The Plasma Membrane and Homeostasis

Homeostasis – Maintaining a Balance 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. This is also known as being partially permeable or semi-permeable.

Structure of the Plasma Membrane Double phospholipid (phosphates and lipids - fats). Double phospholipid membranes are found around the cell (plasma membrane), the nucleus, vacuoles, mitochondria, and chloroplasts. Embedded with proteins and strengthened with cholesterol molecules. This model of what the plasma membrane looks like is called the FLUID MOSAIC MODEL

What’s a Phospholipid? 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 Determine what particles can pass through the membrane. Serve as enzymes (may speed reactions). Act as markers that are recognized by chemicals and molecules from the inside and the outside of the cell (the immune system).

Diagram representing the cell membrane. The above image is from http://www.biosci.uga.edu/almanac/bio_103/notes/may_15.html. But I found it at An On-Line Biology Book: http://gened.emc.maricopa.edu/bio/BIO181/BIOBK/BioBookCELL2.html

Movement in and out of the Cell (Cellular Transport) Passive transport – no energy is needed to move particles across the plasma membrane. There are three main types: Simple diffusion Facilitated diffusion Osmosis

Passive Transport Simple Diffusion – movement of particles from an area of high concentration to an area of low concentration. Continues until an equilibrium is reached (no gradient). Dynamic equilibrium – particles move freely and are evenly distributed. Substances that dissolve in lipids (fats) will cross the plasma membrane easily as they are lipophilic (hydrophobic) e.g. alcohol and ether

Passive Transport cont... Facilitated Diffusion – movement of particles is facilitated or assisted by carrier protein molecules in the plasma membrane. The net direction of flow is also from high concentration to low concentration and therefore, requires no energy. These are substances that do not readily dissolve in the phospholipid layer, e.g. glucose

Passive Transport cont... c. Osmosis– Diffusion of water across a selectively permeable membrane. Occurs until water is balanced on both sides of the membrane.

Cell Concentrations These refer to the comparison between the inside and outside of the cell Hypertonic solutions – more dissolved solute. Hypotonic solutions – less dissolved solute. Isotonic solutions – the same dissolved solute.

(high solute, low water) (low solute, high water) Hypertonic Solution (high solute, low water) Hypotonic Solution (low solute, high water)

Overcoming Osmosis 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).

Cellular Transport Active transport – energy is needed to move particles. It needs energy as it refers to the movement of substances AGAINST a concentration gradient The energy is in the form of ATP – adenosine triphosphate. It requires a carrier protein to transfer the material across the plasma membrane.

Active Transport

Bulk Transport Larger particles entering the cell – formation of vesicles around the material Endocytosis – material moving INTO a cell Phagocytosis – solids entering the cell Pinocytosis – liquid entering the cell Exocytosis – material moving OUT of the cell