1. AP Bio Chapter 5

2. Fluid mosaic model
The plasma membrane has selective permeability – allows some substances to cross easily and others not
The plasma membrane is made mostly of phospholipids
Phospholipids are amphipathic – they have both hydrophilic and hydrophobic regions
The plasma membrane is a mosaic of protein molecules bobbing in a fluid layer of phospholipids
Some proteins move freely, others seem to be anchored in place
As temperatures drop, membranes become more solidified. In order for the membranes to work properly, they should be about as fluid as salad oil
Cholesterol (primarily in animal cells) helps to maintain membrane fluidity in changing temperatures

3. Membrane proteins Six major functions: Types of membrane proteins:
Transport
Enzymatic activity
Attachment to the cytoskeleton and extracellular matrix
Cell-cell recognition
Intercellular joining
Signal transduction
Types of membrane proteins:
Integral proteins – penetrate the hydrophobic interior of the lipid bilayer
Transmembrane protein – spans across the lipid bilayer
Peripheral proteins – loosely bound to the surface of the bilayer

4. Permeability of the lipid bilayer
Hydrophobic (nonpolar) molecules can dissolve in the lipid bilayer and cross it easily
Hydrophilic (polar) molecules (ex: sugar) do not cross the membrane easily
Transport proteins – allow passage of hydrophilic substances
Channel proteins – hydrophilic interior that certain molecules or ions can use as a tunnel
Aquaporins – channel proteins that allow for the passage of water
Carrier proteins – bind to molecules and change shape to shuffle them across the membrane
These are specific for the substance they move

5. Passive transport
Diffusion – the movement of molecules from areas of high concentration to areas of low concentration
Substances move down their concentration gradient
No work must be done
No energy is expended by the cell for substances to diffuse across the plasma membrane – passive transport

6. Types of diffusion Osmosis
Diffusion of water across a selectively permeable membrane
Water moves from high H2O concentration (and therefore low solute concentration) to low H2O concentration (and high solute concentration)
If a cell is placed in a water solution:
Hypertonic (higher solute outside the cell)  water moves out of cell
Hypotonic (lower solute outside the cell)  water moves into the cell
Isotonic (equal concentrations on both sides)  no net water movement
Osmoregulation – the control of solute and water concentrations
Freshwater organisms – some have contractile vacuoles to pump out excess
Plants – rigid cell walls allow them to maintain turgor pressure
Wilting of plants occurs when water moves out of cells - plasmolysis

7. Types of diffusion…
Facilitated diffusion – transport proteins speed up passive transport
These proteins allow a specific molecule or ion to pass
Includes:
Aquaporins
Ion channels
They may change shape as they work
No energy input by the cell is required

8. Active transport
Cells may need to move solutes against their concentration gradients (from low to high)
This type of transport requires energy from the cell – usually in the form of ATP
Sodium-potassium pump – an example of active transport; found in animal nerve cells
Bulk transport – when the cell needs to move large molecules or large amounts of small molecules
Exocytosis – moves material out of cell
Endocytosis – moves material into cell
Phagocytosis = ‘cell eating’
Pinocytosis = ‘cell drinking’
Receptor-mediated endocytosis = requires a receptor to start the process

9. Cell signaling
Direct contact – connect the cytoplasm of adjacent cells; allows for the free passage of substances from one cell to another
Gap junctions – animal cells
Plasmodesmata – plant cells
Paracrine signaling – local signaling when molecules only travel short distances
Ex: Growth factors – stimulate nearby cells to grow and divide
Synaptic signaling – another type of local signaling
Ex: neurotransmitters
Long-distance signaling – hormones are involved

10. Stages of cell signaling
Reception
Signal molecule – ligand – binds to a specific receptor molecule on the plasma membrane
This causes a shape change in the receptor
Two main types of membrane receptors:
G protein-coupled receptors – use the energy of GTP (similar to ATP)
Ligand-gated ion channels – when signal molecule binds, an ion activates the channel
Transduction
Usually multiple steps
Sometimes the signal is amplified to produce a large cellular response
‘falling domino’ effect
cyclic AMP (cAMP) and calcium ions can act as second messengers for amplification
Response
May occur in the cytoplasm or the nucleus – turning genes on or off to regulate protein synthesis