Download Chapter 7: Cells and Their Environment

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 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 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 H2O
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 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 weight
3. Solubility of H2O
4. Concentration of solvent/ solute
5. Temperature
6. Pressure
7. Organic solvents/ lipid solubility
8. Surface area/ volume