Cell Membrane & Cellular Transport

HOMEOSTASIS AND HOMEOSTASIS AND TRANSPORTTRANSPORT

Cell membranes help organisms maintain homeostasis by controlling what substances may enter or leave cellsSome substances can cross the cell membrane without any input of energy by the cell The movement of such substances across the membrane is known as passive transport To stay alive, a cell must exchange materials such as food, water, & wastes with its environment These materials must cross the cell or plasma membrane

Plasma Membrane

Boundary that separates the living cellliving cell from it’s non-livingnon-living surroundings.

Phospholipid bilayer (2 parts)

hydrophilic heads

hydrophobic tails PhospholipidPhospholipid

Phospholipid Bilayer

Plasma Membrane - cont.Plasma Membrane - cont.

Controls traffic into and out of the cell with phospholipids and transport proteins.

Selectively permeable

(allows some molecules to move across cell membrane)

Transport proteinTransport protein

Small molecules like water, oxygen, & carbon dioxide can move in and out freely

Large molecules like proteins & carbohydrates cannot move easily across the plasma membrane

The Cell Membrane is semi permeable or selectively permeable only allowing certain molecules to pass through

DiffusionDiffusionThe net movement of a substance (molecules) down a concentration gradient from an area of high concentration to an area of low concentration.When the molecules are even throughout a space- it is called EQUILIBRIUMEQUILIBRIUM

Passive transport: NO energy is required.

Facilitated diffusion: Type of passive transport which uses transport proteins.

DiffusionDiffusion is the movement of molecules from an area of higher concentration to an area of lower concentration Small molecules can pass through the cell membrane by a process called diffusionDiffusion across a membrane is a type of passive transport because it does not require energyDiffusion continues until the concentration of the molecules is the same on both sides of a membrane

Diffusion is driven by the kinetic energy of the moleculesKinetic energy keeps molecules in constant motion causing the molecules to move randomly away from each other in a liquid or a gasThe rate of diffusion depends on temperature, size of the molecules, & type of molecules diffusingMolecules diffuse faster at higher temperatures than at lower temperaturesSmaller molecules diffuse faster than larger molecules

OsmosisThe diffusion of water across a semi~permeable membrane is called osmosis The water moves from a high concentration to low concentration.

3 kinds of Osmosis in cells:

Hypotonic, Hypertonic, & Isotonic

OSMOSISThis diagram shows water molecules moving across a selectively permeable membrane. Water molecules are the small blue shapes, and the solute is the green.What's happening? The solute (green blobs) is more concentrated on the right side, which pulls the water molecules toward that side. The green blobs would move to the left to spread out evenly, but the membrane won't let those pass

SolutionsSolutions have two parts --- the solute which is being dissolved in the solvent

Water serves as the main solvent in living things

Hypertonic SolutionSolute concentration outside the cell is higher (less water)

Water diffuses out of the cell until equilibrium is reached Cells will shrink & die if too much water is lost

Plant cells become flaccid (wilt); called plasmolysis

Hypotonic Solution Solute concentration is more inside the cell.Water moves into the cell until equilibrium is reachedAnimal cells swell & burst (lysis) if they take in too much water

CytolysisCytolysis is the bursting of cellsPlant cells become turgid due to water pressing outward against cell wall. Turgor pressure in plant cells helps them keep their shape

Isotonic SolutionsConcentration of solutes same inside & outside the cellWater moves into & out of cell at an equal rate so there is no net movement of water

Animal CellsAnimal Cells

Animal cellsAnimal cells placed into a hypotonic solution will HEMOLYSIS (EXPLODE).HEMOLYSIS (EXPLODE).

Animal cellsAnimal cells placed into a hypertonic hypertonic solutionsolution will CRENATE (SHRIVEL).CRENATE (SHRIVEL).

Hemolysis

Crenation

RedRedBloodBloodCellsCells

Plant CellsPlant Cells

Firmness or tension (vacuole full)(vacuole full) that is found in plant cells (cell wall) that are in a hypotonic environment is called TURGIDTURGID..

This process is called TURGOR TURGOR PRESSURE.PRESSURE.

WaterWater

CellWall Water

CentralVacuole

Plant CellsPlant CellsWhen the plasma membraneplasma membrane pulls away from the cell wallcell wall (vacuole empty)(vacuole empty) in a hypertonic hypertonic environmentenvironment (loss of water)(loss of water) is called PLASMOLYSIS.

Water Water

CellWall Water

plasma membraneplasma membrane

Effect of  Solutions on Cells

                                                                                                                                                                                                                                                                                                                                                         

Facilitated DiffusionFaster than simple diffusionConsidered passive transport because extra energy not usedOccurs down a concentration gradientInvolves carrier proteins embedded in a cell’s membrane to help move across certain solutes such as glucoseCarrier molecules change shape when solute attaches to themChange in carrier protein shape helps move solute across the membrane

Active TransportActive Transport The movement of molecules (small or large) across the plasma membrane in which energy (ATP) is required.Moves materials against their concentration gradient from an area of lower to higher concentration

Sodium-Potassium Pump

The mechanism that uses energy (active transport)energy (active transport) released from splitting ATPsplitting ATP to transport Sodium Sodium (Na(Na++)) out out of and Potassium (KPotassium (K++) into cells. ) into cells.

extracellularfluid

intracellularfluid

Na+ Na+

K+ K+

Bulk Transport Moves large, complex molecules such as proteins across the cell membrane

Large molecules, food, or fluid droplets are packaged in membrane-bound sacs called vesicles

Endocytosis moves large particles into a cell

There are two forms of endocytosis:

a) Pinocytosis -  a form of endocytosis that transports liquids into the cell.

b) Phagocytosis - a form of endocytosis that transports solids into the cellExocytosis is used to remove large products from the cell such as wastes, mucus, & cell products

Exocytosis is the movement of a substance out of a cell via vesicles.  Exocytosis is the reverse of endocytosis

Animation of endocytosis & exocytosis