Fluid Mosaic Model And Transport Proteins

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Fluid mosaic model diagram

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Hydrophilic

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Describes a molecule that is water loving. The polar phosphate heads of the Phospholipid is hydrophilic and charged.

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Hydrophobic

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Describes a molecules that is water hating. The inside nonpolar tail of the Phospholipid bilayer of the cell membrane is hydrophobic and not charged.

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Fluid mosaic model

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Theory used to describe the structure of the plasma membrane. It is fluid as phospholipid bilayer is constantly moving. It is a mosaic because the proteins are scattered in a patchy arrangement.

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Plasma Membrane

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Outer barrier of an animal cell. Inner barrier of a plant cell. Controls the transport of molecules into and out of the cell. It is both porous and fluid.

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Phospholipid Bilayer

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The main component of the cell membrane. There are two opposing layers of phospholipid molecules.

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Phospholipid

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Molecule consisting of a hydrophilic head (phosphoric acid called phosphorylated alcohol and Glycerol), which forms the outer part of the membrane and a hydrophobic tail (two strings of hydrogen and carbon atoms called fatty acid chains) which forms the inner part of the cell membrane.

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Phospholipid Structure

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Glycoprotein

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Membrane surface protein that has carbohydrate addition. Often these a used as cell surface markers called antigens. (cell identity markers)

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Receptor Proteins

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Membrane surface proteins that act as receivers for hormones or antibodies.

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Channel Protein

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Transmembrane protein that makes the cell membrane porous and allows the movement of substances across the membrane.

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Integral Protein

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Transmembrane protein that has a supporting function.

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Cell membrane consists of

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* Phospholipds (polar hydrophilic head and nonpoalr hydrophobic fatty acids tail)

* Chlolesterol (maintain the fluidity)

* Proteins (integral or transmembrane, peripheral and inter membrane or lipid bound)

* Carbohydrates (glyco- glycolipids or glycoproteins)

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Fluidity of Membranes

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• Membrane molecules are held in place by relatively weak hydrophobic interactions.
• Most of the lipids and some proteins drift laterally in the plane of the membrane
• Membrane fluidity is influenced by temperature.
• Membrane fluidity is also influenced by its components. Membranes rich in
unsaturated fatty acids are more fluid that those dominated by saturated
fatty acids

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Movement across membrane

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- move freely (water, carbon dioxide, smmonis, oxygen)

- carrier proteins transport some molucules

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Membrane proteins

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- channels or transporters (move molecules in one direction)

- receptors (recognize certain chemicals)

- glycoproteins (identify cell type)

- enzymes (catalyze production of substances)

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Functions of cell membrane

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* regulates passage of substances in and out of cell

* detects chemical messengers arriving at surface

* link adjacent cells together by membrane junctions

* anchor cells to extracellular matrix

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Functions of membrane proteins

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* transport

* enzymatic activity

* signal transduction

* cell-cell recognition

* intercellular joining

* attachment to cytoskeleton and ECM (extracellular matrix)

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plasma membrane proteins function diagram

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Passive transport - No energy

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* simple diffusion

* dialysis - selective diffusion of solutes

(Lipid-soluble materials, small molecules that can pass through membrane pores unassisted)

* osmosis - water from low solute concentration to high solute concentration

* facilitated diffusion - substances require a

protein carrier for passive transport

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Solution

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homogeneous mixture of two or
more components

– Solvent – dissolving medium
– Solutes – components in smaller quantities
within a solution

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Simple Diffusion

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• The net movement of a substance from an area of higher concentration to an area of lower concentration - down a concentration gradient
• Caused by the constant random motion of all atoms and molecules
• substances moves down its own concentration gradient.

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Osmotic Pressure

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• Osmotic pressure of a solution is the
pressure needed to keep it in equilibrium
with pure H20.

• Tonicity is the ability of a solution to cause
a cell to gain or lose water – based on the
concentration of solutes

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Tonicity

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• If 2 solutions have equal [solutes], they are called isotonic
• If one has a higher [solute], and lower [solvent], is hypertonic (high solute concentration in ECF)
• The one with a lower [solute], and higher [solvent], is hypotonic ((high solute concentration in ICF)

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Tonicity diagram

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Osmosis

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Osmosis is the diffusion of water across a
semi-permeable membrane from a
hypotonic solution to a hypertonic solution

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Facilitated Diffusion

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Diffusion of solutes with the help of special transport proteins large polar molecules and ions that cannot pass through membrane
• Two types of transport proteins
– Channel proteins
– Carrier proteins – physically bind to the substance on one side of membrane and release it on the other.

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Characteristics of Facilitated Diffusion

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• Specific – each channel or carrier
transports certain ions or molecules only
• Passive – direction of net movement is
always down the concentration gradient
• Saturates – once all transport proteins are
in use, rate of diffusion cannot be
increased further

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Active Transport

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Uses energy (from ATP) to move a
substance up a concentration gradient.
• Requires the use of carrier proteins
• 2 types:
– Membrane pump (protein-mediated active
transport)
– Coupled transport (cotransport).

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Sodium-potassium Pump

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Coupled transport

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– Carrier protein uses ATP to move a substance across the membrane against its concentration gradient. Storing energy.
– The protein allows the substance to move down its concentration gradient using the stored energy to move a second substance up its concentration gradient

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Coupled transport diagram

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Passive And Active Transport Compared

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Bulk Transport

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• Allows small particles, or groups of
molecules to enter or leave a cell without
actually passing through the membrane.
• 2 mechanisms of bulk transport:
endocytosis and exocytosis.

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Endocytosis

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The plasma membrane envelops small
particles or fluid, then seals on itself to
form a vesicle or vacuole which enters the
cell:
– Phagocytosis
– Pinocytosis
– Receptor-Mediated Endocytosis

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Endocytosis diagram

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Process of Phagocytosis

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Receptor-mediated Endocytosis process

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Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the

membrane called coated pits, which are
lined on their cytoplasmic side. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules.

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Receptor-mediated Endocytosis diagram

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Exocytosis

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• Vesicle moves to cell surface
• Membrane of vesicle fuses
• Materials expelled

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Exocytosis diagram

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Intercellular Communication

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• Coordinates organ systems
• Takes place directly:
– Physical contact between cells

* Gap junctions

* Direct linkage of surface markers
• Or indirectly
– Extracellular chemical messengers or signal
molecules
– Specific to target cell receptors

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Chemical Communication

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Four types of chemical messengers

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– Paracrines • Local chemical messengers • Exert effect only on neighboring cells in immediate environment of secretion site

– Neurotransmitters • Short-range chemical messengers • Diffuse across narrow space to act locally on adjoining target cell (another neuron, a muscle, or a gland)

– Hormones • Long-range messengers • Secreted into blood by endocrine glands in response to appropriate signal • Exert effect on target cells some distance away from release site
– Neurohormones • Hormones released into blood by neurosecretory neurons • Distributed through blood to distant target cells

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Paracrine secretion

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Neurotransmitter secretion

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Hormonal secretion

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Chemical Messengers

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• Cell responses brought about primarily by signal transduction

– Incoming signals conveyed to target cell’s interior
• Binding of extracellular messenger (first
messenger) to receptor brings about intracellular response by either
– Opening or closing channels
• Chemically gated receptor channel
– Activating second-messenger systems
• Activated by first messenger
– Receptor-enzyme
• Relays message to intracellular proteins that carry out dictated response
– G-protein coupled receptor

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Chemical Messengers diagram

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Receptor-enzyme: Tyrosine kinase

pathway

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Hormones

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• Endocrinology

– Study of homeostatic activities accomplished
by hormones
• Two distinct groups of hormones based on
their solubility properties
– Hydrophilic hormones • Highly water soluble • Low lipid solubility
– Lipophilic hormones • High lipid solubility • Poorly soluble in water

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Molecules in second messenger system

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Mechanism of hydrophilic
hormones via cyclic AMP
second messenger pathway

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Comparison of Nervous System and
Endocrine System

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