Chapter 6: A Tour of the Cell

Observation

Is the keystone of science. Need: Techniques to observe cells.

Question ?

Can cells be seen with the naked eye? Yes, a few are large enough, but most

require the use of a microscope.

Microscope History

1590 - Janseen Brothers invent the compound microscope.

1665 - Robert Hooke “discovers” cells in cork.

Early 1700’s - von Leeuwenhoek makes many observations of cells including bacteria.

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

Magnification

Increase in diameter or size.

Resolution

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

As Magnification increases, resolution decreases.

LM working limits are 100 - 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 - 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 - SEM

Excellent views of surfaces. Produces 3-D views. Live specimens possible.

Limitations

Lower magnifications than the TEM.

EM Variations

High Voltage TEM Tunnel SEM Elemental Composition SEM

TEM - interior SEM - surface

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.

Other Tools for Cytology

Cell Fractionation Chromatography Electrophoresis

Cell Fractionation

Disrupt cells. Separate parts by centrifugation at

different speeds. Result - pure samples of cell structures

for study.

Cell Fractionation

Chromatography

Technique for separating mixtures of chemicals.

Separates chemicals by size or degree of attraction to the materials in the medium.

Ex - paper, gas, column, thin-layer

Electrophoresis

Separates mixtures of chemicals by their movement in an electrical field.

Used for proteins and DNA.

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.

1840 - J.E. Purkinje, coined the term “protoplasm”.

Cell Theory

All living matter is composed of one or more cells.

The cell is the structural and functional unit of life.

R. Virchow

“Omnis cellula e cellula” All cells are from other cells.

Types of Cells

Prokaryotic - lack a nucleus and other membrane bounded structures.

Eukaryotic - have a nucleus and other membrane bounded structures.

Prokaryotic Eukaryotic

Nucleus

Prokaryotic

Eukaryotic

How small can a cell be?

Mycoplasmas - bacteria that are .1 to 1.0 m. (1/10 the size of regular bacteria).

Why Are Cells So Small?

Cell volume to surface area ratios favor small size.

Nucleus to cytoplasm consideration (control).

Metabolic requirements.

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

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 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.

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.

Structure Has 2 Faces

Cis face - side toward the nucleus. Receiving side.

Trans faceface - side away from the - side away from the nucleus. Shipping side.nucleus. Shipping side.

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.

Movie

Lysosome

Single membrane. Made from the Trans face of the Golgi

apparatus.

Movie

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.

Mitochondria

Have ribosomes. Have their own DNA. Can reproduce themselves. May have been 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. Contain DNA. Can reproduce themselves. Often contain starch. May have been independent cells at

one time.

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

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.

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 m in length.

Tubulin

Protein in microtubules. Dimer - and 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 and Flagella

Cilia - short, but numerous. Flagella - long, but few. Function - to move cells or to sweep

materials past a cell.

Movie

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

MTOCs

Microtubule Organizing Centers - sites that microtubules grow from.

Assist in cell division by anchoring spindle fibers.

May be anchored by centrioles.

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.

Movie

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 Juctions

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.

Movie

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.

Movie

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.