Cell Transport & the Cell Membrane

Cell Transport & the Cell Membrane

I. Cell Membrane A. Structure Structure of the cell membrane is referred to as the Fluid Mosaic Model. It is made up of lipids, proteins and carbohydrates. The membrane is made up of two layers of lipids (phospholipids) with many proteins embedded within the lipid layer like the individual tiles in a large mosaic picture. Model is best idea we have so far: ESTABLISHED IN 1972:SINGER AND NICOLSON SUPPORTED BY FREEZE-FRACTURE TECHNIQUE

Davson-Danielli v. Fluid Mosaic Could membrane be coated on both sides with hydrophilic protein? Not all membranes look the same under electron microscope Membrane proteins: HYDROPHILIC AND HYDROPHOBIC!

Phospholipids in membranes: -varying % in different organelle membranes -AMPHIPATHIC MOLECULE: -Hydrophobic region -Hydrophilic region Substance soluble in lipids are much more likely to enter membrane- Observation 1895

FLUID -held together by hydrophobic interactions -lateral movement -unsaturated tails= kinks prevents packing of molecules

FLUID CHOLESTEROL: -reduces fluidity at moderate temperatures, reduces movement -At low temperature: prevents solidification

MOSAIC Distinct inside and outside with lipids carbs, and proteins that are different depending on type of cell

MOSAIC Characteristics of the membrane come from the ER: Asymmetrical distribution of membrane components: What was on the inside face of ER becomes the outside of the membrane!

i. Proteins in the cell membrane Proteins perform most of the functions of the cell membrane They act as pumps, channels or pores in the membrane and help to regulate what enters and leaves the cell. INTEGRAL PROTEINS: penetrate membrane Transmembrane proteins: span the membrane PERIPHERAL PROTEINS: not embedded, appendages to surface of membrane, often to exposed parts of integral proteins. Proteins

Role of Proteins: TRANSPORT ENZYMATIC ACTIVITY SIGNAL TRANSDUCTION INTERCELLULAR CONNECTIONS CELL-CELL REGONITION ATTACHMENT POINTS TO ECM

ii.carbohydrates in the membrane Carbohydrate chains are also found attached to proteins or lipids on the outside of the membrane. These carbohydrates ( as well as the proteins) act as identification markers that are recognized by other cells OLIGOSACCARIDES: Fewer than 15 sugar unitsshort polysaccaride Attached to lipids/proteins on cell membrane Carbohydrate chains Important in immune system How body rejects transplants

GYLCOLIPIDS Oligosaccaride attached to a lipid Recognition example: A, B, O blood types

GLYCOPROTEINS: LOTS OF VARIATION: species to species among individuals of same species From cell to cell

I. Cell Membrane B. Function The function of cell membrane is to regulate what enters and leaves the cell: semipermeable, or selectively permeable 1. Cells get what they need from ECF IN/OUT OF CELL: through membrane Examples: Na+, Ca2+ glucose 2. Eukaryotes: transport IN/OUT of membrane bound intercellular compartments Examples: Nucleus (mrna), ER (Ca2+) and Mitochondria (ATP)

Selectively Permeable AQUAPORINS and Water

II. Cellular Transport A. Passive Transport- No energy from the cell is required to perform passive transport Examples: 1. Diffusion 2. Osmosis 3. Facilitated Diffusion B. Active Transport- Can only occur if the cell expends energy Examples: 1. Membrane(Protein) Pumps 2. Endocytosis 3. Exocytosis

Movement across membraneno energy Hydrophobic Molecules: O2 - Oxygen N2 - Nitrogen benzene Small uncharged Polar Molecules: H2O - Water urea glycerol C02 - Carbon Dioxide Large Uncharged Polar Molecules: Glucose Sucrose Ions: H+, Na+, K+, Cl-

1. Diffusion Diffusion is the random net movement of particles from a region of higher concentration to a region of lower concentration until equilibrium is reached Can occur with all types of particles but they must be dissolved to diffuse across a membrane cell Net movement of particles is to the inside; from hi to low Equilibrium is reached Movement in = movement out nucleus 1. 2. 3.

2. Osmosis Osmosis is the diffusion of water from a region of high concentration to a region of lower concentration across a semipermeable membrane until equilibrium is reached. The environment surrounding cells may be Isotonic: water concentration is equal inside and outside the cell Hypotonic:water concentration is high outside the cell (more solute inside the cell) Hypertonic:water concentration is higher inside the cell (more solute outside the cell)

Osmosis What kind of environment does this cell start in? Hypotonic or Hypertonic? Outside the cell!

Osmosis-the diffusion of water Higher Concentration of Water Water molecules Cell membrane Lower Concentration of Water Sugar molecules

What happens to a plant cell in A hypertonic solution A solution with a lower water concentration than the cell A hypotonic solution A solution with a higher water concentration than the cell An isotonic solution A solution with the same concentration of water as the cell PLASMOLYSIS TURGID FLACCID

What type of cell is this? What is happening in each situation?

1 Check yourself! 2 3 5 HYPERTONIC? 4 HYPOTONIC?

3. Facilitated Diffusion Net movement of particles from higher to lower concentrations with the help of proteins in the membrane. Without the special protein channels, the particles may not have passed through at all, or they might diffuse too slowly. Still passive transport because Cell doesn t expend energy!

Facilitated Diffusion-protein channels High Concentration Glucose molecules Outside of cell Cell Membrane Low Concentration Protein channel Inside of cell

Facilitated Diffusion

Examples of Facilitated Dffusion: Ligand-Gated Mechanically Gated 1. Ligand Gated: Ion channels open and close as a result of the binding of small signal molecule- LIGAND Example: Acetycholine 2. Mechanically Gated Example: Sound waves stimulate cilia in ears- leads to nerve impulses, brain interprets as sound

Examples of Facilitated Open or close in response to a change in charge Example: As nerve impulse runs down neuron, change in charge opens Nachannels, influx of Na Goes WITH electrochemical gradient Diffusion: 3.Voltage-Gated

Cellular Transport A. Passive Transport- No energy from the cell is required to perform passive transport Examples: 1. Diffusion 2. Osmosis 3. Facilitated Diffusion B. Active Transport- Can only occur if the cell expends energy Examples: 1. Membrane(Protein) Pumps 2. Endocytosis 3. Exocytosis

1. Membrane (Protein) Pumps Proteins in the cell membrane move molecules across the cell membrane against the concentration gradient (from a region of lower concentration to a higher concentration).

Active Transport- Membrane Pumps Molecule to be carried Low Concentration Specialized for the solute, may have specific binding site Catalyze a physical process Molecule being carried Cell Membrane High Concentration Low Concentration GATED CHANNEL: Cell Membrane Stimulus causes them to open or close Energy Energy High Concentration

Active Transport: Protein Pump Sodium-Potassium Pump 1. Binding of Na+, phosphorylation of ATP, protein changes shape 2. Na+ expelled, K+ binds, phosphate released 3. Restoration of original shape, K+ expelled

Sodium Potassium Pump

Active Transport: Protein Pump Electrogenic Pump Proton Pump: Active transport of H+ out of cell Generates voltage across membrane Plants, bacteria and fungi

2. Endocytosis Movement of molecules into the cell that are too large to get through the cell membrane by diffusion or with the help of proteins. Phagocytosis: cell engulfs large, solid particles and enfolds them into their own vacuole within the cell Pinocytosis:cell membrane pinches in to make a channel for liquids or small particles Receptor Mediated Endocytosis:Coated pits form vesicles when receptors bind with ligands.

Receptor-Mediated Endocytosis THIS PROCESS USED TO GATHER SUBSTANCES NOT HIGHLY CONCENTRATED IN ECF EXAMPLE: CHOLESTEROL

Phagocytosis 1. 2. Food Vacuole 3.

Pinocytosis Food Vacuole

3. Exocytosis Move molecules out of the cell that are too large to diffuse through the membrane.

Exocytosis: