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Molecules can be transported across the plasma membrane via passive or active transport. Active transport is characterized as requiring energy in the form of ATP, while passive transport does not require energy. During active transport, pumps are often used to assist in moving a molecule against a concentration gradient.


We are one huge conglomerate of cells. And although we begin life as only one cell, that cell has differentiated into many specialized cells since then. There are two basic types of cells: eukaryotic cells that have membrane-bound organelles, and prokaryotic cells that do not. Eukaryotic cells are far more complex because they contain compartments, which are also membrane-bound, called organelles. Humans are made entirely of eukaryotic cells. Cells vary in size but can never exceed the volume that can be nourished by materials passing through the surface membrane. Thus it is a cell's surface area-to-volume ratio that determines when it must divide.

The plasma membrane is the outer boundary of the cell and controls the movement of substances in and out of the cell. The primary component of all cell membranes is phospholipids. This lipid bilayer separates the extracellular fluid from the material inside the cell contained in the cytoplasm. Proteins, cholesterol, and carbohydrates are also part of the membrane giving it the qualities of a fluid mosaic.

The primary functions of the membrane are to separate the internal contents of the cell from everything else and to control what moves in and out of the cell. The structure of the membrane allows it to be selectively permeable and to function in cell recognition. Receptors facilitate communication between cells and cell adhesion molecules help cells to form tissues and organs.

Movement across the membrane is accomplished by simple diffusion, facilitated diffusion, and active transport that require the input of energy. Osmosis is the movement of water across a membrane. To facilitate the movement of large particles into the cell, the membrane encloses the substance in a vesicle, a process called endocytosis. Exocytosis is the process whereby large molecules leave the cell.

The plasma membrane is used within the cell to delineate compartments called organelles, each with its own function. The nucleus contains the genetic information of the cell. It is surrounded by a nuclear envelope that allows communication through nuclear pores. Chromosomes contained in the nucleus are made of DNA. The nucleolus is involved in the production of ribosomal RNA, a component of ribosomes used during protein synthesis.

The endoplasmic reticulum is important for cell communication through a network of channels. The rough endoplasmic reticulum contains ribosomes that guide the production of cell products. They are packaged in vesicles and transferred to the Golgi complex for export. The smooth endoplasmic reticulum lacks ribosomes and is involved in the production of phospholipids and detoxification. The enzymes within lysosomes break down macromolecules, diseased cells, and invaders.

Mitochondria are the site of cellular respiration. This double-membrane organelle contains inner foldings, the cristae that provide increased membrane surface for cellular respiration.

Microtubules, a part of the cytoskeleton, provide shape and support for the cell. Microtubules are also responsible for the structure and movement of cilia and flagella. Microfilaments are made of actin and function in muscle contraction. They also are responsible for the movement of pseudopodia. Intermediate filaments are ropelike fibers that maintain cell shape and anchor organelles.

Cellular respiration and fermentation generate cellular energy. Cellular respiration requires oxygen to break down glucose in a three-step process. Glycolysis occurs in the cytoplasm,

Fermentation is the breakdown of glucose without oxygen and takes place entirely in the cytoplasm. It begins with glycolysis and uses pyruvate or one of its derivatives as the final electron receptor. It is very inefficient, resulting in only two ATP. Lactic acid fermentation takes place in the muscles while alcohol fermentation is used in bread making and the production of alcohol.

Passive transport

Passive transport requires no expenditure of energy in simple diffusion, water molecules and gases like oxygen and carbon dioxide need only move down concentration gradients to move into or out of cells. Having been delivered by blood capillaries to an area just outside the cell oxygen exists there in greater concentration than it does inside the cell. Moving down its concentration gradient, it dissolves into the plasma membrane, travels through it and emerges on the other side. Carbon dioxide diffuses out of the cell in a similar way. Molecules that are larger than oxygen but that are fat soluble also move into the cell in this manner.

In facilitated diffusion, transport proteins are required for adequate amounts of water and polar molecules larger than water to pass through the plasma membrane. The transport proteins act as a conduit for only one substance, or at most a small group of associated substances. They area a hydrophilic passageway through a hydrophobic environment. Glucose and other hydrophilic molecules such as amino acids move through the plasma membrane in this way. This form of transport does not require the expenditure of energy because it also has a concentrated gradient working in its favour. For example glucose typically exists in higher concentration outside the cell than inside it.

Passive transport requires no expenditure of energy, but active transport does. in active transport, molecules again move through a transport protein but now energy must be expended to move them against their concentration gradient. a cell move solutes against their concentration gradients by using pumps. Many kind of pumps are in operation in active transport, but each one is specific for one or perhaps two substances. The energy source for such transport is ATP [ adenosine triphosphate].

One of the most important and thoroughly studied of the active transport mechanism is known as the sodium potassium pump. It works this way.

First three molecules of sodium ions located within the cells cytoplasm bind with a transport protein. then, the energy molecule ATP gives up an energetic phosphate group to the transport protein. This binding causes the protein to open its channels to the extracellular fluids. the protein loses its sodium ion binding sites, thus releasing the ions into the fluid. Binding sites are then created for potassium ions.

Two potassium ions move into protein's potassium ion binding sites, which brings



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