B1

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Microscopes

Light Microscopes:

Use a beam of light to form an image, can magnify objects up to 2000 times, can be used to view living objects, cheap and easy to carry around.

Electron Microscopes:

Use a beam of electrons to form an image, can magnify up to 2,000,000 times, can't be used for living objects, expensive and need to be kept in special conditions.

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Calculating magnification, image size, and object

magnification=image size/object size  if you know two of the values in the equation, you can calculate the third.

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Animal Cells

Animal cells contain a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.

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Plant Cells

A plant cell contains a nucleus, cytoplasm, cell membrane, mitochondria, ribosomes, cell wall, chloroplasts and a vacuole.

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Differences in Animal and Plant Cells

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Cell Structures and functions

Nucleus - Controls the cell's activities.

Cytoplasm - A liquid gel in which organelles are suspended, and where many chemical reactions take place.

Cell Membrane - The cell membrane controls the movement of substances such as glucose and mineral ions into the cell, and other substances such as urea and hormones out of the cell.

Mitochondria - Where energy is transferred during aerobic respiration.

Riobosomes - Where protein synthesis takes place.

Cell wall - Cell walls are made of cellulose and they support the cells structure.

Chloroplasts - Chloroplasts contain chlorophyll which is used for photosynthesis; the chloroplasts absorb light to make food.

Vacuole - A permanent vacuole containing cell sap, which keeps the cell rigid and helps to support the plant.

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Eukaryotic Cells

All animals (including human beings), plants fungi and protista are eukaryotes. Eukaryotic cells contain a cell membrane, cytoplasm and a nucleus. The nucleus contains chromosomes which are made of DNA.

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Prokaryotic Cells

Prokaryotic cells consist of cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not in a distinct nucleus. It forms a single DNA loop. Prokaryotes may contain one or more extra small rings of DNA called plasmids.

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Bacteria Cells

Bacteria are very small and can only be seen with a powerful microscope.

Bacteria cells have a cell membrane and a cell wall surrounding the cytoplasm.

The cell wall is different from a plant cell wall because it is not made of cellulose.

Bacteria do not contain a nucleus.

The genetic material is found in the cytoplasm as a long circle of DNA called plasmids.

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Nerve Cells

Nerve cells are specialised to carry electrical impulses to carry electrical impulses around the bodies of animals.

Nerve cells have:

  • many dendrites to make connections to other nerve cells
  • an axon to carry the impulse from one place to another
  • nerve endings or synapses, which pass impulses to other cells by producing transmitter chemicals
  • many mitochondria in the synapses to transfer the energy needed to make the transmitter chemicals.
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Muscle Cells

Muscle cells contract and relax. Striated (striped) muscle cells are found in the muscles that enable your body to move. Smooth muscle cells are found in the tissues of the digestive system and contract to move food along the gut.

So muscle cells can contract they contain:

  • special proteins that slide over each other
  • many mitochondria to transfer the energy needed for chemical reactions
  • a store of glycogen that can be broken down and used in respiration to transfer energy.
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Sperm Cells

Sperm cells carry the genetic information from the male parent. They are specialised to move through water or the female reproductive system to reach the egg.

Sperm cells have:

  • a long tail that whips from side to side to move the sperm
  • a middle section full of mitochondria to transfer the energy needed by the tail to move
  • an acrosome to store digestive enzymes to break down the outer layers of the egg
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Root Hair Cells

Root hair cells occur near the tips of roots. They increase the surface are of the root so that it can absorb water and mineral ions efficiently. Root hair cells are close to xylem tissue that transports water and mineral ions through the plant.

Root hair cells:

  • increase the surface area available for water to move into the cell
  • have a large permanent vacuole - speeds up the movement of water by osmosis from the soil across the root hair cell.
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Photosynthetic Cells

Plants can make their own food by photosynthesis.

Photosynthetic cells:

  • have chloroplasts containing chlorophyll to trap the light needed for photosynthesis
  • are often found in continuous layers in the leaf and outer layers of stems
  • have a large permanent vacuole that helps to keep the cell rigid.
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Xylem Cells

Xylem tissue has two main functions:

1. the xylem cells transport water and mineral ions from the roots to the stem and leaves.

2. Xylem tissue supports the plant.

  • Xylem cells are living when they are first formed
  • then a chemical called lignin builds up in spirals in the cell walls
  • the cells die leaving hollow tubes.
  • water and mineral ions can move up the tubes, the spirals of lignin make the xylem strong.
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Phloem

Phloem is the tissue that transports food made by photosynthesis to the rest of the plant. Phloem cells form tubes but do not become lignified like the xylem.

  • The cell walls between phloem cells break down to form sieve plates,
  • phloem cells lose a lot of their structures but are kept alive by companion cells which contain mitochondria that transfer energy to aid the movement of dissolved food in the phloem
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Diffusion

Diffusion is the spreading out of particles of any substance, in solution or gas, resulting in a net movement from an area of high concentration to and area of low concentration down a concentration gradient.

The rate of diffusion is affected by the difference in concentrations, the temperature, and the available surface area.

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Osmosis

Osmosis is a special case of diffusion. It is the movement of water from a dilute to a more concentrated solution through a partially permeable membrane that allows water to pass through.

Osmosis in animals:

  • Animal cells that are surrounded by a hypotonic solution will swell and possibly burst because the water moves into the cell by osmosis. If the solution around animal cells is hypertonic then water moves out of the cells and they shrink.
  • Animals need complex mechanisms to control the concentration of the solutions around their cells to avoid bursting or shrinking.

Osmosis in plants:

  • Turgor pressure occurs when no more water can enter a cell due to the pressure inside. Water moves into plant cells by osmosis, the vacuole swells, the cytoplasm is pressed against the cell wall, cell becomes rigid, leaves and stem become rigid.
  • As long as the outside solutions is hypotonic, water moves in and keeps the cells rigid to support the plant. Plant cells in a hypertonic solution lose water and become flaccid, so the plant wilts.
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Active Transport

Active transport is the movement of molecules across a membrane from a region of their lower concentration to a region of their higher concentration - in the direction against the concentration gradient.

Active transport requires energy from respiration.

Active transport allows plant root hairs to absorb mineral ions required for healthy growth from very dilute solutions in the soil against a concentration gradient.

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Exchanging Materials

Materials such as oxygen and soluble food molecules need to reach all cells, and metabolic waste materials must be removed efficiently.

  • Small organisms have large surface area to volume ratio. Single-celled organisms are tiny and can gain enough of materials such as oxygen by diffusion through their surface.
  • As organisms increase in size, their surface area to volume ratio decreases.
  • Large, complex, organisms have many cells that are not in contact with the environment; so they have special exchance surfaces to obtain all the food and oxygen they need.
  • Efficient exchange surfaces have a large surface area, thin membranes or a short diffusion path, and an efficient transport system - the blood supply in animals.
  • Gaseous exchange surfaces in animals must be ventilated. Oxygen is absorbed by the alveoli in the lungs when air is drawn in during breathing. The alveoli have a large surface area and a good blood supply to carry the oxygen away and maintain a concentration gradient.
  • The villi of the small intestine have a large surface area, a short diffusion path, and a good blood supply to absorb soluble food molecules.
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