Ch. 5

Cytoplasm

Figure 5.10

Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

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

Phospholipid Bilayer 2 layers of

phospholipids Proteins

Transport Receptors Enzymes

Cholesterol Maintains fluidity

Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings

Figure 5.11

In water, phospholipids form a stable bilayer

Figure 5.11B

Hydrophilicheads

Hydrophobictails

Water

Water

– The heads face outward and the tails face inward

Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

Membrane composed of different molecules Phospholipids Cholesterol Proteins

Phospholipid molecules form a flexible bilayer Cholesterol and protein molecules are

embedded in it Carbohydrates act as cell identification tags

Molecules move laterally

Because of polar and nonpolar regions of the phospholipid bilayer, the membrane allows certain materials in and certain materials out of the cell

Permits exchange of nutrients, waste products, oxygen, and inorganic ions.

Allows some substances to cross more easily than others:

Hydrophobic molecules—hydrocarbons, CO2, and O2 dissolve in and cross membrane

Very small polar molecules, including H2O can cross easily

Diffusion caused by the random movement of particles across a membrane

Movement due to concentration gradient Moving from a higher concentration to

a lower concentration No energy used

Movement continues until equilibrium reached Concentration is the same on both

sides of the membrane

A substance will diffuse from where it is more concentrated to where it is less concentrated

Only small, uncharged particles diffuse without assistance (CO2, and O2)

http://sussexhigh.nbed.nb.ca/swift/biology11/Cell%20Boundaries%20ONLINE.ppt#276,25,Section G

Diffusion of water across a selectively permeable membrane

Water diffuses down its own concentration gradient (from hypotonic solution to hypertonic solution)

Does not use energy

Hypertonic solution Solution contains more solute than the

solution it is compared with Hypotonic solution

Solution contains less solute than the solution it is compared with

Isotonic solution Solution contains same amount of solute

as the solution it is compared with

What kind of solution is inside the cell? Isotonic

Outside the cell? Isotonic

Which direction will water move? In and out at the

same rate

10 % NaCl

10 % NaCl

10 % NaCl

10 % NaCl

10 % NaCl

10 % NaCl

What kind of solution is inside the cell? Hypertonic

Outside the cell? Hypotonic

Which direction will water move? Into the cell

10 % NaCl

20 % NaCl

10 % NaCl

20 % NaCl

10 % NaCl

20 % NaCl

What kind of solution is inside the cell? Hypotonic

Outside the cell? Hypertonic

Which direction will water move? Out of the cell

30 % NaCl

10 % NaCl

30 % NaCl

10 % NaCl

30 % NaCl

10 % NaCl

Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings

Water balance between cells and their surroundings is crucial to organisms

The control of water balance is osmoregulation

Aides transport of many polar molecules and ions that are inhibited by phospholipid bilayer (sugar, some hormones, ions such as K+ and Na+)

Pores—always open Gated channels—

open/close in response to stimuli

Solutemolecule

Transportprotein

Passive transport—no energy usedUses transport proteins embedded in the plasma membrane (ion channels)

http://sussexhigh.nbed.nb.ca/swift/biology11/Cell%20Boundaries%20ONLINE.ppt#276,25,Section G

Transport proteins share similar properties with enzymes:

They are specific for the solutes they transport

They can be saturated with solute—maximum rate occurs when all binding sites are occupied

They can be inhibited by molecules that resemble the solute (similar to competitive inhibition)

Substances are transported across a membrane, against the concentration gradient

Use carrier proteins embedded in the membrane

Use energy (ATP)

http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/images/sppump.gif

Active transport in two solutes across a membrane

Figure 5.18

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

Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

1. 3 Na+ ions inside the cell bind to the pump. ATP donates a phosphate to the pump

2. The pump changes shape, transporting 3 Na+ across the membrane, and are released outside of the cell

3. 2 K+ ions outside the cell bind to the pump

4. The phosphate group is released and

5. 2 K+ ions are transported across the membrane

6. 2 K+ are released inside the cell

http://gotoknow.org/file/somluckv/Cell_membrane06.jpg

Substances that are too large for carrier proteins (proteins and polysaccharides)

Endocytosis—movement of substances into the cell (a.k.a. phagocytosis)

Exocytosis—movement of substance out of the cell

http://www.stanford.edu/group/Urchin/GIFS/exocyt.gif

Vesicle forms from a localized region of cell membrane that sinks inward; pinches off into cytoplasm

Vesicle usually budded from the ER or Golgi and migrates to cell membrane

Used by cells to incorporate extracellular substances

Used by secretory cells to export products (insulin in pancreas; neurotransmitter from neuron)

Process of importing macromolecules into a cell by forming vesicles derived from the cell membrane

Process of exporting macromolecules from a cell by fusion of vesicles with the cell membrane

EndocytosisExocytosis

Figure 5.19A

Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

Figure 5.19B

Phagocytosis—endocytosis of solid particles Forms food vacuoles that

fuse with lysosome to be digested

Pinocytosis—endocytosis of fluid droplets Takes in solutes dissolved

in the droplet

http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/images/phagocyt.gif

http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/images/pinocyt.gif

Imports specific macromolecules into the cell by inward budding of vesicles formed from coated pits Occurs in response to binding specific ligands to receptors on

cell’s surface Harmful levels of cholesterol can accumulate in

the blood if membranes lack cholesterol receptors

Figure 5.19

Three kinds of endocytosis

Figure 5.19C

Pseudopod of amoeba

Food being ingested

Plasma membrane

Material bound to receptor proteins

PIT

Cytoplasm

Protein that binds a specific signal molecule, allowing the cell to respond to the signal molecule

Some receptor proteins are attached to ion channels Changes permeability to a specific ion

Some may cause the formation of a second messenger, which acts as a signal molecule in the cytoplasm

Figure 5.20

Essential Biology with Physiology, 2nd ed., by Campbell, Reece, and Simon, ©2007. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.

BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2003. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.

BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2001. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.