Information

5.4: Bulk Transport - Biology


Skills to Develop

  • Describe endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis
  • Understand the process of exocytosis

In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles (see Table 5.4.1 for examples). Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that the uptake and release of large particles by the cell requires energy. A large particle, however, cannot pass through the membrane, even with energy supplied by the cell.

Endocytosis

Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created intracellular vesicle formed from the plasma membrane.

Phagocytosis

Phagocytosis (the condition of “cell eating”) is the process by which large particles, such as cells or relatively large particles, are taken in by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil (Figure (PageIndex{1})).

In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin, which stabilizes this section of the membrane. The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly formed compartment (endosome). When accessible nutrients from the degradation of the vesicular contents have been extracted, the newly formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane.

Pinocytosis

A variation of endocytosis is called pinocytosis. This literally means “cell drinking” and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid. In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome (Figure (PageIndex{2})).

A variation of pinocytosis is called potocytosis. This process uses a coating protein, called caveolin, on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles or vesicles formed in caveolae (singular caveola) are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.

Receptor-mediated Endocytosis

A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances (Figure (PageIndex{3})).

In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane. If uptake of a compound is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by the failure of receptor-mediated endocytosis. For example, the form of cholesterol termed low-density lipoprotein or LDL (also referred to as “bad” cholesterol) is removed from the blood by receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear LDL particles from their blood.

Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells.

Video (PageIndex{1}): See receptor-mediated endocytosis in action, and click on different parts for a focused animation.

Exocytosis

The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of the processes discussed above in that its purpose is to expel material from the cell into the extracellular fluid. Waste material is enveloped in a membrane and fuses with the interior of the plasma membrane. This fusion opens the membranous envelope on the exterior of the cell, and the waste material is expelled into the extracellular space (Figure (PageIndex{4})). Other examples of cells releasing molecules via exocytosis include the secretion of proteins of the extracellular matrix and secretion of neurotransmitters into the synaptic cleft by synaptic vesicles.

Table (PageIndex{1}): Methods of transport, Energy requirements and types of Material transported.
Transport MethodActive/PassiveMaterial Transported
DiffusionPassiveSmall-molecular weight material
OsmosisPassiveWater
Facilitated transport/diffusionPassiveSodium, potassium, calcium, glucose
Primary active transportActiveSodium, potassium, calcium
Secondary active transportActiveAmino acids, lactose
PhagocytosisActiveLarge macromolecules, whole cells, or cellular structures
Pinocytosis and potocytosisActiveSmall molecules (liquids/water)
Receptor-mediated endocytosisActiveLarge quantities of macromolecules

Summary

Active transport methods require the direct use of ATP to fuel the transport. Large particles, such as macromolecules, parts of cells, or whole cells, can be engulfed by other cells in a process called phagocytosis. In phagocytosis, a portion of the membrane invaginates and flows around the particle, eventually pinching off and leaving the particle entirely enclosed by an envelope of plasma membrane. Vesicle contents are broken down by the cell, with the particles either used as food or dispatched. Pinocytosis is a similar process on a smaller scale. The plasma membrane invaginates and pinches off, producing a small envelope of fluid from outside the cell. Pinocytosis imports substances that the cell needs from the extracellular fluid. The cell expels waste in a similar but reverse manner: it pushes a membranous vacuole to the plasma membrane, allowing the vacuole to fuse with the membrane and incorporate itself into the membrane structure, releasing its contents to the exterior.

Glossary

caveolin
protein that coats the cytoplasmic side of the plasma membrane and participates in the process of liquid update by potocytosis
clathrin
protein that coats the inward-facing surface of the plasma membrane and assists in the formation of specialized structures, like coated pits, for phagocytosis
endocytosis
type of active transport that moves substances, including fluids and particles, into a cell
exocytosis
process of passing bulk material out of a cell
pinocytosis
a variation of endocytosis that imports macromolecules that the cell needs from the extracellular fluid
potocytosis
variation of pinocytosis that uses a different coating protein (caveolin) on the cytoplasmic side of the plasma membrane
receptor-mediated endocytosis
variation of endocytosis that involves the use of specific binding proteins in the plasma membrane for specific molecules or particles, and clathrin-coated pits that become clathrin-coated vesicles

7.4 – Bulk Transport

By the end of this section, you will be able to do the following:

  • Describe endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis
  • Understand the process of exocytosis

In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles (see (Figure) for examples). Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that when a cell uptakes and releases large particles, it requires energy. A large particle, however, cannot pass through the membrane, even with energy that the cell supplies.


28. One type of mutation in the CFTR protein prevents the transport of chloride ions through the channel. Which of the following is most likely to be observed in the lungs of patients with this mutat .

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This text is based on Openstax Biology for AP Courses, Senior Contributing Authors Julianne Zedalis, The Bishop's School in La Jolla, CA, John Eggebrecht, Cornell University Contributing Authors Yael Avissar, Rhode Island College, Jung Choi, Georgia Institute of Technology, Jean DeSaix, University of North Carolina at Chapel Hill, Vladimir Jurukovski, Suffolk County Community College, Connie Rye, East Mississippi Community College, Robert Wise, University of Wisconsin, Oshkosh

This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 Unported License, with no additional restrictions


Exocytosis

Exocytosis is the process by which cells release particles from within the cell into the extracellular space.

Learning Objectives

Describe exocytosis and the processes used to release materials from the cell.

Key Takeaways

Key Points

  • Exocytosis is the opposite of endocytosis as it involves releasing materials from the cell.
  • Exocytosis has five stages, each leading up to the vesicle binding with the cell membrane.
  • Many bodily functions include the use of exocytosis, such as the release of neurotransmitters into the synaptic cleft and the release of enzymes into the blood.

Key Terms

  • secretion: The act of secreting (producing and discharging) a substance, especially from a gland.
  • vesicle: A membrane-bound compartment found in a cell.

Exocytosis

Exocytosis’ main purpose is to expel material from the cell into the extracellular fluid this is the opposite of what occurs in endocytosis. In exocytosis, waste material is enveloped in a membrane and fuses with the interior of the plasma membrane. This fusion opens the membranous envelope on the exterior of the cell and the waste material is expelled into the extracellular space. Exocytosis is used continuously by plant and animal cells to excrete waste from the cells.

Exocytosis: In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents are then released to the exterior of the cell.

Exocytosis is composed of five main stages. The first stage is called vesicle trafficking. This involves the steps required to move, over a significant distance, the vesicle containing the material that is to be disposed. The next stage that occurs is vesicle tethering, which links the vesicle to the cell membrane by biological material at half the diameter of a vesicle. Next, the vesicle’s membrane and the cell membrane connect and are held together in the vesicle docking step. This stage of exocytosis is then followed by vesicle priming, which includes all of the molecular rearrangements and protein and lipid modifications that take place after initial docking. In some cells, there is no priming. The final stage, vesicle fusion, involves the merging of the vesicle membrane with the target membrane. This results in the release of the unwanted materials into the space outside the cell.

Some examples of cells releasing molecules via exocytosis include the secretion of proteins of the extracellular matrix and secretion of neurotransmitters into the synaptic cleft by synaptic vesicles. Some examples of cells using exocytosis include: the secretion of proteins like enzymes, peptide hormones and antibodies from different cells, the flipping of the plasma membrane, the placement of integral membrane proteins(IMPs) or proteins that are attached biologically to the cell, and the recycling of plasma membrane bound receptors(molecules on the cell membrane that intercept signals).


5.4: Bulk Transport - Biology

1 C
2 D
3 D
4 Information for constructing this table can be found on pages 72󈞵.
5 a Information for answering this question can be found on page 77 and in the answer to SAQ 4.5.
b Information for answering this question can be found on page 77 and in the answer to SAQ 4.5.
6 a A phosphate head (of phospholipid)
B fatty acid tail(s) (of phospholipid)
C phospholipid bilayer/membrane
b i hydrophilic
ii hydrophobic
iii hydrophobic
iv hydrophilic
c ions move by diff usion
channel has shape which is specifi c for particular ion
channel is hydrophilic/water-fi lled/allows movement of polar substance
ions move down concentration gradient
d both intrinsic proteins
both have specifi c shape
e channel proteins have a fi xed shape/carrier proteins have a variable shape
f width of C measured in mm
mm converted to μm and μm converted to nm
correct formula used magnifi cation: M = I/A = width of C ÷ 7 accept mm, μm or nm
correct answer in nm

N.B. It could be argued that facilitated diff usion is controllable, because the number of channel proteins in the membrane can aff ect the rate.


9 Description rate of entry of water is rapid at fi rst but slows down gradually
until rate is zero/no further entry of water or water enters until water potential of cell = water potential of pure water = 0 (= equilibrium)
exponential/not linear
rate depends on/proportional to, diff erence in water potential between cell and external
solution [max. 3]

Explanation

water (always) moves from a region of higher water potential to a region of lower water potential
(in this case) by osmosis
through partially permeable cell surface membrane of cell
as cell fi lls with water, cell/protoplast expands and pressure (potential) increases
until water potential of cell = zero/water potential of pure water

cell wall rigid/will not stretch (far), and prevents entry of more water cell is turgid [max. 5]

10 a the greater the concentration diff erence, the greater the rate of transport [1]
b (net) diff usion and facilitated diff usion only occur if there is a concentration, diff erence/gradient, across the membrane
or
at equilibrium/if no concentration diff erence,
there is no, net exchange/transport across membrane/rate of transport, is same in both
directions AW
active transport can occur even if no concentration diff erence
because molecules/ions are being pumped AW [3]
c i active transport [1]
ii active transport depends on a supply of ATP
provided by respiration [2]


Biology 171

By the end of this section, you will be able to do the following:

  • Describe endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis
  • Understand the process of exocytosis

In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles (see (Figure) for examples). Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that when a cell uptakes and releases large particles, it requires energy. A large particle, however, cannot pass through the membrane, even with energy that the cell supplies.

Endocytosis

Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different endocytosis variations, but all share a common characteristic: the cell’s plasma membrane invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle containing itself in a newly created intracellular vesicle formed from the plasma membrane.

Phagocytosis

Phagocytosis (the condition of “cell eating”) is the process by which a cell takes in large particles, such as other cells or relatively large particles. For example, when microorganisms invade the human body, a type of white blood cell, a neutrophil, will remove the invaders through this process, surrounding and engulfing the microorganism, which the neutrophil then destroys ((Figure)).


In preparation for phagocytosis, a portion of the plasma membrane’s inward-facing surface becomes coated with the protein clathrin , which stabilizes this membrane’s section. The membrane’s coated portion then extends from the cell’s body and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for breaking down the material in the newly formed compartment (endosome). When accessible nutrients from the vesicular contents’ degradation have been extracted, the newly formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane.

Pinocytosis

A variation of endocytosis is pinocytosis . This literally means “cell drinking”. Discovered by Warren Lewis in 1929, this American embryologist and cell biologist described a process whereby he assumed that the cell was purposefully taking in extracellular fluid. In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome ((Figure)).


A variation of pinocytosis is potocytosis . This process uses a coating protein, caveolin , on the plasma membrane’s cytoplasmic side, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles or vesicles formed in caveolae (singular caveola) are smaller than those in pinocytosis. Potocytosis brings small molecules into the cell and transports them through the cell for their release on the other side, a process we call transcytosis.

Receptor-mediated Endocytosis

A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances ((Figure)).


In receptor-mediated endocytosis , as in phagocytosis, clathrin attaches to the plasma membrane’s cytoplasmic side. If a compound’s uptake is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. The failure of receptor-mediated endocytosis causes some human diseases. For example, receptor mediated endocytosis removes low density lipoprotein or LDL (or “bad” cholesterol) from the blood. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear LDL particles.

Although receptor-mediated endocytosis is designed to bring specific substances that are normally in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells.

See Receptor-mediated Endocytosis (video) in action, and click on different parts for a focused animation.

Exocytosis

The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of the processes we discussed above in that its purpose is to expel material from the cell into the extracellular fluid. Waste material is enveloped in a membrane and fuses with the plasma membrane’s interior. This fusion opens the membranous envelope on the cell’s exterior, and the waste material expels into the extracellular space ((Figure)). Other examples of cells releasing molecules via exocytosis include extracellular matrix protein secretion and neurotransmitter secretion into the synaptic cleft by synaptic vesicles.


Methods of Transport, Energy Requirements, and Types of Transported Material
Transport Method Active/Passive Material Transported
Diffusion Passive Small-molecular weight material
Osmosis Passive Water
Facilitated transport/diffusion Passive Sodium, potassium, calcium, glucose
Primary active transport Active Sodium, potassium, calcium
Secondary active transport Active Amino acids, lactose
Phagocytosis Active Large macromolecules, whole cells, or cellular structures
Pinocytosis and potocytosis Active Small molecules (liquids/water)
Receptor-mediated endocytosis Active Large quantities of macromolecules

Section Summary

Active transport methods require directly using ATP to fuel the transport. In a process scientists call phagocytosis, other cells can engulf large particles, such as macromolecules, cell parts, or whole cells. In phagocytosis, a portion of the membrane invaginates and flows around the particle, eventually pinching off and leaving the particle entirely enclosed by a plasma membrane’s envelope. The cell breaks down vesicle contents, with the particles either used as food or dispatched. Pinocytosis is a similar process on a smaller scale. The plasma membrane invaginates and pinches off, producing a small envelope of fluid from outside the cell. Pinocytosis imports substances that the cell needs from the extracellular fluid. The cell expels waste in a similar but reverse manner. It pushes a membranous vacuole to the plasma membrane, allowing the vacuole to fuse with the membrane and incorporate itself into the membrane structure, releasing its contents to the exterior.

Free Response

Why is it important that there are different types of proteins in plasma membranes for the transport of materials into and out of a cell?

The proteins allow a cell to select what compound will be transported, meeting the needs of the cell and not bringing in anything else.

Why do ions have a difficult time getting through plasma membranes despite their small size?

Ions are charged, and consequently, they are hydrophilic and cannot associate with the lipid portion of the membrane. Ions must be transported by carrier proteins or ion channels.

Glossary


Biological Membranes #2

Plasma membranes are partially permeable as they allow some but not all substances to pass through them.

  • Very small molecules diffuse through the plasma membrane
  • Some substances dissolve in the lipid layer to pass through
  • Larger substances pass through protein channels or are carried by carrier proteins

Roles of the plasma membrane

  • Separates the cell’s contents from the external environment
  • Regulates transport of materials into and out of the cell
  • May contain specific enzymes involved in metabolic pathways
  • Contains antigens so that the immune system can recognise the cell as being self and not attack it
  • May release chemical signals to other cells and contains receptors for cell communication and signalling (hormone bind to membrane bound receptors)
  • May be the site of chemical reactions

Roles of membranes within cells include:

  • The cristae of mitochondria which provides a large surface area for aerobic respiration
  • The thylakoid of chloroplasts which house chlorophyll and are the site of photosynthesis
  • The plasma membrane of the epithelial cells of the small intestine which contain digestive enzymes that breakdown certain sugars

Fluid mosaic model – theory of cell membrane structure with proteins embedded in a sea of phospholipids

  • Channel proteins – allow ions to mass through
  • Carrier proteins – allow specific molecules across the membrane
  • Glycolipid – lipid/phospholipid with a carbohydrate chain
  • Glycoprotein – protein with a carbohydrate chain
  • Others include: Enzymes, antigens & receptor sites for hormones
  • Cholesterol – regulates fluidity and gives mechanical stability and resists the effect of temperature changes on the membrane
  • Glycocalyx – the hydrophilic area just outside the cell consisting of carbohydrate chains attached to both lipids and proteins

Neuron cell membranes

  • Protein channels and carriers covering the long axon allow the transport of ions to bring the conduction of electrical impulses along their length
  • They have a myelin sheath of flattened cells around them several times to give more membrane layers and to insulate the electrical impulses

Root hair cell membranes

They have many carrier proteins which transport nitrate ions from the soil into the cell as part of the nitrogen cycle.

Cristae of Mitochondria

These contain many electron carriers made of protein and hydrogen ion channels which are associated with ATP synthesis

White Blood Cell Membranes

These contain protein receptors for detection of antigens on foreign cells and pathogens

Diffusion across membranes

Diffusion – movement of molecules from an area of high concentration of that molecule to an area of low concentration across a partially permeable membrane along a concentration gradient. It is passive and does not involve metabolic energy (ATP)

Facilitated diffusion – the movement of molecules from an area of high concentration of that molecule to an area of low concentration across a partially permeable membrane via protein channels or carriers. This still does not require metabolic energy (ATP)

Passive processes only use the kinetic energy of the molecules, not ATP.

When molecules move down their concentration gradient they are still moving randomly but remain evenly dispersed which is called net diffusion. They have reach equilibrium.

Concentration gradient is maintained by the respiring cells using the O2 in animals and the carbon dioxide diffusing into the palisade cell to be used in chloroplasts for photosynthesis and the constant use of these molecules inside the cell maintains a concentration gradient as there is always a higher concentration in the external environment.

Factors the affect the rate of simple diffusion

  • Temperature – as this increases, kinetic energy increases so rate of diffusion increases
  • Diffusion Distance – the thicker the membrane/diffusion distance, the slower the rate of diffusion
  • Surface area – more diffusion can take place of a larger surface area
  • Size of diffusing molecule – smaller molecules/ions diffuse more quickly
  • Concentration gradient – steeper the gradient the faster the diffusion

Neurons have many ion channels at synapses to aid the electrical conductivity between cells.

Epithelial cell membranes always have chloride ion channels as these play a part in regulating mucus composition.

This is the net passage of water molecules down their water potential gradient, across a partially permeable membrane.

Water potential – measure of the tendency of water molecules to diffuse from one region to another

In a solution the solute is dissolved in the solvent. Water molecules can pass directly through the phospholipid bilayer.

If solute molecules dissociate into charged ions water will be attracted to them, as it is a polar molecule.

Water potential

  • Pure water has the highest water potential of 0kPa
  • Solute molecules lower the water potential
  • Water molecules move from a high water potential to a low water potential
  • When water potential is equal on both sides there will be no net movement of osmosis
  • Water with solutes has negative water potential values

Active transport – the movement of substances against their concentration gradient across a cell membrane requiring ATP

Endocytosis – bulk transport of molecules too large to pass through a cell membranes into a cell

Exocytosis – bulk transport of molecules too large to pass through a cell membrane out of a cell

Sodium Potassium pumps

3x Na+ ions are transported in one direction while 2x K+ ions are transported in the opposite direction.

Carrier proteins

ATP allows some carrier proteins to change their conformation to carry the molecule from one side of a gradient to another

Bulk transport

Pinocytosis – cells ingesting liquids

Phagocytosis – cells ingesting solid matter (e.g. WBC ingesting a bacteria)

  1. A membrane bound vesicle, containing substance to be secreted, is moved towards the cell surface
  2. The vesicle fuses to the cell membrane
  3. The fused site splits, releasing the contents of the vesicle to the external environment


Review Questions

What happens to the membrane of a vesicle after exocytosis?

  1. It leaves the cell.
  2. It is disassembled by the cell.
  3. It fuses with and becomes part of the plasma membrane.
  4. It is used again in another exocytosis event.

Which transport mechanism can bring whole cells into a cell?

  1. pinocytosis
  2. phagocytosis
  3. facilitated transport
  4. primary active transport

In what important way does receptor-mediated endocytosis differ from phagocytosis?

  1. It transports only small amounts of fluid.
  2. It does not involve the pinching off of membrane.
  3. It brings in only a specifically targeted substance.
  4. It brings substances into the cell, while phagocytosis removes substances.

Many viruses enter host cells through receptor-mediated endocytosis. What is an advantage of this entry strategy?

  1. The virus directly enters the cytoplasm of the cell.
  2. The virus is protected from recognition by white blood cells.
  3. The virus only enters its target host cell type.
  4. The virus can directly inject its genome into the cell’s nucleus.

Which of the following organelles relies on exocytosis to complete its function?

Imagine a cell can perform exocytosis, but only minimal endocytosis. What would happen to the cell?


A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances (Figure).

In receptor-mediated endocytosis, the cell's uptake of substances targets a single type of substance that binds to the receptor on the cell membrane's external surface. (credit: modification of work by Mariana Ruiz Villareal)

Although receptor-mediated endocytosis is designed to bring specific substances that are normally in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells.

Link to Learning

See receptor-mediated endocytosis in action, and click on different parts for a focused animation.


What Is Bulk Flow?

Bulk flow is a movement of molecules from an area of high pressure to an area of low pressure. In cell biology, it refers to the transport of fluids or electrolytes between cells through openings, or pores, between the cells. Toilets and faucets employ mechanisms that utilize bulk flow, as well as the transport systems found in plants and animals.

Xylem and phloem in plants carry out a function similar to veins and arteries in animals. In the circulatory system, blood flows through the arteries and veins from areas of high pressure to areas of low pressure. Plants also depend on bulk flow to transport materials to various places. Water in the leaves’ xylem evaporates in a process called transpiration, which creates less pressure in the xylem. The water can then flow up through the xylem into the zone of less pressure.

In general, bulk flow in plants is a quicker process than osmosis or diffusion, both of which involve a passive transport of materials from an area of high to low pressure. But diffusion can transfer materials only over short distances, which is a problem for tall plants. Bulk flow can push water from the roots of a plant all the way to its leaves.


Watch the video: Amoeba eats paramecia Amoebas lunch Amoeba Endocytosis. Phagocytosis Part 1 (January 2022).