Comparing the Cell to a Factory

¨       The eukaryotic cell is like a factory.

¨       Organelles are “little organs” which carry out specialized functions within a cell.

¨       Cell biologists divide a eukaryotic cell into two parts: the nucleus and the cytoplasm.

¨       The cytoplasm is the portion of the cell outside the nucleus.

¨       Characteristics of the cytoplasm:

¨       It is a colloidal suspension and looks like a clear thick fluid.

¨       It is in motion in the cell, which makes it appear to be streaming.

¨       It contains cell organelles and stores nutrients.

Nucleus

¨       In the same way that the main office controls a large factory, the nucleus is the control center of the cell.

¨       The nucleus contains all the cell’s DNA and the coded instructions for making proteins and other molecules.

¨       The nuclear envelope surrounds the nucleus.

¨       The nuclear envelope has pores, which allow material to move into and out of the nucleus.

¨       Like an office where messages, instructions and blueprints move in and out, proteins, RNA, and other molecules move through the nuclear pores.

¨       Chromatin is the granular material in the nucleus.

¨       Chromatin consists of DNA bound to protein and is spread throughout the nucleus.

¨       Chromosomes are condensed chromatin, which are threadlike structures that contain genetic information.

¨       The nucleolus is a small dense region inside the nucleus.

¨       The assembly of ribosomes begins in the nucleolus.

Ribosomes

¨       One of the most important jobs carried out in the cellular “factory “ is making proteins.

¨       Proteins are assembled on ribosomes.

¨       Ribosomes are small particles of RNA and protein found in the cytoplasm.

¨       Each ribosome is like a small machine in a factory, making proteins on orders that come from its “boss” – the nucleus.

¨       They are found scattered in the cytoplasm or on the endoplasmic reticulum.

Endoplasmic Reticulum

¨       The endoplasmic reticulum or ER is an internal membrane system (subway system).

¨       The endoplasmic reticulum is where lipid components of the cell membrane are assembled and then exported from the cell.

¨       The rough endoplasmic reticulum or rough ER is involved in the synthesis of protein because ribosomes are found on its surface.

¨       Rough ER is abundant in cells that produce large amounts of protein for export.

¨       The smooth endoplasmic reticulum or smooth ER contains enzymes that help synthesize lipids and detoxify drugs but do not have ribosomes on its surface.

¨       Liver cells contain a large amount of smooth ER.

Golgi Apparatus

¨       The Golgi apparatus receives proteins produced in the rough ER.

¨       It looks like flattened sacs piled on one another.

¨       It acts as a protein-packaging factory modifying, sorting and packaging proteins for storage in the cell or secretion outside the cell.

¨       It wraps protein molecules to be exported from the cell surface like hair, skin and tears.

¨       It puts the finishing touches on proteins before they are ready to leave the “factory”.

¨       Proteins are “shipped” from the Golgi apparatus to their final destinations.

Lysosomes

¨       Lysosomes act like a “cleanup crew”.

¨       Lysosomes are small organelles filled with enzymes that breakdown or digest lipids, carbohydrates, and proteins into smaller molecules used for building materials in the cell.

¨       Lysosomes remove the “junk” that might otherwise accumulate in the cell.

¨       If you bruise yourself and break a blood vessel, lysosomes digest dead RBC, and the WBC make new cells.

Vacuoles

¨       Vacuoles are saclike structures that store materials such as water, salts, proteins, and carbohydrates.

¨       In plant cells, there is a single, large central vacuole filled with liquid.

¨       Contractile vacuoles are also found in single-celled organisms and in some animals where they pump excess water out of the cell.

Mitochondria and Chloroplasts

¨       Cells need energy to survive.

¨       Most cells get energy in two ways – from food molecules or from the sun.

Mitochondria

Ø       Mitochondria are organelles that convert the chemical energy stored in food into compounds that are more convenient for the cell to use.

¨       They are enclosed by two membranes – an outer membrane and an inner membrane.

¨       The outer separates the mitochondria from the cytoplasm.

¨       The inner has many folds called cristae which add surface area for all the important reactions to occur along.

¨       Most numerous in cells that use a lot of energy, such as muscle cells.

¨       All or nearly all of our mitochondria come from the cytoplasm of the egg cell. (Mom!)

Chloroplasts

¨       Chloroplasts are organelles that capture the energy from sunlight and convert it into chemical energy in a process called photosynthesis.

¨       Chloroplasts are like biological solar power plants.

¨       Chloroplasts are surrounded by two membranes with stacks of membranes inside which contain the green pigment chlorophyll.

Organelle DNA

¨       Chloroplasts and mitochondria contain their own genetic information in the form of small DNA molecules.

¨       Lynn Margulis, an American biologist has suggested that mitochondria and chloroplasts are actually descendants of ancient prokaryotes.

¨       She suggests that the prokaryotic ancestors of these organelles evolved a symbiotic relationship with early eukaryotes, taking up residence within the eukaryotic cell.

Cytoskeleton

¨       The cytoskeleton gives support and structure to the eukaryotic cell, just like a factory building is supported by steel or cement beams.

¨       The cytoskeleton is a network of protein filaments that helps the cell to maintain its shape.

¨       The cytoskeleton is also involved in movement.

¨       The principle protein filaments that make up the cytoskeleton are microfilaments and microtubules.

¨       Microfilaments are threadlike structures made up of a protein called actin.

¨       They form a network and produce a tough, flexible framework that supports the cell.

¨       Microfilament assembly and disassembly are responsible for cytoplasmic movement.

¨       Microtubules are hollow structures made up of proteins known as tubulins.

¨       They  play a role in maintaining shape and are important in cell division where they form the mitotic spindles that help separate the chromosomes.

¨       In animal cells, tubulin is used to form a pair of centrioles.

¨       Centrioles are located near the nucleus and help to organize cell division.

¨       Centrioles are not found in plant cells.

¨       Microtubules also help to build projections from the cell surface known as:

Ø       Cilia (singular: cilium)

Ø       Flagella (singular: flagellum)

¨       These structures enable cells to swim rapidly through liquids.