The Plasma Membrane and Homeostasis

2 Functions of Plasma Membrane Protective barrier Regulate transport in & out of cell (selectively permeable) Allow cell recognition Provide anchoring sites for filaments of cytoskeleton Provide Provide a binding site for enzymes Interlocking surfaces bind cells together (junctions)

Homeostasis – Maintaining a Balance Cells must keep the proper concentration of nutrients and water and eliminate wastes. Cells must keep the proper concentration of nutrients and water and eliminate wastes. The plasma membrane is selectively permeable – it will allow some things to pass through, while blocking other things. The plasma membrane is selectively permeable – it will allow some things to pass through, while blocking other things.

Structure of the Plasma Membrane Lipid bilayer – two sheets of lipids (phospholipids). Lipid bilayer – two sheets of lipids (phospholipids). –Found around the cell, the nucleus, vacuoles, mitochondria, and chloroplasts. –Embedded with proteins and strengthened with cholesterol molecules.

What’s a Phospholipid? It’s a pair of fatty acid chains and a phosphate group attached to a glycerol backbone. It’s a pair of fatty acid chains and a phosphate group attached to a glycerol backbone. –Polar (water-soluble) heads face out and the nonpolar fatty acids hang inside.

Membrane Proteins 1. Determine what particles can pass through the membrane. 1. Determine what particles can pass through the membrane. 2. Serve as enzymes (may speed reactions). 2. Serve as enzymes (may speed reactions). 3. Act as markers that are recognized by chemicals and molecules from the inside and the outside of the cell (the immune system). 3. Act as markers that are recognized by chemicals and molecules from the inside and the outside of the cell (the immune system).

9 Simple Diffusion Molecules move from area of HIGH to LOW concentration Molecules move from area of HIGH to LOW concentration Requires NO energy to make the molecules move. Requires NO energy to make the molecules move. passive processpassive process

10 Diffusion of Liquids

11 Diffusion through a Membrane Cell membrane Solute moves DOWN concentration gradient (HIGH to LOW)

12 Osmosis Diffusion of water across a membrane Diffusion of water across a membrane Moves from HIGH water molecule concentration (low solute) to LOW water molecule concentration (high solute) Moves from HIGH water molecule concentration (low solute) to LOW water molecule concentration (high solute) Diffusion across a membrane Semipermeable membrane

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

14 Cell in Isotonic Solution CELL 10% NaCL 90% H 2 O 10% NaCL 90% H 2 O What is the direction of water movement? The cell is at _______________. equilibrium ENVIRONMENT NO NET MOVEMENT

15 Cell in Hypotonic Solution CELL 10% NaCL 90% H 2 O 40% NaCL 60% H 2 O What is the direction of water movement?

16 Cell in Hypertonic Solution CELL 15% NaCL 85% H 2 O 5% NaCL 95% H 2 O What is the direction of water movement? ENVIRONMENT

17 Cells in Solutions copyright cmassengale

18 Which cell is at homeostasis? Cytolysis Plasmolysis

19 Osmosis in Red Blood Cells Isotonic Hypotonic Hypertonic copyright cmassengale

What Happens to Blood Cells? copyright cmassengale20

Cellular Transport Diffusion – movement of particles from an area of high concentration to an area of low concentration. Diffusion – movement of particles from an area of high concentration to an area of low concentration. –Caused by Brownian motion (movement of particles because of the movement of their atoms). –Continues until an equilibrium is reached (no gradient). –Dynamic equilibrium – particles move freely and are evenly distributed.

Cellular Transport [1] Passive transport – no energy is needed to move particles. Passive transport – no energy is needed to move particles. –Facilitated diffusion – embedded proteins act as tunnels allowing particles to “fall” through.

Cellular Transport [2] Active transport – energy is needed to move particles. Active transport – energy is needed to move particles. –Carrier proteins – embedded proteins change shape to open and close passages across the membrane. –Endocytosis – taking something into the cell. –Exocytosis – expelling something from the cell.

Osmosis Diffusion of water across a selectively permeable membrane. Diffusion of water across a selectively permeable membrane. Occurs until water is balanced on both sides of the membrane. Occurs until water is balanced on both sides of the membrane.

Cell Concentrations Hypertonic solutions – more dissolved solute. Hypertonic solutions – more dissolved solute. Hypotonic solutions – less dissolved solute. Hypotonic solutions – less dissolved solute. Isotonic solutions – the same dissolved solute. Isotonic solutions – the same dissolved solute.

Overcoming Osmosis Contractile vacuoles – expel excess water from bacterial cells that live in water. Contractile vacuoles – expel excess water from bacterial cells that live in water. Turgor pressure – water pressure in a plant cell. Loss of turgor pressure causes wilting (plasmolysis). Turgor pressure – water pressure in a plant cell. Loss of turgor pressure causes wilting (plasmolysis).