Chapter 2: How Cells Function 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 2.1: Chemical reactions take place inside cells 2.2: Cells capture and release energy 2.3: Materials move across the cell’s membranes 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

Warm-up questions 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

Warm-up questions 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

Materials move across the cell’s membranes 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

Some materials move by diffusion 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 directionsconstant motion – 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

Diffusion: from higher to lower concentrations 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!

Diffusion in Cells 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

Passive Transport 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

Osmosis 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

tpages/biology/igcsebiology/igcsebiologysimul ations tpages/biology/igcsebiology/igcsebiologysimul ations Grow a plant

Some transport requires energy 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)

Active transport

Endocytosis – into the cell 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

Exocytosis – out of the cell 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

Endocytosis Exocytosis

Cell size affects transport Most cells are too small to be seen without a microscope – Average cell is ~50 micrometers (  m) Most cells on Earth are bacteria ~3 to 5  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…

Cell size – need high: “surface area to volume ratio” 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