Cells and Their Environment

Chapter 4 Section 1

The Plasma MembraneThe Plasma Membrane -

Gateway to the CellGateway to the Cell

TAKS

• (4)  Science concepts. The student knows that cells are the basic structures of all living things and have specialized parts that perform specific functions, and that viruses are different from cells and have different properties and functions. The student is expected to:– (B)  investigate and identify cellular processes

including homeostasis, permeability, energy production, transportation of molecules, disposal of wastes, function of cellular parts, and synthesis of new molecules.

Objectives

• Relate concentration gradients, diffusion, and equilibrium.

• Predict the direction of water movement into and out of cells.

• Describe the importance of ion channels in passive transport.

Key Terms

• Passive Transport• Concentration

Gradient• Equilibrium• Diffusion• Osmosis

• Hypertonic Solution• Hypotonic Solution• Isotonic Solution• Ion Channel• Carrier Protein• Facilitated Diffusion

Structure of the Cell Structure of the Cell MembraneMembrane

Cell Membranes

• Cell membranes help organisms maintain homeostasis by controlling what substances may enter or leave cells.

• 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.

Cell Membrane

Cell Membrane

• 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.

Cell Membrane

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

FLUIDFLUID- because individual phospholipids and proteins can - because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid.move around freely within the layer, like it’s a liquid.

MOSAICMOSAIC- because of the pattern produced by the scattered - because of the pattern produced by the scattered protein molecules when the membrane is viewed from protein molecules when the membrane is viewed from aboveabove..

FLUID MOSAIC MODELFLUID MOSAIC MODEL

Concentration

• The mass of solute in a given volume of solution or mass/volume.

• Concentration Gradient is a difference in the concentration of a substance across a distance

Equilibrium

• Is a condition in which the concentration of a substance is equal throughout a space.

Passive Transport

• Some substances can cross the cell membrane without any input of energy by the cell.

• Diffusion: The movement of a substances from an area of high to an area of low concentration cause by the random motion of particles across the membrane is known as passive transport.

• If diffusiion is allowed to continue cytolysis(cell bursts) eventually results.

3 Types of Diffusion

• 1. Diffusion (simple) 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 diffusion

• Diffusion across a membrane is a type of passive transport because it does not require energy.

Diffusion

Diffusion

• Diffusion is driven by the kinetic energy of the molecules.

• Kinetic energy keeps molecules in constant motion causing the molecules to move randomly away from each other in a liquid or a gas.

The Rate of Diffusion

• The rate of diffusion depends on temperature, size of the molecules, & type of molecules diffusing

• Molecules diffuse faster at higher temperatures than at lower temperatures

• Smaller molecules diffuse faster than larger molecules

• Most short-distance transport of materials into & out of cells occurs by diffusion

Diffusion & the Concentration Gradient

• Diffusion always occurs down a concentration gradient (water moves from an area where it is more concentrated to an area where it is less concentrated)

• Diffusion continues until the concentration of the molecules is the same on both sides of a membrane

• When a concentration gradient no longer exists, equilibrium has been reached but molecules will continue to move equally back & forth across a membrane

2. Osmosis

• The diffusion of water across a semi-permeable membrane is called osmosis.

• Diffusion occurs from an area of high water concentration (less solute) to an area of lower water concentration (more solute)

• Movement of water is down its concentration gradient & doesn’t require extra energy.

Water• Cytoplasm is mostly water containing dissolved

solutes• Concentrated solutions have many solute

molecules & fewer water molecules• Water moves from areas of low solute

concentration to areas of high solute concentration• Water molecules will cross membranes until the

concentrations of water & solutes is equal on both sides of the membrane; called equilibrium

• At equilibrium, molecules continue to move across membranes evenly so there is no net movement

Hypertonic Solutions

• Solute 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 Solutions

• Solute concentration greater inside the cell (less water)

• Water moves into the cell until equilibrium is reached

Hypotonic Solutions

• Animal cells swell & burst if they take in too much water

• Plant cells become turgid due to water pressing outward against cell wall

• Turgor pressure in plant cells helps them keep their shape

• Plant cells do best in hypotonic solutions

Isotonic Solutions

• Concentration of solutes same inside & outside the cell.

• Water moves into & out of cell at an equal rate so there is no net movement of water.

• Animal cells do best in isotonic solutions.

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hypotonic hypertonic isotonic

hypertonic isotonic hypotonic

3. Facilitated Diffusion• Faster than simple diffusion• Considered passive transport because extra energy

not used• Occurs down a concentration gradient• Involves carrier proteins embedded in a cell’s

membrane to help move across certain solutes such as glucose

• Carrier molecules change shape when solute attaches to them

• Change in carrier protein shape helps move solute across the membrane

Facilitated DiffusionFacilitated Diffusion

Molecules will randomly move through the Molecules will randomly move through the pores in Channel Proteins.pores in Channel Proteins.

copyright cmassengale

Facilitated DiffusionFacilitated Diffusion

• Some Carrier proteins Some Carrier proteins do not extend through do not extend through the membrane.the membrane.

• They bond and drag They bond and drag molecules through the molecules through the lipid bilayer and lipid bilayer and release them on the release them on the opposite side.opposite side.

Carrier Proteins

• Carrier proteins in the cell membrane form tunnels across the membrane to move materials

• Channel proteins may always be open or have gates that open & close to control the movement of materials; called gated channels

• Gates open & close in response to concentration inside & outside the cell

• Ion Channel is a transport protein with a polar pore through which ions can pass.

Carrier Proteins

Facilitated Diffusion

Active Transport

• Requires the use of ATP or energy.

• Moves materials against their concentration gradient from an area of lower to higher concentration.

• May also involve membrane proteins

• Used to move ions such as Na+, Ca+, and K+ across the cell membrane.

Objectives

• Compare active transport with passive transport.

• Describe the importance of the sodium-potassium pump.

• Distinguish between endocytosis and exocytosis.

Key Terms

• Active Transport• Sodium-Potassium

Pump• Endocytosis

• Exocytosis• Receptor Protein• Second Messenger

Active Transport

• Plants use active transport to help roots absorb nutrients from the soil (plant nutrients are more concentrated inside the root than outside)

• The movement of a substance across the cell membrane against its concentration gradient.

Types of Active Transport1. Sodium-Potassium Pump

• Sodium-Potassium pump moves 3 Na+ out for every 2 K+ into the cell– Causes a difference in charge inside and outside

the cell– Difference in charge is called membrane

potential

• Ion pumps help muscle & nerve cells work

• Figure 5 pg. 82

2. 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

3. Endocytosis

• Endocytosis moves large particles into a cell

Types of Endocytosisa. Phagocytosis

• Phagocytosis is one type of endocytosis.– Cell membrane extends out forming

pseudopods (fingerlike projections) that surround the particleMembrane pouch encloses the material & pinches off inside the cell making a vesicle

– Vesicle can fuse with lysosomes (digestive organelles) or release their contents in the cytoplasm

– Known as "cell eating"

b. Pinocytosis

• Pinocytosis is another type of endocytosis– Cell membrane surrounds fluid droplets– Fluids taken into membrane-bound vesicle– Known as "cell drinking"

4. Exocytosis

• Exocytosis is used to remove large products from the cell such as wastes, mucus, & cell products