SBI4U Movement Across the Cell Membrane How do particles go in and out of the cell if the membrane is selective?

Animations of Active Transport & Passive Transport Chemicals in our body must be kept in balance in order for us to survive Maintenance of constant conditions is called homeostasis To achieve homeostasis, materials must be exchanged from the inside of the cell to the extracellular environment There are two types of transport mechanisms across the cell membrane: Passive transport (no energy used) Active transport (needs energy) Animations of Active Transport & Passive Transport

The extracellular fluid is a mixture of water and dissolved materials that surrounds the cell ECF (extracellular fluid) cell

Many small and uncharged molecules can move easily through the membrane (i.e. oxygen and water) Brownian motion - molecules are constantly moving randomly and colliding with each other This is the force behind diffusion

Passive Transport Is the movement of materials (randomly) across the cell membrane without the expenditure of cell energy 3 types of passive transport: Diffusion Facilitated Diffusion (diffusion with the help of transport proteins) Osmosis (diffusion of water)

Simple Diffusion Animation Diffusion: random movement of particles from an area of high concentration to an area of low concentration. (High to Low) Diffusion continues until all molecules are evenly spaced (equilibrium is reached) -Note: molecules will still move around but stay spread out. Simple Diffusion Animation Can you think of any examples? Someone sprays cologne in one end of the hallway but eventually can be smelled at the other end Small non-polar molecules (O2, CO2, non-polar steroids, non-polar drugs) Small uncharged molecules (*Water, glycerol) Examples: Oxygen diffuses from blood (high conc.) to the cell (low conc.) Carbon dioxide diffuses from the cell (high conc.) into the blood (low conc.) -membrane is basically impermeable to large molecules and ions. http://bio.winona.edu/berg/Free.htm

2. Facilitated Diffusion 2. Facilitated diffusion: diffusion of specific particles through transport proteins found in the membrane Transport Proteins are specific – they “select” only certain molecules to cross the membrane – WHY? Transports larger or charged molecules Channel Protein hydrophilic channel Na+, K+, Ca2+ and Cl- (voltage-channels in eukaryotes) Used when the slow rate of diffusion cannot keep up with the demand that metabolic processes have for ions and many polar & charged molecules (water, a.a’s, sugar). -still driven by concentration gradients ie. When equilibrium is reached, F.D. stops. Carrier Proteins: bind to a specific solute (eg. glucose) and transports it across the membrane. protein changes shape while carrying in order to do this. Carrier Protein http://bio.winona.edu/berg/Free.htm

Passive Transport: 2. Facilitated Diffusion Glucose molecules Cellular Transport From: High High Concentration Channel Proteins animations Cell Membrane Low Concentration Protein channel Low Transport Protein Through a  Go to Section:

Osmosis diffusion of water through a selectively permeable membrane Osmosis animation diffusion of water through a selectively permeable membrane water from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration) Water moves freely through pores. Solute (green) too large to move across.

Factors that Affect the Rate of Diffusion Any ideas? Molecule size Molecule polarity Charge on particle – charged molecules/ions CANNOT diffuse across the cell membrane Temperature

Which way will water move? HYPOTONIC conditions: when the water concentration outside the cell is greater than inside the cell…water moves INTO the cell H2O Inside Outside

Hypotonic conditions contd. If an animal cell is placed in a hypotonic solution, it could burst through a process called lysis Plant cells become turgid (normal)

Which way will water move? HYPERTONIC conditions: when the water concentration inside the cell is greater than outside the cell…water moves OUT of the cell H2O Inside Outside

Hypertonic conditions contd. Cells placed in hypertonic solutions will shrink/shrivel (water exits) Plasmolysis is a condition in plants where the cell membrane shrinks away from the cell wall Plants will wilt because of a loss of turgor pressure (the pressure of the cell contents on the cell wall) Salt on roads – good thing?

Which way will water move? ISOTONIC conditions: When water concentrations outside and inside the cell are equal, equal amounts of water move in and out of the cell H2O H2O Inside Outside

Isotonic conditions contd.

Isotonic conditions contd. Animal cells in isotonic conditions are normal Plant cells in isotonic conditions are flaccid (lacks stiffness)

B A A B Side A is hypotonic to side A – why? *Solutes cannot pass through the membrane A A B Side A is hypotonic to side A – why? Side A has more water molecules than B Water will always diffuse from a hypotonic solution to a hypertonic solution until both sides have equal concentrations

hypertonic isotonic hypotonic hypertonic hypotonic isotonic

Why are cells so small? Copy the following table for a sphere into your notes and fill it out: Diameter (cm) Radius (cm) Surface Area (cm2) =4r2 Volume (mL) =4/3r3 Surface Area ÷ Volume 2 4 6 8 10

What can you conclude? How does surface area change as the radius increases? As the size of an object increases, does the surface area or volume increase more rapidly? What does this have to do with cells? The amount of nutrients that a cell can take in and the amount of waste that can be expelled depends on the amount of surface area Thus, as the cell size increases, at a certain point, the cell will not have enough surface area to support its volume

Protein changes shape to move molecules: this requires energy - ATP! Active Transport cell uses energy actively moves molecules to where they are needed Movement from an area of low concentration to an area of high concentration (Low  High) Use carrier proteins Three Types: At (1), the molecule enters the carrier. During (2), the breakdown of ATP induces a change in shape that drives the molecule across the membrane. At (3), the carrier protein returns to its former shape or state. Protein changes shape to move molecules: this requires energy - ATP!

Types of Active Transport Sodium Potassium Pumps (Active Transport using proteins) 1. Protein Pumps -transport proteins that require energy to do work Example: Sodium / Potassium Pumps are important in nerve responses. Keeps the concentration of Na+ high outside the cell and K+ high inside the cell This is maintained by transport proteins that pump 3 Na+ ions out of the cell and 2 K+ in to the cell against the concentration gradient

A QUESTION! HOW DO THE REALLY LARGE MOLECULES (Hormones, polysaccharides etc.) move in and out of cells??

An Answer!! By two processes called ENDOCYTOSIS AND EXOCYTOSIS. Both methods require the use of vesicles and ATP!

Types of Active Transport 2. Endocytosis: taking bulky material into a cell Uses energy Cell membrane in-folds around food particle “cell eating” forms food vacuole & digests food This is how white blood cells eat bacteria! Three Types… Phagocytosis (Cell Eating): -Used by white blood cells and amoeba. -The cell moves out and surrounds the solid particle. 2. Pinocytosis (Cell drinking) -Same process as phagocytosis except the cell is moving liquids. 3. RME – Receptor Mediated Endocytosis -Special receptor molecules on the membrane bind with the large molecule to be moved. -When enough receptor molecules have gathered in one place, pinocytosis occurs. -E.g. Moving cholesterol into the cell.

Types of Active Transport 3. Exocytosis: Forces material out of cell in bulk membrane surrounding the material fuses with cell membrane Cell changes shape – requires energy EX: Hormones or wastes released from cell http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html# Endocytosis & Exocytosis animations