cellular biology Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living and ...

, ''active transport'' is the movement of molecules or ions across a

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellular energy to achieve this movement. There are two types of active transport: primary active transport that uses

adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known form ...

( ATP), and secondary active transport that uses an electrochemical gradient. Some examples of active transport include: * Phagocytosis of bacteria by macrophages * Movement of calcium ions out of cardiac muscle cells * Transportation of amino acids across the intestinal lining in the human gut * Secretion of proteins such as enzymes, peptide hormones, and antibodies from various cells * Functioning of white blood cells to defend invading diseases

Active cellular transportation (ACT)

Unlike passive transport, which uses the

kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...

and natural

entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...

of molecules moving down a gradient, active transport uses cellular energy to move them against a gradient, polar repulsion, or other resistance. Active transport is usually associated with accumulating high concentrations of molecules that the cell needs, such as

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

and

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...

s. Examples of active transport include the uptake of glucose in the intestines in humans and the uptake of mineral ions into root hair cells of plants. active transport is the movement of molecules across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellular energy to achieve this movement

History

In 1848, the German physiologist

Emil du Bois-Reymond Emil Heinrich du Bois-Reymond (7 November 181826 December 1896) was a German physician and physiologist, the co-discoverer of nerve action potential, and the developer of experimental electrophysiology. Life Du Bois-Reymond was born in Berlin ...

suggested the possibility of active transport of substances across membranes. In 1926,

Dennis Robert Hoagland Dennis Robert Hoagland (April 2, 1884 – September 5, 1949) was an American chemist and plant scientist working in the fields of plant nutrition, agricultural chemistry, and physiology. He was Professor of Plant Nutrition at the University o ...

investigated the ability of

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae ex ...

s to absorb salts against a

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...

gradient and discovered the dependence of

nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

absorption and translocation on metabolic energy using innovative model systems under controlled experimental conditions. Rosenberg (1948) formulated the concept of active transport based on energetic considerations, but later it would be redefined. In 1997,

Jens Christian Skou Jens Christian Skou (; 8 October 1918 – 28 May 2018) was a Danish biochemist and Nobel laureate. Early life Skou was born in Lemvig, Denmark to a wealthy family. His father Magnus Martinus Skou was a timber and coal merchant. His mother Ane- ...

, a Danish

physician A physician (American English), medical practitioner ( Commonwealth English), medical doctor, or simply doctor, is a health professional who practices medicine, which is concerned with promoting, maintaining or restoring health through t ...

"Jens C. Skou - Biographical". Nobelprize.org. Nobel Media AB 2014. Web. 11 Nov 2017 received the

Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...

for his research regarding the sodium-potassium pump. One category of cotransporters that is especially prominent in research regarding

diabetes Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level (hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ap ...

treatment is sodium-glucose cotransporters. These transporters were discovered by scientists at the National Health Institute.Story of Discovery: SGLT2 Inhibitors: Harnessing the Kidneys to Help Treat Diabetes.” National Institute of Diabetes and Digestive and Kidney Diseases, U.S. Department of Health and Human Services, www.niddk.nih.gov/news/research-updates/Pages/story-discovery-SGLT2-inhibitors-harnessing-kidneys-help-treat-diabetes.aspx. These scientists had noticed a discrepancy in the absorption of glucose at different points in the kidney tubule of a rat. The gene was then discovered for intestinal glucose transport protein and linked to these membrane sodium glucose cotransport systems. The first of these membrane transport proteins was named SGLT1 followed by the discovery of SGLT2. Robert Krane also played a prominent role in this field.

Background

Specialized

transmembrane protein A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequentl ...

s recognize the

substance Substance may refer to: * Matter, anything that has mass and takes up space Chemistry * Chemical substance, a material with a definite chemical composition * Drug substance ** Substance abuse, drug-related healthcare and social policy diagnosis ...

and allow it to move across the membrane when it otherwise would not, either because the

phospholipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vi ...

of the membrane is impermeable to the substance moved or because the substance is moved against the direction of its

concentration gradient Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size (mass) of ...

. There are two forms of active transport, primary active transport and secondary active transport. In primary active transport, the proteins involved are pumps that normally use chemical energy in the form of ATP. Secondary active transport, however, makes use of potential energy, which is usually derived through exploitation of an

electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outc ...

gradient. The energy created from one ion moving down its electrochemical gradient is used to power the transport of another ion moving against its electrochemical gradient.Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 15.6
Cotransport by Symporters and Antiporters
This involves pore-forming

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

that form channels across the

. The difference between passive transport and active transport is that the active transport requires energy, and moves substances against their respective concentration gradient, whereas passive transport requires no cellular energy and moves substances in the direction of their respective concentration gradient. In an

antiporter An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma me ...

, one substrate is transported in one direction across the membrane while another is cotransported in the opposite direction. In a

symporter A symporter is an integral membrane protein that is involved in the transport of two (or more) different molecules across the cell membrane in the same direction. The symporter works in the plasma membrane and molecules are transported across the ...

, two substrates are transported in the same direction across the membrane. Antiport and symport processes are associated with secondary active transport, meaning that one of the two substances is transported against its concentration gradient, utilizing the energy derived from the transport of another ion (mostly Na, K or H ions) down its concentration gradient. If substrate molecules are moving from areas of lower concentration to areas of higher concentration (i.e., in the opposite direction as, or ''against'' the concentration gradient), specific transmembrane carrier proteins are required. These proteins have receptors that bind to specific molecules (e.g.,

) and transport them across the cell membrane. Because energy is required in this process, it is known as 'active' transport. Examples of active transport include the transportation of

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

out of the cell and

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...

into the cell by the sodium-potassium pump. Active transport often takes place in the internal lining of the small intestine. Plants need to absorb mineral salts from the soil or other sources, but these salts exist in very dilute solution. Active transport enables these cells to take up salts from this dilute solution against the direction of the

. For example,

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...

(Cl⁻) and

nitrate Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insolubl ...

(NO₃⁻) ions exist in the cytosol of plant cells, and need to be transported into the vacuole. While the vacuole has channels for these ions, transportation of them is against the concentration gradient, and thus movement of these ions is driven by hydrogen pumps, or proton pumps.

Primary active transport

Primary active transport, also called direct active transport, directly uses metabolic energy to transport molecules across a membrane. Substances that are transported across the cell membrane by primary active transport include metal ions, such as Na⁺, K⁺, Mg²⁺, and Ca²⁺. These charged particles require ion pumps or ion channels to cross membranes and distribute through the body. Most of the

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...

s that perform this type of transport are transmembrane

ATPase ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3−-ATPase, adenosine triphosphatase) are ...

s. A primary ATPase universal to all animal life is the sodium-potassium pump, which helps to maintain the cell potential. The sodium-potassium pump maintains the membrane potential by moving three Na⁺ ions out of the cell for every two K⁺ ions moved into the cell. Other sources of energy for primary active transport are

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

energy and

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are Massless particle, massless ...

energy (

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...

). An example of primary active transport using redox energy is the mitochondrial

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples t ...

that uses the reduction energy of NADH to move protons across the inner mitochondrial membrane against their concentration gradient. An example of primary active transport using light energy are the proteins involved in

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

that use the energy of photons to create a proton gradient across the

thylakoid membrane Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thyla ...

and also to create reduction power in the form of NADPH.

Model of active transport

ATP hydrolysis is used to transport hydrogen ions against the electrochemical gradient (from low to high hydrogen ion concentration).

Phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...

of the carrier protein and the binding of a hydrogen ion induce a conformational (shape) change that drives the hydrogen ions to transport against the electrochemical gradient.

Hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...

of the bound phosphate group and release of hydrogen ion then restores the carrier to its original conformation.

Types of primary active transporters

P-type ATPase The P-type ATPases, also known as E1-E2 ATPases, are a large group of evolutionarily related ion and lipid pumps that are found in bacteria, archaea, and eukaryotes. P-type ATPases are α-helical bundle primary transporters named based upon the ...

: sodium potassium pump, calcium pump,

proton pump A proton pump is an integral membrane protein pump that builds up a proton gradient across a biological membrane. Proton pumps catalyze the following reaction: : n one side of a biological membrane/sub> + energy n the other side of the membr ...

# F-ATPase: mitochondrial ATP synthase, chloroplast ATP synthase #

V-ATPase Vacuolar-type ATPase (V-ATPase) is a highly conserved evolutionarily ancient enzyme with remarkably diverse functions in eukaryotic organisms. V-ATPases acidify a wide array of intracellular organelles and pumps protons across the plasm ...

: vacuolar ATPase # ABC ( ATP binding cassette) transporter: MDR, CFTR, etc. Adenosine triphosphate-binding cassette transporters (

ABC transporters The ATP-binding cassette transporters (ABC transporters) are a transport system superfamily that is one of the largest and possibly one of the oldest gene families. It is represented in all extant phyla, from prokaryotes to humans. ABC tran ...

) comprise a large and diverse protein family, often functioning as ATP-driven pumps. Usually, there are several domains involved in the overall transporter protein's structure, including two nucleotide-binding domains that constitute the ATP-binding motif and two hydrophobic transmembrane domains that create the "pore" component. In broad terms, ABC transporters are involved in the import or export of molecules across a cell membrane; yet within the protein family there is an extensive range of function. In plants, ABC transporters are often found within cell and organelle membranes, such as the mitochondria, chloroplast, and plasma membrane. There is evidence to support that plant ABC transporters play a direct role in pathogen response, phytohormone transport, and detoxification. Furthermore, certain plant ABC transporters may function in actively exporting volatile compounds and antimicrobial metabolites. In petunia flowers (''Petunia hybrida''), the ABC transporter PhABCG1 is involved in the active transport of volatile organic compounds. PhABCG1 is expressed in the petals of open flowers. In general, volatile compounds may promote the attraction of seed-dispersal organisms and pollinators, as well as aid in defense, signaling, allelopathy, and protection. To study the protein PhABCG1, transgenic petunia RNA interference lines were created with decreased ''PhABCG1'' expression levels. In these transgenic lines, a decrease in emission of volatile compounds was observed. Thus, PhABCG1 is likely involved in the export of volatile compounds. Subsequent experiments involved incubating control and transgenic lines that expressed ''PhABCG1'' to test for transport activity involving different substrates. Ultimately, PhABCG1 is responsible for the protein-mediated transport of volatile organic compounds, such as benzyl alcohol and methylbenzoate, across the plasma membrane. Additionally in plants, ABC transporters may be involved in the transport of cellular metabolites. Pleiotropic Drug Resistance ABC transporters are hypothesized to be involved in stress response and export antimicrobial metabolites. One example of this type of ABC transporter is the protein NtPDR1. This unique ABC transporter is found in ''Nicotiana tabacum'' BY2 cells and is expressed in the presence of microbial elicitors. NtPDR1 is localized in the root epidermis and aerial trichomes of the plant. Experiments using antibodies specifically targeting NtPDR1 followed by Western blotting allowed for this determination of localization. Furthermore, it is likely that the protein NtPDR1 actively transports out antimicrobial diterpene molecules, which are toxic to the cell at high levels.

Secondary active transport

In secondary active transport, also known as cotransport or coupled transport, energy is used to transport molecules across a membrane; however, in contrast to

primary active transport In cellular biology, ''active transport'' is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellu ...

, there is no direct coupling of ATP. Instead, it relies upon the electrochemical potential difference created by pumping ions in/out of the cell. Permitting one ion or molecule to move down an electrochemical gradient, but possibly against the concentration gradient where it is more concentrated to that where it is less concentrated, increases

and can serve as a source of

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of h ...

for

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...

(e.g. in

ATP synthase ATP synthase is a protein that catalyzes the formation of the energy storage molecule adenosine triphosphate (ATP) using adenosine diphosphate (ADP) and inorganic phosphate (Pi). It is classified under ligases as it changes ADP by the formation ...

). The energy derived from the pumping of protons across a cell membrane is frequently used as the energy source in secondary active transport. In humans, sodium (Na⁺) is a commonly cotransported ion across the plasma membrane, whose electrochemical gradient is then used to power the active transport of a second ion or molecule against its gradient.Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002
Carrier Proteins and Active Membrane Transport
In bacteria and small yeast cells, a commonly cotransported ion is hydrogen. Hydrogen pumps are also used to create an electrochemical gradient to carry out processes within cells such as in the

, an important function of

cellular respiration Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...

that happens in the

mitochondrion A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used t ...

of the cell. In August 1960, in Prague, Robert K. Crane presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption. Crane's discovery of cotransport was the first ever proposal of flux coupling in biology.

Cotransporter Cotransporters are a subcategory of membrane transport proteins (transporters) that couple the favorable movement of one molecule with its concentration gradient and unfavorable movement of another molecule against its concentration gradient. They ...

s can be classified as

s and

s depending on whether the substances move in the same or opposite directions.

Antiporter

In an antiporter two species of ions or other solutes are pumped in opposite directions across a membrane. One of these species is allowed to flow from high to low concentration, which yields the entropic energy to drive the transport of the other solute from a low concentration region to a high one. An example is the

sodium-calcium exchanger The sodium-calcium exchanger (often denoted Na+/Ca2+ exchanger, exchange protein, or NCX) is an antiporter membrane protein that removes calcium from cells. It uses the energy that is stored in the electrochemical gradient of sodium (Na+) by al ...

, which allows three sodium ions into the cell to transport one calcium out. This antiporter mechanism is important within the membranes of cardiac muscle cells in order to keep the calcium concentration in the cytoplasm low. Many cells also possess

calcium ATPase Ca2+ ATPase is a form of P-ATPase that transfers calcium after a muscle has contracted. The two kinds of calcium ATPase are: *Plasma membrane Ca2+ ATPase (PMCA) *Sarcoplasmic reticulum Ca2+ ATPase (SERCA) Plasma membrane Ca2+ ATPase (PMCA) Pl ...

s, which can operate at lower intracellular concentrations of calcium and sets the normal or resting concentration of this important second messenger. But the ATPase exports calcium ions more slowly: only 30 per second versus 2000 per second by the exchanger. The exchanger comes into service when the calcium concentration rises steeply or "spikes" and enables rapid recovery. This shows that a single type of ion can be transported by several enzymes, which need not be active all the time (constitutively), but may exist to meet specific, intermittent needs.

Symporter

uses the downhill movement of one solute species from high to low concentration to move another molecule uphill from low concentration to high concentration (against its

). Both molecules are transported in the same direction. An example is the glucose symporter SGLT1, which co-transports one

(or

galactose Galactose (, '' galacto-'' + ''-ose'', "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molec ...

) molecule into the cell for every two sodium ions it imports into the cell. This

is located in the small intestines, heart, and brain. It is also located in the S3 segment of the

proximal tubule The proximal tubule is the segment of the nephron in kidneys which begins from the renal pole of the Bowman's capsule to the beginning of loop of Henle. It can be further classified into the proximal convoluted tubule (PCT) and the proximal stra ...

in each

nephron The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure c ...

in the

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...

s. Its mechanism is exploited in glucose rehydration therapy This mechanism uses the absorption of sugar through the walls of the intestine to pull water in along with it. Defects in SGLT2 prevent effective reabsorption of glucose, causing familial renal glucosuria.

Bulk transport

Endocytosis and

exocytosis Exocytosis () is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell ('' exo-'' + '' cytosis''). As an active transport mechanism, exocytosis requires the use ...

are both forms of

bulk transport Bulk cargo is commodity cargo that is transported unpackaged in large quantities. Description Bulk cargo refers to material in either liquid or granular, particulate form, as a mass of relatively small solids, such as petroleum/crude oil, ...

that move materials into and out of cells, respectively, via vesicles. In the case of endocytosis, the cellular membrane folds around the desired materials outside the cell. The ingested particle becomes trapped within a pouch, known as a vesicle, inside the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. T ...

. Often enzymes from

lysosome A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane p ...

s are then used to digest the molecules absorbed by this process. Substances that enter the cell via signal mediated electrolysis include proteins, hormones and growth and stabilization factors. Viruses enter cells through a form of endocytosis that involves their outer membrane fusing with the membrane of the cell. This forces the viral DNA into the host cell. Biologists distinguish two main types of endocytosis:

pinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small molecules dissolved in extracellular fluid are brought into the cell through an invagination of the cell ...

and

phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

. * In pinocytosis, cells engulf liquid particles (in humans this process occurs in the small intestine, where cells engulf fat droplets). * In phagocytosis, cells engulf solid particles.Phagocytosis
Courses.washington.edu. Retrieved on 2011-12-05. Exocytosis involves the removal of substances through the fusion of the outer cell membrane and a vesicle membrane. An example of exocytosis would be the transmission of neurotransmitters across a synapse between brain cells.

References

Notes

External links

*
Secondary Active Transport
{{Authority control Membrane biology Biological matter de:Membrantransport#Aktiver Transport