Chapter 7 Cells and Their Environment Cell Membrane

Chapter 7: Cells and Their Environment

Cell Membrane 1. Function • Selectively (semi-) permeable a. Some substances can pass through but others cannot • Maintains homeostasis a. Used to maintain a stable internal environment b. Cell survives by preventing its interior from mixing w/ a watery environment outside

Cell Membrane cont. 2. Structure • Composed of 2 organic molecules: 1. Phospholipids 2. Proteins • Arranged in a lipid bilayer

Lipid Bilayer

Cell Membrane cont. a. Phospholipid Structure • “Amphipathic molecule” • Consists of: 1. Head (Polar) - Hydrophilic attracts H 2 O (makes hydrogen bonds) 2. Tail (Non-polar fatty acid chain) - Hydrophobic repels H 2 O (pushes H 2 O away from middle)

Cell Membrane cont. b. Arrangement of Lipid Bilayer 1. Membrane fluid-like & flexible like a soap bubble 2. Membrane can grow or change * NOTE: Lipid bilayer arrangement is still maintained as new phospholipids will always have heads toward H 2 O & tails in middle- happens whenever membrane compartments fuse internally

Cell Membrane cont. 3. Forms non-polar interior zone (middle layer) - Polar molecules (glucose, amino acids [AA], ions, cell wastes) can’t pass through b/c repelled by non-polar tails *** Advantage: Forms good barrier! - Problem: If cell membranes were made only of lipids, most substances could not pass into/ out of cells - Solution: Build into lipid bilayer various kinds of passageways composed of proteins (possesses different shapes, sizes, & channels)

Kinds of Cell Surface Proteins • Polar molecules floating in non-polar zone • “Like ice cubes in a punch bowl”

Kinds of Cell Surface Proteins cont. 1. Channel Proteins - Acts like “passageways” - Special proteins having doughnut-shaped channels - Polar substances (glucose, AA, etc) can enter & pass - Specific channels for certain substances doors w/ locks - Allows for “ 2 -way” travel into/ out of cell (facilitated diffusion)

Proteins embedded in cell membrane

Kinds of Cell Surface Proteins cont. 2. Receptor Proteins - Acts like “information receivers” - Sends information from outside the cell to inside the cell - Special outer shape only fits specific substances - If substance matches, information sent inside the cell and a response will occur inside the cell - Carries out communication functions between cells chemically a. Many hormones work this way/ ex: insulin b. Nerve impulses between 2 nerve cells/ between nerve cells & muscle cells/ ex: acetylcholine (ACH)

Signal molecule binds to receptor protein- leads to message being passed on into the cell

Kinds of Cell Surface Proteins cont. 3. Surface Marker Proteins - Acts like “cell identifiers” - Have long external extensions usually made up of CHOs - Both self-markers & tissue markers present - Some markers function in immune response

Cell Membrane

Solutions Mixture of solute & solvent 1. Solute substance being dissolved/ ex: sugar, salt, AA, ions 2. Solvent dissolving substance/ ex: usually H 2 O Equilibrium - Particles evenly distributed in solvent Concentration Gradient - Areas having differences in concentrations - Living cells must have membranes that keep the watery cytoplasm inside the cell different than the watery environment outside the cell

Solutions Outside a Cell vs Solutions inside a Cell

Solutions Outside a Cell VS Solutions inside a Cell cont.

Solutions Outside a Cell VS Solutions inside a Cell cont.

Membrane Transport Processes 1. Passive Transport (3 types) - NO ENERGY used!!! - From high conc low conc a. Diffusion - Random movement of dissolved particles b. Osmosis - Movement of H 2 O through membrane - When extra H 2 O accumulates on 1 side, pressure increases osmotic pressure

Cont. Passive Transport c. Facilitated Diffusion (Channel Proteins) - Substances move through specific channel proteins having pores (differ in size, shape, & polarity) - Substances attaches, pore opens/ ex: how glucose enters most cells

Membrane Transport Processes cont. 2. Active Transport (5 types) - Cells MUST use energy (ATP) - From low conc high conc a. Proton Pumps - Results in production of ATP - Process called chemiosmosis occurs in side both chloroplasts & mitochondria

Cont. Active Transport b. Sodium-Potassium Pump - 2 ions moving in opposite directions at the same time - Process will move 3 sodium ions (Na+) out of cell for every 2 potassium ions (K+) that comes into the cell - Occurs in nerve cells (impulse) & small intestine (food absorption)

Membrane Transport Processes cont. c. Coupled Channels (Cotransport) - 2 ions moving in together in the same direction - Na+ outside diffuse rapidly inside cell & pulls in other substances/ ex: glucose, AA, ions -main way sugar and other food molecules transported into cells

Cont. Active Transport d. Endocytosis - Larger molecules enter as membrane surrounds substance 1. Pinocytosis intake of liquids 2. Phagocytosis intake of solids - Ex: phagocytes- WBC

Cont. Active Transport e. Exocytosis - Removal of cell waste vacuoles & gland secretions through cell membrane/ ex: hormones & enzymes

Cell-Cell Communication - Needed to coordinate body growth & development 1. Direct Communication - Physical contact between cells Ex: tight junctions, gap junctions, plasmodesmata 2. Indirect Communication - No physical contact between cells Ex: endocrine system chemical (hormones) nervous system chemicals (neurotransmitters)

Cell-Cell Communication cont. Involves receptor protein channels - Binding of a signal particle to its specific receptor can influence inside of cell in 3 ways: a. Receptor acts as enzyme causing reaction in cytoplasm b. Receptor causes formation of second messenger that works inside cell/ ex: c. AMP

Cell-Cell Communication cont. c. Receptor can open gates of specific channels proteins allowing rapid movements of ions through membrane via: Gated Channel Proteins - Have a special area that causes a channel to open/ close when it comes into contact with signals from a cell 1. Chemically Gated Channels - Signal that hits special area neurotransmitter molecule from outside the cell - Causes gates to open & Na+ to move rapidly into the cell/ ex: acetylcholine (ACH) (a neurotransmitter)

2. Voltage Gated Channels - Signal that hits special area electric charge from within cell - Impulse moves along a nerve cell, current cause gates to open & Na+ moves rapidly into nerve cells

Factors That Affect the Rate of Diffusion 1. Particle size/ pore size 2. Molecular weight 3. Solubility of H 2 O 4. Concentration of solvent/ solute 5. Temperature 6. Pressure 7. Organic solvents/ lipid solubility 8. Surface area/ volume