Cell Transport

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8.2 Bellringer..
 (1)In your own words, define passive, active, and
transport
 Now, imagine sitting in a boat that is moving
downstream with the current. Then, imagine a small
motor to move the same boat upstream against the
current.
 (2)Determine which action shows passive transport and
which shows active transport
 (3)Which requires an input of energy?
8.2 Cell Transport
 **The cell must move different substances into and out
of the cell
 This happens in a variety of ways….

Sometimes the cell has to use energy, sometimes they do not
 **8.2 discusses the different methods of cell transport
Passive Transport
 Diffusion – passive transport of substances DOWN a
concentration gradient (from high to low
concentration)
 Concentration gradient – one area has a higher
concentration than another area
 Equilibrium – when a space is filled evenly
 Diffusion is simplest type of passive transport
 Some substance diffuse through lipid bilayer
 Others though transport proteins
Diffusion
 Cell membrane is selectively permeable
 Nonpolar interior of lipid bilayer repels ions and most
polar molecules

These substances are prevented from diffusing across
membrane
 Very small or nonpolar molecules can diffuse across the
membrane down their concentration gradient
Simple Diffusion
 Moves small, nonpolar molecules directly through
lipid bilayer
 Ex O2
 CO2
Diffusion
Model of Diffusion
Facilitated Diffusion
 Moves ions and polar molecules that simple diffusion
cannot
 Transport proteins help these substances diffuse
through the cell membrane
 Two types:
 Channel protein
 Carrier protein
Channel Proteins
 Ions, sugars, and amino acids can diffuse with help of
channel proteins
 Proteins, sometimes called pores, serve as tunnels
through the lipid bilayer

Each channel allows the diffusion of a specific substance
 Ex – only sodium ions can pass through sodium ion channels
Carrier Protein
 Transport substances that fit within their binding site
 (1)Carrier protein binds to specific substances on one
side of cell membrane
 (2)Binding causes protein to change shape
 (3)As shape changes, the substance is moved across the
cell membrane and released on other side
Facilitated Diffusion
Osmosis
 Definition – diffusion of water across selectively
permeable membrane
 Type of facilitated diffusion (passive transport)

From high to low concentration
 Allows cells to maintain water balance as environment
changes
 Pass through water channels
 Protein channels specific to water..
Osmosis
Osmosis
 There are three possibilities for the direction of
water movement:
 Water moves out – (hypertonic solution)

Causes cell to shrink
 Water moves in – (hypotonic solution)

Causes cell to swell
 Cell could burst if it swells too much
 Plant cells have rigid cell wall to prevent this
 No net water movement – (isotonic solution)

No change in cell volume
Hypertonic, Hypotonic, and
Isotonic Solutions
Effects of Osmosis
 **Plants and fungi have cell walls to help regulate
 Most plants are healthiest in a hypotonic environment
 Some unicellular eukaryotes have contractile vacuoles
 Force excess water in cell out
 Animals cells do not have cell walls or contractile
vacuoles
 They actively remove solutes from cytoplasm
Active Transport
 Transports of substance against concentration
gradient
 Low to high concentration
 Active Transport – requires energy
 Most energy supplied directly or indirectly in the form
of ATP (adenosine triphosphate)
Pumps
 Substances bind to carrier proteins in low
concentrations, released in higher concentration
 Carrier proteins function as “pumps” – move
substances against their concentration gradient called
membrane pumps
Sodium-Potassium Pump
 One of the most important membrane pumps in
animal cells
 In a complete cycle, the sodium-potassium pump
transports three sodium ions, Na+, out of a cell and two
potassium ions, K+, into the cell.
 Pump actively transports against their concentration
gradients
 Energy supplied by ATP
Sodium-Potassium Pump
**The sodium-potassium pump has four steps:
1. Three sodium ions inside the cell bind to the sodiumpotassium pump.
2. The pump changes shape, transporting the three
sodium ions across the cell membrane and releasing
them outside the cell.
3. Two potassium ions outside the cell bind to the
pump.
4. The two potassium ions are transported across the
cell membrane and are released inside the cell.
Sodium-Potassium Pump
 Important:
 Pump prevents sodium from accumulating in the cell
 Pump helps maintain the concentration gradient of
sodium ions and potassium ions across cell membrane
 Many cells use this pump to help transport other
substances (glucose) across the cell membrane
Movement in Vesicles
 Many substances, like proteins and polysaccharides,
are too large to be transported by carrier proteins
 So, they are moved across the cell membrane by vesicles
 The movement of a substance into a cell by a vesicle is
called endocytosis
 The movement of a substance by a vesicle to the
outside of a cell is called exocytosis
ENDOCYTOSIS
EXOCYTOSIS
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