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Worksheet 2.1
Fifteen summary facts
1
The cell theory has three main principles. These are:
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all organisms are composed of one or more cells
cells are the smallest units of life
all cells come from pre-existing cells.
The cell theory has been developed over the years after long periods of microscope observation
and experimental investigation.
2
Cells are limited in size based on the surface area to volume ratio. This ratio must be high so that
adequate amounts of necessary materials can enter the cell. Also, a relatively large surface area is
needed to get wastes out of the cell so that waste toxicity does not occur.
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Stem cells are cells that occur within organisms and which retain their ability to divide and
differentiate into various cell types. Pluripotent stem cells are also called embryonic stem cells and
they have the ability to form any type of cell in an organism. When stem cells reproduce and go
through differentiation, they continue to produce more stem cells. This maintains a level of stem
cells in organisms. The use of stem cells raises ethical issues, and in some cases, the use of stem
cells is thought to have created cancers.
4
Prokaryotic cells are much smaller and simpler than eukaryotic cells. Because of this and fossil
evidence, prokaryotic cells are thought to have been the first life on Earth. Features of prokaryotic
cells include: the cell wall, the plasma membrane, flagella, ribosomes, and the nucleoid. The
prokaryotic cell wall is composed largely of peptidoglycan.
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Eukaryotic cells range in diameter from 5 to 100 micrometres. They contain organelles and show
greater complexity than prokaryotic cells. The DNA of eukaryotic cells combines with histones
(types of protein) to form chromosomes. The ribosomes of eukaryotic cells are larger in size than
the ribosomes found in prokaryotic cells.
Eukaryotic cells may be differentiated into plant and animal cells. Plant cells contain chloroplasts,
animal cells do not. Plant cells do not have centrioles but animal cells do. Plant cells have cell
walls while animal cells only have plasma membranes. Plant cell walls are composed of cellulose.
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All cellular membranes, including the plasma membrane, are composed of phospholipids and
proteins. The membrane structure is represented by the ‘fluid mosaic model’. This model has a
phospholipid bilayer with proteins, carbohydrates and cholesterol positioned at specific locations.
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The functions of membrane proteins include hormone-binding sites, enzymatic action, cell
adhesion, cell-to-cell communication, channels for passive transport and pumps for active
transport.
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8
Passive transport involves no expenditure of energy on the part of the cell and includes diffusion,
facilitated diffusion and osmosis. Active transport requires the expenditure of energy and it
includes the sodium–potassium pump, endocytosis, and exocytosis.
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The sodium–potassium pump is extremely important in the human body because it brings about
the proper ion levels within nerves and muscles so that they can operate efficiently.
10 Endocytosis occurs when a portion of the plasma membrane is pinched off to enclose
macromolecules or particulates needed by the cell. This results in the formation of a vesicle inside
the cell. Exocytosis is essentially the opposite of endocytosis. The Golgi apparatus is essential to
the process of exocytosis.
11 The cell cycle describes the behaviour of cells as they grow and divide. Cell division is one part of
the cell cycle. The parts of the cycle include G1, S, G2, M, and cytokinesis. Interphase is the phase
of the cell cycle in which G1, S, and G2 occur.
12 Mitosis (M phase) involves four phases, prophase, metaphase, anaphase, and telophase. In
prophase, the chromosomes become visible and the nuclear membrane disappears. During
metaphase, the chromosomes line up along the metaphase plate of the cell and the spindle is
attached to the chromosome centromeres. During anaphase, individual chromatids of the
chromosomes are split and move to opposite poles of the cell. At telophase, the nuclear membrane
reforms and cytokinesis begins.
13 Cytokinesis in plant cells occurs with the formation of a cell plate inside the cell and spreading
out. Cytokinesis in animal cells occurs with the pinching in of the plasma membrane resulting in a
cleavage furrow. It is the presence of the cell wall in plant cells but not in animal cells that brings
about this difference in cytokinesis.
14 Prokaryotic cells divide by a much simpler process called binary fission.
15 Tumours (cancers) are the result of uncontrolled cell division and they may occur in any tissue or
organ.
© Pearson Education Ltd 2010. For more information about the Pearson Baccalaureate series please visit www.pearsonbacc.com 2