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Cell Boundaries
Regulating Cell Environment
 The cell has to be able to regulate its internal
environment.
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Its internal environment is very different than the external
environments that it is bathed in.
 Cells use cell walls and cell membrane to control its
environment.
 Most cells except for animal cells have cell walls.
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They are composed of carbohydrates depending on the
kingdom.
Cell walls are dead and are used to prevent the cells from
bursting and give them structures.
Some Cell Wall Carbohydrates
 Bacteria-Muramic acid (modified sugars held with
peptide cross linkages)
 Plants-Cellulose
 Fungi-Chitin
 Cell walls are porous and allow most anything to
cross-the cell membrane is a barrier that regulates
what gets “into or out of” the cell.
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In plant cells, the first cell wall that is laid down is the primary
cell wall and is elastic. The middle lamella is the layer holding
adjacent cells together.
 Plant cells with soft tissue have only one cell wall.
Harder cells lay down a secondary cell wall.

This is laid inside the primary cell wall and is thicker.
Summary
 Cell walls- found in all cells except animal cells.
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Gives cell shapes and prevents it from bursting.
Porous most everything crosses wall.
Plants made of cellulose
Prokaryotes made of maramic acid
Fungi made of chitin
Plants may have 2 cell walls- primary wall elastic.
Secondary cell wall hardened with lignin. Middle
lamella hold cells together.
 All cells have cell membranes. The basic structure is
a bilayer of phospholipids with proteins and
cholesterol inserted in the layer.

This model is the fluid mosaic layer of cell membranes.
Phospholipids are modified triglycerides
Phospholipid with atoms
illustrated:
Phospholipids usually
illustrated such as:
Summary: All
cells have cell
membranes
And it regulates
what gets into and
out of the cell.
Structure is a
bilayer of
phospholipids with
proteins and
cholesterol in the
layer.
Called fluid mosaic
model
Basic Cell membrane
Has two layers of
phospholipids as seen
below
The phosphate heads are facing
water-inside and outside the
cell, keeping the hydrocarbons
away from it
Summary: Cell membrane
 Phosphate heads face the water-inside and outside,
the hydrocarbon tails face one another
 Proteins are “stuck” in the membrane like a mosaic
(intrinsic) or can be on the surface (extrinsic)
 Functions of membrane proteins
 1.Used as transport
 2.Used as receptors or as enzymes
 3.Used as attachment for cytoskeleton
Glycolipid
Summary: Cell membranes
 Cell membranes are semi permeable or (selectively
permeable) because some molecules can pass the
membrane and others can not
 Permeable are molecules that can pass the
membrane usually small molecules (O2 and CO2)
and lipids like steroids
 Impermeable are molecules that can not cross the
membrane usually large molecules, polar molecules
and ions
Passive Diffusion
 The molecules (CO2, O2, etc) involved are permeable
to the membrane.

Predicting the net movement as if there was no membrane at
all. The molecules move from high to low concentration.
 In the first system, A is more
concentrated on side 2
than side 1.
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A is permeable
Net movement Side 2 to 1
Equilibrium is reached
Equal movement on both sides
Summary:
 Passive diffusion- is the movement of permeable
material across the membrane.
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The net movement of material is from high concentration to
low concentration until equilibrium is reached. Then the
movement of material across the membrane will be
equal in both directions
Summary: Facilitated Diffusion
 Smaller, charged or polar molecules (glucose, amino
acids, etc.) are impermeable to the membrane.
 A transport protein is needed.
 The net movement is like passive diffusion moving
from high to low concentration.
Active Transport
 Active transport-molecules are impermeable to the
membrane
 The molecules are moving against the concentration
gradient (from low to high).
 A transport protein and ATP are needed.
Osmosis
 Is a special case of movement of water.
 When things are impermeable to the membrane,
and not in equilibrium, water will move across the
membrane to help reach equilibrium.
 “Water likes to dilute”
Terms
 Hypertonic-higher concentration of solutes. Lower
concentration of water
 Hypotonic-lower concentration of solutes. Higher
concentration of water
 Isotonic-Two solutions that have equal solutes on
both sides of the membrane
What is happening and Why?
 Think pair share!
 What is happening?
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Animal cell?
Plant cell?
 Why ?
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Name the process
Net movement
Movement of Molecules Using the Cell Membrane
 Endocytosis- is
movement of larger
particles into the
cell by use of
membrane vesicles.
 Two Types:
 Phagocytosis-cell
eating
 Pinocytosis-cell
drinking
 Exocytosis-is the
movement of larger
particles out of the
cell by use of
membrane vesicles.
 http://highered.mcgraw-
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