Chapter 7 – The Cell Membrane
7.1, 7.2
AP Biology
2.b.1 Cell membranes are
selectively permeable due to their
structure.
2.b.2 Growth and dynamic
homeostasis are maintained by the
constant movement of molecules
across membranes.
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7.3, 7.4, 7.5
• Glucose transport
• Na+/K+ transport
Vocabulary:
active transport, carbohydrates, carrier protein, cell membrane, channel protein,
cholesterol, concentration gradient, diffusion, endocytosis, exocytosis, facilitated
transport, fluid-mosaic membrane model, glycolipid, glycoprotein, hydrophilic,
hydrophobic, hypertonic, hypotonic, isotonic, osmosis, passive transport
processes, phagocytosis, phospholipid, phospholipid bilayer, pinocytosis, pressure
gradient, protein, selectively permeable, surface area-to-volume ratio, tonicity
It is expected that students will:
B9
Analyse the structure and function of the cell membrane
B9.1
B9.2
B9.3
B9.4
B9.5
B9.6
B9.7
B9.8
B9.9
apply knowledge of organic molecules – including phospholipids,
proteins, glycoproteins, glycolipids, carbohydrates, and cholesterol – to
explain the structure and function of the fluid-mosaic membrane model
identify the hydrophobic and hydrophilic regions of the phospholipid
bilayer
explain why the cell membrane is described as “selectively permeable”
describe passive transport processes including diffusion, osmosis, and
facilitated transport
explain factors that affect the rate of diffusion across a cell membrane
(e.g., temperature, size of molecule, charge of molecule, concentration
gradient, pressure gradient)
predict the effects of hypertonic, isotonic, and hypotonic environments on
osmosis in animal cells
describe active transport processes including active transport, endocytosis
(phagocytosis and pinocytosis), and exocytosis
compare specific transport processes – including diffusion, osmosis,
facilitated transport, active transport, endocytosis, and exocytosis – in
terms of
– concentration gradient
– use of channel or carrier protein
– use of energy
– types/sizes of molecules transported
devise an experiment using the scientific method (e.g., to investigate the
tonicity of cells)
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B10
Explain why cells divide when they reach a particular surface area-to-volume
ratio
B10.1 differentiate between cells that have a high or low surface area-to-volume
ratio
B10.2 demonstrate an understanding of the significance of surface area-tovolume ratio in cell size
http://nobelprize.org/nobel_prizes/chemistry/laureates/2003/chemanim2.mpg
demonstrates the fluidity of the phospholipid bilayer
The cell membrane is often described as a ‘fluid mosaic’. What could that mean?
The next 3 pages will help you to understand this concept.
Why is it helpful that a phospholipid is an amphipathic molecule?
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Why are membranes are described as
‘fluid-mosaics’?
Evidence for embedded proteins is
gained from freeze-fracturing
techniques:
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Proteins can move within the phospholipid bilayer. Examine the details of the experiment
described on page 128 Fig. 7.7
Here is an overall view of the structure of a typical cell membrane:
 Watch Activity: Membrane Structure on Mastering Biology website
http://multimedia.mcb.harvard.edu/anim_innerlife_lo.html show the interaction of cell
membranes of different cells.
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Hydrophobic portions of membrane proteins help to
anchor them in the phospholipid bilayer:
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What are some of the functions of membrane proteins? Use Figure 7.10 on page 129
to make notes.
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Membrane functions (cont’d)
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The inside and outside of membranes differ and the difference is ‘set up’ when the
membrane is manufactured in the endoplasmic reticulum. Describe what is happening at
1,2,3,4.
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Molecules and ions diffuse across membranes down their concentration gradients
(dialysis):
http://www.northland.cc.mn.us/biology/Biology1111/animations/transport1.html
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9.
Osmosis is the diffusion of water across a selectively permeable membrane from a
hypotonic solution to a hypertonic solution:
Concept 7.2 is essential reading, page 131 to 132. Understand it and make you own
note.
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10.
What is membrane potential? Which side of the membrane is positive?
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What are the two forces that drive the diffusion of ions across the membrane? What is the
combination of these forces called?
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Osmosis is the movement of water across a selectively permeable membrane from high
water potential to low water potential:
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The direction of the net movement of water depends on hydrostatic pressure as well as
osmotic pressure. The hydrostatic pressure gives rise to the pressure potential and
the osmotic pressure gives rise to the solute potential.
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This diagram shows the effects of pressure potential and solute potential on osmosis in a
plant cell:
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5.
Water potential is important in the movement of water from the roots to the leaves
in plants:
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Here is a diagram to show how the water potential is highest in the roots of the plant and
lowest in the air spaces of the leaves. This keeps water moving in the upward direction,
even to the top of very tall trees! See activity in Chapter 36.
Activity: Transport of Xylem Sap
BioFlix: Water Transport in Plants
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Osmosis is important in both animal and plant cells:
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See videos on plasmolysis in Elodea and turgidity in Elodea.
Website ch.36
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Membrane proteins help larger molecules and ions through the membrane.
BioFlix: Membrane Transport
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_ Do concept check 7.3 p.135 _
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14.
Describe active transport. What type of transport proteins are involved, and what is
the role of ATP in the process?
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The sodium-potassium pump is an important system for you to know. Use the following
diagram to understand how it works. Use the following terms to label these figures,
and briefly summarize what is occurring in each figure: extracellular fluid,
cytoplasm, Na+, K+, ATP, ADP, P, transport protein.
Summary
1.
2.
3.
4.
5.
6.
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15.
Here is a summary of mechanisms of transport across the cell membrane:
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An electrogenic pump generates a voltage across the membrane using energy from ATP.
A proton pump is an example of an electrogenic pump:
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In cotransport, as one molecule/ion moves through a membrane protein by facilitated
diffusion it carries another molecule/ion with it. In the following diagram you can
see how energy from ATP is used to pump sucrose into the cell using the proton
gradient set up by the proton pump.
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Endocytosis engulfs material from outside the cell into vesicles/vacuoles. Study Figure
7.22 on page 139
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Exocytosis is used to secrete large molecules through the cell membrane.
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Here you can see a phagocyte (a type of white blood cell), ‘eating’ a worn out red blood
cell:
Find some more detailed information on
the phagacytosis of red blood cells.
The aging erythrocyte undergoes
changes in its plasma membrane,
making it susceptible to
recognition by phagocytes and
subsequent phagocytosis in the
spleen, liver and bone marrow.
Much of the important
breakdown products are
recirculated in the body. The heme constituent of hemoglobin are broken down
into Fe3+ and biliverdin. The biliverdin is reduced to bilirubin, which is released
into the plasma and recirculated to the liver bound to albumin. The iron is
released into the plasma to be recirculated by a carrier protein called transferrin.
Almost all erythrocytes are removed in this manner from the circulation before
they are old enough to hemolyze.
Here you can see how pinocytosis involves the ‘drinking in’ of solute molecules into a
vesicle:
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19.
What will happen?
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Optional reading: Page 130 – Impact: Blocking entry into cells as a form of HIV
treatment.
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Use mastering biology site for the following
 Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins
 Activity: Membrane Structure
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Concept 7.2: Membrane structure results in selective permeability

BioFlix: Membrane Transport
 Activity: Selective Permeability of Membranes

Concept 7.3: Passive transport is diffusion of a substance across a membrane with no
energy investment
 Activity: Diffusion
 Activity: Osmosis and Water Balance in Cells
 Activity: Facilitated Diffusion
 Investigation: How Do Salt Concentrations Affect Cells?
 Video: Paramecium Vacuole
 Video: Chlamydomonas
 Video: Turgid Elodea
 Video: Plasmolysis
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Concept 7.4: Active transport uses energy to move solutes against their gradients
 Activity: Active Transport
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Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and
endocytosis
 Activity: Exocytosis and Endocytosis
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