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4.4A: Vesicles and Vacuoles - Biology

4.4A: Vesicles and Vacuoles - Biology



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Vesicles and vacuoles are membrane-bound sacs that function in storage and transport.

Learning Objectives

  • Summarize the functions of vesicles and vacuoles in cells

Key Points

  • Vesicles are small structures within a cell, consisting of fluid enclosed by a lipid bilayer involved in transport, buoyancy control, and enzyme storage.
  • Lysosomes, which are found in animal cells, are the cell’s “garbage disposal.” The digestive processes take place in these, and enzymes within them aid in the breakdown of proteins, polysaccharides, lipids, nucleic acids, and worn-out organelles.
  • Central vacuoles, which are found in plants, play a key role in regulating the cell’s concentration of water in changing environmental conditions.

Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them: the membranes of vesicles can fuse with either the plasma membrane or other membrane systems within the cell. The membrane of a vacuole does not fuse with the membranes of other cellular components. Additionally, some agents within plant vacuoles, such as enzymes, break down macromolecules.

Vesicles

A vesicle is a small structure within a cell, consisting of fluid enclosed by a lipid bilayer. Vesicles form naturally during the processes of secretion (exocytosis), uptake (phagocytosis) and transport of materials within the cytoplasm. Alternatively, they may be prepared artificially, in which case they are called liposomes. Vesicles can fuse with the plasma membrane to release their contents outside the cell. Vesicles can also fuse with other organelles within the cell.

Vesicles perform a variety of functions. Because they are separated from the cytosol, the inside of a vesicle can be different from the cytosolic environment. For this reason, vesicles are a basic tool used by the cell for organizing cellular substances. Vesicles are involved in metabolism, transport, buoyancy control, and enzyme storage. They can also act as chemical reaction chambers.

Lysosomes

Animal cells have a set of organelles not found in plant cells: lysosomes. Lysosomes are a cell’s “garbage disposal.” Enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and worn-out organelles. These enzymes are active at a much lower pH than that of the cytoplasm. Therefore, the pH within lysosomes is more acidic than the pH of the cytoplasm. Many reactions that take place in the cytoplasm could not occur at a low pH, so again, the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.

Vacuoles

Vacuoles are an essential component of plant cells. If you look at the figure below, you will see that plant cells each have a large central vacuole that occupies most of the area of the cell. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions, and houses the digestive processes.

Have you ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant.

The central vacuole also supports the expansion of the cell. When the central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm.

Contractile vacuoles are found in certain protists, especially those in Phylum Ciliophora. These vacuoles take water from the cytoplasm and excrete it from the cell to avoid bursting due to osmotic pressure.


A vacuole has a broad definition, and includes a variety of membrane-bound sacs. The membranes are composed of phospholipids, but each organism may use slightly different phospholipids. Embedded in the membranes are proteins, which can function to transport molecules across the membrane or give it structure. Various combinations of these proteins allow different vacuoles to handle and hold different materials.

In each organism, different genetics cause different proteins to be embedded in the membrane of the vacuole, which allow different molecules through, and gives the vacuoles different properties. Most plant cells have evolved to use vacuoles as water storage organelles, which provide a variety of functions to the cell. Animals don’t rely on this water storage for the rigidity of their form, and use their vacuoles for the storage of various products, and for exocytosis and endocytosis.


The Central Vacuole (plants)

Previously, we mentioned vacuoles as essential components of plant cells. If you look at Figure 1, you will see that plant cells each have a large, central vacuole that occupies most of the cell.

Figure 1 A generalized plant cell. Note the large grey central vacuole.

The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. In plant cells, the liquid inside the central vacuole provides turgor pressure, which is the outward pressure caused by the fluid inside the cell. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That is because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm and into the soil. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of a plant results in the wilted appearance. Additionally, this fluid has a very bitter taste, which discourages consumption by insects and animals. The central vacuole also functions to store proteins in developing seed cells.


Vacuoles in Cytoplasm: 4 Types | Organelles

They are fluid filled vacuoles or vesicles which are separated from the cytoplasm by a selectively permeable membrane called tonoplast. It has a number of transport systems for the passage of different substances. A number of small sap vacuoles occur in animal cells and young plant cells. In mature plant cells, the small vacuoles fuse to form a single large central vacuole which occupies up to 90% of the volume of the cell.

The large central vacuole spreads the cytoplasm in the form of a thin peripheral layer. This is a device to facilitate rapid exchange between cytoplasm and the surrounding environment. The fluid present in the sap vacuoles is often called sap or vacuolar sap. It contains mineral salts, sugars, amino acids, esters, proteins, waste products and water soluble pig­ments called anthocyanin’s.

Some crystalline deposits may also occur:

(i) Tonoplast has sites for passage of a number of ions and other materials into vacuole against their concentration gradient,

(ii) They may store food reserve, e.g. sucrose,

(iii) Solutes present in cell sap maintain a proper osmotic pressure in the cell for its turgidity and water absorption,

(iv) They play an important role in cell enlargement,

(v) The sap vacuoles store and concentrate waste products. The same are segregated from the living part of the cell,

(vi) Water soluble pigments provide colouration to the cell. The most common water soluble vacuolar pigments are anthocyanins (red, blue, purple) and anthoxanthins (ivory to deep yellow). They provide colouration to flowers in Rose, Violet, Dahlia, etc. The pigments attract pollinating and dispersing agencies. They also absorb light radiations passing through them so that their intensity is decreased,

(vii) Some plant vacuoles have special transport proteins, an acidic pH, a battery of hydrolytic enzymes and function as lysosomes.

(viii) Tannins are stored in vaculoes, cytoplasm and cell walls,

(ix) Latex is stored in vaculoes or vacuolar canals,

(x) Alkaloids and tannins stored in vaculoes provide protection against herbivores.

Type # 2. Contractile Vacuoles:

They occur in some protistan and algal cells found mostly in fresh water. A contractile vacuole has a highly extensible and collapsible membrane. It is also connected to a few feeding canals (e.g., Paramecium). The feeding canals obtain water with or without waste products from the surrounding cytoplasm. They pour the same into the contractile vacuole.

The vacuole swells up. The process is called diastole. The swollen contractile vacuole comes in contact with plasma membrane and collapses. Collapsing is called systole. This throws the vacuolar contents to the outside. Contractile vacuoles take part in osmoregulation and excretion. Osmoregulation is required in fresh water habitats where water has tendency to enter the living cells.

Due to the presence of higher osmotic concentration in the latter, continued entry of water shall cause bursting of the cells. This is prevented by throwing the extra water to the outside with the help of contractile vacuoles.

Type # 3. Food Vacuoles:

They occur in the cells of protozoan protists, several lower animals and phagocytes of higher animals. A food vacuole is formed by fusion of phagosome and a lysosome. The food vacuole contains digestive enzymes with the help of which nutrients are digested. The digested materials pass out into the surrounding cytoplasm.

Type # 4. Air Vacuoles (Pseudo-vacuoles, Gas vacuoles):

They have been reported only in prokaryotes. An air vacuole is not a single entity, neither it is surrounded by a common membrane. It consists of a number of smaller sub-microscopic vesicles. Each vesicle is surrounded by a protein membrane and encloses metabolic gases. Air vacuoles not only store gases but provide buoyancy, mechanical strength and protection from harmful radiations.


Structure and Function of Vacuoles in Plant and Fungi Cells

Unlike animal cells, plant cells typically contain only one vacuole per cell (often referred to as a “central vacuole”), and the vacuole they contain is much larger than those in animal cells. Plant cell central vacuoles take up an enormous percentage of the cell, sometimes over 90% of cell space, although 30-50% is more common.

Surrounding the vacuoles in mature plant cells is an additional thin membrane called a tonoplast. The tonoplast helps the vacuole hold its structure so that the vacuole can retain its shape. Vacuoles in plant and fungi cells perform very similar functions, however fungi cell vacuoles are typically much smaller than plant cell vacuoles, and each fungi cell can contain more than one vacuole (similar to animal cells).

Vacuoles in plant and fungi cells perform more functions than vacuoles in other types of cells they’re a critical part of keeping the plant/fungi alive and healthy. Because fungi and plant cells don’t contain lysosomes, vacuoles in these cells also break down more materials than they do in animal cells. In addition to the functions listed in the previous section, vacuoles in plant and fungi cells also:

Maintain proper pH: The vacuole keeps the cytoplasm in the cell acidic so that enzymes can break down different molecules. The vacuole lowers pH by moving protons from the cell cytosol into the vacuole.

Store water: The vacuole can use proton motive force, a chemical gradient used to move materials in an out of the cell, to store water which allows the plant to survive longer in periods of drought.

Maintain turgor pressure: Turgor pressure is the pressure of the main area of the cell against the cell wall. It’s one of the ways plants and trees avoid being limp and grow tall and strong. Think of fresh, crisp salad greens vs. limp ones. The former have high turgor pressure. Tonoplasts in vacuoles control turgor pressure by maintaining a particular balance of ions, which causes the vacuole to swell against the cell wall.

Adjust size of the cell: Because vacuoles in plant cells can be so large, they are a key part in determining how large or small a certain plant cell is, which can in turn affect the size of different parts of the plant.

In this image, you can see how much larger the vacuole (large blue structure) is in a plant cell compared to an animal cell. Source: Wikipedia commons


Contents

Fatty acid vesicles- aid in the long term storage of the cell

  • Transport vesicles can move molecules between locations inside the cell, e.g., proteins from the rough endoplasmic reticulum to the Golgi apparatus.
  • Synaptic vesicles are located at presynaptic terminals in neurons and store neurotransmitters.
  • Lysosomes are membrane-bound digestive organelles that can digest macromolecules (break them down to small compounds) that were taken in from the outside of the cell by an endocytic vesicle.
  • Matrix vesicles are located within the extracellular space, or matrix. Using electron microscopy but working independently, they were discovered in 1967 by H. Clarke Anderson [1] and Ermanno Bonucci. [2] These cell-derived vesicles are specialized to initiate biomineralization of the matrix in a variety of tissues, including bone, cartilage, and dentin. During normal calcification, a major influx of calcium and phosphate ions into the cells accompanies cellular apoptosis (genetically determined self-destruction) and matrix vesicle formation. Calcium-loading also leads to formation of phosphatidylserine:calcium:phosphate complexes in the plasma membrane mediated in part by a protein called annexins. Matrix vesicles bud from the plasma membrane at sites of interaction with the extracellular matrix. Thus, matrix vesicles convey to the extracellular matrix calcium, phosphate, lipids and the annexins which act to nucleate mineral formation. These processes are precisely coordinated to bring about, at the proper place and time, mineralization of the tissue's matrix.

What are vesicles, and how do they work?

Vesicles are tiny sacs that transport material within or outside the cell. There are several types of vesicle, including transport vesicles, secretory vesicles, and lysosomes.

This article will focus on the functions of vesicles and the different types that are present within the body.

Share on Pinterest Although all vesicles (including lysosomes, pictured here in red) transport material, each type has a specialized role for a biological process.

A vesicle is a self-contained structure consisting of fluid or gas surrounded and enclosed by an outer membrane called the lipid bilayer. This is made up of hydrophilic heads and hydrophobic tails that cluster together.

Thinking of a vesicle as a tiny bubble that stores and transports materials may help people get an idea of how they look and function within a cell.

Each vesicle type has a different function, and different vesicles are necessary for different biological processes.

Vesicles can help transport materials that an organism needs to survive and recycle waste materials. They can also absorb and destroy toxic substances and pathogens to prevent cell damage and infection.

Although they are similar to vacuoles, which also store materials, vesicles have their own unique functions and abilities. For example, they can fuse with the membranes of other cells to carry out a specific role, such as breaking down another cell.

Vesicles also help store and transport materials such as proteins, enzymes, hormones, and neurotransmitters. They are a small but essential part of biological systems and processes such as:

Vesicles can carry out many functions in organisms. There are five main types of vesicle, and each has its own function.

Learn more about the types of vesicle below.

Transport vesicles

Transport vesicles help move materials, such as proteins and other molecules, from one part of a cell to another.

When a cell makes proteins, transporter vesicles help move these proteins to the Golgi apparatus for further sorting and refining. The Golgi apparatus identifies specific types of transport vesicle then directs them to where they are needed.

Some proteins in the transporter vesicles could, for example, be antibodies. So, the Golgi apparatus would package them into secretory vesicles to be released outside of the cell to fight a pathogen.

Some scientists refer to the Golgi apparatus as the cell’s “post office.”

Lysosomes

Lysosomes are vesicles that contain digestive enzymes. They are only present in animal cells. They function as part of the cell’s recycling system and can also help initiate cell death.

When a cell needs to recycle large molecules, lysosomes release their enzymes to break down these bigger molecules into smaller ones. When they have broken up the larger matter, the cell can recycle what is left.

If a cell has absorbed something harmful, such as a pathogen, it can use its lysosomes to ingest those bacteria and destroy them with enzymes.

Scientists are still not sure why lysosomes can survive, given that they are filled with enzymes that can break down cells just like themselves.

Secretory vesicles

Secretory vesicles play an important role in moving molecules outside of the cell, through a process called exocytosis. They are crucial for healthy organ and tissue function. For example, secretory vesicles in the stomach will transport protein-digesting enzymes to help break down food.

Synaptic vesicles are another example of a secretory vesicle, and they are present at the end of nerve cells (neurons).

These vesicles help transmit signals from one nerve cell to another by releasing or secreting neurotransmitters that activate receptors in the next cell along. They are a tiny 30–40 nanometers in diameter.

Peroxisomes

Like lysosomes, peroxisomes contain digestive enzymes. They use enzymes to digest excess nutrients in a cell, such as fatty acids. Peroxisomes also break down alcohol.

Peroxisomes also use an enzyme to break hydrogen peroxide into water and oxygen, which are both harmless and useful to the cell’s function.

Peroxisomes can vary in shape and size, depending on the needs of the cell they serve. They will sometimes increase in number and size if, for example, they have a lot of alcohol to break down.

Extracellular vesicles

Extracellular vesicles can float outside of cells.

For many years, scientists saw extracellular vesicles as insignificant to cell health and functionality. However, recent research has suggested that these vesicles have a vital role to play in communicating between cells and have important evolutionary consequences.

A 2019 literature review in the journal PLOS Biology discusses how viruses and bacteria may be able to interact with healthy cells via extracellular vesicles.

However, more research is necessary to understand why and how this happens.


The Role of the Vacuole in Eukaryotic Cells

Eukaryotic cells include all cells that have a nucleus and other membrane-bound organelles. Eukaryotic cells engage in cell division by the processes of mitosis and meiosis. By contrast, prokaryotic cells are typically unicellular organisms lacking any membrane-bound organelles, and which asexually reproduce through binary fission. All animal and plant cells are eukaryotic cells.

There are a great many number of plant and animal species. Furthermore, for any individual plant or animal, there are typically a number of different organ systems and organs, each with their own types of cells.

A cell’s particular needs for the very adaptable vacuole depend on that cell’s job and on the environmental conditions in the plant or animal body at any given time. A few of these vacuole functions include:

  • Storing water
  • Providing a barrier for substances that need to be separated from the rest of the cell
  • Removing, destroying or storing toxic substances or waste products to protect the rest of the cell
  • Removing improperly folded proteins from the cell

Chapter Summary

A cell is the smallest unit of life. Most cells are so tiny that we cannot see them with the naked eye. Therefore, scientists use microscopes to study cells. Electron microscopes provide higher magnification, higher resolution, and more detail than light microscopes. The unified cell theory states that one or more cells comprise all organisms, the cell is the basic unit of life, and new cells arise from existing cells.

4.2 Prokaryotic Cells

Prokaryotes are single-celled organisms of the domains Bacteria and Archaea. All prokaryotes have plasma membranes, cytoplasm, ribosomes, and DNA that is not membrane-bound. Most have peptidoglycan cell walls and many have polysaccharide capsules. Prokaryotic cells range in diameter from 0.1 to 5.0 μm.

As a cell increases in size, its surface area-to-volume ratio decreases. If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume.

4.3 Eukaryotic Cells

Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning a membrane surrounds its DNA), and has other membrane-bound organelles that allow for compartmentalizing functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleus’s nucleolus is the site of ribosome assembly. We find ribosomes either in the cytoplasm or attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria participate in cellular respiration. They are responsible for the majority of ATP produced in the cell. Peroxisomes hydrolyze fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules.

Animal cells also have a centrosome and lysosomes. The centrosome has two bodies perpendicular to each other, the centrioles, and has an unknown purpose in cell division. Lysosomes are the digestive organelles of animal cells.

Plant cells and plant-like cells each have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives the cell shape. Photosynthesis takes place in chloroplasts. The central vacuole can expand without having to produce more cytoplasm.

4.4 The Endomembrane System and Proteins

The endomembrane system includes the nuclear envelope, lysosomes, vesicles, the ER, and Golgi apparatus, as well as the plasma membrane. These cellular components work together to modify, package, tag, and transport proteins and lipids that form the membranes.

The RER modifies proteins and synthesizes phospholipids in cell membranes. The SER synthesizes carbohydrates, lipids, and steroid hormones engages in the detoxification of medications and poisons and stores calcium ions. Sorting, tagging, packaging, and distributing lipids and proteins take place in the Golgi apparatus. Budding RER and Golgi membranes create lysosomes. Lysosomes digest macromolecules, recycle worn-out organelles, and destroy pathogens.

4.5 The Cytoskeleton

The cytoskeleton has three different protein element types. From narrowest to widest, they are the microfilaments (actin filaments), intermediate filaments, and microtubules. Biologists often associate microfilaments with myosin. They provide rigidity and shape to the cell and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles, flagella, and cilia.

4.6 Connections between Cells and Cellular Activities

Animal cells communicate via their extracellular matrices and are connected to each other via tight junctions, desmosomes, and gap junctions. Plant cells are connected and communicate with each other via plasmodesmata.

When protein receptors on the plasma membrane's surface of an animal cell bind to a substance in the extracellular matrix, a chain of reactions begins that changes activities taking place within the cell. Plasmodesmata are channels between adjacent plant cells, while gap junctions are channels between adjacent animal cells. However, their structures are quite different. A tight junction is a watertight seal between two adjacent cells, while a desmosome acts like a spot weld.


3. Unique organelles in the plant cells

[In this figure] The cell anatomy of animal and plant cells.
The animal cell and plant cell share many organelles in common, such as a nucleus, ER, cytosol, lysosomes, Golgi apparatus, cell membrane, and ribosomes. The organelles that are unique for plant cells are Vacuole, Cell wall, and Chloroplast (shown in orange text).

Cell wall

    is an extra layer of structural support and protection outside the cell membrane of plant cells.
  • Cell wall is made of cellulose, a polymer type of sugars.
  • The structural support of cell walls allows plants to grow to great heights (like pine trees). Wood is made of the reminded cellulose fibers of cell walls after the death of matured xylem tissues of woody plants.
  • When Robert C. Hooke came up with the term “Cell” in the 1660s, he was actually looking at the dead plant cells’ cell walls in a thin cutting of cork.

[In this figure] Cell wall provides additional protective layers outside the cell membrane.

Vacuole

    is a membrane-bound organelle that contains a mass of fluid.
  • Large, central vacuole is only present in the plant cells.
  • Vacuole serves as a storage space for plant cells. It can store a variety of nutrients (including sugars, minerals, amino acids, nucleic acids, ions, and special chemicals) that a cell might need to survive.
  • Vacuole also functions as a reservoir for the cell to store excess water. The amount of water in the vacuole will determine the cell’s turgor pressure (the hydrostatic pressure against the cell wall). A drooping plant has lost much of its water, and the vacuoles are shrinking.

[In this figure] Drawing of a plant cell showing a large vacuole.