Chapter 3
The Cellular Level of
Organization
A Generalized Cell: Animal Model
1.
2.
3.
Plasma membrane
Cytoplasm
Nucleus
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A Generalized Cell
Plasma membrane
1.
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forms the cell’s outer boundary
separates the cell’s internal environment from
the outside environment
is a selective barrier
plays a role in cellular communication
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A Generalized Cell
2. Cytoplasm
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all the cellular contents between the plasma
membrane and the nucleus
cytosol: the fluid portion, mostly water
organelles: sub-cellular structures having
characteristic shapes and specific functions
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A Generalized Cell
3. Nucleus
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large organelle that contains DNA
contains chromosomes, each of which
consists of a single molecule of DNA and
associated proteins
a chromosome contains thousands of
hereditary units called genes
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Plasma Membrane
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Flexible yet sturdy barrier
The fluid mosaic model - the arrangement of
molecules within the membrane resembles a
sea of lipids containing many types of proteins
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Fluid inside of bilayer consistency of olive oil
Mosaic of proteins
The lipids act as a selective barrier to certain
substances
The proteins act as “gatekeepers” to certain
molecules and ions
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Structure of a Membrane
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Consists of a lipid bilayer – made up of
phospholipids (amphipathic), cholesterol and
glycolipids
Integral/transmembrane proteins – extend
into or through the lipid bilayer; most integral
proteins, span the entire lipid bilayer
Peripheral proteins - attached to the inner
or outer surface of the membrane, do not
extend through it
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Structure of a Membrane
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Glycoproteins – membrane proteins with a
carbohydrate group attached that protrudes into
the extracellular fluid
Glycocalyx – the “sugary coating” surrounding
the membrane made up of the carbohydrate
portions of the glycolipids and glycoproteins
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Self recognition
Immune response - foreign antigens
Adherence
Protection
Hydrophilic – attracting water
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Structure of the Plasma Membrane
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Functions of Membrane Proteins
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Some integral proteins are ion channels
Transporters – selectively move substances
through the membrane
Receptors – for cellular recognition; a ligand
is a molecule that binds with a receptor
Enzymes - catalyze chemical reactions
Others act as cell-identity markers (blood
groups ABO)
Attachment to neighboring cells
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Selective Membrane Permeability
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The cell’s membrane is either permeable or
impermeable to certain substances
The lipid bilayer is permeable to oxygen,
carbon dioxide, water and steroids, but
impermeable to glucose
Transmembrane proteins act as channels
and transporters to assist the entrance of
certain substances, for example, glucose and
ions
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Passive vs. Active Processes
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Passive:
1.
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substances move across cell membranes
without the input of any energy
with or down the gradient
lipid soluble
Active:
1.
2.
3.
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energy usually from hydrolysis of ATP
carrier protein
gradient independent usually against a
concentration gradient
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03_table_01
Diffusion
Factors affecting diffusion:
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Steepness of concentration
gradient
Temperature
Mass of diffusing substance
Surface area
Diffusion distance
Can substance readily cross
membrane?
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Simple Diffusion
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Osmosis
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1.
2.
Net movement of water through a
selectively permeable membrane from an
area of high concentration of water (lower
concentration of solutes) to one of lower
concentration of water
Water can pass through plasma membrane
in 2 ways:
through lipid bilayer by simple diffusion
through aquaporins, integral membrane
proteins
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Tonicity
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Active Transport
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Solutes are transported across plasma
membranes with the use of energy
Gradient independent: from an area of
lower concentration to an area of higher
concentration
Carrier molecule / protein
Example: sodium – potassium pump
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Transport in Vesicles
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Vesicle - a small spherical sac formed by budding off
from a membrane
Endocytosis - materials move into a cell in a vesicle
formed from the plasma membrane
1.
receptor-mediated endocytosis
2.
Phagocytosis
3.
bulk-phase endocytosis (pinocytosis)
Exocytosis vesicles fuse with the plasma
membrane, releasing their contents into the
extracellular fluid
Transcytosis - a combination of endocytosis and
exocytosis
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Organelles
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Ribosomes - sites of protein synthesis
Endoplasmic reticulum - network of membranes in
the shape of flattened sacs or tubules
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Rough ER – connected to the nuclear
envelope, a series of flattened sacs, surface is
studded with ribosomes, produces various
proteins
Smooth ER – a network of membrane bound
tubules without ribosomes, synthesizes fatty
acids and steroids, detoxifies certain drugs,
and stores calcium
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Endoplasmic Reticulum
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Organelles
Golgi complex - consists of 3-20 flattened,
membranous sacs called cisternae
- modify, sort, and package proteins for
transport to different destinations
- proteins are transported by various
vesicles
 Lysosomes - vesicles that form from the
Golgi complex, contain powerful digestive
enzymes
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Golgi Complex
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Organelles
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Mitochondria - the “powerhouses” of the cell
 Generate ATP
 More prevalent in physiologically active cells:
muscles, liver and kidneys
 Inner and outer mitochondrial membranes
 Cristae - the series of folds of the inner
membrane
 Matrix - the large central fluid-filled cavity
 Self-replicate during times of increased cellular
demand or before cell division
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Mitochondria
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Organelles
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Nucleus
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Spherical or oval shaped structure
Usually most prominent feature of a cell
Nuclear envelope – a double membrane that
separates the nucleus from the cytoplasm
Nuclear pores – numerous openings in the nuclear
envelope, control movement of substances between
nucleus and cytoplasm
Nucleolus – spherical body that produces ribosomes
Genes – are the cell’s hereditary units, control
activities and structure of the cell
Chromosomes – long molecules of DNA combined
with protein molecules
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End of Chapter 3
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