Cell to cell transport is movement of substances into or out of the cell through cell membrane. It’s also called cell transport. It is the basic process of how things move into or out of the cell through the cell membrane.  

Things can sometimes just pass through the phospholipid bilayer of plasma membrane. Other times, a protein, such as a channel protein or another transmembrane protein, is required to help molecules pass the cell membrane. Based on this phenomenon, molecules can flow through a membrane in two ways: Passive transport and Active transport.

• Passive transport is automatic and does not require energy or ATP to proceed. Osmosis, Simple Diffusion and facilitated diffusion are the three types of passive transport. 
• Active transport requires energy to pull in or remove specific molecules and ions. Proteins called pumps are involved in active transport.

Molecules moving against a concentration gradient or other sort of resistance, such as from a lower to a higher charge area, is known as active transport. Storage of essential chemicals like glucose and amino acids in the cells occurs by active transport. 

Examples of active transport in animals and humans:

1. Pump for sodium and potassium (exchange of sodium and potassium ions across cell walls)
2. Calcium ions are being transported from heart muscle cells
3. Glucose entering or exiting a cell
4. Amino acids moving through the human intestine
5. Ingesting a bacterial cell by a macrophage
6. Secretion of enzymes
7. Release of antibodies
8. Neurotransmitters communicating through the synapse of brain cells
9. Attack of white blood cells on infection or disease causing germs

Some of the examples of active transport in plants:

1. Ions moving from the soil to the roots of plants
2. Photosynthesis sugars make their way from the leaves to the fruit.
3. Active transport of calcium moves in cells is done with the help of ATP.
4. Minerals pass through a stem on their way to different areas of the plant.
5. Chloride and nitrate transport from the cytosol to the vacuole
6. Root pressure transports water from plant roots to other plant cells.

How does active transport work?

Proteins that cross a cell membrane are known as transmembrane proteins. They usually have one or more transmembrane domains that cross the lipid bilayer of the membrane, as well as domains that bind to ligands both inside and outside the membrane. The transmembrane region of the protein identifies the molecule or ion to be transported and passes it through.

Two types of active transport:

Primary active transport 

Secondary active transport 

Primary active transport:

Primary active transport is defined as active transport driven by adenosine triphosphate (ATP). All animal cells contain the enzyme sodium-potassium adenosine triphosphatase (ATPase). It keeps cell membrane potential constant by pumping three sodium ions out of the cell for every two potassium ions it brings in.

Examples of primary active transport:

1. The mitochondrial electron transport chain, which is based on NADH reduction, is another major example of active transport. It transports protons from a lower to a greater quantity across the mitochondrial membrane. This produces energy, which is used to sustain life. 
2. Active transport is also used in photosynthesis. It creates redox potential by moving protons across the chloroplast’s thylakoid membrane using photon energy.

Secondary active transport:

Secondary active transport is defined as transfer that uses an electrochemical gradient.

Secondary active transport is independent of ATP. Pumping ions into or out of the cell creates electrochemical potential. This potential can be used to provide energy to the body’s metabolism. 

Examples of Secondary active transport:

1. Sodium ions, for example, are transported across the plasma membrane, and an electrochemical gradient drives the active transport of another ion or molecule. To power cellular respiration, hydrogen pumps create an electrochemical gradient of H+ ions in cells.

Endocytosis and exocytosis:

Endocytosis (things entering a cell) and exocytosis (items leaving a cell) are two more types of active transport (items exiting a cell). Endocytosis, also known as pinocytosis, occurs when a cell may engulf and ingest liquid molecules. The opposite type of endocytosis is phagocytosis, which involves a cell absorbing a solid object. Exocytosis is the process by which a cell discharges a substance through the cell wall when it has enough of it or wants it to go somewhere else.

Active transport Vs passive transport:

Energy is required for active transports to move molecules from lower to higher concentrations whereas Passive transfer works in the opposite way, from higher to lower concentrations, and requires no additional energy.