Movement into and out of Cells

Simple Diffusion

Simple diffusion is the random movement of molecules from an area of high concentration to an area of low concentration, along their concentration gradient.
It is a passive process, requiring no input of energy.
Non-polar molecules such as oxygen and carbon dioxide are able to move directly through the phospholipid bilayer by this method.
The rate of diffusion is affected by factors such as temperature, surface area, concentration gradient, and the size of the diffusing molecules.

This is a type of passive transport where substances move across the cell membrane with the help of transport proteins.
Channel proteins allow certain ions or molecules to pass through via a water-filled pore.
Carrier proteins bind to specific molecules, inducing a conformational change in the protein and allowing the substance to pass through.
Unlike simple diffusion, facilitated diffusion allows polar and charged species to cross the membrane.

Osmosis is the diffusion of water from an area of low solute concentration to an area of high solute concentration, across a semi-permeable membrane.
Like simple diffusion, it is a passive process.
The direction of water movement is determined by comparing the osmolarity of the cell’s contents with its surroundings.

Active transport moves substances against their concentration gradient, from low concentration to high concentration.
It involves the use of energy from ATP.
Primary active transport utilises ATP directly, typically through the phosphorylation and conformational change of transport proteins.
An example is the sodium-potassium pump, which maintains cell polarity by pumping three sodium ions out of the cell and two potassium ions in, for each ATP used.
Secondary active transport uses the energy stored in the electrochemical gradient established by primary active transport to transport another substance.

Endocytosis is the process by which cells take in materials by engulfing them in a portion of the cell membrane, which then pinches off to form a vesicle in the cytoplasm.
Phagocytosis (cell eating) is the uptake of solid particles, while pinocytosis (cell drinking) is the uptake of liquids.
Exocytosis is the reverse process, utilising vesicles to expel materials out of the cell. It’s used for waste removal and the secretion of cell products.

Larger particles, or larger quantities of substances, can be transported into or out of the cell via vesicles in processes called bulk transport.
This includes both the endocytosis and exocytosis mechanisms, which are both active processes, requiring energy input.