Chapter 5 - Cell Membrane Structure and Function

• 5.1 How is the structure of a membrane related to its function?

• 5.2 How do substances move across membranes?

• 5.3 How do specialized junctions allow cells to connect and communicate?

5.1 How Is the Structure of a Membrane Related to Its Function?

1. Selectively isolates cell’s contents from outside (phospholipids)

2. Regulates exchange of substances in and out (proteins)

3. Communicates with other cells (proteins)

4. Create attachments within & between cells (proteins)

5. Regulate many biochemical reactions (proteins)

Fluid Mosaic Model

head(hydrophilic)

tails(hydrophobic)

C

CHCH2

CH2

CH2

CHC

O

O

N OPH3C

CH3

O

O

OH2C

O

HC

CH2

O

CH2

CH2

CH2

CH2

CH3

CH2CH2CH2CH2CH2CH2CH2

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2

CH2CH2

CH3

unsaturated

phospholipid

hydrophilicheads

hydrophobictails

hydrophilicheads

extracellular fluid(watery environment)

cytoplasm(watery environment)

bilayer

(This picture is not in edition 7)

Kinks increase membrane fluidity

Membrane Proteins Form a Mosaic

• Proteins are embedded in the phospholipid bilayer

– Some proteins can float and drift– Other proteins are anchored by protein

filaments in the cytoplasm– Many proteins have attached carbohydrates

(glycoproteins)

5.1 How Is the Structure of a Membrane Related to Its Function?

• 5 major types of membrane proteins– Receptor Proteins

– Recognition Proteins

– Enzymatic Proteins

– Attachment Proteins

– Transport Proteins

• (This section has changed slightly from ed. 7)

Proteins in the Membrane

• Glycoprotein are a protein with a carbohydrate attached (diverse functions not mentioned in this book)

• Receptor Proteins: hormones or nutrients attach and trigger cellular responses

(Picture not in ed. 7)

Receptor Proteins

Proteins in the Membrane

• Recognition Proteins – Many are glycoproteins– ID tags and surface attachment sites– Immune system

• Enzyme Proteins– Promote chemical reactions that synthesize

or break apart biological molecules

Proteins in the Membrane

• Attachment Proteins – Anchor the cell membrane to inner

cytoskeleton, to proteins outside the cell, and to other cells

• Transport Proteins– Channel Proteins create pores for water-

soluble molecules to pass through– Carrier Proteins attach to molecules and

help them across

5.2 How Do Substances Move Across Membranes?

• Know the following definitions:– Fluid: any substance whose molecules

move freely past each other (both liquid and gas)

– Solutes: a substance that can be dissolved– Solvent: Fluid capable of dissolving solute– Concentration: number of molecules of a

substance in a given volume of fluid

More Definitions

• A gradient is a physical difference in properties such as temperature, pressure, electrical change and/or concentrations between 2 adjoining regions

• Diffusion: movement of particles from high to low concentration driven by a gradient

Principles of Diffusion• Net movement of molecules down a gradient

from high to low concentration

• The greater the concentration gradient, the faster the rate of diffusion

• The higher the temperature, the faster the rate

• If no other processes intervene, diffusion will continue until gradient is gone

• Diffusion cannot move molecules rapidly over long distances

5.2 How Do Substances Move Across Membranes?

• Movement Across Membranes Occurs by Both Passive and Active Transport

• Active transport requires energy to be added

• Passive transport doesn’t need energy

5.2 How Do Substances Move Across Membranes?

• Diffusion– Diffusion can happen in a fluid or across a

membrane– Plasma membranes are selectively

permeable– Some molecules can permeate (pass

through)– To some molecules, the membrane is

impermeable (cannot pass through)

Many ions are transported this way

Facilitated diffusion only works with (or down) the gradient

Principles of Osmosis

• Diffusion of water across selectively permeable membranes

• Water moves across a membrane down (or with) the concentration gradient

• From high to low water concentration• Dissolved substances reduce the

concentration of free water molecules in solution

• “Having the same strength”• Extracellular fluid of animals is usually isotonic to cytoplasmic fluid• Gradients of dissolved particles equal (but not types)

• “Having greater strength”• Dissolved particles higher outside• In this case, salt is higher on the outside

• “Having lesser strength”• Dissolved particles lower outside• In this case, salt is lower or non-existent on the outside

Active Transport

• Active transport uses cellular energy to move molecules against their concentration gradients

• ATP (usually) donates energy to change the shape of the protein and move it across

• These are often called ATP pumps

Cells Engulf Particles or Fluids by Endocytosis

• Endocystosis is Greek for “into the cell”

• Three types:

– Pinocytosis

– Receptor-mediated endocytosis

– Phagocytosis

A dimple forms in the plasma membrane, which deepens and surrounds the extracellular fluid. The membrane encloses the extracellular fluid, forming a vesicle.

Pinocytosis “cell drinking”

3

3

(extracellular fluid)

(cytoplasm)

vesicle containing extracellularfluid

2

2

1

1

Receptor proteins for specific molecules or complexes of molecules are localized at coated pit sites. The receptors bindthe molecules and the membrane dimples inward. The coatedpit region of the membrane encloses the receptor-bound molecules. A vesicle ("coated vesicle") containing the bound molecules is released into the cytoplasm.

(extracellular fluid)

(cytoplasm)

1

1

2

3 4

4

32

1

Receptor-mediated endocytosis

nutrientsreceptors

coated pit

coated vesicle

proteincoating

coated pit

extracellular particlesbound to receptors

plasma membrane

(cyto-plasm)

(extracellular fluid)

coated vesicle

0.1 micrometer

The plasma membrane extends pseudopods toward an extracellular particle (for example, food). The ends of the pseudopods fuse, encircling the particle. A vesicle calleda food vacuole is formed containing the engulfed particle.

(extracellular fluid)

(cytoplasm)

pseudopods

food vacuole

food particle

12

3

1 2

3

Phagocytosis “cell eating”

5.2 How Do Substances Move Across Membranes?

• Pinocytosis moves liquids into the cell

• Receptor-mediated endocytosis moves specific molecules into the cell

• Phagocytosis moves large particles into the cell

• Exocytosis moves material out of the cell (Greek for “out of the cell”)

plasma membrane

(cytoplasm)

vesicle

secretedmaterial

(extracellular fluid)

plasma membrane

vesicle

0.2 micrometer

secretedmaterial

4.3 How Are Cell Surfaces Specialized?

• Various Specialized Junctions Allow Cells to Connect and Communicate– Desmosomes (animals) attach cells

together– Tight junctions (animals) make the cell

leakproof– Gap junctions (animals) communication

between cells– Plasmodesmata (plants) communication

between cells

desmosome

small intestine

plasma membranes(edge view)

cells lining smallintestine

proteinfilaments incytoplasm

Desmosome

urinary bladder

plasma membranes(edge view)

cells lining bladder

Tight junction

liver

liver cells

plasma membrane

Gap junctions

Hormones, nutrients, ions and electrical signals

plasma membrane cell wall

rootcells

root

Plasmodesmata

Water, nutrients and hormones