Cell Membrane Transport:

Substances move across the cell membrane through two primary mechanisms:

Passive Diffusion: Movement of molecules from an area of higher concentration to an area of lower concentration, down the concentration gradient.

Active Transport: Movement of molecules from an area of lower concentration to an area of higher concentration, against the concentration gradient, using the energy in the form of ATP(energy currency) using protein carrier pumps.

Diffusion: 

Diffusion is the passive movement of particles or molecules from an area of higher concentration to an area of lower concentration, down the concentration gradient. It is  the random motion of particles. This process continues until equilibrium is reached, resulting in an even distribution of particles throughout the available space.

In biological systems, molecules exhibit constant movement. Whether they are part of a gas, liquid, or dissolved in a solution like sugar in water, molecules tend to spread out evenly to fill all available space due to this movement. This fundamental process is known as diffusion. In organisms, where substances often need to traverse cell membranes, diffusion plays a crucial role

For instance, consider a scenario where a cell has a higher concentration of molecules, such as oxygen, outside compared to inside. This concentration gradient prompts the molecules to diffuse into the cell until equilibrium is achieved. However, whether diffusion occurs depends on the permeability of the cell membrane. Fortunately, small molecules like water (H2O), carbon dioxide (CO2), and oxygen (O2) can easily pass through the cell membrane, facilitating diffusion and balancing the concentration of these molecules inside and outside the cell.

Importance of Diffusion: 

Gasses: 

Oxygen Uptake in Respiration:

• Most living organisms require a constant supply of oxygen for respiration. Oxygen moves into organisms through diffusion down a concentration gradient.
• Small animals with a high surface area to volume ratio can absorb oxygen through their body surface, while larger animals utilize specialized gas exchange organs such as lungs or gills. These organs provide a large surface area for efficient gas exchange.
• A circulatory system is essential for larger animals to transport oxygen from gas exchange organs to cells throughout the body.
• Similarly, carbon dioxide, a byproduct of aerobic respiration, is removed from organisms by diffusion.

Photosynthesis in Plants:

• Photosynthetic plants require carbon dioxide for the synthesis of carbohydrates. Carbon dioxide diffuses through stomata in the leaves into the air spaces in the mesophyll, reaching the palisade cells where photosynthesis primarily occurs.
• Oxygen produced during photosynthesis, as well as water vapor from the transpiration stream, diffuses out of the leaf through stomata.

Rates of Diffusion:

Molecules and ions in liquids and gasses exhibit random movement driven by kinetic energy derived from their motion. The rate of diffusion through a cell wall or membrane is influenced by various factors, including:

• Surface area available for diffusion
• Temperature
• Difference in concentration inside and outside the cell
• Distance the molecules need to travel.

Surface Area:

Surface area refers to the total area available for diffusion to occur. In biological systems, a larger surface area provides more space for molecules to interact and diffuse. For example, in organisms with specialized gas exchange organs like lungs or gills, an increased surface area allows for efficient exchange of gasses such as oxygen and carbon dioxide between the organism and its environment. Similarly, in cells, microvilli and other cellular protrusions increase the surface area of the cell membrane, facilitating the uptake of nutrients and removal of waste products through diffusion.

Temperature:

Temperature:

Temperature affects the kinetic energy of molecules. If the temperature increases, it causes an increase in the kinetic energy of the molecule. Kinetic energy is the energy of motion. This means particles will move at a faster speed and frequency. As temperature increases, molecules gain energy and move more frequently, resulting in increased diffusion rates. Conversely, lower temperatures decrease molecular movement, leading to slower diffusion. This relationship between temperature and diffusion is vital in various biological processes, such as enzymatic reactions and cellular metabolism, where temperature regulation ensures optimal rates of diffusion and physiological function.

Concentration Gradients:

The concentration gradient, or the difference in concentration of particles between two regions, directly influences the direction and rate of diffusion. A steeper concentration gradient, where the difference in concentration is more significant, results in faster diffusion rates. As particles move from regions of higher concentration to regions of lower concentration, the concentration gradient drives diffusion until equilibrium is reached.
​​

Distance:

Shorter distances result in faster diffusion rates, as particles have less space to cover. For example if you have a constant speed of 80km/hr, covering a distance of 10m will be faster than 50 meters. Therefore the rate of diffusion will be faster. In biological systems, factors such as cell size and membrane thickness influence diffusion distances. Cells with smaller dimensions and thinner membranes facilitate more rapid diffusion, ensuring efficient exchange of molecules across cellular barriers. Conversely, larger distances or thicker barriers may impede diffusion, necessitating additional mechanisms such as active transport to transport molecules across membranes.

Osmosis:
Osmosis is a process of movement of water molecules from a region of higher concentration to a region of lower concentration, down the concentration gradient, using a semipermeable membrane.

Like diffusion, osmosis is also the movement from higher concentration to lower concentration, it is also regarded as a type diffusion. Solute molecules are large enough, so due to the limited space in the cell surface membrane, solute molecules cannot move. Cell membrane is a partially permeable membrane, that means, it allows few molecules to pass through and some not. Solute molecules are confined within space.