The CellThe CellThe Cell

ByDr. Carmen Rexach

PhysiologyMt San Antonio College

ChocWalk 2006

Size ranges

0.1nm - 1nm atoms, small molecules

1nm-10nm proteins, lipids

1μm to 10μm nucleus, most bacteria, mitochondria

10μm to 100μm plant and animal cells

1mm to 1cm frog egg

1m - 2m human height

10nm to 100nm viruses, ribosomes

Prokaryotic vs. eukaryotic

• Prokaryotic• <5μm• simple internal

structure• homogenous cytoplasm• secreted cell wall• some with glycocalyx• single, circular DNA

• Eukaryotic• >10μm• membrane-bound

organelles• cytoplasm: cytosol +

organelles• cytoskeleton• more complex

By definition, all living things are made up of cells.Cells are the structural and functional unit of life.

eukaryotic prokaryotic

Human Cells

Eukaryotic cell structure

• 3 principal parts:– Plasma membrane– Cytoplasm and

organelles– Nucleus

Organelles

• nucleus• ribosomes• endomembrane system

– ER– Golgi– lysosomes

• mitochondria• peroxisomes

Nucleus = “control center”

Nuclearpore

Doublemembrane

nucleolusNuclearpore

Nucleoplasm with DNA

Nucleolus

• Nucleolus– Most prominent structure within nucleus– Contains genes that code for ribosomal RNA

(rRNA)– Site of rRNA transcription and processing– Site of ribosome assembly

Nuclear envelope• Separates nuclear compartment from

cytoplasm• Provides structural framework for nucleus• Composed of two nuclear membranes

– Outer membrane continuous with endoplasmic reticulum

– Inner membrane unique proteins found only in nucleus

• Nuclear pores– Regulate which molecules enter and leave

nucleus– Important role in regulating gene expression

Ribosomes

• Sites of protein synthesis

• Composed of protein and rRNA

• Found free in the cytoplasm and associated with endoplasmic reticulum

• Eukaryotic cells contain about 10 million ribosomes each!

Large subunit

Small subunit

Endomembrane system

• Series of organelles connected by vesicular transport

• Include three structures– Endoplasmic reticulum– Golgi apparatus– lysosomes

Endomembrane system: Endoplasmic reticulum

• Series of intracellular channels• Two types

– smooth• lipid synthesis• Ca++ storage in muscle cells

– rough• “sprinkled” with ribosomes• proteins processing & sorting

Endomembrane system: Golgi apparatus

• Stack of flattened sacks• three functions

– protein modification• ex) add carbohydrates to

produce glycoproteins– separation of products based

on function and destination– packaging and shipping

Golgi apparatus

Lysosomes• Membrane bound

sacks of digestive enzymes

• Work at low pH• three functions

– breakdown of food particles in the cell

– destroys invaders in phagocytic cells of the immune system

– autophagy

Mitochondria: the powerhouse

Outer membrane

Inner membrane

matrixcrista

Inner membranousspace

Peroxisomes• Single membrane vesicles • Contain approximately 50

different enzymes depending on cell

• Produce and degrade hydrogen peroxide, (H2O2) which is toxic to cells– Contain catalase

• Decomposes H2O2 to water and oxygen, or

• Utilizes H2O2 to oxidize other compounds

• Also involved in biosynthesis of certain lipids

• They are not part of the endomembrane system

The cytoskeleton• Function

– structural support– motility– regulation

• Components – microtubules– microfilaments– intermediate filaments– muscle thick filaments

Cytoskeletal filaments

Microtubules

• Structure– hollow tubes

• diameter– 25nm

• Subunits– Composed of α & β tubulin

• function– cell shape– motility– spindle fibers– organelle movement

Centrosomes

• Region containing the centrioles and the microtubule organizing center (MTOC)

• centrioles– bundles of microtubules

involved in cell division– replicate during S phase

of interphase• MTOC

– origin of the cytoskeleton

Cilia and flagella• cilia:

– 10-25μm, move parallel to plasma membrane

– moves fluid over the surface– examples

• respiratory epithelium• fallopian tubes

• flagella:– 15-25μm, move perpendicular to

plasma membrane– human sperm is the only flagellated

cell in human body

Microfilaments• Structure

– two intertwined actin strands• diameter

– 7nm• subunits

– actin• functions

– cell shape & shape changes– muscle contraction– cell motility = pseudopodia– cell division = cleavage furrow

Intermediate filaments• Structure

– super-coiled fibrous proteins• Diameter

– 8-12nm• subunits

– various proteins of the keratin family• Functions

– Provide mechanical strength to cells and tissues– cell shape– anchorage of nucleus and other organelles– formation of nuclear lamina

Muscle thick filaments

• Structure– myosin filaments

• Diameter– 15nm

• Function– muscle contraction

Plasma membrane structure: Fluid Mosaic Model

• Phospholipidbilayer

• Associated protein• Cholesterol

• stronger• more flexibility• less permeable to

polar substances

Plasma membrane functions

• Isolate cell cytoplasm from external environment

• Regulate exchange of essential substances

• Communicate with other cells• Bear markers that identify cell

Functions of proteins associated with the plasma

membrane• Transport• enzymatic activity• signal transduction• intercellular joining• cell to cell recognition• attachment of cytoskeleton and

extracellular matrix (ECM)

Membrane proteins• Integral

– Integral part of membrane structure– Amphipathic– Can move laterally, like phospholipids– Most transmembrane

• Peripheral– Not amphipathic– Predominately located in cytosolic

surface of plasma membrane– Function

• Associated with cytoskeleton• Influence cell shape and motility

Types of membrane proteins

• Three major categories– transport proteins

• channel proteins• carrier proteins

– receptor proteins• signal transduction• protein channel regulation

– recognition proteins and glycoproteins• antigens

Cytosolic side

Extracellular side

Fluidity

• Lateral movement = 2μm per second• Flip flops• Unsaturated phospholipids

– kinked tails = enhanced fluidity• Saturated phospholipids

– straight tails = increased viscosity• Cholesterol

Microvilli• Purpose: to increase the surface area• Examples

– small intestines• increased absorption

Endocytosis

• Phagocytosis: “cell eating”– only in cells capable of forming pseudopods

• Pinocytosis: “cell drinking”– uptake of smaller molecules and liquids

• Receptor mediated endocytosis– coated pits– binding to receptor triggers endocytosis

Endocytosis

Exocytosis is the opposite!

Extracellular matrix: cell surface

• Composition– Fibrous structural and adhesion proteins

embedded in gelatinous polysaccharide ground substance

• Function– Binds cells and tissues together

Extracellular matrix: cell surface

proteoglycan complex Collagen fiber

PhospholipidbilayerMicrofilaments

(cytoskeleton)integrinfibronectin

Cell Junctions

tight junction

Intermediatefilaments

desmosomes

Gap junctions

Adjacent cells

Intercellular space

Cell junctions

Bullous pemphigoid

disease resulting from defective or absent desmosomes