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Cell Features
Section 3-2
What is the Cell Theory?
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All living things are made of one or more cells.
Cells are the basic units of structure and function in
an organism.
All cells arise from other living cells.
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Remember from Section 3-1 the scientists that contributed to
development of the cell theory – Virchow, van Leeuwenhoek,
Schwann, Schleiden, and Hooke
Surface Area – to Volume Ratio
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Small cells function more efficiently than larger cells
Human body cells range from 5-20 μm
All substances that enter or leave a cell must cross
the cell’s surface. If the cell’s surface to volume ratio
is too low, the substance cannot enter or leave the
cell in numbers large enough to meet the cell’s needs.
Surface Area – to Volume Ratio (con’t)
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Small cells can exchange substances more readily
than large cells because small objects have higher
surface area to volume ratio than larger objects.
The result is substances do not need to travel as far
to reach the center of a small cell.
Common Cell Features
Cell Membrane
1)
a.
b.
c.
Outer boundary of cell
Encloses cell interior from cell’s surroundings
Regulates what enters and leaves cell
Cytoplasm
2)
a.
b.
c.
The cell’s interior contents
Made of various membrane covered organelles and the
cytosol
Cytosol – soluble portion of cytoplasm; includes small
molecules and small particles.
Common Cell Features (con’t)
3) Cytoskeleton
a.
Cytoplasmic network of protein filaments that plays an
essential role in cell movement, shape, and division
4) Ribosomes
a.
Cellular structures on which proteins are made
5) DNA (deoxyribonucleic acid)
a.
b.
c.
Provides instructions for making proteins
Regulates cellular activities
Enables a cell to reproduce
Prokaryotes
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The smallest and simplest cells – informally called
bacteria
Single-celled organism that lacks a true nucleus and
other internal compartments. Because of this, they
cannot carry out many specialized functions.
Size ranges from 1 to 15 μm
Can exist in a broad range of environments
Cytoplasm contains everything inside the cell
membrane
Have no internal structures dividing cell into
compartments – ribosomes move freely in cell
Prokaryotes (con’t)
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Have circular DNA that is suspended in cytoplasm; no
nuclear membrane – NO TRUE NUCLEUS!!!
Cell Wall – surrounds cell membrane; provides
structure and support needed because no cytoskeleton;
made of polysaccharides connected by short strands of
amino acids
Some cell walls are surrounded by capsules. Capsules
are made of polysaccharides. These capsules allow
bacteria to cling to almost anything.
Prokaryotes (con’t)
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Flagella
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Long, thread-like structures that protrude from the cell’s
surface
Enable movement
Rotate and propel bacteria through its environment at
speeds of up to 20 cell lengths per second
Eukaryotes
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Organism that has a true nucleus – surrounded by
nuclear membrane
Have internal compartments – organelles –
structures that carry out specific activities in the cell
Nucleus – internal compartment that houses the
cell’s DNA
Organelles enable eukaryotic cells to function in ways
different from bacteria
Range from single-celled to multicellular
Eukaryotes (con’t)
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Some have flagella; others have cilia protruding from surface.
Cilia – hair-like structures that can either propel the cell or
can move substances across the cell’s surface.
Cytoplasm – includes everything inside the cell membrane,
but outside the nucleus
Cytoskeleton – web of protein fibers that hold cell together
and keep cell membrane from collapsing or folding.
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Microtubules – long, hollow tubes that extend through the
cytoplasm, supporting the cell
Microfilaments – extremely thin protein strands that also support
cell’s shape and structure
Eukaryotes (con’t)
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Ribosomes – moving freely in cell or attached to
the rough endoplasmic reticulum
Cell membrane – contains cytoplasm and organelles
Cell Membrane
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Fluid (like a soap bubble), not rigid.
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This is caused by lipids.
The lipids form a barrier that separates the inside of the cell
from the outside of the cell and only allows certain substances
to pass through – SELECTIVELY PERMEABLE!!!
Barrier
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Selective permeability caused by the way the phospholipids
interact with water
Phospholipid – lipid made of phosphate group and two
nonpolar fatty acid tails which make up the interior of the cell
membrane/lipid bilayer.
Cell Membrane (con’t)
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Forced to inside of bilayer because of interaction with water.
Water on both sides of cell
Ions and most polar molecules (sugars and some proteins) are
repelled by the nonpolar interior.
Proteins are embedded in the cell membrane
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Nonpolar part of membrane protein attracted to interior of the
lipid bilayer, but repelled by water on either side.
Polar part of membrane protein attracted to water on either
side of lipid bilayer
Dual attraction holds protein in lipid bilayer
Can move around because of fluidity of membrane
Cell Membrane (con’t)
Different Types of Membrane Proteins
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1.
2.
3.
4.
Marker Protein – attached to a carbohydrate on cell’s
surface to identify the cell type
Receptor Protein – recognizes and binds to specific
substances
Enzymes – involved in important biochemical
reactions in the cell
Transport Proteins – aid in movement of substances
in and out of the cell