Download Lectures 6 & 7: Powerpoint

Biology 102
Lectures 6 & 7:
Biological Membranes
Lecture outline
1.
Relationship of membrane structure and
function


2.
Movement of substances across
membranes




3.
Functions
Structure: The fluid-mosaic model of membranes
Principles of Diffusion
Passive and active transport of solutes
Osmosis
Endocytosis and exocytosis
Specialization of cell surfaces
1. Membrane structure
and function

Biological membranes
Thin
barrier separating the inside of the cell
(or structure) from the outside environment

Functions (focus on plasma membrane)
Selectively
isolate the cell’s contents from
the external environment
Regulate the exchange of substances
between the inside and outside of the cell
Communicate with other cells
Fluid-mosaic model of
membrane structure
The phospholipid bilayer is the
fluid portion of the membrane




Double layer
Polar head group: hydrophilic exterior
Non-polar hydrocarbon tails: hydrophobic interior
Unsaturated hydrocarbon chains: maintains fluidity
Phospholipid bilayer as a barrier

Hydrophilic molecules cannot pass freely
through the membrane’s hydrophobic
interior
amino acids, charged ions (i.e. Na+ and Cl-)
are some examples
 Though polar, H20 is so small it does pass through.
 Sugars,

Many hydrophobic molecules can pass freely
through the membrane’s hydrophobic
interior
 Steroid
hormones and other lipids are some examples
Cholesterol molecules are part
of the lipid bilayer
Adds strength
 Adds flexibility
 Affects fluidity

Decreases
fluidity at
“moderate” temperatures

Harder for phospholipids
to move
Prevents
solidification at
low temperatures

Keeps phospholipids from
binding to each other
A mosaic of proteins is embeded
in the membrane

Glycoproteins: proteins with attached carbohydrates
Types of membrane proteins

Transport proteins
 For
passage of materials through the
plasma membrane


Channel vs. carrier proteins
Receptor proteins
 Bind
molecules and trigger cellular
responses


Example: hormones
Recognition proteins
 Self
vs. non-self (glycoprotein-based)
recognition
 Markers during development
2. Movement of substances
across membranes

Definitions
Concentration

Number of molecules in a given volume
Gradient

Differences in concentration between two regions of
space.

This causes molecules to move from one region to the
other (if no barrier to movement)
Diffusion

Net movement of molecules from regions of high
concentration to regions of low concentration

Considered as movement “down” its concentration gradient
Diffusion of Dye in Water
Dispersing
Random
Dispersal
Time 0
Time 1
Time 2
Steep
Concentration
Gradient
Reduced
Concentration
Gradient
No
Concentration
Gradient
Passive vs. active transport

Passive transport
Movement
of molecules down their
concentration gradients
Requires no net energy expenditure


The gradients themselves provide energy
Active transport
Movement
of molecules against their
concentration gradients
Requires energy!
Focus: Passive transport
1. Simple diffusion
2. Facilitated diffusion
3. Osmosis
 Remember that no energy is required, and
molecules move down their concentration
gradients
Focus: Passive transport
1. Simple diffusion

Molecules simply cross cell membrane on their own,
down their concentration gradients

Possible only for molecules that can cross the lipid
bilayer on their own

Lipid-soluble molecules


Very small molecules


Examples: ethyl alcohol, vitamin A, steroid hormones
Examples: water, carbon dioxide
Rate depends upon



Concentration gradient
Size
Lipid solubility
Focus: Passive transport (cont.)
2. Facilitated diffusion



Molecules move down their concentration
gradients (as for simple diffusion), but…
Transport proteins assist these molecules in
crossing the membrane
No net energy expenditure! (This is a type
of diffusion…)
Focus: Passive transport (cont.):
Facilitated diffusion via a channel
Focus: Passive transport (cont.):
Facilitated diffusion via a carrier protein
Diffusion
Channel
Protein
Molecule in
Transit
Diffusion
Gradient
(Outside
Cell)
Carrier protein
has binding site
for molecule
Molecule enters
binding site
(Inside Cell)
Carrier protein changes
shape, transporting
molecule across membrane
Carrier protein resumes
original shape
Focus: Passive transport (cont.)
3. Osmosis

Movement of water from a high [water] to an
area of low [water] concentration across a semipermeable membrane

Note here that water can pass through, but glucose
cannot
Think about which
way water will move
(blackboard demo)
The effects of osmosis

Compare solute and water concentrations
outside vs. inside the cell (sketches)
Focus: Active Transport
1. Movement via active transport proteins
2. Endocytosis
3. Exocytosis
 Remember that energy is required, and
molecules are moved against their
concentration gradients
Focus: Active transport
1. Movement via active transport proteins
ATP required (has own binding site)
 Note movement of particles (Ca++)
against their concentration gradient

Focus: Active transport
2. Endocytosis

Three types of endocytosis
 Pinocytosis


“cell drinking”
Extracellular fluid taken in
 Receptor-mediated



endocytosis
Specific for particular molecules
Molecules bind to receptors.
Receptor-molecule complex taken in
 Phagocytosis

Large particles engulfed
Focus: Active transport
3. Exocytosis
3. Specialization of cell surfaces

Connections between cells
 Desmosome:
Membranes of adjacent cells glued
together by proteins and carbohydrates
 Tight junction: Cells sealed together with proteins
3. Specialization of cell surfaces (cont.)

Communication between cells
 Gap
junctions: Channels connect adjacent cells
 Plasmodesmata: Continuous cytoplasm bridges
between two cells (plants)

Note also cell walls. Only certain cell types have cell walls!