Transport of Materials Across Cell Boundaries Part I
Cell Boundaries • All cells are in a liquid environment • Cytoplasm is a solution of many substances • Cell Boundaries help maintain constant stable environment inside cell = homeostasis • Must regulate substances moving into & out of cell
• Cell Membrane: Thin flexible barrier that surrounds all cells (phospholipid bi-layer) – Regulates what enters and leaves the cell – Provides protection and support – Hydrophyllic = water-loving – Hydrophobic = water-fearing
Cell Membrane • Cell Membranes also contain other embedded molecules with specific jobs • Protein channels for transport (doorways) • Carbohydrate receptors (security guard)
• Cell Wall: Strong supporting layer around the cell membrane in plant cells and prokaryotes – Provides support and protection for the cell – Porous enough to allow some substances to pass through
• Solution: Mixture of 2 or more substances – Solute: Dissolved substances • Example = Kool-Aid – Solvent: Dissolving substance • Example = Water
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• Dissolved particles (solute) move around in space to reach equilibrium – Diffusion: Movement of particles from an area of high concentration to an area of low concentration – Equilibrium: Concentration of the solute is the same throughout the system
Concentration Gradient Low Concentration High Concentration
Diffusion • Depends on random particle movement down a concentration gradient (cg) – CG = Measurement of concentration change over a distance • Requires no energy = passive transport • Small particles = able to pass through cell membrane
Permeability • Permeable: A substance can pass across it • Impermeable: A substance cannot pass across it • Most cell membranes are semi or selectively permeable – Some substances can pass through
• Cells regulate water exchange to maintain homeostasis – Osmosis: The diffusion of water through a selectively permeable membrane – Water will move across the membrane until equilibrium is reached = isotonic
• Isotonic: Equal concentration – Same strength – Water moves equally – Cell stays the same
Hypertonic: Higher concentration of solute outside the cell – “Above Strength” – Water moves out of the cell – Cell shrinks
Hypotonic: Lower concentration of solute outside the cell – “Below strength” – Water moves into the cell – Cell expands
Osmotic Pressure • Pressure difference across a membrane caused by different concentrations of particles in and out of the cell
Facilitated Diffusion • Molecules that are too large to cross the cell membrane must use special “doorways” • Protein Channels – designed to allow specific molecules to diffuse across the membrane easily • Example - Glucose
Transport of Materials Across Cell Boundaries Part II
• What if a cell needs to move particles against a concentration gradient? – From low to high concentration – Allows cells to concentrate molecules in a particular location regardless of concentration gradient
– Requires energy = Active Transport • Examples – Transport proteins – Endocytosis – Exocytosis
• Transport Protein: Transmembrane protein that helps a certain class of substances cross the membrane – Think of them like doors
• Endocytosis: Materials are moved into the cell via folds in the cell membrane – Endo = Within – Two examples • Phagocytosis: Extensions of cytoplasm surround a particle and package it within a food vacuole • Pinocytosis: Tiny pockets form along the cell membrane, fill with liquid, and pinch off to form vacuoles within the cell
• Phagocyte: “Eating Cell” – Important white blood cell – Consumes and destroys cellular debris & pathogens (germ)
Endocytosis
• Exocytosis: Materials are moved out of the cell via folds in the cell membrane – Exo = Outside
Cell Differentiation & Life Organization Ch. 7 Section 4 In Textbook
Cell Differentiation In unicellular organisms the single cell will develop the structures necessary to live its life. Some may grow appendages for movement: 1) Cilia – tiny hairs on cell surface that move to allow mobility of the cell 2) Flagella – whip-like appendage used for movement
What about multi-cellular eukaryotes? Some cells develop differently than others for specific jobs: 1) Muscle Fiber Cells: Highly developed cytoskeleton for movement (remember actin? ) 2) Pancreatic Cells: Need to produce enzymes for digestion so the Rough ER and Ribosomes are more highly developed 3) Red Blood Cells: No nucleus, oxygen transport
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