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Cell Membrane
• Phospholipid: 1 head and 2 tails– Polar head attract water - hydrophilic
– Non-polar tails repel water -hydrophobic
Cell Membrane
• Bilayer: 2 layers of phospholipids– The polar heads stay on the outside and the
tails stay on the inside.
Cell Membrane
• Cell surface proteins– a. Channel proteins - transport food and other
molecules into the cell and transport wastes out of the cells.
– b. Receptor proteins - gather information about the cell’s surroundings.
– c. Cell surface markers - identify the type of cell, important for cell recognition.
Cell Membrane
• Permeability of the cell membrane– 1. Semi permeable/selectively permeable -
only certain substances can pass across the membrane.
– 2. Factors that determine whether a molecule can pass through a membrane or not:
• a. size • b. type (polar, non-polar)
Transport Mechanisms
• Transport Mechanisms - moving material in and out of the cell
• Concentration gradient - the difference in the amount of a substance inside and outside of the cell– 1. Going “with the gradient”-moving from high to low
concentration– 2. Going “against the gradient”-moving from low to high
concentration– 3. Equilibrium exists when the concentration of molecules
is the same throughout a space (inside and outside the cell)
Transport Mechanisms
• Two categories of transport based on concentration gradient and the need for energy– 1. Passive transport that does not require
energy, goes with the gradient.• Example = diffusion and osmosis
– 2. Active transport requires energy, goes against the gradient.
Passive Transport Mechanisms
• Diffusion - movement from an area of high concentration to an area of low concentration– Example: smoke across a room, food coloring
dropped into water, oxygen in lungs
Passive Transport Mechanisms
• Osmosis - diffusion of water– Direction of osmosis-the direction of water
flow depends upon the concentration of solute and solvent (water)
– Tonicity-term used to compare 2 solutions (usually the inside and the outside of the cell)
• a. Hypotonic - less solute (more water)• b. Hypertonic - more solute (less water)• c. Isotonic - equal amounts of solute and water
Passive Transport Mechanisms
• Osmosis - diffusion of water– Water will flow from a hypotonic solution to a
hypertonic solution
– Will water move in an isotonic solution? Yes, but it won’t be noticeable-equilibrium is established
Passive Transport Mechanisms
• Osmosis - diffusion of water– Turgor pressure/osmotic pressure-pressure
due to water in cell.• Analogy: air pressure in a tire.
– When a cell has high turgor pressure, it is bigger and stiffer. When a cell has low turgor pressure, it is smaller and flimsy.
Passive Transport Mechanisms
• Plasmolysis - wilting of a cell due to loss of turgor pressure
• Cytolysis - bursting of a cell due to an increase in turgor pressure.– How do plant cells avoid cytolysis? Cell walls– How do unicellular freshwater organisms
avoid bursting? Contractile vacuoles.
Passive Transport Mechanisms
• Facilitated diffusion - transport of specific molecules across a membrane with the help of a channel protein– An example of a molecule that is often
transported in this manner is glucose
Active Transport Mechanism
• Active Transport Mechanism(requires energy in the form of ATP)
• Sodium-potassium pump - pumps sodium ions out and forces potassium ions in. Important for conducting nerve impulses and muscle contraction.
Active Transport Mechanism
• Proton pump-transport protons through membranes of chloroplasts and mitochondria
Active Transport Mechanism
• Endocytosis - process of bringing larger molecules into the cell by engulfing them.– 1. Pinocytosis – liquids– 2. Phagocytosis - solids (amoebas eat this
way, white blood cell engulf bacteria
Active Transport Mechanism
• Excocytosis - discharge of wastes, hormones or other larger molecules out of the cell (opposite of endocytosis)
How Cells Communicate
• Organ system involved in communication– 1. Endocrine system - releases hormones
(protein) for communication– 2. Nervous system - releases
neurotransmitters to other nerve cells or to muscle cells.
How Cells Communicate
• Receptors– 1. Receptors - are proteins that are or the cell
membrane surface - they send messages to the inside of the cell when they encounter certain molecules outside of the cell.
– 2. Chemically gated channels - a chemical trigger transmits information.
– 3. Voltage gated channel - an electrical signal transmits information along nerve cells.