75 Membranes are Selectively Permeable
Plasma membranes act not only as a barrier, but also as a gatekeeper. It must allow needed substances to enter and cell products to leave the cell, while preventing entrance of harmful material and exit of essential material. In other words, plasma membranes are selectively permeable—they allow some substances through but not others (Figure 75.1). If the membrane were to lose this selectivity, the cell would no longer be able to maintain homeostasis, or to sustain itself, and it would be destroyed. Some cells require larger amounts of specific substances than other cells; they must have a way of obtaining these materials from the extracellular fluids.
This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that ensure transport. Most cells expend most of their energy, in the form of adenosine triphosphate (ATP), to create and maintain an uneven distribution of ions on the opposite sides of their membranes. The structure of the plasma membrane contributes to these functions.
Selective Permeability
Plasma membranes are asymmetric, meaning that despite the mirror image formed by the phospholipids, the side of the membrane facing the inside of the cell is not identical to the exterior of the membrane. Proteins that act as channels or pumps work in one direction. Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane.
These carbohydrate complexes help the cell bind substances in the extracellular fluid that the cell needs. This adds considerably to the selective nature of plasma membranes.
Recall that plasma membranes have hydrophilic and hydrophobic regions. This characteristic helps the movement of certain materials through the membrane and hinders the movement of others. Lipid-soluble material can easily slip through the hydrophobic lipid core of the membrane. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Fat-soluble drugs also gain easy entry into cells and are readily transported into the body’s tissues and organs. Molecules of oxygen and carbon dioxide have no charge and pass through by simple diffusion.
Polar substances, with the exception of water, present problems for the membrane. While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane. Additionally, whereas small ions could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have a special means of penetrating plasma membranes. Simple sugars and amino acids also need help with transport across plasma membranes.
Video Transcript
A semi-permeable membrane, also called a selectively permeable membrane, is a membrane that allows certain molecules or ions to pass through it while blocking others.
One example of a semi-permeable membrane is a phospholipid bilayer – a group of phospholipids consisting of a phosphate head and two fatty acid tails, arranged into a double layer, with the hydrophilic phosphate heads exposed to the water content outside and within the cell, and the hydrophobic tails hidden on the inside.
In general, small, non-charged molecules such as oxygen or carbon dioxide, can freely cross the membrane without an input of energy. They’re able to slip between the heads of the phospholipids and pass through the hydrophobic tails through a process known as diffusion. These molecules are able to move down their concentration gradient as they move from an area of high concentration to an area of lower concentration.
In contrast, ions and polar molecules have difficulty crossing a membrane. To move as quickly as is necessary, they are often assisted across the plasma membrane by carrier proteins. The proteins that conduct this form of transport are often called pumps because they use energy to force molecules or ions to move from an area of lower concentration to an area of higher concentration. This is commonly referred to as up, or against, the concentration gradient.
References
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Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016. http://cnx.org/contents/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.10
