Physics Concepts Overview

Subatomic Particles in Atoms

• Protons, neutrons, and electrons are the three main subatomic particles in an atom.

Net Force on a Moving Object

• If an object moves with constant velocity, the net force acting on it is zero.

Acceleration

• Acceleration is a vector quantity.
• It is the rate of change of speed.
• Acceleration can occur even if the speed is not constant.
• Acceleration can be positive or negative.

Center of Mass (CM)

• The center of mass (CM) of an object is the average position of its mass distribution.

Class 2 Lever Components

• The components of a Class 2 lever, from one end to the other, are:
  • Fulcrum
  • Load
  • Applied force

Study of Friction, Lubrication, and Wear in Biological Systems

• Bio-tribology is the study of friction, lubrication, and wear in biological systems.

Laminar or Turbulent Fluid Flow

• Fluid flow is classified as laminar or turbulent based on the velocity of flow.
• Density of fluid is another factor.

Snell's Law

• Snell's Law relates the angles of incidence and refraction to the refractive indices.
• It determines the relationship between pressure and volume of gases.

Exam (Biophysics)

• Penalty session exam (September 2025) is worth 100 raw exam points and 100 credit points.
• The minimum condition is 51 raw exam points.
• No activity points are needed.

Biological Membranes and Molecular Transport

• Topics covered include:
  • Introduction to membrane structure and function
  • Understanding molecular transport mechanisms

Membrane Properties and Functions

• Membranes form the boundaries of cells and control molecular traffic.
• Membranes are flexible, self-sealing, and selectively permeable to polar solutes.
• Membrane flexibility allows for changes in cell shape during growth and movement.
• Membranes organize cellular functions, including the synthesis of lipids and proteins.
• Membranes facilitate energy transfer within mitochondria and chloroplasts.

Fluid Mosaic Model of Membranes

• The fluid mosaic model describes the structure of biological membranes.
• The model proposes that membranes consist of a fluid phospholipid bilayer with proteins embedded in or associated with it.
• Phospholipids are in a fluid state, allowing them to move laterally within the membrane.
• Integral proteins span the bilayer, while peripheral proteins are associated with the surface.
• Cholesterol is also a component and helps modulate membrane fluidity and stability.

Biological Membrane Functions

• Membranes divide cells and organelles from their environments.
• Membranes control what enters and exits cells.
• They facilitate the transport of substances.
• Membranes host receptors for signaling. Also, they are involved in energy conversion processes.

Major Components of Plasma Membranes

• The composition of plasma membranes varies among different organisms.
• Human myelin sheath has higher protein content while bacteria have a higher proportion of phospholipids.

Membrane Evidence and Fluid Mosaic Model

• Combined evidence from electron microscopy and studies of chemical composition led to the development of the fluid mosaic model to describe membrane structure.
• This provides a description of the trilaminar appearance of cell membranes.

Phospholipid Distribution and Membrane Structure

• Phospholipids are asymmetrically distributed between the inner and outer layers of a membrane, contributing to membrane structure and function.

Structure of Biological Membranes

• Phospholipid bilayer: Hydrophilic heads outward, hydrophobic tails inwards
• Proteins: Integral (embedded) and peripheral (surface-associated)
• Cholesterol: Modulates membrane fluidity and stability
• Glycocalyx: Carbohydrate-rich layer on outer surface involved in cell recognition.

Peripheral Membrane Proteins

• Integral proteins are firmly embedded in the membrane, requiring agents to remove them like detergents.
• Peripheral proteins are associated with the membrane through electrostatic interactions and hydrogen bonding.

Types of Membrane Transport

• Passive transport: no energy needed
  • Simple diffusion (small, nonpolar molecules)
  • Facilitated diffusion (requires membrane proteins)
  • Osmosis (water across a semipermeable membrane)
• Active transport: needs energy
  • Primary active transport (direct energy use, e.g., Na+/K+ pump)
  • Secondary active transport (uses electrochemical gradients created by primary transport)
• Bulk transport: Involves larger molecules or particles
  • Endocytosis (cells engulf materials)
  • Exocytosis (vesicles expel materials)

Passive Transport

• Substances move from an area of higher concentration to one of lower concentration.
• Passive transport does not require the cell to expend any energy.

Facilitated Diffusion

• Specialized proteins, like channel and carrier proteins, aid the movement of substances across the cell membrane.
  • Molecules can travel down their concentration gradient in this type of passive transport.

Active Transport

• Active transport involves the movement of substances against their concentration gradient, requiring energy (typically ATP).
• Primary active transport describes the direct use of energy for this purpose.
• Secondary active transport uses electrochemical gradients generated by primary active transport.

Primary Active Transport - Sodium-Potassium Pump

• The sodium-potassium pump is a primary active transport that uses ATP to move sodium out of cells and potassium into them.

Secondary Active Transport

• Secondary active transport uses the energy stored in electrochemical gradients to move other substances against their own gradients.

Symporters and Antiporters

• Symporters move substances in the same direction.
• Antiporters move substances in opposite directions.

Osmosis

• Osmosis is the diffusion of water through a selectively permeable membrane from an area of high water potential to an area of low water potential.
• It equalizes solute concentrations across the membrane
  • It involves the movement of water, not solutes, to equalize the concentration of solutes on both sides of the membrane.

Semi-Permeable Membranes

• Semi-permeable membranes allow certain substances to pass through while blocking others based on size, charge, or polarity.

Osmotic Pressure

• Osmosis creates osmotic pressure, the tendency of water to flow across a membrane.
• Adding solutes like sugar to the water decreases the water concentration.
• The water flows from the section with higher water concentration to the section with lower concentration.
• The osmosis process continues until the water concentration on both sides is equal across the membrane, resulting in a change in volume of those fluid-containing sections.

Solutions

• Water mixed with other substances form solutions.
• The substance that dissolves is the solute; the substance doing the dissolving is the solvent.
• For example, water mixed with sugar forms a sugar solution.

Isotonic Solution

• An isotonic solution has an equal concentration of solute as the cells.
• The rate of water entering the cell equals the rate leaving the cell

Hypertonic Solution

• A hypertonic solution has a higher concentration of solute than the cells.
• Water flows out of the cells, causing them to shrink.

Hypotonic Solution

• A hypotonic solution has a lower concentration of solute than the cells.
• Water flows into the cells, causing them to swell and potentially rupture.

Membrane Dynamics

• Biological membranes have flexibility, and do not break while changing shape; maintained by noncovalent interactions between lipids (not covalent bonds)
• Lipid bilayer structure is stable, but lipids have freedom to move.