The Cell Membrane Mader Biology, Chapter 5

Membrane Structure: Fluid Mosaic Model

The Membrane is Selectively Permeable The ability to allow some substances to cross more easily than others Allows the cell to maintain an internal environment different from the external one

Membrane Components Phospholipids Proteins Cholesterol Carbohydrates (glucose)

Phospholipids

Cholesterol Steroid (lipid) found within the phospholipid bilayer Keeps membrane solid when under heat

Membrane Proteins Integral Proteins: Peripheral Proteins: Glycoproteins:

Glycocalyx Polysaccharides (chains of glucose) attached to glycoproteins Functions:

Membrane Transport

Modes of Membrane Transport Passive Transport Diffusion Osmosis Facilitated Diffusion Active Transport Protein Pumps Endocytosis Exocytosis

Goes “with” the concentration gradient; no ATP energy required Passive Transport Goes “with” the concentration gradient; no ATP energy required

Simple Diffusion The tendency for molecules of any substance to spread out into the available space Molecules move from a region of high concentration to low concentration moving down the concentration gradient

Osmosis Osmosis is the diffusion of water across a semi- permeable membrane Water travels through protein pores called aquaporins

Effects of Tonicity on Cell Membranes Hypotonic solution higher concentration of water outside of cell results in hemolysis Hypertonic solution lower concentration of water outside of cell causes crenation Isotonic solution water concentration the same inside & outside of cell results in no net movement of water across cell membrane

Osmosis Water always moves from a hypotonic solution to a hypertonic solution What happens to cells when placed in: Hypotonic solutions? Hypertonic solutions? Isotonic solutions?

(equal amounts entering & leaving) Cells in Solutions Isotonic Solution Hypotonic Solution Hypertonic Solution No net movement of H2O (equal amounts entering & leaving) CYTOLYSIS PLASMOLYSIS

Facilitated Diffusion Diffusion with the assistance of integral transport proteins. (Still Passive transport) Note: Carrier proteins allow both passive and active transport

Facilitated Diffusion Transport proteins Provide corridors allowing a specific molecule or ion to cross membrane Undergo a shape change that translocates the molecule across the membrane Still Passive Transport Process requires no energy Particles are moving from an area of high concentration to low concentration

Channel Proteins Channel Proteins – allow certain materials to flow in and out (down their concentration gradient.) Aquaporin – channel protein that only lets water in. Ion Channels – also called gated channels, open and close in response to a chemical or electrical stimulus

Carrier Proteins Carrier Proteins – change shape to escort certain material in and out of the cell. (Passive- down their concentration gradients; Active- against). Passive is shown below. How do you know?

Active Transport Goes “against” the concentration gradient (“low to high”); requires ATP energy

Active Transport Movement of solutes AGAINST the concentration gradient. Goes from low to high concentration. Requires energy input by the cell ATP supplies this energy

Active Transport Carrier proteins are needed to transport molecules across the membrane against the concentration gradient and require an expenditure of energy ATP ADP +P (ATP must release energy to go [low] to [high] Often referred to as “pumps”. Ex: Na+ and K+ for nerve (neuron) impulses

Active Transport Active transport - requires energy Proteins act as pumps to move material from low to high concentration. Ex. Proton Pumps (Hydrogen Pumps) going against the gradient

Pumps continued Sodium-Potassium pump – pumps sodium out and potassium in. (low to high concentration)

Moving Large Objects Endocytosis – Cells take in materials by forming vesicles from the plasma membrane Exocytosis – Cell secretes macromolecules by the fusion of vesicles with plasma membrane

3 Types of Endocytosis 1. Phagocytosis: engulfing solid particles 2. Pinocytosis: engulfing liquid substances (extracellular fluid)

3 Types of Endocytosis Receptor mediated endocytosis – particles called ligands bind to receptors, which signals the engulfing of the particles.

Membrane Proteins

Membrane Proteins Integral Proteins – go through the whole membrane Peripheral Proteins – sit on the outside of the membrane (inside of the cell).

Membrane Proteins and their Functions Membrane proteins do six main things: Transport Enzymatic Activity Signal Transduction Cell Recognition Cell-Cell Joining Attachment to the cytoskeleton and extracellular matrix

Transport Proteins Channel Proteins: Passive Have a pore for materials to cross Carrier Proteins: Passive and Active Can change shape to move material from one side of the membrane to the other

Enzymatic Proteins Initiate reactions directly at membrane site. Ex- converting ATP->ADP +P (and vice versa)

Receptor Proteins Glycoproteins: “Alert” if being invaded by pathogens Initiate immune response

Cell Recognition Proteins May be embedded in membrane or in cytoplasm Ligand (signal molecule) binds to receptor (H bonding) Changes shape and starts cellular response (further reactions inside cell called signal transduction). Ex- hormones.

Cell Signaling

The Three Stages of Cell Signaling There are 3 stages at the “receiving end” of a cellular conversation: Reception Transduction Response

Transport across membrane: Signal Transduction A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response Molecules that are small enough or hydrophobic and pass through the membrane - directly activate intracellular receptors in the cytoplasm or nucleus of target cell Molecules that are too large or too hydrophilic to cross the plasma membrane - rely on membrane receptors Ligand- Signaling Molecule