Membrane
transport:
-
In
the most basic of senses, a solution is a homogenous mixture of multiple
substances, such as the air that we breathe, or salt water. The component
which the dissolving medium is referred to as the solvent, while the other
is a solute. Intracellular fluid is a solution of water, small amounts
of gasses (oxygen and carbon dioxide), nutrients, and salts. The extracellular
fluid, or interstitial fluid, contains the same ingredients. It also contains
amino acids, sugars, fatty acids, vitamins, hormones, neurotransmitters,
and waste products
-
The
plasma is a selectively permeable membrane. It allows some substances to
pass through, while others are turned away. When a cell is damaged, or
dies, the membrane becomes permeable.
-
In
passive transport, substances pass through the membrane without any energy
being used. In active transport, the cell provides adenosine triphosphate,
the metabolic energy, that drives the substance in question through
-
There
are two kinds of passive transport:
-
Diffusion
is the process through which molecules and ions scatter themselves throughout
all available space. All of these molecules possess kinetic energy, and
are in constant motion. The overall effect of this wandering is a general
movement away from the area of highest concentration, towards that of lower
concentration. This is called moving down the concentration gradient. The
diffusion of solute through a membrane is called dialysis, or simple diffusion.
Some solutes transported in this manner are lipid soluble fats, vitamins,
oxygen, carbon dioxide, or particles small enough to fit through the pores
of the membrane, such as small ions. Diffusion of water through a selectively
membrane is known as osmosis. Because the water molecules are highly polar,
they are repelled by the nonpolar lipid core of the membrane, but can pass
easily through the pores. Another example of diffusion is called facilitated
diffusion, which provides means for certain substances, most notably glucose,
to pass. These molecules are not lipid soluble, nor small enough to pass
in the manner water does. This form of diffusion follows all the "rules"-
substances move down their concentration gradient- a protein carrier is
needed as a transport. Thus, channel proteins passively allow the glucose
in. Water and glucose continually move in, down the gradient, because they
are constantly being used up within the cell. Likewise, carbon dioxide
constantly move out, always being produced as a waste by the cells metabolism
-
Filtration
is the process which forces solutes to pass by hydrostatic, or fluid pressure.
Like the previous, this is also a passive transport, using a pressure gradient
that actually pushes the solute-containing fluid through to the low pressure
area
-
The
tendency of a solution to hold water is called it's osmotic pressure. The
higher the solute's concentration, the higher it's osmotic pressure. The
solution's ability to change the size and shape of a cell is it's tonicity.
Isotonic solutions have the same amount of solutes as the inner fluid of
the cell. Hypertonic solutions have more, and cause the cells to crenate,
or shrink. Hypertonic solutions are given to sufferers of edema, swollen
feet of hands. Hypotonic is the opposite: too few solutes, causing the
cell to expand and burst, or lyse. These solutions are sometimes given
intravenously or as tea and juices to those who are dehydrated
-
Whenever
a cell uses adenosine triphosphate to move a substance through the membrane,
it is active transport. Those that must be moved by these means are to
large to pass, or must move against their concentration gradient. There
are two main types of active transport:
-
Solute
pumping, or simply active transport, is similar to facilitated diffusion.
However, the latter is moved by kinetic energy, while the fist is driven
by atp. The atp is used to energize protein carriers, called solute pumps.
Amino acids, some larger sugars, and most ions are moved this way, and
mostly against the concentration gradient. Amino acids are too large to
pass through the channel proteins, and are not lipid soluble. Sodium ions
are moved out by the solute pumps, while potassium is moved in. These two
must be constantly moved back and forth against the gradient for cells
to conduct nerve impulses. Each pump in the plasma membrane transports
only one specific substance
-
The
other form of active transport is bulk transport, made up of exocytosis,
and endocytosis. Excocytosis does what is sounds like: it moves substances
out of the cells. The product is "packaged" (usually by the Golgi) and
sent to the cellular membrane, where it fuses and releases it's contents.
Endocytolosis is the process by which the plasma membrane engulfs outside
substances and pinches part of itself off, forming a small vesicle. The
vesicle them moves into the cytoplasm and fuses with a lysosome and it's
contents are then digested. If the engulfed substances are large, such
as bacteria or dead body cells, the process is known as phagocytosis, most
commonly found in the white blood cells, and other phagocytes. Pinocytolosis
is utilized in taking in liquids that contain fats or proteins. Unlike
phagocytosis, it is a routine activity for most cells, especially in cells
that engage in large amount of absorption, such as the cells lining the
small intestine
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