CHAPTER 4

CELLS

CELLS

• Cells are the smallest unit of life.• Because they are

so small, no one observed them until the microscope was invented. Robert Hooke was the first to describe a cell in 1665.

CELL THEORY

• Cell Theory – idea that all organisms are composed of cells.• All organisms

composed of 1+ cells within which life processes occur.• Cells are the smallest living things.

• Life evolved only once, 3.5 billion years ago.• Cells arise only by division of a previously

existing cell.

TINY CELLS

• Cells are usually very small because larger cells can’t function as efficiently.• Larger cells have

a smaller surface area to volume ratio.

CELL STRUCTURE

• Cells have a delicate cell (plasma) membrane surrounding them that controls permeability to water and dissolved substances.• A semi-fluid matrix called cytoplasm fills the inside of the cell.

Cholesterol

Cell identitymarker

Receptor protein

Cholesterol

Protein channel

Phospholipid

Polarhydrophilic

heads Nonpolarhydrophobic

tails

Polar areasof proteinPhospholipids

Nonpolar areasof protein

Polarhydrophilic

heads

PLASMA MEMBRANE

• Plasma membrane is made up of a variety of proteins in a lipid framework – the fluid mosaic model.• All cells have the same basic type of outer

membrane.

http://www.youtube.com/watch?v=Qqsf_UJcfBc

PHOSPHOLIPIDS

• The lipid layer is composed of phospholipids.• Polar phosphate

group on one end.• Two nonpolar fatty

acid chains on the other.

PHOSPHOLIPID BILAYER

• When lots of phospholipids are placed in water, they form a bilayer with fatty acid tails pointing in together – away from water.• Note that the layer is double – no fatty acid tails

in contact with water.• Polar molecules can’t pass through nonpolar

area without assistance.

MEMBRANE PROTEINS

• Proteins float in the lipid bilayer and provide channels that will allow certain molecules to pass through.

CELL SURFACE PROTEINS

• Cell surface proteins project up from the membrane surface and may have carbohydrates or lipids attached to them.• Identify type of cell.• Binding sites for particular hormones or

proteins.

TRANSMEMBRANE PROTEINS

• Transmembrane proteins span the entire lipid bilayer providing channels for polar ions and molecules.

PROKARYOTIC CELLS

• Cytoplasm is not compartmentalized.• Prokaryotes include the two kingdoms of

bacteria.• Bacteria are the simplest cellular

organisms.• Ribosomes for protein construction are

present. They are not organelles – no membrane.

PROKARYOTIC CELLS

• Bacteria have a plasma membrane like all cells.• Cell wall

(different than that found in plants or fungi)• Capsule

encloses cell wall sometimes.

PROKARYOTIC CELLS

• Bacteria have diverse array of shapes.• They can

adhere in chains and masses, although the cells remain separate.

EUKARYOTIC CELLS

• Eukaryotes include all protists, fungi, plants, and animals.• Much larger than prokaryotic cells –

complex internal compartmentalization.• Membrane bound organelles – specialized

structures where particular cell processes occur.• Largest organelle is usually the nucleus.

• Cells Alive!

CYTOSKELETON

• Cytoskeleton – a dense network of protein fibers which supports the shape of the cell and anchors organelles in place.

CYTOSKELETON

• Three types of protein fibers make up the cytoskeleton.• Long, slender

microfilaments made of actin.

• Hollow tubes called microtubules made of tubulin.

• Thick ropes called intermediate fibers.

CYTOSKELETON

• Cytoskeleton is important in determining the shape of an animal cell (no cell wall).• Filaments can form and dissolve quickly so

the shape of an animal cell can change rapidly.

NUCLEUS

• The nucleus is the control center of the cell and genetic library where hereditary information is stored.

NUCLEUS

• The nuclear envelope is actually 2 membranes.• Nuclear pores are depressions where the

two membranes pinch together.• The pores contain many embedded proteins

that permit proteins and RNA to pass into or out of the nucleus.

NUCLEUS

• In bacteria and eukaryotes, all hereditary information is encoded in DNA.• In eukaryotes, the DNA is divided into

several segments and is associated with protein, forming chromosomes.• The protein allows the condensing of the

chromosomes during cell division.• Uncoiled, threadlike strands of DNA are called

chromatin.

NUCLEUS

• The darkest region of the nucleus is called the nucleolus.• Ribosomal subunit assembly.• Subunits leave through nuclear pores –

ribosomes are assembled in cytoplasm.

RIBOSOMES

• Ribosomes read the RNA copy of a gene and uses the information to construct a protein.• Ribosomes are made up of several special

forms of RNA – ribosomal RNA (rRNA) bound up with proteins.

Endoplasmic Reticulum – The Transportation System

• Endoplasmic Reticulum - “Little net within the cytoplasm” - an extensive system of internal membranes.• Sometimes forms

membrane enclosed sacs called vesicles.

ENDOPLASMIC RETICULUM

• Carbohydrates and lipids are manufactured on the surface of the ER.• Manufacture of proteins intended for

export occurs on ER that is studded with ribosomes and called rough ER.• ER with few ribosomes is called smooth

ER.

Golgi Complex – The Delivery System

• New molecules made on the ER surface are passed through the ER membrane and into flattened stacks of membranes called Golgi bodies.• Function – collection, packaging, and

distribution of molecules manufactured in the cell.• Collectively Golgi bodies are called the Golgi

complex.

GOLGI COMPLEX

• Proteins & lipids manufactured on ER membranes are transported through the channels of the ER or as vesicles budded off of it and passed into Golgi bodies.• Inside Golgi bodies, carbohydrates may be

attached.

GOLGI COMPLEX

• Vesicles pinch off the Golgi and carry molecules to other parts of cell or to the plasma membrane so they can be released outside the cell.

Lysosomes – Recycling Centers

• Lysosomes arise from the Golgi complex and contain a concentrated mix of powerful enzymes that break down macromolecules.• They act as recycling centers by digesting

worn out cell components to make way for newly formed ones while recycling the proteins of the old components.

LYSOSOMES

• They also eliminate particles engulfed by the cell.• Enzymes that occur inside a lysosome

digest cell parts that are engulfed – if not confined to the lysosome, they would digest the cell!

ORGANELLES THAT CONTAIN DNA

• Eukaryotic cells contain some organelles derived from ancient bacteria assimilated by ancestral eukaryotes.• Mitochondria – occur in all but a few

eukaryotes.• Chloroplasts – occur only in plants & some

protists (algae).• Centrioles – relict organelles with no

membrane that occur in all animals & most protists.

Mitochondria – Powerhouses of the Cell

• Eukaryotic organisms extract energy from food through a complex series of chemical reactions called oxidative metabolism which takes place in mitochondria.

MITOCHONDRIA

• Mitochondria have two membranes• Outer membrane is smooth • Inner membrane bent into numerous folds

called cristae.• Cristae partition mitochondrion into two

compartments, an inner matrix and an outer compartment – the intermembrane space.

MITOCHONDRIA

• Mitochondria still have some of their original genes, contained in a circular, closed molecule of DNA (mtDNA).• This DNA loop contains genes that code for

proteins essential to oxidative metabolism.

Chloroplasts – Energy Capturing Centers

• All photosynthesis in plants and algae takes place within chloroplasts.• Likely derived

from an ancient symbiotic bacteria.

CHLOROPLASTS

• Two membranes, as in mitochondria.• Inner membranes are fused to form stacks

of closed vesicles called thylakoids.• Light powered reactions of

photosynthesis take place within thylakoids.

• Stacks of thylakoids are called grana.• Interior fluid is called the stroma.

CHLOROPLASTS

• Like mitochondria, chloroplasts contain a circular DNA molecule containing genes that code for proteins essential to the process of photosynthesis.

CENTRIOLES

• Centrioles assemble microtubules from tubulin subunits in animals and most protists.

CENTRIOLES

• Centrioles occur in pairs in the cytoplasm.• Often at right angles• Usually near nuclear envelope

• Cilia and flagella are anchored by a type of centriole called a basal body.

CENTRIOLES

• Centrioles lack a membrane, but contain a circular DNA molecule involved in the production of structural proteins.• They resemble a type of bacteria.• May have originated as symbiotic bacteria.

CELL MOVEMENT

• Cell motion is tied to movement of actin filaments, microtubules, or both.• Actin filaments form and dissolve quickly.

CELL MOVEMENT - CRAWLING

• The arrangement of actin filaments in the cell cytoplasm allow a cell to crawl.• Motion essential to inflammation, clotting,

wound healing, and the spread of cancer.• White blood cells move this way.• Produced in bone marrow, released in

circulatory system, they crawl out of capillary into tissue to destroy pathogens.

CELL MOVEMENT

• During animal cell reproduction, chromosomes move to opposite sides of a dividing cell because they are attached to shortening microtubules.• Cells pinch in two because the belt of actin

filaments contracts.• Also essential for muscle contraction.

FLAGELLA

• Flagella are long, threadlike organelles protruding from the cell surface.• Each flagellum is anchored at a basal body and consists of 9 microtubule pairs surrounding 2 central microtubules (9+2 arrangement).• 9+2 arrangement is fundamental feature of

eukaryotes.• Examples: human sperm cell, many single

celled organisms - used for locomotion.

CILIA

• When flagella are very numerous and organized in dense rows they are called cilia.• Cilia have the same structure as flagella, but

are usually short.• Examples: lining of human trachea to move

dust and mucus out of the respiratory tract to the throat, protists such as the Paramecium.

PLANT CELL SPECIALIZATIONS

• Vacuoles – a central storage compartment for water, sugars, ions, & pigments.• Also functions to

increase surface area to volume ratio.

PLANT CELL SPECIALIZATIONS

• Cell walls provide protection & support.• In plants, cell walls made of cellulose.• In fungi, cell walls made of chitin.• Both different from the composition of

bacterial cell walls.

TRANSPORT ACROSS CELL MEMBRANES

• Food particles, water, and other materials must pass into a cell, waste must be eliminated.• Water can diffuse through a membrane.• Food particles can be engulfed by membrane

folding around them.• Proteins in the membrane act as doors allowing

only certain molecules through.

DIFFUSION

• Molecules move randomly.• Random motion tends to create uniform

mixtures.• Net movement of molecules toward the area

where they are scarce – down the concentration gradient.

• Diffusion allows O2, CO2, & nonpolar liquids to cross plasma membrane.

DIFFUSION

• Diffusion – net movement of molecules from region of higher concentration to region of lower concentration.

http://www.youtube.com/watch?v=VY0mZUDvbH4

OSMOSIS

• Water molecules are small enough to pass through the plasma membrane.• Diffusion of water

across the plasma membrane toward the side with more polar molecules is called osmosis.

http://www.youtube.com/watch?v=w3_8FSrqc-I

SOLUTIONS

• Solutes are molecules dissolved in a solution.• Osmotic concentration – concentration

of all molecules dissolved in a solution.• Hyperosmotic – solution with higher

concentration.• Hypoosmotic – solution with lower

concentration.• Isoosmotic – solutions with equal

concentration.

OSMOSIS

• Movement of water into a cell creates pressure – osmotic pressure.• Can cause cell to swell and burst.

• Cell walls protect cells from bursting.

ENDOCYTOSIS

• Endocytosis - cells can extend their plasma membranes around food particles or liquid engulfing them in vesicles.• Phagocytosis –

food• Pinocytosis –

drinkhttp://www.youtube.com/watch?v=4gLtk8Yc1Zc

EXOCYTOSIS

• Exocytosis – the process of ridding a cell of material by discharging it from vesicles at the cell surface.

SELECTIVE PERMEABILITY

• Endocytosis is energetically expensive.• It also is not picky – whatever is there gets

engulfed.• Proteins in plasma membrane can allow

only certain molecules through – selective permeability.

SELECTIVE DIFFUSION

• Some channels act like open doors – if a molecule fits, it can pass through in either direction.• The concentrations tend to equalize

through the process of diffusion.• Selective diffusion.

FACILITATED DIFFUSION

• Facilitated diffusion - most diffusion channels use a special carrier protein.• These proteins bind only to certain molecules,

binding them on one side of the membrane, releasing them on the other.

• This process requires no energy.

FACILITATED DIFFUSION

• Net movement is from region of higher concentration to lower concentration.• Just like simple

diffusion, but facilitated by carrier proteins.• Facilitated

Diffusion

http://www.youtube.com/watch?v=vKGN_Zhz8AY

ACTIVE TRANSPORT

• Some channels through the plasma membrane are like closed doors – they require energy to pass through.• Active transport channels only work in

one direction.• Movement of molecules from a region of

lower concentration to a region of higher concentration.

ACTIVE TRANSPORT

• Two types of active transport channels.• Sodium-Potassium pump• Proton pump

SODIUM-POTASSIUM PUMP

• Sodium ions (Na+) pumped out of cell. (300/second)• Potassium ions

(K+) pumped into cell.• Energy to run

pump derived from ATP.• Important in

nerve signal conduction.

http://www.youtube.com/watch?v=yz7EHJFDEJs

PROTON PUMP

• Proton pump – a complex channel that expends energy to pump protons (H+) across the membrane.• The pump is the

key to cell metabolism – the way cells convert photosynthetic energy or chemical energy from food to ATP.• This activity is

called chemiosmosis.

http://www.youtube.com/watch?v=rd1xYSy6s2A

HOW CELLS GET INFORMATION

• Some cells can sense light or pressure changes.• Most use chemical or electrical signals.• Use cell surface proteins embedded in plasma

membrane.• Cell’s only contact with outside world.

SENSING CHEMICAL INFORMATION

• Cells sense chemical information using cell surface proteins called receptor proteins.• Bind to particular molecules – no channel.• Pass information about concentration of certain

molecules which may indicate the presence of another cell.

SENSING VOLTAGE

• Voltage sensitive channels for sodium or other ions that are usually closed.• Open in response to

voltage change.• Voltage sensitive “door” contains charged amino acids.• Important in muscle

& nerve tissue.

http://www.youtube.com/watch?v=mKalkv9c2iU