CELLS & THEIR ENVIRONMENT

• Review: Cell (or plasma) membrane – is composed of a lipid bi-layer with embedded proteins that float. The cell membrane is a selectively permeable membrane, which means it controls what enters and what leaves the cell.

• Remember that the cell membrane allows the cell to remain separate from its environment.

State Test Question

What kind of organic compound makes up the double-layered membrane of cells?

a) Cellulose

b) Carbohydrate

c) Amino acid

d) Lipid

• The currently accepted model is the Fluid Mosaic Model.• According to this model, the cell membrane consists of a

double layer of phospholipids and large protein molecules that float in between.

• This model allows molecules to “fluidly” pass through, and the make-up resembles tiles, like a mosaic, because of the various proteins and carbohydrates that “fit” together-just as mosaic tiles fit together.

State Test Question

The plasma membrane of a cell is selectively permeable, which means it—

a) controls all cellular activities.

b) is responsible for asexual reproduction.

c) allows some materials to pass.

d) has a carbohydrate foundation.

Cellular Transport• The cell membrane helps organisms maintain homeostasis.

• Homeostasis is the maintenance of the internal environment despite changes in the external environment.

• Smaller molecules, like H20, CO2, and 02, pass easily through the cell membrane.

• Larger molecules, like glucose, amino acids, ions, and most polar molecules, cannot pass through.

• Ions (which have a charge—Na+, Cl-) and most polar molecules cannot diffuse across the cell membrane because they cannot pass through the non-polar interior of the lipid bilayer.

Cellular Two Types of Cellular Transport:1. Passive Transport and 2.Active Transport

1. Passive Transport (goes WITH the concentration gradient/down the grad.)

– The passage of substances through the membrane from regions of higher concentration into regions of lower concentrations.

– Movement across the cell membrane that does NOT require energy from the cell.

Three Types of Passive Transport:1. Diffusion 2. Facilitated Diffusion 3. Osmosis

In all 3, the movement of a substance is from a region of higher concentration to regions of lower concentration

High

Low

Three Types of Passive Transport: 1. Diffusion 2. Facilitated Diffusion 3. Osmosis

1. Diffusion– Diffusion is the SIMPLEST type of passive transport.– If there is a concentration gradient in the solution, the

substance will move from an area of higher concentration to an area of lower concentration.

– This movement

results because of

the random move-

ment of particles.

Three Types of Passive Transport:1. Diffusion 2. Osmosis 3. Facilitated Diffusion

• Diffusion depends on a concentration gradient, which is the difference in concentration of a substance across a space.

• Equilibrium is a state that exists when the concentration of a substance is the same throughout a space.

• In order for diffusion to occur, there must be a concentration gradient. Why? If there is no concentration gradient, then the substance would be in equilibrium.

• Ion channel - a doughnut-shaped transport

protein with a polar pore through which ions

can pass. An ion that enters the pore can

cross the cell membrane without contacting

the non-polar interior of the lipid bi-layer.

State Test Question

The diffusion of a substance into or out of a cell requires—

a) a concentration gradient.

b) an active transport system.

c) a carrier molecule.

d) a functioning mitochondrion.

Three Types of Passive Transport:1. Diffusion 2. Facilitated Diffusion 3. Osmosis

2. Facilitated Diffusion – -the passage of substances through the cell

membrane by means of transport proteins, called carrier proteins, from regions of higher concentration into regions of lower concentration.

– A carrier protein is a type of transport protein, which binds to a specific substance on one side of the cell membrane, carries the substance across the cell membrane, and releases it on the other side.

– Substances, such as amino acids and sugars (glucose), cross the cell membrane by facilitated diffusion.

– Carrier proteins transport molecules across the cell membrane. They bind to the molecule, change shape, and push it into or out of the cell.

– This process is used for molecules that cannot diffuse through the cell membrane.

• An example of Facilitated Diffusion is Osmosis.– In Facilitated Diffusion, Aquaporins are water channel

proteins that allow water to pass right through them.– The movement of water through cell membranes by

facilitated diffusion is an extremely important biological process—the process of osmosis.

Three Types of Passive Transport:1. Diffusion 2. Facilitated Diffusion 3. Osmosis

3. Osmosis– The diffusion of water molecules passes through a

selectively permeable membrane from an area of higher concentration to an area of lower water concentration.

– In living

organisms,

water enters

and leaves

cells by

osmosis.

Contractile vacuoles are organelles that remove excess water from cells like in this paramecium.

– A solute is a substance dissolved in a solution.– A solvent is a substance a solute is dissolved in.– A solution is a mixture of a solute and a solvent.

Solvation = Solvent + Solution:

Osmosis

Three Types of Solutions Describing Water Balance in Regions Surrounding a Cell:

1) Isotonic solution– No net water movement.– The concentration of dissolved substance or particles inside the cell is

equal to the concentration outside the cell.– Example: if a cell is placed in an isotonic solution, water still moves

into and out of a cell at random but there is not new movement of water.

– **The cell stays the same size.**

OsmosisThree Types of Solutions Describing Water Balance in Regions

Surrounding a Cell:2) Hypotonic solution

– Water moves into the cell.– The concentration of dissolved substance or particles in water

outside the cell is lower than the concentration inside the cell; therefore, the concentration of water is higher outside the cell than inside the cell.

– **The cell swells.**– Turgor pressure is the pressure that exists in a cell. As water

diffuses into the cell, the cell swells and its internal pressure increases.

– The swelling and bursting of an animal cell when water enters is called cytolysis.

State Test Question

The diagram is showing the process of—

a) osmosis.

b) active transport.

c) phagocytosis.

d) exocytosis.

OsmosisThree Types of Solutions Describing Water Balance in Regions

Surrounding a Cell:

3) Hypertonic solution– Water moves out of the cell.

– The concentration of the dissolved substance or particles is greater in the water outside the cell than the water inside the cell.

– **The cells shrinks.** – Plasmolysis is the loss of water from a cell resulting in a drop

in turgor pressure. This causes a plant to wilt. – The shrinking of animal cells that are placed in a hypertonic solution is

called crenation.

State Test Question

What is the solution in this process called?

a) Diffusion

b) Isotonic

c) Active transport

d) Mitosis

Cellular Two Types of Cellular Transport:1. Passive Transport and 2.Active Transport

Active Transport **Requires energy from the cell**• Some substances move in and out of a cell against a

concentration gradient. The passage of substances across a membrane using energy—

ATP. Substances are moved from regions of lower concentration into regions of higher concentrations.

Some active-transport processes involve carrier proteins. Like the carrier proteins used in facilitated diffusion, the carrier

protein used in active transport binds the specific substances on one side of the cell membrane.

In active transport, the substances bind to carrier (channel) proteins where they are in low concentration and are released where they are higher in concentration.

Thus, carrier proteins in active transport function as “pumps” and move substances against their concentration gradient.

State Test Question

When a molecule binds to a channel protein in order to move across a selectively permeable membrane against a concentration gradient, the process is called—

a) passive diffusion.

b) osmosis.

c) facilitated transport.

d) active transport.

State Test Question

The process by which molecules move across a cell membrane from low to high concentration is called—

a) diffusion.b) osmosis.c) active transport.d) passive transport.

State Test Question

The channels in cell membranes that help substances to move in and out of cells during active transport are made of—

a) protein.

b) chlorophyll.

c) cytoplasm.

d) carbohydrates.

Three Types of Active Transport:1)Endocytosis 2)Exocytosis 3)Sodium-Potassium

PumpEndocytosis

• The process in which cells TAKE IN large materials that cannot pass through the cell membrane, like proteins and polysaccharides.

• The movement of a substance INTO a cell by a vesicle is called endocytosis.

• Large molecules, groups of molecules, or even whole cells are engulfed and enclosed by a portion of the cell’s membrane. That portion of the mem-

brane then breaks away, and the result-

ing vesicle, along with its contents

moves into the cell.

Endocytosis Cont.Endocytosis

• Two Types of Endocytosis:

1) Pinocytosiso Involves the transport of solutes or fluids.

2) Phagocytosiso Involves the transport of large particles or whole cells.

o Example: An amoeba engulfs food particles by phagocytosis.

o Example: White blood cells engulf bacteria and viruses

Pinocytosis Phagocytosis

1) Exocytosis The process where certain substances are moved

OUT of the cell. Vesicles travel to

the cell membrane

and substances in-

side vesicles are

ejected from the cell. An example would be

waste products.

Sodium-Potassium Pump• A chemical mechanism that

uses carrier proteins to move 3 sodium ions out of the cell and 2 potassium ions into the cell.

• The sodium potassium pump is important for the conduction of nerve impulses and contraction of muscles.

• The sodium-potassium pump can transport 450 sodium and 300 potassium ions in and out of the cell in just one second.

• 1/3 of an animal’s at rest energy is used to power the sodium-potassium pump!

State Test Question

Which of the following substances can MOST easily travel into and out of the cell?

a) Sodium ions

b) Potassium ions

c) Water

d) Glucose

State Test Question

Which of the following would a cell MOST likely use to move materials by active transport?

a) A channel protein

b) A marker protein

c) A phospholipid molecule

d) A transfer RNA molecule