Organelles are small structures within cells that perform dedicated functions. As the name implies, you can think of organelles as small organs. There are a dozen different types of organelles commonly found in eukaryotic cells.
a. Mitochondria: Mitochondria is a membrane bound cellular structure and is found in most of the eukaryotic cells. The mitochondria range from 0.5 to 1.0 micrometer in diameter. The mitochondria are sometimes described as power plants of the cells. These organelles generate most of the energy of the cell in the form of adenosine triphosphate (ATP) and it is used a source of chemical energy. The mitochondria also involved in other cellular activities like signaling, cellular differentiation, cell senescence and also control of cell cycle and cell growth. Mitochondria also affect human health, like mitochondrial disorder and cardiac dysfunction and they also play important role in the aging process. The term 'mitochondrion' is derived from a Greek word 'mitos' which means 'thread' and 'chondrion' which means 'granule'. Mitochondria are rod shaped structure found in both animal and plant cells. It is a double membrane bound organelle. It has the outer membrane and the inner membrane. The membranes are made up of phospholipids and proteins.
The components of mitochondria are as follows:
Outer membrane:
* It is smooth and is composed of equal amounts of phospholipids and proteins.
* It has a large number of special proteins known as the porins.
* The porins are integral membrane proteins and they allow the movement of molecules that are of 5000 Daltons or less in weight to pass through it.
* The outer membrane is freely permeable to nutrient molecules, ions, energy molecules like the ATP and ADP molecules.
Inner membrane:
* The inner membrane of mitochondria is more complex in structure.
* It is folded into a number of folds many times and is known as the cristae.
* This folding help to increase the surface area inside the organelle.
* The cristae and the proteins of the inner membrane aids in the production of ATP molecules.
* Various chemical reactions take place in the inner membrane of the mitochondria.
* Unlike the outer membrane, the inner membrane is strictly permeable, it is permeable only to oxygen and ATP and it also helps in regulating transfer of metabolites across the membrane.
Intermembrane space:
* It is the space between the outer and inner membrane of the mitochondria, it has the same composition as that of the cell's cytoplasm.
* There is a difference in the protein content in the intermembrane space.
Matrix:
* The matrix of the mitochondria is a complex mixture of proteins and enzymes. These enzymes are important for the synthesis of ATP molecules, mitochondrial ribosomes, TRNAs and mitochondrial DNA.
Functions:
Functions of mitochondria depend on the cell type in which they are present.
* The most important function of the mitochondria is to produce energy. The simpler molecules of nutrition are sent to the mitochondria to be processed and to produce charged molecules. These charged molecules combine with oxygen and produce ATP molecules. This process is known as oxidative phosphorylation.
* Mitochondria help the cells to maintain proper concentration of calcium ions within the compartments of the cell.
* The mitochondria also help in building certain parts of blood and hormones like testosterone and estrogen.
* The liver cells mitochondria have enzymes that detoxify ammonia.
* The mitochondria also play important role in the process of apoptosis or programmed cell death. Abnormal death of cells due to the dysfunction of mitochondria can affect the function of organ.
Mitochondria DNA:
Mitochondrial DNA or mtDNA or mDNA is the DNA in the mitochondria, rest of the DNA present in the eukaryotic cells is in the nucleus, in plants DNA is also found in chloroplasts.
The mitochondria have a small amount of DNA of their own. Human mitochondrial DNA spans about 16,500 DNA base pairs, it represents a small fraction of the total DNA in cells. The mtDNA contains 37 genes. All these genes are essential for normal function of the mitochondria.
These DNA help the mitochondria divide independently from the cell. mtDNA is maternally inherited. The fact that mtDNA is maternally inherited enables to trace the maternal lineage far back in time.
The mtDNA in most multicellular organisms is circular, covalently closed, double-stranded DNA. mt DNA is susceptible to free oxygen radicals. Mutations in the mitochondrial DNA leads to a number of illness like exercise intolerance.
b. Chloroplast: The word chloroplast is derived from the Greek word "chloros" meaning "green" and "plastes" meaning "the one who forms". The chloroplasts are cellular organelles of green plants and some eukaryotic organisms. These organelles conduct photosynthesis. They absorb sunlight and convert it into sugar molecules and also produce free energy stored in the form of ATP and NADPH through photosynthesis. Chloroplasts are unique organelles and are said to have originated as endosymbiotic bacteria.
Chloroplast structure: Chloroplasts found in higher plants are generally biconvex or planoconvex shaped. In different plants chloroplasts have different shapes, they vary from spheroid, filamentous saucer-shaped, discoid or ovoid shaped.
They are vesicular and have a colorless center. Some chloroplasts are in shape of club, they have a thin middle zone and the ends are filled with chlorophyll. In algae a single huge chloroplast is seen that appears as a network, a spiral band or a stellate plate.
The size of the chloroplast also varies from species to species and it is constant for a given cell type. In higher plants, the average size of chloroplast is 4-6 Âμ in diameter and 1-3 Âμ in thickness.
The chloroplast is double membrane bound organelles and are the site of photosynthesis the chloroplasts have a system of three membranes: the outer membrane, the inner membrane and the thylakoid system. The outer and the inner membrane of the chloroplast enclose a semi-gel-like fluid known as the stroma. This stroma makes up much of the volume of the chloroplast, the thylakoids system floats in the stroma.
Outer membrane: It is a semi-porous membrane and is permeable to small molecules and ions, which diffuses easily. The outer membrane is not permeable to larger proteins.
Intermembrane Space: It is usually a thin intermembrane space about 10 - 20 nanometers and it is present between the outer and the inner membrane of the chloroplast.
Inner membrane: The inner membrane of the chloroplast forms a border to the stroma. It regulates passage of materials in and out of the chloroplast. In addition of regulation activity, the fatty acids, lipids and carotenoids are synthesized in the inner chloroplast membrane.
Stroma: Stroma is an alkaline, aqueous fluid which is protein rich and is present within the inner membrane of the chloroplast. The space outside the thylakoid space is called the stroma. The chloroplast DNA chloroplast ribosomes and the thylakoid sytem, starch granules and many proteins are found floating around the stroma.
Thylakoid System: The thylakoid system is suspended in the stroma. The thylakoid system is a collection of membranous sacks called thylakoids. The chlorophyll is found in the thylakoids and is the sight for the process of light reactions of photosynthesis to happen. The thylakoids are arranged in stacks known as grana.
Each granum contains around 10 - 20 thylakoids.
Thylakoids are interconnected small sacks, the membranes of these thylakoids is the site for the light reactions of the photosynthesis to take place. The word 'thylakoid' is derived from the Greek word "thylakos" which means 'sack'.
Important protein complexes which carry out light reaction of photosynthesis are embedded in the membranes of the thylakoids. The Photosystem-I and the Photosystem-II are complexes that harvest light with chlorophyll and carotenoids, they absorb the light energy and use it to energize the electrons.
The molecules present in the thylakoid membrane use the electrons that are energized to pump hydrogen ions into the thylakoid space, this decrease the pH and become acidic in nature. A large protein complex known as the ATP synthase controls the concentration gradient of the hydrogen ions in the thylakoid space to generate ATP energy and the hydrogen ions flow back into the stroma.
Thylakoids are of two types - granal thylakoids and stromal thylakoids. Granal thylakoids are arranged in the grana are pancake shaped circular discs, which are about 300 - 600 nanometers in diameter. The stromal thylakoids are in contact with the stroma and are in the form of helicoid sheets.
The granal thylakoids contain only photosystem-II protein complex, this allows them to stack tightly and form many granal layers with granal membrane. This structure increases stability and surface area for the capture of light.
The photosystem-I and ATP synthase protein complexes are present in the stroma. These protein complexes act as spacers between the sheets of stromal thylakoids.
Functions of chloroplast:
* In plants all the cells participate in plant immune response as they lack specialized immune cells. The chloroplasts with the nucleus and cell membrane and ER are the key organelles of pathogen defense.
* The most important function of chloroplast is to make food by the process of photosynthesis. Food is prepared in the form of sugars. During the process of photosynthesis sugar and oxygen are made using light energy, water, and carbon dioxide.
* Light reactions take place on the membranes of the thylakoids.
* Chloroplasts, like the mitochondria use the potential energy of the H+ ions or the hydrogen ion gradient to generate energy in the form of ATP.
* The dark reactions also known as the Calvin cycle take place in the stroma of chloroplast.
* Production of NADPH2 molecules and oxygen as a result of photolysis of water.
* By the utilization of assimilatory powers the 6-carbon atom is broken into two molecules of phosphoglyceric acid.
c. Ribosomes: Ribosomes are small particles, present in large numbers in all the living cells. They are sites of protein synthesis. The ribosome word is derived - ‘ribo’ from ribonucleic acid and 'somes' from the Greek word 'soma' which means 'body'. The ribosomes link amino acids together in the order that is specified by the messenger RNA molecules. The ribosomes are made up of two subunits - a small and a large subunit. The small subunit reads the mRNA while the large subunit joins the amino acids to form a chain of polypeptides. Ribosomal subunits are made of one or more rRNA (ribosomal RNA) molecules and various proteins.
Proteins are necessary for the cells to perform cellular functions. Ribosomes are the cellular components that make proteins from all amino acids. Ribosomes are made from complexes of RNAs and proteins. The number of ribosomes in a cell depends on the activity of the cell. Ribosomes are freely suspended in the cytoplasm or attached to the endoplasmic reticulum forming the rough endoplasmic reticulum. On an average in a mammalian cell there can be about 10 million ribosomes.
When the ribosomes are attached to the same mRNA strand, this structure is known as polysome. The existence of ribosomes is temporary, after the synthesis of polypeptide the two sub-units separate and is reused or broken up. Amino acids are joined by the ribosomes at a rate of 200 per minute. Therefore small proteins can be made quickly but two or three hours are needed for proteins which are as large as 30,000 amino acids.
The ribosome presents in the prokaryotes function differently in protein production than the ribosomes of the eukaryote organisms. The ribosomes of bacteria, archea and eukaryotes differ significantly from each other in structure and RNA sequences. The difference in the ribosomes allows the antibiotic to kill the bacterial ribosome by inhibiting the activity of the bacterial ribosomes, the human ribosome sramin unaffected. The ribosomes of the eukaryotic callas are similar to the ribosomes of the bacterial cells, showing the evolutionary origin of the organelle.
Characteristics of ribosomes:
* Typically ribosomes are composed of two subunits: a large subunit and a small subunit.
* The subunits of the ribosome are synthesized by the nucleolus.
* The subunits of ribosomes join together when the ribosomes attaches to the messenger RNA during the process of protein synthesis.
* Ribosomes along with a transfer RNA molecule (tRNA), helps to translate the protein-coding genes in mRNA to proteins.
Ribosomes Structure:
* Ribosomes in a cell are located in two regions of the cytoplasm.
* They are found scattered in the cytoplasm and some are attached to the endoplasmic reticulum.
* When the ribosomes are bound to the ER there is known as the rough endoplasmic reticulum.
* The bound and the free ribosomes are similar in structure and are involved in protein synthesis.
* Ribosomes are composed of both RNA and proteins.
* About 37 - 62% of RNA is made up of RNA and the rest is proteins.
* Ribosome is made up of two subunits. The subunits of ribosomes are named according to their ability of sedimentation on a special gel which the Sevdberg Unit.
* Prokaryotes have 70S ribosomes each subunit consisting of small subunit is of 30S and the large subunit is of 50S. Eukaryotes have 80S ribosomes each consisting of small (40S) and large (60S) subunit.
* The ribosome found in the chloroplasts of mitochondria of eukaryotes consists of large and small subunits bound together with proteins into one 70S particle.
* The ribosomes share a core structure which is similar to all ribosomes despite differences in its size.
* The RNA is organized in various tertiary structures. The RNA in the larger ribosomes is into several continuous insertion as they form loops out of the core structure without disrupting or changing it.
* The catalytic activity of the ribosome is carried out by the RNA, the proteins reside on the surface and stabilize the structure.
* The differences between the ribosomes of bacterial and eukaryotic are used to create antibiotics that can destroy bacterial infection without harming human cells.
d. Endoplasmic reticulum(ER): Endoplasmic reticulum is a continuous membrane, which is present in both plant cells, animal cells and absent in prokaryotic cells. It is the membrane of network tubules and flattened sacs, which serves a variety of functions within the cell. The space, which is present in the endoplasmic reticulum, is called as the lumen.
It can be defined as a eukaryotic organelle, which forms a network of tubules, vesicles and cisternae within the cells. There are two regions of the Endoplasmic reticulum, which differ in both structure and function. One region is called as rough Endoplasmic reticulum, as it contains ribosome attached to the cytoplasmic side of the membrane and they are the series of flattened sacs. The other region is called as smooth Endoplasmic reticulum as it lacks the attached ribosome and they are tubule network.
The major functions of Endoplasmic reticulum are:
* It is mainly responsible for the transportation of proteins and other carbohydrates to another organelle, which includes lysosomes, Golgi apparatus, plasma membrane, etc.
* They play a vital role in the formation of the skeletal framework.
* They provide the increased surface area for cellular reactions.
* They help in the formation of nuclear membrane during cell division.
* They play a vital role in the synthesis of proteins, lipids, glycogen and other steroids like cholesterol, progesterone, testosterone, etc.
Endoplasmic reticulum structure:
* Endoplasmic reticulum is an extensive membrane network of cisternae (sac-like structures), which are held together by the cytoskeleton. The phospholipid membrane encloses a space, the lumen from the cytosol, which is continuous with the perinuclear space.
* The surface of the rough endoplasmic reticulum is studded with the protein manufacturing ribosome, which gives it a rough appearance. Hence it is referred as a rough endoplasmic reticulum.
* The smooth endoplasmic reticulum consists of tubules, which are located near the cell periphery. This network increases the surface area for the storage of key enzymes and the products of these enzymes.
* Rough endoplasmic reticulum synthesizes proteins, while smooth endoplasmic reticulum synthesizes lipids and steroids. It also metabolizes carbohydrates and regulates calcium concentration, drug detoxification, and attachment of receptors on cell membrane proteins.
* Endoplasmic reticulum varies extensive extending from the cell membrane through the cytoplasm and forming a continuous connection with the nuclear envelope.
Plant endoplasmic reticulum: In plant cell, the endoplasmic reticulum acts as a port for the entry of proteins into the membrane. It also plays a vital role in the biosynthesis and storage of lipids. There are number of soluble membrane, which are associated with the enzymes and the molecular chaperones. The general functions of the endoplasmic reticulum in plant cell are protein synthesis and maturation. Endoplasmic reticulum of plant cell possesses some additional functions, which is not found in animal cells. The additional function involves cell to cell communication between specialized cells and also it serves as a storage site for proteins. Endoplasmic reticulum of plant cell contains enzymes and structural proteins, which are involved in the process of oil body biogenesis and lipid storage. In plants, the endoplasmic reticulum is connected between the cells via the plasmodesmata.
Animal endoplasmic reticulum: In animal cells, the endoplasmic reticulum is a network of sacs, which play a vital role in manufacturing, processing and transporting different types of chemical compounds for use of both inside and outside of the cell. It is connected to the double-layered nuclear envelope, which provides the pipeline between the nucleus and the cytoplasm of a cell.
In animal cells, the endoplasmic reticulum is a multifunctional organelle, which synthesis the membrane lipids, proteins and also regulates the intracellular calcium.
e. Golgi apparatus: Golgi apparatus was discovered in the year 1898 by an Italian biologist Camillo Golgi. It was one of the first cellular organelles to be discovered and observed in detail due to its large size. The term Golgi apparatus was used in 1910 and in 1913 it first appeared in the scientific literature.
With the aids of special staining techniques the Golgi bodies were seen as densely stained region of the cytoplasm under the optical microscope. Under the electron microscope the Golgi apparatus is seen to be composed of stacks of flattened structures which contain numerous vesicles containing secretory granules. The newly synthesized proteins, found in the channels of the rough endoplasmic reticulum are moved to the Golgi body where the carbohydrates are added to them and these molecules are enveloped in a part of the Golgi membrane and then the enveloped molecules leave the cell. The Golgi apparatus hence acts as the assembly factory of the cell where the raw materials are directed to the Golgi apparatus before being passed out from the cell.
Golgi complex is referred to as the manufacturing and the shipping center of the eukaryotic cell. The Golgi apparatus or the Golgi body or Golgi complex or Golgi is a cellular organelle present in most of the cells of the eukaryotic organisms. The Golgi bodies were identified by an Italian biologist Camillo Golgi in the year 1897 and were maned after him in the year 1898. The Golgi complex is responsible inside the cell for packaging of the protein molecules before they are sent to their destination. This organelles helps in processing and packaging the macromolecules like proteins and lipids that are synthesized by the cell, it is known as the 'post office' of the cell. The major function of the Golgi body is to modify, sort and package the macromolecules. It also helps in transportation of lipids around the cell and the creation of lysosomes.
The Golgi apparatus Structure:
* The Golgi apparatus is a major organelle in most of the eukaryotic cells.
* They are membrane bound organelles, which are sac-like. They are found in the cytoplasm of plant and animal cells.
* The Golgi complex is composed of stacks of membrane-bound structures, these structures are known as the cisternae. An individual stack of the cisternae is sometimes referred as dictyosome.
* In a typical animal cell, there are about 40 to 100 stacks. In a stack there are about four to eight cisternae.
* Each cisternae is a disc enclosed in a membrane, it possess special enzymes of the Golgi which help to modify and transport of the modified proteins to their destination.
* The flat sacs of the cisternae are stacked and are bent and semicircular in shape.
* Each group of stacks is membrane bound and its insides are separated from the cytoplasm of the cell.
* The interaction in the Golgi membrane in responsible for the unique shape of the apparatus.
* The Golgi complex is polar in nature.
* The membrane of one end of the stack is different in composition and thickness to the membranes at the other end.
* One end of the stack is known as the cis face, it is the 'receiving department' while the other end is the trans face and is the "shipping department". The cis face of the Golgi apparatus is closely associated with the endoplasmic reticulum.
The Golgi apparatus Function:
* The cell synthesize a huge amount of variety of macromolecules. The main function of the Golgi apparatus is to modify, sort and package the macromolecules that are synthesized by the cells for secretion purposes or for use within the cell.
* Mainly modifies the proteins that are prepared by the rough endoplasmic reticulum.
* They are also involved in the transport of lipid molecules around the cell.
* They also create lysosomes.
* The Golgi complex is thus referred as post office where the molecules are packaged, labelled and sent to different parts of the cell.
* The enzymes in the cisternae have the ability to modify proteins by the addition of carbohydrates and phosphate by the process of glycosylation and phoshphorylation respectively.
* In order to modify the proteins the Golgi complex imports substances like nucleotides from the cytosol of the cell. The modifications brought about by the Golgi body might form a signal sequence. This determines the final destination of the protein.
* The Golgi complex also plays an important role in the production of proteoglycans. The proteoglycans are molecules that are present in the extracellular matrix of the animal cells.
* It is also a major site of synthesis of carbohydrates. These carbohydratres includes the synthesis of glycoasaminoglycans, Golgi attaches to these polysaccharides which then attaches to a protein produced in the endoplasmic reticulum to form proteoglycans.
* The Golgi involves in the sulfation process of certain molecules.
* The process of phosphorylation of molecules by the Golgi requires the import of ATP into the lumen of the Golgi.
f. Nucleus: Nucleus the most prominent organelle of the cell. The number of nuclei may vary, they may be uni-nucleate (single nucleus), bi-nucleate (two nuclei) or even multi-nucleate.
Nucleus is present in all eukaryotic cells, they may be absent in few cells like the mammalian RBCs.The shape of the nucleus is mostly round, it may be oval, disc shaped depending on the type of cell.
Nucleus was the first cell organelle to be discovered. Antonie von Leeuwenhoek (1632 - 1723) observed lumen (nucleus) in the red blood cells of salmon. Nucleus was also described by Franz Bauer in 1804 and by Robert Brown in 1831. Robert Brown in 1833 named and discovered nucleus in plant cells.
The word nucleus is derived from a Latin word nucleus or nucleus which means 'kernel'. Nucleus a double-membrane bound cell organelle present in eukaryotic cells. The nucleus constitutes most of the genetic material of the cell is the DNA.
The nucleus maintains the integrity of the genes which regulate the gene expression, in-turn regulating the activities of the cell. Therefore, the nucleus is known as the control center of the cell.
Nucleus structure: The nucleus is the largest organelle of the cell. The nucleus appears to be dense, spherical organelle. It occupies about 10% of the total volume of the cell.
In mammalian cells the average diameter of the nucleus is approximately 6 micrometers. A semi-fluid matrix nucleoplasm is seen inside the nucleus which is a viscous fluid and is similar to the composition of the cytoplasm.
Nuclear Envelope:
* The nuclear envelope is also known as the nuclear membrane.
* It is made up of two membranes the outer membrane and the inner membrane.
* The outer membrane of the nucleus is continuous with the membrane of the rough endoplasmic reticulum.
* The space between these layers is known as the perinuclear space.
* The nuclear envelope encloses the nucleus and separates the genetic material of the cell from the cytoplasm of the cell.
* It also serves as a barrier to prevent passage of macro-molecules freely between the nucleoplasm and the cytoplasm.
Nuclear Pore:
* The nuclear envelope is perforated with numerous pores called nuclear pores.
* The nuclear pores are composed of many proteins known as nucleoproteins.
* The nuclear pores regulate the passage of the molecules between the nucleus and cytoplasm.
* The pores allow the passage of molecules of only about 9nm wide. The larger molecules are transferred through active transport.
* Molecules like of DNA and RNA are allowed into the nucleus. But energy molecules (ATP), water and ions are permitted freely.
Chromosomes:
* The nucleus of the cell contains majority of the cells genetic material in the form of multiple linear DNA molecules.
* These DNA molecules are organized into structures called chromosomes.
* The DNA molecules are in complex with a large variety of proteins (histones) which form the chromosome.
* In the cell they are organized in a DNA-protein complex known as chromatin.
* During cell-division the chromatin forms well-defined chromosomes.
* The gene within the chromosomes consists of the cells nuclear genome.
* Mitochondrion of the cell also contains a small fraction of genes.
* Human cells have nearly 6 feet of DNA, which is divided into 46 individual molecules.
Nucleolus:
* The nucleolus is not surrounded by a membrane, it is a densely stained structure found in the nucleus.
* The nucleoli are formed around the nuclear organizer regions.
* It synthesizes and assembles ribosomes and r RNA.
* The number of nucleoli is different from species to species but within a species the number is fixed.
* During cell division, the nucleolus disappears.
* Studies suggest that nucleolus may be involved in cellular aging and senescence.
Functions of Nucleus:
* It controls the heredity characteristics of an organism.
* It is responsible for protein synthesis, cell division, growth and differentiation.
* Stores heredity material in the form of deoxy-ribonucleic acid (DNA) strands.
* Also stores proteins and ribonucleic acid (RNA) in the nucleolus.
* It is a site for transcription process in which messenger RNA (m RNA) are produced for protein synthesis.
* Aids in exchange of DNA and RNA (heredity materials) between the nucleus and the rest of the cell.
* Nucleoli produces ribosomes and are known as protein factories.
* It also regulates the integrity of genes and gene expression.
Animal Cell Nucleus: Animal cell nucleus is a membrane bound organelle. It is surrounded by double membrane. The nucleus communicates with the surrounding cell cytoplasm through the nuclear pores. The DNA in the nucleus is responsible for the hereditary characteristics and protein synthesis. The active genes on the DNA are similar, but some genes may be turned on or off depending on the specific cell type. This is the reason why a muscle cell is different from a liver cell. Nucleolus is a prominent structure in the nucleus. This aids in ribosomes production and protein synthesis.
Plant Cell Nucleus: Plant cell nucleus is a double-membrane bound organelle. It controls the activities of the cell and is known as the master mind or the control center of the cell.
The plant cell wall has two layers - the outer membrane and the inner membrane, which encloses a tiny space known as perinuclear space.
The nucleus communicates to the cell cytoplasm through the nuclear pores present in the nuclear membrane. The nuclear membrane is continuous with the endoplasmic reticulum. The DNA is responsible for cell division, growth and protein synthesis.
g. Lysosomes: Lysosomes are in animals like eukaryotic cell. Lysosomes hold enzymes that were created by the cell. The purpose of the lysosome is to digest things. They might be used to digest food or break down the cell when it dies.
A lysosome is basically a specialized vesicle that holds a variety of enzymes. The enzyme proteins are first created in the rough endoplasmic reticulum. Those proteins are packaged in a vesicle and sent to the Golgi apparatus. The Golgi then does its final work to create the digestive enzymes and pinches off a small, very specific vesicle. That vesicle is a lysosome. From there the lysosomes float in the cytoplasm until they are needed. Lysosomes are single-membrane organelles.
Lysosome structure:
Since lysosomes are little digestion machines, they go to work when the cell absorbs or eats some food. Once the material is inside the cell, the lysosomes attach and release their enzymes. The enzymes break down complex molecules that can include complex sugars and proteins. The lysosomes go to work even if there is no food for the cell. When the signal is sent out, lysosomes will actually digest the cell organelles for nutrients.
h. Vacuole: Vacuoles are storage bubbles found in cells. They are found in both animal and plant cells but are much larger in plant cells. Vacuoles might store food or any variety of nutrients a cell might need to survive. They can even store waste products so the rest of the cell is protected from contamination. Eventually, those waste products would be sent out of the cell.
The structure of vacuoles is fairly simple. There is a membrane that surrounds a mass of fluid. In that fluid are nutrients or waste products. Plants may also use vacuoles to store water. Those tiny water bags help to support the plant. They are closely related to objects called vesicles that are found throughout the cell.
In plant cells, the vacuoles are much larger than in animal cells. When a plant cell has stopped growing, there is usually one very large vacuole. Sometimes that vacuole can take up more than half of the cell's volume. The vacuole holds large amounts of water or food. Don't forge that vacuoles can also hold the plant waste products. Those waste products are slowly broken into small pieces that cannot hurt the cell. Vacuoles hold onto things that the cell might need, just like a backpack.
