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Cell Structure and Function

Organelles and Their Functions

In this lab you will look at the eukaryoticcells of plants and animals.

Eukaryotic cells are distinguished from the more primitive prokaryotic cells

by the presence of 1) cytoplasmic membranous organelles, 2) a nuclear

membrane (i.e. a true nucleus), and 3) chromosomal proteins. In this lab we

will focus primarily on organelles, their functions within the cell and howthey differ between plant and animal cells.

Think of the cell as a microscopic city. Like a real city it requires many

services to keep it clean and running smoothly. Think of some of the services

a real city needs: traffic control, waste disposal, and authority figure just to

name a few. Like our imagined city a cell needs the same services. Organelles

are the workers that provide these services. The following is a list

describing the various functions of some common organelles.

The NUCLEUS (mayor of city hall)

The nucleus houses the majority of genetic material of a cell. The nucleus is

the brain of the cell and controls all activity within the cell. Using DNA as a

blueprint

(like the blueprints of a city) the nucleus directs the production of proteins.

You will learn about this process in the DNA Transcription and Translation

lab.

RIBOSOMES (lumber or brick yard)

The ribosomes carry out manual labor in the form of protein synthesis for the

nucleus. They bring together all the raw ingredients such as RNA (copies of

the original DNA blueprints) and amino acids to assemble proteins. The

proteins created are essential to cell and organismal function. Think of

proteins as machinery for cell functions much like electricity and plumbing

are essential in a real city. For example, enzymes are a type of protein

without which life could not exist.

The ENDOPLASMIC RETICULUM (highways and road systems)

There are two types of endoplasmic reticulum (ER) Smooth ER and Rough

ER. This extensive network makes up approximately one half of all

membranous tissue of the cell and is the site of membrane and protein

synthesis. The ER system is much like a road system along which industry can

be found. Goods are manufactured and shipped to needed areas via the road

system. Rough ER is named for the presence of ribosomes along its

membrane and is the source of proteins. Smooth ER lacks ribosomes and is

responsible for lipid synthesis and processes a variety of metabolic processes

such as drug detoxification.

CELL MEMBRANE (City Border) and CELL WALL (City Wall)

Cell membranes are found in animal cells whereas cell walls are found in

plant cells. Cell walls and membranes have similar functions. Like a city

perimeter, cell membranes surround the cell and have the ability to regulate

entrance and exit of substances, thereby maintaining internal balance. These

membranes also protect the inner cell from outside forces. Cell walls, as the

city analogy implies, are much stronger than cell membranes and protect

cells from lysing (exploding) in extremely hypotonic (diluted) solutions. You

will learn more about these concepts in the Biological Membranes lab.

CYTOSKELETON (steel girders)

The cytoskeleton makes up the internal framework, like the steel girders that

are the framework for buildings in a city that gives each cell its distinctive

shape and high level of organization. It is important for cell movement and

cell division (mitosis).

CYTOSKELETON (steel girders)

The cytoskeleton makes up the internal framework, like the steel girders tha

are the framework for buildings in a city that gives each cell its distinctive

shape and high level of organization. It is important for cell movement and

cell division (mitosis).

CYTOPLASM (lawns and parks)

Cytoplasm is a semi-fluid substance (think gelatin) found inside the cell. The

cytoplasm encases, cushions and protects the internal organelles. It is the cel

landscape found in any space where organelles are not and therefore is

much like the lawns and parks of our city.

GOLGI APPARATUS (post office)

Like a post office, the golgi apparatus is used for shipping those good

created by the ER and ribosomes to the rest of cell.

MITOCHONDRIA (energy plant)

Mitochondria are found in both plant and animal cells and is the site o

cellular respiration. Through this process that will be covered in the

Photosynthesis and Respiration lab ATP is created which is used for energy

by the cell.

LYSOSOMES (waste disposal and recycling)

The lysosomes are digestive sacs that can break down macromolecules in the

cell using the process of hydrolysis. The digestion is carried out with

lysosomal enzymes found in the lysosome. Like waste disposal in a city

lysosomes help keep excessive or bulky macromolecules from building up in

the cell.

VACUOLES and VESICLES (warehouses, water towers or garbage dumps)

Think of these membrane sacs that have a variety of functions as

containment units for anything in excess in a city. They can hold many

substances from organic molecules to simple excess water. Plant cells have a

central vacuole that is important in maintaining plant turgidity. You can read

more about this phenomenon in the Biological Membranes Lab.

The Plasma MembraneA Cell's Protective Coat

The outer lining of a eukaryotic cell is called the plasma membrane. Thimembrane serves to separate and protect a cell from its surrounding

environment and is made mostly from a double layer of proteins and lipids

fat-like molecules. Embedded within this membrane are a variety of othe

molecules that act as channels and pumps, moving different molecules into

and out of the cell. A form of plasma membrane is also found in prokaryotes

but in this organism it is usually referred to as the cell membrane.

The Plasma MembraneA Cell's Protective Coat

The outer lining of a eukaryotic cell is called the plasma membrane. Thi

membrane serves to separate and protect a cell from its surrounding

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environment and is made mostly from a double layer of proteins and lipids,

fat-like molecules. Embedded within this membrane are a variety of other

molecules that act as channels and pumps, moving different molecules into

and out of the cell. A form of plasma membrane is also found in prokaryotes,

but in this organism it is usually referred to as the cell membrane.

The CytoplasmA Cell's Inner Space

Inside the cell there is a large fluid-filled space called the cytoplasm,

sometimes called the cytosol. In prokaryotes, this space is relatively free of

compartments. In eukaryotes, the cytosol is the "soup" within which all of

the cell's organelles reside. It is also the home of the cytoskeleton. The

cytosol contains dissolved nutrients, helps break down waste products, and

moves material around the cell through a process called cytoplasmic

streaming. The nucleus often flows with the cytoplasm changing its shape as

it moves. The cytoplasm also contains many salts and is an excellent

conductor of electricity, creating the perfect environment for the mechanics

of the cell. The function of the cytoplasm, and the organelles which reside in

it, are critical for a c ell's survival.

The NucleusA Cell's Center

The nucleus is the most conspicuous organelle found in a eukaryotic cell. It

houses the cell's chromosomes and is the place where almost all DNA

replication and RNA synthesis occur. The nucleus is spheroid in shape and

separated from the cytoplasm by a membrane called the nuclear envelope.The nuclear envelope isolates and protects a cell's DNA from various

molecules that could accidentally damage its structure or interfere with its

processing. During processing, DNA is transcribed, or synthesized, into a

special RNA, called mRNA. This mRNA is then transported out of the nucleus,

where it is translated into a specific protein molecule. In prokaryotes, DNA

processing takes place in the cytoplasm.

The RibosomeThe Protein Production Machine

Ribosomes are found in both prokaryotes and eukaryotes. The ribosome is a

large complex composed of many molecules, including RNAs and proteins,

and is responsible for processing the genetic instructions carried by an

mRNA. The process of converting an mRNA's genetic code into the exact

sequence of amino acids that make up a protein is called translation. Protein

synthesis is extremely important to all cells, and therefore a large number of

ribosomessometimes hundreds or even thousandscan be found

throughout a cell.

Mitochondria and ChloroplastsThe Power Generators

Mitochondria are self-replicating organelles that occur in various numbers,

shapes, and sizes in the cytoplasm of all eukaryotic cells. As mentioned

earlier, mitochondria contain their own genome that is separate and distinct

from the nuclear genome of a cell. Mitochondria have two functionally

distinct membrane systems separated by a space: the outer membrane,

which surrounds the whole organelle; and the inner membrane, which is

thrown into folds or shelves that project inward. These inward folds are

called cristae. The number and shape of cristae in mitochondria differ,

depending on the tissue and organism in which they are found, and serve toincrease the surface area of the membrane.

Membranes

1. Cell membranes are composed of proteins and lipids. Themembranes of human cells protect them and regulate what goes

in and out of them. Other roles of cell membranes include

allowing motility (movement of the cell's independent parts),

signal transduction and selective receptivity (the ability of cells to

communicate with each other), cell recognition (the ability of cells

to recognize the purpose and type of other cells they come in

contact with) and serving as an anchoring site (a place for proteins

to attach).

Nucleus

2. The nucleus of the human cell stores and organizes our genes(also known as chromosomes). This essential part of the human

cell also transports regulatory factors, makes ribosomes

(organelles that help produce proteins), uncoils our DNA and

produces messenger ribonucleic acid (also known as mRNA).

Microtubules

3. Microtubules are essentially the cytoskeleton of a cell; they give acells its shape. They are composed of the globular protein tubulin,

which makes up their protofilaments (tubular structures that help

a cell's components travel). With these protofilaments,microtubules move organelles such as chromosomes and

mitochondria throughout the cell.

Mitochondria

4. Mitochondria are a cell's source of chemical energy, supplyingthem with ATP (adenosine triphosphate). In addition,

mitochondria control the growth, death, signaling and

differentiation of human cells. It is because of this that

mitochondria have become associated with various diseases and

human aging.

Cilia

5. The cilia of a human cell project from the outer membrane. Theirpurpose is to provide cells with mobility, allowing them to move

throughout the body. Cilia also move mucus and bodily fluids over

the surface of the cell.

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