Download Notes Cell Membrane and Cell Transport

The Plasma Membrane
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I. Maintaining Balance
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I. Maintaining Balance
•How do cells maintain balance?
•Cells need to maintain a balance by
controlling material that move in and out
of the cell  HOMEOSTASIS
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I. Maintaining Balance
•Small molecules, like water, oxygen, and
carbon dioxide can move in and out of the
cell freely.
•Large molecules like proteins and
carbohydrates cannot.
•Eliminating wastes
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I. Maintaining balance
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Structure of the plasma (cell) membrane
• All cells are surrounded by a plasma membrane.
• Functions like a GATE, controlling what ENTERS
and LEAVES the cell.
• The cell membrane is semipermeable or
selectively permeable.
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Structure of the plasma (cell) membrane
• A semipermeable
membrane only
allows certain
molecules to pass
through
• Some substances
easily cross the
membrane, while
others cannot cross
at all.
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Structure of the plasma (cell) membrane
• Made of a thin layer of
lipids and proteins
• Made mostly of
phospholipid molecules
(phosphate and lipid)
• Phospholipids are a kind
of lipid that consists of 2
FATTY ACIDS (tails) and a
PHOSPHATE GROUP
(heads)
Hydrophilic head
Hydrophobic tail
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Structure of the plasma (cell) membrane
• Cell membranes consist of TWO phospholipid layers called a
PHOSPHOLIPID BILAYER
Cytoplasm
inside cell
Phosphate head
Outside cell
Phosphate head
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Structure of the plasma (cell) membrane
•Water molecules surround both sides of the
cell membrane.
• Polar phosphate heads sticking TOWARD the
water (hydrophilic)
• Nonpolar lipid tails pointing AWAY from the
water (hydrophobic)
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Structure of the plasma (cell) membrane
• The cell membrane is constantly being formed and broken down in
living cells.
cytoplasm
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III. Lipid Bilayer
• Moving with and among the phospholipids are
cholesterol, proteins, and carbohydrates.
• Cholesterol:
• Nonpolar, found among the phospholipids to help prevent
the fatty acid tails from sticking together
• Helps with structure and homeostasis
• Gives cell its fluidity
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III. Lipid Bilayer
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III. Lipid bilayer
•Proteins:
• Found on the surface of the plasma
membrane = transmits signals to the inside of
the cell
• Embedded in the plasma membrane =
structure and support of cell’s shape, and
move large substances in and out of cell
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III. Lipid Bilayer
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III. Lipid bilayer
• Carbohydrates
• Attached to proteins; helps cells identify chemical signals
• Ex: help disease fighting cells recognize and attack a potentially harmful cell
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Cellular Transport
Types
Active vs Passive
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Cellular Transport
• All particles move and have kinetic energy
(energy of motion).
• Movement is random and usually in a water
solution
• Cells are mostly made of water and there is a
constant flow of ions and particles.
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IV. 2 types of cellular transport
1. Passive transport = movement of molecules
across the membrane by using the molecules
kinetic energy. The cell exerts NO energy!
2. Active transport = transport of materials
against the concentration gradient and requires
cellular energy.
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V. Passive Transport
• 3 types of passive transport
1. Diffusion = the net movement of
particles from an area of higher
concentration of particles to an
area of LOWER concentration of
particles.
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Diffusion…
•Molecules move randomly until they are
equally distributed (equilibrium).
•Diffusion continues until the
concentration of substances is uniform
throughout.
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Diffusion…
• Dynamic equilibrium =
continual movement but no
overall change in
concentration.
• Movement of materials into
and out of the cell at equal
rates maintains its dynamic
equilibrium with its
environment.
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Diffusion…
• Diffusion depends on the concentration gradient.
• Concentration gradient is the difference between the
concentration of a particular molecule in one area and the
concentration in an adjacent area.
• Ex: gas exchange in the lungs (oxygen from air to
blood and carbon dioxide from blood to air)
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V. Passive Transport
2. Facilitated Diffusion = type of passive transport
that increase the rate of diffusion with the use
of a carrier protein (transport)
• Usually because molecules are too large to pass
through the membrane.
• Would pass too slowly without the use of a carrier
protein
• Ex: Facilitated diffusion of glucose
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Facilitated Diffusion
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V. Passive Transport
3. Osmosis = the diffusion of water molecules from an area of HIGH
water concentration to an area of LOW water concentration across a
semi-permeable membrane
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V. Passive Transport: Osmosis
• Occurs in response to the concentration of
solutes dissolved in water!
• Solutes are dissolved substances in a solution.
• Solvent is usually water in cells.
• Cytoplasm of cells is mostly water containing many
dissolved solutes.
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V. Passive Transport: Osmosis
• Because no TWO molecules can occupy the
same space at the same time, the MORE solutes
there are in a certain volume of water; the
FEWER water molecules there can be in the
same volume.
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V. Passive Transport: Osmosis
• Plant and animal cells behave differently because plant cells have a
large water vacuole and a cell wall
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V. Passive Transport: Osmosis
A. Isotonic Solution = a solution in which the concentration of
dissolved substances (solutes) is the SAME as the
concentration of solutes inside the cell.
• Osmosis DOES NOT occur since a concentration
gradient(difference) is not established.
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What happens to cells when places in an
isotonic solution?
• Plant cells – become flaccid (limp)
• Plant wilts because no net tendency for water to enter
• Animal Cell – normal (preferred condition)
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Animal cell
Plant cell
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V. Passive Transport Osmosis
B. Hypotonic Solution = a solution in which the
concentration of solutes is LOWER than the
concentration of solutes inside the cell.
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What happens to cells when placed in a
hypotonic solution?
• Animal cell – water moves through plasma membrane into the cell.
This causes the cell to swell and the internal pressure increases.
• Cell lyses (bursts)!
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What happens to cells when placed in a
hypotonic solution?
• Plant cell – normal
• The vacuole and cytoplasm increase in volume
• The cell membrane is pushed harder against the cell wall causing it to stretch
a little
• The plant tissue becomes stiffer (turgid).
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V. Passive Transport: Osmosis
C. Hypertonic Solution = a solution in which the
concentration of dissolved substances is
HIGHER than the concentration inside the cell
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What happens to cells when placed in a
hypertonic solution?
• Animal cell - will shrivel because of decreased turgor pressure
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What happens to cells when placed in a
hypertonic solution?
• Plant Cell – will lose water from vacuole and a
decrease in turgor pressure will occur: so it is
plasmolyzed.
• Turgor pressure = internal pressure of a cell due to water
held there by osmotic pressure
• Plasmolysis = the loss of turgor pressure causing the
plasma membrane to pull away from the cell wall.
• Causes the plant to wilt.
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Plant cell
Animal cell
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Summary of Cell Behavior in Different
Environments
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VI. Active Transport
• Movement of molecules from an area of Low to
an area of HIGH concentration (opposite of
passive transport!)
• REQUIRES cellular energy!
• Moves large, complex molecules such as
proteins across the cell membrane.
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VI. Active Transport
• Large molecules, food, or fluid droplets are packaged in membranebound sacs called vesicles.
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3 types of active transport
1. Endocytosis = process by which a cell surrounds and takes in
material from its environment
• Used by ameba to feed and white blood cells to kill bacteria
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Endocytosis
• Two Types
1. Pinocytosis = cell taking in liquids
2. Phagocytosis = cell taking in solids (food)
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3 types of active transport
2. Exocytosis = expels materials out of the cell, reverse of endocytosis
• Used to remove wastes, mucus, and cell products
• Proteins made by the ribosomes in a cell are packaged into transport vesicles
by the Golgi apparatus
• Transport vesicles fuse with the cell membrane and then the proteins are
secreted out of the cell (ex: insulin)
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3 Types of Active Transport
3. Pumps
• Proteins pump ions against concentration gradient.
(Ca +2, Na +1, K+1)
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Summary of the types of transport
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Which process is used to transport these?
• Liquids, gasses, dyes, small uncharged molecules
• Water
• Glucose
• Ions such as sodium, potassium, calcium
• Hormones, enzymes, solids, food entering
• Hormones, enzymes, wastes leaving
Diffusion
Osmosis
Facilitated diffusion
Active (pumps)
Active (Endocytosis)
Active (Exocytosis)
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• Crash Course Video: Membranes and Transport
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