TIPS FOR SUCCESS IN BI 314

CHAPTER 1 — OVERVIEW OF CELLS AND CELL RESEARCH

Cells are the fundamental units of life

- cell biology explains how life works

All cells =

Small, membrane-bounded units
Are filled with concentrated aqueous solution of chemicals
Can copy themselves by growing and dividing
Employ DNA as their genetic material
Use the same basic mechanisms for energy metabolism

These similarities indicate that all present-day cells descended from single primordial ancestor

Simplest organisms = single cells

- viruses can’t replicate themselves, so not cells, not living

Higher organisms = communities of cells

- highly diverse, but function in coordinated manner

- due to intricate communication systems.

Two main classes of cells

Prokaryotic ("before nucleus) — lack membrane around genetic material

= bacteria, the smallest cells, simple, mostly single cells

Eukaryotic ("true nucleus) — genetic material separated from cytoplasm

by nuclear envelope.

= yeast, animals, plants, etc., more complex, have organelles,

cytoskeleton.

- often multicellular

How did first cell originate?

From fossil record, know life emerged ~3.8 billion yrs ago

- 750 million yrs after Earth formed.

Will never know exactly what happened — conditions different today.

But, evidence from several directions gives hint what happened:

- can add energy to presumed early atmosphere and get organic

molecules

- other early conditions result in polymerization of organic

molecules to form macromolecules;

- RNA can catalyze its own replication

- would allow reproduction and evolution

Early cells originated in sea of organic molecules

- could directly obtain food and energy from environment

Eventually evolved mechanisms for generating energy and synthesizing

molecules on their own — less limiting

ATP = adenosine-5’triphosphate = basic chemical fuel of all cells

Glycolysis = 1^st energy-generating process to evolve

= anaerobic breakdown of glucose to lactic acid (net gain = 2ATP)

Photosynthesis — came next (~3 billion yrs ago)

= uses energy of sunlight to produce glucose from CO₂ and H₂O

- changed Earth’s atmosphere because O₂ is released.

Oxidative metabolism — able to evolve once O₂ was abundant

- O₂ allows much more efficient conversion of organic molecules

to energy — e.g. 36-38 ATP/glucose instead of 2.

- almost all present-day cells use it as principal source of energy.

Present-day prokaryotes = eubacteria and archaebacteria

Single compartment w/cytoplasm and DNA

Simple shapes, e.g. rods, spheres, spirals

Archaebacteria evolved first and many still live in extreme environments similar to early Earth — e.g. sulfur hot springs

Both types of bacteria have remarkable chemical diversity

- can exploit wide range of habitats

Some eubacteria are photosynthetic = cyanobacteria
A few form chains, clusters, or other multicellular structures

Eukaryotic cells have nuclei and other organelles

Nucleus = information store of cell — enclosed by 2 membranes

- contains DNA = extremely long polymers, packed in chromosomes

- stores genetic information in copyable, useable form

Mitochondria — generate energy from food to power cell

- present in all eukaryotic cells

- enclosed by 2 membranes, inner has many extensions to interior

- site of oxidative metabolism

Chloroplasts — capture energy from sunlight

- large organelles — green from chlorophyll = light-harvesting pigment

- 2 surrounding membranes + stacks of membrane sacs

present in plants, algae, some bacteria, but not animals or fungi

Mitochondria and chloroplasts are both thought to originate by

endosymbiosis between two organisms, e.g an aerobic or

photosynthetic bacterium and an anaerobic eukaryote.

- mitochondria thought to evolve first, then chloroplasts.

Endoplasmic reticulum (ER) = irregular maze of spaces enclosed

by a single membrane

- extends from nucleus through cytoplasm

- site of synthesis of most cell membrane components

Golgi apparatus = stacks of flattened single membrane sacs

- receives and modifies molecules made in ER

- traffic director for rest of cell — sorts and transports

- in plants, synthesizes polysaccharides for cell wall

Lysosomes = small, irregularly-shaped, single membranes

- digest food particles, recycle unwanted molecules

Peroxisomes = small, single membrane-bounded vesicles

- provide contained environment for generation and degradation of

hydrogen peroxide

Vacuole = large central organelle in plants — important to growth

— performs variety of functions including digestion of

macromolecules, storage of wastes and nutrients

Cytoskeleton = matrix of long, fine filaments of protein that give structure to cytoplasm

responsible for transport, movement, positioning of cell components

Development of Multicellular Organisms

Even the simplest single-celled eukaryotes (e.g. yeast) are more

complex than prokaryotes and some single-celled eukaryotes are

very complex (e.g. amoebae — specialized for movement)

Multicellular organisms evolved from unicellular eukaryotes ~1.7 billion

years ago.

early version was multicellular aggregates of unicellular

eukaryotes — e.g. Dictyostelium, Volvox

cells became increasingly specialized, leading to division of labor

and lifestyles and diversity that weren’t available before

Cells as Experimental Models

Read about the various model organisms in the text, but they will be

covered individually in lecture as each is used later in the term.

Tools of Cell Biology

Progress in cell biology research has depended on technical advances

Since cells are usually too small to be seen by the eye, the beginning of

cell biology began with the development of microscopes

Light microscopy

1665 — Robert Hooke discovered "cells" by observing cork slices

with a simple light microscope.

1670s — Antony van Leeuwenhoek visualized bacteria, sperm and

blood cells using microscope that could magnify 300x

1838 — Matthias Schleiden (plant) and Theodor Schwann (animal)

propose that all organisms are composed of cells

= cell theory

Soon after, scientists realized all cells come from division of

pre-existing cells — no spontaneous generation

Light microscope still basic tool of cell biologists

modern light microscopes can magnify ~1,000x

resolution = ability of microscope to distinguish objects separated by short distances — needed to see detail

limit of resolution of light microscope ~0.2 µm

determined by 2 factors — wavelength of visible light

and light-gathering power of lens = numerical aperature

other tricks needed to see intracellular structure —

Bright-field = normal, sections usually stained

Phase contrast and differential-interference microscopy convert variations in density or thickness between different parts of cell to differences in contrast

- good for live, unstained cells

Video-enhanced differential interference-contrast microscopy — adds electronic image processing

Greatly enhances contrast —allows visualization of smaller objects

Fluorescence microscopy uses fluorescent dyes to visualize where specific cell components (e.g. proteins) are in cell, even though they can’t be seen themselves

Confocal laser-scanning microscopy combines fluorescence microscopy with electronic image analysis.

Gives 3-D images

Two-photon excitation microscopy — similar to confocal

Especially good for 3-D imaging of living cells

Electron Microscopy

Wavelength of electrons is much shorter than light, so can get much

greater resolution than can be obtained with light microscope

Not as great as theoretically possible due to limits imposed by available lenses (NA), but still much better than light microscopy - ~ 1-2 nm

Two types — transmission (specimens fixed and stained)

and scanning (3-D images of surfaces)

Allows visualization in internal cellular structure and even individual molecules

Subcellular Fractionation

Isolation of organelles of eukaryotic cells, so they can be used for

biochemical studies

Differential centrifugation — separates components of cells based on

their size and density

Density-gradient centrifugation — organelles are separated by

sedimentation through a gradient of a dense substance such as

sucrose.

Cell Cultures — allow study of regulation of cell growth and differentiation

Animal cell cultures retain their differentiated identity — e.g. fibroblast

or epithelial cell in cell cultures

Attach themselves to plastic surfaces
Not all animal cells can be cultured
Complex culture media required — including growth factors

Plant cell cultures become undifferentiated (= callus), then can develop different cell types

Totipotent = entire plant can be regenerated from single cell
Facilitates genetic engineering
Also requires specific growth factors