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In animal cells, the lysosomes are the cell’s “garbage disposal.” Digestive enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. In single-celled eukaryotes, lysosomes are important for digestion of the food they ingest and the recycling of organelles. These enzymes are active at a much lower pH (more acidic) than those located in the cytoplasm. Many reactions that take place in the cytoplasm could not occur at a low pH, thus the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.

Lysosomes also use their hydrolytic enzymes to destroy disease-causing organisms that might enter the cell. A good example of this occurs in a group of white blood cells called macrophages, which are part of your body’s immune system. In a process known as phagocytosis, a section of the plasma membrane of the macrophage invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen ( [link] ).

In this illustration, a eukaryotic cell is shown consuming a bacterium. As the bacterium is consumed, it is encapsulated into a vesicle. The vesicle fuses with a lysosome, and proteins inside the lysosome digest the bacterium.
A macrophage has phagocytized a potentially pathogenic bacterium into a vesicle, which then fuses with a lysosome within the cell so that the pathogen can be destroyed. Other organelles are present in the cell, but for simplicity, are not shown.


Vesicles are membrane-bound sacs that function in storage and transport. Vesicles can fuse with other membranes within the cell system.

Art connection

This figure shows the nucleus, rough ER, Golgi apparatus, vesicles, and plasma membrane. The right side of the rough ER is shown with an integral membrane protein embedded in it. The part of the protein facing the inside of the ER has a carbohydrate attached to it. The protein is shown leaving the ER in a vesicle that fuses with the cis face of the Golgi apparatus. The Golgi apparatus consists of several layers of membranes, called cisternae. As the protein passes through the cisternae, it is further modified by the addition of more carbohydrates. Eventually, it leaves the trans face of the Golgi in a vesicle. The vesicle fuses with the cell membrane so that the carbohydrate that was on the inside of the vesicle faces the outside of the membrane. At the same time, the contents of the vesicle are released from the cell.
The endomembrane system works to modify, package, and transport lipids and proteins. (credit: modification of work by Magnus Manske)


Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, free ribosomes appear as either clusters or single tiny dots floating freely in the cytoplasm. Ribosomes may be attached to either the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum. Electron microscopy has shown that ribosomes consist of large and small subunits. Ribosomes are enzyme complexes that are responsible for protein synthesis.

Because protein synthesis is essential for all cells, ribosomes are found in practically every cell, although they are smaller in prokaryotic cells. They are particularly abundant in immature red blood cells for the synthesis of hemoglobin, which functions in the transport of oxygen throughout the body.


Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. The formation of ATP from the breakdown of glucose is known as cellular respiration. Mitochondria are oval-shaped, double-membrane organelles ( [link] ) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae, which increase the surface area of the inner membrane. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria because muscle cells need a lot of energy to contract.

This transmission electron micrograph of a mitochondrion shows an oval, outer membrane and an inner membrane with many folds called cristae. Inside of the inner membrane is a space called the mitochondrial matrix.
This transmission electron micrograph shows a mitochondrion as viewed with an electron microscope. Notice the inner and outer membranes, the cristae, and the mitochondrial matrix. (credit: modification of work by Matthew Britton; scale-bar data from Matt Russell)

Section summary

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 its DNA is surrounded by a membrane), and has other membrane-bound organelles that allow for compartmentalization of functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleolus within the nucleus is the site for ribosome assembly. Ribosomes are found in the cytoplasm or are attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria perform cellular respiration and produce ATP. Vesicles are storage and transport compartments.

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

Art connections

[link] Why does the cis face of the Golgi not face the plasma membrane?

[link] Because that face receives chemicals from the ER, which is toward the center of the cell.

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Questions & Answers

atom, molecules, organelle, cell, tissue, organ, organ system
Cheila Reply
sub atom atom molecular molecules and cells.
cells structural level.
what is microorganisms I am a laboratory student.
explain why is it that old peoples as from the age of 60 usually have eye problems?
Kenn Reply
most old people suffere eye problems.because of short sight where individuals with this kind of problems see only near objects clearly but not distance objects, this is cause by elongated eyeball so light from distance is focus infront the retina so the image on the retina is blurred .
plasma DNA cytoplasm Ribosome
Pearl Reply
cell, tissue,organ, organ systems,organism
Pearl Reply
cell is the build block of Life
also a smallest unit of life
consistsof cytoplsm enclosed with in a membrane which contains many biomolecules such as proteins and nucleic acids
differences in the effects of infection by HIV and influenza A virus on host genetic variability.
Yo Reply
i dont know
hiv is direct contact of blood or body fluid and if not treated can break down the immune system faster and lead to acids with complications of pneumonia skin infections inflammation of the brain etc.while influenza A is a air borne virus which can cause the common cold pain fever and can be cured
suggest bioingormatics research article plz
what makes the heart to pumps blood?
Jamer Reply
sa node
i do think its the nerve tranmission to the brain
The contracting and relaxing of the atria and vertricles but the electrical system of the heart is what makes this possible.
yes which is called autorhymicity
adrenoline action in brains
Frankline Reply
please describe the human heart brefly
Shah Reply
the size of the heart is like a fist. it has four chambers. the left and right upper atrium which push blood in the lower left and right ventricles which in turns pump blood to the lungs or the rest of the body.
how do u prove it
Maureen is correct ,
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What is housing?
Baldwin Reply
is where located you
Hi, my name is Syedul Mostafa from Bangladesh Refugees camp. I would like to study under your.
Mostafa Reply
what is Endocytosis
Alice Reply
Endocytosis is the process where by organism lives inside it host and feed on it
Endocytosis is the process where by substances are taken in to the cell of an organism
the taking in of matter by a living cell by invagination of its membrane to form a vacuole.
Exocytosis... is the durable, energy-consuming process by which a cell directs the contents of secretory vesicles out of the cell membrane and into the extracellular space. Endocytosis is an energy-using process by which cells absorb molecules (such as proteins) by engulfing them.
There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. In phagocytosis or “cellular eating,” the cell's plasma membrane surrounds a macromolecule or even an entire cell from the extracellular environment and buds off to form a food vacuole or phagosome.
what are hormones
what the meaning of adenosine triphosphate
Alice Reply
what the full meaning of ADP
ATP as simply called is substance which provide energy during active transport
what is biology
Alice Reply
biology simply means the study of life
pls HV u get it
what is the cardiovascular disease
Kami Reply
heart disease
heart disease
kidney failure
heart failure
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Aboagye Reply
tanx wae
what is the name of the largest virus
this is in female body which have ovum which is without fertilization if ovum is fertilize then it is called egg
Am new
what is cell division
in female lagest cell in overy cell

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