Homeostasis is a term that is used to both describe the survival of organisms in an ecosystem and to describe the successful survival of cells inside of an organism. Organisms and populations can maintain homeostasis in an environment when they have a steady level of births and deaths.  describes what happens when a cell uses energy to transport something. We're not talking about phagocytosis (cell eating) or pinocytosis (cell drinking) in this section. We're talking about the movement of individual molecules across the cell membrane. The liquids inside and outside of cells have different substances. Sometimes a cell has to work and use some energy to maintain a proper balance of ions and molecules. A proton pump is an integral membrane protein that is capable of moving protons across a biological membrane. Mechanisms are based on conformational changes of the protein structure or on the Q cycle.Endocytosis {Endo (within) cytosis (cell) } is a process in which a substance gains entry into a cell without passing through the cell membrane. This process is subdivided into three different types:

• pinocytosis-In cellular biology, pinocytosis (pino- + cytosis), otherwise known as cell drinking, fluid endocytosis, and bulk-phase pinocytosis, is a mode of endocytosis in which small particles are brought into the cell, forming an invagination, and then suspended within small vesicles.

• phagocytosis-In cell biology, phagocytosis is the process by which a cell—often a phagocyte or a protist—engulfs a solid particle to form an internal vesicle known as a phagosome.

• receptor mediated endocytosis-Receptor mediated endocytosis is an endocytotic mechanism in which specific molecules are ingested into the cell. The specificity results from a receptor-ligand interaction. Receptors on the plasma membrane of the target tissue will specifically bind to ligands on the outside of the cell.

In each case endocytosis results in the formation of an intracellular vesicle by virtue of the invagination of the plasma membrane and membrane fusion. The different catagories differ in the exact mechanisms by which this process occurs.

 

Exocytosis is a process in which an intracellular vesicle (membrane bounded sphere) moves to the plasma membrane and subsequent fusion of the vesicular membrane and plasma membrane ensues. Many cellular processes involve exocytosis.

 

Passive transport is a movement of biochemicals and other atomic or molecular substances across cell membranes without need of energy input.

 

-Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion. Consider two containers of gas A and B separated by a partition. The molecules of both gases are in constant motion and make numerous collisions with the partition.

 

If two solutions of different concentration are separated by a semi-permeable membrane which is permeable to to the smaller solvent molecules but not to the larger solute molecules, then the solvent will tend todiffuse across the membrane from the less concentrated to the more concentrated solution. This process is called osmosis.

Osmosis is of great importance in biological processes where the solvent is water. The transport of water and other molecules across biologicalmembranes is essential to many processes in living organisms. The energy which drives the process is usually discussed in terms of osmotic pressure.