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Passive transport

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Passive transport

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Passive diffusion across a cell membrane.

Passive transport is a type of membrane transport that does not require energy to move substances across cell membranes.[1][2] Instead of using cellular energy, like active transport,[3] passive transport relies on the second law of thermodynamics to drive the movement of substances across cell membranes.[1][2][4] Fundamentally, substances follow Fick’s first law, and move from an area of high concentration to one of low concentration because this movement increases the entropy of the overall system.[4][5] The rate of passive transport depends on the permeability of the cell membrane, which, in turn, depends on the organization and characteristics of the membrane lipids and proteins. The four main kinds of passive transport are simple diffusion, facilitated diffusion, filtration, and/or osmosis.

Diffusion

Main article: Diffusion
Passive diffusion on a cell membrane.

Diffusion is the net movement of material from an area of high concentration to an area with lower concentration. The difference of concentration between the two areas is often termed as the concentration gradient, and diffusion will continue until this gradient has been eliminated. Since diffusion moves materials from an area of higher concentration to an area of lower concentration, it is described as moving solutes “down the concentration gradient” (compared with active transport, which often moves material from area of low concentration to area of higher concentration, and therefore referred to as moving the material “against the concentration gradient”).
However, in many cases (e.g. passive drug transport) the driving force of passive transport can not be simplified to the concentration gradient. If there are different solutions at the two sides of the membrane with different equilibrium solubility of the drug, the difference in the degree of saturation is the driving force of passive membrane transport.[6] It is also true for supersaturated solutions which are more and more important owing to the spreading of the application of amorphous solid dispersions for drug bioavailability enhancement.

Simple diffusion and osmosis are in some ways similar. Simple diffusion is the passive movement of solute from a high concentration to a lower concentration until the concentration of the solute is uniform throughout and reaches equilibrium. Osmosis is much like simple diffusion but it specifically describes the movement of water (not the solute) across a selectively permeable membrane until there is an equal concentration of water and solute on both sides of the membrane. Simple diffusion and osmosis are both forms of passive transport and require none of the cell’s ATP energy.

Example of diffusion: Gas Exchange

A biological example of diffusion is the gas exchange that occurs during respiration within the human body.[7] Upon inhalation, oxygen is brought into the lungs and quickly diffuses across the membrane of alveoli and enters the circulatory system by diffusing across the membrane of the pulmonary capillaries.[8] Simultaneously, carbon dioxide moves in the opposite direction, diffusing across the membrane of the capillaries and entering into the alveoli, where it can be exhaled. The process of moving oxygen into the cells, and carbon dioxide out, occurs because of the concentration gradient of these substances, each moving away from their respective areas of higher concentration toward areas of lower concentration.[7][8]Cellular respiration is the cause of the low concentration of oxygen and high concentration of carbon dioxide within the blood which creates the concentration gradient. Because the gasses are small and uncharged, they are able to pass directly through the cell membrane without any special membrane proteins.[9] No energy is required because the movement of the gasses follows Fick’s first law and the second law of thermodynamics.

Facilitated diffusion

Main article: Facilitated diffusion
Depiction of facilitated diffusion.

Facilitated diffusion, also called carrier-mediated osmosis, is the movement of molecules across the cell membrane via special transport proteins that are embedded in the plasma membrane by actively taking up or excluding ions. Active transport of protons by H+ ATPases[10] alters membrane potential allowing for facilitated passive transport of particular ions such as potassium [11] down their charge gradient through high affinity transporters and channels.

Example of facilitated diffusion: GLUT2

An example of facilitated diffusion is when glucose is absorbed into cells through Glucose transporter 2 (GLUT2) in the human body.[12][13] There are many other types of glucose transport proteins, some that do require energy, and are therefore not examples of passive transport.[13] Since glucose is a large molecule, it requires a specific channel to facilitate its entry across plasma membranes and into cells.[13] When diffusing into a cell through GLUT2, the driving force that moves glucose into the cell is still the concentration gradient.[12] The main difference between simple diffusion and facilitated diffusion is that facilitated diffusion requires a transport protein to ‘facilitate’ or assist the substance through the membrane.[14] After a meal, the cell is signaled to move GLUT2 into membranes of the cells lining the intestines called enterocytes.[12] With GLUT2 in place after a meal and the relative high concentration of glucose outside of these cells as compared to within them, the concentration gradient drives glucose across the cell membrane through GLUT2.[12][13]

Filtration

Main articles: Filtration and Ultrafiltration (renal)
Filtration.

Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system. Depending on the size of the membrane pores, only solutes of a certain size may pass through it. For example, the membrane pores of the Bowman’s capsule in the kidneys are very small, and only albumins, the smallest of the proteins, have any chance of being filtered through. On the other hand, the membrane pores of liver cells are extremely large, but not forgetting cells are extremely small to allow a variety of solutes to pass through and be metabolized.

Osmosis

Main articles: Osmosis and Tonicity
Effect of osmosis on blood cells under different solutions.

Osmosis is the movement of water molecules across a selectively permeable membrane. The net movement of water molecules through a partially permeable membrane from a solution of high water potential to an area of low water potential. A cell with a less negative water potential will draw in water but this depends on other factors as well such as solute potential (pressure in the cell e.g. solute molecules) and pressure potential (external pressure e.g. cell wall). There are three types of Osmosis solutions: the isotonic solution, hypotonic solution, and hypertonic solution. Isotonic solution is when the extracellular solute concentration is balanced with the concentration inside the cell. In the Isotonic solution, the water molecules still moves between the solutions, but the rates are the same from both directions, thus the water movement is balanced between the inside of the cell as well as the outside of the cell. A hypotonic solution is when the solute concentration outside the cell is lower than the concentration inside the cell. In hypotonic solutions, the water moves into the cell, down its concentration gradient (from higher to lower water concentrations). That can cause the cell to swell. Cells that don’t have a cell wall, such as animal cells, could burst in this solution. A hypertonic solution is when the solute concentration is higher (think of hyper – as high) than the concentration inside the cell. In hypertonic solution, the water will move out, causing the cell to shrink.

See also

Active transport
Transport phenomena

References

^ a b .mw-parser-output cite.citation{font-style:inherit}.mw-parser-output .citation q{quotes:”\”””\”””‘””‘”}.mw-parser-output .id-lock-free a,.mw-parser-output .citation .cs1-lock-free a{background:linear-gradient(transparent,transparent),url(“//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg”)right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited a,.mw-parser-output .id-lock-registration a,.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration a{background:linear-gradient(transparent,transparent),url(“//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg”)right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription a,.mw-parser-output .citation .cs1-lock-subscription a{background:linear-gradient(transparent,transparent),url(“//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg”)right 0.1em center/9px no-repeat}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-ws-icon a{background:linear-gradient(transparent,transparent),url(“//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg”)right 0.1em center/12px no-repeat}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-maint{display:none;color:#33aa33;margin-left:0.3em}.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}”5.2 Passive Transport – Biology 2e | OpenStax”. openstax.org. Retrieved 2020-12-06.

^ a b “5.2A: The Role of Passive Transport”. Biology LibreTexts. 2018-07-10. Retrieved 2020-12-06.

^ “5.3 Active Transport – Biology 2e | OpenStax”. openstax.org. Retrieved 2020-12-06.

^ a b Skene, Keith R. (2015). “Life’s a Gas: A Thermodynamic Theory of Biological Evolution”. Entropy. 17 (8): 5522–5548. Bibcode:2015Entrp..17.5522S. doi:10.3390/e17085522.

^ “12.7 Molecular Transport Phenomena: Diffusion, Osmosis, and Related Processes – College Physics for AP® Courses | OpenStax”. openstax.org. Retrieved 2020-12-06.

^ Borbas, E.; et al. (2016). “Investigation and Mathematical Description of the Real Driving Force of Passive Transport of Drug Molecules from Supersaturated Solutions”. Molecular Pharmaceutics. 13 (11): 3816–3826. doi:10.1021/acs.molpharmaceut.6b00613. PMID 27611057.

^ a b Wagner, Peter D. (2015-01-01). “The physiological basis of pulmonary gas exchange: implications for clinical interpretation of arterial blood gases”. European Respiratory Journal. 45 (1): 227–243. doi:10.1183/09031936.00039214. ISSN 0903-1936. PMID 25323225.

^ a b “22.4 Gas Exchange – Anatomy and Physiology | OpenStax”. openstax.org. Retrieved 2020-12-06.

^ “3.1 The Cell Membrane – Anatomy and Physiology | OpenStax”. openstax.org. Retrieved 2020-12-06.

^ Palmgren, Michael G. (2001-01-01). “PLANT PLASMA MEMBRANE H+-ATPases: Powerhouses for Nutrient Uptake”. Annual Review of Plant Physiology and Plant Molecular Biology. 52 (1): 817–845. doi:10.1146/annurev.arplant.52.1.817. PMID 11337417.

^ Dreyer, Ingo; Uozumi, Nobuyuki (2011-11-01). “Potassium channels in plant cells”. FEBS Journal. 278 (22): 4293–4303. doi:10.1111/j.1742-4658.2011.08371.x. ISSN 1742-4658. PMID 21955642. S2CID 12814450.

^ a b c d Kellett, George L.; Brot-Laroche, Edith; Mace, Oliver J.; Leturque, Armelle (2008). “Sugar absorption in the intestine: the role of GLUT2”. Annual Review of Nutrition. 28: 35–54. doi:10.1146/annurev.nutr.28.061807.155518. ISSN 0199-9885. PMID 18393659.

^ a b c d Chen, Lihong; Tuo, Biguang; Dong, Hui (2016-01-14). “Regulation of Intestinal Glucose Absorption by Ion Channels and Transporters”. Nutrients. 8 (1): 43. doi:10.3390/nu8010043. ISSN 2072-6643. PMC 4728656. PMID 26784222.

^ Cooper, Geoffrey M. (2000). “Transport of Small Molecules”. The Cell: A Molecular Approach. 2nd Edition.

Alcamo, I. Edward (1997). “Chapter 2–5: Passive transport”. Biology coloring workbook. Illustrations by John Bergdahl. New York: Random House. pp. 24–25. ISBN 9780679778844.
Sadava, David; H. Craig Heller; Gordon H. Orians; William K. Purves; David M. Hillis (2007). “What are the passive processes of membrane transport?”. Life : the science of biology (8th ed.). Sunderland, MA: Sinauer Associates. pp. 105–110. ISBN 9780716776710.
Srivastava, P. K. (2005). Elementary biophysics : an introduction. Harrow: Alpha Science Internat. pp. 140–148. ISBN 9781842651933.
vteMembrane transportMechanisms for chemical transport through biological membranesPassive transport
Simple diffusion (or non-mediated transport)
Facilitated diffusion
Osmosis
Channels
CarriersActive transport
Uniporter
Symporter
Antiporter
Primary active transport
Secondary active transportCytosisEndocytosis
Efferocytosis
Non-specific, adsorptive pinocytosis
Phagocytosis
Pinocytosis
Potocytosis
Receptor-mediated endocytosis
TranscytosisExocytosisDegranulation

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Passive membrane transport processes include _.

Passive membrane transport processes include _. – Passive membrane processes are those that do not require energy and move substances down a concentration gradient; namely, these are diffusion, facilitated diffusion and osmosis. Passive membrane transport include the movement of a substance down its concentration gradient.Considering the cell membrane itself, processes include diffusion, osmosis, facilitated diffusion (passive processes that do not require Osmosis is a type of passive transport. It is specialized diffusion. It can occur through the membrane itself. Or through an aquaporin channel protein.While passive transport is the simple option for moving molecules across the membrane, active transport is no less essential to cell function and Of the two types of movement across cellular membranes, passive transport is certainly the easier option. Utilizing the process of diffusion…

Passive membrane transport processes include? – Answers – Because passive transport does not need ATP (cell energy) to operate. So a, and b are correct. Option a) is the definition of passive transport. This Site Might Help You. RE: Passive membrane transport processes include:? a) movement of a substance down its concentration gradient.Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration). Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis). There are three main types of passive…College Cell Structure and Function. Passive membrane transport processes include . movement of a substance down its concentration gradient. consumption of ATP. the use of transport proteins when moving substances from areas of low concentration to high concentration.

Passive membrane transport processes include? - Answers

Active Vs Passive Transport In The Plasma Membrane – The cards are meant to be seen as a digital flashcard as they appear double sided, or rather hide the answer giving you the opportunity to think about the question at hand and answer it in your head or on a sheet before revealing the correct answer to yourself or studying partner. Some questions will include…Passive transport is independent of membrane proteins and the catabolism of biological molecules for energy. Diffusion is a passive process of transport. A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across space.The role of membrane transport processes is to deliver essential nutrients to cells, to As we shall see, both active and passive processes are involved in the transport of substances across cell The protein portion is diverse and includes structural proteins, transporters, enzymes, hormone…

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Active Transport vs Passive Transport – .

Cell Transport – تعليق هي على! إيقاف بالنقر على "CC" في أسفل اليمين.
تابعونا على تويتر (AmoebaSisters) والفيسبوك! هل سبق لك أن تساءلت عن ما يجب أن يكون مثل
أن يكون داخل الخلية؟ تخيل المادة الوراثية، السيتوبلازم، والريبوسومات — سوف تجد
أنه في ALL تقريبا خلايا بدائيات النوى و—- حقيقيات النواة. خلايا يوكاريوت بالإضافة لها الغشاء العضيات ملزمة. كل تلك العضيات لديها وظائف مختلفة. ولكن الخلايا ليست عوالم صغيرة معزولة. لديهم الكثير من الذهاب في داخلها، لكنها تتفاعل أيضا مع
بيئتهم. فمن المنطقي أن الحفاظ على بيئة مستقرة
داخلهم — والمعروف باسم التوازن — هم يجب أن يكون بعض السيطرة على ما يجري في و
للخروج منها. هيكل مهم جدا ل ذلك أن تحتوي جميع الخلايا هو غشاء الخلية.
من خلال التحكم في ما يجري داخل وخارج، الخلية غشاء يساعد على تنظيم التوازن. دعونا نلقي نظرة على غشاء الخلية.
هل يمكن أن يكون بالطبع على غشاء الخلية نفسها — لديها بنية مذهلة والإشارات
قدرات. ولكن التمسك أساسيات جدا، فإنه مصنوع من طبقة ثنائية فوسفورية. طبقة ثنائية
يعني طبقتين، ولذلك عليك أن هاتين الطبقتين الدهون. جزء منهم — الرأس هو القطبي.
الجزء الذيل اقطبي. بعض جزيئات ليس لديهم مشكلة يمر بها
غشاء الخلية وتذهب مباشرة من خلال طبقة ثنائية فوسفورية. صغيرة جدا غير القطبية
جزيئات تناسب في هذه الفئة وهي كبيرة مثال. مثل بعض الغازات. الأكسجين والكربون
غاز ثاني أكسيد أمثلة كبيرة. هذا هو المعروف نشر بهذه البساطة. أيضا، لأنها لا تأخذ
أي طاقة لإجبار هذه الجزيئات أو من ذلك وهو ما يعرف النقل السلبي.
يتحرك الانتشار البسيط مع تدفق. المعنى، وهو يتحرك مع التدرج التركيز.
الجزيئات تتحرك من تركيز عال لل تركيز منخفض. هذا هو الطبيعي
الجزيئات الطريق مثل لنقل — من الأعلى إلى منخفضة — حتى عندما تسمع أحدهم يقول انها
الذهاب مع التدرج بعد ذلك أن ما وهم يقصدون بذلك. نتذكر كيف قلنا غشاء الخلية
في الواقع بنية معقدة جدا؟ حسنا، الشيء الوحيد الذي لم يذكر حتى الان هي البروتينات
في الغشاء، وبعضها النقل البروتينات. بعض البروتينات النقل بمثابة القنوات.
بعض هذه البروتينات تغيير الواقع على تشكيل لتحصل على البنود في جميع أنحاء. بعض منهم مفتوحة
وعلى أساس وثيقة على التحفيز من نوع ما. وهذه هي الأشياء الجيدة، لأنها
مساعدة مع الجزيئات التي قد تكون كبيرة جدا عبور الغشاء من تلقاء نفسها أو جزيئات
التي هي القطبية — وبالتالي بحاجة إلى مساعدة من بروتين النقل. وهذا ما يعرف سهلت
تعريف. انها لا تزال نشرها، وذلك لا يزال يتحرك مع التدرج تركيز
من الأعلى إلى الأقل. أنها لا تتطلب طاقة لذلك هو نوع من الرياضة تران السلبي. انها
مجرد أن البروتينات هي تسهيل، أو مساعدة، الأمور تمر. وغالبا ما تتطلب الأيونات
قناة البروتين من أجل بالمرور. الجلوكوز يحتاج مساعدة من البروتين النقل
بالمرور. في التناضح، عن الماء ل السفر بمعدل سريع عبر الغشاء،
أنها تمر عبر قنوات بروتين يسمى aquaporins. هذه كلها أمثلة على سهلت
نشر، وهو نوع من النقل السلبي ويتحرك مع التدرج تركيز
من الأعلى إلى تركيز منخفض. الآن كل وسائل النقل ذكرنا له
كانت سلبية في الطبيعة، وهذا يعني انها الانتقال من التركيز العالي لتركيز منخفض.
ولكن ماذا لو كنت تريد أن تذهب في الاتجاه الآخر؟ على سبيل المثال، فإن الخلايا المبطنة الحاجة أمعائك
لتأخذ في الجلوكوز. ولكن ما إذا كان تركيز من الجلوكوز في الخلية أعلى من
بيئة؟ نحن بحاجة للحصول على الجلوكوز في وانها ستكون لدينا ليضطر ضد
تدفق التدرج المنتظم. حركة الجزيئات من الأقل إلى تركيز عال يأخذ الطاقة
لأن هذا ضد التيار. عادة ATP الطاقة. للتذكير فقط أن ATP أدينوسين —
ثلاثي — فقد 3 الفوسفات. عندما يتم تقسيم السندات للفوسفات الماضي،
يطلق كمية كبيرة من الطاقة. انها جزيء صغير رائع جدا. ATP يمكن
السلطة النقل النشط لإجبار تلك الجزيئات للذهاب ضد التدرج تركيزهم،
وطريقة واحدة يمكن أن تفعل ذلك هو تنشيط الواقع البروتين النقل نفسها. واحدة من المفضلة لدينا
أمثلة على النقل النشط هو الصوديوم والبوتاسيوم مضخة ذلك وهذا بالتأكيد شيء يستحق
التحقق من! –
هناك أوقات أخرى تحتاج الخلية إلى ممارسة طاقة للنقل – ونحن ما زلنا في
النقل النشط في الوقت الراهن. ولكن دعونا نقول خلية تحتاج جزيء كبير جدا — دعونا
أقول السكاريد كبير (إذا كنت تحقق من لدينا شريط فيديو جزيء حيوي، وهذا هو الكربوهيدرات كبيرة) — جيدا
قد تحتاج غشاء الخلية لتندمج مع جزيئات انه أخذ في لجعله
في داخل. وهذا ما يسمى الإلتقام — التفكير إندو ل "في" في كثير من الأحيان، وهذا التفجير من المواد
مع غشاء الخلية وتكوين الحويصلات التي يمكن اتخاذها داخل الخلية. الإلتقام
هو مصطلح عام، ولكن هناك الفعلية مختلفة أنواع الإلتقام اعتمادا على كيفية
خلية تجلب المواد في الداخل. الأميبا على سبيل المثال تعتمد على شكل الإلتقام. Pseudopods تمتد في انحاء مختلفة
ما تريد أن تبتلع وبعد ذلك يحصل سحبت في فجوة. هناك أشكال أخرى
أيضا مثل يتوهم الإلتقام مستقبلات بوساطة — حيث خلايا يمكن أن يكون جدا، جدا، من الصعب إرضاءه جدا حول
ما سيأتي في ذلك لأن المواد الواردة فعلا لربط المستقبلات حتى
الحصول على، أو الإحتساء — الذي يسمح لل خلية لتأخذ في السوائل. حتى لجوجل ل
معرفة المزيد من التفاصيل عن أنواع مختلفة الإلتقام. إيماس هو الاتجاه العكسي من الإلتقام،
لذلك هذا هو عندما جزيئات خروج — يعتقدون إكسو والخروج. ويمكن أيضا أن تستخدم إيماس للحصول على
التخلص من النفايات الخلية ولكن من المهم أيضا حقا للحصول على المواد الهامة إلى أن
جعلت الخلية. تريد مثال رائع؟ تفكير العودة إلى تلك السكريات — هل تعلم
أن الكربوهيدرات كبيرة هي أيضا ذات أهمية كبيرة لصنع جدران الخلايا النباتية؟ جدران الخلايا هي
مختلفة من أغشية الخلايا —- كل الخلايا لدينا الأغشية ولكن ليس كل الخلايا لديها جدار.
ولكن إذا كنت تسير على جعل جدار الخلية، وأنت تسير في حاجة للحصول على تلك الكربوهيدرات
التي يتم إنتاجها في الخلية النباتية من الخلية لجعل الجدار. لذا فإن هناك
مثال عظيم على عندما كنت بحاجة إيماس هناك مباشرة. حسنا هذا كل شيء للأخوات الاميبا و
نذكر لك بالبقاء غريبة! تابعونا على تويتر (AmoebaSisters) والفيسبوك! .

Active, Passive, and Bulk Cell Transport – The plasma membrane regulates the passage of molecules into and out of the cell.
It is capable of carrying out this function because it is selectively permeable, meaning that it allows certain substances to pass while preventing others. Basically, substances enter the cell in one of three ways: passive transport, active transport or bulk transport. Let’s look at each of these in turn. Passive transport is the movement of substances into or out of a cell without the expenditure of energy by the cell. One form of passive transport is diffusion. During diffusion, molecules move across a membrane from an area of high concentration to an area of low concentration. The molecules are therefore said to be moving down a concentration gradient. This continues until equilibrium is reached and the molecules are distributed equally. Another form of passive transport is facilitated diffusion. Facilitated diffusion occurs when an ion or molecule diffuses across a membrane faster than expected, either by way of a specific channel protein or with the assistance of carrier proteins that change shape as they pass through. The diffusion of water across a membrane, or osmosis, is another example of passive transport. In many cases, specialized proteins called aquaporins allow for the more rapid transport of water molecules. The second type of transport, active transport, requires the input of energy in the form of ATP. The proteins that conduct this form of transport are often called pumps, because they force molecules or ions to move from an area of low concentration to an area of high concentration. This is commonly referred to as up, or against, the concentration gradient. One of the more common examples is the sodium-potassium pump, which moves sodium ions back out of the cell – and potassium ions into the cell. Notice that the sodium potassium pump undergoes a change in shape that allows it to combine alternately with sodium ions and potassium ions. The third type of transport, bulk transport, is used for molecules that are too large to be moved by transport proteins. Instead, vesicles take them into or out of the cell. During this process, the plasma membrane surrounds and engulfs the particle. This is known as endocytosis. Cells use three basic types of endocytosis depending on the size and nature of the material to be digested: phagocytosis, pinocytosis and receptor-mediated endocytosis. If the material taken in is large, such as bacteria or a food particle, the process is called phagocytosis. Pinocytosis occurs when vesicles form around a liquid, or very small particles. During receptor-mediated endocytosis, molecules bind to specific receptor proteins embedded in a coated pit within the plasma membrane. When enough molecules accumulate in the coated pit, the pit deepens, seals, and is incorporated into the cell as a vesicle. Exocytosis is the opposite of endocytosis. During exocytosis, membrane bound vesicles move to the surface of the plasma membrane, fuse with the membrane, and then release their contents to the outside of the cell. To recap, there are three main types of cell transport: passive transport, active transport and bulk transport. Passive transport is the movement of substances in or out of a cell without the expenditure of energy by the cell. Active transport requires energy in the form of ATP to move molecules against their concentration gradient. Bulk transport requires vesicle formation and metabolic energy. Forms of bulk transport include endocytosis, pinocytosis, receptor-mediated endocytosis, and exocytosis. .