Biology Unit 1 Cells, exchange and transport

Covering the learning objectives in Biology AS OCR book unit 1

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Cells- Structure and function


  • Largest organelle
  • Surrounded by nuclear envelope with nuclear pores
  • Dense spherical structure
  • Has a nucleolus inside
  • House nearly all the cell's genetic material
  • Cromatic consists of DNA and proteins
  • The nucleolus makes RNA and ribosomes

Golgi apparatus

  • A stack of membrane-bound, flattened sacs
  • Receives proteins from the ER and modifies them
  • Packages the modified proteins into vesibles that can be transported
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Cells- Structure and function

Endoplasmic reticulum (ER)

  • Series of flattened, membrane-bound sacs called cisternae
  • The rough endoplasmic reticulum (RER) is studded with ribosomes
  • RER transports proteins that were made on the attached ribosomes
  • SER is involved in making the lipids needed by cells


  • Two membranes separated by a fluid-filled space
  • Ineer membrane is highly folded to form cristae
  • The central part is called matrix 
  • Site where ATM is produced during respiration
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Cells- Structure and function (2)


  • Only found in plant cells and some protoctists
  • Two membranes separated by a fluid-filled space
  • Have an elaborate network of flattened membrane sacs called thylakoids
  • A stack of thylakoids is called a granum
  • Clorophyl molecules are present on the thylakoids
  • Site of photosynthesis in plant cells


  • Spherical sacs surrounded by a single membrane
  • Contain powerful digestive enzymes to break down materials
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Cells- Structure and function (2)


  • Tiny organelles
  • Some in the cytoplasm and some bound to the RER
  • They consist of two subunits
  • Site of protein synthesis
  • Acts as an assembly line where mRNA is used to assemble proteins


  • They are microtubules (small tubes of protein fibres
  • They form the spindle fibres which move chromosomes during nuclear division
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Cells- Microscopes

Light microscope

  • Maximum magnification of x1500
  • low resolution (200nm)

Transmission electron microscope (TEM)

  • Final image is two dimensional
  • Maximum magnification of x500 000
  • Electrons pass through the sample and show contrast in different tissues
  • Resolution of 0.1 nm

Scanning electron microscope (SEM)

  • Final image is a three dimensional image of the surface
  • Maximum magnification is x100 000
  • Electrons are bounced off the surface
  • Resolution of 0.1 nm
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Cells- Microscopes

  • A microscopic ruler (stage micrometer) can be placed on the microscope
  • The eye piece can be fitted with a graticule
  • The graticule and stage micrometer can be used to measure samples
  • Actual size = image size/ magnification  (I AM triangle)
  • Magnification- the number of times greater an image is than the object
  • Resolution- the ability to distinguish that two points are separated
  • Staining and sectioning are used to prepare samples
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Crossing membranes

  • Roles of membranes:
    • separate cell contents from the outside environment
    • separate cell contents from the cytoplasm
    • cell recognition and signalling (with complementary receptors)
    • holding the components of some metabolic pathways in place
    • regulating the transport of materials in and out of cells
  • Membranes are about 7-10 nm thick
  • The phospholipid bilayer is the basic structural component of plasma membranes consist of two layers of phospholipid molecules
  • A pospholipid molecule is formed of a hydrophilic phosphate head and two hydrophobic fatty acid tails, forming the fluid mosaic model
  • Increasing the temperature of membranes makes the molecules have more kinetic energy and makes the membrane more fluid and leaky
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Crossing membranes- transport

  • They are partially permeable
  • Involved in cell signalling:
    • Glycolipid- a lipid with a carbohydrate molecules attached
    • Glycoprotein- a protein with carbohydrate molecules attached
  • Cholesterol gives the membranes stability and makes the membrane impermeable to water molecules and ions
  • Passive processes (no ATP used)
    • Diffusion- down a concentration gradient, for lipid soluble or very small molecules, through the lipid bilayer
    • Facilitated diffusion- down a concentration gradient, charged or hydrophilic molecules or ions, via carrier or channel proteins
    • Osmosis- down a water potential gradient, through bilayer or protein pores
  • Active processes (using ATP)
    • Active transport- against a concentration gradient, via carrier proteins which change shape using ATP
    • Endocytosis and exocytosis- bulk transport of materials via vesicles that fuse with or break from the cell surface membrane
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Crossing membranes- osmosis

  • There is a net movement from the region with a higher water potential to a region with lower water potential
  • If the water potential is the same there will be movement across, just no net movement
  • The more solutes in a solution, the lower the water potential is
  • Water moves in the cell
    • Animal cell- the cell will swell and eventually burst open (haemolysed)
    • Plant cell- the cell will swell and eventually be turgid (due to support of the cell wall)
  • Water moves out of the cell
    • Animal cell- the cell will shrink and eventually be crenated
    • Plant cell- cell membrane pulls away from the cell wall (it is plasmolysed)
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Cells- Mitosis

  • Mitosis is the process of nuclear division where two genetically identical nuclei are formed from one parent cell nucleus
  • The daughter cells are genetically identical both to each other and to the parent cell
  • Chromatin- DNA wrapped around proteins called histones, they remain together at a point called centromere
  • A pair of two replica DNA strands is called a pair of sister chromatids
  • In plant cells only meristem cell can divide this way
  • In plant cells there are no centrioles, tubulin protein threads are used
  • In animal cells cytokinesis starts from the outside (nipping in) but in plant cells cytokinesis starts with the formation of a cell plate where the spindle equator was (outwards)
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Cells- Mitosis

  • Stages:
    • Interphase- It has two growth stages (G1 and G2) and stage S which is DNA synthesis (replication of DNA)
    • Mitosis
      • Prohase- chromosomes shorten and thicken and are now visible, nuclear envelope breaks down, centriole divides into two and each moves to opposite poles
      • Metaphase- chromosomes attach to a spindle by their centromere
      • Anaphase- spindle fibres shorten pulling the sister chromatids towards the poles
      • Telophase- spindle breaks down and disappears, chromosomes uncoil and are no longer visible, nuclear envelope reforms
    • Cytokinesis- the cytoplasm divides or cleaves
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Cells- specialisation

  • Differentiation- the changes occurring in cells of a multicellular organism so that each different type of cell becomes specialised to perform a specific function (e.g. erythrocytes, lysosomes)
  • Tissues- a collection of cells that are similar to each other and perform a common function (e.g. xylem, phloem, epithelial, nervous)
  • Organs- a collection of tissues working together to perform a particular function (e.g. leaves, liver)
  • Organ systems- made up a number of organs working together to perform an overall life function (e.g. excretory system, reproductive system)
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Really good notes :) thank you

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