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Cell Boundaries/Active and
Passive Transport
CHAPTER 7, SECTION 3
Cell Boundaries
All cells are surrounded by a thin, flexible barrier
known as the cell membrane.
Many cells also produce a strong supporting layer
around the membrane known as a cell wall.
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Cell Membrane: Lipids
 The cell membrane regulates what enters and leaves the
cell and also provides protection and support.
 It is made of a lipid bilayer, or a double-sheet of lipids.
 These are often phospholipids, with an outer, polar
phosphate “head” of each lipid, which is hydrophilic (water
loving), and an inner, nonpolar fatty acid tail that is
hydrophobic (water repellent).
 The lipid bilayer gives the cell membranes a flexible
structure that forms a strong barrier between the cell and
its surroundings.
Cell Membrane
Cell Membrane
Outside of
cell
Proteins
Carbohydrate
chains
Cell
membrane
Inside of cell
(cytoplasm)
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Protein
channel
Lipid bilayer
Cell Membrane: Proteins and Carbohydrates
 Most cell membranes contain protein molecules
inside the lipid bilayer. Carbohydrates are attached
to many of these proteins.
 Proteins form channels and pumps to help move
materials back and forth across the cell membrane.
 Many carbohydrates are like chemical
identification cards that allow cells to identify one
another.
Cell Wall Importance
 Cell walls are in plants, fungi, and most
prokaryotes.
 Cell walls are outside cell membranes.
 Most cell walls have enough space to allow water,
oxygen, carbon dioxide, and other substances
through.
 The cell wall’s main function is to provide
support and protection for the cell.
Cell Wall Construction
 Most cell walls are made from fibers of
carbohydrates and proteins.
 These substances are made inside the cell and then
used to construct wall.
 Plant cell walls are mostly made up of cellulose, a
tough carbohydrate.
Cell Walls
Cell Wall
Cell walls are found in plants, algae, fungi, and many
prokaryotes.
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Concept Review
 What are the similarities and differences between the
function and composition of the cell wall and cell
membrane?
 What is the main reason cell membranes are
flexible?
 How do carbohydrates assist with the cell
membrane’s tasks?
Diffusion and Osmosis
 Diffusion is when particles move from an area of
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higher concentration to an area of lower
concentration.
When the concentration of the solute is the same,
equilibrium has been reached.
Diffusion does not require energy.
Osmosis is the diffusion of water through a
selectively permeable membrane.
These types of movement that require no energy
are part of passive transport.
Diffusion Through Cell Boundaries
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Osmosis
 Osmosis is the diffusion of water through a
selectively permeable membrane.
 When the concentration of water is the same on
both sides of the membrane, the solutions are
isotonic.
 When there is less water on one side of the
membrane, it is known as being in a hypertonic.
 The other side of the membrane is then
hypotonic
Osmosis
How Osmosis Works
Dilute sugar
solution (Water
more
concentrated)
Concentrated
sugar solution
(Water less
concentrated)
Sugar
molecules
Selectively permeable
membrane
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Movement of
water
Facilitated Diffusion
 Some materials are too large to fit through the
membrane, so they need to be aided in being brought
through
Glucose
Facilitated Diffusion
molecules
Facilitated Diffusion
Protein
channel
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Active Transport
 Cells sometimes have to move molecules from lower
to higher concentrations.
 When this happens, energy needs to be used, making
it active transport.
 Proteins generally take care of this transport.
 *** Active transport requires energy.
Active Transport
Molecule to be carried
Active Transport
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Concentration
 The mass of solute in a given volume of solution is known
as concentration.
 A concentration gradient is the amount of the
concentration of a substance that changes as you move
from one area to another.
 If the solute is moving from an area of higher
concentration to an area of lower concentration, it is
moving with its concentration gradient. If moving from
an area of lower to higher concentration, it is moving
against the gradient.
Diffusion and Osmosis Review
 Diffusion and osmosis require no energy to occur.
 Because of this they are known as forms of passive
transport.
Passing Through the Cell Membrane
 The following materials can easily pass through the cell
membrane:
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Oxygen
Carbon dioxide
Amino acids
Lipids
 Some materials require more help. They include:
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Glucose
Protein
Sodium
Potassium
Facilitated Diffusion
 Some molecules pass through protein channels
during facilitated diffusion
 No energy is required.
 The molecules move from an area of higher
concentration to an area of lower concentration.
 Glucose is an example of a molecule that needs help
in this way.
Active Transport
 Some molecules cannot pass through the membrane easily
due to their polarity, which prevents them from going past the
fatty acid tails of phospholipids.
 These molecule are then moved via active transport.
 Energy is required for this, since the molecules are being
moved from an area of lower concentration to an area of
higher concentration (against their concentration gradient of
higher to lower).
 Two examples of molecules that do this are:
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Sodium (Na+)
Potassium (K+)
Other Active Transport
 Endocytosis occurs when the cell membrane brings
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materials into the cell by forming pouches to bring it in. These
materials include proteins.
Phagocytosis is cell-eating, when cells bring materials in
that are then digested by lysosomes. The materials eaten may
be bacteria, as happens in white blood cells.
Pinocytosis brings liquid in a similar fashion to endocytosis.
Both phagocytosis and pinocytosis are examples of
endocytosis.
Exocytosis occurs when the cell sends materials outside of
the cell in pouches. An example is proteins.
Endocytosis
Exocytosis