Homeostasis and the Plasma Membrane The Plasma Membrane

Homeostasis and the Plasma Membrane

The Plasma Membrane n Cells maintain a balance by controlling materials that enter and leave the cell n Concentrations of H 2 O, glucose, and nutrients must be kept up while also eliminating wastes n Plasma/Membrane is a lipid bilayer made of phospholipids (2 fat layers)

Basic Cell Structures n Cell Membrane: cells of all organisms are surrounded by a cell membrane – Thin layer of lipid and protein that separates the cell’s contents from its environment Thickness: 5 -10 nm, (a stack of 10, 000 membranes is about equal to a page in the text book) n Controls what enters and exits the cell (the gatekeeper) n Made of a phospholipid bilayer. A phospholipid is a type of lipid made of glycerol, two fatty acids, and a phosphate group. n

Phospholipid Bilayer Phospholipid molecule has a hydrophillic (water loving) head. It dissolves easily in water. The phospholipid tail is hydrophobic (water-fearing). The lipid tails do not dissolve in water. Water Hydrophillic phosphate head Hydrophobic lipid tail Water

Plasma Membrane Cont. Fluid Mosaic Model Free to move sideways

Membrane Proteins and Their Functions A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer n Proteins determine most of the membrane’s specific functions n

Peripheral proteins are bound to the surface of the membrane n Integral proteins penetrate the hydrophobic core n Integral proteins that span the membrane are called transmembrane proteins n The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices n

n Six major functions of membrane proteins – – – Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)

The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane n Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) n Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual •

Membrane structure results in selective permeability Nonpolar molecules- such as hydrocarbons, CO 2, and O 2 - are hydrophobic and can dissolve in bilayer and cross easily n Hydrophobic core of membrane prevents passage of ions and polar molecules, which are hydrophilic. However, they can use transport proteins n Water moves through special proteins (aquaporins), accelerate speed (3 billion H 2 O per protein per second n

Cellular Transport How does stuff get in and out of cells? n The cell membrane is selectively permeable n Types of cellular transport or needed materials enter and wastes leave n Active Transport- energy must be used to move substances from low to high concentration (up the hill) ∕ High ∕ low ∕ n

Cellular Transport Cont n Passive Transport- Does not require energy to move substances across the cell membrane (down the hill) ∕ high ∕ ∕ low

Types of Passive n Diffusion- movement from high to low concentration – Ex. Sugar cube in H 2 O / Perfume n Facilitated Diffusion- The proteins provide a passage (facilitate means to help- helps diffusion) n Osmosis- Movement of WATER ONLY from high to low concentration

Passive Transport

Passive Transport

Osmosis

Osmosis Cont. n 3 Ways n Isotonic- water concentration equal in and out of the cell (cell stays the same) n Hypotonic- water concentration higher outside cell than inside (H 2 O moves in cell) –Swello n Hypertonic- water concentration higher inside cell than outside (H 2 O moves out of cell) -Shrinker

Tonicity

Transport Proteins and Facilitated Diffusion n Many substances that are necessary for viability of the cell do not freely diffuse across the membrane (ions and polar molecules) – They require the help of specific transport proteins – These proteins assist in facilitated diffusion, a type of passive transport that does not require energy

Passive Transport Cont. n Some proteins function by becoming a hydrophilic tunnel for passage – Other proteins bind their passenger, change shape, and release their passenger on the other side – In both of these situations, the protein is specific for the substrate, which can be sugars, amino acids, ions, and even water

Active Transport n Cells have a mechanism for moving a solute against its concentration gradient – It requires the expenditure of energy in the form of ATP – The mechanism alters the shape of the membrane protein through phosphorylation using ATP

Transport protein Protein changes shape Solute 1 Solute binding 2 Phosphorylation 3 Transport Phosphate detaches 4 Protein reversion

Sodium-Potassium Pump

How Ion Pumps Maintain Membrane Potential Membrane potential is the voltage difference across a membrane n Voltage is created by differences in the distribution of positive and negative ions across a membrane n

n Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane – A chemical force (the ion’s concentration gradient) – An electrical force (the effect of the membrane potential on the ion’s movement)

• • n n An electrogenic pump is a transport protein that generates voltage across a membrane The sodium-potassium pump is the major electrogenic pump of animal cells The main electrogenic pump of plants, fungi, and bacteria is a proton pump Electrogenic pumps help store energy that can be used for cellular work

ATP Proton pump H CYTOPLASM EXTRACELLULAR FLUID H H H H H

Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when active transport of a solute indirectly drives transport of other solutes n Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell n

Cellular Transport Cont. How cells get large molecules or whole cells across the membrane n 1. Endocytosis –take into cell § Pinocytosis- taking in liquid § Phagocytosis- taking in solids § Receptor-mediated endocytosis- binding of ligands to receptors triggers vesicle formation § ligand is any molecule that binds specifically to a receptor site of another molecule n n 2. Exocytosis- release from cell (the reverse) – Most single celled organisms use the above methods

Passive Vs. Active Passive No energy required n Active Energy req. as ATP!energy molecule (fuel) Use transport Proteins (facilitated diffusion) moves nutrients from low to high Moves with gradient moves against gradient