Cell Membrane & Cellular Transport

Homeostasis – the maintenance of internal stable conditions The cell membrane keeps the cell in balance.

Membrane Functions Provides a selectively permeable barrier around the cell. Controls the passage of substances in and out of the cell. Maintains homeostasis for the cell.

Membrane Structure Phospholipid bilayer (nonpolar) Membrane proteins: act as transport channels. Carbohydrate chains: act as “ID” tags for the cell. Cholesterol: provides stability for the membrane. http://library.thinkquest.org/C004535/media/cell_membrane.gif

Phospholipid Bilayer Phospholipid Bilayer Phospholipids: phosphate head and lipid tail. Phospholipid Bilayer: two layers. Hydrophilic = attracts water inside cell outside cell Phosphate Head Lipid Tails Phospholipid Bilayer Hydrophobic = repels water

So what needs to get across the membrane? The cell membrane is semipermeable, which means only some material can get in or out. So what needs to get across the membrane? Sugar Lipids Amino Acids O2 H2O Salt Waste

How do you build a semi-permeable cell membrane? Channels are made of proteins. Proteins act as doors in the membrane. Bilipid Membrane Protein Channels in Bilipid Membrane

Diffusion Molecules move from HIGH to LOW concentration; this continues until equilibrium is reached. Simple diffusion: directly through membrane. Facilitated diffusion: help through a protein channel. NO energy needed!! HIGH LOW

Active Transport Cells use energy to move against concentration gradient - from LOW to HIGH. Particles that are polar (charged) must move through a protein channel. Use protein pump requires energy (ATP) Plants have nitrate & phosphate pumps in their roots. Why? Nitrate for amino acids Phosphate for DNA & membranes Not coincidentally these are the main constituents of fertilizer. ATP

Transport of Molecules Endocytosis: takes material into cell by cell membrane making pockets. Phagocytosis – “cell eating”; cell engulfs particles Pinocytosis – cell takes in liquid from surrounding environment. Exocytosis: process forcing contents out of the cell.

Endocytosis PHAGOCYTOSIS PINOCYTOSIS

Exocytosis

Transport Summary simple diffusion facilitated diffusion (Smaller molecules) facilitated diffusion ATP active transport (Larger molecules)

Types of Cellular Transport PASSIVE Does NOT require energy Goes with the concentration gradient (high to low) Simple Diffusion, Facilitated Diffusion ACTIVE Requires energy from ATP Goes against the concentration gradient (low to high) Active Transport, Endocytosis, Exocytosis

Osmosis movement of water across cell membrane. HIGH H2O LOW H2O

Effects of Osmosis on Cells (osmotic pressure) Hypertonic: higher concentrations of solute; less H2O. Isotonic: equal concentrations of solute. Hypotonic: lower concentrations of solute; more H2O.

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