White Diffused
2008
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White Diffused
Diffusion and Osmosis
Cell membrane structure
The cell (plasma) membrane is made up of a variety of molecules, including phospholipids, proteins, and cholesterol. Most of these are able to move about within the membrane. Phospholipids A double layer, about 5 nm thick, of phospholipid molecules comprises much of the cell membrane.
Phospholipids are modified lipids (fats) that contain phosphorus. They are divided into two main parts: a head and a tail. As the polar heads are waterseeking (hydrophilic) and the nonpolar tails are water-repelling (hydrophobic), the phospholipid molecules orient themselves with their heads pointing outward and their tails in contact.
Proteins
Intrinsic proteins pass through the membrane and protrude on either side. Extrinsic proteins are attached to the surface or one half of the lipid bilayer. Structural (or fibrous) proteins are strandlike and connect to the cell's cytoskeleton. They may also connect the cell to adjacent cells or to the extracellular matrix. Functional (or globular) proteins are generally spherical and include sensors and receptors; for example, glycoproteins, enzymes, and transport proteins (channel proteins and carrier proteins).
Cholesterol
Cholesterol is a steroid that gives the bilipid layer extra strength.
Osmosis
The contents of a cell form a highly concentrated solution. Large molecules, such as proteins, are blocked from diffusing in and out of the cell by the membrane. However, smaller molecules, such as water, can move freely. When a cell is placed in weak solution, its concentrated contents cannot disperse. Instead water diffuses into the cell to make the contents less concentrated, and in the process make the external solution stronger. This type of diffusion is called osmosis.
This process continues until the cell contents have the same concentration as the solution outside the cell.
Since water has flowed into the cell, it will swell up. If the cell is in a very weak solution or pure water, so much water will diffuse into it that the cell will eventually burst. The opposite happens if a cell is placed in a stronger solution. The cell loses water by osmosis, and its contents become more concentrated. The cell shrinks and becomes crinkled, a process known as crenation.
Facilitated diffusion
Most substances cannot dissolve in both lipids and water, so they cannot cross a cell membrane. Some, such as water, diffuse through channels made of protein. Large items bind to a carrier protein straddling the membrane. This chemical reaction makes the protein change shape. As it does so it moves the item to the other side of the membrane.
Active transport
In active transport, the cell uses energy supplied by ATP (adenosine triphosphate) to transport substances across its membrane. Active transport is used if a particle:
• is too large to pass through the membrane pores;
• cannot dissolve through the lipid bilayer (fatty double layer) of the cell membrane; or
• needs to be moved against the direction of diffusion, from an area of lower concentration to an area of higher concentration.
Solute pumping
A carrier protein, called a solute pump, powered by ATP, combines with the particle. It then transports the particle across the cell membrane. Amino acids and most types of ion are transported across the membrane in this way.
Exocytosis
This is how substances, such as hormones, mucus, and waste products are secreted from a cell. A membrane-covered vesicle (sac) carries the substance. The sac fuses with the cell membrane. The particle is ejected from the cell.
Phagocytosis
Engulfing other cells or particles by a cell. This is generally used by white blood cells to destroy foreign bodies such as bacteria and viruses. Part of the cell membrane protrudes around the particle.
Pinocytosis
This process follows a similar sequence to phagocytosis, but is used to transport fluids into the cell across the cell membrane. The fluids include water and the various solutes dissolved in it and liquids containing proteins and fats.
Receptor-mediated endocytosis
Molecules, generally glycoproteins, in the cell membrane act as receptor sites for certain hormones, minerals, and fats, such as cholesterol. Once a substance is attached to the receptor, the membrane folds inward and the small sac created separates from the inside of the cell membrane. The membrane encloses the particle. The sac formed separates from the inner surface of the cell membrane. Enzyme-containing sacs (lysosomes) then destroy the contents.
About the Author
Christine still studies molecular biology. She has deep interest in molecular cloning techniques. She believes that the cellular and physiological functions of spliceosome and the alternative splicing defines the diversity of the phenotypes but also determines the molecular basis of some diseases. She maintaines a biology blog,where she shares interesting biomedical facts and hopes to share her future research findings.
in low light everything appears black or white because?
few action potentials are stimulated at low light
only white light is available at night
rods are more sensitive to light than are cones
no colored light is reflected to the eye
the diffused light of night does not fall on the fovea
rods are more sensitive to light than are cones.
Rods are for black and white sight as well as night vision.
Cones are for colour.
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