What are cells? 1 Cells and movements across membranes Topic overview Living organisms are made up of building blocks called cells. Cells are usually too small to see with the naked eye. They are microscopic. Cells are formed from pre-existing cells during cell division. The chemical reactions that create life occur within cells. Hereditary information (deoxyribonucleic acid, DNA) is passed on from cell to cell when cell division occurs. © Hodder & Stoughton 2017

1 Cells and movements across membranes Topic overview Plant and animal cells All cells have a cytoplasm, where most of life’s chemical reactions go on. The cytoplasm is surrounded by a cell membrane, which controls what enters and leaves the cell. The nucleus contains DNA and controls the cell’s activities. Mitochondria carry out respiration to provide energy. Plant cells have a cellulose cell wall for support. Plant cells have a central vacuole, filled with cell sap. Some plant cells contain chloroplasts, which absorb light for photosynthesis © Hodder & Stoughton 2017

Plant and animal cells (continued) 1 Cells and movements across membranes Topic overview Plant and animal cells (continued) © Hodder & Stoughton 2017

Observing cells with a microscope 1 Cells and movements across membranes Topic overview Observing cells with a microscope The eyepiece lens is of fixed magnification The objective lenses have different magnifications and are interchangeable. The slide is placed on the stage. The iris diaphragm and condenser control and concentrate the light. The coarse and fine focus controls get the image into focus. © Hodder & Stoughton 2017

Observing cells with a microscope (continued) 1 Cells and movements across membranes Topic overview Observing cells with a microscope (continued) There are certain ‘rules’ about producing scientific diagrams. Always use a sharp pencil. Make sure lines are thin, clear and do not overlap. Do not shade your drawing. Always use a rule for labelling lines. The lines should never cross each other. Keep the labels outside the drawing. Make sure the proportions of the drawing are correct. © Hodder & Stoughton 2017

Observing cells with a microscope (continued) 1 Cells and movements across membranes Topic overview Observing cells with a microscope (continued) Cells often become specialised for a particular function. These specialisations develop as a cell matures. The process is called differentiation. Examples of specialised cells are shown below. © Hodder & Stoughton 2017

How are cells organised into a whole body? 1 Cells and movements across membranes Topic overview How are cells organised into a whole body? Cells are organised into tissues. Tissues are a group of similar cells with similar functions. Organs are a collection of tissues that perform a specific function. Organ systems are a collection of several organs which work together. An organism is a whole living thing. © Hodder & Stoughton 2017

Movement into and out of cells 1 Cells and movements across membranes Topic overview Movement into and out of cells Substances move into or out of cells by one of three methods: diffusion, osmosis or active transport. Diffusion is a passive process where particles move from a high concentration to a low concentration. Osmosis is the diffusion of water through a selectively permeable membrane. Active transport requires energy, and moves substances from an area of lower concentration to an area of higher concentration, i.e. against a concentration gradient. © Hodder & Stoughton 2017

Movement into and out of cells (continued) 1 Cells and movements across membranes Topic overview Movement into and out of cells (continued) Osmosis moves water from a dilute solution (where there is more water) to a more concentrated solution (where there is less water). The water moves through a selectively permeable membrane. Osmosis is net movement of water, i.e. it is not one- directional. If plant cells lose a lot of water, their vacuoles and cytoplasm shrink, and they become plasmolysed. © Hodder & Stoughton 2017

How are the activities of cells controlled? 1 Cells and movements across membranes Topic overview How are the activities of cells controlled? All the chemical reactions in cells are controlled by enzymes. Enzymes are biological catalysts, which speed up reactions. Enzymes are specific – they will only catalyse one type of reaction. The substance an enzyme works on is called its substrate. The enzyme and substrate molecules fit together, like a lock and key. The place on the enzyme where the substrate attaches is called the active site. © Hodder & Stoughton 2017

1 Cells and movements across membranes Topic overview How are the activities of cells controlled? (continued) Enzymes do not work well at cold temperatures. As temperature increases, the rate of enzyme activity will increase. High temperatures denature enzymes (destroy them), because the shape of the active site is changed. Each enzyme has an optimum pH. If the pH varies too much away from that, the enzyme denatures. The effect of pH on enzymes The effect of temperature on enzymes © Hodder & Stoughton 2017