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cells living unit.docx

Biological Sciences
Course Code
Kenneth Welch

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Cells: The Living Units
Cell Theory
The cell is the basic structural and functional unit of life
Organismal activity depends on individual and collective activity of cells
Biochemical activities of cells are dictated by subcellular structure
Continuity of life has a cellular basis
Structure of a Generalized Cell
Plasma Membrane
Separates intracellular fluids from extracellular fluids
Plays a dynamic role in cellular activity
Glycocalyx is a glycoprotein area abutting the cell that provides highly specific biological
markers by which cells recognize one another
Fluid Mosaic Model
Double bilayer of lipids with imbedded, dispersed proteins
Bilayer consists of phospholipids, cholesterol, and glycolipids
Glycolipids are lipids with bound carbohydrate
Phospholipids have hydrophobic and hydrophilic bipoles
Functions of Membrane Proteins
Enzymatic activity
Receptors for signal transduction
Intercellular adhesion
Cell-cell recognition
Attachment to cytoskeleton and extracellular matrix
Membrane Junctions
Tight junction impermeable junction that encircles the cell
Desmosome anchoring junction scattered along the sides of cells
Gap junction a nexus that allows chemical substances to pass between cells
Passive Membrane Transport: Diffusion
Simple diffusion nonpolar and lipid-soluble substances
Diffuse directly through the lipid bilayer
Diffuse through channel proteins
Facilitated diffusion large, polar molecules such as simple sugars
Combine with protein carriers
Passive Membrane Transport: Osmosis
Occurs when the concentration of a solvent is different on opposite sides of a membrane
Diffusion of water across a semipermeable membrane
Osmolarity total concentration of solute particles in a solution
Passive Membrane Transport: Filtration
The passage of water and solutes through a membrane by hydrostatic pressure
Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-
pressure area
Isotonic solutions with the same solute concentration as that of the cytosol
Hypertonic solutions having greater solute concentration than that of the cytosol

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Hypotonic solutions having lesser solute concentration than that of the cytosol
Sodium-Potassium Pump
Active Transport
Uses ATP to move solutes across a membrane
Requires carrier proteins
Types of Active Transport
Symport system two substances are moved across a membrane in the same direction
Antiport system two substances are moved across a membrane in opposite directions
Primary active transport hydrolysis of ATP phosphorylates the transport protein causing
conformational change
Types of Active Transport
Secondary active transport use of an exchange pump (such as the Na+-
K+ pump) indirectly to drive the transport of other solutes
Vesicular Transport
Transport of large particles and macromolecules across plasma membranes
Exocytosis moves substance from the cell interior to the extracellular space
Endocytosis enables large particles and macromolecules to enter the cell
Phagocytosis pseudopods engulf solids and bring them into the cell’s interior
Vesicular Transport
Bulk-phase endocytosis the plasma membrane infolds, bringing extracellular fluid and
solutes into the interior of the cell
Receptor-mediated transport uses clathrin-coated pits as the major mechanism for
specific uptake of macromolecules Chapter 3
Membrane Potential
Voltage across
a membrane
Ranges from
20 to 200 mV
Results from Na+ and K+ concentration gradients across the membrane
Differential permeability of the plasma membrane to Na+ and K+
Cell Adhesion Molecules (CAMs)
Anchor cells to the extracellular matrix
Assist in movement of cells past one another
Rally protective white blood cells to injured or infected areas
Roles of Membrane Receptors
Contact signaling important in normal development and immunity
Electrical signaling voltage-regulated “ion gates” in nerve and muscle tissue
Chemical signaling neurotransmitters bind to chemically gated channel-linked receptors in
nerve and muscle tissue
G protein-linked receptors ligands bind to a receptor which activates a G protein, causing
the release of a second messenger, such as cyclic AMP
Operation of a G protein
An extracellular ligand (first messenger), binds to a specific plasma membrane protein
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