Cells : The Living Units

Chapter 3

Cell Physiology

• Metabolism ( build to cell material, breakdown substances, make ATP)

• Digest foods• Dispose of wastes• Grow• Move• Irritability• Reproduce

• Flexible, transparent and separates the cell contents from the surrounding environment: intracellular and extracellular fluids

• Fluid mosaic model:– Two lipid layers arranged tail to tail– Mainly phospholipids but also contains cholesterol

give fluidity

Plasma Membrane

• The polar heads are hydrophilic:interact with water and other polar molecules

• Nonpolar tails are hydrophobic: makes the plasma membrane relatively impermeable to most water soluble molecules

• Proteins: Responsible for the specialized functions ( enzymes, receptors, transporters, carriers)

Plasma Membrane Components

Cell Membrane

Membrane proteins

• Integral and peripheral membrane proteins

Membrane junctions: Desmososome

• Desmosomes: Mechanical couplings scattered like rivets along the sides of abutting cells to prevent separation. Anchoring function

• Cytoplasmic surface of each plasma membrane is a plaque ( button)

Membrane junctions: Gap junctions

• Gap junctions, or nexus: Seen in heart and between embryonic cells: function mainly to allow communication for chemical molecules.

• Neighboring cells are connected by connexons: hollow cylinders composed of transmembrane proteins

Membrane junctions: Tight Junctions

• Impermeable• bind cells together, integral

proteins of adjacent cells fuse together

The Plasma Membrane

• Selectively permeable: allows some substances to pass through and keeps other substances out

• Transport across the membrane– Passive Transport– Active Transport

Passive Transport Active Transport

• Substances are transported without the use of energy

• The cell provides ATP that drives the transport process

Diffusion and Filtration

• Diffusion: Process whereby molecules scatter

themselves throughout the available space by means of their own kinetic energy. When molecules move from an area of greater concentration to an are of lesser concentration they move down the concentration gradient.

Simple Facilitated

• What? – Lipid soluble ( fats, fat-soluble

vitamins, O2,CO2) or small enough to pass through the pores

– Osmosis- Water is highly polar and passes through the membrane pores (Aquaporins), as well as through the bilayer by moving from gap to gap

• What?– Substances that are lipid

insoluble and too large to pass through the pores

• How? – Protein carrier is used,

(permease) transmembrane integral protein

– Channels: proteins allow ions or water through aqueous channels

Osmosis• Osmosis is diffusion of water from am area of higher

concentration to a lower concentration across plasma membranes

• Osmolarity: the total of all solute particles in a solution

• When equal volumes of aqueous solutions of different osmolarity are separated by a membrane that is permeable to all molecules in a system, then net diffusion of both water and solute occurs until equilibrium is reached

Osmosis

• Animal cells lack cell walls• Osmotic imbalance will cause these cells to swell or

shrink• Tonicity: the ability of a solution to change the

shape or tone of a cell by altering their internal water volume

• Isotonic: 0.9% saline or 5% glucose, same concentrations of nonpenetrating solutes as those found in the cells. Cells exposed to this solution retain their normal shape

Osmosis• Extracellular fluids and are

isotonic• Hypertonic: Solutions with

higher concentrations of nonpenetrating solutes than as seen in cells. Causes the cells to shrink or crenate when immersed in this solution

• Hypotonic: Solutions that are more dilute. Causes the cells to swell and finally lyse or burst

Filtration• In the body filtration occurs because blood exerts

hydrostatic pressure• Pressure gradient: Pushes the filtrate from high-

pressure areas to low pressure areas• Kidneys• Only blood cells and large proteins are held back- not

very selective process

Vesicular Transport

• Exocytosis

• Endocytosis

Move substances out of the cell. Secretion of hormones, mucus and other cell products as well as the ejection of wastes

Includes all those ATP-requiring processes that take up or engulf the extra cellular substances. Includes phagocytosis and pinocytosis( cell drinking) which is a routine activity of most cells. (cells lining the walls of the small intestine and kidney tubules)

Phagocytosis & Pinocytosis

Active Transport

• Cells use its own supply of ATP to move substances across the membrane

• Solute pumping( active transport): requires protein carriers like facilitated diffusion but here the cell uses ATP to energize its solute pumps (proteins)

• Cytosol• Organelles• Inclusion– Dissolved– Insoluble

Cell Cytoplasm

Nonmembranous Organelles

• Ribosomes– Free– Fixed– Protein synthesis

• Vaults

Nonmenbranous OrganellesRibosomes are nonmembranous organelles

composed of RNA and protein

Cytoskeleton

• Strength• Support• Shape• Transport• Cell to cell links• Protein fibers– Microfilaments– Intermediate – Microtubules

Cytoskeleton