vesicles within the cytoplasm.
  • Pinocytosis: movement of fluids and small dissolved substances into the cell.
  • Phagocytosis: movement of solids, large particles, or cells into the cell.
• Exocytosis: Transports molecules from the vesicles within the cell to the outside.

Cell Organelles

• Cytoplasm is the semi-fluid medium inside the cell, primarily composed of water and various organic and inorganic components.
  • Eukaryotic cells include various organelles, categorized by their membrane composition.
  • Organelles with one membrane include endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, and vacuoles.
  • Organelles with two membranes include mitochondria and chloroplasts.
  • Additional organelles consist of proteins (cytoskeleton - microtubules, microfilaments, and intermediate filaments; flagella and cilia) and nucleic acids (ribosomes and nucleolus).

Mitochondria

• Mitochondria are essential organelles involved in energy production in eukaryotic cells.
• They generate energy by breaking down organic molecules.
• Number and shape vary based on cell type.
• They possess two membranes: an outer membrane and an inner membrane with cristae (folds) to maximize surface area.
• ATP (adenosine triphosphate) synthesis occurs in the inner membrane.
• Mitochondria contain their own DNA and ribosomes.

Endoplasmic Reticulum

• Endoplasmic reticulum (ER) is a network of membranes (tubules and cisterns) linked to the nuclear membrane.
• It plays a crucial role in protein and lipid synthesis and is involved in compartmentalization within the cell.
• Two types of ER are distinguished based on structure and function:
  • Rough ER: Studded with ribosomes, responsible for protein synthesis.
  • Smooth ER: Lacks ribosomes and deals with lipid production, steroid hormone synthesis, and detoxification.

Golgi Apparatus

• Golgi apparatus modifies, packages, and sorts proteins and lipids produced by the ER, often repackaging and targeting them to specific locations.
• Composed of stacks of flattened membranes (cisternae).
• Post-translational modifications of proteins in Golgi are performed.

Lysosomes and Vesicles

• Lysosomes containing enzymes for digestion of materials internalized via endocytosis.
• They break down excess or worn-out organelles, cellular debris.
• They play a critical role in cell apoptosis.

Peroxisomes

• Peroxisomes are single-membrane-bound organelles containing enzymes, especially catalase, for detoxification and lipid metabolism.

Ribosomes

• Ribosomes are essential particles responsible for protein synthesis in cells.
• They consist of large and small subunits.
• They may be free in the cytoplasm or bound to the rough ER.