1.1 Introduction to Cells (with special thanks to Stephen Taylor)

According to the cell theory, living organisms are composed of cells. Nature of science: Looking for trends and discrepancies— although most organisms conform to cell theory, there are exceptions. Most human cells are 0.03 mm in length Our striated muscle cells have an average length of 30 mm and can have as many as several hundred nuclei…in a SINGLE cell! Fungi consist of narrow thread-like structures called hyphae. In some types of fungi, these hyphae have small cell-like sections, but not these…they are aseptate, a long, uninterrupted tube with many nuclei. Most algae are single-celled and microscopic. Not this one. This one grows up to 100 mm and is a single cell with one nucleus!

Organisms consisting of only one cell carry out all functions of life in that cell: nutrition, metabolism, growth, response, excretion, homeostasis and reproduction. Nucleus can divide to produce two when the cell is ready to divide. Food vacuoles contain food that has been absorbed and will be digested Cell membrane controls what substances enter and leave, such as O 2 in and waste out. Contractile vacuole that fills up with water and expels it to keep the amount of water balanced. Metabolic reactions take place in the cytoplasm, including respiration. Enzymes are the catalysts behind these reactions. Beating of the cilia moves Paramecium through the water so it can choose what direction to move in.

Organisms consisting of only one cell carry out all functions of life in that cell: nutrition, metabolism, growth, response, excretion, homeostasis and reproduction. Nucleus can divide to produce two when the cell is ready to divide. These nuclei can also fuse with anothers for a form of sexual reproduction. Photosynthesis occurs inside the chloroplasts. In the dark other compounds are sometimes absorbed. Cell wall is freely permeable and controls what substances enter and leave. O 2 is a waste product of photosynthesis and diffuses out. Contractile vacuole near the flagella fills up with water and expels it to keep the amount of water balanced. Metabolic reactions take place in the cytoplasm, including respiration. Enzymes are the catalysts behind these reactions. Beating of the two flagella move the Chlamydomonas through the water. A light-sensitive eyespot allows the cell to sense where the brightest light is and to move towards it.

Viruses are not classified as living organisms. Why not?

Surface area to volume ratio is important in the limitation of cell size. Volume = l × w × h Surface area = l × w × 6 Surface area to volume ratio = surface area ÷ volume

Stem cells and differentiation 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.

Use of stem cells to treat Stargardt’s disease and one other named condition. Stargardt’s macular dystrophy is a genetic disease that develops in children between the ages of 6 and 12. A mutation causes retina cells to malfunction and photoreception to degenerate. These are the cells that detect light, so vision becomes worse and worse and can lead to legal blindness. In mice, embryonic stem cells were made to develop into retinal cells. They were injected into the eyes of mice that had a condition similar to Stargardt’s disease. The cells were not rejected, did not develop into tumors, and did not cause other problems. The cells moved to the retina, attached themselves, and the vision of the mice improved. In 2010, human trials were approved in the US. A woman in her 50’s had 50,000 retina cells derived from embryonic stem cells injected into her eyes. The results were the same as in the mice; the cells moved to the retina, attached themselves, and her vision improved. Further trials are needed, but the results are optimistic!

Use of stem cells to treat Stargardt’s disease and one other named condition. A large needle is inserted into a large bone, usually the pelvis, and fluid is removed from the bone marrow of a donor. These adult stem cells are extracted and stored by freezing them. Kill the cancer cells with chemotherapy. Return the stored stem cells to the patient’s body. They re-establish themselves in the bone marrow, multiply, and start to produce red and white cells. Leukemia is a type of cancer. Cancer cells have lost their ability to control their reproduction, so they reproduce uncontrollably. These normally form lumps or tumors, But not leukemia. Instead, huge numbers of white blood cells are created, 10× as many as there should be! Chemotherapy kills tumor cells, but we need stem cells in our marrow to make blood cells.