Unit 2

The Cell

The cell is: the most basic structural and

functional unit of life. the smallest structure capable

of carrying on all vital life functions.

I. Cell Structure and Function

A. Plasma (cytoplasmic) membrane

Envelopes the cell Serves as a barrier between

intra- and extra-cellular environments

Is usually selectively permeable

Enables the cell to maintain homeostasis by regulating the movement of materials into and out of the cell.

Modifications:1. Microvilli: slender projections

created by extensive folding of the free surfaceServe to increase the surface area of the cell.Prominent in cells responsible for absorption.

2. Cilia: slender projections containing supportive microtubules.

Serve to move (redistribute/relocate) body fluids

3. Flagellum: single “whip like” projection similar in structure to cilia, but longer.

Provides the cell with mobility – propulsion.

Usually moves the entire cell.

B. Cytoplasm Protoplasmic material

contained within the cell by the plasma membrane

Organelles are suspended in the cytoplasm

C. Organelles Distinct structures found in the

cytoplasm They play specific roles in the life

process of the cell and, therefore, the organism.

Their abundance is determined by the specific function of each cell.

1. Endoplasmic Reticulum An extensive system of

interconnected tubes and membranes that coil through the cell connecting the cytoplasmic and nuclear membranes

a. Smooth ER Enzymes catalyze reactions

involved in: metabolism and synthesis of lipids, synthesis of steroid-based

hormones, detoxification of drugs, some

pesticides, and carcinogens,

breakdown of stored glycogen to form free glucose

and calcium ion storage and release

(specific to muscle cells and called sarcoplasmic reticulum).

b. Rough ER

The external surface is abundant in ribosomes which are small, dark granules made of proteins and RNA

Ribosomes manufacture all proteins that are secreted from cells.

Some ribosomes float free in the cytoplasm…and make soluble proteins that function in the cytoplasm

Ribosomes can attach to and detach from the endoplasmic reticulum as necessary.

The ribosomes on rER manufactures the proteins and phospholipids that form all cellular membranes (so it is called the “membrane factory”)

2. Golgi Apparatus

Stacked and flattened vesicles “Traffic director” for cellular

proteins… modifies, concentrates and

packages the proteins and lipids made at the rER

to aid in their release to the exterior of the cell

They also pinch off vesicles that contain lipids and transmembrane proteins to send to the plasma (or other organelle’s) membrane

Numerous in cells active in secretion

3. Mitochondria

The energy powerhouse Contain large amounts of enzymes

to break down nutrients providing the cell with energy forming ATP…aerobic cellular respiration

They contain their own DNA and RNA

making them able to reproduce themselves.

An interesting side note (an IFAF):

Because mitochondria and

mitochondrial DNA are similar to the purple phylum bacteria, it is widely accepted that mitochondria came from bacteria that invaded the ancient ancestors of plant and animal cells!

Mitochondria are abundant in cells with high energy needs (muscle cells)

and increase in abundance as energy needs increase.

The relative density of mitochondria reflects the energy requirements of the cell.

4. Lysosomes

“cellular garbage disposals” “disintegrator bodies” Contain enzymes capable of

breaking down the components of the cell…worn out or nonfunctional organelles

Responsible for the digestion of dead cells

Digest ingested bacteria, viruses, and toxins

Perform metabolic functions such as the breakdown and release of glycogen

Break down nonuseful tissues (i.e.: the webs between fingers and toes during fetal development; the uterine lining during menstruation)

Break down bone to release calcium ions into the bloodstream

5. Microtubules Stiff, bendable and hollow tubes

made of spherical protein subunits determine the overall shape of the

cell and

the distribution of cellular organelles.

6. Centrosomes and Centrioles

Centrosomes act as a microtubule organizing center because the microtubules radiate out from here.

Contain paired centrioles

Generates microtubules and

organizes the mitotic spindle in cell division

Centrioles form the bases of cilia and flagella

D. The Nucleus

Is surrounded by a nuclear membrane which contains the nucleoplasm

Two Vital Functions Controls and regulates

metabolic activities Essential to the process of cell

division

Openings in the nuclear membrane connect the nucleus to the ER

The nucleus manufactures nucleic acids needed for protein synthesis

The nucleus contains chromatin (a nucleoprotein) which become rod like chromosomes during cell division.

Nucleoproteins are combinations of proteins and nucleic acids.

The nucleic acid found in chromatin is deoxyribonucleic acid…DNA!

Inside the nucleus are one or more nucleoli.

Each nucleolus is a cluster of protein, DNA and RNA that are not enclosed by a membrane

It is the site where ribosomal subunits are assembled and stored.

It is also where one type of RNA is synthesized.

The nucleolus is found in the nuclei of cells that are not undergoing cell division; it disperses and disappears during cell division and reappears when division is complete and new cells are formed.

Nucleoli are abundant in cells that synthesize large amounts of proteins such as the liver and muscles cells.

II. Cell Transport Physical processes: Entails the net movement of

ions or molecules through a cell membrane.

This movement occurs across a concentration gradient

1. Diffusion: the scattering of particles…the movement of molecules from a region of high concentration to a region of lower concentration.

2. Osmosis: the diffusion of (primarily) water through a selectively permeable membrane.

3. Active transport: the process by which substances (ions) are moved across the plasma membrane due to the expenditure of energy.

A.T. typically occurs from areas of low concentration to high (as opposed to diffusion and osmosis).

4. Transport in vesicles: vesicles are spherical sacs that “bud” off from a membrane. They transport substances from one structure to another within cells; take in from (endocytosis), and/or release substances to (exocytosis), extracellular fluid

Cell membranes are not completely permeable to any substance…

however, they do allow some substances to pass through with more ease than others.

III. Homeostasis And Cells

All of the components of the cell coordinate to maintain homeostasis.

A. Homeostatic Imbalances

1. Cystic fibrosis

2. Tay-Sachs Disease

3. Mitochondrial myopathies

4. Progeria

5. Werner Syndrome

6. Cancer (carcinogenesis)

Cellular Disorders

IV. CELL DIVISION As cells become damaged,

diseased or worn out, they are replaced through the process of cell division…

the process by which cells reproduce themselves.

I. Somatic Cell Division Somatic cell division replaces

dead or injured cells and adds new ones for tissue

growth.

Cells undergo nuclear cell division …

called Mitosis and a cytoplasmic division

called…

cytokinesis

The combination of mitosis and cytokinesis results in two identical cells each having the same number and kinds of chromosomes as the original cell.

Somatic cells (with the exception of sex cells, liver cells and red blood cells) all have 23 pairs of chromosomes.

One member of each pair is inherited from each parent.

The two chromosomes that make up each pair are called

homologous chromosomes or

homologs They contain similar genes

arranged in the same (or similar) order

These cells are diploid cells because they contain two sets of chromosomes.

When cell duplication occurs, all of the chromosomes must be replicated so that its genes can be passed on

The cycle of cell division consists of two major periods: Interphase Mitotic phase

A. Interphase There are three subphases of

interphase: G1

S G2

1. G1 Phase

Also referred to as the

growth phase At this time, the cell is

metabolically active

2. S Phase Also referred to as the

synthesis phase At this time, DNA replication

occurs Two identical cells are formed

having the same genetic material

Once this phase begins, the cell must complete the cycle of division!

The centrosome replicates during the S phase.

3. G2 Phase This is the interval between the S

phase and the mitotic phase. Cell growth continues Enzymes and proteins are

synthesized Centrosomes are fully replicated

At the end of the G2 phase, the cell is ready for division.

Throughout interphase, the cell continues to grow and carry on its normal functions.

B. Mitosis AKA the M phase This is the series of events that

distributes the replicated DNA of the mother cell to the two daughter cells.

Mitosis is a continuous process with the four phases smoothly transitioning from one to the next.

The entire process usually takes about an hour or less to take place

1. Prophase This is the first, and the longest

phase of mitosis. Chromatin fibers condense and

shorten into chromosomes Each chromosome is made up of a

pair of identical, double-stranded chromatids

The central centromere holds the pairs together.

Towards the end of prophase, the centrioles create the mitotic spindle which pushes the centrioles farther apart towards opposite ends of the cell

The nuclear membrane fragments and disperses to the ER

This allows the spindle to occupy the center of the cell and interact with the chromosomes.

Some of the spindles attach to the chromosome’s centromere.

The rest of them are linked by their tips near the center

So that together they can draw the chromosomes to the center of the cell.

2. Metaphase Once the microtubules have

aligned the chromosomes in the center of the cell, the cell is in metaphase

Enzymes trigger the separation of the chromatids as anaphase begins.

3. Anaphase This phase begins as soon as

the centromeres of the chromosomes split

This is the shortest stage of mitosis…usually lasting only a few minutes.

Each chromatid becomes an individual chromosome

The spindles attached to the centromeres pull the chromosomes towards its pole.

This makes the chromosomes look “v” shaped

The other spindles push the two poles of the cell apart causing it to elongate.

It is theorized that the chromosomes are short and compact to prevent their tangling during anaphase

Tangling would result in an imperfect distribution of genetic material to the daughter cells.

4. Telophase As soon as the chromosomes

stop moving, telophase begins. This is the reverse of prophase The chromosomes at the

opposite poles extend,

a new nuclear envelope (made from the bits of the old membrane that have been stored) forms around each chromatin mass.

Nucleoli reappear in the nuclei, the spindles break down and

disappear Mitosis is ended! For just a bit…the cell is

binucleate with each nucleus identical to the original one

C. Cytokinesis The process of splitting the cell

into two daughter cells begins during anaphase, and continues through and after telophase

Cytokinesis occurs as peripheral microfilaments form at the cleavage furrow and squeezes the cell apart.

D. Cell Destiny Basically, cells have three

options:

1. To remain alive and functioning without dividing

2. To grow and divide

3. To die

Homeostasis is maintained by a balance between cell proliferation and cell death.

Signals trigger cell division, death and stability.

Cell death is regulated… The process of apoptosis is an

orderly, genetically programmed cellular death.

This is triggered by agents outside or inside of the cell

Necrosis is a pathological cellular death that results from injury.

Inflammation occurs in necrosis, but not in apoptosis.

II. Sexual Reproduction Cell division that occurs in the

gonads Each new organism is the

result of the union of two different gametes…

one produced by each parent.

Gametes contain a single set of 23 chromosomes

They are haploid. Fertilization restores the cells

to their diploid state.

A. Meiosis Occurs in two successive stages:

I and II. In meiotic interphase, the

chromosomes of the diploid begin chromosomal replication similar to somatic division.

Meiosis I

Begins once chromosomal replication is complete and consists of

Prophase I, Metaphase I, Anaphase I and Telophase I.

1. Prophase I Chromosomes shorten, and

thicken Nuclear membrane and

nucleoli disappear Mitotic spindle forms

In meiotic prophase, the chromatids of each pair pair off (synapsis) resulting in a tetrad (four chromatids)

Also, crossing over frequently occurs: a process in which parts of chromatids of homologous chromosomes can be exchanged resulting in genetically different chromatids…

This accounts for much of the genetic variations among humans.

2. Metaphase I Tetrads line up in the middle of

the cell with homologous chromosomes side by side.

3. Anaphase I Each homolog separates and is

pulled to opposite poles The paired chromatid remain

attached to the centromere.

4. Telophase I Nuclear envelope forms around

homologs at poles. Nucleoli reappear Cleavage furrow forms

5. Cytokinesis Cell divides at cleavage furrow

Meiosis II

Consists of Prophase II, Metaphase II, Anaphase II and Telophase II and a second cytokinesis.

This results in 4 haploid gametes that differ from the diploid cell of origin.