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 H2O (makes hydrogen bonds)

2. Tail (Non-polar fatty acid chain) - Hydrophobic repels H2O (pushes H2O away from middle)

Cell Membrane cont.

b. Arrangement of Lipid Bilayer1. Membrane fluid-like & flexible like a soap bubble2. Membrane can grow or change * NOTE: Lipid bilayer arrangement is still maintained as new phospholipids will always have heads toward H2O & 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 H2OEquilibrium - Particles evenly distributed in solventConcentration 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 Processes1. Passive Transport (3 types)

- NO ENERGY used!!! - From high conc low conc

a. Diffusion- Random movement of dissolved particles

b. Osmosis

- Movement of H2O through membrane - When extra H2O 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: cAMP

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 weight3. Solubility of H2O4. Concentration of solvent/ solute5. Temperature6. Pressure7. Organic solvents/ lipid solubility8. Surface area/ volume