Passive movements

•Requires no energy from cell

•Diffusion

•Facilitated Diffusion

•Osmosis

•Filtration

Active Movements

•Requires energy from cell

•Ion pumps

•Endocytosis

•Exocytosis

Diffusion

• Tendency of atoms or molecules in water or air to move from areas of higher concentration to lower concentration

• Moving substances eventually become diffuse, or • Moving substances eventually become diffuse, or evenly distributed

• Diffusion occurs because all substances are in constant motion

Diffusion

• Substances move down their concentration gradient (from high to lower concentration)

• Concentration gradient = difference in concentrations

Diffusion of sugar In water

Requirements for diffusion across cell membrane

2. A concentration gradient must exist across the cell

1. Cell membrane must be permeable to substance.

• Oxygen, Carbon Dioxide, and Steroid Hormones easily diffuse across the cell membrane.

Oxygen enters cells & Carbon Dioxide leaves cellsby diffusion

2. A concentration gradient must exist across the cell

membrane

Facilitated Diffusion

•Diffusion that uses membrane proteins, called carrier proteins.

•Carrier proteins include ion channels and other proteins that “carry” substances across the cell membrane.

•Similar to diffusion in that substances only move down their concentration gradient.

membrane.

•Transports ions, glucose, and some hormones across the cell membrane.

The number of carrier proteins limits the rate of facilitated diffusion.

Example of facilitated diffusion transporting substances into a cell.

Osmosis

• Water diffuses across the cell membrane•Movement of water from areas of higher

water concentrations to areas of lower water concentrations.

• Rule of thumb: Water follows salt.

Osmosis

• Solutes such as salts, proteins, sugars are too large to

• Aquaporins (channel proteins) allow water to freely diffuse across the cell membrane.

• H2O moves into areas containing higher solute concentrations.

• Solutes such as salts, proteins, sugars are too large to cross membrane.

60% H O 80% H O

A B

Membrane is impermeable to proteins

60% H2O

40% protein80% H2O

20% protein

Higher H2O concentration

A B

Higher soluteconcentration

Osmotic Pressure = Pressure generated by osmosis.

Osmotic PressurePressure

H2O moved down its concentration gradient

Osmotic Pressure exerted on Cells

Intracellular fluid (fluid inside cells) = 0.9% NaCl

Extracellular Solutions (fluid outside cells) may exert osmotic pressure onto cells.osmotic pressure onto cells.

• Extracellular solution of 0.9%NaCl = Isotonic

• Extracellular Solution above 0.9%NaCl = Hypertonic

• Extracellular Solution below 0.9%NaCl = Hypotonic

Isotonic Solution

• Extracellular [NaCl] is equal to Intracellular [NaCl].

• Results in no osmotic pressure

Red Blood Cell in isotonic solutionNo osmotic pressure = normal shape & size

Hypertonic Solution

• Extracelluar [NaCl] is greater than Intracellular [NaCl]

• Water moves out of cell & cell may shrink

Red Blood Cell in hypertonic solution.Water leaves the cell causing it to shrink.

Hypotonic Solution

• Extracellular [NaCl] is less than Intracellular [NaCl].

• Water moves into cell & the cell swells

• Cell may lyse (burst)• Cell may lyse (burst)

Red Blood Cell in hypotonic solution.Water enters the cell causing it to swell.

Filtration

• Molecules are forced through membranes

• Force created by hydrostatic pressure, such as blood pressure.

• Commonly separates solids from liquids.

In filtration of water and solids, gravity forces water through filter paper.

Filtration in body

•Blood pressure forces water and smaller solutes through tiny openings in capillary wall.

•Larger molecules, proteins, and cells remain inside the capillaries.

Active Transport

• From lower concentration to higher concentration.

• Movement against a concentration gradient.

• Cell must use energy from cellular metabolism

• Up to 40% of a cell’s energy supply is used for active transport.

• Cell must use energy from cellular metabolism (usually in the form of ATP).

• Active transport requires carrier proteins.

Active Transport

•Pumps Na+ out of the cell

•Creates a high extracellular [Na+]

•Pumps K+ into the cell

•Creates a high intracellular [K+]

Sodium/Potassium Pump: Active Transport

ATP = currency of energy for cell

•Cell engulfs larger substances.

•Cell membrane invaginates and surrounds a substance.

•Forms a vesicle from a portion of the cell membrane.

Endocytosis

• Pinocytosis = endocytosis of a fluid

Endocytosis Examples

• Phagocytosis = endocytosis of a solid

Endocytosis Examples

• Receptor-Mediated Endocytosis

Endocytosis Examples

Moves specific types of molecules into the cell.Moves specific types of molecules into the cell.

Specific target molecules are called ligands.

Receptor-Mediated Endocytosis

• Think of the receptor as a lock, and only a ligand “key” with the correct shape can bind to it.

Ligand = Key

Receptor = Lock

• The receptor-ligand complex is engulfed by cell

• The ligand fits onto a receptor and forms a receptor-ligand complex.

Receptor-Mediated Endocytosis

• Provides specificity

• Removes substances even in very low concentrations

Exocytosis

•Reverse of Endocytosis

•Cell organelles produce chemicals and proteins, •Cell organelles produce chemicals and proteins, and then package them in vesicles.

•The vesicles fuse with the cell membrane releasing the chemicals from the cell.

Exocytosis

Example of Exocytosis:Neurotransmitters are secreted from neurons by exocytosis

Transcytosis

•Combines endocytosis & exocytosis

•Quickly transports substances across cell•Quickly transports substances across cell

•Trancytosis transports HIV across linings of the mouth, anus, and female reproductive tract

Trancytosis of HIV

End of Section 2, Chapter 3