MEMBRANE STRUCTURE AND FUNCTION

• 2.4.2 Membranes organize the chemical activities of cells

– Membranes provide structural order for metabolism

• Form most of the cell's organelles

• Compartmentalize chemical reactions

– The plasma membrane forms a boundary between a living cell and its surroundings

• Exhibits selective permeability

• Controls traffic of molecules in and out

LE 5-10

Outside

of cell

Cytoplasm

LE 5-11a Hydrophilic head

Phosphate

group

Symbol

Hydrophobic tails

Membrane phospholipids form a bilayer

Phospholipids are the main structural components of membranes

Two nonpolar hydrophobic fatty acid "tails" One phosphate group attached to the hydrophilic glycerol "head"

Hydrophilic

heads

Hydrophobic

tails

Water

Water

In membranes, phospholipids form a bilayer Two-layer sheet Phospholipid heads facing outward and tails facing inward Selectively permeable

Polar lipid-soluble molecules pass through Nonpolar molecules not soluble in lipids do not pass through

• 2.4.3 The membrane is a fluid mosaic of phospholipids and proteins

– A membrane is a mosaic

• Proteins and other molecules are embedded in a framework of phospholipids

– A membrane is fluid

• Most protein and phospholipid molecules can move laterally

– Membrane glycoproteins and glycolipids function in cell identification

LE 5-12

Extracellular

matrix Glycoprotein

Carbohydrate

Plasma

membrane

Microfilaments

of cytoskeleton

Phospholipid

Cholesterol

Proteins

Cytoplasm

Glycolipid

1.3A.1 Cholesterol in mammalian membranes helps prevent extremes. Reducing fluidity, but also preventing fatty acid chain crystalizatioon

• 2.4.3 Proteins perform most membrane functions

• Transporters of substances across the membrane

• Receptors of chemical messages from other cells (signal transduction)

• Anchorage

• Cell recognition

• Intercellular junctions between adjacent cells

• Enzymes

LE 5-13a

Enzyme activity

LE 5-13b

Messenger molecule

Receptor

Activated

molecule

Signal transduction

LE 5-13c

Transport

ATP

• 2.4.4 Passive transport is diffusion across a membrane

– Diffusion is the tendency for particles to spread out evenly in an available space

• From an area of high concentration to an area of low concentration

– Passive transport across membranes occurs when a molecule diffuses down a concentration gradient

– Small nonpolar molecules such as O2 and CO2 diffuse easily across the phospholipid bilayer of a membrane

LE 5-14a

Molecules of dye Membrane Equilibrium

LE 5-14b

Equilibrium

• 2.4.5 Transport proteins may facilitate diffusion across membranes

– In facilitated diffusion

• Transport proteins that span the membrane bilayer help substances diffuse down a concentration gradient

– To transport the substance, a transport protein may

• Provide a pore for passage

• Bind the substance, change shape, and then release the substance

LE 5-15

Solute

molecule

Transport

protein

• 2.4.6 Cells expend energy for active transport

– Active transport requires energy to move solutes against a concentration gradient

• ATP supplies the energy

• Transport proteins move solute molecules across the membrane

Transport

protein

Solute

ATP P

ADP

Protein

changes shape

P

Solute binding Phosphorylation Transport Protein reversion

Phosphate

detaches

P

2.4.7 Cells transport materials in vesicles

• Proteins made in ER are sent to the Golgi via vesicles

• Vesicles go from the Golgi to the plasma membrane

• 2.4.8 Exocytosis and endocytosis transport large molecules

– To move large molecules or particles through a cell membrane

• A vesicle may fuse with the membrane and expel its contents outside the cell (exocytosis)

• Membranes may fold inward, enclosing material from the outside (endocytosis)

LE 5-19a

Vesicle

Fluid outside cell

Protein

Cytoplasm

LE 5-19b

Vesicle forming

– Endocytosis can occur in three ways

• Phagocytosis ("cell eating")

–Surrounding food and engulfing it

• Pinocytosis ("cell drinking")

–Absorbing fluid into tiny vesicles

• Receptor-mediated endocytosis

–Like pinocytosis, but very specific

LE 5-19c

Pseudopodium of amoeba

Phagocytosis

Plasma membrane

Food being ingested

Material bound to receptor proteins

PIT

Cytoplasm

Receptor-mediated endocytosis Pinocytosis

• Osmosis is the diffusion of water across a membrane

– In osmosis water, molecules diffuse across a selectively permeable membrane

• From an area of low solute concentration

• To an area of high solute concentration

• Until the solution is equally concentrated on both sides of the membrane

– The direction of movement is determined by the difference in total solute concentration

• Not by the nature of the solutes

LE 5-16

Water

molecule

Selectively

permeable

membrane

Solute

molecule

H2O

Lower

concentration

of solute

Higher

concentration

of solute

Equal

concentration

of solute

Solute molecule with

cluster of water molecules

Net flow of water

• Water balance between cells and their surroundings is crucial to organisms

– Osmoregulation is the control of water balance

– Tonicity is the tendency of a cell to lose or gain water in solution

• Isotonic solution: solute concentration is the same in the cell and in the solution – No osmosis occurs

– Animal cell volume remains constant; plant cell becomes flaccid

• Hypotonic solution: solute concentration is greater in the cell than in the solution – Cell gains water through osmosis

– Animal cell lyses; plant cell becomes turgid

• Hypertonic solution: solute concentration is lower in the cell than in the solution – Cell loses water through osmosis

– Animal cell shrivels; plant cell plasmolyzes

LE 5-17

Isotonic solution Hypotonic solution Hypertonic solution

H2O H2O

(1) Normal (2) Lysed

H2O

H2O H2O H2O

Animal

cell

Plant

cell

(4) Flaccid (5) Turgid (6) Shriveled

(plasmolyzed)

(3) Shriveled

Plasma

membrane

H2O

H2O

The difference between passive and active transport is:

• A: Passive doesn’t take energy

• B: Active transport is for larger molecules

• C: Passive transport aloows things to come in and out of the cell

• D: All of the above are true

Which of the following is not a form of active transport?

• A: Exocytosis

• B: Endocytosis

• C: Facilitated diffusion

• D: Protein pumps

• Which of the following represents a picture of a cell placed in an environment that has more solutes outside of the cell?

A B C