Cells In Their Environment

Review Facts

• Solution: a uniform mixture of 2 or more substances.

• Solute: the dissolved substance (salt)• Solvent: the dissolving substance (water)• When placed in water, NaCl will dissolve

into sodium and chloride ions.• The mixture would be considered a

solution.

What Does This Have To Do With Cells???

• All cells live at least partly in touch with a water solution.

• To survive, cells must take in nutrients and eliminate waste materials.

• To do this, nutrients and waste materials must cross the cell membrane between the solution outside of the cell, and the solution inside the cell.

What Kinds of Membranes Are There?

• Permeable membrane: membrane that allows for almost any substance to penetrate and pass through.

• Semi permeable membrane: allows certain molecules to pass through and prevents others from passing.

• Non permeable membranes: membranes that allow no passage.

Semi Permeable Cell Membrane

• Small molecules like water can enter and leave the cell freely.

• However, small particles with strong electrical charge such as ions cannot pass easily through the membrane.

• The electrical charge prevents the ion from moving through the bilayer.

• Larger molecules such as proteins and carbohydrates cannot enter and leave the cell freely due to their size.

Transportation Processes (2)

• Passive Transport: movement of a substance across a cell membrane without the input of the cell’s energy.

• Active Transport: uses cellular energy to move substances across the cell membrane.

Passive Transport (3 types)

• Simple Diffusion

• Facilitated Diffusion

• Osmosis

All three involve the movement of a substance across a cell membrane without the input of the cell’s energy.

Factors To Consider

• The rate of diffusion depends on the size and temperature of the molecule involved.

• Molecules diffuse faster at a higher temperature than at a lower temperature

• Smaller molecules are diffused more easily than larger ones.

Factors To Consider (2)

• Diffusion always occurs down a concentration gradient.

• A concentration gradient is the difference between the concentration of a particular molecule in one area and its concentration in an adjacent area.

• When molecules have been dispersed evenly, there is no concentration gradient, an equilibrium has been reached, and diffusion stops.

Simple Diffusion

• Most common form of passive transport.

• The random movement of molecules from an area of higher concentration (more molecules) to an area of lower concentration (fewer molecules).

• Enables oxygen and carbon dioxide to cross the membranes.

Simple Diffusion

Initially, the concentration of a substance outside the cell is higher than the inside.

Later, the substance has diffused into the cell so that the concentration is the same on both sides of the membrane.

If a lump of sugar is dropped into a beaker of water, the molecules dissolve (A) and diffuse (B and C).

Eventually, diffusion results in an even distribution of sugar molecules throughout the water (D).

Facilitated Diffusion

• Molecules diffuse across a cell membrane with the help of carrier proteins in the membrane.

• Always move down the concentration gradient from a higher level of concentration to a lower level.

• Increases the rate at which larger molecules such as glucose can cross the cell membrane.

• Allows the glucose molecule into the blood cells.

Facilitated Diffusion

During facilitated diffusion, carrier proteins shuttle molecules across the lipid bilayer of the cell membrane.

Facilitated Diffusion

Osmosis

• The third form of passive transport.• The diffusion of water across a semi permeable

membrane• Occurs when the concentrations of solutions on the 2

sides of a semi permeable membrane are different.• Again water moves down its concentration gradient.• Moves from solutions with higher water

concentration (higher solute) to solutions with lower water concentration (lower solute).

Osmosis

Types of Solutions

• Hypertonic Solution: concentration of solutes outside is higher than it is inside the cell in the cytoplasm.

• Water diffuses out.

• Ex: lettuce in a bowl of salt water.

Types of Solutions (2)

• Isotonic Solution: concentration of solutes outside the cell is equal to that found inside the cell.

• Osmosis does not occur because solution is said to be in equilibrium.

Types of Solutions (3)

• Hypotonic Solution: Solution in which the concentration of solutes outside the cell is lower than it is inside the cell.

• Water moves into the cell causing the cell to swell.

Very Hypotonic Solution

• If the cell swells too much, it will burst due the the large amount of water entering it.

• This is referred to as being a very hypotonic solution.

• Distilled water is hypotonic because it contains no solutes.

• When animal cells are placed in distilled water, the cell bursts due to osmosis.

Solution Types

Active Transport

• Sodium Potassium Pump

Uses cellular energy to move substances across the cell membrane.

Active Transport Factors

• Cells must use energy to move molecules across a membrane against a concentration gradient from areas of low concentration to areas of high concentration.

• Involves carrier proteins as seen in facilitative diffusion.

• Carrier proteins use energy to pump ions and molecules across the membrane.

• The energy comes from ATP (adenosine triphosphate)

The Sodium Potassium Pump

• Active transport is especially important in maintaining the ion concentration in cells.

• Ex: Animal cells pump sodium ions out and potassium ions into the cell.

• Results in high concentration of potassium ions inside of the cell.

• High concentration of sodium ions outside of the cell.

• Uses 1/3 of the cells available energy (active).

Sodium Potassium Pump

Sodium Potassium Pump

• Important for the contraction of muscles, transmission of nerve impulses, and the absorption of nutrients.

• Ex: plants use the pump in its roots to absorb nutrients from the soil

• Nutrients are more concentrated in the roots than in the surrounding soil itself.

• Without active transport, the nutrients would diffuse out of the roots and back into the soil.

Bulk Movement

• The use of several processes to transport large molecules such as polysaccharides or proteins across its membrane.

• Large molecules move across the cell membrane by being packaged in membrane bound sacs.

Bulk Movement (2 types)

• Exocytosis

• Endocytosis– Phagocytosis– Pinocytosis

Exocytosis

• Wastes and cell products are packaged by the golgi apparatus in sacs called golgi vesicles.

• The vesicles then fuse with the cell membrane and the materials in the vesicles is secreted out of the cell.

• Ex: When we cry, tear glands use exocytosis to secrete a salty solution containing proteins.

Golgi vesicle

Wastes

Endocytosis

• The process by which a portion of the cell membrane surrounds a desirable macromolecule that is outside of the cell.

• The cell then pinches off the saclike portion of its outer membrane to form a tiny new vesicle.

• Membranous vesicle moves into the cell where it can fuse with other organelles or release its contents into the cytoplasm.

Desired macromolecules

Endocytosis (2 types)

• Pinocytosis (cellular drinking)

• Phagocytosis (cellular eating)

Pinocytosis

• Cell membrane encloses a product of fluid and its solutes and brings the droplet into the cell.

Phagocytosis

• The cell engulfs the food particle or solid substance rather than a liquid.

• Many unicellular organisms such as amoebas obtain food by means of phagocytosis.

• Human white blood cells are phagocytes that engulf and destroy bacteria.

Three tumor cells being engulfed by a white blood cell!

The End