Cell Membrane& Cellular Transport

Homeostasis –the maintenance of internal stable conditionsThe cell membrane keeps the cell in balance.

Membrane Functions• Provides a selectively permeable barrier

around the cell. • Controls the passage of substances in and out

of the cell.• Maintains homeostasis for the cell.

Membrane Structure• Phospholipid bilayer

(nonpolar)• Membrane proteins:

act as transport channels.

• Carbohydrate chains: act as “ID” tags for the cell.

• Cholesterol: provides stability for the membrane.

http://library.thinkquest.org/C004535/media/cell_membrane.gif

Phospholipid Bilayer– Phospholipids: phosphate head and lipid tail.– Phospholipid Bilayer: two layers.

inside cell

outside cellLipid Tails

Hydrophobic = repels water

Hydrophilic = attracts water

PhosphateHead

Phospholipid Bilayer

The cell membrane is semipermeable, which means only some material can

get in or out.

AminoAcids H2OSugar Lipids Salt Waste

So what needs to get across the membrane?

O2

How do you build a semi-permeable cell membrane?

• Channels are made of proteins.• Proteins act as doors in the membrane.

BilipidMembrane

Protein Channelsin Bilipid Membrane

Diffusion Molecules move from HIGH to LOW concentration; this continues until equilibrium is reached.

• Simple diffusion: directly through membrane.• Facilitated diffusion: help through a protein channel.

NO energy needed!!

HIGH

LOW

Active Transport• Cells use energy to move against

concentration gradient - from LOW to HIGH.• Particles that are polar (charged) must move

through a protein channel.– Use protein pump– requires energy (ATP)

ATP

Transport of Molecules• Endocytosis: takes material into cell by cell

membrane making pockets.– Phagocytosis – “cell eating”; cell engulfs particles– Pinocytosis – cell takes in liquid from surrounding

environment.• Exocytosis: process forcing contents out of the

cell.

Endocytosis

PINOCYTOSISPHAGOCYTOSIS

Exocytosis

Transport Summarysimplediffusion

facilitateddiffusion

activetransport

ATP

(Smaller molecules)

(Larger molecules)

Types of Cellular TransportPASSIVE• Does NOT require

energy• Goes with the

concentration gradient (high to low)

• Simple Diffusion, Facilitated Diffusion

ACTIVE• Requires energy from ATP• Goes against the

concentration gradient (low to high)

• Active Transport, Endocytosis, Exocytosis

Osmosis movement of water across cell membrane.

HIGH H2O

LOW H2O

Effects of Osmosis on Cells (osmotic pressure)

• Hypertonic: higher concentrations of solute; less H2O.• Isotonic: equal concentrations of solute.• Hypotonic: lower concentrations of solute; more H2O.

http://www.glencoe.com/sites/common_assets/science/virtual_labs/LS03/LS03.html