Cell Division, Diversity and Organisation

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Cell Division, Diversity and Organisation

Cell Division - Mitosis

  • Cell cycle - process that all body cells from multicellular organisms use to grow and divide
  • Cycle starts when a cell has been produced by cell division and ends with cell dividing to produce two identical cells
  • Cyle consists of period of cell growth, interphase, and a period of cell division, mitosis - mitosis occupies small percentage of cell cycle - most taken up by interphase, where DNA is copied and checked  for errors that may have occured during copying - if errors occur, cell kills itself to prevent mutations from being passed on
  • Stages of interphase:
    - C = cytokinesis
    - G1 = growth phase, protein synthesis, replication of organelles
    - S = DNA replication to ensure there are two identical copies to go into each new cell during next cell cycle
    - G2 = growth phase, protein synthesis, replication of organelles
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Cell Division, Diversity and Organisation

Cell Division - Mitosis

Stages of Mitosis

  • Interphase - Cell carries out normal functions and prepares to divide. Cell's DNA unravelled and replicated to double genetic content. Organelles replicated and ATP content increases to provide energy for cell division
  • Prophase - Chromosomes condense, getting shorter and fatter, become visible. Centrioles start moving towards opposite ends of cell, forming network of protein fibres called the spindle. Nuclear envelope breaks down and chromosomes lie free in cytoplasm. DNA supercoiled. No nucleolus
  • Metaphase - Chromosomes line up along middle of cell and attach to spindle by centromere. Chromosomes starting to be pulled apart at centromeres towards each pole
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Cell Division, Diversity and Organisation

Cell Division - Mitosis

Stages of Mitosis

  • Anaphase - Centromeres divide, separating each pair of sister chromatids. Spindles contract, pulling chromatids to opposite ends of cell, centromere first - requires energy, lots of mitochondria
  • Telophase - Chromatids reach opposite poles on spindle, uncoil and become long and thin again, now called chromosomes. Nuclear envelope forms around each group of chromosomes, there are two nuclei. Cytoplasm divides to produce two daughter cells that are identical to themselves and the parent cell. Each daughter cell starts interphase process ready for more mitosis. Spindle breaks down, nucleolus and nuclear membrane reform

You can stain chromosomes with acetic orcene so they are visible under a microscope

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Cell Division, Diversity and Organisation

Cell Division and Reproduction

  • Some organisms reproduce asexually using mitosis.

Budding - Yeast cells (single-celled, eukaryotic)

  • Bud forms at surface of cell
  • Cell undergoes interphase - DNA and organelles replicated ready for cell to divide
  • Cell begins to undergo mitosis
  • Nuclear division complete - budding cell contains nucleus that has identical copy of parent cell's DNA
  • Bud separates off from parent cell, producing new, genetically identical yeast cell
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Cell Division, Diversity and Organisation

Cell Division and Reproduction

Meiosis

  • Some organisms reproduce sexually using meiosis
  • In sexual reproduction two gametes (egg and sperm) join at fertilisation to form a zygote - zygote divides and develops into new organism
  • Meiosis happens in reproductive organs to produce gametes
  • Cells that divide by meiosis have full number of chromosomes to start with, but cells formed have half the number
  • Cells genetically different as there is different combination of chromosomes
  • Humans have 46 chromosomes - 23 pairs
  • One chromosome in each pair came from mum and one from dad - there are two number 1s, two number 2s etc
  • Chromosomes that make up each pair are same size and have same genes, although they could have different versions of those genes, alleles - homologous pairs
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Cell Division, Diversity and Organisation

Cell Division and Reproduction

Stages of Meiosis

  • DNA replicates and coils to form chromosomes
  • Chromosomes arrange themselves into homologous pairs
  • Chromosome pairs swap alleles with each other
  • In first division, homologous pairs split up. Any chromosome from each pair go into either cell, as long as each cell gets one number 1, one number 2 etc
  • In second division, each chromosome splits in half. Any half can go into any cell
  • Four new genetically different cells produced
  • Genetically different as chromosomes swap alleles during process and each gamete gets combination of half of them, at random
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Cell Division, Diversity and Organisation

Stem Cells and Differentiation

Stem cells are unspecialised cells

  • Multicellular organisms made up from many different cell types that are specialised for their function - muscle cells, liver cells, white blood cells
  • Specialised cell types originally came from stem cells
  • Stem cells are unspecialised - capable of becoming differentiated to a number of possible cell types
  • In humans, stem cells are found in early embryos and in a few places in adults
  • In first few days of embryo's life, any of its cells can develop into any type of human cell - stem cells
  • In adults, stem cells found in a few places e.g. bone marrow, but their not as flexible - they can only develop into a limited range of cells
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Cell Division, Diversity and Organisation

Stem Cells and Differentiation

Stem cells differentiate into specialised cells

  • Stem cells divide to become new cells, which become specialised
  • The process is differentiation

Cells in bone marrow differentiate into blood cells:
- Bones are living organs, containing nerves and blood vessels
- Main bones of body have bone marrow in the centre
- Adult stem cells divide and differentiate to replace worn out blood cells - erythrocytes and neutrophils help fight infection
Cells in cambium differentiate into xylem and phloem
- In plants, stem cells found in cambium
- In the root and stem, stem cells of vascular cambium divide and differentiate to become xylem and phloem
- Vascular cambium forms ring inside root and shoots
- Cells divide and grow out of ring, differentiating as moved away from cambium

 

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Cell Division, Diversity and Organisation

Stem Cells and Differentiation

Animal cells

  • Neutrophils - Defend body against disease. Flexible shape allows them to engulf foreign pathogens. Lysosomes in cytoplasm contain digestive enzymes which break down engulfed particles
  • Erythrocytes - Carry oxygen in blood. Biconcave disc shape provides large surface area for gas exchange. No nucleus to make more room for haemoglobin, protein that carries oxygen
  • Epithelial cells - Cover surface of organs. Cells joined by interlinking cell membranes and a membrane at base. Some epithelia (lungs) have cilia that beat to move particles away. Other epithelia (small intestine) have microvilli, folds in cell membrane that increase surface area
  • Sperm cells - Male sex cells. Have flagellum so they can swim to egg. Lots of mitochondria to provide energy to swim. Acrosome contains digestive enzymes to enable sperm to penetrate surface of egg
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Cell Division, Diversity and Organisation

Stem Cells and Differentiation

Plant cells

  • Palisade mesophyll cells - Involved in photosynthesis. Contain chloroplasts so they can absorb lots of sunlight. Thin walls, so carbon dioxide can easily diffuse into cell
  • Root hair cells - Absorb water and mineral ions from soil. Large surface area for absorption and thin permeable cell wall for entry of water and ions. Cytoplasm contains extra mitochondria to provide energy needed for active transport
  • Guard cells - Line stomata. Tiny pores in surface of leaf used for gas exchange. In light, guard cells take up water and become turgid. Thin outer walls and thickened inner walls force them to bend outwards, opening stomata. Allows leaf to exchange gases for photosynthesis
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Cell Division, Diversity and Organisation

Tissues, Organs and Systems

  • Tissue - Group of cells specialised to work together to carry out particular function. Can contain more than one cell type
  • Squamous epithelium tissue - Single layer of flat cells lining a surface. Found in many places including alveoli and lungs
  • Xylem tissue - Plant tissue with two jobs - transports water around plant and supports it. Contains xylem vessel cells and parenchyma cells
  • Ciliated epithelium tissue - Layer of cells covered in cilia. Found on surfaces where things need to be moved - in trachea where cilia beat mucus along
  • Phloem tissue - Transports sugars around plant. Arranged in tubes and made up of sieve cells, companion cells, and some ordinary plant cells. Each sieve cell has end walls with holes in them, so sap can move easily through them. End walls called sieve plates
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Cell Division, Diversity and Organisation

Tissues, Organs and Systems

Different tissues make up organ

  • Organ - Group of tissues that work together to perform a particular function
  • Lungs - Contain squamous epithelium tissue (alveoli) and ciliated epithelium tissue (bronchi). Also have elastic connective tissue and vascular tissue in blood vessels
  • Leaves - Contain palisade tissue for photosynthesis, as well as epidermal tissue, and xylem and phloem tissues in veins

Different organs make up organ system

  • Respiratory system - Made up of all the organs, tissues and cells involved in breathing. Lungs, trachea, larynx,all part of respiratory system
  • Circulatory system - Made up of all the organs involved in blood supply. Heart, arteries, veins and capillery all part of circulatory system
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Cell Division, Diversity and Organisation

Tissues, Organs and Systems

Tissues, organs and systems cooperate

  • Multicellular organisms work efficiently as they have different cells specialised for different functions
  • Each different cell type can carry out specialised function more effectively than unspecialised cell could
  • Each cell type depends on other cells for functions it can't do
  • Cells, tissues and organs within multicellular organisms cooperate together to keep organism alive
  • Palisade cell - good at photosynthesising, but can't absorb water and minerals from soil. Depends on root hair cells for this and vice versa
  • Muscle cell - good for getting you to where you want to go, but to do this they need oxygen. Depend on erythrocytes to carry oxygen to them from lungs

 

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Cell Division, Diversity and Organisation

Tissues, Organs and Systems

  • Multicellular organisms have developed different systems of cooperation between different cells
  • Transport systems - Used to carry substances between different cells. For example, xylem cells carry water and minerals from root hair cells to palisade cells and phloem cells carry sugars around plant. In humans, circulatory system helps to move substances around body in blood
  • Communication systems - Allow communication between cells in different parts of organism. Both plants and animals have chemical communication systems that use messenger molecules like hormones. Animals have nervous system for communication, sending electrical signals to different tissues and organs
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