UNIT II - CYTOLOGY

Cell Structure and Function

Cell Theory: A history

• Hooke (1655) observes cells in cork tree w/ primative scope

• Leewenhoek builds a microscope • Discovers Protozoa (1674) and Bacteria (1683)

• Schleiden and Schwann (1838) propose cell theory

• Kolliker describes mitochondria in muscle cells (1857)

Cell theory

• All organisms are composed of one or more cells

• Cells are the basic unit of structure and function in organisms

• All cells come only from other cells

Biological organization

• All living things are constructed of cells. • Is this a true statement?

• Living things may be unicellular or multicellular.

• Cell structure is diverse, but all cells share common characteristics.

Relative sizes

• Human eye resolution = 100um

• Is there an area of this slide that is now obsolete, or at least incomplete?

Why are cells small?• Ability to exchange materials with surroundings.

Prokaryotic cells

• Lack a nucleus

• Some can generate own food

• Capable of sexual and asexual reproduction

• Used in genetic research

• Used to produce insulin

Eukaryotic cells

• Contain a nucleus

• Surrounded by a plasma membrane containing • Phospholipids

• Proteins

• Glycolipids

• Cholesterol

• Plasma membrane regulates intra to extracellular movement of material

• Plants have a cell wall extracellular to the plasma membrane

Cell membrane

Eukaryotic cells

• Internal environment • Cytosol • Organelles

• Organelles compartmentalize functions within the cell

• Many organelles are present in both animal and plant cells • Differences: Plants contain chloroplasts and animal cells

contain centrioles

Animal cell

Plant cell

nucleus

• Chromatin: DNA and proteins (histones)

• Nucleolus: Chromatin and ribosomal subunits

• Nuclear envelope: Double phospholipid membrane with pores

• Nucleoplasm: Semifluid medium inside the nucleus

Ribosomes

• Responsible for protein synthesis

• Composed of 2 sub-units (40s and 60s) which are comprised of a mixture of rRNA and proteins (40+)

• Subunits produced in the nucleolus and transported into the cytosol

• Ribosomes can be found: • Alone or in groups called polyribosomes • Attached to the ER

Ribosome production

Endomembrane system

• Consists of: • Nuclear envelope

• Endoplasmic reticulum

• Golgi apparatus

• Vesicles

• Connected via direct physical contact and/or by the transfer of vesicles from one part to another.

Rough er

• Ribosomes are attached to the rough E.R. exterior

• Site of much protein synthesis in the cell

• Site of Glycolsylation

• Protein sorting start point in the cell

Smooth er

• Lacks ribosomes

• Location of phospholipid and steroid synthesis

• Protein packaging

• Cell dependent functions: • Testosterone production in testes • Detoxifying drugs in liver • Hormone synthesis in adrenal gland

golgi• Golgi complex consists of a series of flattened sacs

called cisternae

• 2 faces with functional and structural differences • cis face: receives vessicles from the ER • trans face: vessicles exit towards their destination

• 2 Models of protein movement • Vessicle transport model

• Cisternae maturation model

• Further biochemical cell product modifications occur

• Secretory proteins are packaged and transported to the plasma membrane for exocytosis.

golgi

Lysosomes

• Produced by the golgi complex

• Contain hydrolytic enzymes (approx. 40)

• Involved with both intracellular digestion and autodigestion • Degrade extracellular material and worn-out organelles

• Plays a role in apoptosis (programmed cell death)

Vacuoles & vessicles

• VACUOLES

• Large membrane enclosed sac in eukaryotic cells

• Maintains turgor pressure in plant cells

• VESSICLES

• Inter-golgi transportation

• Cellular transport for exocytosis

peroxisomes

• Self-replication occurs through binary fission

• Contain enzymes that carry out oxidation reactions • Produce hydrogen peroxide as by-product

• Abundant in Liver cells • Break down fats and alcohol • Produce bile salts and cholesterol

• Convert fatty acids to carbohydrates in seeds

• Aids in photosynthesis in plant cells

Energy organelles

mitochondria

• Double membrane organelle found in both plant and animal cells.

• Site for cellular respiration

• Internal space called the matrix that contain enzymes responsible for breakdown of glucose and fatty acids

• Fold in the inner membrane called cristae are the sites of ATP generation

• Mitochondrial DNA is located in the matrix. All this DNA is derived from maternal DNA and contain genes for ATP production

Cellular Respiration

Citric Acid CycleGlycolysis

Electron Transport Chain

Cell. Resp. Formula

• Aerobic Energy Production = Net 38 ATP • Glycolysis – 4 ATP • NADH Transport into Matrix – (-2 ATP) • Citric Acid Cycle – 2 ATP • ETC – 34 ATP

• Anaerobic Energy Production = Net 4 ATP • Glycolysis Only

chloroplasts• Double membrane bound organelle with three distinct

compartments

• 1. – Inter-membrane Space

• Internal thylakoid membrane that separates the two internal spaces:

• 2. Stroma: site of carbohydrate synthesis

• 3. Lumen: contains chlorophyll that absorbs solar energy for photosynthesis reactions

• Chloroplasts contain genetic material used in the production of photosynthesis proteins

CYTOSKELETON

• A network of protein filaments found through out the cytoplasm of the cell

• FUNCTIONS

• Structure of the cell • Cell movement (whole cell and internal structures)

• Composed of 3 types of elements: • Actin Filaments • Microtubules • Intermediate Filaments

Actin filaments

• Major cytoskeleton protein within cells.

• Two polymerized globular actin chains twist together to form each filament

• Actin filaments are organized into two groups: • Bundles – found mainly at cell periphery • Networks – 3D network spaced throughout cytoplasm

• Function

• Cell Shape • Cytokinesis • Muscle contraction • Structure of microvilli in intestine

microtubules

• Tubulin is the protein that forms microtubules

• Tubulin dimers are made of α-tubulin and β-tubulin assembled around a hollow core

• Polymerization of tubulin begins at the microtubule organizing centre (MTOC) which is the centrosome in animal cells.

• Function

• Involved in mitosis • Stable forms are found in nerve cells • Allow for organelle movement with the aid of proteins

Dynein and Kinesin

Microtubule motor proteins

Intermediate filaments

• Named for their size (10ηm). The diameter is between actin filaments and microtubules.

• Made from more than 50 proteins that vary within cells

• Function

• Play a structural role in the cell. • Anchor the position of the nucleus in the cell

Structure of Intermediate Filaments

Types of Intermediate Filaments

• Keratins are found in epithelial cells and also form hair and nails.

• Lamins form a meshwork that stabalizes the nuclear envelope.

• Neurofiliaments strengthen long axons

• Vimentins give muscles their mechanical strength.

Centrioles

• Nine bundles of microtubule triplets arranged in a ring

• Allows for a more efficient processing of chromosomes in cell division.

Cilia and Flagella

• Cilia (small and numerous) and flagella (large and single) have a 9 + 2 pattern of microtubule doublets.

• Each cilium and flagellum has a basal body at its base--these are like centrioles (9+0)

Cilia and flagella movement

• Cilia and flagella move when the microtubule doublets slide past one another aided by the motor protein dynein

Prokaryotic cells

• 2 Kingdoms – Monera (simple bacteria) and Archaea

• Some capable of photosynthesis (cyanobacteria)

• Most have flagella (structurally different from eukaryotic cells)

• DNA found in a singular circular chromosome as well as small circular plasmids.

• Pili allow for conjugation (swapping of genetic information)

Eukaryotic evolution

• Invagination of plasma membrane enclosed genetic material creating a nucleus

• Endosymbiosis hypothesis – the idea that organelles resulted from the association of prokaryotic cells within eukaryotic ancestors

• EVIDENCE of mitochondria and chloroplast prokaryote evolution • Reproduce by dividing in two • Similar in size to bacteria • Contain DNA • rRNA on organelles are closely related to bacteria