CytologyThe Study of Cell Structure

and Function

The Cell Theory

1. All living things are composed of cells 2. The cell is the simplest structural unit of all living things.3. All cells come from other cells

• Robert Hooke (1500’s) was first to use the term “cell”• Schleiden (botanist) and Schwann (zoologist) (1800’s) make individual observations about cells and form the cell theory.• Its postulates state that:

• Two Major Types of Cells in the Body– Sex cells (gametes) - sperm or egg– Somatic cells - ALL other cells in body

• Basic Structure of the Cell – Vary widely in shape - squamous, spheroid, stellate, fibrous– Major components include:

• Plasma (cell) membrane - the outermost envelope• Cytosol - fluid component within the cell• Organelles - ultramicroscopic structures in cytosol that carry out

functions of the cell

• Functions of the Cell– Physical isolation - a barrier that separates inside of cell from

surrounding extracellular fluid– Communication - cells produce and receive electrical and chemical

signals– Cell metabolism and energy release– Inheritance - each cell contains DNA. Specialized cells known as

gametes used for exchange of DNA during sexual reproduction

Limits to Cell Size: Why small is better!Surface area of a cell is proportional to the square of its diameter.Volume is proportional to the cube of its diameter.Thus, for any increase in diameter of the cell, the volume increases faster than surface area.

E.g. Small Cell of 10 m diameterSA = 10 m x 10 m x 6 = 600 m2

Vol = 10 m x 10 m x 10 m =1000 m3 ---------------------------------------------------------Large Cell of 20 m diameterSA = 20 m x 20 m x 6 = 2,400 m2

Vol = 20 m x 20 m x 20 m =8000 m3

--------------------------------------------------------Note, that the larger cell has 8 times as much cytoplasm needing nourishment (1000 m3 vs. 8000 m3) and waste removal, but only 4 times as much membrane surface (600 m2 vs. 2,400 m2) through which wastes and nutrients can be exchanged.

Plasma membrane: Fluid Mosaic Model

• Separates the intracellular from the extracellular environments

• A fluid mosaic model– A bilayer of lipids with mobile

globular proteins• Membrane lipids

– make up 90-99% of molecules in membrane

– Phospholipids - 75% of lipids– Cholesterol - 20%– Glycolipids - 5%

Membrane Lipids• MEMBRANE LIPIDS

– Phospholipids: form a bilayer. • Polar heads are hydrophilic• Nonpolar tails are hydrophobic• Drift laterally from place to place

– Cholesterol: interspersed among phospholipid tails.

• Amount determines fluid nature of the membrane

– Glycolipids • Phospholipids with short carbo-

hydate chains attached• Only on extracellular surface• Form the glycocalyx

• Fluid nature provides/allows: – Redistribution of molecules within

the membrane– Phospholipids automatically

reassembled if membrane is damaged

Membrane Proteins

• Constitute 1-10% of total molecules but 50% of the weight because of their larger size

• Types of proteins– Integral or intrinsic proteins

• Pass through the mem- brane

• Can form channels through the membrane

– Peripheral or extrinsic• Attached to integral

proteins or lipids at either the inner or outer surfaces of the lipid bilayer

Functions of Membrane Proteins

1. Transport - by channel or carrier proteins2. Enzymatic activity3. Receptors4. Cell-cell recognition5. Cell-adhesion functions

Channel Proteins & Carrier Proteins

• Channel proteins– Integral proteins with pores– Nongated ion channels:

always open– Gated ion channels: can be

opened by stimuli• Ligand gated channel • Voltage-gated channel• Mechano-gated channel

• Carrier proteins: integral proteins move solutes from one side of membrane to the other– Carrier pumps consume ATP

•

1. Transport Proteins

2. Enzymatic Proteins

• Enzymes: catalyze reactions at outer/inner surface of plasma membrane. – Cells lining part of small intestine produce enzymes that digest

disaccharides; Called brush border enzymes – Some breakdown hormones or neurotransmitters once their job is

done (e.g., acetylcholinesterase)

3. Receptor Proteins• Proteins in membranes

with an exposed receptor site

• Can attach to specific ligand molecules and act as an intercellular communication system– Ligand - a hormone or

neurotransmitter• Ligand can attach only

to cells with that specific receptor; e.g., insulin binds only to insulin receptors

4. Cell Identity Markers: Glycoproteins

• Glycoproteins are part of the glycocalyx

• Act as “identification tags”– Enables our bodies to

tell which cells belong to it and which are foreign invaders

5. Cell-Adhesion Molecules

• Cells adhere to one another to extracellular material through CAMs– (cell-adhesion molecules)

• Integrins• Attachment sites to other

cells or to extracellular matrix (ECM) or intracellular molecules (cytoskeleton)

• Help maintain cell shape, bind cells together, aid in cell movement.

Cytoplasm• Cytoplasm - the cellular material outside the nucleus but

within the plasma membrane; consists of the following:

– Cytosol - cellular fluid (mainly water) with dissolved proteins, salts, sugars, and other solutes

– Organelles - ultramicroscopic structures that perform various cellular functions; ribosomes, ER, mitochondria, etc

– Cytoskeleton - protein filaments and tubules that provide support, movement within the cell; cellular skeleton

– Cytoplasmic Inclusions - chemicals such as glycogen, fat, and pigments

Cytoplasmic Organelles• Specialized internal structures with

specialized functions– Membranous organelles - have bilipid membrane

• Nucleus, mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, Golgi apparatus

– Nonmembranous - no membrane present• Ribosomes, centrosome, centrioles, basal bodies

• Related to specific structure and function of the cell– e.g., much energy neededmany mitochondria

Nucleus • Largest organelle– Only one visible with light micro.

• Surrounded by two membranes with pores– Encloses jellylike nucleoplasm – Outer membrane continuous

with rough ER • Chromatin - threadlike strands

of DNA dispersed in nucleus– Form condensed bodies of

chromosomes when nucleus starts to divide

– Contains the genetic library with blueprints for nearly all cellular proteins

• Nucleolus - site of ribosome production

Ribosomes• Not surrounded by a lipid membrane• Composed of protein and ribosomal RNA (rRNA)• Made in the nucleolus• Site of protein synthesis• Two major types based on location

– Free ribosomes• Synthesize proteins used intracellularly• Very abundant in embryonic cells

– Membrane-bound ribosomes • synthesize proteins that are packaged and secreted from the cell or

incorporated into the plasma membrane or membranes of different organelles

Endoplasmic Reticulum• Interconnected tubes and parallel membranes enclosing cisternae

(spaces isolated from rest of cytoplasm)• ER membrane continuous with outer nuclear membrane• 2 Types - rough ER and smooth ER

– Rough ER - external surface has ribosomes attached to it• Manufactures all secreted proteins

– ER first packages these proteins in transport vesicles which carry them to the Golgi body

• Synthesizes integral membrane proteins – Smooth ER - NO attached ribosomes. No protein-synthesis functions!

• Manufactures phospholipids and cholesterol• In the liver – lipid and cholesterol metabolism, breakdown of

glycogen and, along with the kidneys, detoxification of drugs • In the testes – synthesis of steroid-based (fat) hormones

(testosterone) • In the intestinal cells – absorption, synthesis, and transport of fats• In skeletal and cardiac muscle – storage and release of calcium

Endoplasmic Reticulum

Golgi Apparatus• Stacked and flattened

membranous sacs with cisternae

• Functions in modification, packaging, distribution of proteins and lipids for secretion or internal use

• Transport vesicles from ER fuse with Golgi body

• After modification protein is packaged in secretory vesicle (a budding of the Golgi) and move to designated parts of the cell

• Some secretory vesicles may accumulate in cell waiting for signal to release their contents (e.g.,blood sugar levels and insulin release)

Role of the Golgi Apparatus

Figure 3.21

Lysosomes• Membranous vesiclesthat pinch off from the

Golgi body• Contain variety of hydrolytic enzymes that

function in intracellular digestion• Degrade nonfunctional organelles• Breakdown nonuseful tissue(“suicide bags”)• Breakdown bone to release Ca2+

• Secretory lysosomes are found in white blood cells, immune cells, and melanocytes– Digest ingested bacteria, viruses, and toxins

Peroxisomes

• Membranous sacs containing oxidases and catalases (enzymes)

• Detoxify harmful or toxic substances– Abundant in liver and kidney cells

• Neutralize dangerous free radicals– Free radicals – highly reactive chemicals

with unpaired electrons (i.e., O2–)

Mitochondria • Double membrane structure with shelf-like cristae

• Matrix: Substance located in space formed by inner membrane

• Provide most of the cell’s ATP via aerobic cellular respiration

• Mitochondria increase in number when cell energy requirements increase.

• Mitochondrial DNA codes for some of the proteins needed for mitochondria production.– Mutations of mDNA result in

mitochondrial diseases– Effects most obvious in nerve

and muscle tissue (need most energy)

– Passed only from mother to offspring

Centrosome and Centrioles

• Centrosome - area near nucleus– Contains 2 centrioles

• Centrioles– Small barrel-shaped

organelles– Pinwheel array of 9

triplets of micro-tubules

– Before cell division, centrosome divides, move to ends of cell and centrioles form spindle fibers (also micro-tubules)

Cytoskeleton• A collection of protein filaments and

cylinders• Connected to integral proteins of plasma

membrane• Supports the cell but has to allow for

movements like changes in cell shape and movements of cilia– Microtubules: hollow tubes, made of

protein tubulin.• Determine overall shape of the cell,

form internal scaffold, transport of organelles (railroad tracks for motor proteins, cell division (spindle fibers), in cilia and flagella

– Microfilaments: made of actin protein• Provide structure, support for microvilli,

contractility, movement– Intermediate filaments: help hold epi-

thelial cells together• Thicker and stiffer than others• Resist pulling forces on the cell• Made of protein keratin in epidermal

cells

Motor Molecules

Figure 3.25a

Cilia • Hair-like structure projecting from cell surfaces

• Formed by micro-tubules• 7-10 m long • Capable of movement• Moves materials in one

direction across the cell surface

• Abundant in upper respiratory (mucus) and reproductive tracts (eggs)

Flagella

• Similar to cilia but longer• Usually only one per cell• Move the cell itself in wave-like fashion• Example: sperm cell

Microvilli • Extension of plasma membrane

• Abundant in intestine and kidneys where they increase the cell surface area and thus increase absorption)

• Highly modified in some special sense organs like the ears

• Normally many on each cell

• One tenth to one twentieth size of cilia

Mitosis• Process by which a cell divides into 2 daughter

cells with identical copies of its DNA• Four main functions:

– Formation of a multicellular embryo from a fertilized egg– Tissue growth– Replacement of old and dead cells– Repair of injured tissue

• Gametes (egg and sperm) produced by meiosis– Cells contain one-half of their original chromosomes

Cell Life Cycle

• Interphase: phase between cell divisions– Ongoing normal cell

activities– Replication of DNA

• Mitosis: series of events that leads to the production of two cells by division of a mother cell into two daughter cells. Cells are genetically identical.– Prophase– Metaphase– Anaphase– Telophase

• Cytokinesis: division of cell cytoplasm into 2 new cells

Chromosome Structure

• Chromatin: DNA complexed with proteins (histones)

• During cell division, chromatin condenses into pairs of chromatids called chromosomes. Each pair of chromatids is joined by a centromere

DNA Replication• DNA molecule first

unwinds• Complementary

strands separate from one another

• Each nucleotide strand serves as a template for building a new complementary strand

• Semiconservtive repli-cation

Early and Late Prophase• Nuclear membrane

disintegrates• Nucleoli disappear• Centriole pairs separate,

move to opposite poles of the nucleus and form spindle fibers

• Replicated chromo-somes attach to individual spindle fibers by their centromere

Metaphase plate

Spindle

Metaphase

Figure 3.32.4

• Replicated chromosomes aligned along equator.

Anaphase• Centromeres of the

chromosomes split• Motor proteins pull

chromosomes toward opposite poles

Telophase and Cytokinesis

• New sets of chromosomes extend into chromatin

• New nuclear membrane is formed from the rough ER

• Nucleoli reappear• Generally cytokinesis (splitting

of the cell into two) completes cell division

Cellular Aspects of Aging• Cellular clock. After a certain amount of time or

certain number of cell divisions, cells die.• Death genes. Turn on late in life, or sometimes

prematurely causing cells to deteriorate and die. Apoptosis - ‘programmed cell death’

• DNA damage. Telomeres at ends of chromosomes TTAGGG. During replication, nucleotides are lost. Telomerase protects telomeres, enzymes seem to be lost with aging.

• Free radicals. DNA mutation caused by free radicals (atoms or molecules with an unpaired electron.

• Mitochondrial damage. Mitochondrial DNA may be more sensitive to free radicals. Loss of energy, cell death.