Biology Department
Mário's Homepage
Biology 1115 Outline
Biology 1115 Lecture Notes
Chapter 1: Introduction: Themes in the Study of Life
Outline











Life is organized on many structural levels
Each level of organization has emergent properties
Properties of life
Cells are an organism's basic unit of structure and function
The continuity of life is based on heritable information in the form of
DNA
Structure and function are correlated at all levels of biological
organization
Organisms are open systems that interact continuously with their
environment
Diveristy and unity are the dual faces of life on earth
Evolution is core theme of biology
Science as a process of inquiry often involves hypothetico-deductive
reasoning
Biology is connected to our lives in many ways
Biology is the study of life. It is deeply rooted in the human spirit. Its scope is immense. Rate of
knowledge extremely rapid. Certain unifying themes pervade all biology. This chapter explores
some broad themes in the study of life.
LIFE IS ORGANIZED ON MANY STRUCTURAL LEVELS
Biological organization is based on a hierarchy of structural levels, each level building on those
levels below.
Proceeds upward from atoms, molecules, organelles, cells, tissues, organs, organ systems,
organisms, population, community, ecosystem, biosphere. Most biologists specialize in the study
of life at a particular level, but they gain broader perspective when they integrate their discoveries
with processes occurring at lower or higher levels.
EACH LEVEL OF ORGANIZATION HAS EMERGENT PROPERTIES
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html

With each step upward in the hierarchy of biological order, novel properties emerge that
were not present at the simpler levels of organization.
o These emergent properties result from interactions between components.
o A living organism is a whole greater than the sum of its parts. One cannot fully
explain a higher level of order by breaking it down into its parts (holism).
o Reductionism, reducing complex systems to simpler components that are more
manageable to study, is a powerful strategy in biology.
SOME PROPERTIES OF LIFE







1. Order
2. Reproduction
o Life begets life.
o Biogenesis = life comes only from life.
3. Growth and development
o Programmed by DNA
4. Energy utilization
o Organisms take in energy (light or chemical) and transform it to do many kinds of
work.
5. Response to the environment
6. Homeostasis
o Regulatory mechanisms maintain an organism's internal environment within
tolerable limits, even though the external environment will fluctuate.
7. Evolutionary adaptation
o Life evolves as an interaction between organisms and their environment.
o Adaptation is a consequence of evolution.
CELLS ARE AN ORGANISM'S BASIC UNITS OF STRUCTURE AND FUNCTION

The cell is the lowest level of structure capable of performing all the activities of life.
Fundamental unit of life.
o Robert Hooke : first to describe cells in 1665 ( cork cells @ 30X)
o Anton van Leeuwenhoek : discovered single celled organisms (300X).
o Cell theory (Shleiden and Schwann): All living things consist of cells, and all cells
are derived from preexisting cells. Ability of cells to divide to form new cells is the
basis for all reproduction and development.
o All cells are enclosed by a membrane which regulates passage of materials between
the cell and its surroundings.
o All cells are have DNA at some point in their life cycle. DNA, an information
encoding molecule, is the heritable material that directs the cell's activities.
o Two major kinds of cells (based on structural organization):(Fig 1.8)
 Prokaryotes: includes eubacteria, and archaea. Lack internal
compartmentalization. Simpler in structure.
 Eukaryotes: Have extensive compartmentalization due to presence of
organelles. More complex.
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html
THE CONTINUITY OF LIFE IS BASED ON HERITABLE INFORMATION IN THE
FORM OF DNA

Life is encoded in DNA, a large, linear polymer with a tremendous capacity to store
information.
o Genes, the units of inheritance that transmit information from parents to offspring,
are made of DNA.
o Each DNA molecule is a long chain made up of four chemical building blocks
called nucleotides.
o Nucleotides are the alphabet of inheritance. The sequence of nucleotides encodes
the precise function of a gene.
o All organisms use the same genetic code i.e. the language for programming
biological order is common to all organisms. Only the programs change.
o All life on earth traces its ancestry to DNA molecules which have been replicating
since life first evolved, 4 billion years ago.
STRUCTURE AND FUNCTION ARE CORRELATED AT ALL LEVELS OF
BIOLOGICAL ORGANIZATION

How a molecule, organelle, cell, tissue, organ, etc.. works is correlated with its structure.
o "Form fits function" is a major theme in biology.
o Many examples of this
 birds wing shaped for flight
 arrangement of atoms in chlorophyll molecule allows absorption of light and
anchoring into lipid bilayer.
 highly branched bronchioles increase surface area for gas exchange.
ORGANISMS ARE OPEN SYSTEMS THAT INTERACT CONTINUOUSLY WITH
THEIR ENVIRONMENTS

Organisms are open systems i.e. there is exchange of materials and energy between system
and its surroundings (Figure 1.4).
o This theme is essential to understanding life on all levels of organization
o Each organism interacts continuously with its environment (biotic and abiotic).
o Both organisms and environment are affected by interaction between them.
o The many interactions between organisms and their environment are interwoven to
form the fabric of an ecosystem.
o The dynamics of any ecosystem include two major processes.
 1. Cycling of nutrients
 2. Energy flow from sunlight to photosynthetic life to organisms that feed
directly or indirectly on plants
DIVERSITY AND UNITY ARE THE DUAL FACES OF LIFE ON EARTH
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html




Diversity is a hallmark of life. Estimates of the total diversity of life range from 5-30
million species. To make diversity more comprehensible, taxonomists have devised ways
of grouping species that are similar.
Taxonomists, biologists concerned with naming and classifying species, group living
things into a hierarchical classification scheme (Figure 1.14).
A good taxonomy should reflect evolutionary relationships.
Tree of life has 3 Domains: Eubacteria, Archaea, and Eukaryotes.
Underlying the diversity of life is a striking unity, based on common origins:
o Universal genetic code
o Translation machinery
o Similarities of cell structure (organelles, cillia)
o Primary metabolism
EVOLUTION IS CORE THEME OF BIOLOGY









Life evolves. Each species is one tip on a branching tree of life extending back in time
through ancestral species more and more remote.
Similar species have more recent common ancestors than more divergent species.
Evolution, i.e. descent with modification, is the one biological theme that ties together all
others.
Charles Darwin, in 1859, published "The origin of species".
o Provided much evidence that species arose from a succession of ancestors through a
process of "descent with modification".
o Proposed that life evolves by the mechanism of "natural selection".
Key observations led to theory of natural selection:
o 1. Individuals vary in many heritable traits.
o 2. Struggle for existence
 any population has the potential to produce more offspring than the
environment can possibly sustain. This overproduction makes struggle for
existence inevitable.
Inference: Differential reproductive success. Those individuals with traits best suited to the
local environment will leave more progeny.
Darwin believed that this differential reproductive success, which he called natural
selection, was the cause of evolution.
Adaptation = a trait that allows individuals to survive and reproduce in a certain
environment better than individuals that lack that trait. Adaptations result from natural
selection.
Darwin proposed that over many generations, natural selection could produce new species
from ancestral ones
Today, we know that there are other mechanisms of evolution (genetic drift, neutral
theory of evolution).
SCIENCE AS A PROCESS OF INQUIRY OFTEN INVOLVES HYPOTHETICODEDUCTIVE REASONING
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html







Science is a way of knowing. Uses a hypothetico-deductive method process known as the
scientific method. Characterized by 5 key steps:
1. Observations
o from others or results from previous experiments.
2. Questions
o about unclear aspects of observations: How? When? Why?
3. Hypotheses
o tentative explanations of a phenomena phrased in such a way as to be testable
4. Predictions
o logical, testable outcomes of the hypotheses developed by the use of deductive
reasoning (e.g. if (statement of hypotheses) is true, then (predictions).
5. Tests
o determine if predictions are supported (fail to falsify)
o experimental tests differs from the control test by a single factor, called the
variable.
o a control is a replica of the experiment in which the special treatment being studied
is omitted. In controlled experiments, the experimental subjects are arranged in two
groups, a control and an experimental group. Both groups are treated identically in
every way except that the special condition under consideration is not applied to the
control group (i.e. the variable).
 controls clarify experimental results by focusing on a single testable
variable. Make it possible to draw clear conclusions from the results of
experiments.
 the control group represents a standard to which the treated group is
compared. Since they are treated identically in every way except for the
single variable, any difference in the results can be attributed to that
variable.
The scientific process involves the rejection of hypothesis that are inconsistent with
experimental results or observations. Hypothesis that are consistent with available data are
conditionally accepted. The formulation of a hypothesis often involves creative insight. A
theory is a hypothesis that is supported by a great deal of evidence.
SCIENCE

All scientific inquiry is based on a small set of assumptions that have been so thoroughly
tested and found valid that we call them scientific principles.
o Natural causality: all events can be traced to natural causes
o Uniformity: in space and time: The natural laws that govern events apply
everywhere and for all time
o Common perception: scientific inquiry is based on the assumption that people
perceive the natural events in similar ways.
 Common perception is a principle unique to science. Value systems, such as
those involved in the appreciation of art, poetry, and music do not assume
common perception. Because value systems are subjective not objective,
science can not solve certain types of philosophical or moral problems, such
as the morality of abortion.
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html








adaptable process, not rigid. (e.g., controls can not always be used especially where
experiments are done in nature).
Science is tentative, it does not prove, only falsifies.
requires critical thinking at every step
cumulative (often observations are simply previous results)
self-critical: always open to revision by additional data. It is a social activity with a selfcorrecting mechanism.
Science itself is devoid of moral content. However, the knowledge that we gain from
applying scientific method to the uncovering of the mysteries of nature does help humans
make better decisions. Do not look to nature for a moral compass.
Many questions lie outside the realm of science. There are limits to science. Only deals
with observable, quantifiable, testable phenomena in the natural world. Not for use with the
"supernatural". Will not answer the question "Does God Exist?"
Is freedom in science is essential?
BIOLOGY IS CONNECTED TO OUR LIVES IN MANY WAYS







Some of the biggest challenges facing humanity have biological underpinnings. A basic
understanding of biology is necessary for an informed position on many issues.
These issues include:
o human population growth
o climate change
o pollution
o endangered species (fisheries)
o genetic engineering
o nutrition
o medical advances
o disease
o etc...
Biology- from molecular to ecosystem levels- is directly connected to our lives.
Evaluating reports on problems of this magnitude requires critical thinking and familiarity
with many aspects of biology (politicians should be biologically literate).
Biology offers us a deeper understanding of ourselves and the planet.
Biology may provide solutions, as well as create other problems.
Practical implications of biology: Technology = application of scientific knowledge
Biology is multidisciplinary.
[Back] This Page last updated Sept 2005. For more information contact Mário Moniz de Sá
Biology 1115 Lecture Notes
Chapter 2 Chemical Context of Life
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html
Outline













Matter consists of chemical elements in pure form and in
combinations called compounds
Life requires about 25 elements
Atomic structure determines the behavior of an element
Subatomic Particles
Energy levels
Electron orbitals
Electron configuration and chemical properties
Atoms combine by chemical bonding to form molecules
Covalent bonds
Ionic bonds
Weak chemical bonds play important roles in the chemistry of life
A molecules biological function is related to its shape
Chemical reactions make and break chemical bonds
Organisms are natural systems to which basic concepts of chemistry and physics apply. One of the
main themes of biology is the organization of life on a hierarchy of structural levels, with
additional properties emerging at each successive level. In this chapter, we will see how the theme
of emergent properties applies to the lowest level of biological organization.
Matter consists of chemical elements in pure form and in combinations called
compounds


All life is composed of matter.
Matter = anything that takes up space and has mass. Matter exists in diverse forms, each
with its own characteristics.
All matter is made up of chemical elements.
o Element = a substance that cannot be broken down to other substances by chemical
reactions.
o Compound = combination of two or more elements in a fixed ratio. Compounds
have emergent properties.
Life requires about 25 elements


Just 4 elements make up 96% of living matter (carbon, oxygen, hydrogen, nitrogen)( table
2.1). These elements form stable cobalent bonds.
Trace elements = those required by an organism in minute quantities.
o Defficiencies in trace elements can cause illness. e.g. lack of iron and iodine cause
anemia and goiter, respectively.
Atomic structure determines the behavior of an element
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html








Atoms = fundamental unit of matter. Smallest possible amount of an element that retains
that element’s properties.
Each element consists of a certain kind of atom, which is different from the atoms of other
elements.
Atoms are composed of even smaller parts. Only three kinds of subatomic particles are
relevant from a biological perspective: protons, neutrons, and electrons.
o Neutrons and protons are packed together tightly at the center of an atom to form a
nucleus. The electrons move about this nucleus at almost the speed of light.
o Electrons (-) and protons (+) are electrically charged, whereas the neutron is
neutral. Protons give the nucleus a positive charge, and it is the attaction between
opposite charges that keeps the rapidly moving electrons orbiting around the
nucleus.
The unit of measurement for atomic particles is the dalton. Neutrons and protons have a
mass of 1 dalton each. Electron mass is negligible.
Atoms of the various elements vary in their number of subatomic particles. All atoms of a
particular element have the same number of protons in their nuclei. This is their atomic
number.
Mass number = sum of protons and neutrons in the nucleus of an atom.
Atomic weight = since neutrons and protons have a mass close to 1 dalton, the mass
number tells us the approximate mass of the whole atom.
Isotopes = variant forms of elements. Have same number of protons and electrons, but
different number of neutrons.
e.g. carbon has three isotopes (12C, 13C, 14C). The nucleus of 14C is unstable and
therefore radioactive.
o Radioactive isotopes = nucleus decays spontaneously giving off particles and
energy. Dangerous to life because it causes mutations in DNA. However,
radioactive isotopes can also be useful in biological research and medicine as
tracers. Living cells cannot distinguish radioactive isotopes from nonradioactive
atoms of the same elements.
Energy levels




Atoms are mostly empty space. When two atoms approach each other during a chemical
reaction, their nuclei do not come close enough together to interact. Only electrons are
directly involved in the chemical reactions between atoms.
An atom’s electrons vary in the amount of energy they possess (Fig 2.7).
o Energy = ability to do work.
o Potential energy = energy that matter stores because of its position or location.
Matter has a natural tendency to move to the lowest possible state of potential energy.
Electrons of an atom also have potential energy because of their position in relation to the
nucleus. The negatively charged electrons are attracted to the positively charged nucleus.
The more distant the electrons are from the nucleus, the greater their potential energy.
Changes in the potential energy of electrons can only occur in steps of fixed amounts. The
different states of potential energy for electrons in an atom are called energy levels or
electron shells. Electrons in first shell closest to nucleus have the lowest energy. Electrons
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html

in the second shell have more energy, electrons in third shell have more energy still, and so
on.
An electron can change its shell, but only by absorbing or losing an amount of energy equal
to the difference in potential energy between the old shell and the new shell
Electron orbitals




We can never know the exact trajectory of an electron. Instead, we describe the volume of
space in which an electron spends most of its time(Fig 2.9).
o Orbital = three-dimensional space where an electron is found 90% of the time.
No more than two electrons can occupy the same orbital.
First shell has a single spherical orbital and can hold only 2 electrons. An atom with more
electrons must use higher shells.
The second electron shell can hold 8 electrons, two in each of four orbitala (1 spherical and
3 dumbbel-shaped).
Electron configuration and chemical properties


The chemical properties of an atom depends mostly on the number of electrons in its
outermost shell (Fig 2.8).
o valence electrons = electrons in outer most shell
o valence shell = outermost energy shell.
o Valence = an atom’s bonding capacity (# of electrons needed to fill outer shell).
Atoms with a complete valence shell are unreactive. All other atoms are chemically
reactive because they have incomplete valence shells with unpaired electrons.
Atoms combine by chemical bonding to form molecules



When atoms with incomplete outer shells react, each atom gives up or aquires electrons so
that partners end up with completed outer shells.
Atoms do this by either sharing (covalent bonds) or transferring outer electrons (ionic
bonds) resulting in chemical bonds.
The strongest chemical bonds are covalent bonds and ionic bonds.
Covalent bonds






Covalent bond = two atoms sharing one or more pairs of outer shell electrons( Fig 2.10 and
11).
Molecule = two or more atoms held together by covalent bonds.
The number of single covalent bonds an atom can form is equal to the number of additional
electrons needed to fill its outer shell (i.e. it's valence).
Double bond = sharing of 2 of pairs of electrons. Stronger than single bonds.
Atoms in a covalently bonded molecule are constantly in a tug-of-war for the electrons of
their covalent bonds.
Electronegativity = an atom’s attaction for the shared electrons of the bond. The more
electronegative an atom, the more strongly it pulls electrons towards itself.
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html



Nonpolar covalent bonds = electrons shared equally between the atoms of equal
electronegativity (H2, O2, CH4 ).
Water is made up of 2 kinds of atoms with differeing electronegativity (O>H). Oxygen
attracts electrons more strongly than hydrogen.
Polar covalent bond = chemical bond in which shared electrons are pulled closer to the
more electronegative atom, making it partially negative and the other atom partially
positive(Refer to Fig 2.12).
H2O, even though is neutral overall, has a slightly negative pole and two slightly positive
poles, making it a polar molecule.
Ionic bonds



Refer to Fig 2.13
Ionic bonds = attractions between ions of opposite charge (e.g. table salt, NaCl). Much
weaker than covalent bonds.
When atoms of chlorine and sodium collide, chlorine atom strips sodiums’s outer electron
away. This results in sodium having a positive charge and chlorine having a negative
charge. Two ions of opposite charge attract each other; when the attraction holds them
together, it is called an ionic bond.
Ion = atom or molecule with an electrical charge resulting from a gain or loss of one or
more electrons.
anion = ion with negative charge
cation = ion with a positive charge
NaCl is a type of salt. Salts are ionic compounds that often exist as crystals in nature.
Weak chemical bonds play important roles in the chemistry of life



Weak bonds, unlike covalent bonds, allow interactions between molecules to be brief;
molecules may come together, change in some way and then separte.
The most imporatnt weak bond in living matter is the hydrogen bond.
Hydrogen bond = occurs when a hydrogen atom covalently bonded to one electronegative
atom is also attracted by another electronegative atom. In living cells, the electronegative
partner involved are usually oxygen and nitrogen atoms. (refer to Fig 2.15)
Hydrogen bonds, ionic bonds, and other weak bonds, form between and within molecules.
Although these bonds are individually weak, their cumulative effect can re-enforce the 3-D
shape of a large molecule.
A molecules biological function is related to its shape


Molecular shape is important in biology because it is the basis for how most molecules of
life recognize and respond to one another.
Recognition and binding of neurotransmitters to cell surface receptors in synapses of brain
cells is basis on interecellular communication in vervous system (Fig 2.16 and 17).
Chemical reactions make and break chemical bonds
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html



Living matter is not static. There is constant flux, as new molecules are being built and
others are being broken down. The goal of biochemistry is not simply to catalogue the
molecules that make up the living world, but to understand how these molecules are
transformed into others in biochemcial pathways. These transformations always involve
chemical reactions.
In a chemical reaction, reactants interact, atoms rearrange, and products result.
o Matter is conserved in a chemical reaction. Reactions cannot create nor destroy
matter but can only rearrange it.
Living cells carry out thousands of chemical reactions that rearrange matter in significant
ways.
Some chemical reactions go to completion, others are reversible
Chemical equilibrium = point at which rate of forward reaction equals that of reverse reaction

elated Links
o
This Page last updated Sept 2005. For more information contact Mário Moniz de Sá
Biology 1115 Lecture Notes
Chapter 7: Membrane structure and function
Outline











Introduction
Fluid mosaic model
A membrane's molecular organization results in selective permeability
Passive transport is diffusion across a membrane
Osmosis is the passive transport of water
Cell survival depends on balancing water uptake and loss
Specific proteins facilitate the passive transport of selected solutes
Active transport is pumping of solutes against their concentration gradients
Cotransport: membrane protein couples the transport of one solute to another
Endocytosis and exocytosis transport large molecules
Membrane proteins and signal transduction
Introduction

The plasma membrane is the edge of life, a boundary 8 nm thick that separates the living
cell from its nonliving surroundings. It controls traffic into and out of the cell (Fig 7.1).
Membranes are selectively permeable, i.e allow some substances to cross but not others.
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html

This ability of the cell to discriminate in its chemical exchanges with the environment is
fundamental to life.
In this chapter we study how biological membranes control passage of substances, with
emphasis on the plasma membrane.
Fluid mosaic model



The main ingredients of membranes are lipids, proteins, and some carbohydrates. The Fluid
Mosaic Model is the most widely accepted model for the arrangement of these molecules in
membranes.
Phospholipids
o most abundant lipid in membranes
o form bilayers spontaneously in water (entropy increases).
amphipathic = has both a hydrophobic and hydrophilic regions.
o phospholipid bilayer is 2 molecules thick (8 nm)
o Bilayer is a stable boundary between two aqueous solutions because the molecular
arrangement shelters the hydrophobic tails of the phospholipids from water, while
exposing hydrophilic heads to water.
(Fig 7.2)
The fluid mosaic model proposes that proteins are dispersed and individually inserted into
the phospholipid bilayer, with only their hydrophilic regions protruding far enough from
the bilayer to be exposed to water ( Fig 7.3). The membrane is seen as a mosaic of protein
molecules bobbing in an oily bilayer of phospholipids.
Fluidity of membranes








Lipids move laterally in a membrane, but flip-flopping is rare(Fig 7.5).
Unsaturated hydrocarbon tails of phospholipids have kinks that keep molecules from
packing together, enhancing fluidity.
Cholesterol reduces fluidity at moderate temps, but prevents membrane solidification at
cold temps.
Membranes must be fluid to work. When it solidifies, its permeability changes. Cells can
regulate fluidity by controlling type of lipids present in membrane.
Some Proteins also drift in the bilayer, as shown by cell fusion studies (7.6). Others are
anchored to cytoskeleton and do not move much .
A membrane is a mosaic because it is a collage of many different proteins embedded in the
fluid matrix of the lipid bilayer (Fig 7.7).
The lipid bilayer is the main fabric of the membrane, but it is the proteins which determine
most of the membranes specific functions
The plasma membrane and the other membranes in the cell vary in their proteins, thus
affording them unique functions (Fig 7.9). These functions include:
o Anchoring to cytoskeleton
o Enzymes
o Receptors
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html
o


Intercellular junctions
Ccell-cell recognition (some glycoproteins serve as identification tags which are
specifically recognized by other cells)
o Transport proteins.
Two type of membrane proteins.
o 1. Integral proteins - penetrate far enough into the hydrophobic regions to be
surrounded by the hydrocarbon tails of lipids (Fig 7.8).
o 2. Peripheral proteins - not embedded in lipid bilayer. They are appendages
attached to surface of membrane, often the exposed portions of integral proteins.
Membrane has distinct cytoplasmic and extracellular sides. This quality is determined
when membrane is first synthesized by ER and Golgi ( Fig 7.10).

A membrane's molecular organization results in selective permeability



Membranes aren't just barriers to movement, they are also selective in what moves in and
out. Transport of materials across a membrane is a central aspect of cell function. So much
so, that, in E.coli, 20% of genes encode proteins involved in some aspect of transport.
Selective permeability results from two factors:
o 1. Permeability of lipid bilayer
 hydrophobic core of membrane impedes transport of ions and polar
molecules.
 hydrophobic molecules, (small hydrocarbons, O2) dissolve in membrane and
dissolve with ease.
 permeable to very small polar molecules (H2O, CO2), but impermeable to
larger uncharged polar molecules (sugars) or even small ions (H+, Na+
etc...)
o 2. Transport proteins
 span membrane
 some are channels, others actually bind solute and move it to other side of
membrane.
 each transport protein is specific for only one solute.
 Two basic types of transport:
1. Passive transport = no energy expended
2. Active transport = energy expended
The rest of this chapter studies transport across membranes in more detail.
Passive transport is diffusion across a membrane




Diffusion = tendency for molecules of any substance to spread out into the available space
(Fig7.11).
Dynamic equilibrium = no net movement across a membrane
The rule of diffusion is: " any substance will diffuse from where it is concentrated to where
it is less concentrated" i.e substances diffuse down their concentration gradients.
No work is required. Diffusion is a spontaneous process because it decreases free energy (
increase in entropy).
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html


** Note: each substance diffuses down it own concentration gradient, and is unaffected by
the concentration of other substances.
Much of the traffic across cell membranes occurs by diffusion.
In passive transport, the concentration gradient itself represents potential energy and
drives diffusion.
Osmosis is the passive transport of water






Osmosis = diffusion of water across a selectively permeable membrane(Fig 7.12). (passive)
Hypertonic = solution with high [solute]
Hypotonic = solution with low [solute]
Isotonic = equal [solute]
The direction of osmosis is determined only by the difference in total solute concentration,
not by the nature of the solutes.
Water moves from a hypotonic solution to a hypertonic solution, even if the hypotonic
solution has more kinds of solutes.
Cell survival depends on balancing water uptake and loss


Cells without walls (Fig 7.13):
o shrivel in hypertonic solutions
o lyse in hypotonic solutions
o do well in isotonic solutions
o many unicellular organisms have special adaptations to living in hypotonic
environments
o many organisms are isotonic with their environment, e.g. crabs, sea stars.
o Animals living in hypertonic or hypotonic environments must have special
adaptations for osmoregulation (i.e. control of water balance)(Fig 7.14).
Cells with walls:
o plasmolysis (plasma membrane pulls away from cell wall) in hypertonic solutions
o flaccid in isotonic solutions (wilting)
o turgid in hypotonic solutions. Plant cells are healthiest in hypotonic solutions.
Specific proteins facilitate the passive transport of selected solutes




Facilitated diffusion = passive transport of solutes across a membrane by transport
proteins (Fig 7.15).
Some may bind solute and undergo conformational change such that solute is translocated
from one side of membrane to another.
Some may act as channels. Some of these can be gated channels, responding to electrical
or chemical stimuli.
There are several diseases associated with defective transport systems, e.g. cystinuria,
cystic fibrosis.
Active transport is pumping of solutes against their concentration gradients
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html








Active transport = movement of solutes by transport proteins against their concentration
gradients (7.16).
major factor in the ability of a cell to maintain internal concentrations of small molecules
that differ from concentrations in the surrounding medium (e.g. Na+/K+ pump)
Active transport is performed by proteins embedded in the membranes. Energy is supplied
by ATP.
Some ion pumps generate a voltage across membranes, such that inside is usually more
negative than outside. This is called a membrane potential.
Membrane potential is an energy source that affects the traffic of all charged substances
across the membrane. Membrane potential favors passive transport of cations into the cell
and anions out of the cell.
Thus the two forces that drive diffusion across a membrane are:
1. chemical force (i.e ion's concentration gradient)
2. electrical force (i.e effect of membrane potential on movement of ions)
These two forces combined are known as the electrochemical gradient. Thus passive
transport can be restated as: ' ions diffuse down their electrochemical gradient"
Some membrane proteins contribute to membrane potential. A transport protein that
generates voltage across a membrane is called an electrogenic pump (Fig 7.18). In animals,
Na+/K+ pump is major electrogenic pump. In plants, bacteria and fungi, the main
electrogenic pump is a proton pump .
Cotransport: membrane protein couples the transport of one solute to another

In cotransport, a specialized transport protein can couple the "downhill" diffusion of a
substance to the "uphill" transport of a second substance(Fig 7.19).
o e.g. use of proton motive force to move sucrose against its concentration gradient.
This is important in phloem loading in plants.
Endocytosis and exocytosis transport large molecules







Transport of large molecules, such as protein and polysaccharides, cross the membrane by
endocytosis and exocytosis(Fig 7.20).
Exocytosis = secretion of macromolecules by the fusion of vesicles with the plasma
membrane (many secretory cells export their products this way, e.g. mammary cells).
Endocytosis = cell takes in macromolecules and particulate matter by forming vesicles
derived from the plasma membrane.
Three type of endocytosis
1. Phagocytosis
o cell engulfs a particle by wrapping pseudopodia around it and packaging it within a
membrane-enclosed sac large enough to be classified as a vacuole.
2. Pinocytosis
o cell "gulps" droplets of extracellular fluid in tiny vesicles.
o unspecific to what it brings into cell.
3. Receptor-mediated endocytosis
o ligands bind to receptor in cell membrane, which are then engulfed.
o receptors often clustered in regions of membrane called coated pits.
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html
o
this method allows cell to acquire bulk quantities of specific substances, even
though those substances may not be very concentrated in the extracellular fluid.
Specialized membrane proteins transmit extracellular signals to the inside of the
cell.

Some membrane proteins function in sensing their external environment. These receptor
proteins transmit chemical signals from extracellular environment to the inside of the cell.
The binding of ligand to these receptors is first step in a chain of molecular interaction
known as signal-transduction pathways.

Related Links
o
o
o
o
o
o
o
Cell Membranes tutorial
Membrane structure
Transport across membranes
Cell structure
Virtual cell
Osmosis
Osmosis and owermeability tutorials
http://www.langara.bc.ca/biology/mario/Biol1115notes/biol1115chap7.html