Topic 2.1 – Cell Theory & Stem Cells

2.1.1 - 2.1.10

Text pg 7-21

What is a cell?

• The usually microscopic unit from which living things are built

• A “bag” of gel-like cytoplasm inside a plasma membrane

• The smallest unit capable of all the functions of life

Egg and sperm cells

Discovery of Cells

• Linked to developments in technology like the microscope

• Sequence of discoveries by different scientists

Discovery of Cells• 1590 - Jansen invents microscope• 1665 - Hooke studies cork and names the

structures “cells”• 1675 - van Leeuwenhoek discovers

unicellular organisms• 1838 - Schleiden - plants are made of cells• 1839 - Schwann - animals made of cells• 1855 - Virchow - all cells come from cells

Theory

• System of ideas used to explain observations

• Can be modified or replaced with new theory as new evidence is found that supports or rejects original theory

Cell Theory: first point•Living organisms are composed of cells

Evidence:All organisms, no matter their size, are made of one or more cells.

Cholera bacteria Elephant cells

These cells all have similar chemicals and structures (membrane, ribosomes, etc.)

Cell Theory: first point•Living organisms are composed of cellsChallenges:Are single-celled organisms like bacteria truly cellular?In some fungus and muscles, the cytoplasm is not separated into cells. These “cells” can be very large.

Cell theory: second point• A single cell is the smallest unit that can

carry out all the functions of life

Evidence:The cell is smallest unit known to science that can carry out all of the functions of life.

Functions of life: Homeostasis – maintaining a balanced internal

environment

Reproduction – creating genetically related offspring

Metabolism – controls the chemical reactions of life

Nutrition – acquires the chemical building blocks needed to sustain life

Growth – increase in size and mass through nutrition and metabolism

Responsiveness – changing actions or behavior due to environmental signals

Cell theory: second point• A single cell is the smallest unit that can

carry out all the functions of life

Challenges:Viruses (much smaller than cells) are made of the molecules of life (like RNA or DNA) and are similar to living things, but are non-living.

Cell theory: third point• Cells are formed only from pre-existing cells

Evidence:When no cells are present (like in sterilized soup) no new cells can form.

Cell theory: third point

• Cells are formed only from pre-existing cells

Challenges:Where did the first cells come from?In a different environment, like that of the early Earth, it is thought that cells once evolved from non-living cell-like ancestors

Unicellular v. Multicellular

• Unicellular organisms: cells are generalists -- each cell capable of performing every life function.

• Multicellular organisms: cells are specialists -- each cell is adapted to a specific, specialized function.

Differentiation• Every cell in a multicellular organism

has the same DNA but cells develop in different ways in order to have a specialized function

• Eg. Muscle cells - single function is to contract

Differentiation

• What do these cells do?

Differentiation• Once a cell specializes, it has

committed and cannot change to another change to another type of cell.

All cells have the same genes but different genes are activated (turned on) or inactivated (turned off)

Emergent properties• When all cells in the multicellular organism

work together, new abilities appear• These abilities are not found in any of the

individual cells or groups of cells• Ex. Thinking, seeing

Stem Cells• Found in embryos• Undifferentiated• These cells have the ability to differentiate

along different pathways into any type of cell• Not committed• Small # in adult tissues that allow

regeneration and repair (heart, brain)

Stem Cell Possibilities• Potential to cure many diseases

– Paralysis from spinal cord injuries– Type I Diabetes– Multiple sclerosis– Parkinson’s disease

• Other difficulties– Tissue rejection, cells not accepted– Cells difficult to program to differentiate into the

correct type of cell

Stem Cell Therapy

• Leukemia is a cancer of white blood cells (leukocytes)

• Made in bone marrow and released to bloodstream where they fight off infection.

• Abnormal leukocytes can be cancerous and must be destroyed through chemotherapy and radiation (destroys normal cells too)

Stem Cell Therapy

• Bone marrow transplant can provide patient with a sample of new bone marrow stem cells

• If successful, stem cells will differentiate into leukocytes and replace the abnormal cells

Ethics pg 19

• Most usable stem cells are from embryos• Using early-stage embryos sometimes from

aborted embryos• Harm vs help decrease the suffering of

patients• Culture, religion• Uncontrolled differentiation of cells into

tumours

TOK Link

• How do scientists convey information to general public

• Who makes the decisions?• Are they informed?• How do we know what we know about

stem cells?