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Carrier proteins

Another type of protein embedded in the plasma membrane is a carrier protein    . This aptly named protein binds a substance and, in doing so, triggers a change of its own shape, moving the bound molecule from the outside of the cell to its interior ( [link] ); depending on the gradient, the material may move in the opposite direction. Carrier proteins are typically specific for a single substance. This selectivity adds to the overall selectivity of the plasma membrane. The exact mechanism for the change of shape is poorly understood. Proteins can change shape when their hydrogen bonds are affected, but this may not fully explain this mechanism. Each carrier protein is specific to one substance, and there are a finite number of these proteins in any membrane. This can cause problems in transporting enough of the material for the cell to function properly. When all of the proteins are bound to their ligands, they are saturated and the rate of transport is at its maximum. Increasing the concentration gradient at this point will not result in an increased rate of transport.

This illustration shows a carrier protein embedded in the membrane with an opening that initially faces the extracellular surface. After a substance binds the carrier, it changes shape so that the opening faces the cytoplasm, and the substance is released.
Some substances are able to move down their concentration gradient across the plasma membrane with the aid of carrier proteins. Carrier proteins change shape as they move molecules across the membrane. (credit: modification of work by Mariana Ruiz Villareal)

An example of this process occurs in the kidney. Glucose, water, salts, ions, and amino acids needed by the body are filtered in one part of the kidney. This filtrate, which includes glucose, is then reabsorbed in another part of the kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than the proteins can handle, the excess is not transported and it is excreted from the body in the urine. In a diabetic individual, this is described as “spilling glucose into the urine.” A different group of carrier proteins called glucose transport proteins, or GLUTs, are involved in transporting glucose and other hexose sugars through plasma membranes within the body.

Channel and carrier proteins transport material at different rates. Channel proteins transport much more quickly than do carrier proteins. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second.

Osmosis

Osmosis is the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes. While diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the diffusion of solutes in the water. Not surprisingly, the aquaporins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules.

Mechanism

Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration to one of low concentration. An obvious question is what makes water move at all? Imagine a beaker with a semipermeable membrane separating the two sides or halves ( [link] ). On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute    , that cannot cross the membrane (otherwise the concentrations on each side would be balanced by the solute crossing the membrane). If the volume of the solution on both sides of the membrane is the same, but the concentrations of solute are different, then there are different amounts of water, the solvent, on either side of the membrane.

Questions & Answers

write a short note on how the kidney carry out osmoregulation in man
Rhoda Reply
Kidney play a very large role in human osoregulation by regulating the amount of water reabsorbed from glomerular filtrate in kidney as tubules, which is controlled by hormones such as antrdiuvetic hormone (ADH) , aldosterone and angiotensin.
Evelyn
kidney is capable of osmoregulation in mammal ,
Judith
simple definition for respiratory system
Gift Reply
Respiratory system is a network of organs and tissues that helps you to breathe or help in getting the rids of oxygen and discharge of carbon dioxide in the body.
Ruhiyatu
how is a aerenchyma tissue adapted to its function
fatuma Reply
what does DNA mean
innocent Reply
dioxiribo nucleic acid
Lekan
what is a zygote
Victor Reply
zygote is developed or fertilized egg cell
Lekan
what is the difference between cell wall and cell membrane
Ruhiyatu Reply
cell wall is found in plant while cell membrane is found in animal cell
Lekan
please this is the main answer to that question okay Cell wall gives shape and support to the cell whiles Cell membrane support the movement of substances into and out of the cell. This question is very tricky that's why I asked.
Ruhiyatu
how cell I form
Elijah Reply
what difference between animal cell and plant cell
Lazarus Reply
what is animal call
Rita
Animal cell does not have a cell wall but plant cell have a cell wall
Ruhiyatu
plant have cell Wall well animal have not cell wall
Yula
what is fertilization?
Muhamed Reply
Fertilization is the fusion of male sex cell and female sex cell to form a zygote.
Ruhiyatu
What kind of nutrients is composed of plants
Annie Reply
phosphorus,potassium,nitrogen
Kritika
NPK
Ruhiyatu
what is hormones
Igwe Reply
hormon is the chemical messanger
Sneha
Genes can make someone dull?
Taperah
21ecological instrument and their diagrams
Ayomide Reply
cell biology
Muhamed
I am sorry
onuoha
for wat
ezeadaugo
no reason
KP
nothing but speculate
KP
Sorry without reason is nothing but speculate
Leilah
what are the kidney disease
Immaculate Reply
kidney stones
Gracelyne
it is a disease that affects the kidney
Miriam
what are some lung diseases
Gracelyne
kidney cancer
Victor
what is the other way of treating kidney failure
Mirriam
After having kidney transplant or dialysis
Evelyn
Kidney stone is a hard deposits of mineral and acid salt that stick together in concentrated urine and they are painful when passing through the urinary tract, but usually don't cause permanent damage
Evelyn
what is micro-organism
Jackson Reply
what is the hypothesis
Jackson
hypothesis is a proposed explanation for a phenomenon
Miriam
hypothesis is raw materials
KP
what is biology
KP
biology is the study of living things and their interaction with their environment
Miriam
what is zyogot?
Muhamed
what is fertilization?
Muhamed
zygote is an unfertilized eggs
KP
fertilization refers to the fusion of a sperm and ovum
KP
organs that can't be seen physically
Victor
what does mean stigma
Amira Reply
the stigma receives pollen grains during pollination
Ruhiyatu

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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