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Endosymbiont Theory

• It is believed that eukaryotic cells arose from groups of prokaryotic cells living together ­ smaller ones inside larger ones. This theory of a cooperative arrangement is called endosymbiont theory or endosymbiosis.

o Some eukaryotic organelles resemble bacteria ­ mitochondria, chloroplasts, and bacteria all have a similar size and shape.

o Mitochondria and chloroplasts are surrounded by a double membrane – the inner membrane resembles the plasma membrane of free­living prokaryotes.

o Mitochondria and chloroplasts have their own DNA and it is circular like bacterial DNA.

o Ribosomes found in mitochondria and chloroplasts resemble the ribosomes within bacteria.

o Mitochondria and chloroplasts divide as bacteria do.

Endosymbiont Theory

It is believed that eukaryotic cells arose from groups of prokaryotic cells living together ­smaller ones inside larger ones. These cooperative arrangements provided advantages to the cellsin them. Evidence comes from:

Some eukaryotic organelles resemble bacteria.Mitochondria and chloroplasts surrounded by a double membrane.Mitochondria and bacteria have similar size.Mitochondrial ribosomes resemble bacterial ribosomes.Mitochondria and chloroplasts have their own DNA and it s circular like bacterial DNA.Mitochondria divide as bacteria do.

http://www.youtube.com/watch?v=bBjD4A7R2xU

Membrane model

The Cell Membrane

MEMBRANE STRUCTURE• Our current model of the cell membrane, or plasma membrane, is called the fluid mosaic model -the membrane is fluid-like and it contains several different components embedded in it.

Phospholipids

• It is a lipid bilayer, meaning it is made of two layers of fatty molecules (called phospholipids) sandwiched together.

o Phospholipids have a hydrophilic (or polar) head and two long, hydrophobic (or nonpolar) tails. This means the heads like to be near (or interact with) water while the tails prefer to be away from (or not interact with) water. Molecules like this are called amphipathic.

o In general, nonpolar molecules do not interact with polar molecules. Polar molecules interact with other polar molecules and ions while nonpolar molecules interact with other nonpolar molecules.

o The easiest way to satisfy both heads and tails is for the phospholipids to form two layers with the tails between them and the heads facing water on both sides. Phospholipids do this spontaneously.

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Membrane Components

• The fluidity of the bilayer allows...o a small tear in the membrane to be spontaneously repaired. o two membranes to fuse together very easily.o components within the membrane to move sideways like buoys floating in

water.

Membrane transport

• The membrane contains proteins that have a variety of functions, such as…o channels for moving materials into and out of the cell.o pumps for transporting materials across the membrane.o receptors for hormones.o binding adjacent cells together.o markers used for cell recognition.

§ Glycoproteins are proteins with sugars attached. They function in cell recognition, allowing cells to identify other cells. This is particularly important in the immune system where cells need to be able to identify and destroy foreign invaders such as bacteria or viruses.

Selectively Permeability

SELECTIVE PERMEABILITY• The cell membrane forms a barrier between the cell and the external

environment. It allows the cell to...o retain the molecules it requires.o exclude unwanted molecules.o recognize other cells by molecules attached to their membranes.

• Membranes are selectively permeable barriers. This means that some molecules are able to pass through while others cannot.o Substances able to pass through a membrane:

§ Nonpolar molecules like (fats) lipids.§ Small polar molecules like water.

o Substances that are unable to pass through a membrane:§ Ions and charged molecules (like salts) dissolved in water.§ Polar molecules like glucose.§ Macromolecules.

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Permeability

Fat H2ONa Cl

GlucoseMacromolecule

ExtracellularFluid

(Cytosol)

Diffusion

o Diffusion§ Heat energy causes molecules to move randomly. This motion is called

Brownian motion.

§ All molecules move continuously by random simple diffusion. This movement is spontaneous and does not require energy. Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration.

§ The greater the concentration difference, the more rapid the net diffusion.

§ Diffusion evens out the concentrations so they are equal everywhere. This stage of uniform concentration is called equilibrium.

TRANSPORT ACROSS MEMBRANES - PASSIVE TRANSPORT• The cell membrane prevents equal motion of water and dissolved substances

from inside to outside the cell. Many substances cannot pass through biological membranes. The transport of materials across a cell membrane falls into two general categories: Passive transport and active transport.

• Passive transport does not require energy use by the cell. There are three types of passive transport:

Passive Movement of Solutes

Dye in Beaker

Rate of diffusion

§ The movement is due to collisions between particles and a few factors affect the rate

− Temperature - the rate of diffusion increases as temperature increases.

− Pressure - the rate of diffusion increases as pressure increases.

− Concentration - the rate of diffusion increases as concentration increases.

− Size - the rate of diffusion decreases as molecule size increases.

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Facilitated Diffusion

o Facilitated Diffusion§ Some important molecules, like glucose, are helped across the cell

membrane so that they move into a cell faster.§ Special protein channels help move these substances across the

membrane.§ Each protein channel is specific for the molecule it is transporting.§ This process is spontaneous and does not require energy.

Transport Proteins

Osmosis

o Osmosis§ Remember that, in a solution, the substance present in the greatest

amount is called the solvent. In biology the solvent is almost always water. Solutes are the substances dissolved in the solvent.

§ When water diffuses across a selectively permeable membrane, it is called osmosis. It occurs when a solute cannot pass through a membrane but the solvent (water) can.

§ Because it is a special kind of diffusion, osmosis occurs spontaneously and requires no energy.

§ Water moves from an area where there is more water to an area where there is less water. In general, water moves toward the area with a higher solute concentration because it has a lower water concentration.

§ The amount of water in a solution is indirectly proportional to the amount of solute in the solution...

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Example

§ The amount of water in a solution is indirectly proportional to the amount of solute in the solution. − Think of a dilute solution (call it A) as having a high water

concentration (because it has few solutes) and a concentrated solution (call it B) as having a lower water concentration (because it has lots of solutes). If these two solutions were separated by a selectively permeable membrane, water would flow from high water to low water concentration (i.e. from A to B).

Hypo

− Hypotonic - a solution which is more dilute (i.e. less solutes) than the cytosol. The cell gains water and swells. This is a problem for many freshwater organisms.

Hyper

− Hypertonic - a solution which is more concentrated (i.e. more solutes) than the cytosol. The cell loses water and shrinks. This is a problem for many marine organisms.

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Iso

− Isotonic - a solution which has the same concentration of solutes as the cytosol. The cell neither gains nor loses water and thus remains unchanged.

Turgor Pressure

§ Osmotic swelling dilutes the cytosol and can eventually cause the cell to burst (lyse). The opposite problem, the cell shrinking, would occur in a hypertonic solution. Cells have different ways of dealing with these differences in concentration:

− Cell walls of plant, fungal and bacterial cells are rigid and prevent swelling. The walls are strong enough to allow a fairly high pressure gradient. In plants this is called turgor pressure. Plant cells placed in a hypertonic solution will undergo plasmolysis, a condition where the plasma membrane pulls away from the cell wall as the cell shrinks. The cell wall is rigid and does not shrink.

Contractile Vacuole

− Some simple, single-celled organisms have contractile vacuoles which store excess water and then squirt it out.

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Blood Cells

− In complex organisms such as humans, the blood is isotonic to cytosol so that cells do not have to face these problems.

− Most cells pump ions out of the cell. This increases the solute concentration outside the cell and water follows by osmosis.

Active Transport

TRANSPORT ACROSS MEMBRANES - ACTIVE TRANSPORT• Active transport requires energy use by the cell.

o Often, a cell requires substances that are at a lower concentration outside the cell than inside the cell. These substances will not move by diffusion.

o Active transport enables a cell to concentrate materials inside itself that are a low concentration in the environment.

o In humans, active transport can account for 30% of your resting energy use.

o There are two main types of active transport:§ Molecular transport

− Special proteins in the membrane use energy to transport these substances into the cell. Note the similarity to facilitated diffusion.

Active transport of solutes

Endo/Exocytosis

§ Endocytosis and Exocytosis− These processes are used for macromolecules that are too big

to diffuse through the cell membrane normally (ex. proteins).− The cell membrane bends inward, forming a vesicle containing

extracellular fluid and other substances dissolved in it. This is called endocytosis.o The vesicle is then fused with a lysosome to digest

macromolecules.o Phagocytosis is bringing particles into the cell while pinocytosis is bringing in fluid.

− Exocytosis is the opposite of endocytosis; materials are excreted from a cell but the mechanism is the same.

Endo/Exocytosis

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Transport review

Property Simple Diffusion

Facilitated Diffusion

Active Transport

Requires special membrane proteins

Highly selective

Transport saturates

Can be inhibited

“Uphill” transport

Requires ATP energy