Cell Membranes and Diffusion Concepts

Cell Membranes

• All cells possess cell membranes, which regulate the entry and exit of substances.
• Small molecules readily cross the membrane; medium-sized molecules pass with difficulty, and large molecules cannot.
• Membrane types include:
  • Permeable: allows any substance to pass.
  • Impermeable: blocks all substance passage.
  • Semi-permeable: allows only specific substances.

Diffusion

• Particles are constantly in motion, colliding and dispersing from high-concentration areas to low-concentration areas.
• This movement is along a concentration gradient, which is the term used to describe the gradual change in the concentration of solutes in a solution as a function of distance in a specific direction. This gradient is essential for various biological processes, as it drives the movement of particles, such as ions and molecules, across cell membranes. It serves as a key concept in understanding how substances enter and exit cells, influencing cell communication, nutrient uptake, and waste removal. In essence, the concentration gradient acts as a motivating force for diffusion, leading to the eventual equalization of concentrations in an environment.t.
• Diffusion continues until equilibrium is reached, meaning equal concentrations across the space.
• Equilibrium results in uniform dispersion.

Cell Size and Function

• Single-celled organisms are typically small due to their need to absorb all necessary substances through their cell membrane. They generally reside in nutrient-rich environments.
• Multicellular organisms exhibit greater diversity, size, and dietary versatility.
• Smaller cells provide an efficient surface area to volume ratio for substance exchange and waste removal.
• Larger cells would struggle to obtain adequate substances while producing excess waste and needing more energy.

Cell Membrane Structure and Function

• The cell membrane acts as a selective barrier, controlling the passage of substances.
• Permeability types:
  • Permeable: all substances can pass.
  • Impermeable: no substances can pass.
  • Semi-permeable: only specific substances can pass.
• The membrane prevents harmful substances from entering and removes waste products.
• The membrane comprises two layers of particles with embedded proteins.
• Channel proteins and carrier proteins facilitate the movement of substances across the membrane.

Active Transport

• Active transport utilizes energy to move substances across the cell membrane via proteins.

Diffusion in Cells

• Particles move from high to low concentration regions.
• Diffusion is crucial for nutrient absorption and waste removal.
• Example: Oxygen diffuses from blood to cells during respiration.
• The process continues until oxygen concentrations are equal on both sides of the membrane.

Osmosis

• A specific diffusion form for water.
• Water is essential for all living things.
• Water readily diffuses across cell membranes (until equilibrium).
• This maintains cell size by balancing water concentrations inside and outside the cell.