IB BIOLOGY
TOPIC 2: CELLS
2.4 AND 2.1.6
I. Membrane Structure-refer to TOPIC 2: CELLS 2.1.1-2.1.5, 2.7-2.10, 2.2 , 2.3
A. Function of Glycolipids
1. cell recognition/identification
B. Function of Glycoproteins
1. cell recognition/identification
C. Function of Integral Proteins
1. channels for passive transport
2. pumps for active transport
D. Function of Peripheral Proteins
1. hormone binding sites
2. cell adhesion
3. sites for immobilizing enzymes
4. cell-to-cell communication
5. enzymes
II. Polarity
A. describes the distribution of electrons/charges around a molecule
1. non-polar: the electrons/charges are evenly distributed
Figure 1: The molecular structure of ethene (ethylene).
http://chemwiki.ucdavis.edu/Organic_Chemistry/Hydrocarbons/Alkenes/Structure_a
nd_Bonding_in_Ethene-The_Pi_Bond#Structure_of_Ethene
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2. polar: the electrons/charges are not evenly distributed
Figure 2: The molecular structure of water.
http://che11109.wikispaces.com/Titanium
(note that this image is used commonly but I did not find the original document)
B. affects how molecules behave around each other
1. polar molecules mix easily with other polar molecules or charged species
such as ions but not with non-polar molecules
a. known as hydrophilic or water-loving
b. thus the phosphate portion of the phospholipid faces either to the
outside of the cell (usually watery) or toward the inside of the cell
(cytoplasm is mostly water)
2. non-polar molecules tend to mix easily with other non-polar molecules but
not with polar molecules or charged species such as ions
a. known as hydrophobic or water-fearing
b. thus the lipid portion of the phospholipid is sandwiched between the
phosphate portions and thus separated from the watery internal and
external environments
III. Permeability of the Cell Membrane
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A. selectively permeable: permeability changes depending on the cell’s
requirements
B. contrast with semi-permeable: permeability is constant, size is the main
variable that controls whether a material will be able to pass through the
membrane
1. all membranes have “holes” or spaces between the component
molecules so anything smaller than the spaces can pass through the
membrane
Figure 3: Permeability of the cell membrane.
C. Factors Affecting a Molecule’s/Particle’s Ability to Cross the Cell Membrane
1. Size
a. smaller molecules tend to cross more readily
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2. Polarity
a. non-polar molecules tend to cross more easily
3. Charge
a. uncharged particles tend to cross more easily
4. Interactions with water
a. hydrophobic particles tend to cross more easily than hydrophilic
i. are often non-polar
IV. Passive Transport
A. used to describe methods of moving materials across the cell membrane that
don’t require energy in the form of ATP
B. Diffusion: the net movement of particles from an area of higher concentration
to an area of lower concentration
1. note that molecules will still move from an area of lower concentration to
an area of higher concentration but the number of molecules is smaller
relative to the number of molecules moving in the other direction
2. the net movement is down the concentration gradient
3. is a random process
4. explained by the Kinetic Molecular Theory
5. often occurs in solutions as the solute mixes with the solvent
a. e.g. put a sugar cube in water and do not stir...eventually the sugar
dissolves and mixes evenly with the water such that the solution is
uniformly sweet-tasting
6. some molecules that diffuse easily across the cell membrane include
oxygen, carbon dioxide, alcohol (ethanol)
C. Osmosis: the net movement of water from an area of lower solute
concentration to an area of higher solute concentration across a semi- or
selectively permeable membrane
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1. note that water will still move from an area of higher solute concentration
to an area of lower solute concentration but the net movement will be
from an area of lower solute concentration to an area of higher solute
concentration
2. is a random process
3. explained by the Kinetic Molecular Theory
4. becomes especially important when some molecules are too large to cross
the membrane
5. Osmotic Pressure: the ability of a solution to “attract” water molecules
a. the greater the solute concentration, the greater the osmotic pressure
b. osmotic pressure often works against turgor pressure
6. Relative Tonicity of Solutions
a. tonicity: the strength of a solution relative to osmosis
b. always compare the tonicity of two solution
i. isotonic: having a solute concentration equal to that of the
comparison solution
ii. hypotonic: having a solute concentration lower than that of the
comparison solution
iii. hypertonic: having a solute concentration higher than that of the
comparison solution
a. Effect of Solutions of Differing Tonicity on Cells
i. Cells Placed in Isotonic Solutions
a. no net movement of water
b. assuming no toxic solutes, cells remains alive
ii. Cells Placed in Hypotonic Solutions
a. net movement of water is from the outside of the cell to the
inside of the cell
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b. cell expands and may burst
c. lysis: bursting of the cell
d. hemolysis: the bursting of red blood cells
e. in plant cells, turgor pressure increases but the cell wall prevents
bursting
iii. Cells Placed in Hypertonic Solutions
a. net movement of water is from the inside of the cell to the
outside of the cell
b. causes the cell to shrink
c. crenation: shrinking in animal cells resulting in a ruffled edge
d. plasmolysis: shrinking in plant cells
D. Facilitated Diffusion/Facilitated Transport: net movement of a
molecule/particle from an area of higher concentration to an area of lower
concentration with the aid of a carrier or channel protein
1. molecules/particles move down their concentration gradient
2. requires specific proteins i.e. each protein moves only one molecule or one
class of molecules
3. glucose and amino acids cross the cell membrane using this method
a. probably too large for simple diffusion to be efficient
Animations:
1. Passive Transport:
http://bcs.whfreeman.com/thelifewire9e/default.asp#542578__591836__
2. Crenation in Red Blood Cells: http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter21/animation__hemolysis_a
nd_crenation.html
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E. Factors Affecting Rates of Passive Transport
1. Temperature
a. an increase in temperature usually increases the rate of transport
i. Kinetic Molecular Theory: molecules are moving faster
2. Concentration gradient
a. a greater concentration difference usually increases the rate of
transport
3. Size of particle
a. smaller particles tend to be transported more quickly
4. Surface Area
a. since movement depends on the spaces and assuming a relatively even
distribution of these spaces, a larger surface area means that more
spaces are available for materials to travel through
V. Active Transport
A. refers to methods of moving materials across the cell membrane that do
require energy in the form of ATP
B. Active Transport: molecules/particles are moved across the membrane from
an area of lower concentration to an area of higher concentration with the aid
of a carrier/pump protein
1. molecules move against their concentration gradient
2. The sodium-potassium pump
a. 3 sodium ions are transported from the inside of the cell to the outside
of the cell and 2 potassium ions are transported from the outside of the
cell to the inside of the cell
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Animation:
1. This one is more simple but the animation is more literal relative to the
description: http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/007243731
6/120068/bio03.swf::Sodium-Potassium%20Exchange%20Pump
2. This one is more complex but the animation is less detailed:
http://bcs.whfreeman.com/thelifewire/content/chp05/0502002.html
Table 1: Examples of some molecules moved by active transport in the human body.
MOLECULE
SODIUM ION
POTASSIUM ION
SUGARS
AMINO ACIDS
SITE IN THE BODY/CELL TYPE
NERVE CELLS
NERVE CELLS
SMALL INTESTINE
SMALL INTESTINE
Animation:
http://bcs.whfreeman.com/thelifewire9e/default.asp#542578__591837__
C. Endocytosis: the formation of a vesicle around a material to bring it into the
cell
1. phagocytosis: endocytosis of a solid particle
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2. pinocytosis: endocytosis of a liquid particle(s)
D. Exocytosis: the joining of a vesicle with the cell membrane in order to release
a material from the cell
1. essentially the opposite of endocytosis
2. used by cells to export products or secrete materials to the outside of the
cell or to other cells/other parts of the body
Animations:
1. Endocytosis and exocytosis:
http://bcs.whfreeman.com/thelifewire9e/default.asp#542578__591206__
2. ER and Golgi functional relationship:
http://bcs.whfreeman.com/thelifewire/content/chp04/0402002.html
Review:
1. http://wps.prenhall.com/esm_audesirk_bloe_7/17/4453/1140182.cw/index.ht
ml
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