Cell StructureChapter 3 Part 2

Eukaryotic Cells Include protists, fungi, plants, and animals These organisms can be unicellular or

multicellular Begin life with a true nucleus A typical eukaryotic cell contains, not only a

nucleus, but also other membrane bound organelles, including: An endomembrane system (ER, Golgi bodies, various

vesicles) Mitochondria (and plants also contain chloroplasts) Cytoskeletal elements (including structures that

support the cells, move things within the cell, or move the entire cell)

Controlling the Cell The nucleus is a double membrane bound

structure that houses and protects the cell’s DNA. Since DNA is used to make RNA, which is then

used to make proteins that carry out every cell process and are also used to build cell structures, the nucleus is considered the control center of the cell.

The Nuclear Envelope The double membrane surrounding the nucleus is called

the nuclear envelope. The nuclear envelope contains proteins (much like the cell

membrane) that form pores to allow the nucleus to control the passage of certain molecules into and out of the nucleus.

Molecules needed for the production of RNA from DNA are selectively transported into the nucleus and out of the nucleus. In addition, the RNA that is made in the nucleus is transported out of the nucleus.

Selectively regulating the passage of these molecules into and out of the nucleus is one way the cell regulates the amount of RNA and proteins it makes.

The Nucleus

The Endomembrane System The Endomembrane System is a series of interacting

organelles between the nucleus and the plasma membrane.

Endoplasmic Reticulum (ER) is an extension of the nuclear envelope that forms a continuous compartment of flattened sacs and tubes.

There are two kinds of ER: Rough and Smooth Rough ER is so named because it appears rough

under the electron microscope. This is due to the fact that thousands of ribosomes are attached to its outer surface.

These ribosomes produce proteins that are then transported through the ER to other destinations (such as the smooth ER) or become part of the ER membrane.

The Endomembrane System Smooth ER does not have ribosomes

attached, and so no proteins are produced on this organelle.

Instead, proteins from the rough ER become enzymes in the smooth ER. These enzymes make lipids that form the plasma membrane. They also can break down carbohydrates, fatty acids, and some drugs and poisons.

The Endoplasmic Reticulum

The Endomembrane System Another part of the Endomembrane

System are vesicles. Vesicles are various types of small,

membrane enclosed sacs that can form on their own, by budding from other organelles (such as the ER or Golgi), or by budding from the plasma membrane.

Different types of vesicles have different functions.

Vesicles Peroxisomes: vesicles that contain enzymes

that can inactivate toxins. (ie., peroxisomes in the liver and kidneys detoxify alcohol)

Vacuoles: (like “trash cans”)collect waste, debris. Toxins, and dispose of them by fusing with other vesicles called lysosomes

Lysosomes: contain powerful digestive enzymes that break down the waste contents of vacuoles

Vesicles

The Endomembrane System Some vesicles fuse with a Golgi body and empty their

contents into it. The Golgi body is made of a stack of flattened

membranes that resemble pancakes. Enzymes in the Golgi body put finishing touches on

proteins and lipids that have been delivered by vesicles from the ER.

These finishing touches may include the addition of phosphate groups or sugars.

These “finished” proteins and lipids are then packaged into new vesicles that transport them to the plasma membrane or to lysosomes.

Golgi Body

Powering the Cell The organelle that specializes in powering

the cell is called the mitochondrion. The mitochondrion produces energy for the

cell in the form of ATP. The mitochondrion has a double membrane. The inner membrane, called the cristae, is

highly folded and houses the machinery upon which ATP is produced.

Nearly all eukaryotic cells have mitochondria.

Mitochondria Mitochondria are about the size of bacteria. They have their own single, circular molecule of

DNA like bacteria. They also have bacterial ribosomes inside of them. Finally, they divide independently of the cell using a

process just like bacteria use, called binary fission. For these reasons, scientists have theorized that

these organelles (mitochondria) evolved from what used to be aerobic (oxygen using) bacteria.

The Endosymbiotic Theory Says that organelles like mitochondria

and chloroplasts evolved from an aerobic bacterium that permanently took up residence inside of another host cell.

This formed the first eukaryotic cell and explains why mitochondria and chloroplasts resemble bacterial (prokaryotic ) cells.

Mitochondrion

Chloroplasts Plants and many protists are able to perform the

biochemical process of photosynthesis because they contain organelles called chloroplasts.

Most are oval or disk-shaped enclosed by a double membrane.

Inside of the inner membrane is a semifluid interior called the stroma.

The stroma contains enzymes important for one part of photosynthesis and the chloroplasts’ own DNA.

A third, highly folded membrane inside of the chloroplast (called the thylakoids) is where another part of photosynthesis takes place.

Like mitochondria, chloroplasts are believed to have evolved according to the Endosymbiotic Theory.

Chloroplasts

The Cytoskeleton A system of interconnected protein

filaments that are present between the nucleus and plasma membrane

Responsible for reinforcing the cell, as well as organizing and moving cell structures or the whole cell

Some of its elements are permanent while others form only at certain times during the cell’s life cycle

The Cytoskeleton: Microtubules

Microtubules: long, hollow cylindrical structures that are made of the protein tubulin

Can rapidly assemble and disassemble as needed in the cell

For example, microtubules rapidly assemble before cell division to separate the cells duplicated chromosomes then disassemble when division is over.

The Cytoskeleton: Microtubules

Cytoskeleton: Microfilaments Microfilaments: fibers that are composed

mainly of the protein actin Strengthen or change the shape of of

eukaryotic cells Form a reinforcing mesh underneath the

plasma membrane called the cortex Can form at the edge of a cell to drag the

cell or extend the cell in a certain direction Also cause contractions in muscle cells

Cytoskeleton: MicrofilamentsThe red lines are actin microfilaments.The green dots are Golgibodiesaligned onactin cables, allowingGolgi bodies to movelike atrain on tracks.

Cytoskeleton: Intermediate filaments Intermediate filaments are the most

stable part of the cell’s cytoskeleton. Can lock cells and tissues together, giving

them physical strength Help to maintain cell shape and rigidity Anchor some cell organelles (such as the

nucleus) in place Support the inner nuclear membrane and

give the nucleus its shape

Cytoskeleton: Intermediate filaments

Cytoskeleton: Motor Proteins All eukaryotic cells have similar microtubules

and microfilaments; however, they may function in very different roles in different cells.

Their roles are determined when they interact with motor proteins, which move cell parts when energized with ATP.

If a cell is a train station during the busy holiday season, then the microtubules and microfilaments are train tracks that are constantly being changed, assembled, and disassembled, and the motor proteins are the freight trains that move along the tracks.

The Cytoskeleton: Motor Proteins

TRACK

FREIGHT TRAIN

ORGANELLE

Dyenin and Kinesin = motor proteins (FREIGHT TRAIN)

Cell Structures: Locomotion Organized arrays of cytoskeletal components

such as microtubules form cilia and flagella, which are responsible for the locomotion of cells.

Flagella are whip-like structures that propel cells through fluid. An example would be the flagella present on sperm cells.

There is usually only one flagellum present on a cell.

Prokaryotic flagella are different from eukaryotic flagella.

Cell Structures: Locomotion

Cell Structures: Locomotion

Cilia are short, hair-like structures. There are usually many present on a single

cell. Their coordinated beating propels cells

through fluid. These not only move cells but can also move

materials over the cell’s surface. For example, the cilia on the cells lining your

airway sweep inhaled particles away from the inside of your lungs.

Cell Structures: Locomotion

Cell Structures: Locomotion

Pseudopods (or “false feet”) are responsible for the movement of cells like amoeba.

This kind of locomotion is called amoeboid movement. Pseudopods are temporary, irregular lobes that bulge

out from the cell. These allow the amoeba to move and engulf prey. They also allow white blood cells such as macrophages

to engulf pathogens such as bacterial cells. Elongating microfilaments force the bulging in one

direction while motor proteins attached to the microfilaments drag the plasma membrane along with the microfilaments.

Cell Structures: Locomotion

Cell Surface Specializations

Cells in multicellular organisms are surrounded and organized by a nonliving mixture of proteins and fluid called extracellular matrix (ECM).

It is secreted by the cells themselves and supports and anchors cells, separates tissues, and assists with cell to cell communication.

Cells are able to communicate with one another across the ECM by means of different kinds of cell junctions.

These junctions connect cells to one aother and to their environment.

Cell Junctions Tight junctions: found in epithelial tissues

that line body surfaces and internal cavities and are formed by rows of proteins

Tight junctions prevent body fluids from seeping between adjacent cells.

For example, they are present between cells in the stomach lining and prevent stomach acid from seeping out into underlying layers, resulting in an ulcer.

They also seal the lining of ducts and kidney tubules.

Tight Junctions

Cell Junctions Adhering junctions: made of adhesion

proteins Hold cells to each other and to the ECM Tissues such as skin that are subject to a

lot of mechanical stress have many adhering junctions.

They are also found in abundance in tissues with a lot of contractile stress such as heart and skeletal muscle.

Adhering JunctionsThe blue indicatesadhering junctionsin epithelial tissuefrom the wing ofa fruit fly.

Cell Junctions Gap junctions: form channels that

connect adjoining cells together to permit the transport of ions and small molecules from one cell to another

This allows cell to cell communication

Gap Junctions