Cell – the basic unit of life

Cells

• Smallest living unit• Most are microscopic

Discovery of Cells

• Robert Hooke (mid-1600s)– Observed bark of oak tree– Saw “row of empty boxes”– Coined the term cell

Cell theory

• (1839)Theodor Schwann & Matthias Schleiden

“ all living things are made of cells”

• (50 yrs. later) Rudolf Virchow

“all cells come from cells”

Principles of Cell Theory

• All living things are made of cells

• Smallest living unit of structure and function of all organisms is the cell

• All cells arise from preexisting cells

Cell Size

The origin and evolution of cells

• On the basis of presence of nucleus cells are divided

into two groups:

– Prokaryotic cells: lack a nuclear envelope

– Eukaryotic cells: have a nucleus in which the genetic material

is separated from the cytoplasm.

• Prokaryotic cells are generally smaller and simpler than

eukaryotic cells; their genomes are less complex and

they do not contain cytoplasmic organelles.

Cell Types

• Prokaryotic

• Eukaryotic

Prokaryotic Cells

• First cell type on earth

• Cell type of Bacteria and Archaea

Prokaryotic Cells

• No membrane bound nucleus

• Nucleoid = region of DNA concentration

• Organelles not bound by membranes

Prokaryotic Cells• Features shared by all prokaryotic cells:

– All have a plasma membrane.

– All have a region called the nucleoid where the DNA is concentrated.

– The cytoplasm (the plasma-membrane enclosed region) consists of the nucleoid, ribosomes, and a liquid portion called the cytosol.

Prokaryotic Cells• Specialized features of some prokaryotic

cells:

– A cell wall just outside the plasma membrane.

– Some bacteria have an outermost slimy layer made of polysaccharides and referred to as a capsule.

• Some bacteria have flagella, locomotory structures.

• Some bacteria have cillia, threadlike structures that help bacteria adhere to one another during mating or to other cells for food and protection.

Figure 4.5 A Prokaryotic Cell

Eukaryotic Cells• Nucleus bound by membrane

• Include fungi,, plant, and animal cells

• Possess many organelles

Protozoan

Eukaryotic Cells• Eukaryotes, animals, plants, fungi

have a membrane-enclosed nucleus in each of their cells.

• Eukaryotic cells:

– tend to be larger than prokaryotic cells.

– have a variety of membrane-enclosed compartments called organelles.

– have a protein scaffolding called the cytoskeleton.

Eukaryotic Cells• Compartmentalization is the key to

eukaryotic cell function.

• Each organelle has a specific role defined by chemical processes.

• Membranes surrounding these organelles keep away inappropriate molecules and also act as traffic regulators for raw materials into and out of the organelle.

Representative Animal Cell

Structure Animal cells Plant cells

cell membrane Yes yes

nucleus Yes yes

nucleolus yes yes

ribosomes yes yes

ER yes yes

Golgi yes yes

centrioles yes no

cell wall no yes

mitochondria yes yes

cholorplasts no yes

One big vacuole no yes

cytoskeleton yes Yes

How did organelles evolve?• many scientists theorize

that eukaryotes evolved from prokaryote ancestors.

• in 1981, Lynn Margulis popularized the “endosymbiont theory.”

Endosymbiont theory:

• a prokaryote ancestor “eats” a smaller prokaryote

• the smaller prokaryote evolves a way to avoid being digested, and lives inside its new “host” cell kind of like a pet.

• the small prokaryotes that can do photosynthesis evolve into chloroplasts, and “pay” their host with glucose.

• The smaller prokaryotes that can do aerobic respiration evolve into mitochondria, and convert the glucose into energy the cell can use.

• Both the host and the symbiont benefit from the relationship