Notes on Unit 4 – Nature’s Principles
CELLS
I.
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The Cell Theory
Cells are the basic unit of life – all life functions can be explained on the cellular level
All living organisms are composed of cells
Cells come from other cells
II.
The Size of Cells
 Most cells are between 1-100 µm in size. Prokaryotic cells are about 1-10 µm in size, while
eukaryotic cells are 10-100 µm in size.
 Cell size is limited by constrains of their surface area. If the SA:V ratio becomes too small, cells
are not able to take in nutrients and release harmful waste quickly enough. As the cell grows, its
volume grows at a faster rate than its surface area, so the SA:V ratio decreases. This results in
less efficient transport into and out of the cell.
 Cells have various adaptations to increase the SA:V ratio.
 There are exceptions to this rule.
o Egg cells are generally larger than usual. This is possible because they are metabolically
inactive, produce little waste and don’t really take in too many nutrients from their
environment.
o Nerve cells can be very long (1 meter or more) but they are extremely thin, so all parts
of the cell are always close to the cell membrane.
o Cells can increase their surface area by having lots of membrane folds.
Do Activity on SA:V ratio.
 Large size has its evolutionary benefits. Larger organisms have a better chance of defending
themselves and larger plants can compete more effectively for sunlight. Multicellular body
structure makes larger size possible. However, multicellular structures also result in cell
specialization – when cell specialize with their structure (shape, organelles, processes) to
perform a certain function. This is going to be a central theme in this unit.
III.
Cell Types
 There are certain properties that all cells have. These are:
o Cell membrane – to separate the cell from its environment
o DNA – for storing genetic information for building proteins
o Ribosomes – to build proteins for the cell
o Cytoplasm – to conduct all chemical life processes
 All these structures support the common origin of life hypothesis.
 There are two major types of cells that are quite different in many ways from each other:
A. Prokaryotes – Bacteria and Archaea domains belong here.
 These organisms don’t have membrane-bound organelles and nucleus.
 Evolved about 3.5 billion years ago
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They are about 1-10 µm sized
Most of the life processes of these cells are carried out either in the cytoplasm or in folds of the
cell membrane, because they are lacking organelles.
 They are usually single-celled, however, can form biofilms, thin layers of millions of cells of
different species that help each other with various functions.
Be able to draw and label the parts of these cells.
 The following structures are found in prokaryotes:
o Nucleoid region – holds the circular shaped single bacterial chromosome (DNA) without
a nuclear envelope
o Plasmid – small, circular pieces of DNA, substantially smaller than the chromosome,
which can be passed on from one organism to another – horizontal gene transfer. They
usually contain only a few genes that are not vital but beneficial for the bacterium, such
as genes for antibiotic resistance.
o Pilus – short extensions of the cell membrane that help with conjugation – passing on
genes from one bacterium to another.
o Ribosomes – assemble the primary structure of proteins
o Cell wall – made up of peptidoglycan in bacteria, but made up of different materials in
Archaea. The cell wall helps with keeping the cell’s shape and protection
o Glycocalyx – an outer capsule that is mostly carbohydrate based for extra protection.
o Fimbriae – also called pili, but these make bacteria stick on surfaces
o Flagella – long projections that make the bacterial cells able to move in water
B. Eukaryotic Cells
 First eukaryotes appeared 2.1 billion years ago.
 They have membrane-bound organelles including a nucleus. The evolution of these is explained
partially by the endosymbiotic theory
 Their size range is about 10-100 µm.
 Because of the compartments that the organelles create, the larger size is possible for these
cells. The compartments make the cell processes more efficient.
 The organisms with eukaryotic cells include Protists, Fungi, Plants and Animals. These organisms
can be single-celled but for the most part they are multicellular and could reach very large sizes.
C. The Endosymbiotic Theory
 They have membrane-bound organelles. The evolution of these is explained partially by the
endosymbiotic theory
 The Endosymbiotic Theory – Mitochondria and chloroplasts evolved from free living bacteria,
when they were engulfed by simple but larger cells. Evidence:
1. These organelles have circular DNA
2. They reproduce by binary fission like bacteria do
3. Some of their metabolic enzymes are similar to bacterial enzymes
4. They are about the same size as bacteria
5. They have double membranes, one of which was the original bacterial
membrane, the other was the host’s cell membrane
EUKARYOTIC CELLS
I.
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Evolution
These cells evolved 2.1 billion years ago as a result of nutrient shortage pressures. These
cells are more efficient at using available sources because of the compartmentalization of
their life processes. These organelles (the compartments) restrict chemical reactions to
certain areas and with that more complex reactions can go on in each organelle, where the
required enzymes are concentrated.
The more complex organization also requires more chromosomes – chromosomal structure
changed to linear chromosomes and there are multiple of them in each eukaryotic cell.
II.
The Role of the Nucleus
 It is a control center of the cell, which is processing inputs from the cytoplasm, stores genetic
information and carries out instructions to regulate cell functions. The instructions are coded in
DNA. The instructions include genes for making proteins, making RNA and to perform
reproduction.
 Parts of the nucleus:
o Nuclear envelope – made up of TWO phospholipid bilayer with nuclear pores. These
pores are large enough to transport RNA or proteins in and out of the nucleus.
o Nucleolus – synthesizes rRNA to assemble the large and small subunits of the
ribosomes. These subunits only combine later in the cytoplasm.
o DNA – always remains in the confined and protected area of the nucleus
 Ribosomes are used to synthesize proteins. If the ribosomes are free floating in the cytoplasm,
they synthesize proteins for the cell. If they are suspended on the rough ER, they form proteins
that exit the cell.
III.
The Endomembrane System
 A set of organelles, which are functionally interrelated and located in the cytoplasm. Includes
the nuclear envelope, rough and smooth endoplasmic reticulum, Golgi apparatus, cell
membrane and other organelles that form from these listed ones (vesicles, vacuoles).
All other organelles are listed in your book and you can take notes on them on your review sheets.
http://www.youtube.com/watch?v=TYxDoP9ABHc – Harvard cell animation
CYTOSKELETON
I.
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Structure of Cytoskeletal Proteins
There is a network of fibers within the cell that is only visible with very high power electron
microscopes. This network collectively is called the cytoskeleton. This network organizes
cellular components and cellular activities. They support and preserve the shape of the cell.
They participate in moving the cell organelles, substances in or on the cell and the entire cell.
The network is composed of three different fibers:
o Microtubules
o Microfilaments
o Intermediate filaments
This system is very dynamic, easily reorganizes and breaks apart.
The fibers of the cytoskeleton are made up of proteins.
II.
Microfilaments
 The thinnest cytoskeletal structures, that help to support the cell membrane from the inside,
compose microvilli in the small intestinal cells, they form muscle fibers
 They are made up of a protein called actin which forms two chains that twist together.
 In muscle fibers, actin is joined by an other protein called myosin. Myosin slides along the actin
filaments to generate muscle contraction.
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Amoeba uses actin filaments to form pseudopods for motion -http://www.youtube.com/watch?v=PsYpngBG394
http://www.youtube.com/watch?v=pvOz4V699gk
Actin filaments also produce cytoplasmic streaming that moves the cytoplasm of the cell
around. http://www.youtube.com/watch?v=pFsty-XyLZc
III.
Intermediate Filaments
 Medium width fibers which are made up of several different types of proteins. They generally
form permanent structures, because they are not as dynamic as the other two types.
 They form the surface structures of skin cells that they retain even after their death. These
keratin layers protect underlying tissue. They can also support and anchor various organelles in
position.
IV.
Microtubules
 They are hollow tubes that are made up of a protein called tubulin. These units come as dimers
(alpha and beta tubulins).
 Microtubules form tracks within the cell for vesicles to move on, they form cilia and flagella with
a 9+2 microtubule arrangement. They also form centrioles with a 9+0 arrangement of
microtubules.
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Microtubules use ATP to bend protein arms that connect them to each other. The bending of
these protein arms results in an undulating motion.
http://programs.northlandcollege.edu/biology/Biology1111/animations/flagellum.html
V.
Cilia and Flagella
 They are strictly speaking not organelles, because they are outside of the cytoplasm. However,
they are made up of microtubules, so they are part of the cytoskeleton.
 Cilia are short, many structures outside of the cell. They are made up of the 9+2 microtubule
structures
 Flagella are long, few structures that have the same arrangement.
 They are both used to move the cell along.
 Prokaryotic cells have a different type of flagella arrangement.
EXTRACELLULAR STRUCTURES
I.
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Extracellular Structures of Plant and Animal cells
Beyond the cell membrane there is a wide range of structures that can cover the cell’s surface.
In plants the cell wall is the most significant, while in animal cells there is an extracellular matrix
(ECM).
The basic composition of these structures is similar in plant and animal cells. They both have
fibrous components imbedded in a gel-like substance:
o In plants cellulose fibers are imbedded in a substance of polysaccharides and proteins
o In animals collagen protein fibers are imbedded into a matrix of carbohydrate
molecules.
These structures help with cell identification, cell attachment and anchoring (cells cannot divide
unless they are attached to a solid surface), they also connect cells to other cells for transport
purposes and for cell communication.
II.
Animal Cell Junctions
 Cell junctions – direct connections of cells to each other.
 Types of cell junctions in animal cells:
o Tight junctions – connections between cells that are made by adhesive connections of
the cytoskeleton. This usually protects materials from moving from cell to cell
o Desmosomes – Formed by intermediate filaments, which permanently attach cells to
each other and form functional units like muscle fibers.
o Gap junctions – passageways between cells that allow the transport of certain
substances like sugars, amino acids, ions to pass from one cell to the next.
III.
Plant Cell Walls
 Cellulose fibers are formed from β-glucose monomers. These fibers bind to each other by Hbonding and various proteins and carbohydrates are deposited in between them to form a gellike matrix.
 The young plant cell only produces a primary cell wall – thin, flexible wall that is able to expand
and grow with the cell.
 As the plant cell matures, it forms a secondary cell wall – forms between the cell membrane
and the primary cell wall, more rigid and does not grow any more.
 Plant cells use plasmodesmata for communication. These are holes on the cell wall that allow
the cytoplasm of one cell to flow into the next cell and are surrounded by a cell membrane.
These cytoplasmic bridges unify an entire plant. Essential materials, such as water, nutrients,
and minerals can pass through the plasmodesmata.
PROKARYOTES
I.
Prokaryotic Diversity
Don’t forget to be able to draw and label the parts of a prokaryotic cell
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Prokaryotes are single-celled organisms with a wide range of metabolic properties.
The diversity of prokaryotes relates to their ability to perform horizontal gene transfer – a gene
or part of a gene moves from one organism to another.
Types of horizontal gene transfer:
o Transformation – uptake of foreign DNA from the surroundings. In this case, live
bacteria can take in pieces of DNA from a decomposing, dead bacterium.
o Transduction – prokaryotic genes are carried from a donor bacterium to a recipient
bacterium by using a virus (bacteriophage)
o
See next page
Conjugation – DNA transfer from a donor to a recipient bacterium by using two cells
that connect through a sex pilus.
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Once the gene is incorporated by any of the methods, it remains in the genome of the recipient
and is then transferred during reproduction.
Because of the horizontal gene transfer, it is very hard to set up evolutionary relationships
among bacteria. As a result, today, metagenomics is used to identify groups of bacteria that
tend to live together in the same area. They are genetically related even if they have a more
distant ancestry.
Prokaryotes divide by binary fission – a type of asexual reproduction in which the cells double in
size, then copy their circular chromosome and split into two new daughter cells that are
identical to the parent cell.
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In harsh environments, many bacteria produce endospores – dormant forms that survive under
adverse conditions. Some of these endospores can remain dormant for thousands of years.
Endospores can withstand even boiling for several hours.
II.
Prokaryotic Structure
 Prokaryotic cells can have a wide range of shapes. They are categorized into the following 3
basic ones:
o Spheres (cocci)
o Rods (bacilli)
o Spirals (spirillum)
 Bacterial cell walls always contain peptidoglycan, while Archaeal cell wall can vary in structure
but usually contain polysaccharides and proteins in various combinations
 By using Gram staining, bacteria can be categorized into two basic groups:
o Gram positive bacteria possess simple cell walls with peptidoglycan and colors blue or
purple in gram stains
o Gram negative bacteria possess a more complex cell wall with an outer membrane over
the peptidoglycan cell wall. These bacteria only faintly color to red or pink with gram
stains.
 Gram staining is important in determining the possible treatment for a bacterial infection. Gram
positive bacteria are usually easier to treat with antibiotics.
III.
Quorum Sensing in Bacteria
 Explains how bacteria can communicate with each other and act together in unison when
certain chemicals are detected.
Take notes while listening to
http://www.ted.com/talks/bonnie_bassler_on_how_bacteria_communicate.html
NUTRITIONAL AND METABOLIC ADAPTATIONS OF PROKARYOTES
I.
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Nutritional Adaptations
Prokaryotes adapted to use a wide range of carbon and energy sources. Carbon is used to build
their organic molecules and the energy is required to invest to fuel life processes and perform
synthesis of these molecules. According to the sources that they use they can be:
o Photoautotrophs – use energy of sunlight as energy source and CO2 as a carbon source
o Chemoautotrophs – use chemical energy derived from oxidation reactions of a variety
of chemicals (S, H2S, Fe2+, NH3 etc.) and CO2 as a carbon source
o Photoheterotrophs – use the energy of sunlight as an energy source, but use organic
substances as a carbon source
o Chemoheterotrophs – use organic molecules as their energy and carbon source
Prokaryotes can also be categorized by their way of responding to the presence of oxygen
(remember the ancient atmosphere):
o Obligate anaerobs are organisms that are not able to tolerate oxygen and die in the
presence of it
o Facultative anaerobs/aerobs are organisms that are able to tolerate oxygen but are also
able to survive without it
o Obligate aerobs are organisms that are not able to survive without oxygen.
Some prokaryotes are vital because of their metabolism of nitrogen.
Think back to the nitrogen cycle and review the following:
o Nitrogen fixing bacteria
o Nitrifying bacteria
o Ammonifying bacteria
o Denitrifying bacteria
II.
Can Prokaryotes Work Together?
 Bacteria developed a wide range of mechanisms to deal with environmental changes.
 Some bacteria, like Anabaena contains genes for both photosynthesis and nitrogen fixation.
However, nitrogen fixation cannot take place in the presence of oxygen which is the product of
photosynthesis. So the bacteria can only perform one or the other process at a time. Usually
the bacterium performs photosynthesis, but if surrounding bacteria (of different species) run
become low on fixed nitrogen and signal to Anabaena, it can start to perform nitrogen fixation.
 Prokaryotes can also form biofilms – a community of organisms on a living or non-living surface,
surrounded by a slime-like structure. The colony can grow its capsule as a protection for the
entire colony, it can burst eventually and start new colonies close by.
Check out Interpreting a Graph of Microbial Fitness in Biofilms in Module 83
CELL MEMBRANES
I.
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The Fluid Mosaic Model of Biological Membranes
This model describes the composition and molecular characteristics of biological membranes
You need to be able to draw and label the parts of the cell membrane. Draw it here:
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Because of the molecular composition of the membrane it is said to be selectively permeable –
meaning that it only allows certain substances to cross but it serves as a barrier for other
substances.
The hydrophobic fatty acid tails make the membrane’s inner region nonpolar. It is hard for polar
substances to pass this layer. So the membrane forms a barrier for polar molecules.
Large molecules also are stopped by the membrane because they cannot fit through in between
the bilayer molecules.
A. Phospholipids in the membrane:
 The phospholipids are able to move around in the membrane. They can move sideways or flipflop upside down (very rarely).
 The fluidity of the membrane depends on several factors:
o Unsaturated fatty acids make the membrane more fluid, but saturated ones make it less
fluid
o High temperature makes the membrane more fluid
o Cholesterol make the membrane less fluid on high temperatures, but on lower
temperatures it slows down the solidification of the membrane
B. Proteins in the membrane:
 Large protein molecules can float in the phospholipid bilayer (integral proteins) or on the
surface of the bilayer (peripheral proteins)
 The proteins use their side chains to have varying polarity to fit into the nonpolar bilayer or the
polar environment around the membrane
 Proteins perform a wide range of functions in the membrane:
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Transport proteins – facilitate the movement of materials into and out of the cell. These
substances are generally not able to move across the bilayer because they are polar,
large or move against the concentration gradient. These proteins are very specific and
can move only certain molecules or ones that are similar to their target molecules
Structural proteins – these anchor the plasma membrane to the cytoskeleton and to the
extracellular matrix. Also can coordinate the migration of cells for example during
embryonic development.
Enzymes – catalyze reactions like chemiosmosis in cellular respiration
Facilitate cell-to-cell interactions – proteins can link cells together by tight junctions (ex.
between skin cells)
Cell communication – gap junctions can act as communication channels between cells.
These gap junctions are also formed by membrane proteins
Cell identification – glycoproteins
Receptors – bind to outside signal molecules and generate changes inside of the cell as a
result of the binding.
C. Carbohydrates of the membrane
 Carbohydrates are polar molecules that are attached to the outside of the cell membrane.
 They attach themselves to either a phospholipid – glycolipid or attached to a protein –
glycoprotein.
 Carbohydrates are used to indicate a cell of the organism (“self”) and also indicate infected,
damaged or foreign cells in the body for the immune system.
D. Cholesterols
 Membranes should have the consistency of oils. To keep this consistency under different
temperatures, sterols are used.
 At lower temperatures, the membrane would become too solid and brittle. Cholesterol is
wedged into the hydrophobic fatty acid tails and prevents the fatty acids to pack up too tightly
to form solids.
 On moderate temperatures, cholesterol prevents the membrane from becoming too fluid and
moves the phospholipids around too much.
 Cholesterol or some other sterols are used in animal cells, fungal cells and plant cells. However,
bacteria and archaea use other substances to accomplish membrane fluidity.
II.
The Function of the Cell Membrane:
 Protect the cell’s internal environment by being selectively permeable. Only allows certain
substances in or out of the cell
 Maintains the cell’s shape
 Helps the immune system by presenting ID tags for immune cells to recognize self, defective or
infected cells
 Connect cells to other cells
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Participates in cell communication.
MEMBRANE TRANSPORT – Passive Transport
I.
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Passive Transport
The cell must balance its input and output of materials and energy. This balance is partially
accomplished by the cell membrane
Passive transport – the process that moves substances across the membrane without using
energy from the cell. This process always moves the molecules down the concentration
gradient (from high to low concentration).
Passive transport continues until equilibrium is reached. Random movement of molecules may
continue but there is no directionality after equilibrium is reached.
A type of passive transport is diffusion – the random movement of particles due to their kinetic
energy. This movement however, can be directional down the concentration gradient (from
higher to lower concentration). Usually small, nonpolar substances use diffusion to cross the
phospholipid bilayer. Factors that influence diffusion include:
o Temperature
o Concentration gradient
o Polarity (when moving across a barrier like the phospholipid bilayer)
o Size (when moving across a barrier like the phospholipid bilayer)
Osmosis – a specialized form of diffusion that moves only water molecules. Water can cross the
phospholipid bilayer with low efficiency because of its polarity. However, specialized channel
proteins (aquaporins) help water to move across the membrane with a higher efficiency.
The pressure that is needed to prevent water from moving across a membrane is osmotic
pressure. If the osmotic pressure is too high, water is forced out of an otherwise low water
concentration area.
Facilitated diffusion – a special kind of passive transport that requires the help of a transport
protein to carry dissolved substances across the membrane that are too polar or larger to get
through the phospholipid bilayer. Substances such as glucose or chlorine ions frequently use
this process – polar, larger and/or ionic substances. The proteins that are used during facilitated
diffusion are generally integral proteins that are constantly open and move substances from the
higher to the lower concentration area.
II.
The Regulation of Water Content and Homeostasis
 Controlling the concentration and type of substances that enter and leave the cell is vital for
keeping homeostasis in cells.
 Osmoregulation – the process by which living organisms control their internal concentration of
water and dissolved substances. On the cellular level, the water level in the cell compared to
the environment is measured by tonicity – the measure of water concentration of two solutions
separated by a semipermeable membrane. Cells can be the following compared to their
environment:
o Hypertonic – higher concentration of dissolved substances and less water inside of the
cell than in the environment
o
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Hypotonic – lower concentration of dissolved substances and more water inside of the
cell than in the environment
o Isotonic – equal concentrations of dissolved substances and water inside and outside of
the cell
The effect of tonicity on cells depends on the type of cells:
o In isotonic solutions animal cells will hold their normal shape, while plant cells are
flaccid – organelles dispersed, low pressure on the cell wall
o In hypotonic solutions animal cells swell with water and may lyse (burst), while plant
cells swell up but the cell wall will protect them from bursting (turgid).
o In a hypertonic solution animal cells shrink because they lose water, while plant cells
plasmolyse – their cell membranes move away from the cell wall.
You will need to be able to determine the direction of water movement and consequences of this
movement across a membrane.
III.
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Water Potential (not in the book)
In plants it is more complicated to predict which direction water is moving to because of the
presence of a cell wall. In the case of plants pressure and solute concentration together determines
the direction of water movement. The combined measure of solute concentration and physical
pressure are incorporated into water potential (abbreviated with the Greek letter psi, Ψ). Water
potential determines the direction of movement of water.
Free water that is not bound by solute or by surfaces always moves from the higher water potential
region to the lower water potential region if there is no barrier to its flow.
The unit of water potential is megapascals (MPa). Pure water has a water potential of zero if the
container is open to the atmosphere, under standard conditions.
The equation for water potential shows both of its components:
Ψ = Ψs + Ψp
Where Ψs is the solute potential and ΨP is the pressure potential.
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Solute potential of a solution is proportional to the number of dissolved solute molecules. Because
solutes bind to water molecules and lower the capacity of water molecules to do work, adding
solute molecules to water always lowers water potential. Because pure water has zero water
potential, solutions always have a negative solute potential.
Pressure potential is the physical pressure on a solution. Pressure potential can be both positive
and negative relative to atmospheric pressure. The pressure potential in living cells is usually under
positive pressure. This positive pressure that presses the plasma membrane against the cell wall is
called turgor pressure.
B. Analysis of Water Potential in an Artificial System
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Part a: The membrane that separates the two sides of the U-tube is permeable to water but not to
solutes. If one side is filled with pure water and the other side is filled with a 0.1M solution, the net
water movement will be toward the solution from the pure water, because water always moves
from higher water potential to the lower water potential area.
Part b: If a pressure of 0.23 MPa is applied to the solution that the water potential is raised to 0
MPa. There will be no net flow between the two sides.
Part c: If the pressure is further increased to 0.3 MPa than the solution has a water potential of
+0.07 MPa. This solution will actually lose water to the pure water part of the U-tube.
Part d: If the plunger would be pulled back on the pure water side, it would create a negative
pressure and water would move to the pure water side.
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In plant cells, if a plant cell is limp (flaccid) it would have a ΨP of 0 and if it is sitting in a more
concentrated environment, the cell would lose water and would plasmolyze. However, if the plant
cell is in pure water, it would gain water and become turgid until the cell wall would produce
enough pressure to compensate for the concentration difference and Ψ = 0.
Turgor pressure is an important tool of plant support in nonwoody plants.
Solute potential can also be calculated by using the following equation:
Ψs = - i C R T
(look in your formula sheet for the meaning and units of these terms)
Problems are on a separate worksheet but here is one for the class to try:
What is the solute potential of the solution that has -0.5 M of sugar dissolved in 1 L water on 20 °C
temperature? What is the water potential of this same solution if it is in an open container?
ACTIVE TRANSPORT
I.
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Membrane-Mediated Transport
Some passive transport processes such as facilitated diffusion and osmosis to some extend
require proteins. These proteins are called channels, because they are usually open and simply
allow molecules to move from high-concentration area to low concentration area.
Some proteins that are involved only in active transport are called gated channels, these are
usually closed and require an electrical or a chemical signal to open.
Both channel proteins and gated channels are highly selective and only allow the transport of
one or a few similar substances.
Three types of transports can be performed by these proteins:
o Uniport – one molecule or ion type is transported either in or out of the cell
o Symport – two or more different molecules or ions are transported in the same
direction
o Antiport – two different molecules or ions are transported in opposite directions
II.
Active Transport
 Active transport is a process that moves molecules against their concentration gradient cross
the membrane. This process requires energy provided by the hydrolysis of ATP molecules. This
type of transport always requires a transport protein
 A special type of active transport is the sodium-potassium ion pump – this process maintains a
low sodium and high potassium ion concentration in the cell by carrying 3 Na+ ions out of the
cell and 2 K+ ions into the cell. One third of the cell’s energy is used to keep these ions at their
optimal concentrations. Steps of this process:
o 3 Na+ ions bind to the protein in the cytoplasm
o This stimulates the hydrolysis of an ATP molecule and the addition of a phosphate to the
protein. This changes the shape of the protein
o The protein releases the sodium ions outside and takes in two potassium ions.
o This triggers the release of the phosphate group from the protein and the change of the
protein’s shape back to its original shape.
o
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The potassium ions are released into the cytoplasm and the protein is ready to perform
the same action again.
The transport of various ions across the membrane can set up a charge difference between the
opposite sides of the membrane. This also results in an electrical difference called membrane
potential. This charge difference can be used by the cell as a potential energy source. The
difference in concentration of charges on the inside and outside of the cell is called
electrochemical gradient.
This electrochemical gradient can be used to move another molecule or ion against its own
gradient – secondary active transport. An example of this is the sucrose-proton cotransport
(sucrose – H+ cotransport). In this case a proton pump is used to set up a electrochemical
gradient and then this gradient is used to cotransport sucrose into the cell with H+ ions in a
symport to get sucrose into the plant cells.
http://www.youtube.com/watch?v=2UPqLm-uDnI – summary of all
ENDOCYTOSIS AND EXOCYTOSIS
I.
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Bulk Transport
Cells use vesicles to move large materials or large quantities of materials.
Endocytosis is used to move substances into the cell. There are three types of endocytosis:
o Phagocytosis – nonspecific intake of solid materials
o Pinocytosis – nonspecific intake of liquid materials
o Receptor-mediated endocytosis – brings in specific molecules that need to bind
with the receptors on the outer surface of the membrane to be able to enter, than
the receptors must be able to move back to the outer surface to function again.
Exocytosis is used to move substances out of the cell. This process is used to secrete
materials into the external environment. Hormones, digestive enzymes, neurotransmitters,
cell wall components are all transported this way.
Vesicles are moved with the targeted materials on tracks of microtubules with the help of
motor proteins.
Some cells continuously release molecules while others release molecules, like hormones,
only if some signal molecules activate receptors to stimulate release of substances.
Endocytosis is important for the functioning of phagocytes – specialized white blood cells
that engulf pathogens.
Sometimes, however bacteria, through natural selection evolved to avoid to be recognized
by phagocytes. The glycocalyx can serve as a defensive layer that prevents the binding of
phagocytes to bacterial cells and also protects the cells from being digested.