Have already learned:
-all cells have an outer covering called the cell membrane
-cells need starting materials for life sustaining processes
-cells need to get rid of waste products
Will learn:
-how materials move into and out of the cell through the cell membrane
-how energy is involved in transporting some materials into and out of cells
-how surface area affects transport in cells

• Match the number with the letter…
• Definitions:
1. proteins, lipids, and carbohydrates
2. lipid layers that repel water
3. source of chemical energy
• Terms: a. cell membrane b. glucose
C. large molecules
D. small molecules
E. water

• Match the number with the letter…
• Definitions:
1. proteins, lipids, and carbohydrates C
2. lipid layers that repel water A
3. source of chemical energy B
• Terms: a. cell membrane b. glucose
C. large molecules
D. small molecules
E. water

• Beaker of tap water
• 3 drops of food coloring
• Observe what happens
• The food coloring gradually colors all the water in the beaker
• Food coloring diffused through the beaker
• Diffusion: process by which molecules spread out, or move from areas where there are many of them to areas where there are fewer of them

• Smell a fragrance from across a room:
– The scent has traveled through the air via diffusion
– Other examples?
• Molecules in gases, liquids, and even solids, are in constant motion in all directions
– Random movement of molecules spreads them out until they are evenly distributed
• Cells use diffusion to help them maintain conditions necessary for life
– Oxygen (for respiration) enters the cell, and carbon dioxide leaves the cell, via diffusion

• Particles move from an area of higher concentration to an area of lower concentration
– Concentration = # of particles of that substance in a specific volume: 9 g/L of sugar
• Generally the greater the concentration difference, the faster diffusion occurs
•  occurs until the concentration of the given substance is balanced throughout the whole
• Example: a 9 g sugar cube is dropped in 1 L of water
– The sugar slowly dissolves until the concentration of sugar in water is the same throughout…but it is still 9 g/L in total!

• Small molecules, such as oxygen, can pass through tiny gaps in the cell membrane by diffusion
– Example:
• photosynthesis produces oxygen inside the cell
• Oxygen in cell > oxygen outside cell
• Diffusion occurs to balance oxygen inside = oxygen outside
– In plants, some of this oxygen is used for cellular respiration, some to air, and some to other cells with lower oxygen concentrations

• Diffusion is one form – it doesn’t use the cell’s energy (therefore is “passive”)
• A particle’s ability to diffuse across the cell membrane depends on:
– how well the substance dissolves in the lipids that make up the cell membrane
• Special form of passive transport:
– Allows some polar substances, such as glucose, salts, and amino acids to pass
• Glucose is produced in just some plants cells
• Polar substances move into other cells through “protein channels” (openings) that are specific for each substance

• Special case of diffusion: diffusion of WATER through a membrane
– If the concentration of water outside the cell is greater than inside, water moves into the cell
• Example:
– A wilted plant: the soil is dried out so the plant’s cells lose water through osmosis and shrink
– Water the soil and the water returns to the plant cells through osmosis

• http://www.cambridgestudents.org.uk/subjec tpages/biology/igcsebiology/igcsebiologysimul ations
• Grow a plant

• Some materials must move from an area with
lower concentrations to higher concentrations
– Requires energy
• Active transport: process of using energy to move materials through a membrane
– (diffusion and osmosis do NOT require energy)

• Such as: Removal of excess salt from the body
– Example:
• Marine iguanas swim and feed in salty ocean and soak up a lot of salt – must get rid of the excess salt
» Cells in glands above its eyes use active transport and chemical energy to remove salts from the blood
» The gland forms a droplet of salt, the iguana easily blows it out through its nostrils

• Some materials are too large to go through the cell membrane or protein channel
• A large bit of material is captured in a pocket of the membrane
– it breaks off, forming a package that moves into the cell
– Ex: cells can fight bacteria and virus by absorbing them

• Membrane within the cell encloses the material that needs to be removed
– The package moves to the cell membrane, joins with it, the material is expelled
• Ex: flush out waste materials or to expel proteins or hormones made by the cell

• Most cells are too small to be seen without a microscope
– Average cell is ~50 micrometers ( m m)
• Most cells on Earth are bacteria ~3 to 5 m m
• Small size is necessary…
– All the cell needs or gets rid of goes through the cell membrane
– The amount (surface area) of cell membrane limits the ability of the cell to get what it needs in or transport waste out
– The ratio between surface area and volume controls cell size: as a cell gets larger, its volume increase faster than its surface area if the cell is maintaining the same shape…

• As the cell gets bigger, eventually the surface area is not large enough to allow resources to travel to all parts of the cell so the cell stops growing
• Bird eggs and frog eggs get around this restriction: the cells divide
– Surface area to volume ratio stays high enough that materials can still effectively be exchanged within the cell and cell membrane
• Cell shape is also important – longer thin and flat cells have an increased surface area