Download Cell Membrane - hrsbstaff.ednet.ns.ca

Cell Membrane
In order to survive, cells must take in nutrients from the nonliving environment
outside of the cell and dispose of the wastes that build up inside of the cell. These
processes occur through the cell membrane. Regulating what enters and exits the cell is
the main function of the cell membrane.
The cell membrane is made of two layers of lipids and a variety of protein molecules
are embedded with it. The embedded proteins have several functions, including:
1 - Gatekeeper proteins - open and close paths through the cell membrane allowing
some molecules to pass freely through the membrane. This is known as facilitated
diffusion. It allows materials to move in and out of the cell much more quickly than regular
diffusion because the cell does not expend its own energy.
2 - Hormone receptor proteins - attach to hormones and pass the chemical
message to the cell. The mechanism for these receptors is not fully understood, but it is
thought to be similar to a lock and key. Hormones have a special shape that fits into the
protein receptor on the cell membrane.
3 - Identification proteins - recognize the type of a nearby cell. If the cell is not
recognized as a part of the organism, it may be attacked by the organism’s immune system.
4 - Carrier proteins - aid in active transport, which requires the use of energy by
the cell. Carrier proteins receive energy and then capture specific molecules to move
either into or out of the cell. Active transport moves molecules against the concentration
gradient, meaning from an area of low concentration to an area of higher concentration.
Passive Transport
The movement of nutrients and wastes across a cell membrane without expending
energy is called passive transport. There are three types of passive transport:
1- Diffusion - occurs when there is an unequal concentration of molecules. The
molecules move across the concentration gradient between an area of high concentration
to an area of lower concentration until a dynamic equilibrium is reached.
2 – Osmosis - is the diffusion of water through a selectively permeable membrane.
The membrane is permeable to water, but impermeable to the larger molecules. Water
diffuses from an area of high water concentration to an area of lower water concentration
until the equilibrium between water and solute is reached.
3 - Facilitated diffusion – see notes on gatekeeper proteins.
Active Transport
The movement of materials from an area of lower concentration to an area of
higher concentration by expending energy is called active transport. Carried proteins
receive energy that allows them to aid in active transport. There are two other types of
active transport:
1 - Endocytosis - movement of large particles into the cell. The cell engulfs (takes
in) the particle by extending its cytoplasm around it. Pinocytosis occurs is when cells
engulf liquid droplets (Example: nutrients dissolved in a solution). Phagocytosis is when
cells engulf large molecules, or even whole cells, and incorporate them into the cytoplasm.
2 - Exocytosis - movement of large particles out of the cell. Small vesicles break
off the Golgi apparatus and fuse to the cell membrane, releasing the interior of the vesicle
in the process. Waste is an example of a material that is released by exocytosis.
The above diagram shows an example of endocytosis and exocytosis.
Task – Use ALL bolded terms in the above reading (16 in total) to create a concept map
about the cellular membrane and cellular transportation. Hint: You may want to draft a
rough copy and then complete your good copy.