Download All cells are surrounded by a membrane that separates the inside of

All cells are surrounded by a membrane that separates the inside of the cell from
the outside
I.
Membranes organize the chemical activity of the cell
A. A cell must be highly ORGANIZED to carry out many metabolic
reactions SIMULTANEOUSLY.
B. Membranes provide the structural basis for metabolic order
1. Membranes form most of the cell’s ORGANELLES.
2. Membranes PARTITION the cell into compartments that contain
enzymes in solution.
C. The PLASMA MEMBRANE is literally the edge of life, forming a
BOUNDARY between the living cell and its surroundings.
1. The PLASMA MEMBRANE controls what molecules enter and
leave the CELL.
2. The
plasma
membrane,
like
all
membranes,
exhibits
SELECTIVE PERMEABILITY.
a. Allows some substances to CROSS more easily than others
b. BLOCKS passage of some substances altogether
3. The PLASMA MEMBRANE takes in substances the cell needs
and disposes of the cells WASTE.
II.
Membrane phospholipids form a bilayer
A. PHOSPHOLIPIDS are the main structural components of membranes
B. Recall the structure of a phospholipids molecule
1. Two FATTY ACID chains attach to a GLYCEROL molecule
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2. A PHOSPHATE group attached to glycerol instead of third fatty
acid
C. The phosphate group
1. Is POLAR
2. Is HYDROPHILIC
3. Referred to as the HEAD
D. The two fatty acid chains
1. Are NONPOLAR
2. Are HYDROPHOBIC
3. Referred to as TAILS
E. In the WATERY environment within living organisms, phospholipids
spontaneously form a stable two-layer framework called a phospholipid
BILAYER.
1. The hydrophilic heads face OUTWARD, exposed to the water
2. The hydrophobic tails point INWARD, shielded from the water.
F. The hydrophobic interior of the bilayer is one reason membranes are
selectively PERMEABLE.
1. NONPOLAR, hydrophobic molecules are SOLUBLE in lipids
and can easily pass through MEMBRANE.
2. POLAR, hydrophilic molecules are not soluble in LIPIDS.
G. Whether polar molecules or ions pass through the membrane depends on
PROTEIN molecules in the PHOSPHOLIPID bilayer
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H. Much of a membrane’s SELECTIVE permeability depends on membrane
PROTEINS.
III.
The membrane is a fluid mosaic of phospholipids and proteins
A. Membranes are FLUID like, rather than sheets of molecules locked
rigidly in place.
B. Membranes are commonly described as a “FLUID MOSAIC.”
1. MOSAIC means a surface made of small fragments like pieces
of colored tile cemented together in a mosaic picture.
2. A membrane is a MOSAIC in having many different PROTEIN
molecules embedded in a phospholipid framework
3. The membrane is FLUID in that most of the individual
PHOSPHOLIPIDS and PROTEIN molecules can drift sideways
in the membrane
IV. Proteins make the membrane a mosaic of function
A. Proteins PERFORM most of the functions of the membrane
1. Attach membrane to CYTOSKELETON
2. Provide IDENTIFICATION to help other cells recognize it.
3. PROTEINS (ENZYMES) built into membrane carry out important
chemical reactions
4. Act as signals to help cells COMMUNICATE with each other
5. Help move SUBSTANCES across the membrane
V. Passive transport is diffusion across a membrane
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A. The nature of phospholipids and the kinds of PROTEINS in a membrane
DETERMINE whether a particular substance can cross the membrane.
1. Some molecules can cross without the cell doing any WORK.
2. Some molecules can only cross membranes if the cell expends
ENERGY to move the molecule across the membrane
B. DIFFUSION is the tendency for particles of any kind to spread spontaneously
from where they are more concentrated.
1. DIFFUSION requires no ENERGY
2. It results from the RANDOM motion (THERMAL energy) of
atoms and molecules
C. Because a cell does not perform WORK when molecules DIFFUSE across its
MEMBRANE, the diffusion of a substance across a biological membrane is
called PASSIVE transport.
D. Molecules will diffuse down their CONCENTRATION gradient until they
reach DYNAMIC equilibrium
1. Molecules tend to DIFFUSE from the side of the membrane
where they are MORE concentrated to the side where they are
LESS concentrated
2. This continues until the concentration of the molecule is
EQUAL on both sides of the membrane.
3. At EQUILIBRIUM, molecules continue to move back and
forth, but there is no net change in CONCENTRATION on
either side of the membrane
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E. Two or more substances diffuse INDEPENDENTLY of each other
1. Each diffuses DOWN its own concentration gradient
2. May diffuse in different DIRECTIONS.
F. PASSIVE transport is extremely important to all cells
1. OXYGEN enters cells solely by diffusion
2. CARBON DIOXIDE leaves cells solely by diffusion
VI. Osmosis is the passive transport of water
A. A cell contains an AQUEOUS solution and is SURROUNDED by one
B. The plasma membrane is PERMEABLE to water; water molecules can readily
pass into and out of cells.
C. DIFFUSION of water molecules across a SELECTIVELY permeable
membrane is a special case of passive transport called OSMOSIS
1. If two solutions are separated by a selectively permeable membrane
and the solutes are not PERMEABLE
a. The solution with the HIGHER concentration of solute is said
to be HYPERTONIC
b. The solution with the LOWER solute concentration is
HYPOTONIC
2. Water crosses the membrane until the solute concentrations are
EQUAL on both sides
3. There is a net movement of water from the HYPOTONIC side to the
HYPERTONIC side of membrane
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a. HYPOTONIC side has more free water molecules and
LOWER solute concentration
b. HYPERTONIC side has LESS free water molecules but
HIGHER solute concentration
c. Free water molecules move from area of HIGH concentration
to area of LOW concentration
4. The direction of OSMOSIS is determined by the difference in
TOTAL SOLUTE CONCENTRATION.
5. Solutions of equal solute concentration are said to be ISOTONIC
a. EQUAL amounts of water move in both directions through
membrane
b. No net CHANGE in the amount of WATER on each side of
membrane
VII. Water balance between cells and their surroundings is crucial to organisms
A. If one placed an animal cell into an isotonic solution, the cell volume would
remain CONSTANT.
1. The cell gains WATER at the same rate that it LOSES it
2. The cell and its SURROUNDING are in equilibrium
3. The two solutions have the same TOTAL CONCENTRATION of
solutes
B. If one placed an animal cell into a hypotonic solution, the cell GAINS water,
SWELLS and may burst (CYTOLYSIS)
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1. Water moves by diffusion through the membrane from the area of
HIGHER WATER concentration (OUTSIDE the cell) to the area of
LOWER WATER concentration (INSIDE the cell)
2. Initially the total concentration of SOLUTE was GREATER inside the
cell than OUTSIDE the cell
C. If an animal cell is placed into a HYPERTONIC solution, the cell shrivels and
can die from water loss.
1. Water moves by diffusion through the membrane from the area of
HIGHER WATER concentration (INSIDE the cell) to the area of
LOWER WATER concentration (OUTSIDE the cell)
2. Initially the total concentration of SOLUTES was GREATER outside
the cell
D. Animals must have ways to prevent excessive UPTAKE or excessive LOSS
of water
1. The control of water balance is called OSMOREGULATION
2. In many MULTICELLULAR organisms KIDNEYS provide this
function
3. In UNICELLULAR organisms CONTRACTILE vacuoles help
provide this function
E. Because plant cells have a rigid CELL WALLS, water balance problems are a
little different
1. A plant cell placed in an isotonic solution is FLACCID, and a plant
WILTS.
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2. A plant cell placed in a hypotonic solution is TURGID, and the plant is
HEALTHIEST.
a. To become turgid, a plant cell needs a net INFLOW of water
b. The rigidity of the CELL WALL prevents the plasma
membrane from expanding too much and BURSTING.
3. A plant cell placed in a HYPERTONIC environment loses water
a. The plasma membrane pulls away from the CELL WALL.
b. This is called PLASMOLYSIS.
c. Usually the cell DIES.
VIII. Specific proteins facilitate diffusion across a membrane
A. Many molecules move across a membrane with the help of transport
PROTEINS in the membrane
1. When one of these proteins makes it possible for a substance to move
DOWN
its
concentration
gradient,
the
process
is
called
FACILITATED diffusion
2. Without the PROTEIN, the molecule does not cross the membrane or
it diffuses across too SLOWLY to be useful to the cell
B. Like osmosis, FACILITATED diffusion is a type of PASSIVE transport
because it does not expend ENERGY.
C. The most common way membrane PROTEINS facilitate diffusion is by acting
as a CHANNEL or passage way for a particular substance
a. The transport PROTEIN spans the entire membrane
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b. The rate of FACILITATED diffusion depends on the number
of transport PROTEINS in membrane
IX. Cells expend energy for active transport
A. In contrast to passive transport, ACTIVE transport requires that a cell expend
ENERGY to MOVE molecules ACROSS a membrane
1. A transport PROTEIN actively pumps a specific solute across a
membrane AGAINST the solutes concentration gradient
2. Substances move from area of LOWER concentration to area of
HIGHER concentration
3. Membrane proteins usually use ATP as their energy source for
ACTIVE transport
X. Exocytosis and endocytosis transport large molecules
A. Cells use the process of EXOCYTOSIS to EXPORT bulky
materials from its cytoplasm
1. First, a membrane enclosed VESICLE filled with
MACROMOLECULES such as PROTEINS moves to the plasma
membrane
2. The VESICLE fuses with the PLASMA MEMBRANE, and the
VESICLE contents spill out of the cell.
3. INSULIN is released from pancreatic cells into the
BLOODSTREAM by EXOCYTOSIS.
4. TEARS are released from cells of tear glands by EXOCYTOSIS.
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B. Cells use the process of ENDOCYTOSIS to take in macromolecules or other
particles by forming VESICLES from its plasma membrane.
C. There are two types of ENDOCYTOSIS
1. PHAGOCYTOSIS
a. Cellular EATING
b. Membrane engulfs prey and packages it within VESICLE
c. Fuses with a LYSOSOME and contents are digested
2. PINOCYTOSIS
a. Cellular DRINKING
b. Takes droplets of fluid from its surroundings into tiny
VESICLES
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