Unit 5- Cell Membrane & Cell Transport

5b- Cellular Transport

About Cell Membranes

• All cells have a cell membrane• Functions:

a. Maintains homeostasisb. Provides protection and

support for the cell

TEM picture of a real cell

membrane.• Animations of Active Transport & Passive Transport

What is the purpose of cellular transport?

• Homeostasis depends upon appropriate movement of materials across the cell membrane.– Required materials must pass into the cells so they can be

used.• Ex. Oxygen and glucose for cellular respiration

– Waste materials must pass out of the cells as they are produced• Ex. The CO2 produced as a waste product of cellular respiration

How?

• Each individual cell exists in a fluid environment, and the cytoplasm within the cell is a fluid environment. – The presence of a liquid makes it possible for substances (such as

nutrients, oxygen, and waste products) to move into and out of the cell.

• A cell membrane is semipermeable (selectively permeable).• Materials can enter or exit through the cell membrane by

passive transport or active transport.

Membrane movement animation

Passive Transport• Cell uses no energy • The movement of molecules

occurs along a concentration gradient

– meaning molecules move from an area of high concentration to an area of low concentration

• 3 types of passive transport

high

low

Weeee!!!

Passive Transport: 1. Diffusion: spreading out of molecules across a cell

membrane until they are equally concentrated (equilibrium) *molecules never stop moving, just stay spread out*

– results from the random motion of molecules– Occurs along a concentration gradient– Molecules can diffuse across a membrane by dissolving in the

phospholipid bilayer (ex O2, CO2) or by passing through pores in the membrane

Simple Diffusion Animation

2. Osmosis: diffusion of water across a membrane• Water moves from a high

concentration of water to a low concentration of water

• Net direction of osmosis is determined by the relative solute concentrations on the two sides of the membrane.

Osmosis animationPassive Transport:

Aquaporin

Solutions

• Solute- substance being dissolved

• Solvent- substance solute dissolves in

• Solution= solute+solvent

Passive Transport-OsmosisHypotonic Solution: The solute concentration outside the cell is lower than the solute concentration inside the cell.

(Low solute; High water)

Result: Water moves from the solution to inside the cell. Cell swells & may burst (cytolysis).Animation

Passive Transport-OsmosisHypertonic Solution: The solute concentration outside the cell is higher than the solute concentration inside the cell.

(High solute; Low water)

Result: Water moves out of the cell into the solution. Cell shrinks! Animation

shrinks

Plasmolysis

Passive Transport-OsmosisIsotonic Solution: The solute concentrations outside the

cell & inside the cell are equal.

Result: No net movement. Water moves equally in both directions & the cell remains same size. (Dynamic

Equilibrium)

Hypertonic, Hypotonic, Isotonic Solutions

In what type of solution are these cells?

A CB

Hypertonic

Isotonic

Hypotonic

How Cells Deal with Osmosis• Cells must compensate for the water that enters the cell in

hypotonic environments & leaves the cell in hypertonic environments.

• Protists, like paramecia, have contractile vacuoles that collect & pump out excess water.

• Plants & bacteria have cell walls that prevent them from over-expanding.– In plants the pressure exerted on the cell

wall is called turgor pressure. – Plasmolysis- in a hypertonic environment the

cells shrink away from the cell walls & turgor pressure is lost.

• Paramecium (protist) removing excess water video

How Cells Deal with Osmosis

• Animal cells are bathed in blood. Kidneys keep the blood isotonic by removing excess salt & water.

• Many multicellular organisms have outer surfaces that are waterproof to prevent water loss on land or water gain in freshwater.

3. Facilitated diffusion: diffusion of specific particles along a concentration gradient with the help of transport proteins • Transport Proteins are specific –

they “select” only certain molecules to cross the membranea. Transports larger or charged

molecules that cannot pass through the membrane on their own

b. Glucose is an example of a molecule that passes into the cell through facilitated diffusion

Facilitated diffusion (Channel Protein)

Diffusion (Lipid Bilayer)

Passive Transport:

Carrier Protein

A B

• http://bio.winona.edu/berg/Free.htm

High Concentration

Low Concentration

Cell Membrane

Glucosemolecules

Proteinchannel

Passive Transport: 3. Facilitated Diffusion

Transport Protein

Through a

Cellular Transport From a- Hig

h

Low

• Channel Proteins animations

Active Transport

• Requires the cell to use energy • Molecules move against the

concentration gradient – – from an area of low concentration to an

area of high concentration • Active transport is used to:

– move large molecules– concentrate molecules within the cell– remove waste from the cell

high

low

This is gonna

be hard

work!!

Types of Active Transport

1. Protein Pumps (aka cell membrane pumps) –transport proteins that require energy to do work

• Example: Sodium / Potassium Pumps are used in nerve cells to transmit messages.

Sodium Potassium Pumps

(Active Transport using

proteins)

Active Transport- Protein Pumps

• Sodium-Potassium Pump– ATP supplies the energy that drives the pump

Types of Active Transport

2. Endocytosis: • Cells take in large molecules by

folding around them & forming a pouch– Pouch then pinches off from

the membrane & becomes an organelle called a vesicle

• This is how white blood cells ingest bacteria

Types of Active Transport

3. Exocytosis: – vesicles made by the cell fuse

with the cell membrane, releasing their contents into the external environment

– Used to release large molecules, waste products, or toxins

– Used to release proteins after Golgi apparatus packages them into vesicles

Endocytosis & Exocytosis

animations

Endocytosis & Exocytosis