8-2: Cell Transport What determines the direction a substance will

move during passive transport ? Why is osmosis important? How do substances move against their

concentration gradients?

The cell’s membrane is a little like a country’s border.

It both barriers regulate who or what enters

(imports/ immigration)

and who or what leaves

(exports/ emigration/ deportation).

Passive Transport• Random movement of molecules because of

collisions between molecules.• Requires NO cell energy (doesn’t cost ATP)• When particles fill a space evenly, and then

maintain that spacing, its called a state of equilibrium. (for every molecule out, one comes in/ for every molecule up or right, one goes down or left, etc)

• Concentration is the amount of substance per given volume…. “how crowded”. An example would be a blood sugar reading of 85 mg/dL

• A difference in the concentrations of two areas is called a concentration gradient

Diffusion• Diffusion the random movement of molecules

from an area of higher concentration to an area of lower concentration. (the smell of cookies diffusing when you open the oven door, from near the cookies, to the kitchen, to the rest of the house )

– Directly through CM– through transport proteins in the CM

• Simple diffusion; small, nonpolar substances (Oxygen from RBC to muscle)

• Facilitated diffusion; needs help from a protein to cross the CM (sugar does this)

Facilitated Diffusion

• Channel proteins (imagine a slide on the playground)

– CM has integral proteins that are a specific size and shape to fit molecule

– Used for sugars, ions and amino acids– Only sodium gets in the Na channel protein

• Carrier Proteins (imagine a revolving door)

– Molecule must fit into a binding site– Protein changes shape and – Drops molecule on inside of CM (could also move

substance out of cell)

Osmosis• Osmosis the movement of water, through a

selectively permeable membrane to maintain homeostasis.

• It is a specialized form of diffusion.• The amounts of other solute sets up the

concentration gradient that drives the movement of water… based on overall ratios

• Special channel proteins in CM help regulate water balance during digestion, for regulation of body temperature and for water conservation in the kidneys

Predicting Water Movement• Water moves out of cells into ….

– Hypertonic solutions have high amounts of solutes. – If the cell is in a hypertonic solution (like salt water) then water

moves out of the cytoplasm and into the hypertonic solution until equilibrium is reached

• Water moves into a cell…..– Solutions with relatively lower concentrations of solutes are

known as hypotonic solutions– Cells in these solutions tend to gain water from their

environments

• No net change– Solutions with similar concentrations of solutes are known as

isotonic (equal) – The cell both loses and gains water, but at the same rates.

Effects of Osmosis

• Plants lose water to things like road salt (wilt)• Plants water vacuoles gain water and this

creates pressure in the stem – organisms without cell walls could actually burst

• You need to get out of the pool (fresh water) frequently because your kidneys are regulating water balance

• You get really dry skin and feel thirsty when your cells try to reach equilibrium with ocean water

Active Transport• Cells must spend energy (ATP) to move some

substances against their concentration gradients (‘up hill’).

• One example is the sodium-potassium pump.– Membrane carrier protein– Bonds to Na which the cell doesn’t want on inside – Spends ATP to change shape of protein– Flips Na to outside and protein is now the right shape

to bond to K (potassium)– Carrier Protein with K changes back to original shape

(no ATP) and the necessary K is now inside the cell– Remember direction with “nah” for Na and “okay” for K

Active Transport Using Vesicles

• Endocytosis is the formation of a vesicle around something like food and then bringing it into the CM ( white blood cell getting bacteria). CM folds in like a pocket.

• Exocytosis is when a vesicle merges with the CM and dumps something out of the cell (protista release waste this way). “turning a plastic bag inside out”

Vesicle

Endo = enter; inside; inner

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* Was outside of CM, now the inside of the vesicle

Exo = exit; outside; outer

* Was inside of vesicle, now part of CM

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ALL membranes are flexible and inter-changeable