Chapter 6

A Tour of the Cell

Cell Biology or Cytology

• Cyto = cell -

ology = study of

• Should use observations from

several types of microscopes to

make a total picture of how a cell is

put together.

Light Microscope - LM

• Uses visible light to illuminate the

object.

• Relatively inexpensive type of

microscope.

• Can examine live or dead objects.

Light Microscope

Occular Lens

Objective Lens

Stage with specimen

Light Source

Resolution

• Ability to detect two discrete points as separate from each other.

• As Magnification increases, Resolution decreases.

• LM working limits are100 - 1000X.

Limitations - LM

• Miss many cell structures that are

beyond the magnification of the light

microscope.

• Need other ways to make the

observations.

Light Microscope Variations

• Fluorescence: uses dyes to make parts of cells “glow”.

• Phase-contrast: enhances contrasts in density.

• Confocal: uses lasers and special optics to focus only narrow slides of cells.

Electron Microscopes

• Use beams of electrons instead of light.

• Invented in 1939, but not used much

until after WWII.

TEM SEM

Advantages

• Much higher magnifications.

• Magnifications of 50,000X or higher are

possible.

• Can get down to atomic level in some

cases.

Disadvantages

• Need a Vacuum.

• Specimen must stop the electrons.

• High cost of equipment.

• Specimen preparation.

Transmission Electron Microscope

or TEM

• Sends electrons through thinly sliced

and stained specimens.

• Gives high magnification of interior

views. Many cells structures are now

visible.

TEM Limitations

• Specimen dead.

• Specimen preparation uses extreme

chemicals so artifacts are always a

concern.

Scanning Electron Microscope

or SEM

• Excellent views of surfaces.

• Produces 3-D views.

• Live specimens possible.

Other Tools for Cytology

• Cell Fractionation – break the cell apart

and separate out the pieces.

• Chromatography – separates mixtures

based on their solubility.

• Electrophoresis – separates mixtures of

protein or DNA using gels and

electricity.

Cell Fractionation

History of Cells

• Robert Hooke - Observed cells in cork.

• Coined the term "cells” in 1665.

History of Cells

• 1833 - Robert Brown, discovered the nucleus.

• 1838 - M.J. Schleiden, all plants are made of cells.

• 1839 - T. Schwann, all animals are made of cells.

Cell Theory

• All living matter is composed of one or

more cells.

• The cell is the structural and functional

unit of life.

• All cells come from cells.

Types of Cells

• Prokaryotic - lack a nucleus and other

membrane bounded structures.

• Eukaryotic - have a nucleus and other

membrane bounded structures.

Both Have:

• Membrane

• Cytosol

• Ribosomes (but the size is different)

• DNA

Prokaryotic Eukaryotic

Nucleus

Prokaryotic

Eukaryotic

Why Are Cells So Small?

• Cell volume to surface area ratios favor

small size.

• Nucleus to cytoplasm consideration

(control).

• Metabolic requirements.

• Speed of diffusion.

Tuesday, September 22

• The paramecium and euglena are both

single-celled organisms. Identify two

pieces of evidence that make them

eukaryotic as opposed to prokaryotic.

Basic Cell Organization

• Membrane

• Nucleus

• Cytoplasm

• Organelles

Animal Cell

Plant Cell

Membrane

• Separates the cell from the

environment.

• Boundary layer for regulating the

movement of materials in/out of a cell.

Cytoplasm or Cytosol

• Cell substance between the cell

membrane and the nucleus.

• The “fluid” part of a cell. Exists in two

forms: • gel - thick

• sol - fluid

Organelle

• Term means "small organ”.

Formed body (or compartment) in a cell

with a specialized function.

• Important in organizational structure of

cells.

Organelles - function

• Way to form compartments in cells to

separate chemical reactions.

• Keeps various enzymes separated in

space.

You must be able to:

• Identify the major organelles

• Give their structure

• Give their function

Nucleus

• Most conspicuous organelle.

• Usually spherical, but can be lobed or

irregular in shape.

Structure

• Nuclear membrane

• Nuclear pores

• Nucleolus

• Chromatin

Nuclear Membrane

• Double membrane separated by a 20-

40 nm space.

• Inner membrane supported by a protein

matrix which gives the shape to the

nucleus.

Nuclear Pores

• Regular “holes” through both

membranes.

• 100 nm in diameter.

• Protein complex gives shape.

• Allows materials in/out of nucleus.

Nucleolus

• Dark staining area in the nucleus.

• 0 - 4 per nucleus.

• Storage area for ribosomes.

Chromatin

• Chrom: colored

• - tin: threads

• DNA and Protein in a “loose” format.

Will form the cell’s chromosomes.

Nucleus - Function

• Control center for the cell.

• Contains the genetic instructions.

Ribosomes

• Structure: 2 subunits made of protein

and rRNA. No membrane.

• Function: protein synthesis.

Subunits

• Large: • 45 proteins

• 3 rRNA molecules

• Small: • 23 proteins

• 1 rRNA molecule

Locations

• Free in the cytoplasm - make proteins

for use in cytosol.

• Membrane bound - make proteins that

are exported from the cell.

Endomembrane System

• Membranes that are related through

direct physical continuity or by the

transfer of membrane segments called

vesicles.

Endomembrane System

Endoplasmic Reticulum

• Often referred to as ER.

• Makes up to 1/2 of the total membrane

in cells.

• Often continuous with the nuclear

membrane.

Structure of ER

• Folded sheets or tubes of membranes.

• Very “fluid” in structure with the

membranes constantly changing size

and shape.

Types of ER

• Smooth ER: no ribosomes.

• Used for lipid synthesis,

carbohydrate storage, detoxification

of poisons.

• Rough ER: with ribosomes.

• Makes secretory proteins.

Golgi Apparatus or Dictyosomes

• Structure: parallel array of flattened

cisternae. (looks like a stack of Pita

bread)

• 3 to 20 per cell.

• Likely an outgrowth of the ER system.

Function of Golgi Bodies

• Processing - modification of ER

products.

• Distribution - packaging of ER products

for transport.

Golgi Vesicles

• Small sacs of membranes that bud off

the Golgi Body.

• Transportation vehicle for the modified

ER products.

Lysosome

• Single membrane.

• Made from the Golgi apparatus.

Function

• Breakdown and degradation of cellular

materials.

• Contains enzymes for fats, proteins,

polysaccharides, and nucleic acids.

• Over 40 types known.

Lysosomes

• Important in cell death.

• Missing enzymes may cause various

genetic enzyme diseases.

• Examples: Tay-Sachs, Pompe’s

Disease

Vacuoles

• Structure - single membrane, usually

larger than the Golgi vesicles.

• Function - depends on the organism.

Protists

• Contractile vacuoles - pump out excess

water.

• Food vacuoles - store newly ingested

food until the lysosomes can digest it.

Plants

• Large single vacuole when mature

making up to 90% of the cell's volume.

• Tonoplast - the name for the vacuole

membrane.

Function

• Water regulation.

• Storage of ions.

• Storage of hydrophilic pigments.

(e.g. red and blues in flower petals).

Function: Plant vacuole

• Used to enlarge cells and create turgor

pressure.

• Enzymes (various types).

• Store toxins.

• Coloration.

Microbodies

• Structure: single membrane.

• Often have a granular or crystalline

core of enzymes.

Function

• Specialized enzymes for specific

reactions.

• Peroxisomes: use up hydrogen

peroxide.

• Glyoxysomes: lipid digestion.

Enzymes in a

crystal

Mitochondria

• Structure: 2 membranes. The inner

membrane has more surface area than

the outer membrane.

• Matrix: inner space.

• Intermembrane space: area between

the membranes.

Inner Membrane

• Folded into cristae.

• Amount of folding depends on the level

of cell activity.

• Contains many enzymes.

• ATP generated here.

Function

• Cell Respiration - the release of energy

from food.

• Major location of ATP generation.

• “Powerhouse” of the cell.

Comment – be careful NOT to overuse

this phrase.

Mitochondria

• Have ribosomes (small size).

• Have their own DNA.

• Can reproduce themselves.

• Likely were independent cells at one

time.

Chloroplasts

• Structure - two outer membranes.

• Complex internal membrane.

• Fluid-like stroma is around the internal

membranes.

Inner or Thylakoid Membranes

• Arranged into flattened sacs called

thylakoids.

• Some regions stacked into layers called

grana.

• Contain the green pigment chlorophyll.

Function

• Photosynthesis - the use of light energy

to make food.

Chloroplasts

• Contain ribosomes (small size).

• Contain DNA.

• Can reproduce themselves.

• Often contain starch.

• Likely were independent cells at one

time (cyano-bacteria).

Plastids

• Group of plant organelles.

• Structure - single membrane.

• Function - store various materials.

Examples

• Amyloplasts/ Leucoplasts - store starch.

• Chromoplasts - store hydrophobic plant

pigments such as carotene.

Ergastic Materials

• General term for other substances

produced or stored by plant cells.

• Examples: • Crystals

• Tannins

• Latex

• Resins

Thursday, September 24

• Which of these statements accurately

reflects the relationship between cell size

and surface area? • Larger cells are most efficient at transporting materials across

the membrane since their surface area is increased.

• Smaller cells must have more phospholipids per area in order

to adequately transport materials into the cell.

• Cells must maximize their surface area to volume ratio in

order to maintain homeostasis.

• Cells must minimize their surface area exposure to the

extracellular matrix in order to retain cytosol.

Cytoskeleton

• Network of rods and filaments in the

cytoplasm.

Functions

• Cell structure and shape.

• Cell movement.

• Cell division - helps build cell walls and

move the chromosomes apart.

Cytoskeleton Components

• Microtubules

• Microfilaments

• Intermediate Filaments

Microtubules

• Structure - small hollow tubes made of

repeating units of a protein dimer.

• Size - 25 nm diameter with a 15 nm

lumen. Can be 200 nm to 25 mm in

length.

Tubulin

• Protein in microtubules.

• Dimer - a and b tubulin.

Microtubules

• Regulate cell shape.

• Coordinate direction of cellulose fibers

in cell wall formation.

• Tracks for motor molecules.

Microtubules

• Form cilia and flagella.

• Internal cellular movement.

• Make up centioles, basal bodies and

spindle fibers.

Cilia vs. Flagella

• Cilia - short, but numerous.

• Flagella - long, but few.

• Function - to move cells or to sweep

materials past a cell.

Cilia and Flagella

• Structure - 9+2 arrangement of

microtubules, covered by the cell

membrane.

• Dynein - motor protein that connects

the tubules.

Dynein Protein

• A contractile protein.

• Uses ATP.

• Creates a twisting motion between the

microtubules causing the structure to

bend or move.

Centrioles

• Usually one pair per cell, located close

to the nucleus.

• Found in animal cells.

• 9 sets of triplet microtubules.

• Help in cell division.

Basal Bodies

• Same structure as a centriole.

• Anchor cilia and flagella.

Basal Body

Microfilaments

• 5 to 7 nm in diameter.

• Structure - two intertwined strands of

actin protein.

Microfilaments

are stained green.

Functions

• Muscle contraction.

• Cytoplasmic streaming.

• Pseudopodia.

• Cleavage furrow formation.

• Maintenance and changes in cell

shape.

Intermediate Filaments

• Fibrous proteins that are super coiled

into thicker cables and filaments

8 - 12 nm in diameter.

• Made from several different types of

protein.

Functions

• Maintenance of cell shape.

• Hold organelles in place.

Cytoskeleton

• Very dynamic; changing in composition

and shape frequently.

• Cell is not just a "bag" of cytoplasm

within a cell membrane.

Cell Wall

• Nonliving jacket that surrounds some

cells.

• Found in: • Plants

• Prokaryotes

• Fungi

• Some Protists

Plant Cell Walls

• All plant cells have a Primary Cell Wall.

• Some cells will develop a Secondary

Cell Wall.

Primary Wall

• Thin and flexible.

• Cellulose fibers placed at right angles to

expansion.

• Placement of fibers guided by

microtubules.

Secondary Wall

• Thick and rigid.

• Added between the cell membrane and the primary cell wall in laminated layers.

• May cover only part of the cell; giving spirals.

• Makes up "wood”.

Middle Lamella

• Thin layer rich in pectin found between

adjacent plant cells.

• Glues cells together.

Cell Walls

• May be made of other types of

polysaccharides and/or silica.

• Function as the cell's exoskeleton for

support and protection.

Extracellular Matrix - ECM

• Fuzzy coat on animal cells.

• Helps glue cells together.

• Made of glycoproteins and collagen.

• Evidence suggests ECM is involved with cell behavior and cell communication.

Intercellular Junctions

• Plants-Plasmodesmata

Plasmodesmata

• Channels between cells through

adjacent cell walls.

• Allows communication between cells.

• Also allows viruses to travel rapidly

between cells.

Intercellular Juctions

• Animals: • Tight junctions

• Desmosomes

• Gap junctions

Tight Junctions

• Very tight fusion of the membranes of

adjacent cells.

• Seals off areas between the cells.

• Prevents movement of materials around

cells.

Desmosomes

• Bundles of filaments which anchor junctions between cells.

• Does not close off the area

between adjacent cells.

• Coordination of movement between

groups of cells.

Gap Junctions

• Open channels between cells, similar to

plasmodesmata.

• Allows “communication” between cells.

Chapter Summary

• Answer: Why is Life cellular and what

are the factors that affect cell size?

• Be able to identify cellular parts, their

structure, and their functions.

Cell Animation Link

• http://multimedia.mcb.harvard.edu/anim

_innerlife_hi.html

• You may need to replay this several

times to catch all of the parts.