CELLS

Cell Theory, Microscopy, Prokaryotes

CELL THEORY1. All living things are composed of

cells and cell products.2. New cells are formed only by the

division of existing cells.3. The cell contains inherited

information (genes) that are used as instructions for growth, functioning and development.

CELL THEORY, continued

4. The cell is the functioning unit of life; the chemical reactions of life take place within cells.

Possible Exceptions to the Cell Theory

The structure of skeletal muscle

Muscle

Bundle ofmuscle fibers

Single muscle fiber(cell)

Plasma membrane

Myofibril

Lightband Dark band

Z line

Sarcomere

TEM 0.5 mI band A band I band

M line

Thickfilaments(myosin)

Thinfilaments(actin)

H zoneSarcomere

Z lineZ line

Nuclei

•Muscle consists of bundles of long cells called fibers.

•Each fiber has a membrane around the outside like a single cell

•But each fiber has hundreds of nuclei.

Figure 31.2: Structure of a multicellular fungus

Hyphae. The mushroom and its

subterranean mycelium are a

continuous network of hyphae.

Reproductive structure.

The mushroom produces

tiny cells called spores.

Spore-producing

structures

20 m

Mycelium

Figure 31.3: Structure of fungal hyphae

Nuclei

Cell wall

Septum

Pore

(a) Septate hypha (b) Coenocytic hypha

Cell wall

Nuclei

SOME (but not most!) biologists consider unicellular organisms to be acellular.

• Examples: Amoeba and Acetabularia

– instead of separate cells for separate functions, cytoplasm must carry out all vital functions

– usually much larger than typical cells

Acetabularia > 7 cm in length

Amoeba proteus ~ 700 -800 µm

The animal cell

The plant cell

Cell differentiation

www.cellsalive.com

www.brianpop.com

Task • Look at the diagram

and list similarities and differences between the two cells.

Prokaryotic Cells

Prokaryotic Cells

• Pro – before

• Karyon – nucleus

• Comprise domains Bacteria and Archaea (Archaebacteria)

Electron microscope views of prokaryotic cells

Functions of the parts of a prokaryotic cell• Cell wall – forms protective outer

layer which prevents damage from outside and bursting if internal pressure is high

• Plasma membrane – controls entry and exit of substances, pumping some in by active transport

Functions of the parts of a prokaryotic cell, cont’d.• Mesosome – increases the area of

membrane for ATP production; may move DNA to poles during cell division

• Cytoplasm – contains enzymes that catalyze chemical reactions of metabolism; also contains cell’s DNA, in region known as nucleoid

Functions of the parts of a prokaryotic cell, cont’d.• Ribosomes – synthesize proteins

by translating mRNA; some proteins remain in cell and others are exported

• Naked DNA – stores genetic information that controls cell and is passed onto daughter cells.

Metabolic activities in prokaryotes• Photosynthesis – carried out by

Cyanobacteria (sometimes called blue-green “algae”)

Metabolic activities in prokaryotes, cont’d.• Nitrogen fixation – nitrogen fixing

bacteria convert nitrogen gas from the air into nitrogen compounds–some are free-living bacteria–some live in symbiosis with

plants, such as Rhizobium, forming root nodules on legumes

Metabolic activities in prokaryotes, cont’d.• Fermentation - process by which an

sugar is broken and energy released.

–occurs during times of low oxygen supply and is therefore known as a type of anaerobic respiration