CHAPTER 9

MEMBRANES, DIFFUSION, OSMOSIS, ACTIVE TRANSPORT,

ETC.

• Membranes organize the chemical reactions making up metabolism

Membranes organize the chemical activities of cells

MEMBRANE STRUCTURE AND FUNCTION

Cytoplasm

• Membranes are selectively permeable

– They control the flow of substances into and out of a cell

• Membranes can hold teams of enzymes that function in metabolism

• Phospholipids are the main structural components of membranes

• They each have a hydrophilic head and two hydrophobic tails

Membrane phospholipids form a bilayerHead

Symbol

Tails

• In water, phospholipids form a stable bilayer

Hydrophilicheads

Hydrophobictails

Water

Water

– The heads face outward and the tails face inward

• Phospholipid molecules form a flexible bilayer– Cholesterol and protein molecules are embedded in it – Carbohydrates act as cell identification tags

The membrane is a fluid mosaic of phospholipids and proteins

• The plasma membrane of an animal cell

Fibers of the extracellular matrix

Glycoprotein Carbohydrate (of glycoprotein)

Microfilaments of the cytoskeleton

Phospholipid

Cholesterol

Proteins

CYTOPLASM

Glycolipid

• Some membrane proteins form cell junctions

• Others transport substances across the membrane

Proteins make the membrane a mosaic of function

Transport

• Many membrane proteins are enzymes

• Some proteins function as receptors for chemical messages from other cells

– The binding of a messenger to a receptor may trigger signal transduction

Enzyme activity Signal transduction

Messenger molecule

Receptor

Activated molecule

• In passive transport, substances diffuse through membranes without work by the cell– They spread from

areas of high concentration to areas of lower concentration

Passive transport is diffusion across a membrane

EQUILIBRIUMMolecule of dye Membrane

EQUILIBRIUM

• In osmosis, water travels from an area of lower solute concentration to an area of higher solute concentration

Osmosis is the passive transport of water

Hypotonicsolution

Solutemolecule

HYPOTONIC SOLUTION

Hypertonic solution

Selectivelypermeablemembrane

HYPERTONIC SOLUTION

Selectivelypermeablemembrane

NET FLOW OF WATER

Solute molecule with cluster of water molecules

Water molecule

• Osmosis causes cells to shrink in a hypertonic solution and swell in a hypotonic solution

• The control of water balance(osmoregulation) is essential for organisms

Water balance between cells and their surroundings is crucial to

organisms

ISOTONIC SOLUTION

HYPOTONIC SOLUTION

HYPERTONIC SOLUTION

(1) Normal

(4) Flaccid

(2) Lysing

(5) Turgid

(3) Shriveled

(6) Shriveled

ANIMALCELL

PLANTCELL

Plasmamembrane

• Small nonpolar molecules diffuse freely through the phospholipid bilayer

• Many other kinds of molecules pass through selective protein pores by facilitated diffusion

Transport proteins facilitate diffusion across membranes

Solutemolecule

Transportprotein

• Transport proteins can move solutes across a membrane against a concentration gradient– This is called active transport – Active transport requires ATP

Cells expend energy for active transport

• Active transport in two solutes across a membrane

Transportprotein

1

FLUIDOUTSIDECELL

Firstsolute

First solute, inside cell, binds to protein

Phosphorylated transport protein

2 ATP transfers phosphate to protein

3 Protein releases solute outside cell

4 Second solute binds to protein

Second solute

5 Phosphate detaches from protein

6 Protein releases second solute into cell

• To move large molecules or particles through a membrane– a vesicle may fuse with the membrane and expel its

contents (exocytosis)

Exocytosis and endocytosis transport large molecules

FLUID OUTSIDE CELL

CYTOPLASM

– or the membrane may fold inward, trapping material from the outside (endocytosis)

• Three kinds of endocytosis

Pseudopod of amoeba

Food being ingested

Plasma membrane

Material bound to receptor proteins

PIT

Cytoplasm

• Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors

Connection: Faulty membranes can overload the blood with

cholesterol

LDL PARTICLEPhospholipid outer layer

Protein

Cholesterol

Plasma membraneCYTOPLASM

Receptor protein

Vesicle

• Enzymes and membranes are central to the processes that make energy available to the cell

• Chloroplasts carry out photosynthesis, using solar energy to produce glucose and oxygen from carbon dioxide and water

• Mitochondria consume oxygen in cellular respiration, using the energy stored in glucose to make ATP

Chloroplasts and mitochondria make energy available for cellular

work

• Nearly all the chemical energy that organisms use comes ultimately from sunlight

• Chemicals recycle among living organisms and their environment

Sunlight energy

Chloroplasts,site of photosynthesis

CO2

+H2O

Glucose+O2

Mitochondriasites of cellular

respiration

(for cellular work)

Heat energy