Chapter 3
An Introduction to
The Cellular Level of Organization
I. An Introduction to Cells
A. The cell theory states:
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Cells are the building blocks of all plants and animals
Cells are produced by the division of preexisting cells
Cells are the smallest units that perform all vital physiological functions
Each cell maintains homeostasis at the cellular level
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Homeostasis at higher levels reflects combined, coordinated action of many cells
B. Cell biology
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Cytology, the study of the structure and function of cells
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The human body contains both somatic and sex cells
C. A typical cell
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Is surrounded by extracellular fluid, which is the interstitial fluid of the tissue
Has an outer boundary called the cell membrane or plasma membrane
II. The Cell Membrane
A. Cell membrane functions include:
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Physical isolation
Regulation of exchange with the environment
Structural support
B. Membrane proteins include:
• Integral proteins
• Peripheral proteins
Membrane proteins perform the following functions:
• Anchoring proteins
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Recognition proteins
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Receptor proteins
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Carrier proteins
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Channels
C. Membrane carbohydrates form the glycocalyx
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Proteoglycans
Glycolipids
Glycoproteins
D. Membrane lipids lend stability
•Phospholipids make up the bilayer membrane
•Hydrophilic head, hydrophobic tails
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Cholesterol – important membrane lipid for structural integrity of membrane
III. The Cytoplasm
A. The cytoplasm contains:
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The fluid (cytosol)
The organelles the cytosol surrounds
IV. Cellular Organelles
A. Organelles
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Nonmembranous organelles are not enclosed by a membrane and always in touch with the cytosol
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Cytoskeleton, microvilli, centrioles, cilia, ribosomes, proteasomes
Membranous organelles are surrounded by lipid membranes
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Endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria
B. Cytoskeleton provides strength and flexibility
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Microfilaments
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Intermediate filaments
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Microtubules
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Thick filaments
Microvilli increase surface area
C. Centrioles
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Direct the movement of chromosomes during cell division
Organize the cytoskeleton
Cytoplasm surrounding the centrioles is the centrosome
D. Cilia
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Is anchored by a basal body
Beats rhythmically to move fluids across cell surface
E. Ribosomes
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Are responsible for manufacturing proteins
Are composed of a large and a small ribosomal subunit
Contain ribosomal RNA (rRNA)
Can be free or fixed ribosomes
F. Proteasomes
• Remove and break down damaged or abnormal proteins
• Require targeted proteins to be tagged with ubiquitin
G. Endoplasmic reticulum
• Intracellular membranes involved in synthesis, storage, transportation and detoxification
• Forms cisternae
• Rough ER (RER) contains ribosomes
• Forms transport vesicles
• Smooth ER (SER)
• Involved in lipid synthesis and detoxification
H. Golgi Apparatus
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Forms secretory vesicles
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Discharged by exocytosis
Forms new membrane components
Packages lysosomes
I. Lysosomes and Peroxisomes
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Lysosomes
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Filled with digestive enzymes
Responsible for autolysis of injured cells
Peroxisomes
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Carry enzymes that neutralize toxins
J. Mitochondria
• Responsible for ATP production through aerobic respiration
• Matrix = fluid contents of mitochondria
• Cristae = folds in inner membrane
K. Membrane flow
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Continuous movement and recycling of membranes
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Vesicles
Golgi apparatus
Cell membrane
V. The Nucleus
A. The nucleus is the center of cellular operations
• Surrounded by a nuclear envelope
• Perinuclear space
• Communicates with cytoplasm through nuclear pores
B. Contents of the nucleus
• A supportive nuclear matrix
• One or more nucleoli
• Chromosomes
• DNA bound to histones
• Chromatin
VI. Membrane Transport
A. Permeability
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The ease with which substances can cross the cell membrane
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Nothing passes through an impermeable barrier
Anything can pass through a freely permeable barrier
Cell membranes are selectively permeable
B. Diffusion
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Movement of a substance from an area of high concentration to low
Continues until concentration gradient is eliminated
C. Osmosis
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Diffusion of water across a semipermeable membrane in response to solute differences
Osmotic pressure = force of water movement into a solution
Hydrostatic pressure opposes osmotic pressure
Water molecules undergo bulk flow
D. Tonicity
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The effects of osmotic solutions on cells
Isotonic = no net gain or loss of water
Hypotonic = net gain of water into cell
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Hemolysis
Hypertonic = net water flow out of cell
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Crenation
E. Passive transport
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Carrier mediated transport
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Binding and transporting specific ions by integral proteins
• Cotransport
• Counter-transport
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Facilitated diffusion
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Compounds to be transported bind to a receptor site on a carrier protein
F. Active transport
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Active transport
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Consumes ATP
Independent of concentration gradients
Types of active transport include
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Ion pumps
Secondary active transport
G. Vesicular transport: material moves into or out of cells in membranous vesicles
• Endocytosis is movement into the cell
• Receptor mediated endocytosis (coated vesicles)
• Pinocytosis
• Phagocytosis (pseudopodia)
• Exocytosis is ejection of materials from the cell