Chapter 5 – The Plasma Membrane and Transport

State Standard Standard 1.a. – Cells are enclosed within semi-permeable membranes that regulate their interaction with their surroundings

Membrane Function Forms a boundary between living cells and their surroundings. Controls the movement of molecules into and out of the cell - Selective permeability – allows some substances to cross more easily than others and blocks passage of some substances altogether

Membrane Function 3. Takes up substances the cell needs and disposes of the cell’s wastes. 4. Membranes can contain enzymes that function in various chemical reactions.

Membrane Structure The plasma membrane is 8 nm thick. It is mainly made up of phospholipids and proteins.

Membrane Structure carbohydrate protein cholesterolc Phospholipid bilayer

Membrane phospholipids Phospholipids have a hydrophilic head and two hydrophilic tails.

Membrane phospholipids The phospholipids form a bilayer - The hydrophilic heads face outward and the hydrophobic tails face inward Hydrophobic tails

Membrane Proteins Membrane proteins are found within the phospholipid bilayer They can extend all the way through the bilayer – integral proteins They can be found embedded on either surface – peripheral proteins Phospholipid bilayer Integral Protein Peripheral protein

Functions of Membrane Proteins

Some proteins transport substances across the membrane by forming channels or by physically moving them across the membrane.

Membrane Proteins Functions of membrane proteins include: Transport of substances across the membrane Join cells to one another Act as enzymes Bind to chemical messengers from other cells Act as identification tags so cells can recognize one another Attach to cytoskeleton and the extracellular matrix

Membrane cholesterol and carbohydrates Cholesterol is found within the phospholipid bilayer It helps stabilize the phospholipids Carbohydrates Located on outer layer of membrane only Bound to proteins (glycoproteins) or phopholipids (glycolipids) Act as identification tags for cell cell recognition

Fluid Mosaic Model

Fluid Mosaic Model The membrane is a fluid mosaic Fluid – the phospholipids and the proteins drift within the membrane Mosaic – the diversity of proteins found within the membrane and the arrangement of these proteins in the membrane

Transport Across the Membrane Diffusion The tendency of particles of any kind to spread from regions of high concentration to regions of low concentration Requires no energy, results from the random motion of atoms and molecules.

Transport Across the Membrane Passive Transport Substances diffuse through the membrane without work by the cell They spread from areas of high concentration to areas of lower concentration At equilibrium the molecules continue to move but there is no net change in the concentration on either side of the membrane Molecule of dye Membrane EQUILIBRIUM

Transport Across the Membrane Osmosis is the passive transport of water Water travels from an area of lower solute concentration to an area of higher solute concentration

Transport Across the Membrane Hypertonic - the solution with the higher solute concentration Hypotonic - the solution with the lower solute concentration Water moves from the hypotonic solution to the hypertonic solution.

Transport Across the Membrane Water continues to cross the membrane until the solute concentrations are equal on both sides of the membrane Isotonic - solutions with equal solute concentrations Water molecules continue to move across the membrane but at the same rate in both directions

Osmosis Problems – Draw a diagram illustrating the problem, identify the solutions as hyper, hypo or isotonic, and show the direction osmosis will occur 1. A cell containing 2% solute is placed in a solution containing 10% solute. 2. A candy containing 15% solute is placed in a solution containing 12% solute. 3. A cell containing 7% solute is placed in a solution containing 9% solute. 4. A candy containing 10% water is placed in a solution containing 90% water. 5. A fish contains 95% water lives in freshwater that contains 68% water.

How Does Osmosis Affect Cells? Distilled Water 3x Saline Induces turgor in cells Induces turgor in animal cells Makes plant cell pull away from cell wall Makes plant cell lyse Makes animal cell lyse Makes animal cell shrivel

How Does Osmosis Affect Cells? Explain the different outcomes of adding distilled water to the plant and animal cells. Which conditions best model the effects of drought on plant cells? Explain your answer. Hypothesize why plants grow better when the water surrounding their cells is slightly hypotonic to the cell.

Water balance between cells and their surroundings is crucial to organisms Osmosis causes cells to shrink in a hypertonic solution and swell in a hypotonic solution.

Transport Across the Membrane Small non-polar molecules diffuse freely through the phospholipid bilayer Many other kinds of molecules diffuse through pores created by membrane proteins This type of transport is called facilitated diffusion Solute molecule Transport protein

Transport Across the Membrane Active Transport

Transport Across the Membrane Transport proteins can move solutes across the membrane against a concentration gradient. - This is called active transport Active transport requires ATP – cells use energy to move the molecule across the membrane. The solute is moved from low concentration to high concentration.

Transport Across the Membrane To move large molecules or particles across the membrane a vesicle may fuse with the membrane and expel its contents – exocytosis

Transport Across the Membrane Or the membrane may fold inward, trapping material from the outside, and form a vesicle that carries the material into the cell – endocytosis

Transport Across the Membrane There are 3 kinds of endocytosis Pseudopod of the amoeba Food being ingested Plasma membrane Material bound to receptor proteins

Transport Across the Membrane There are 3 kinds of endocytosis: Phagocytosis – a cell brings in macromolecules, other cells, or particles Pinocytosis – the cell takes in fluid and dissolved solutes Receptor-mediated endocytosis – specific molecules bind to receptors on the membrane that signal to the cell to take the molecule in

Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors