Lecture 4: Cell Theory, Structure and Function

Covers 4.1, 4.2, 4.3, 5.1

Cell Theory

• Our current understanding of cell theory is the result of hundreds of years of research on animal and plant material.

• Three basic tenets:*– Every living organism is made up of one or more cells– The smallest living organisms are single cells and cells

are the functional units of multicellular organisms– All cells arise from preexisting cells (and all cells are

derived from common ancestors. Ex: we come from 1 cell, but have hair cells, liver cells, heart cells, etc)

mitochondrion

cytoplasmicfluid

flagellum

vesicle

ribosomeson rough ER

centriole

intermediatefilaments(cytoskeleton)

Golgiapparatus

cytoplasm

lysosome

exocytosis ofmaterial fromthe cell

polyribosome

nuclear pore

basal body

nuclear envelope

chromatin (DNA)nucleolus

nucleus

plasmamembrane

roughendoplasmicreticulum

free ribosome

smoothendoplasmicreticulum

micro- (cytoskeleton)tubules

microfilaments

Fig. 4-3

Fig. 4-4

centralvacuole

cytoplasmic fluid plastid

vesicle

lysosome

cytoplasm

plasmodesmatacell wall plasma

membrane

intermediatefilaments(cytoskeleton)

free ribosome

ribosomesnucleus

nucleolus

nuclear porechromatin

nuclear envelope

smoothendoplasmicreticulum

roughendoplasmicreticulum

Golgi apparatus

chloroplast

mitochondrion

microtubules(cytoskeleton)

cell walls of adjoiningplant cells

Functions of Cells*

• To survive, cells must– Obtain energy and nutrients from environment– Synthesize a variety of proteins and other

molecules necessary for growth and repair– Eliminate waste– Reproduce

Common Features*

• 1.) Plasma membrane

• 2.) All cells contain cytoplasm

• 3.) All cells use DNA as hereditary blueprint

• 4.) All cells obtain raw materials and energy from their environment

1.) PLASMA MEMBRANE*

• Every cell surrounded by thin, rather fluid membrane called plasma membrane

• Proteins in a “sea” of phospholipids• Functions:

– Isolates inside of cell from external environment (phospholipids)

– Regulates flow of material in and out of cell (proteins in the membrane allow exchange of material)

– Allows communication and interaction between cells and with the external environment (proteins in the membrane allow for communication/interaction)

Structure of Plasma Membrane-phospholipids

• Called the phospholipid bilayer, this part of the membrane is composed of 2 layers of phospholipids stacked together

• Each phospholipid has a polar hydrophilic head and two non-polar, hydrophobic tails

• Outside layer has heads pointing to outside of cell with tails facing inside

• Inside layer has tails facing out and heads facing in

• Hydrophilic heads interact with watery contents inside and outside the cell while hydrophobic tails face each other in the middle of the membrane, keeping water out

• Bilayer is fluid; it can move, change shape

head(hydrophilic)

tails(hydrophobic)

A Phospholipid

Fig. 5-2

Phospholipid Bilayer

Structure of Plasma Membrane-proteins

• Called the “fluid mosaic” model, proteins exist in a “sea” of phospholipids

• They are embedded within the phospholipid layers

• There are many types of proteins embedded in the cell membrane

Proteins in plasma membrane

•

Types of proteins in plasma membrane*

• Receptor proteins: trigger cell response when molecules (from other cells) attach to receptor proteins ex: hormones

• Recognition proteins: glycoproteins (protein with sugar component) that are identification tags on the outside of a cell

• Transport proteins: allow hydrophilic molecules to move into the cell. TWO TYPES:– Channel proteins: make channels for molecules to

move through the protein and into the cell– Carrier proteins: attach to molecules on outside of cell

and move them through to the inside

Fig. 5-5

receptor

hormone

(cytoplasm)

(extracellular fluid)

A hormone bindsto the receptor

Hormone bindingactivates the receptor,changing its shape The activated receptor

stimulates a responsein the cell

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2

3

•

The whole picture

•

Movement of material across plasma membrane

• Due to phospholipids, water-soluble substances CANNOT cross the hydrophobic portion of the plasma membrane (salts, amino acids, sugars)

• However, very SMALL molecules can cross (water, O2, CO2)

• Hydrophobic molecules can cross

2.) CYTOPLASM

• All fluid and structures that lie inside plasma membrane but outside of nucleus

• Contains water, salt, and organic molecules (proteins, sugars, lipids, amino acids, nucleotides)

3.) DNA AS HEREDITARY BLUEPRINT

• More in lecture 8

• Each cell stores all of the genetic material needed to make exact copies of itself

• During cell division, DNA is copied and a complete copy of that DNA is given to the new cell.

4.) OBTAIN MATERIALS AND ENERGY FROM ENVIRONMENT

• More in lecture 5

• Cells must continually acquire and expend energy to live.

2 types of cells*

• Procaryotic (no nucleus)

• Eucaryotic (have a nucleus)

Characteristics of eucaryotic cells*

• Larger than procaryotic• Cytoplasm contains many organelles

– elaborate system of membranes– membrane enclosed structures that perform specific

functions • Contain cytoskeleton (protein fibers that help it maintain

its shape)• Some have cell walls• Some have cilia/flagella• Nucleus is control center of cell

CYTOPLASM: MEMBRANE SYSTEMS

• Three types of membrane systems create internal compartments housing specialized machinery. This separates different reactions that must occur. Material can be moved between these compartments via vesicles (membrane sacs)– Endoplasmic Reticulum*– Golgi Apparatus*– Lysosomes*

Endoplasmic Reticulum (ER)

• Series of interconneted membranes that form sacs and channels in cytoplasm

• PROTEINS AND PHOSPHOLIPIDS ARE MADE HERE*

• Move through ER structure and are modified (folded into final protein shape, sometimes chemically modified)

• Then they accumulate in pockets of ER membrane which bud off as vesicles and are carried to Golgi apparatus

GOLGI APPARATUS

• Set of membranes derived from ER• Stack of interconnected sacs• Modify, sort and package proteins made

in ER (large proteins cleaved into smaller pieces, carbohydrates added to proteins)

• Modified proteins bud off golgi surface and deliver finished products to other parts of cell or to surface to leave cell.

Fig. 4-13

Vesicles merge with theplasma membrane andrelease antibodies into theextracellular fluid

Vesicles fuse with theGolgi apparatus, andcarbohydrates are addedas the protein passesthrough the compartments

The protein ispackaged into vesiclesand travels to the Golgiapparatus

Antibody protein issynthesized on ribosomesand is transported intochannels of the rough ER

Completed glycoproteinantibodies are packagedinto vesicles on the oppositeside of the Golgi apparatus

(extracellular fluid)

(cytoplasm)

vesicles

Golgi apparatus

formingvesicle

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4

3

2

1

Lysosomes

• Membrane-enclosed vesicles that contain digestive enzymes

• Enzymes break food down into small molecules (amino acids, monosaccharides) that can be used by the cell for food

• Lysosomes can also digest worn out organelles, recycling the basic components to be re-used

Fig. 4-14

Golgi apparatus

digestiveenzymes

lysosome

food vacuoles

The Golgiapparatus modifiesthe enzymes asthey pass throughits compartments

The enzymesare packaged intolysosomes, whichbud from theGolgi apparatus

food(extracellular fluid)

Digestiveenzymes aresynthesized onribosomes andtravel throughthe rough ER

(cytoplasm)

The enzymesare packaged intovesicles and travel tothe Golgi apparatus

A lysosome fuseswith a food vacuole,and the enzymesdigest the food

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2

1

CYTOPLASM-ORGANELLES

• Mitochondria are double-membraned organelles that extract energy from food molecules and package it into ATP, which can be used by the cell.

• Mitochondria exist in large numbers in some cells (where lots of energy needed-muscle) and smaller numbers in other cells (cartilage, for instance)

– More about production of ATP in Lecture 7– Chloroplasts are the “mitochondria” of plant cells. More in Lecture 22

• Many other organelles:– Nucleus– Nucleolus– Peroxisome– Ribosome

•

CELL WALLS

• Some eucaryotic cells have non-living, stiff coatings called cell walls

• Support and protect plasma membrane

• Ex: single celled marine organisms, plants

Fig. 4-4

centralvacuole

cytoplasmic fluid plastid

vesicle

lysosome

cytoplasm

plasmodesmatacell wall plasma

membrane

intermediatefilaments(cytoskeleton)

free ribosome

ribosomesnucleus

nucleolus

nuclear porechromatin

nuclear envelope

smoothendoplasmicreticulum

roughendoplasmicreticulum

Golgi apparatus

chloroplast

mitochondrion

microtubules(cytoskeleton)

cell walls of adjoiningplant cells

CILIA/FLAGELLA

• Extensions of plasma membrane which allow the cell to move

• Ex of cilia: fallopian tubes of female mammals, respiratory tract of land mammals

• Ex of flagella: sperm

How Cilia and Flagella Move

Fig. 4-7

return stroke

cilia liningtrachea

flagellum ofhuman sperm

surface ofhuman eggcell

(a) Cilium

continuous propulsion

plasma membrane

direction of locomotion

power stroke

(b) Flagellum

propulsion of fluid

propulsion of fluid

NUCLEUS-Control Center

• More in Lecture 8