Membrane Structure and Function

Membrane Structure and Function

• Membranes are composed of phospholipids and proteins, described as a fluid mosaic.
• Proteins drift in the phospholipid bilayer, which is kept liquid due to unsaturated fatty acids with kinks in their tails.
• Cholesterol helps maintain membrane fluidity at lower temperatures.
• Integrins attach to the extracellular matrix and cytoskeleton, providing structural support.
• Glycoproteins act as identification tags for cell-cell recognition, important for immune system function.
• Membranes exhibit selective permeability, allowing some substances to cross more easily than others.
• Nonpolar molecules, like carbon dioxide and oxygen, cross easily, while polar molecules, like glucose, struggle.

Evolution Connection: Membranes

• Phospholipids spontaneously assemble into simple membranes, a crucial step in the origin of life.
• Formation of a membrane enclosing necessary molecules was critical for the evolution of life.

Passive Transport

• Diffusion is the movement of particles from an area of higher concentration to lower concentration, down the concentration gradient.
• Passive transport across the membrane doesn't require energy, utilizing the potential energy of the concentration gradient.
• Equilibrium is reached when the concentration is uniform throughout the space.

Osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane.
• Water moves down its concentration gradient until the solute concentration is equal on both sides.
• Tonicity describes the ability of a solution to cause a cell to gain or lose water.
• Isotonic solution has equal solute concentration inside and outside the cell.
• Hypertonic solution has a higher solute concentration outside the cell, causing water to move out.
• Hypotonic solution has a higher solute concentration inside the cell, causing water to move in.
• Osmoregulation is the process that maintains water balance within cells, preventing excessive water gain or loss.

Passive Transport: Facilitated Diffusion

• Membrane proteins facilitate the diffusion of certain substances across the membrane.
• This process does not require energy.
• Aquaporins, water-channel proteins, facilitate the passage of water through the membrane.

Active Transport

• Active transport moves a solute against its concentration gradient, requiring energy expenditure.
• This process alters the shape of membrane proteins through phosphorylation using ATP.

Exocytosis and Endocytosis

• These processes transport large molecules across membranes.
• Exocytosis exports bulky molecules like proteins or polysaccharides by packaging them in vesicles that fuse with the membrane.
• Endocytosis imports substances by taking them into vesicles, forming through invagination of the membrane.
• Three types of endocytosis are:
  • Phagocytosis: engulfing a particle by wrapping the membrane around it.
  • Pinocytosis: engulfing fluids into small vesicles.
  • Receptor-mediated endocytosis: specific proteins bind to receptors, initiating vesicle formation.

Energy and the Cell

• Cells transform energy as they perform work, maintaining the cell, manufacturing parts, and replicating.
• Energy is the capacity to cause change and is either kinetic (energy of motion) or potential (energy stored due to location).
• Chemical energy is a form of potential energy stored in chemical bonds.
• Thermodynamics studies the relationship between energy transformations and matter, crucial for understanding energy exchange in organisms.
• First Law of Thermodynamics: energy in the universe is constant.
• Second Law of Thermodynamics: energy conversions increase the disorder (entropy) of the universe.
• Exergonic reactions release energy, often producing heat and products with lower potential energy. Examples: burning wood, cellular respiration.
• Endergonic reactions require energy, the products containing more potential energy than the reactants. Example: photosynthesis.
• Metabolism is the sum of all chemical reactions occurring in an organism.
• Metabolic pathways are sequences of reactions breaking down or building complex molecules.
• Cells perform three main types of work:
  • Chemical work: driving endergonic reactions.
  • Transport work: pumping substances across membranes.
  • Mechanical work: movement of organelles, cilia, etc.
• Energy coupling uses an exergonic process to drive an endergonic one.

ATP

• ATP (adenosine triphosphate) is the primary energy currency of cells.
• It consists of adenine, ribose sugar, and three phosphate groups.
• Hydrolysis of ATP releases energy by transferring its third phosphate to other molecules, a process called phosphorylation.
• ATP is a renewable resource. Energy released in exergonic reactions is used to regenerate ATP from ADP.

Enzyme Function

• Enzymes are protein catalysts that speed up biological reactions by lowering their activation energy (EA) without being consumed in the process.
• Each enzyme has a unique three-dimensional shape with an active site that binds to a specific substrate.
• Enzymes function best at optimal temperatures and pH levels.
• Cofactors (inorganic) and coenzymes (organic) can assist enzymatic activity.

Enzyme Inhibition

• Enzyme inhibitors block enzyme activity.
• Competitive inhibitors bind to the active site, preventing substrate access.
• Noncompetitive inhibitors bind to a different site on the enzyme, changing its shape and hindering substrate binding.
• Feedback inhibition: the product of a metabolic pathway can inhibit an early enzyme in the pathway, regulating metabolic activity.

This quiz covers the essential aspects of membrane structure and function, including the fluid mosaic model and the roles of phospholipids, proteins, and cholesterol. Additionally, it explores the concepts of selective permeability and the evolutionary significance of membranes in the origin of life. Test your knowledge and understanding of these critical biological concepts.