CHAPTER 3

A TOUR OF THE CELL

Concept 3.3:

CELL MEMBRANE

MEMBRANE STRUCTURE

Membranes keep eukaryotic cells organized

Partition the cell Control the movement of substances into

and out of the cell Help maintain chemical environment for

the cell

MEMBRANE STRUCTURE

Membranes are mainly composed of 2 layers of phospholipid molecules with protein molecules embedded in them

Also contain small amounts of cholesterol and carbohydrate molecules

PHOSPHOLIPIDS Similar to lipid molecules, but have only

two fatty acids instead of three

Two fatty acids at one end of molecule—tails—are hydrophobic

Other end—head—is hydrophilic; has phosphate group attached

Membrane Phospholipids

PHOSPHOLIPID STRUCTURE ALLOWS MEMBRANES TO FORM

BOUNDARIES

Plasma membrane separates watery inside of cell from watery environment outside the cell

Phospholipids form a two layer “sandwich” of molecules—phospholipid bilayer—at boundaries that surround the cell or organelle

MEMBRANE PROTEINS Two main types —Fig. 3.4; p. 80:

1. Intracellular Receptors- Inside the Cell

2. Membrane Receptors – Located in the cell membrane.

MEMBRANE PROTEINS Perform most specific membrane

functions 1. Enzyme activity 2. Cell-to-cell recognition 3. Cell signaling 4. Transport of materials

Concept 3.4:

Diffusion and Osmosis

DIFFUSION Diffusion: net movement of particles from

an area of greater concentration to an area of lesser concentration

Permeable membranes: membranes that particles to freely pass through them until the concentration of particles is equal on both sides of the membrane—balance or equilibrium

Example of Diffusion

PASSIVE TRANSPORT Selectively permeable membranes:

allow some substances to cross more easily than others; block some completely

Cell membranes are selectively permeable; O2 and CO2 pass freely; H2O must pass through protein channels; other molecules pass only under specific conditions

PASSIVE TRANSPORT Passive transport: diffusion of a

substance across a membrane with no energy expenditure by the cell

Simple Diffusion

TYPES OF PASSIVE TRANSPORT

Simple diffusion: passage of substance directly through the membrane—O2, CO2

Facilitated diffusion: substances pass through a channel in a transport protein in the membrane—sugars, H2O, some ions

Passive Transport

TYPES OF PASSIVE TRANSPORT

OSMOSIS Passive transport of water is so

common that it is given a special name

Osmosis: passive transport of H2O across a selectively permeable membrane

Aqueous (water is the solvent) solutions are classified into 3 categories called osmotic solutions

Movement of Water

TYPES OF OSMOTIC SOLUTIONS

Hypertonic Solution: a solution whose solute concentration is higher than the solute concentration inside the cell

- Animal cells will lose water and shrivel when placed in a hypertonic solution— water moves out of the cell

Cells placed in a hypertonic solution

TYPES OF OSMOTIC SOLUTIONS

Hypotonic Solution: a solution whose solute concentration is lower than the solute concentration inside the cell

- Animal cells will swell and possibly burst when placed in a hypotonic solution—water moves into the cell

Cell placed in a hypotonic solution

TYPES OF OSMOTIC SOLUTIONS

Isotonic Solution: a solution whose solute concentration is equal to the solute concentration inside the cell

- Animal cells retain their shape when placed in isotonic solutions—no net gain or loss of water inside the cell

Isotonic solution

Water molecules move in and out of the cell at the same rate, and the cell retains it’s normal shape.

BEHAVIOR OF RED BLOOD CELLS IN DIFFERENT OSMOTIC

SOLUTIONS

OSMOSIS & PLANT CELLS

Hypotonic environment: plant cells are firmest and healthiest

- Cell wall prevents cell membrane from bursting Isotonic environment: non-woody plants

wilt Hypertonic environment: membrane

pulls away from cell wall—kills the cell

PLANT & ANIMAL CELLS IN DIFFERENT OSMOTIC

SOLUTIONS

Concept 3.5:

Active Transport, Endocytosis, and

Exocytosis

ACTIVE TRANSPORT Substance is moved from area of lower

concentration to an area of higher concentration

Requires the cell to expend energy

ACTIVE TRANSPORT Plays major role in maintaining cell’s

chemical environment

Two major types: 1) solute pumping 2) large molecule transport

SOLUTE PUMPING Specific membrane transport protein

pumps a solute across the membrane

Energy is used to move solute from area of lower concentration to area of higher concentration

Solutes moved into cells and out of cells

SOLUTE PUMPING

LARGE MOLECULE TRANSPORT

Some particles are too large to move through the membrane

Must be packaged in membrane sacs called vesicles

Particle fuses with plasma membrane and is moved either into (endocytosis) or out of (exocytosis) the cell

Both processes require energy

TYPES OF ENDOCYTOSIS

Phagocytosis: cell membrane engulfs and takes in particle of solid material;

“cell eating”

Pinocytosis: cell membranes engulfs and takes in small droplet of liquid; “cell drinking”

EXOCYTOSIS Product of cell is exported Requires energy

ENDOCYTOSIS Material is brought into the cell Requires energy

ENDOCYTOSIS & EXOCYTOSIS

PASSIVE vs. ACTIVE TRANSPORT