Cell Membrane

and Transport

Mechanisms

Cell Membrane

• Fluid Mosaic Model - proteins embedded in a phospholipid bilayer.

AS Biology. Foundation. Cell membranes and Transport

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Fluid mosaic model

FLUID- because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid.

MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above.

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

• Diffusion -

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

• Click to: See osmosis in action

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

Passive Transport Mechanisms

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

Passive Transport Mechanisms

• Click to: see facilitated diffusion

Passive Transport Mechanisms

• Click below to:Go to the passive transport animation!!!

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

• Sodium-potassium pump -

1.

2. 3.

4.

Active Transport Mechanism

• Click to: Check out the animation of the Sodium – Potassium Pump

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

• Click to: see phagocytosis

Active Transport Mechanism

• Excocytosis - discharge of wastes, hormones or other larger molecules out of the cell (opposite of endocytosis)

Active Transport Mechanism

• Click to: see exocytosis

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.

Active Transport Mechanisms

• Click below to:Go to the active transport animation!!!