Chapter 4

Cell Membrane Structure & Function

4.1 – How Is the Structure of a Membrane Related to

its Function?

• The plasma membrane isolates the cell while allowing communication with its surroundings

• Three General Functions:1.Selectively Permeable2.Regulates exchange 3.Communication

Membranes are Fluid Mosaics in Which

Proteins Move•Fluid Mosaic Model was developed in 1972

•Phospholipids act as grout for membrane proteins, which represent tiles

Phospholipid Bilayer is the Fluid Portion of the

Membrane•Phospholipid Review–Have a polar head

•Hydrophilic–Have 2 nonpolar tails

•Hydrophobic•Double bonds in the tail increase fluidity of membrane

The Plasma Membrane is a Phospholipid Bilayer

•The tails point inward –away from the watery environment

•The heads point outward –toward the watery environment

•Membrane is “fluid” because the phospholipds are not bonded together

•Phospholipids bilayer selectively isolates internal environment from external environment–Most biological molecules are hydrophilic and cannot pass through the membrance easily

–Some molecules can freely pass through the membrane

•Cholesterol in animal cell membranes make the bilayer stronger, more flexible, less fluid & less permeable to water soluble substances

•Flexibility and fluid nature of the bilayer is important to its function

The Plasma Membrane

•Proteins embedded within or attached to the surface of the bilayer regulate the movement of substance across the membrane and communicate with the environment

•Many membrane proteins are glycoproteins

A Mosaic of Proteins is Embedded in the

Membrane

Three Categories of Membrane Proteins

1. Transport Proteins – regulate the movement of hydrophilic molecules through the membrane

1. Channel Proteins – form pores and channels for small water-soluble molecules

2. Carrier Proteins – bind molecules and move them across the membrane

Channel proteins do not change shape

Carrier proteins change shape

Three Categories of Membrane Proteins

(con’t)2. Receptor Proteins – bind

molecules in the environment, triggering changes in the metabolism of the cell

3. Recognition Proteins – serve as identification tags and cell-surface attachment sites

4.2 – How Do Substances Move Across

the Membrane

Molecules Move in Response to Gradients

•Characteristics of a fluid–Fluid – any substance that can move or change shape in response to external forces without breaking apart

–Concentration – number of molecules in a given unit of volume

–Gradient – physical difference in properties such as temperature, pressure, or concentration

Diffusion

•The movement of molecules from regions of high concentration to regions of low concentration

•Movement down the concentration gradient

Diffusion Con’t

•Molecules move randomly and continuously, colliding with each other, until a dynamic equilibrium exists in which there is no concentration gradient

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

Example of DiffusionA drop of food coloring in a glass of water

Movement Across Membranes Occurs by Both Passive & Active Transport

•There are significant concentration gradients of ions and molecules across the plasma membrane because the cytoplasm is very different from the extracellular fluid

Movement Across the Membrane Occurs by:

•Passive Transport–Substances move down conc. gradient

–No energy is required

•Active Transport–Substance move up conc. gradient –Energy is required

Types of Passive Transport

1. Simple Diffusion2. Facilitated Diffusion3. Osmosis

Simple Diffusion•Membranes are selectively

permeable to diffusion of molecules

•Lipid-soluble molecules & very small molecules can easily diffuse across the membrane

•Rate of simple diffusion depends on conc. gradient, the size of the molecule & its lipid solubility

Facilitated Diffusion

•Molecules cross the membrane with the help of membrane transport proteins–Channel proteins –Carrier proteins

•No energy required–molecules move down the conc. gradient

Osmosis

•Osmosis is the diffusion of water•Water moves down the conc.

gradient across a selectively permeable membrane

•Dissolved substances reduce the concentration of water molecules in solution

Osmosis is Important in the Life of Cells

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

• Three environments exist due to varying water concentrations1. Isotonic2.Hypertonic3.Hypotonic

Isotonic Cell Environment

• Water concentration around the cell is the same as the water concentration inside the cell

• No net movement of water occurs• Cell remains the same size

– The type of dissolved particles does not have to be the same, but the total concentration of all dissolved particles is equal

Water is moving in to and out of the cell at an equal rate.

Isotonic

Hypertonic Cell Environment

•The solution outside the cell has a higher concentration of solutes than the interior of the cell–Lower water concentration

•Water will flow out of the cell by osmosis–Cells shrivel and shrink

Hypertonic

Net movement of water out of the cell

Cell shrinks

Hypotonic Cell Environment

•The solution outside the cell has a lower solute concentration than the solution inside the cell–Higher water concentration

•Water will flow into the cell by osmosis–Cells will swell and sometimes burst

Hypotonic

Net movement of water into the cell

Cells swell

Active Transport Uses Energy to Move

Molecules•All cells need to move some substances against their conc. gradient

•Membrane proteins that require energy are used to move molecules against their conc. gradient

•Active transport proteins are sometimes called “pumps” because they move substances uphill

Active Transport Proteins

• Active transport proteins span the width of the membrane and have 2 active sites1.One site binds the substance

to be transported2.Second site binds an energy

carrier molecule, usually ATP

Example of Active Transport

Cells Engulf Particles or Fluids by Endocytosis

• Types of Endocytosis1.Pinocytosis2.Phagocytosis3.Receptor-Mediated

Endocytosis

Pinocytosis

•Moves liquids into the cell•Means “cell drinking”•A small patch of membrane

dimples inward to form a vesicle surrounding the fluid

•The acquired material has the same concentration as extracellular fluid

Pinocytosis

Phagocytosis

•Moves large particles into the cell•Means “cell eating”•Extensions of the membrane fuse

around the large particle and carry it to the interior of the cell in a vacuole for intracellular digestion

Phagocytosis

Cells Move Material Out of the Cell by Exocytosis•A membrane-enclosed vesicle

carrying the material to be expelled moves to the cell surface

•The vesicle then fuses with the plasma membrane and releases its contents

Exocytosis

4.3 – How Are Cell Surfaces Specialized?

Various Specialized Junctions Allow Cells to

Connect and Communicate• Four types of

connections occur between cells, depending on the organism and cell type

1.Desmosomes2.Tight Junctions3.Gap Junctions4.Plasmodesmat

a

Desmosomes• Membranes of

adjacent cells are held together by proteins and carbohydrates

• Further strengthened by protein filaments that extend from inside the desmosome to the interior of each cell

Tight Junctions

•Membranes of adjacent cells are fused together to create leak-proof junctions

Gap Junctions

•Cell-to-cell cytoplasmic connections found in animal cells that need to communicate with each other

Plasmodesmata

•Cell-to-cell cytoplasmic connections between plant cells