Chapter 3Cells

I. Cells A. History

1. Robert Hooke a. English physicist b. Examined cork slices, used the term "cell" (1665) to describe

compartments seen in cork tissue

2. Anton van Leeuwenhoek • single celled "animalcules", 1670's

3. Nucleus discovered in 1831 by Robert Brown

4. Matthias Schleiden and Theodor Schwann credited with the cell theory, 1838-1839

5. Rudolf Virchow, 1858, "omnis cellula ecellula", (every cell comes from a preexisting cell), i.e., no spontaneous generation of cells

6. Louis Pasteur's experiments disprove spontaneous generation

7. Discovery of enzymes from dead yeast cells (1897)

B. Modern Microscopes

1. Light microscopes • compound and dissecting microscope

2. Electron microscopes a. Useful magnifications of 200,000 b. Transmission (TEM) and scanning electron

microscopes (SEM) c. Scanning tunneling microscope (STM) • Nobel Prize in Physics (1986) awarded for

development of STM

III. Eukaryotic versus Prokaryotic Cells

A. Prokaryotic cells (bacteria)

• lack membrane-bound nuclei and other organelles

B. Eukaryotic cells

• Protists, Plants, Fungi, and Animals have nuclei and other organelles

IV. Cell Size and Structure

A. Size and Number

1. Higher plant cells, 10-100 µm long

2. Bacterial cells, 0.5-1.0 µm long

3. Pear tree leaf contains ~ 50 million cells

4. Full grown pear tree contains ~ 15 trillion cells

B. The Cell Wall

1. Found outside plasma membrane 2. Consists of mostly cellulose • cellulose molecules grouped into bundles known as

microfibrils 3. Pectins form the middle lamella 4. First formed walls called primary walls, then secondary walls

may be formed 5. Lignin may be part of cell wall giving mechanical strength to

wall 6. Plasmodesmata interconnect living plant cells

C. Cytoplasm

1. The Nucleus a. Most conspicuous organelle, generally spherical or

ellipsoidal in shape b. Control center of the cell, stores hereditary information c. DNA in nucleus makes a copy of itself (RNA) as a blueprint

for directing protein synthesis d. Contains 2 membranes (outer and inner), called the nuclear

envelope, which contains pores to allow passage of certain molecules

e. Outer membrane connected to endoplasmic reticulum f. Nucleoplasm is the granular-appearing fluid inside the

nuclear envelope g. Nucleoli are larger bodies noticeable in the nucleoplasm • involved in RNA synthesis h. Chromatin is the thread-like DNA strands which shorten and

condense into chromosomes when the cell is ready to divide • chromatin composed of DNA and protein

2. Plastids a. Types 1) Chloroplasts a) Most common type, green in color b) Stroma, enzyme containing region c) Grana membranes called thylakoids, suspended in the

stroma 2) Chromoplasts a) Contain colored pigments such as carotenoids b) Give orange, red, and yellow colors to various plant

parts 3) Leucoplasts a) Colorless b) Types

i) Amyloplasts, store starch granules ii) Elaioplasts, store oils

b. Proplastids • all plastids develop from these

3. The Cytoskeleton

a. Microtubules 1) Unbranched, thin, hollow tube-like structures 2) Composed of protein 3) 15-25 nm in diameter and of various lengths 4) Found inside plasma membrane, help control the addition

of cellulose to the cell wall 5) Other functions, vesicle transport, motility of flagella and

cilia, and component of mitotic spindle

b. Microfilaments 1) Long protein filaments approximately 6 nm in diameter 2) Often grouped together in bundles 3) Function in cytoplasmic streaming and as part of the

framework of the cell

4. The Plasma Membrane

a. Outer boundary of living cell, extremely thin

b. Mosaic structure composed of phospholipids and proteins

c. Unit membrane structure seen with electron microscope

5. The Endoplasmic Reticulum a. Complex system of membrane channels b. Outer surface of membranes may be covered with

ribosomes c. Called "rough" ER because of pebbly appearance d. "Smooth" ER has no ribosomes e. Protein synthesis, secretion and storage associated with

rough ER

6. Ribosomes a. Occur free in cytoplasm, mitochondria and chloroplast, or associated with rough ER b. Roughly ellipsoidal in shape, approximately 17-25 nm in

diameter c. Function in protein synthesis by linking amino acids

together d. Ribosome composed of two subunits, made up of RNA and proteins

7. The Golgi Apparatus (Dictyosomes)

a. Groups of flat, roundish sacs, organized into stacks b. Collecting and packaging centers for proteins and other

molecules like carbohydrates

8. Mitochondria

a. "Power-houses" of the cell b. Function in generation of ATP, the energy molecule of the

cell c. Small organelles, 1-3 µm in length, look like tiny membranes d. Outer and inner membranes present • inner membrane has inward extensions or folds called

cristae

9. Other Organelles

a. Microbodies

1) Small spherical organelles, bounded by a single membrane

2) Contain enzymes which function in photosynthesis, photorespiration, and conversion

of fats to carbohydrates

b. Lysosomes

• store digestive enzymes (more typical of animal cells)

10. Vacuoles

a. Membrane bound sacs filled with a watery fluid called cell sap

• vacuolar membrane called the tonoplast

b. Cell sap contains salts, sugars, organic acids, and may contain water-soluble pigments called anthocyanins

c. Vacuoles are small and numerous in size in newly formed cells but increase in size as cell matures

V. Cyclosis

A. Defined • cytoplasmic streaming exhibited by all living cells

B. Function • facilitates exchange and movement of materials within a cell

C. Movement • microfilaments and microtubules are thought to be

responsible for generating the movement of the cytoplasm

VI. Cellular Reproduction

A. The Cell Cycle:

The dividing cell goes through a cycle of events known as the cell cycle. Such cycle is divided into interphase and mitosis

B. Interphase:

1. Period of DNA replication in preparation of nucleus dividing

2. Divided into 3 periods, G1 (1st growth or gap period), S (synthesis), and G2 (2nd growth or gap period)

C. Mitosis:

1. Terms

a. Mitosis = division of the nucleus

b. Cytokinesis = division of the cytoplasm

c. Meristems = regions in plants where mitosis occurs

2. Prophase a. Chromosomes become shorter, thicker (by now they are

doubled in DNA content) 1) Chromatids • each consist of a DNA double helix. In prophase,

each chromosome consists of two chrromatids. 2) Centromeres

• hold chromatids together 3) Kinetochore • near centromere, spindle fibers attach here 4) Other constrictions may occur on individual

chromosomes • called satellites b. Nucleolus and nuclear envelope disappear c. Centrioles and asters form (only in algae, fungi, and

animal cells)

3. Metaphase

a. Spindle forms b. Chromosomes become aligned in a plane in the center of

the cell (the equator)

4. Anaphase

• two chromatids of each chromosome separate and move to opposite poles

5. Telophase

a. Each group of daughter chromosomes becomes surrounded by a new nuclear envelope

b. Daughter chromosomes become longer and thinner

c. New nucleoli appear

d. Phragmoplast appears

e. Cell plate forms • vesicles from the Golgi fuse to form the cell plate

VII. Communication Between Cells

A. Plasmodesmata

• fine strands of cytoplasm interconnect plant cells

B. Pits

• discontinuities (gaps) in cell walls create pits

VIII. Higher Plant Cells versus Animal Cells

A. Unique Structures of Plant Cells 1. Cell walls 2. Plasmodesmata 3. Cell plate

B. Minor Differences 1. Plant cells lack centrioles • animal cells have these 2. Plant cells contain plastids • animal cells have none 3. Vacuoles, common in plant cells • either small or absent in animal cells