Cell transport

The cell membrane

A flexible boundary that regulates movement of substances in and out of the cell to keep the cell in homeostasis

cell (plasma) membrane- roles in homeostasis

1) Regulates the movement of substances into and out of the cell (“gatekeeper”)

2) semi-permeable: some substances can pass through, others cannot

3) physically protects the cell by forming a boundary around it

three major parts of cell membrane

1) phospholipid bilayer

2) proteins

3) cholestrol

fluid mosaic model

-membrane fluidity allows proteins to function and the cell to change shape without breaking

-the membrane is fluid because phospholipids and proteins can move sideways within the layer (lateral movement is common; flip-flop is rare)

-the membrane is a mosaic because it is made of many different components embedded in the bilayer (phospholipids, proteins, cholesterol)

phospholipid bilayer

-phospholipids arrange themselves in a bilayer because their heads are hydrophilic and their tails are hydrophobic

-water is on both sides of the membrane, so the hydrophobic tails orient inward

-this structure creates a stable barrier between the inside and outside of the cell

the role of cholesterol in the cell membrane

-cholesterol helps control membrane fluidity

-it adds structure and stability to the membrane

-by fitting between phospholipids, cholesterol limits how much they can move

-increased cholesterol leads to decreased membrane flexibility

this balance allows the cell membrane to stay flexible while still protecting the cell

what type of substances need to get into the cell?

nutrients

what type of substances need to get out of the cell?

waste

cellular transport

-the movement of water and other substances into and out of the cell

-helps the cell maintain homeostasis

-the cell membrane is not a static wall, it is highly dynamic

passive transport

-down the concentration gradient

-no energy is needed to move substances across the cell membrane

-molecules move from high to low concentrations

active transport

-against the concentration gradient

-energy (ATP) is needed to move substances across the cell membrane

-molecules move from low to high concentrations

active transport: membrane pumps

-ATP is required to move substances against the concentration gradient

-membrane pumps: pumps ions across the membrane

-substances are moved “uphill”

types of active transport: bulk transport

highly energy (ATP) demanding

endocytosis- brings particles into the cell

exocytosis- sending particles out of the cell

diffusion

-the molecules “spread out”

-over time concentration becomes more uniform- equilibrium

-at equilibrium, particles are still moving randomly but there is no net movement of the population in only one direction

solution concentration

high concentration: many solute particles per volume of solvent

low concentration: fewer solute particles per volume of solvent

solute

what is being dissolved

solvent

what does the dissolving

passive transport types

-no energy required

-equal concentrations on either side = equilibrium

diffusion: net movement of particles is from areas of high to low concentration

osmosis: water molecules from high water concentration to low water concentration

simple diffusion

-small nonpolar molecules are able to diffuse directly through the mostly hydrophobic lipid bilayer

-at equilibrium, particles are still moving just with no net charge

-95% hydrophobic cell membrane

Facilitated diffusion

-net movement of particles is from areas of high to low concentration

-facilitated diffusion uses membrane proteins (channels or carrier proteins) to help particles move across the membrane

-used for large, polar, or charged molecules that cannot cross the lipid bilayer on their own

-no ATP required, particles still move down their concentration gradient

osmosis

-water moves toward the side with the higher solute concentration (lower water concentration)

-solute particles cannot pass

how to know whether water flows into or out of the cell

depends on the concentration of solute (tonicity) outside of the cell compared to the inside of the cell

hypotonic solution

lower concentration outside of the cell, water flows into cell

isotonic solution

equal concentration outside of the cell, no net water movement

hypertonic solution

higher concentration outside of the cell, water flows out of cell

cytolysis

hypotonic solution in an animal cell

-cell swells until it bursts

what tonicity does a animal cell like?

isotonic, it means the salt/solute concentration is equal inside and outside the cell, allowing water to move in and out at the same rate, maintaining a stable volume, normal shape, and optimal function

high turgor pressure =

healthy plant

what tonicity do plant cells prefer?

hypotonic, because water flows in, creating turgor pressure against the rigid cell wall, which makes the plant firm, upright, and supports growth

plasmolysis

cell membrane pulls away from cell wall (wilted plant)

flaccid plant

reduced turgor pressure

key difference between plant and animal cell

presence or absence of a cell wall

why do cells want a high surface area to volume ratio

to efficiently exchange vital substances like nutrients, oxygen, and waste with their environment, because as a cell grows, its volume (needs) increases much faster than its surface area (exchange capacity), limiting how quickly materials can move in and out, which is essential for survival. A high SA:V ensures the cell's membrane can adequately supply its internal volume, preventing it from becoming metabolically starved or overwhelmed with waste

how to calculate surface area

6s²

what does SA:V ratio mean

• High SA:V ratio → very efficient exchange

  • Materials move in and out quickly

  • Small cells have this

• Low SA:V ratio → inefficient exchange

  • Not enough surface for the cell’s needs

  • Big cells struggle unless they have folds (like lungs or intestines)