Chapter 7.2 & 8.1

The Plasma Membrane

Review of the Plasma Membrane Structure and Function• Function

• the plasma membrane is the flexible lipid boundary of the cell and it helps the cell to maintain homeostasis through selective permeability

• homeostasis is the regulation of the internal environment of the cell regardless of the external environment

• remember, cells need certain chemicals and molecules in certain amounts to function correctly

• selective permeability is the act of allowing certain molecules to move into and out of the cell and not others

• Structure• Remember, the plasma membrane is a flexible

lipid bilayer• It is composed of 3 types of molecules

– the main type of molecule that composes the membrane is the phospholipids

• these phospholipids are composed of 2 parts; the polar (they like water – hydrophilic) head and the nonpolar (they don’t like water - hydrophobic) fatty acid tails

• these phospholipids line up in 2 layers to makeup the membrane; the polar heads on the outside while the nonpolar fatty acid tails point in because there is water found both inside and outside of the cell (see diagram)

• there are also proteins found in the bilayer– there are several different types of proteins found in

the membrane• some go only partway through the bilayer – these

serve as enzymes and chemical “markers” • some go the entire way through the bilayer – these

allow larger molecules to pass through the membrane

• there is also cholesterol found in the bilayer– the cholesterol serves 2 purposes; it gives the

membrane stability and helps to keep the fatty acid tails separated

• We call our model of the plasma membrane the fluid mosaic model

– it is made of many tiny parts that are free to move around

• Review of Diffusion– Diffusion is the random movement of particles from

an area of high concentration to low concentration– Diffusion allows particles in a solution to become

evenly spread out• once the particles are evenly spread out, the

random movement of particles continues but there is no overall change in the distribution of particles

• this condition is called dynamic equilibrium• If nothing interferes with diffusion, it will continue

until dynamic equilibrium is reached and there is no longer a difference in concentration

• Diffusion depends on three things• concentration gradient

– diffusion cannot occur unless there is a difference in the concentration of a substance

– this difference in concentration in different areas called a concentration gradient

– since particles randomly move from high concentration to low concentration it is said that they move with the gradient

– if something makes particles move from low concentration to high concentration they are said to move against the gradient

• size – it is difficult for large molecules to pass through the

membrane by diffusion because there is not much room between the phospholipids to diffuse

• charge– it is also difficult for charged particles to pass

through the memebrane by diffusion because of the nonpolar fatty acid tails

– the nonpolar tails do not “like” polar molecules and they will not allow them to pass through the membrane by diffusion

• there are only a few molecules that can pass through the membrane by diffusion

– water, oxygen, nitrogen, carbon dioxide and a few other small nonpolar molecules

•Osmosis: Diffusion of Water- osmosis is the diffusion of water into and out of a cell

• remember, diffusion occurs from an area of high concentration to low concentration

• this means within a cell osmosis occurs when water moves into or out of a cell to even out the concentration of water

• concentration refers to how many molecules are in one area relative to another area

• for example, we can make salt water by adding sodium chloride to water. If we do this in 2 separate beakers, we can add more salt to one than to the other. This gives each beaker a different concentration of dissolved particles.

• The concentration of dissolved particles can help us to classify the solution, but we must look at the water concentration to see in which direction the water molecules will diffuse (see diagram)

• Water concentration is opposite of the dissolved substance concentration

• Unequal distribution of particles, called a concentration gradient, is one factor that controls osmosis.

What controls osmosis?

BeforeOsmosis

AfterOsmosis

Water molecule

Sugar moleculeSelectively permeable membrane

The concentration of dissolved substance is opposite of the water concentration.

• Remember, water is one of the only molecules that can diffuse across a membrane

• Water will move out of the cell if the water concentration is greater inside the cell than outside

• Water will move into the cell if the water concentration is greater outside of the cell than inside

• Cells can be present or placed into three types of environments that are classified by the type of solution they are

• these three types of solutions are classified by the concentration of dissolved substance outside of the cell compared to the concentration of dissolved substance inside the cell

• these three types are called isotonic, hypotonic and hypertonic

• Isotonic solutions are solutions that have the same concentration of dissolved substance that the inside of the cell has. This means that their water concentrations are equal as well.

– if there is no difference in water concentration, then there will be no diffusion of water (osmosis)

• water molecules will still be moving in and out, but since the concentrations are equal dynamic equilibrium is occurring

• cells in isotonic solutions maintain their “normal” shape

• most solutions that are injected into our bodies are isotonic so the cell is not damaged by the loss or gain of water

• Hypotonic solutions are solutions in which the dissolved substance concentration is less outside the cell than inside the cell.

– this means that the water concentration is greater outside the cell than inside, so water will move into the cell

– this will cause the cell to swell, because water will move into it, the pressure inside the cell will increase

• in plant cells; – this pressure is called turgor pressure. – Because plants have cell walls, they can resist turgor

pressure and not be damaged– This turgor pressure helps to maintain plant cell shape and

support– When turgor pressure is lost, plants wilt

• In animal cell;– animal cells have no cell walls, therefore, animal cells can

continue to swell until they burst (called lysis)– therefore, organisms that live in freshwater have to get rid

of excess water from their cells– fish in fresh water constantly urinate– some single-celled organisms have organelles called

contractile vacuoles that help them excrete excess water

• Hypertonic solutions are solutions in which the dissolved substance has a higher concentration outside the cell than inside the cell.

– this means that the water concentration is less outside the cell than inside, so water will move out of the cell.

– This will cause the cell to shrink, because water is leaving the cell and the pressure decreases

• In plant cells;– will lose water mainly from their large vacuole– this loss in pressure allows the cell membrane

to pull in and away from the cell wall which is rigid and cannot move; this is called plasmolysis

– this results in wilting of the plant

• In animal cells;– will shrivel up, called crenation– animals that live in salt water must compensate

for the constant loss of water (ex. Salt water fish drink the water and excrete the salt)

• see page 197 for diagrams

• Summary– Hypertonic – more solute, less water outside;

water will move out and cell will shrink– Hypotonic – less solute, more water outside;

water will move into the cell and the cell will swell

– Isotonic – same concentrations inside and out; water will move in and out in equal rates, so cell will remain “normal”

– Plant cells “shrinkage” = plasmolysis, “swelling” causes turgor pressure

– Animal cells shrinkage = crenation, swelling to bursting = lysis

• Passive Transport• passive transport is the movement of

materials into and out of the cell that does not require the cell to use energy to accomplish the movement

• there are 2 types of passive transport• diffusion

– remember, diffusion occurs with the concentration gradient and therefore it happens randomly so the cell doesn’t have to use energy to make it happen

– diffusion only occurs for a few small molecules

• facilitated diffusion– this type of diffusion takes place through the

membrane transport proteins, but it is still diffusion and therefore the particles are moving with the gradient and the movement does require the cell to expend energy

– there are 2 types of transport proteins• channel proteins – these proteins are like tubes

that go the entire way through the membrane and allow larger molecules to flow right in (they are like an open doorway)

• carrier proteins – these proteins are like doors that open and close (by changing shape) to allow substances to pass through the membrane

channel proteins – these proteins are like tubes that go the entire way through the membrane and allow larger molecules to flow right in (they are like an open doorway)

carrier proteins – these proteins are like doors that open and close (by changing shape) to allow substances to pass through the membrane

see figure 8.5 page 198

Passive Transport

• Active Transport

• active transport is the movement of materials through a membrane against a concentration gradient

• this means that the cell is moving materials from an area of low concentration to an area of high concentration

• this can be done, it just means the cell has to use energy to move the materials

• active transport is accomplished by the use of carrier proteins

• remember, they are like doors that open and close by changing shape

• generally, each carrier protein has a specific shape that fits a specific molecule or ion

• when the proper molecule “binds” with the protein, cellular energy is used to change the shape of the protein allowing the material to be released on the other side of the membrane

• once the material is released, the proteins original shape is restored

Active Transport

– Transports of Large Molecules• some cells can move very large molecules, groups

of molecules or even whole cells in and out • when large particles are moved in, it is called

endocytosis• when large particles are moved out it is called

exocytosis• in these processes, the materials do not pass

directly through the membrane; instead they either “engulfed” by the cell during endocytosis or put in vacuoles and pushed out of the cell in exocytosis

• both of these processes require cellular energy• see figure 8.7, page 200