Membranes

Membrane Structure and Function

The Cell Membrane and Interactions with the Environment
Cells interact with their environment in a number of ways. Each cell needs to obtain oxygen and other nutrients (carbohydrates, amino acids, lipid molecules, minerals, etc.) from the environment, maintain water balance with its surroundings, and remove waste materials from the cell. The boundary between any cell and its environment (through which substances must pass) is the plasma membrane, composed of phospholipid and protein molecules.

Plasma Membrane
The plasma membrane has a number of functions for a cell.

Serves as the boundary between the cytoplasm of the cell and the external environment, and selectively isolates the cell from the external environment.
Maintains the cell's environment by regulating materials that enter or leave the cell. (Anything that enters or leaves the cell must pass through the membrane). We often say that a membrane is selectively or differentially permeable for this reason.
Provides mechanisms for cell-to-cell communication.
Genetically unique cell recognition markers embedded in the plasma membrane provide mechanisms for a cell to recognize itself and other cells of its particular individual organism versus non-self (foreign materials). This is important to the immune system and defense of the organism.

Although the plasma membrane forms the boundary of the cell, and surrounds the cell, many internal structures of most cells also have their own membrane boundaries. Much of what we say about membrane structure and function at this time applies to all membranes.

The Fluid Mosaic Membrane Structure
The typical membrane structure consists of a phospholipid bilayer with a number of proteins scattered throughout, and some carbohydrates (glycoproteins) found on the exterior side, similar to the way in which one does a mosaic tile, hence the name.

Phospholipid Bilayer
A phospholipid has both polar and non-polar regions. The fatty acid "tails" of the two phospholipid layers are oriented towards each other so that the hydrophilic "heads", which contain the phosphate portion, face out to the environment as well as into the cytoplasm of the cell's interior, where they form hydrogen bonds with surrounding water molecules. Because the individual phospholipid molecules are not bonded to each other, a membrane is flexible (or “fluid”), something which is pretty important to its functions.

As your text discusses, the fluidity of a membrane is crucial to its function. In caribou, circulation is reduced in the lower legs to prevent excess heat loss during cold winters. The membranes of the lower legs have more unsaturated fatty acids than those of the upper legs to retain more fluidity in reduced temperatures. Brain cell membranes in ground squirrels become more solid during hibernation. Phospholipids containing more polyunsaturated fatty acids are more fluid than those with fewer polyunsaturated fatty acids. Cholesterol in membranes reduces fluidity as well. There are times when membranes need more or less fluidity, and molecular composition provides for sure membrane flexibility.

Many materials that enter or leave the cell are water-soluble; the fatty acid layers serve as a barrier to their free entry. Proteins in the membrane are required to move these substances through the membrane. Lipids generally pass through the membrane more easily.

Membrane Proteins
The membrane proteins have a number of functions. Some are embedded in the phospholipid layers; others move (literally) throughout the membrane layers. Other membrane proteins are complexed to carbohydrate molecules, forming glycoproteins. Generally, there are three categories of membrane proteins:

Transport Proteins
Some membrane proteins function as carriers, which have binding sites that attract specific molecules. When a molecule binds to the carrier protein, the protein changes shape and moves the substance through the membrane. This process often requires energy. Many ions, such as K⁺and Na⁺ are transported through carrier proteins.
Other transport proteins form channels within the phospholipid bilayer, which allows small water-soluble molecules to pass through.
Receptor Proteins
Receptor proteins serve as binding or attachment sites, especially for hormones or other molecular messengers. Once activated, they trigger certain cell responses, often using signal transduction pathways.

Some receptors work in conjunction with carrier proteins, opening gated channels for ion movement. Other receptors are sensitive to nutrient levels, affect metabolic rate or even affect cell division.
Recognition Proteins
Glycoproteins (carbohydrate-protein hybrids) serve as surface receptors for cell recognition and identification. They are important to the immune system.

Recognition proteins are also very important in guiding cell attachments in developmental processes.
Other Membrane Proteins
Some proteins are responsible for the cell junctions that permit cells to adhere to each other.
Many enzymes are embedded in membranes, which attract reacting molecules to the membrane surface. Typically, enzymes needed for a series of chemical reactions can be aligned adjacent to each other to act like an assembly line.

Moving Materials Through Membranes
A significant part of membrane activity involves transporting materials through it in one direction or the other. Recall that the plasma membrane is selectively, or differentially, permeable. This means that:

Some materials freely pass - the membrane is permeable to such molecules and whether they are inside or outside of the cell depends on other factors
Some materials are excluded
Some materials enter or leave the cell only by the using cell energy

For example, small hydrophobic molecules, such as CO₂, O₂ and small lipids, dissolve in the membrane and pass through readily. Tiny polar molecules, such as H₂O and alcohol, can also slip between the phospholipid molecules. Ions and most nutrient molecules do not move freely through membranes, but are often carried by the transport protein channels, either with or without the use of energy. Most large molecules are excluded and must be manufactured within the cell, or moved by significant alterations of the membrane itself.

Before we talk about how molecules move through membranes, however, it is useful to have some definitions:

Fluid
Any substance that can move or change shape in response to external forces without breaking apart. Gases and liquids are fluids.

Concentration
The number of molecules of a substance in a given volume

Gradient
A physical difference between two regions so that molecules will tend to move from one of the regions toward the other. Concentration, pressure and electrical charge gradients are common in cells.

In general, the movement of any substance is subject to "physical rules" of molecule behavior. All molecules are in motion and make random collisions with other molecules. However, when the distribution of molecules is not equal, and we have a gradient, there is a net movement of molecules along the gradient. Many gradients exist between a cell's environment and the cytoplasm of the cell. These gradients are important in moving materials through membranes, both passively (without the use of energy by the cell) and actively (transport requiring cell energy).

Passive Transport involves moving things through membranes without the expenditure of cell energy down gradients. Passive transport in cells involves the process of diffusion.

Simple Diffusion

Diffusion is the movement of a substance from where there is more of it along a concentration gradient to where there is less of it, until molecules are equally distributed (and the gradient no longer exists).

Diffusion is a means of passive transport, since no additional energy is expended for the process.

In terms of cellular activity, diffusion:

Requires no energy
But the cell has no control over diffusion, and the rate of diffusion is pretty slow and can not cover much distance.

The Rate of Diffusion can be affected by:

Temperature (Higher temperature, faster molecule movement)
Molecule size (Smaller molecules often move more easily)
Concentration (Initial rate faster with higher concentration)
Gradient of the two regions (Greater the gradient differential, the more rapid the diffusion (again, initially))

Materials which may move through membranes by diffusion include:

H₂0 (water)
CO₂ (carbon dioxide)
O₂ (oxygen)
Some lipid-soluble molecules (alcohol)

Note: The movement of water through a differentially permeable membrane in response to solute concentrations, the phenomenon of osmosis, is a special case of diffusion that we shall discuss later

Facilitated Diffusion
Some small molecules can not move freely through the membrane, but do cross membranes with the help of membrane transport proteins, which temporarily bind to the substance to be moved through the membrane, a process called facilitated diffusion. No energy is involved, so it is still a passive process. Both carrier proteins and channel proteins are involved in facilitated diffusion.

Materials which may move through membranes by facilitated diffusion include:

Glucose
Many small ions
Amino acids

The movement of water through membranes also involves facilitated diffusion. There are special channel proteins, called aquaporins that facilitate the movement of water at a rate needed for cell activities.

Energy-Requiring Transport Across Membranes
All cells need to move some substances through membrane in a direction counter to the gradient, or move substances that are too large or bulky be moved without the use of cell energy. Cells have a number of ways to move things with the use of energy.

Active Transport
Transport proteins (carrier proteins) can move substances through the membrane against the concentration gradient. Active transport typically requires two carrier protein active sites, one to recognize the substance to be carried, and one to release ATP to provide the energy for the protein carriers or "pumps". Much energy is expended by the cell to do this!

In other cases, concentration gradients of ions, typically H⁺ or Na⁺ ions, can be used to provide the energy needed to move something through a membrane. For example, the substance to be moved is "coupled" to the concentration of H⁺, and while to H+ is moving "down" through the carrier channel, the substance is transported "up".

Membrane Interactions with the Environment
Larger substances may require changes in membrane shape and the fusion of membranes to move into the cell. To do so, the membrane engulfs the material, pinches off a membrane sac forming a vesicle, which is then released into the cytoplasm.

Endocytosis
Substances which enter the cell in this manner move by endocytosis. Endocytosis can move:

Methods of Endocytosis

Pinocytosis Membrane invaginates, substance "falls" in cavity, used for moving fluids into or out of a cell.
Receptor-Mediated Endocytosis Receptor molecules in the membrane attract the substance to be moved into the cell, creating a membrane depression in that area (or coated pit). When sufficient molecules have been attracted, the pocket will be pinched off forming a coated vesicle in the cytoplasm.
Phagocytosis Membrane surrounds and engulfs object, used for larger objects, such as engulfing prey by the Amoeba, and bacteria by white blood cells.

Exocytosis
Materials can be exported from the cell by fusing vesicles with the plasma membrane, a process called exocytosis. For example, insulin, made in cells of the pancreas, leaves the cells of the pancreas by exocytosis.

Now to a Complication of water, membranes and diffusion: Osmosis
Osmosis is the movement (diffusion) of water across a differentially permeable membrane in response to solute (dissolved substances) gradients that are maintained by the membrane. The "force" to move water through membranes is called osmotic pressure. It is comparable to physical pressure. Osmotic pressure may be resisted by the cell membrane (if it is strong enough) or the cell wall, in organisms that have cell walls. The wall or membrane exerts a mechanical pressure. The difference in the osmotic pressure and the wall or membrane pressure is known as water potential. Water potential is very important in a number of processes.

For the process of osmosis:

The membrane is permeable to water.
The membrane is not permeable to the solute(s), and the solutes will be substances which can "bind" to water, affecting the free flow of water.
A water gradient exists, in part because dissolved substances always lower the concentration of water in a solution. (Pure water would have the highest concentration of water – any substance that is added to pure water will “displace” some water molecules, lowering the content of the water.)
Since osmosis depends of the differences in the concentration of water, the specific types of solutes do not matter; it's their collective effect on the concentration of water than counts. Or, it's not so much the number of molecules, or volume, but the proportions of solutes to water.

There are terms that are used to describe the ratio of water to solutes in osmosis, and whether we are discussing the inside condition or the outside condition. They are in your text. (We do not need to use these terms in Biology 101, but we do need to define them in order to understand your book, and to succeed in courses that expect you to understand them):

Hypertonic
The solution has a higher solute concentration (less water) than the cell
Hypertonic solutions will cause water to leave cells by osmosis, and cells may shrink.
Hypotonic
The solution has a lower solute concentration (more water) than the cell.
Hypotonic solutions will cause water to enter cells by osmosis, causing the cells to swell.
Isotonic
Equal proportions of solutes to water on both sides of the membrane.
Isotonic solutions are osmotically balanced.

Although we do not need to remember the specific terms which describe the ratio of solutes to water inside and outside of cells, we do need to understand the impact of these different conditions on how cells function. Cells can not afford to either lose water, or gain excess water. They must maintain an equal proportion of solutes both inside and outside of the cells, a condition called osmotic balance, to function. The process by which organisms regulate their osmotic balance is called osmoregulation. Here are some examples:

Hypertonic Environments
An environment which has a higher proportion of solutes than found inside the cell will cause water to leave the cell. Salt water, for example, is hypertonic to the cells of freshwater organisms. A cell placed in this environment will lose water and shrivel, a phenomenon called plasmolysis, unless it has special mechanisms to prevent this.

Hypotonic Environments
An environment which has a lower proportion of solutes than found inside the cell will cause water to enter the cell. Fresh water, for example, is hypotonic to the cells of all organisms.

Animal cells may swell to bursting when placed in fresh water. Animal cells, therefore, require some method to prevent this and maintain osmotic balance.

One method of doing so is through vacuoles. The contractile vacuoles found in protists are used to collect excess water which moves into their cell, and periodically, "spit" the water back out into the environment.

Plant cells use osmotic pressure to their advantage, using the cell wall and central plant vacuole. As mentioned earlier, stored substances in the vacuole attract water that increases fluid pressure within the vacuole. This pressure forces the cytoplasm against the plasma membrane and cell wall, helping to keep the cell rigid, maintaining a condition of turgor. Turgor provides support and strength for herbaceous plants and other plant parts lacking secondary cell walls. When plant cells lose turgor, they wilt, a condition known biologically as plasmolysis.

Cell to Cell Attachment and Communication
Before we leave the cell, we should take a look at how cells communicate with each other through extracellular connections.

We earlier mentioned that cell membranes have receptor proteins, molecules which chemically detect signals from other cells or chemicals in the environment. Hormones often "work" by binding to receptors, or passing through protein channels and binding to chemicals within the cell to direct specific functions. Such chemical communications are vital to the functioning of all cells and organisms.

Cells typically have methods of "physically" communicating with adjacent cells as well. These connections are often called cell junctions.

Desmosomes
Many epithelial cells must adhere to adjacent membranes to prevent free passage or free movement, and to not break apart under stress. Such junctions of adjacent plasma membranes of these types of membranes are called desmosomes. Protein fibers in the desmosomes help strengthen the junction.

Tight Junctions
Tight junctions are composed of protein fibers that seal adjacent cells to prevent leakage, something that can be useful in organs such as the bladder.

Gap Junctions
Gap junctions are protein channels between adjacent cells that permit the transfer of substances between the cells. They are common in brain cells, forming the synapse, and in many glands.

Plasmodesmata
Recall that plasmodesmata are membrane connections (little channels) between adjacent plant cells that pass through the wall layers. Plasmodesmata provide for intercellular cytoplasm communication.

The Cell Surface – External Structures
Although the boundary of any cell is its cell or plasma membrane, the cells of many types of organisms, such as plants, fungi, bacteria and many protists, have one or more rigid surface layers exterior to the plasma membrane. These surface layers are called the cell wall. Cell walls are secreted by the cell they surround, and are composed of a number of different kinds of materials, depending on the Kingdom or Domain. Cell walls support, protect and provide shape to the cells they surround.

Let's look a bit at the wall structure of plants.

Plant Cell Wall Layers
Primary Wall

All plant cells secrete a primary cell wall exterior to the plasma membrane.
The primary wall is composed extensively of cellulose, a polysaccharide of glucose discussed previously. Cellulose is non digestible for most organisms. It is the main constituent of wood, pulp and paper, and cotton and linen. It is also the bulk of the "fiber" of our diets.

Secondary Walls

Secondary walls consist of layers secreted interior to the primary wall and provide additional strength to those cells.
Secondary walls will have lignin as well as cellulose materials and are important in woody tissues, and in the support and conducting tissues of plants

Middle Lamella
When plant cells divide, vesicles secrete pectins (Calcium pectate) and other "gummy" substances between the adjacent cell membranes of the divided cell. This material forms the middle lamella that "glues" plant cells together (Unlike membranes, walls are not sticky.)

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