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Channel proteins

Provide open passageways through the membrane for certain hydrophilic (water-soluble) substances such as polar and charged molecules.


Ion channels

Allow the passage of ions across the membrane. In nerve and muscle cells, ion channels called gated channels open and close in response to specific chemical or electrical stimuli to allow the passage of specific ions (such as Na+ and K+).



Are proteins that allow the passage of certain ions and small polar molecules through membranes. Aquaporins, found in the plasma membranes of certain cells (such as those found in kidneys and plant roots), dramatically increase the passage rate of H2O molecules.


Carrier proteins

Bind to specific molecules, which are then transferred across the membrane after the carrier protein undergoes a change of shape. The passage of glucose into a cell is by a carrier protein.


Transport proteins

Use energy (ATP) to transport materials across the membrane. When energy is used for this purpose, the materials are said to be actively transported, and the process is called active transport. The Na+-K+ pump, for example, uses ATP to maintain higher concentrations of Na+ and K+ on opposite sides of the plasma membrane.


Recognition proteins

Give each cell type a unique identification. This provides for a distinction between cell types, between self-cells and foreign cells, and between normal cells and cells infected with viruses. Recognition proteins are glycoproteins because they have short polysaccharide chains (oligosaccharides) attached. The oligosaccharide part of the glycoprotein extends away from the surface of the membrane.


Adhesion proteins

Attach cells to neighboring cells or provide anchors for the internal filaments and tubules that give stability to the cell.


Receptor proteins

Provide binding sites for hormones or other trigger molecules. In response to the hormone or trigger molecule, a specific cell response is activated


The glycocalyx

IS a carbohydrate coat that covers the outer face of the cell wall of some bacteria and the outer face of the plasma membrane of certain animal cells. It consists of various oligosaccharides that are attached to membrane phospholipids (glycolipids) and proteins (such as the glycoproteins of recognition proteins). Depending upon the cell, it may provide adhesive capabilities, a barrier to infection, or markers for cell-cell recognition


Golgi apparatus (Golgi complex or Golgi body)

Is a group of flattened sacs arranged like a stack of bowls. They modify and package proteins and lipids into vesicles, small, spherically shaped sacs that bud from the outside surface of the Golgi apparatus. Vesicles often migrate to and merge with the plasma membrane, releasing their contents to the outside of the cell.



Are vesicles from a Golgi apparatus that contain digestive enzymes. They break down food, cellular debris, and foreign invaders such as bacteria. A low pH (acidic), favorable to the activity of the enzymes, is maintained inside the lysosome. As a result, any enzyme that might escape from the lysosome remains inactive in the neutral pH of the cytosol. Lysosomes do not occur in plant cells.



Break down various substances, including hydrogen peroxide (H2O2) (forming H2O and O2.), fatty acids, and amino acids. Peroxisomes are common in liver and kidney cells where they break down toxic substances. In plant cells, peroxisomes modify by-products of photorespiration, a process by which CO2 is diverted
from its use in photosynthesis. In germinating seeds, peroxisomes (called glyoxysomes) break down stored fatty acids to help generate energy for growth



Carry out aerobic respiration, a process in which energy (in the form of ATP) is obtained from carbohydrates.



carry out photosynthesis, the plant process of incorporating energy from sunlight into carbohydrates.



Are made of the protein tubulin and provide support and motility for cellular activities. They are found in the spindle apparatus, which guides the movement of chromosomes during cell division, and in flagella and cilia (described in the following section), structures that project from the plasma membrane to provide motility to the cell.


Intermediate filaments

Provide support for maintaining the shape of the cell.



Are made of the protein actin and are involved in cell motility. They are found in muscle cells and in cells that move by changing shape, such as phagocytes (white blood cells that wander throughout the body attacking bacteria and other foreign invaders)


Transport vesicles

Move materials between organelles or between organelles and the plasma membrane.


Food vacuoles

Are temporary receptacles of nutrients. Food vacuoles often merge with lysosomes, whose digestive enzymes break down the food


Storage vacuoles

in plants store starch, pigments, and toxic substances (nicotine, for example)


Central vacuoles

Are large bodies occupying most of the interior of certain plant cells. When fully filled, they exert turgor, or pressure, on the cell walls, thus maintaining rigidity in the cell. They also store nutrients and carry out functions otherwise assumed by lysosomes in animal cells.


Contractile vacuoles

Are specialized organelles in single-celled organisms that collect and pump excess water out of the cell


Anchoring junctions

Are protein attachments between adjacent animal cells. One such junction, the desmosome, consists of proteins (including the protein keratin) that bind adjacent cells together, providing mechanical stability to tissues. Desmosomes are also associated with protein filaments that extend into the interior of the cell and serve to hold cellular structures together


Tight junctions

Are tightly stitched seams between animal cells. The junction completely encircles each cell, producing a seal that prevents the passage of materials between the cells. Tight junctions are characteristic of cells lining the digestive tract where materials are required to pass through cells (rather than intercellular spaces) to penetrate the blood stream.


Gap junctions

Are narrow tunnels between animal cells that consist of proteins called connexins. The proteins prevent the cytoplasms of each cell from mixing, but allow the passage of ions and small molecules. In this manner, gap junctions allow communication between cells through the exchange of materials or through the transmission of electrical impulses. Gap junctions are essentially channel proteins of two adjacent cells that are closely aligned. Because the proteins of each cell extend beyond the plasma membranes before they meet, a small gap occurs between the two plasma membranes.



Are narrow channels between plant cells. A narrow tube of endoplasmic
reticulum, called a desmotubule, surrounded by cytoplasm and the plasma membrane, passes through the channel. Material exchange through a plasmodesma apparently occurs through the cytoplasm surrounding
the desmotubule.