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By the end of this section, you will be able to:
  • Explain why and how passive transport occurs
  • Understand the processes of osmosis and diffusion
  • Define tonicity and describe its relevance to passive transport

Plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving. In other words, plasma membranes are selectively permeable    —they allow some substances to pass through, but not others. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed. Some cells require larger amounts of specific substances than do other cells; they must have a way of obtaining these materials from extracellular fluids. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that facilitate transport. Some materials are so important to a cell that it spends some of its energy, hydrolyzing adenosine triphosphate (ATP), to obtain these materials. Red blood cells use some of their energy doing just that. All cells spend the majority of their energy to maintain an imbalance of sodium and potassium ions between the interior and exterior of the cell.

The most direct forms of membrane transport are passive. Passive transport is a naturally occurring phenomenon and does not require the cell to exert any of its energy to accomplish the movement. In passive transport, substances move from an area of higher concentration to an area of lower concentration. A physical space in which there is a range of concentrations of a single substance is said to have a concentration gradient    .

Selective permeability

Plasma membranes are asymmetric: the interior of the membrane is not identical to the exterior of the membrane. In fact, there is a considerable difference between the array of phospholipids and proteins between the two leaflets that form a membrane. On the interior of the membrane, some proteins serve to anchor the membrane to fibers of the cytoskeleton. There are peripheral proteins on the exterior of the membrane that bind elements of the extracellular matrix. Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane. These carbohydrate complexes help the cell bind substances that the cell needs in the extracellular fluid. This adds considerably to the selective nature of plasma membranes ( [link] ).

This illustration shows that the inside and outside of a plasma membrane are different.
The exterior surface of the plasma membrane is not identical to the interior surface of the same membrane.

Recall that plasma membranes are amphiphilic: They have hydrophilic and hydrophobic regions. This characteristic helps the movement of some materials through the membrane and hinders the movement of others. Lipid-soluble material with a low molecular weight can easily slip through the hydrophobic lipid core of the membrane. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Fat-soluble drugs and hormones also gain easy entry into cells and are readily transported into the body’s tissues and organs. Molecules of oxygen and carbon dioxide have no charge and so pass through membranes by simple diffusion.

Questions & Answers

What is the Molecular Biology?
Service Reply
The branch of biology that deals with the structure and function of the macromolecules (e.g. proteins and nucleic acids) essential to life.
Sam
what is the difference between mitosis and meiosis?
Fatima Reply
what is an element
Saidu Reply
Element is any one of the simplest chemical substances that cannot be decomposed in a chemical reaction.
Lucky
an element is a substance that cannot be broken down into any other substance
Akosua
explain why only one sperm fertilisers the ovum
Frazzy Reply
its because other sperms are destroyed on the way
Nasib
what is cell
Dau Reply
Cell is the smallest unit of life
Yodit
cell refers of the basic, structural and function unit of an organism
jacob
smallest functional unit of an organism
Nasib
explain for me for ecological factors affecting each of aquatic and terrestrial habitats
Ikeh Reply
Temperature, high humidity
Afriyie
50%
Yodit
temperature,light intensity,Co2
Nasib
what's meosis
Umoke Reply
meiosis this is the type of cell division which reduces the number of chromosomes in order to form a gamete
Caren
meiosis refers to the form of cell division in which a cell divides into four daughter nuclei each containing the same number of chromosomes as the parent cell
Bigenis
yes
Marah
cell division where one cell sprits into 4 daughter cells each having half number of chromosomes as parent cell
Nasib
The end of the linear chromosomes are maintained by
birabwa Reply
Gene
Afriyie
Telomerase
birabwa
is the process of undigested food in body
Enoch Reply
Hi
Afriyie
hello
birabwa
hi
Boame
Hello
kanahiya
What is biology
Hope Reply
What is DNA in full
birabwa
Deoxyribo Nucleic Acid
ibrahim
deoxyibonucleic acid
jacob
Deoxyribonucleic acid
Afriyie
is the study of living things and their interactions with the environment
jacob
is the energy production of studying in life (every living things)
Enoch
de-oxybonuclic acid
Fafali
Biology is the study of life.
Euchy
Biology is the study of plant, animal and it's environment
Eze
biology is the branch of science that deal with plants and living organism
Dominic
what's the difference between bi-enial crops and perennial crops
Johnson Reply
Bi-enial crops are crops that live/exist/can last for as long as 2 years while perennial crops are crops that are evergreen and can last eternity or an inestimable amount of time.
Joanna
what isrespiration
Massa Reply
Mg is very important part of enzymes which deal with DNA. It is a very simple reason but overlooked.
Eric Reply
what mitochondria means
ADEDAYO Reply
its is the most universal organelle among eukaryotes,it is surrounded by double membrane and in it are foldings and invaginationscalled cristae where areobic respiration takes place
Alex
an organelle found in large number in most cell ,in which the biochemical process of respiration and energy production occur
jacob
Mitochondria is the power house of a cell
Joanna
is the energy production in a female hormones
Enoch
how does a cell look like
Clara Reply

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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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