Passive and Active transport

Passive transport

Movement of substances across the plasma membrane without energy expenditure by the cell

Direction of passive transport

Substances move from high concentration to low concentration (down the concentration gradient)

Selectively permeable

Plasma membrane allows some substances through but not others

Integral proteins

Act as channels or pumps, usually in one direction

Lipid-soluble materials

Pass easily through the plasma membrane (e.g., fat-soluble vitamins, oxygen, carbon dioxide)

Polar substances and ions

Need special transport because they cannot pass through the lipid core easily

Diffusion

Passive movement of a substance from high to low concentration until equilibrium is reached

Energy requirement for diffusion

No energy required

Example of diffusion

Perfume spreading in a room

Factors affecting diffusion

Extent of concentration gradient, mass of molecules, temperature, solvent density

Facilitated transport

Movement of substances across membrane with help of transmembrane proteins, no energy needed

Substances using facilitated transport

Polar substances and others that cannot diffuse easily

Role of proteins in facilitated transport

Act as channels or carriers to facilitate passage

Osmosis

Diffusion of water across a semipermeable membrane down its concentration gradient

Solute movement in osmosis

Only water moves; solutes cannot cross the membrane

Direction of water in osmosis

From an area of higher concentration to lower concentration

Tonicity

The amount of solute in a solution

Hypotonic solution effect on animal cells

Water enters the cell, which may burst (lyse)

Hypertonic solution effect on animal cells

Water leaves the cell, causing it to shrivel (crenate)

Isotonic solution effect on animal cells

No net water movement; cell remains normal

Concentration gradient

Area of high concentration across from area of low concentration

Osmolarity

Total amount of substances dissolved in a specific amount of solution

Solute

Substance dissolved in another to form a solution