cells and movements across membranes

• cytoplasm - where most of the chemical reactions that make up life go on
• cell membrane - which controls what enters and leaves the cell
• nucleus - which contains DNA, the chemical that controls the cell's activities
• mitochondria - which are the structures that carry out aerobic respiration, supplying cells with energy
• cell wall - made of cellulose and supports the cell and provides strength and rigidity
• permanent central vacuole - contains cell sap, keeps the cell rigid and prevents it from bursting
• chloroplasts - which absorb light for photosynthesis - chloroplasts are found in the palisade layer
• microscope diagram -

  A) eyepiece lens

  B) fine focus

  C) coarse focus

  D) objective lens

  E) specimen

  F) stage

  G) iris diaphragm and condenser

  H) lamp

  
• the eyepiece lens - which has a fixed magnification
• the objective lenses - are of different magnifying powers are interchangeable, to adjust the magnification of the image that you see down the microscope
• the stage - is where the microscope slide is placed
• below the stage of a microscope is a part that is usually made up of two components - an iris diaphragm, which can be opened or closed to adjust the amount of light entering the objective lens, and (sometimes) a condenser, which concentrates the light into a beam directed precisely into the objective lens
• at the base of the microscope is a lamp, or possibly a mirror, which is used to shine light through the condenser and iris diaphragm
• The microscope is focused using two focus controls. The coarse focus control is used to get the image roughly into focus using the lowest-power objective, and then the fine focus control is used to fine tune the image and make it as clear as possible.
• Microscope slides hold thin specimens or sections, which may be stained using a variety of dyes so that structures can be seen more clearly
• Newly formed cells undergo a process of differentiation into specialised cells, which have features which are modified to suit their function.
• sperm cell - the cell has very little cytoplasm and a tail, to help it swim fast towards the egg
• red blood cells - the cells have lost their nuclei and have become packed with a red pigment, haemoglobin, which carries oxygen around the body
• xylem cells - the xylem cells form tubes which carry water up a plant, and also strengthen it. To do this, the cells have perforated end walls, the cell wall is very thick, and the cytoplasm has died off to leave a hollow tubet
• tissue --> organ --> organ system --> organism -->
• tissue - a group of similar cells with similar functions
• organ - a collection of two or more tissues that perform specific functions
• organ system - a collection of several organs that work together
• organism - a whole animal or plant
• The cell membrane is selectively permeable, and will allow some substances to pass through but not other. The movement through the membrane is by one of three processes - diffusion, osmosis or active transport
• diffusion - the net movement of particles form an area of high concentration to an area of low concentration, as a result of random movement
• In diffusion we say particles move down a concentration gradient
• diffusion is a passive process which means it does not require energy
• The speed of diffusion is affected by 2 factors -

  1. temperature - increasing the temperature will speed up diffusion, because it makes the particles move faster.
  2. The size of the concentration gradient. The bigger the concentration gradient, the faster diffusion will be.
• the cell membrane has two main functions; to control what enters and leaves the cell and to maintain homeostasis within the cell
• Osmosis is a specific type of diffusion. It is the diffusion of water molecules through a selectively permeable membrane. From a region of high water concentration to an region of low water concentration.
• If an animal cell is put into a solution that is more dilute than its cytoplasm, water will go in by osmosis and the cell will burst. Plant cells do not burst in dilute solutions because their cell wall prevents it.
• Active Transport


  When cells need to move substances against a concentration gradient (from an area of lower concentration to an area of higher concentration), this will not happen by diffusion, and the cell has to use energy to 'pump' the particles in the direction they need to go. This type of transport requires energy, and is called active transport.
• Enzymes


  • Different enzymes are made up of different sequences of amino acids
  • The bonds formed between the amino acids means that each type of enzyme has a specific molecular shape
• Enzymes


  Catalysts that speed up chemical reactions, without taking part in them
• Enzymes


  • Specific - they will only catalyse one reaction or one type of reaction
• The rate of an enzyme-controlled reaction

  Is affected by temperature, pH and the concentrations of the enzyme and the substrate
• High temperatures and extreme pH can alter the enzyme's structure so that it becomes denatured and can no longer work
• lock and key theory

  to catalyse a reaction, the enzyme molecule must collide with, and 'lock together' with its substrate. The place where the substrate fits into the enzyme is called the active site. The shapes of the enzyme's active site and of the substrate must match, so that they fit together like a lock and key. That is why enzymes are specific - they can only work with substances that fit into their active site.
• enzyme-substrate complex


  the structure formed when an enzyme and its substrate are fitted together