Chapter 5

Cell Membrane

Homeostasis Maintenance of stable internal

conditions (“inside different from outside”)

Cell membrane controls movements of substances

Structure of the Membrane

Mainly a lipid bilayer

Hydrophilic phospholipid “heads” point out and in

Hydrophobic phospholipid “tails” point towards interior

Structure of the Membrane

Membrane contains steroid cholesterol

Fluidity increases with increasing temperatures

Cholesterol also helps prevent freezing

Structure of the Membrane

Only small, non-charged molecules can pass through the membrane

Examples: Oxygen Carbon Dioxide

Structure of the Membrane

Proteins embedded in membranes have many functions

1. Cell-surface markers act as “ID” tags

Structure of the Membrane

2. Receptor proteins receive signals from out of the cell

3. Enzymes cause chemical reactions to occur

Structure of the Membrane

4. Transport proteins move substances across membrane

Other Important Terms “Peripheral

Proteins” lie on only one side of the membrane

“Integral Proteins” pass completely through

SOLUTES AND SOLVENTS

Chapter 5

Important Terms

Solute: type of molecule dissolved in another type of substance; that substance is called a…

Solvent: substance that dissolves the solute

Examples

Put one tablespoon of sugar into a glass of water and stir.

Solute = sugar

Solvent = water

Examples

Fish use gills to extract oxygen dissolved in seawater

Solute = oxygen

Solvent = water

CONCENTRATION

Concentration refers to how much of some substance is present, compared to another substance.

CONCENTRATION EXAMPLES

High solute concentration: lots of sugar dissolved in a relatively small amount of water

Low solute concentration: little sugar dissolved in a relatively high amount of water

REMEMBER!

Solutes can be many different kinds of molecules (sugars, gases, nutrients, proteins, and lipids)

Solvents can vary as well (solids, liquids, or gases), but are usually H2O

DIFFUSION

Diffusion

Diffusion is the process by which solute passes through pores in a cell membrane

Diffusion also occurs within one area (no membrane) as solute travels from one area of space to another

Diffusion

Molecules ALWAYS diffuse in both directions at once (into and out of a cell)

NET FLOW OF SOLUTES IS ALWAYS FROM AREAS OF HIGHER CONCENTRATION TO AREAS OF LOWER CONCENTATION!

Diffusion

Molecules diffuse through a solvent

Example: aromas

Types of Diffusion

“Simple Diffusion”

Directly through membrane “Facilitated

Diffusion”

Done by specific carrier proteins in membrane

OSMOSIS

Osmosis Osmosis is the process by which

WATER passes through a cell membrane

Water molecules always flow in both directions at once (into and out of a cell)

NET FLOW OF WATER IS ALWAYS FROM AREAS OF LOWER SOLUTE CONCENTRATION TO AREAS OF HIGHER SOLUTE CONCENTRATION

Osmosis: Net Flow of Water Hypotonic solution – Lower than

cell’s solute concentration

Hypertonic solution – Higher than…

Isotonic – The same as…

Results of Osmosis Plant cells in hypotonic environment

experience “turgor pressure” due to cell wall

In a hypertonic environment, plasmolysis (wilting) occurs

Important Terms• Equilibrium

• Concentration Gradient

• Diffusion

• Osmosis

• Concentration of a substance is the same throughout

• Difference in concentration across a membrane

• Process by which molecules move from areas of high to low concentration

• Transport of water across a membrane

CELLULAR TRANSPORT

Cellular Transport Passive Transport

Movement across a membrane without using energy (ex.: simple diffusion, facilitated diffusion, and osmosis)

Active Transport Movement across a

membrane using energy

Cellular Transport

Passive transport allows flow of molecules down concentration gradient

Active transport uses energy to force molecules against concentration gradient

Active Transport

Passive Transport

Other Important Terms

Homeostasis Stable internal

conditions of a living thing

This is another characteristic of all living things

Living things actively keep their “insides” different than their “outsides”

Cellular Transport

Maintain Homeostasis by…

Passive Transport

Diffusion Osmosis

Active TransportExample:

Sodium/Potassium Pump

Sodium/PotassiumIon Pump

Sodium/Potassium Ion Pump

Sodium = Na+

Potassium = K+

Ions are electrically charged atoms

Sodium/Potassium Ion Pump

Pump is a protein in the membrane of cells.

Example of ACTIVE TRANSPORT (pumps AGAINST the concentration gradient)

Sodium/Potassium Ion Pump

Pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell. (Uses ATP, a molecule with lots of stored energy)

750 ions transported per second

High concentrations of Na+ and K+ build on opposite sides of cell membrane

Importance of Na+/K+ Pump

Electrical signal sent through neuron when gated ion channels open and Na+ and K+ are able to rapidly diffuse across membrane (which is passive transport)

Chapter 5 / 8

Test Review

Terms to Know:

Solute Solvent Concentration Homeostasis Hydrophilic Hydrophobic Concentration

gradient

Diffusion Osmosis Equilibrium Active Transport Passive

Transport Net flow ATP Na+/K+ Pump

Concepts to Study

What is the structure of the cell membrane?

Which way do solutes travel during diffusion?

Which way does water travel during osmosis?

The egg lab and the dialysis bag lab

Example Questions

A cell is placed in pure water. What would you expect to happen to the mass of the cell?

A cell is placed in water with the same solute concentration as inside the cell. What is this called? Which way will solutes flow?