section 2 of chapter 3

movements into & out of cells

The cell membrane creates a barrier through which molecules must cross into and out of the

cell.

Passive movementsRequires no energy from cell

DiffusionFacilitated DiffusionOsmosisFiltration

Active MovementsRequires energy from cell

Ion pumpsEndocytosisExocytosis

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

Moving substances eventually become diffuse, or evenly distributed

Diffusion occurs because all substances are in constant motion

Diffusion of sugar In water

DiffusionSubstances move down their concentration gradient (from high to lower concentration)

Concentration gradient = difference in concentrations

Requirements for diffusion across cell membrane

2. A concentration gradient must exist across the cell membrane

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 cells by diffusion

Illustrating diffusion across cell

Substances move down their concentration gradient.

Diffusion through carrier proteins within the cell membrane

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

Facilitated diffusion transports ions, glucose, and some hormones across the cell membrane.

facilitated diffusion

Diffusion of water across a semipermeable membrane

Osmosis

Water freely crosses the membrane, but solutes (sugars, salts, and proteins) cannot cross the membrane.

Remember: Water follows salts!

water moves down its concentration gradient, i.e. from area of higher [water] into area of lower [water]

60% H2O40% protein

80% H2O20% protein

Higher H2O concentration

A B

Higher soluteconcentration

Membrane is impermeable to proteins

Osmotic Pressure = Pressure generated by osmosis.

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.

Isotonic = extracellular solution of 0.9%NaCl

Hypertonic = extracellular solution above 0.9%NaCl

Hypotonic = extracellular Solution below 0.9%NaCl

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

Isotonic Solution

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

Results in no osmotic pressure

H2O

H2O

Water moves into and out of the cell

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

Hypertonic Solution

Extracelluar [NaCl] is greater than Intracellular [NaCl]

Water moves out of cell & cell may shrink

H2O

net water movement out of the cell. Cell shrinks

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

Hypotonic Solution

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

Water moves into cell & the cell swells

Cell may lyse (burst)

H2O

net water movement into the cell. Cell swells and my lyse (burst)

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

FiltrationMolecules are forced through membranesForce created by hydrostatic pressure, such as blood pressure.Commonly separates solids from liquids.

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

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

i.e. From lower concentration to higher concentration.

Movement against a concentration gradient.

Requires cellular energy (usually in the form of ATP).

Active transport uses energy to “pump” particles through a carrier protein

ATP = currency of energy for cell

Sodium/Potassium Pump: example of active transport

3 Na+ are pumped out of the cell, while 2 K+ are pumped into the cell.

The Na+/K+ pump creates a high extracellular [Na+] and a high intracellular [K+]

endocytosisCells take in larger substances by engulfing them.

The cell membrane surrounds the substance and forms a vesicle from a portion of the cell membrane.

Pinocytosis –cell takes in fluids

Phagocytosis –cell takes in solid particles

Receptor-Mediated Endocytosis

Cell takes in specific molecules

Removes substances even in very low concentrations

Reverse of Endocytosis

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

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

End of Section 2, Chapter 3Transcytosis of HIV