Chapter 3: Exchanging Materials with the Environment

Cellular Transport

Transport across the Membrane

Transport? • Cells need things…water, oxygen, balance of ions,

nutrients (amino acids, sugars..building blocks), nucleotides, hormones

• Cells need to get rid of things…waste (carbon dioxide, ammonia, any material that has been broken down)

• Transport…allows these materials into a cell and allows these materials to leave the cell across the cell membrane.

The Cell Membrane

• ALL cells have a cell membrane (also called the plasma membrane, phospholipid bilayer, fluid mosaic model).

5

FLUID- because individual phospholipids and proteins can move side-to-side within the layer, like it’s a liquid.

MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above.

Why is it called the FLUID MOSAIC MODEL?

Functions of the Cell Membrane

• Selectively Permeable – allows certain things to enter and leave the cell (transport).

• Acts as a protective barrier.

• Recognizes other cells/things nearby.

Parts to the Cell Membrane

proteins

proteins

8

Phospholipids

Make up the cell membrane

Contains 2 fatty acid chains that are nonpolar (repel charged particles but let fat-soluble molecules to pass)

Head is polar & contains a Phosphate group & glycerol

• Polar heads (phosphate heads) are hydrophilic (water loving)

• Nonpolar tails (fatty acid tails) are hydrophobic (water fearing)

• This makes the cell membrane “selective” in what crosses.

10

Cell Membrane

Hydrophobic molecules pass easily; hydrophilic DO NOT

The cell membrane is made of 2 layers of phospholipids called the lipid bilayer

Another Look at the CM

proteins

proteins

Cholesterol?

• Major membrane lipid

• Similar to phospholipids-one end is hydrophilic and one end is hydrophobic

• Makes the membrane less permeable to most biological molecules

• Lowers the temperature that a membrane solidifies and it decreases fluidity at higher temperatures

Importance of Protein Channels

Importance of Glycoproteins & Glycolipids

• Embedded in cell membrane

• Sometimes have sugar chains attached to the proteins or sugar chains attached to the phospholipids

• Function: act as antennae that receive chemical messages from other cells and function in cell recognition

Getting through the Cell Membrane

Can pass easily • Small hydrophobic nonpolar

molecules move through easily.

e.g. O2, CO2, N2

• Small uncharged polar molecules that are soluble in lipids (fat-soluble) can pass easily

e.g. H2 O, ethanol, glycerol

Cannot pass easily • Ions (charged particles),

hydrophilic molecules larger than water, and larger uncharged polar molecules do not move through the membrane on their own.

e.g. H+ ions, Ca+ ions, Na+ ions, Cl- ions, amino acids, glucose, nucleotides

So what determines if material can cross?

• Polarity – – In general, nonpolar molecules do not interact with

polar molecules (oil-nonpolar, water-polar)

– Polar molecules interact with polar molecules (salt –ion, water-polar)

• Hydrophobicity- – lipid bilayer is hydrophobic so would repel hydrophilic

molecules. Hydrophobic molecules can pass

• Charge- – the hydrophobic tails will not allow charged particles

to cross...they need special transport proteins

Review Questions

1. What materials need to be exchanged across a cell membrane?

2. In what way is an organism a protected compartment?

3. What is the significance of a membrane being selectively permeable?

Answers to Review Questions 1. CM must control the exchange of nutrients,

waste, water, and communication signals

2. An organism is a protected compartment in the sense that whether it is made of a single cell or many, each cell is completely surrounded by a protective membrane

3. A cell membrane must be selectively permeable so that the cell can exert some level of control over which types of molecules can pass through the membrane

3.3 Diffusion and Osmosis

• Movement of molecules from high concentration to low concentration (down the concentration gradient –like balls rolling down a hill).

Concentration Gradient

• Refers to a change in solute concentration, either from a low concentration to a high one or the opposite.

• Diffusion moves molecules down a concentration gradient, but it cannot transport molecules against a concentration gradient.

Diffusion

• Diffusion is a PASSIVE process – requires no energy from the cell

• Molecules will diffuse until the concentration is the same throughout

• Does diffusion increase

or decrease entropy of

a system?

Osmosis

• Diffusion of water across the cell membrane

• Water moves from high to low concentration

• Water will always move

to where there is more

solute!

• Does not require energy

from the cell (passive)

Water will always move to where there is more solute!

low solute high solute

Osmotic States of a Cell • Isotonic state – solute concentration inside

the cell and outside the cell is equal. No movement of water

• Hypotonic state – the solute concentration inside the cell is lower than outside the cell. Water will move out of the cell. The cell will shrink

• Hypertonic state – the solute concentration inside the cell is higher than outside the cell. Water will move into the cell. The cell will swell/expand

30

Cell in Isotonic Solution

CELL

10% NaCL 90% H2O

10% NaCL 90% H2O

What is the direction of water movement?

The cell is at _______________.

ENVIRONMENT

NO NET MOVEMENT

31

Cell in Hypotonic Solution

CELL

10% NaCL 90% H2O

20% NaCL 80% H2O

What is the direction of water movement?

32

Cell in Hypertonic Solution

CELL

15% NaCL 85% H2O

5% NaCL 95% H2O

What is the direction of water movement?

ENVIRONMENT

33

Cells in Solutions

copyright cmassengale

34

Isotonic Solution

NO NET MOVEMENT OF H2O (equal amounts entering &

leaving)

Hypotonic Solution

Cell bursts (CYTOLYSIS)

Hypertonic Solution

Cell shrinks & shrivels

(PLASMOLYSIS)

Aquaporins

• Aquaporins are membrane water channels that play critical roles in controlling the water contents of cells. These channels are widely distributed in all kingdoms of life, including bacteria, plants, and mammals.

37

Cytolysis & Plasmolysis

Cytolysis Plasmolysis copyright cmassengale

38

Osmosis in Red Blood Cells

Isotonic Hypotonic Hypertonic

copyright cmassengale

What Happens to Blood Cells?

copyright cmassengale 39

What happens in plant cells? • Plant cells have a cell wall (which means extra

protection from bursting)…animal cells do not.

• Turgor – is the outward pressure of a cell against its cell wall.

• Flaccid- opposite. Lacking firmness or turgor

Plasmolysis- when the cell membrane of a plant cell pulls away from the cell wall due to the loss of water through osmosis

Plasmolysis

43

hypotonic hypertonic isotonic

hypertonic isotonic hypotonic

copyright cmassengale