ACTIVE TRANSPORT
In many cases, cells must move materials up their concentrated gradient, from and area of lower concentration to an area of higher concentration. Such movement of materials is known as ACTIVE TRANSPORT. Unlike passive transport, active transport requires a cell to use energy (ATP).
Active transport is very similar to that of facilitated diffusion for it uses transport proteins to allow molecules to pass through the cell membrane. However, the difference this time is that the ion or molecules physical binds to the protein and then is pumped across the membrane.
CELL MEMBRANE PUMPS / HOW THEY WORK
Cells often move molecules across the membrane AGAINST a Concentration Gradient. These molecules or ions go form an area of LOW concentration to areas of HIGH concentration.
To move molecules AGAINST the concentration gradient REQUIRES ENERGY. (Active transport)
The CARRIER PROTEINS act as PUMPS that USE ENERGY to move IONS and MOLECULES across the membrane. The carrier proteins that serve in active transport are often called CELL MEMBRANE PUMPS. Active transport is especially important in maintaining ion concentration in the cell and between cells.
SODIUM-POTASSIUM PUMPS are important for muscle contractions, the transmission of nerve impulses, and the absorption of nutrients. Sodium-Potassium pumps in animal cells pump sodium ions out and potassium ions in, against (up) the concentration gradient. ATP supplies the energy needed by the carrier proteins to do this job. (See diagram in textbook.)
In Plants, ACTIVE TRANSPORT enables roots to absorb nutrients from the soil. Plant nutrients are more concentrated inside the roots than in the surrounding soil. Without active transport, nutrients would diffuse out of the roots. Active Transport in the root cell membrane enables the plant to absorb the nutrients against the
Concentration Gradient.
BULK TRANSPORT- ENDOCYTOSIS AND EXOCYTOSIS
Some molecules, such as COMPLEX PROTEINS, are too LARGE to cross the cell
membrane. In order for these molecules to cross the membrane they need something to transport them. These proteins are too large even to fit through the protein channels, so the cell must use a unique technique called BULK TRANSPORT.
In BULK TRANSPORT, large molecules, food, and other substance are packed in membrane-bound sacs called a vesicle, and these move across the cell membrane.
There are several types of bulk transport, including ENDOCYTOSIS,
EXOCYTOSIS, PINOCYTOSIS, and PHAGOCYTOSIS.
During ENDOCYTOSIS the Cell Membrane folds into a POUCH that encloses the particles. (Figure 1.40 in text). The Pouch pinches off INSIDE the Cell to form a VESICLE/ VACULE (membrane-wrapped bubbles).
The VESICLE/ VACULE can then fuse with other organelles (lysosomes) or release its contents into the cytoplasm.
PINOCYTOSIS AND PHAGOCYTOSIS are two types of ENDOCYTOSIS.
PINOCYTOSIS is sometimes called “CELL DRINKING". This involves the intake of a small droplet of ECF, together with any dissolved substances or small particles that it may contain. This happens in nearly all cells, and occurs all the time.
PHAGOCYTOSIS IS LIKE PINOCYTOSIS, EXCEPT THE CELL ENGULFS A FOOD
PARTICLE OR OTHER CELLS INSTEAD OF A DROP OF LIQUID. "CELL EATING"
The Food Vesicle can then Fuse with a LYSOSOME that contains DIGESTIVE ENZYMES.
White Blood Cells (WBC, PHAGOCYTES) Destroy Bacteria and other Unwanted Cells by
Phagocytosis.
EXOCYTOSIS IS THE OPPOSITE OR REVERSE OF ENDOCYTOSIS.
DURING EXOCYTOSIS, WASTE AND CELL PRODUCTS LEAVE THE CELL.
Products MADE IN the Cell are Packaged in GOLGI VESICLES, which then FUSE with the Cell Membrane and Secrete Material OUT OF THE CELL.
MUCUS AND WASTE PRODUCTS ARE MATERIALS SECRETED BY EXOCYTOSIS.
