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The Cell Membrane and Interactions with the EnvironmentCells interact with their environment in a number of ways. Each cell needs toobtain oxygen and other nutrients (carbohydrates, amino acids, lipid molecules,minerals, etc.) from the environment, maintain water balance with itssurroundings, and remove waste materials from the cell. The boundary betweenany cell and its environment (through which substances must pass) is the plasmamembrane, composed of phospholipid and protein molecules.

Plasma MembraneThe 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 themembrane). We often say that a membrane is selectively or differentiallypermeable 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 itsparticular individual organism versus non-self (foreign materials). This isimportant to the immune system and defense of the organism.

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

The Fluid Mosaic Membrane StructureThe typical membrane structure consists of a phospholipid bilayer with anumber of proteins scattered throughout, along with some carbohydrates(glycoproteins), glycolipids and sterols, similar to the way in which one does amosaic tile, hence the name.

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Phospholipid BilayerA phospholipid has both polar and non-polar regions. The fatty acid "tails" of thetwo phospholipid layers are oriented towards each other so that the hydrophilic"heads", which contain the phosphate portion, face out to the environment as wellas into the cytoplasm of the cell's interior, where they form hydrogen bonds withsurrounding water molecules. Because the individual phospholipid molecules are notbonded to each other, a membrane is flexible (or “fluid”), something which ispretty important to its functions.

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

Phospholipid Movement Unsaturated/Saturated With Cholesterol

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

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Membrane ProteinsThe membrane proteins have a number of functions. Some are embedded in thephospholipid layers; others move (literally) throughout the membrane layers.Other membrane proteins are complexed to carbohydrate molecules, formingglycoproteins. Generally, there are a number of different membrane proteins.

Cell Adhesion (Intercellular Joining) Proteins.

Some proteins are responsible for the cell junctionssuch as tight junctions that permit cells to adhere to each other. Some adhesionproteins attach to the cytoskeleton or extracellular matrix to help maintain cellshape (particularly for animal cells) and fix into position some membrane proteins.Collagen is an important glycoprotein of the extracellular matrix.

Communication Proteins

Communication proteins are responsible for the cell gap junctions thatpermit cells to communicate with each other. Gap junctions are common in heartmuscle cells so that the contraction can be coordinated.

Receptor Proteins

Receptor proteins serve as binding or attachment sites, especially forhormones or other molecular messengers. Once activated, they trigger certaincell responses, often using signal transduction pathways.

Some receptors work in conjunction with carrier proteins, opening gated channelsfor ion movement. Other receptors are sensitive to nutrient levels, affectmetabolic rate or even affect cell division.

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Transport Proteins

Transport Proteins function as carriers, which have binding sites thatattract specific molecules. When a molecule binds to the carrier protein, theprotein changes shape and moves the substance through the membrane. Thisprocess often requires energy (ATP), and the ATP complex is a part of thetransport protein. When ATP is involved with actively moving molecules throughthe protein channel the process is called Active Transport. Most of our ions(Ca++, Na+, Cl-, K+, etc.), along with amino acids, sugars and other small nutrientmolecules are moved through transport proteins.

Other transport proteins form channels within the phospholipid bilayer, whichallows small water-soluble molecules to pass through. Aquaporins are importantwater channels that facilitate the movement of water through membranes. Somechannels are gated.

Recognition Proteins

Glycoproteins (carbohydrate-protein hybrids) and some glycolipidsserve as surface receptors for cell recognition and identification. They areimportant to the immune system so that immune system cells can distinguishbetween one’s own cells and foreign cells. Recognition proteins are also used toguide cell attachments/adhesions in developmental processes.

Enzymatic Proteins

Many enzymes are embedded in membranes, which attract reactingmolecules to the membrane surface. Enzymes needed for metabolic pathways canbe aligned adjacent to each other to act like an assembly line for the reactions.

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Moving Materials Through MembranesA significant part of membrane activity involves transporting materials through itin one direction or the other. Recall that the plasma membrane is selectively, ordifferentially, 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, dissolvein the membrane and pass through readily. Tiny polar molecules, such as H2O andalcohol, can also slip between the phospholipid molecules. Ions and most nutrientmolecules do not move freely through membranes, but are often carried by thetransport protein channels, either with or without the use of energy. Most largemolecules are excluded and must be manufactured within the cell, or moved bysignificant alterations of the membrane itself.

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

• FluidAny substance that can move or change shape in response to externalforces without breaking apart. Gases and liquids are fluids.

• ConcentrationThe number of molecules of a substance in a given volume

• GradientA physical difference between two regions so that molecules will tend tomove from one of the regions toward the other. Concentration, pressureand electrical charge gradients are common in cells.

In general, the movement of any substance is subject to “physical rules” ofmolecule behavior. All molecules are in motion and make random collisions withother molecules. However, when the distribution of molecules is not equal, and wehave a gradient, there is a net movement of molecules along the gradient. Manygradients exist between a cell's environment and the cytoplasm of the cell.

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

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Gradients are important in moving materials through membranes, both passively(without the use of energy by the cell) and actively (transport requiring cellenergy).

Passive Transport involves moving things through membranes without theexpenditure of cell energy down gradients. Passive transport in cells involves theprocess of diffusion.

Simple Diffusion• 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)• Electrical and pressure gradients of the two regions (Greater the

gradient differential, the more rapid the diffusion (again, initially))

Materials that may move through membranes by simple diffusion include:• H20 (water)• CO2 (carbon dioxide)• O2 (oxygen)• Some small lipid-soluble molecules (alcohol)

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

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Facilitated DiffusionMost molecules can not move freely through the membrane, but do crossmembranes with the help of membrane transport proteins, which temporarilybind to the substance to be moved through the membrane, a process calledfacilitated diffusion or passive transport. No energy is involved, so it is stilla passive process. Both carrier proteins and channel proteins are involved infacilitated diffusion.

Materials that 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 themovement of water at a rate needed for cell activities.

Energy-Requiring Transport Across MembranesAll cells need to move some substances through membrane in a direction counterto the gradient, or move substances that are too large or bulky be moved withoutthe use of cell energy. Cells have a number of ways to move things with the use ofenergy.

Active TransportSome transport proteins (carrier proteins) can move substances through themembrane against the concentration gradient. Active transport typically requirestwo carrier protein active sites: one to recognize the substance to be carried, andone 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 beused to provide the energy needed to move something through a membrane. Forexample, the substance to be moved is "coupled" to the concentration of H+, andwhile to H+ is moving "down" through the carrier channel, the substance istransported "up".

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Active Transport

Membrane Interactions with the EnvironmentLarger substances may require changes in membrane shape and the fusion of theplasma membrane with vesicles containing the substances to successfully movethe needed substance. Such changes in membranes occur throughout the lifetimeof the cell, so that membrane sections are constantly being formed and reformedin the cell. Movement out of the cell involving changes of the membranes andformation of vesicles is exocytosis. Movement of materials into the cell isendocytosis.

ExocytosisMaterials can be exported from the cell by fusing vesicles with the plasmamembrane, a process called exocytosis. For example, insulin, made in cells of thepancreas, leaves the cells of the pancreas by exocytosis.

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EndocytosisSubstances that enter the cell in this manner move by endocytosis. There are avariety of endocytosis processes: pinocytosis, receptor mediatedendocytosis and phagocytosis.

Methods of Endocytosis• Pinocytosis

Membrane invaginates, substances "fall" in cavity, used for moving fluids into orout of a cell. Whatever molecules were in the fluid will be moved into the cell.

• Receptor-Mediated EndocytosisReceptor molecules in the membrane attract the substance to be moved intothe cell, creating a membrane depression in that area (or coated pit). Whensufficient molecules have been attracted, the pocket will be pinched off forminga coated vesicle in the cytoplasm.

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• PhagocytosisMembrane surrounds and engulfs object, used for larger objects, such asengulfing prey by the protist, Amoeba, and bacteria by white blood cells.

Now to a Complication of water, membranes and diffusion: OsmosisOsmosis is the movement (diffusion) of water across a differentially permeablemembrane in response to solute (dissolved substances) gradients that aremaintained by the membrane. The "force" to move water through membranes iscalled osmotic pressure. It is comparable to physical pressure. Osmoticpressure may be resisted by the cell membrane (if it is strong enough) or the cellwall, in organisms that have cell walls. The wall or membrane exerts a mechanicalpressure. The difference in the osmotic pressure and the wall or membranepressure is known as water potential. Water potential is very important in anumber 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 highestconcentration 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, thespecific types of solutes do not matter; it's their collective effect on theconcentration of water than counts. Or, it's not so much the number ofmolecules, or volume, but the proportions of solutes to water.

There are terms that are used to describe the ratio of water to solutes inosmosis, and whether we are discussing the inside condition or the outsidecondition. They are in your text. (We do not need to use these terms in Biology101, but we do need to define them in order to understand your book, and tosucceed in courses that expect you to understand them):

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• Hypertonic The solution has a higher solute concentration (less water) thanthe cellHypertonic solutions will cause water to leave cells by osmosis, and cells mayshrink.

• Hypotonic The solution has a lower solute concentration (more water) thanthe cellHypotonic solutions will cause water to enter cells by osmosis, causing the cellsto swell.

• Isotonic Equal proportions of solutes to water on both sides of themembrane.Isotonic solutions are osmotically balanced.

Human Red Blood Cells

Typical Plant CellHypotonic Isotonic Hypertonic

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

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Hypertonic EnvironmentsAn environment which has a higher proportion of solutes than found inside the cellwill cause water to leave the cell. Salt water, for example, is hypertonic to thecells of freshwater organisms. A cell placed in this environment will lose water andshrivel, a phenomenon called plasmolysis, unless it has special mechanisms toprevent this.

Plasmolysis in Plants

Hypotonic EnvironmentsAn environment that has a lower proportion of solutes than found inside the cellwill cause water to enter the cell. Fresh water, for example, is hypotonic to thecells 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 inprotists are used to collect excess water which moves into their cell, andperiodically, "spit" the water back out into the environment.

Contractile Vacuole in the Paramecium

Full Empty

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