Essential idea: The evolution of multicellular organisms allowed cell specialization and cell replacement.
Understandings:
According to the cell theory, living organisms are composed of cells.
Organisms consisting of only one cell carry out all functions of life in that cell.
Surface area to volume ratio is important in the limitation of cell size.
Multicellular organisms have properties that emerge from the interaction of their cellular components.
Specialized tissues can develop by cell differentiation in multicellular organisms.
Differentiation involves the expression of some genes and not others in a cell’s genome.
The capacity of stem cells to divide and differentiate along different pathways is necessary in embryonic development and also makes stem cells suitable for therapeutic uses.
Applications and skills:
Application: Questioning the cell theory using atypical examples, including striated muscle, giant algae and aseptate fungal hyphae.
Application: Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism.
Application: Use of stem cells to treat Stargardt’s disease and one other named condition.
Application: Ethics of the therapeutic use of stem cells from specially created embryos, from the umbilical cord blood of a new-born baby and from an adult’s own tissues.
Skill: Use of a light microscope to investigate the structure of cells and tissues, with drawing of cells. Calculation of the magnification of drawings and the actual size of structures and ultrastructures shown in drawings or micrographs. (Practical 1)
. All organisms are composed of one or more cells . Cells are the smallest unit of life . All cells come from pre-existing cells
Most organisms consists of cells but there are some cases where the idea of living organsims consisting of tiny-like structures does not seem to fit (like skeletal muscle which is made up of muscle fibers)
. Cells taken from an organism often survive for a time, smaller parts of an organism do not. Cells do therefore seem to be the smallest units of life that are capable of survival
. Some of the classic experiments in biology showed that spontaneous generation of life is imposible
. The first cells must've been formed from non-cellular material, but today there isn't any evidence that cells can be formed except by cell division
Unicellular organisms- organisms such as Amoeba, Chlorella and Euglena have only one cell. this single cell has to carry out all the functions of life:
. Metabolism- all the
chemical reaction that occur within an organism . Growth-
irreversible increase in life . Reproduction-
producing offspring either sexually or axesually . Response- to the
environment is imperative to the survival of the organism . Homeostasis-
keeping conditions inside the organisms within tolerable limits . Nutrition-
obtaining food, to provide energy and the materials needed for growth
Multicellular organisms- consists of many cells. These cells so not have to carry out many different functions. They can become specialized for one particular function and carry it out very efficiently. They can differentiate
Why do cells look so different?
Stem Cells- populations of cells within organisms that that retain their ability to divide and differentiate into various cell types.
Small numbers of embryonic cells remain as stem cells however and they are
still present in the adult body
They are found in most human tissues: bone marrow, liver, and skin
The stem cells only allows limited repair- brain, kidney, and heart
Diseases such as Parkinson, multiple sclerosis, and strokes all involve the loss of neurons or other cells in the nervous system
• Multicellular organisms can differentiate into a variety of interdependent cell types –each specialized to carry out a subset of functions à greater efficiency
• Cellular differentiation –achieved through differential gene expression –all cells in an organism have identical DNA
Therapeutic use of stem cells
1. The placenta and umbilical cord of a baby is used as a source of stem cells. The blood drains out of the umbilical cord and is collected; the blood contains many hematopoietic stem cells, which means the cells can divide and differentiate into any type of blood cells
2. The red blood cells are removed from the cord blood and the remaining fluid is tested to find its tissue type, checked for disease-causing organisms and stored in liquid nitrogen, in a special bank of cord blood
3. Cord blood is used to treat patients, especially children who have leukemia. The patient's tissue type is matched with cord blood in the bank. If a suitable cord blood is available, the patient is given chemotherapy drugs that kill bone marrow cells, including the cells causing the leukemia
4. The selected cord blood is taken from the bank, thawed and introduced into the patient's blood system. The hematopoietic stem cells establish themselves in the patient's bone marrow, where they divide repeatedly to build up a population of bone marrow cells to replace those killed by the chemotherapy drugs.
Why is the cell small?
•Surface area determines rate of exchange •Volume determines metabolism •As cell size increases, volume increases proportionally faster than SA => SA: V ratio decreases –Low SA reduces rate of exchange while high V increases metabolic demands –Cell cannot meet needs of metabolic demands
( picture of convertion of units , bottom of the page)
um = micrometers
1mm = 1000 um
1 um = 1000 nm
A molecule = 1 nm Thickness of cell membrane = 10 nm Viruses = 100 nm Bacteria = 1μm Organelles = up to 10 μm Eukaryotic cells = up to 100 μm
Magnification can be used to calculate the actual size of a specimen if the , magnification and size of the image are known , this equation can be used : magnification = Size of image Size of specimen