Chapter 5: Biological
Membranes
AP Biology
Chapter 5
Plasma Membrane
Functions:
 separates the cell's insides from outside
 regulates passage of materials into/out of
cell
 transmitting signals and info. between the
cell and environment
 participates in chemical reactions
 essential part of energy transfer and storage
systems

Composition:
- lipid bilayer
- proteins
- in constant motion
Membrane Proteins
*Important area of research: how
membrane proteins function in
health/disease
 many are enzymes
 function in transport of
materials/information
 connect cells together to form tissues

Phospholipid Bilayer


Phospholipid: 2 fatty acid chains linked to a
glycerol molecule
- nonpolar, hydrophobic ends (fatty acids)
- hydrophilic ends (phosphate group)
- hydrophobic "tails" turn to inside of
membrane
- hydrophilic "heads" turn to outer ends
of membrane
- amphipathic - have distinct
hydrophilic and hydrophobic regions
Phospholipid Bilayer
Hydrogen bonds form between the
phospholipid "heads" and the watery
environment inside and outside of the cell
 Hydrophobic interactions force the "tails"
to face inward
 Phospholipids are not bonded to each
other, which makes the double layer fluid

Fluid Mosaic Model
Cell membrane consists of a fluid bilayer
of phospholipid molecules in which
proteins are embedded
- like the tiles in a mosaic picture
 NOT static - proteins can move


p.107 diagram of fluid mosaic
model
Membrane Proteins

Functions:
◦
◦
◦
◦
Transport of small molecules
Enzymes
Information transfer
Identification tags - allow for cell-cell
recognition
Membrane Proteins
◦ Integral Proteins - firmly bound to the
membrane, usually do not extend all
the way through
 amphipathic
◦ hydrophilic regions extend out of the cell or
into
cytoplasm
- hydrophobic regions interact with fatty acid
tails of the phospholipids
Membrane Proteins

Transmembrane Proteins - extend
through the membrane, also amphipathic
Membrane Proteins

Peripheral Proteins - not embedded in
the lipid bilayer
◦ located on the inner or outer surfaces of the
plasma membrane
◦ can be removed from the membrane without
disrupting the structure
Cell Membrane is Selectively
Permeable
Most membranes are permeable to small
molecules and lipid-soluble or polar
molecules
 Water molecules may pass through the
lipid bilayer

◦ gases: such as O2 and CO2
Transport Across Membranes

Passive Transport - does not require
energy, moves with the concentration
gradient

Active Transport - requires ATP, moves
against the concentration gradient
Simple Diffusion
Process based on random motion
 Particles move down concentration
gradient -from an area of high
concentration to low concentration

◦ can occur rapidly
◦ occurs until equilibrium is reached
Types of Diffusion:

Osmosis - diffusion of water across a
selectively permeable membrane
◦ p.114 Figure 5-11

Dialysis - diffusion of a solute across a
selectively permeable membrane
◦ p.113 Figure 5-10
High H2O potential
Low solute
concentration
Low H2O potential
High solute
concentration
Osmotic Pressure
the tendency of water to move into a
solution by osmosis
 solution with high solute concentration,
low water, has a high osmotic pressure
 solution with a low solute concentration,
high water, low osmotic pressure


Isotonic - equal solute concentration
◦ ex. blood plasma isotonic to blood cells

Hypertonic - higher solute concentration
- if a cell is placed in a hypertonic environment,
water will leave the cell, the cell shrinks
- plasmolysis occurs: plasma membrane separates
from cell wall

Hypotonic - lower solute concentration
- a cell placed in a hypotonic environment will
gain water, swell, and possibly burst
Turgor Pressure

Turgor Pressure - internal pressure of cells
with cell walls
◦ Plants, Algae, and Bacteria




Enables them to withstand a low solute
concentration outside the cell
Cell is hypertonic to environment
Water moves into cell, cell swells, building
pressure -> turgor pressure against cell wall
Cell does not burst b/c of cell wall, resist
stretching and water molecules must stop
moving into the cell
Carrier-Mediated Transport
Membrane proteins move ions or
molecules across a membrane
 2 types:
1. facilitated diffusion (passive)
2. carrier-mediated active transport

Carrier-Mediated Transport
Facilitated diffusion - with
concentration gradient, requires
transport protein
 energy comes from concentration
gradient
 ex. glucose permease - transports glucose
into red blood cells

Carrier-Mediated Active Transport




Carrier-mediated Active Transport movement of solutes across
membrane against concentration
gradient
particles must be "pumped" from region or
low conc. to region of high conc.
requires energy source - ATP, and transport
protein
ex. sodium-potassium pump: in all animal
cells, pump sodium ions out of cell and
potassium ions into cell
Other Types of Active Transport
large particles such as food, cell parts
 requires ATP
 Endocytosis and Exocytosis


Active Transport video clip
Exocytosis
Cell ejects waste products or hormones
by the fusion of a vesicle with the plasma
membrane
 Vesicle releases contents from the cell

Endocytosis
Materials are taken into the cell
 Types: phagocytosis, pinocytosis


Endocytosis Video clip
Phagocytosis
"cell eating"
 cell ingests large solid particles such as
bacteria and food
 ex. protists, white blood cells
 plasma membrane folds enclose particle,
forms a vacuole, fuses, then enters the cell
and fuses with lysosomes

Pinocytosis
"cell drinking"
 cell takes in dissolved materials
 droplets of fluid are trapped by folds in
the membrane
 pinch off into the cytosol as tiny vesicles
 liquid is slowly transferred into the
cytosol
 vesicles become smaller, then disappear
