Cell Transport Review

Overview

• Homeostasis: need of an organism to stay stable by regulating internal conditions.

– A dynamic equilibrium is maintained = it isn’t always the same, but things stay within a range.

– Organisms constantly take in stimuli and have to respond to them in order to maintain homeostasis.

• Stimulus = change in environment
• Response = a change in the organism, as a result of a stimulus

• Feedback mechanisms evolved to help maintain homeostasis in organisms as they respond to stimuli.

– These mechanisms use the output of a system to signal a change in input so that a system response can be stabilized or amplified.

– Can be positive or negative

Overview

• In a positive feedback loop, the output (or product) of a system intensifies the response.

(+) Feedback Mechanisms

AMPLIFICATION

• Examples:

– Human child birth
• Hormones → Contractions → Pressure → Release of more hormones → More contractions → More pressure...etc.
– Fruit ripening
• Fruit ripens → Releases ethylene → Signals surrounding fruit to ripen → Neighboring fruit ripen → Release more ethylene…etc.

• In a negative feedback loop, the output (or product) of a system causes a counter response to return to a set point.

(-) Feedback Mechanisms

• Examples:

– Human body temperature (thermoregulation)
– Water concentration (osmoregulation)
– Blood sugar regulation

STABILIZATION

Cell Membrane

• Homeostasis is maintained through regulation at the organ system level all the way down to the cellular level.

• Much of homeostasis on
the cellular level
is maintained by
the cell
membrane
controlling the
movement of
things in and out
of the cell.

Cell Membrane

• The cell membrane is selectively permeable = picky about what goes in and out.

• The transport of materials into and out of the cell is classified as either passive or active.

– CAN pass easily: things that are small, nonpolar,
hydrophobic and/or neutral, as
well as water

– CANNOT pass easily: polar and/or large molecules

Cell Transport

• Passive Transport requires no extra energy by the cell because molecules move from high concentration (squished together) to low concentration (spread out) areas down the concentration gradient.

– Ex. Simple diffusion, facilitated diffusion, and osmosis

• Active Transport requires extra energy (ATP) to be spent to bring materials into the cell or expel materials out of the cell moving from low to high concentration against the concentration gradient.

– Ex. Molecular pumps, exocytosis, and endocytosis

Words to Know

• Solute – what gets dissolved (Ex. Lemonade powder)

• Solvent – does the dissolving (Ex. Water)

• Solution – uniform mixture of two or more substances (Ex. Lemonade)

• Concentration – amount of solute dissolved in solvent

Symbol for abbreviation = [ ]

Passive Transport:
Simple Diffusion

• The spreading out of molecules across a membrane until equilibrium is reached

– Equilibrium = equally concentrated on both sides of the membrane

• Molecules move down a concentration gradient, from high [ ] to an area of low [ ]

– Ex. O2 and CO2, and other small, nonpolar molecules

Passive Transport:
Facilitated Diffusion

• A transport protein helps to facilitate the diffusion of molecules that normally couldn‘t pass through the cell membrane

– Transport proteins can act as a channel or a carrier

• Molecules move down a concentration gradient, from high [ ] to an area of low [ ]

– Ex. Large molecules like glucose(sugar = C6H12O6) and polar molecules like calcium(Ca+2)

Passive Transport:

Osmosis

• The simple diffusion of water across the cell membrane.
• Water molecules move down a concentration gradient, from high [water] to an area of low [water] until equilibrium is reached.

– High water concentration means low solute concentration.
– Low water concentration means high solute concentration.

Passive Transport:

Osmosis

• Hypertonic solutions: water [ ] is lower than the cell’s cytoplasm.

  • Net movement of water out of cell → Cell shrivels

• Hypotonic solutions: water [ ] is higher than the cell’s cytoplasm.

  • Net movement of water into a cell → Cell swells

• Isotonic solutions: identical water [ ] to cell’s cytoplasm →Cell stays the same

Passive Transport:

Osmosis

Active Transport:
Molecular Pumps

• When a cell uses energy to pump molecules across the membrane, against the [ ] gradient, through a protein channel.

• This allows a cell to concentrate key molecules within the cell, or remove waste quickly from the cell.
– Ex. Potassium (K+), chlorine (Cl-) and sodium (Na+) = ions (charged particles)

Active Transport

Using Vesicles

• Endocytosis uses vesicles to move large particles into the cell.

– Ex. When white blood cells engulf bacteria in order to fight infection.

• Exocytosis uses vesicles to export materials out of the cell.

– Ex. When nerve cells secrete neurotransmitters to send signals throughout the body.