Cells
I. Cell Structure and Function
A. Cell Theory
1. A cell is the basic structural and functional unit of living organisms
2. All cells come from pre-existing cells
3. The activity of an organism depends on both the individual and the collective
activities of its cells
A) Principle of Complementarity – the biochemical activities of cells are dictated
by their specific subcellular structures
4. Most cells are capable of self-reproduction
B. Typical cells are made of three main parts:
1. Cell (plasma) membrane
2. Nucleus
3. Cytoplasm and cytoplasmic organelles
C. Cell Membrane
1. A flexible, selectively permeable membrane that defines the extent of a cell by
separating intracellular fluid (ICF) from the extracellular fluid (ECF)
2. The Fluid Mosaic Model – 1972
A) Composed of a phospholipid bilayer
1) Hydrophilic heads face outward
2) Hydrophobic tails face inward
a) creates a hydrophobic core that gives the membrane selective
permeability
3) Two layers are moveable (fluid)
B) The plasma membrane contains many embedded structures
1) Proteins:
a) Transmembrane proteins – proteins that typically extend all the way
across the membrane and play a role in many functions
i) Transport proteins
(a) Channels, pores, carriers, and pumps
ii) Enzymes
iii) Receptors
iv) Intercellular junctions
v) Cell-cell recognition
b) Surface (peripheral) proteins – proteins that are attached to either side of
the membrane and are mostly involved with:
i) Acting as enzymes & receptors
ii) Attaching to the cytoskeleton for movement and support
iii) Aid in changing the shape of the cell
2) Cholesterol & Steroids – lipid structures that help to stabilize the membrane
a) Necessary due to the lack of a cell wall
, 3. Associated Structures
A) Microvilli (“little shaggy hairs”) – finger-like projections that increase the
surface area of the plasma membrane
B) Membrane junctions
1) Tight junctions – membrane proteins fuse the two adjacent cell membranes
together
a) Prevent the movement of substances between cells
2) Desmosomes – anchoring junctions that are associated with protein filaments
a) Provide strength
3) Gap junctions – membrane proteins form channels between adjacent cells
a) Allow impulse transmission between cells
D. Nucleus
1. The nucleus is the control center of the cell that also contains DNA in the form of
chromatin/chromosomes
A) Chromatin – relaxed DNA; not visible
B) Chromosomes – condensed DNA; visible during mitosis/meiosis
2. The nucleus is bound by a dual-walled nuclear envelope that possesses many
nuclear pores
3. Within the nucleus is a jelly-like fluid called the nucleoplasm
4. Nucleoli – are darkly staining areas within the nucleus that indicate RNA
synthesis/storage
E. Cytoplasm and Cytoplasmic Organelles
1. Cytoplasm (a.k.a. cytosol) – is the viscous semitransparent fluid in which the other
cytoplasmic elements are suspended
2. Organelles – specialized cellular compartments
A) Mitochondria – site of cellular respiration and ATP production; often called the
“powerhouse”
B) Ribosomes – used for protein synthesis
1) Composed of two subunits
a) Small subunit – holds mRNA during translation
b) Large subunit – composed of 2 functional sites
i) A-site – where tRNA brings the amino acids
ii) P-site – site of peptide bond formation; also holds the developing
protein
C) Endoplasmic Reticulum – extensive system of interconnected fluid-filled tubes
that are continuous with the nuclear membrane
1) Rough ER – studded with ribosomes; responsible for protein production
2) Smooth ER – no ribosomes; responsible for lipid synthesis
D) Golgi apparatus – series of membranous sacs (cisternae); responsible for the
processing & packaging of metabolic products
E) Vesicle – membrane-bound sac containing cellular products
1) Lysosomes – a vesicle containing digestive enzymes for intracellular
digestion
I. Cell Structure and Function
A. Cell Theory
1. A cell is the basic structural and functional unit of living organisms
2. All cells come from pre-existing cells
3. The activity of an organism depends on both the individual and the collective
activities of its cells
A) Principle of Complementarity – the biochemical activities of cells are dictated
by their specific subcellular structures
4. Most cells are capable of self-reproduction
B. Typical cells are made of three main parts:
1. Cell (plasma) membrane
2. Nucleus
3. Cytoplasm and cytoplasmic organelles
C. Cell Membrane
1. A flexible, selectively permeable membrane that defines the extent of a cell by
separating intracellular fluid (ICF) from the extracellular fluid (ECF)
2. The Fluid Mosaic Model – 1972
A) Composed of a phospholipid bilayer
1) Hydrophilic heads face outward
2) Hydrophobic tails face inward
a) creates a hydrophobic core that gives the membrane selective
permeability
3) Two layers are moveable (fluid)
B) The plasma membrane contains many embedded structures
1) Proteins:
a) Transmembrane proteins – proteins that typically extend all the way
across the membrane and play a role in many functions
i) Transport proteins
(a) Channels, pores, carriers, and pumps
ii) Enzymes
iii) Receptors
iv) Intercellular junctions
v) Cell-cell recognition
b) Surface (peripheral) proteins – proteins that are attached to either side of
the membrane and are mostly involved with:
i) Acting as enzymes & receptors
ii) Attaching to the cytoskeleton for movement and support
iii) Aid in changing the shape of the cell
2) Cholesterol & Steroids – lipid structures that help to stabilize the membrane
a) Necessary due to the lack of a cell wall
, 3. Associated Structures
A) Microvilli (“little shaggy hairs”) – finger-like projections that increase the
surface area of the plasma membrane
B) Membrane junctions
1) Tight junctions – membrane proteins fuse the two adjacent cell membranes
together
a) Prevent the movement of substances between cells
2) Desmosomes – anchoring junctions that are associated with protein filaments
a) Provide strength
3) Gap junctions – membrane proteins form channels between adjacent cells
a) Allow impulse transmission between cells
D. Nucleus
1. The nucleus is the control center of the cell that also contains DNA in the form of
chromatin/chromosomes
A) Chromatin – relaxed DNA; not visible
B) Chromosomes – condensed DNA; visible during mitosis/meiosis
2. The nucleus is bound by a dual-walled nuclear envelope that possesses many
nuclear pores
3. Within the nucleus is a jelly-like fluid called the nucleoplasm
4. Nucleoli – are darkly staining areas within the nucleus that indicate RNA
synthesis/storage
E. Cytoplasm and Cytoplasmic Organelles
1. Cytoplasm (a.k.a. cytosol) – is the viscous semitransparent fluid in which the other
cytoplasmic elements are suspended
2. Organelles – specialized cellular compartments
A) Mitochondria – site of cellular respiration and ATP production; often called the
“powerhouse”
B) Ribosomes – used for protein synthesis
1) Composed of two subunits
a) Small subunit – holds mRNA during translation
b) Large subunit – composed of 2 functional sites
i) A-site – where tRNA brings the amino acids
ii) P-site – site of peptide bond formation; also holds the developing
protein
C) Endoplasmic Reticulum – extensive system of interconnected fluid-filled tubes
that are continuous with the nuclear membrane
1) Rough ER – studded with ribosomes; responsible for protein production
2) Smooth ER – no ribosomes; responsible for lipid synthesis
D) Golgi apparatus – series of membranous sacs (cisternae); responsible for the
processing & packaging of metabolic products
E) Vesicle – membrane-bound sac containing cellular products
1) Lysosomes – a vesicle containing digestive enzymes for intracellular
digestion
