CHAPTER 8 SELECTIVITY C4:- The Intracellular environment of cells differs in composition from the extracellular environment of cells

Also includes compounds-atoms of two elements combined. C4.1 Know that the proportions of chemicals in the intracellular environment of cells are different from those in the extracellular environment of cells Matter is any substance that makes up living and non living things-solid, liquid or gas. Elements are the units of matter that cannot be converted into simpler substances-made of one type of atom In nature matter is arranged in molecules-one or two types of atoms i.e. water Also includes compounds-atoms of two elements combined.

Organisms- Made of mostly water Most common atoms in the body-hydrogen and oxygen Carbon and hydrogen and oxygen-very important as account for most of the living matter on Earth Carbon important as forms strong covalent bonds by sharing electrons with other atoms-especially each other and to hydrogen and oxygen. Remember these form the major groups of macromolecules-lipids, proteins, carbs, nucleic acids.

Organic Molecules Remember! Organisms built from 35 smaller units-an arrangement of 20 amino acids, lipids, phospholipids, sugars, bases and phosphate groups that make up nucleic acids. Living organisms-living cells-large amount of chemicals continually reacting with each other There are differences between the proportions of the elements on earth and in your body

MOLECULES- manufactured by cells are called organic See pg 71 table These molecules constantly involved in reactions Making new molecules –transforming energy To proceed efficiently must control amount of chemicals used I.E respiration needs just the right amount of glucose and oxygen-and waste products must be removed to maintain the correct intracellular environment. The environment is maintained by the cellular membrane-outside the cell varies greater than inside the cell

C4.2 Understand why the internal composition of the cell is regulated For cellular metabolic processes to function correctly the internal environment must remain stable Enzymes in the cell convert one or more substrates to other substances constantly Remember it’s the 3 dimensional shape of proteins/enzymes that assist in this reaction control-and the factors which influence the shape of these molecules-temp/pH/Concentration etc

All internal conditions must be regulated for reactions to proceed efficiently Efficiency depends on the relative concentrations of substrates and products i.e if a cell runs out of oxygen- respiration slows down This may then mean conditions are suitable for FERMENTATION- Similarly if waste products like Urea or CO2 accumulate the reaction slows down It is the cell membrane which controls the ins and outs in order to ensure all reactions proceed correctly and efficiently- ensures enzymes can operate effectively under the right conditions

C4.3 Explain how selectivity exchange occurs at the cell membrane Selective exchange is when the membrane restricts some substances and freely allows others in or out= differential permeability -Or SEMI PERMEABLE Some molecules pass by diffusion- some require energy = active transport-e.g mineral ions like potassium and sodium- charged molecules have more difficulty passing through Some facilitated diffusion-no energy proteins in the cell membrane – allow the passage- sometimes by having receptors that recognise the molecule , cause structural change in the protein membranes that facilitates its passage across. Eg, insulin see pg 73

C5 Movement of soluble materials across membranes may be passive or require the expenditure of energy C5.1 Understand that the movement of substances by diffusion and osmosis is passive Unless molecules at -273 degrees Celsius molecules are in constant motion Kinetic energy of these particles is also heat energy The amount depends on the temp of the substance and its mass Gas and liquid- freer particles- more collisions- occurs until the particles fill a space evenly-passive- no energy required

Diffusion Movement of particles from a region of high concentration to low concentration with the concentration gradient The greater the difference in concentration-steeper the gradient-faster the diffusion process Small molecules diffuse quicker than large Water, oxygen and carbon dioxide diffuse easily No energy required

Diffusion Simple diagram-copy if you must!

Osmosis Diffusion of water- movement of water molecules form a region of high concentration to low concentration or defined as movement of water from a region of low solute concentration to high solute concentration to reach equilibrium. Osmosis has a more dramatic effect on animal cells than plant cells due to animal cells having NO cell wall and therefore can burst or shrivel easily!

OSMOSIS

C5.2 Understand that the active transport of substances against the concentration gradient requires energy. This energy is supplied by ATP. Active transport is movement of solutes against the gradient from high to low Transport proteins in the cell membrane actively pump specific substances across the membrane moving them against the gradient with energy supplied Energy is supplied by ATP-Adenosine Tri-Phosphate Nerve cells provide a good example:- Transmission of nerve signals requires the exchange of sodium and potassium ions across the membranes by diffusion but they become fatigued

They need to be re-charged by pumping them against the gradient back to the other side of the membrane This is why you need regular study breaks- to pump the ions back!