Cells and Their Environment Chapter 4 Section 1

The Plasma Membrane The Plasma Membrane - Gateway 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 Membrane

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

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.

FLUID- because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid. MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above. FLUID MOSAIC MODEL

Hydrophobic molecules pass easily; hydrophilic DO NOT phospholipid bilayer The cell membrane is made of 2 layers of phospholipids called the lipid bilayer

Ions, hydrophilic molecules larger than water, and large molecules such as proteins do not move through the membrane on their own.

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 Passive Transport 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

Solute moves DOWN concentration gradient (HIGH to LOW)

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

Osmosis Diffusion of water across a membraneDiffusion of water across a membrane Moves from HIGH water potential (low solute) to LOW water potential (high solute)Moves from HIGH water potential (low solute) to LOW water potential (high solute)

Diffusion of H 2 O Across A Membrane Low H 2 O potential High solute concentration High H 2 O potential Low solute concentration

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.

31 hypotonichypertonic isotonic hypertonicisotonic 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 Diffusion Molecules will randomly move through the pores in Channel Proteins. copyright cmassengale

Facilitated Diffusion Some Carrier proteins do not extend through the membrane.Some Carrier proteins do not extend through the membrane. They bond and drag molecules through the lipid bilayer and release them on the opposite side.They bond and drag molecules through the lipid bilayer and release them on the 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 Transport 1. 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 Endocytosis a. Phagocytosis Phagocytosis is one type of endocytosis. –Cell membrane extends out forming pseudopods (fingerlike projections) that surround the particle Membrane 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 most common form of endocytosis –Cell membrane surrounds fluid droplets –Fluids taken into membrane-bound vesicle –Known as "cell drinking"

50 Example of Pinocytosis pinocytic vesicles forming mature transport vesicle Transport across a capillary cell (blue).

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

Exocytic vesicle immediately after fusion with plasma membrane.