The Cell Membrane (transport)

SBI4U - Birrell

In order for a cell to perform life functions, the ________ inside the cell must remain in a ____________ _________ with that of the extracellular fluid

environment, dynamic balance

the structure responsible for maintaining dynamic balance is the:

cell membrane

Passive Transport

a molecule crosses the membrane and moves down its concentration/electrochemical gradient without using energy

molecules that use passive transport include

H2O, CO2, O2

Electrochemical Gradient

means particles are charged: difference in charge across a membrane

3 types of passive transport

🤎 simple diffusion

🧱 facilitated diffusion

💧osmosis

Simple Diffusion

• small molecules move directly thru cell membrane from an area of high⟶low concentration (gradient)

• driven by Brownian Motion

• slows down over long distances

since diffusion slows down over _____ distances, this limits ____ ____

longer, cell size

Brownian Motion

🤎 The movement of particles in a liquid

Facilitated Diffusion

• polar, charged, or big particles cannot diffuse directly thru membrane

• they rely on transport proteins to enter/exit

• still moving down the gradient (no energy needed)

Transport proteins that do not require energy (passive) are divided into two types:

Carrier Proteins: transport non-charged molecules (ex: glucose)

Channel Proteins: transport charged particles like ions (also water)

Shapes of channel and carrier proteins

Channel: tunnel-like shape that forms a hydrophilic pathway

Carrier: since they bind to a specific molecule, they change shape

Osmosis

• diffusion of water

• moves from low ⟶ high solute concentration (ie. high ⟶ low water)

3 osmotic conditions

1. hypertonic: less water and more solute outside = water leaves cell = cell shrinks

2. hypotonic: more water outside and less solute outside = water enters cell = cell grows

3. isotonic: no net movement, water leaving equal to water entering

Equilibrium in animal vs plant cells

🐻: maintain isotonic environment (else will lyse or shrivel)

🌱: maintain hypotonic environment, turgid (else will become flaccid or even plasmolyzed)

some molecules are too big to cross the membrane via diffusion or a transport protein — they must undergo ____ transport, in which the cell membrane will:

bulk, fold in on itself to create a vesicle

bulk transport can either be:

• Endocytosis: into the cell

• Exocytosis: out of the cell

3 kinds of endocytosis

🧃 pinocytosis: inake of fluid

🧫 phagocytosis: intake of organic material

⚾ receptor-assisted endocytosis: intake of a specific molecule once bound to a receptor on the membrane

Active Transport

transport across a membrane against the concentration gradient (low⟶high), requiring both a transport protein and ENERGY (ATP)

Active transport can be considered either __________, like the Sodium-Potassium Pump, or __________

primary, secondary

Primary Active Transport

• uses ATP directly

• moves positively-charged ions (ex: H+, Na+, Ca2+, K+)

• Establishes electrochemical gradients, which store potential energy (which can be used for mechanisms like nerve impulse transmission)

The Sodium-Potassium Pump:

• animals have __ Na+ and ___ K+ ______ their cells

• these concentrations are maintained by pumping __ Na+ out and __ K+ in

• the pump is driven by ___ energy

• pumping action involves changes to ______ ____ and is very _____

low, high, inside, 3, 2, ATP, protein shape, fast

what, specifically, causes the change in shape in the Na-K pump?

⇨ after 3 Na+ bind, ATP transports its phosphate to the protein ⟶ causes the shape to open to the outside of the cell

⇨ after 2 K+ bind, phosphate is released ⟶ causes the shape to open back to the inside of the cell

Secondary Active Transport

• uses the gradient established by primary active transport to move other molecules against their concentration gradient

• amino acids and glucose are accumulated inside the cell

• special channels allow Na+ to move inside the cell if it brings a sugar or amino acid with it

two types of secondary active transport

1. Symport: transported solute movies in same direction as driving ion

2. Antiport: transported solute moves in opposite direction to gradient of driving ion

Driving Ion vs Solute

Driving Ion: an ion that moves down its conc. gradient

Solute: molecile that moves against its conc. gradient

the components of ATP

• 3 phosphate groups

• a sugar

• a nitrogenous base