Living systems must be able to efficiently remove waste, acquire nutrients, and exchange chemicals and energy with their surroundings.

• The surface area to volume ratio of a cell is an important component in influencing the cell's capacity to perform these functions.

• The greater the surface area to volume ratio of a cell, the more effectively it can do these activities.

Plasma membranes are crucial in helping cells to maintain a suitable interior environment for existence.

• Plasma (or cell) membranes are selectively permeable, which implies that certain materials may pass through while others cannot.

• This selective permeability enables the cell to keep its internal environment stable.

The plasma membrane is composed of a phospholipid bilayer.

• Phospholipids feature a hydrophilic (or polar) phosphate "head" and two nonpolar (or hydrophobic) "tails."

• When a phospholipid bilayer forms in an aquatic environment.

Proteins in the plasma membrane perform a variety of activities, including material transport, cell signaling, attaching the cell to its environment, and catalyzing chemical reactions.

• Glycoproteins and glycolipids play important roles in cell recognition.

• Steroids in the plasma membrane can change the fluidity of the membrane in response to changing environmental circumstances and cell demands.

Individual phospholipids may also migrate across the plasma membrane's surface.

• Because the plasma membrane's components are mobile, the membrane's structure is typically referred to as a fluid mosaic model.

• The use of water potential to describe solutions has the benefit of not being a relative phrase; rather, water potential can be determined using a few simple formulae.

  • The computed water potential values of solutions allow for more precise comparisons.

  • This aids in anticipating the movement of water between solutions.

Water potential is described as the potential energy of water in a solution, or water's ability to do work.

• The higher the water potential of a solution, the more water it contains.

The concentration of water in a solution is the focus of water potential.

• When there is more solute in a solution, the water concentration decreases.

• A more soluted hypertonic solution will have less concentrated water and a lower water potential.

• A hypotonic solution has less solute and hence more concentrated water, as well as a larger water potential.

Because water moves from places of greater water potential to areas of lower water potential, water will flow from hypotonic (higher water potential) solutions to hypertonic solutions (which have a lower water potential).

• When you sip through a straw, you are utilizing negative pressure potential.

• You produce a negative pressure in the straw when you suck air through it.

• The liquid in your drink then goes from a greater pressure potential location (in the cup) to a lower pressure potential area (in the straw).

Super Soaker water toys are an example of positive pressure potential.

• By pumping air into the toy, you may produce positive pressure inside the water toy.

• When you pull the trigger on the water toy, water flows from the toy's higher pressure potential environment into the toy's lower pressure potential habitat.

A freshwater paramecium has a greater internal solute content and a lower water potential than its surroundings.

• Water will continually flow from the higher water potential in the freshwater environment into the paramecium because water moves from locations of higher water potential to areas of lower water potential.

• If this was not controlled, excess water would accumulate inside the paramecium, causing the cell to explode.

• To counteract this, paramecia have a unique organelle called a contractile vacuole, which stores excess water entering the cell and subsequently pumps it out.

• This permits the paramecium to keep its internal solute concentration constant.

• A saltwater fish's cells have a lower solute content and a higher water potential than their freshwater counterparts.