B1 Topics (AQA Biology)

Microscopy, size and magnification Even larger human cells - like the skin cell - are 20 times smaller than a grain of salt. A red blood cell is much smaller than that. To allow us to see detail in these cells, we need the help of a microscope. Making slides A slide is a thin piece of glass used to hold objects which are examined under a microscope. Most slides will already be made up for you. If you do get the chance to prepare your own slide, here’s what to do: Plant cells 1. Peel a thin, transparent layer of epidermal cells from the inside of an onion. 2. Place cells on a microscope slide. 3. Add a drop of water or iodine (a chemical stain). 4. Lower a coverslip onto the onion cells using forceps or a mounted needle. This needs to be done gently to prevent trapping air bubbles. Animal cells 1. Remove cells from the inside of your cheek using a cotton bud. 2. Smear the cotton bud onto a microscope slide. 3. Add a drop of methylene blue (a chemical stain). 4. Lower a coverslip onto th
Cells, tissues and organs In order of increasing complexity, multicellular organisms consist of: Organelles → cells → tissues → organs → organ systems
Plant and animal cells How different types of animal cell are adapted to carry out their function There are many different types of cells in animals. Each type is specialised for a particular role. These ensure that the organism functions as a whole. The head of the sperm contains the genetic material for fertilisation. The acrosome in the head contains enzymes so that the sperm can penetrate an egg. The middle piece is packed with mitochondria to release energy needed to swim and fertilise the egg. The tail enables the sperm to swim.
Prokaryotes Bacteria are amongst the simplest of organisms - they are made of single cells. Their cell structure is simpler than the cells of eukaryotes and cells are smaller, most are 0.2 μm - 2.0 μm. These cells do not contain membrane bound organelles such as a nucleus and mitochondria. Bacterial cells are called prokaryotic cells.
Eukaryotes Cells of animals, plants and fungi are called eukaryotic cells. They contain membrane bound organelles such as a nucleus and mitochondria. Animal cells Almost all animals and plants are made up of cells.
Diffusion Diffusion is the process by which particles of a substance spread out from each other, moving from a region where they are in high concentration to a region of low concentration. In the same way as a ball will roll from a high point to a low point down a gradient, particles of a substance will move down a concentration gradient until they are evenly spread. In order to do this, particles of a substance must be free to move. This is the case for particles of a gas or particles of a dissolved substance.
Osmosis Water can move across cell membranes because of osmosis. For osmosis to happen you need: • Two solutions with different concentrations • A partially permeable membrane to separate them Partially permeable membranes let some substances pass through them, but not others. The animation shows an example of osmosis. Osmosis is the movement of water from a less concentrated solution to a more concentrated solution through a partially permeable membrane. This is shown in the animation above. Eventually the level on the more concentrated side of the membrane rises, while the one on the less concentrated side falls. When the concentration is the same on both sides of the membrane, the movement of water molecules will be the same in both directions. At this point, the net exchange of water is zero and there is no further change in the liquid levels. Osmosis is important to plants. They gain water by osmosis through their roots. Water moves into plant cells by osmosis, making them turgid
Plant cells Isolated plant cells placed in a dilute solution or water will take in water by osmosis. Root hair cells, if the soil is wet or moist, will also take up water by osmosis. Leaf cells of land plants, unless it is raining or the humidity is high, will have a tendency to lose water. Plant cells have a strong cellulose cell wall outside the cell membrane. The cell wall is fully permeable to all molecules and supports the cell and stops it bursting when it gains water by osmosis.
Active transport Active transport is the movement of dissolved molecules into or out of a cell through the cell membrane, from a region of lower concentration to a region of higher concentration. The particles move against the concentration gradient, using energy released during respiration. Sometimes dissolved molecules are at a higher concentration inside the cell than outside, but, because the organism needs these molecules, they still have to be absorbed. Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient.
Diffusion Particles (molecules and ions) in a liquid and a gas move continuously. Because of this movement, particles will spread themselves evenly throughout a liquid or a gas. If there is a situation where particles of a substance are in a higher concentration, they will move from this region to where they are in a lower concentration. This is called diffusion. It is important to remember that the particles: • will move in both directions, but there will be a net movement from high to low concentration • will end up evenly spread throughout the liquid or gas, but will continue to move

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	Criado por Maeesha Hussain aproximadamente 6 anos atrás