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Notes: Cell membrane and the Cell Environment:
Cells must maintain a biological balance with its environment to survive. To do this
cells achieve homeostasis by controlling and regulating what gets into and out of the
cell. There can be limits on how much the cell can adjust.
Understanding how molecules move, what states of environmental conditions they
exist best in and how far they are wiling to adjust determines how efficient cells are.
Molecules travel in constant motion. As molecules travel they tend to move into
areas where the molecules are less concentrated. This tells us the direction of the
When direction of molecules moves from areas of high concentration to areas of low
concentration this is called passive transport. Passive transport requires no need
of energy to move the molecules . This also means it is moving from greater
concentration to lesser concentration. The difference in concentration of a
substance across space is called a concentration gradient.
There are 3 types of passive movement.
1. Diffusion
2. Facilitated Diffusion
3. Osmosis
Carrier Molecules= help transport molecules across the cell membrane. Carrier
molecules are embedded in the membrane and some are extended across the
Each carrier molecule is specialized to allow the movement of only one type of
molecules across the membrane
Gated channels- are also a form of passive transport that occurs when proteins in
the membrane form a gated channel across the cell membrane and this allows the
membrane to be permeable as needed. Again these gated channels are active in
nerve impulses and contraction of muscles.
Diffusion – is molecules spreading out across a concentration gradient from an area
of high to low concentration. The example is the movement of oxygen across the cell
Some kinds of molecules can diffuse across a membrane. The ability of a molecule
to pass through a membrane depends on the sixe and type of molecule and the
molecular structure of the membrane.
Membranes that let some substances through are called selectively permeable
membranes or semipermeable. If they are semipermeable they regulate what gets
into and out of a cell.
Facilitated diffusion-These are carrier molecules that transport molecules across
the cell membrane without energy. Carrier molecules speed up diffusion rate. Ex:
glucose which is to large to pass on its own needs the help of a carrier molecules.
Osmosis- is the movement of water from an area of high concentration to an area of
low concentration. So we say diffusion of water. The movement of water depends
on the concentration of solutes in the solution or water.
Water will diffuse or move to the area where the solutes are.
Direction of Osmosis:
solution outside is
Hypo means low
Cell solution is
Water will move
Hyper means high
Into the cell
If the solute
concentration in
the environment is
higher than in the
Out of the cell
If the solute
concentration in
the environment is
equal to that in the
Iso means equal
No movement
water will not
If the solute
concentration in
the environment
outside the cell is
lower than that in
the cell
Hypotonic solution exists in a coexisting hypertonic solution.
Isotonic solution exists in a coexisting isotonic solution.
If a plant cell swells s it fills with water it will not swell endlessly, because it is
restricted by cell walls. When water pressure increases it is called turgor pressure.
This pressure forces the cytoplasm and the cell membrane against the cell wall and
the cell becomes rigid.
Active transport – is the movement of any substance across a cell membrane with
the use of chemical energy.
Materials will cross the cell membrane by either passive or active transport,
depending on the size and chemical makeup of the material. The structure of the
cell membrane also plays an important roll in both types of transport.
Cell Membrane- consists of 2 layers, each layer containing lipid molecules. This is
known as the lipid bilayer. The outside surface of the membrane is hydrophilic. (not
afraid of water).
The inside layer of the membrane is hydrophobic. (Afraid of water) This particular
arrangement ensures that molecules that dissolve in water do not automatically
pass through the membranes. A layer of lipids separates the 2 aquatic
environments, one of the cell and one of the environments.
Carrier molecules also can act and function in the active transport of molecules
across the cell membrane.
One type of active transport system is sodium-potassium pump. This is a
mechanism that moves sodium ions out of the cell and forces potassium ions into
the cell. Movement in both cases is against the concentration gradient, hence the
reason it is active transport.
This process depends on the ability of the protein carrier molecule to change shape.
3 Na ions fit into a carrier molecule. A chemical reaction using high-energy ATP
molecules changes the shape fo the carrier molecule which causes the sodium to be
released outside the cell. This pump is essential to all animal muscle and nerve
When large molecules (food and waste) are too large to pass through the cell
membrane the processes of endocytosis and exocytosis will enable particles to enter
and exit the cell membrane. These processes are part of the cells maintenance of
homeostasis to stay in balance with their environment by taking in nutrients,
exporting proteins, and excreting wastes.
Endocytosis- is the process y which cells engulf substances that are much too large
to enter the cell by passing through the membrane. The large molecule outside the
cell are enclosed by a portion of the cell, which folds into itself and makes a pouch
called a vesicle, which pinches off from the membrane and enters the cytoplasm.
There are two types of endocytosis- pinocytosis and phagocytosis. These two
types of active transport engulf liquids; (pinocytosis) and engulf solids;
Exocytosis-helps the cell maintain homeostasis by exporting larger molecules out of
the cell.
Ex: the ribosomes make the proteins that the Golgi processes prepares and packages
that move to the cell membrane. The vesicle membrane fuses with the cell
membrane and dumps its contents out of the cell.
Diversity of Cellular Life:
All living things are composed of cells, use the same basic chemistry, follow the
same genetic code, and even contain the same kinds of organelles.
A single celled organism is also called a unicellular organism. Unicellular organisms
grow, respond to the environment, transform energy, and reproduce. The also
dominate life on earth.
Multicellular organisms are made up of many cells. All multicellular organisms
depend on communication and cooperation among specialized cells.
Cells throughout an organism can develop in different ways to perform
different tasks. This is called cell specialization.