Cell Transport & Cell Division

Cell Membranes

Cell membranes act as barriers and can control what passes into and out of cells and organelles
It is composed of phospholipids, proteins, glycoproteins, glycolipids and cholesterol
Gylcolipids and glycoproteins allow cell adherence, stability and acts as recognition sites
Cholesterol has a hydrophillic end and a hydrophobic end & regulates membrane fluidity by interclataing between the phospholipids
At high temperatures the phospholipid bilayer increases its fluidity and permeability, increasing the risk of proteins being denatured.
- At low temperatures, the phospholipds are compressed, making the membrane stiff
Organic solvents disrupt the bilayer and dissolves the membrane

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Involves the exchange of substances without requiring metabolic energy from the cell
Diffusion = the net movement of paricles form an area of higher concentration to an area of lower concentration (down the concentration gradient)
Facilitated diffusion = the net movement of particles down their concentration gradient across a partially permeable membrane via channel or carrier proteins
Water potential is a measure of the tendency of water molecules to move from one area to another area and describes the pressure created by these water molecules
- the more dilute a solution the high the water potential
Osmosis is the net movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane
The rate of diffusion can be increased by increasing the number of channel & carrier proteins, the surface area of the cell membrane, reducing the diffusion distance and creating a steeper concentration gradient

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Active transport is the movement of particles from an area of low concentration to an area of high concentration (against their concentration gradient) across a cell membrane, using ATP and carrier proteins
Bulk transport transports large substances across the cell membrane
Endocytosis is the bulk transport of large moleculesinto a cell. The membrane invaginates and encloses the material being taken up into a vesicle, which pinches off
Exocytosis is the bulk transport of large molecules out of cell.
- A vesicle fuses witht he cell membrane, releasing its material

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Within multicellular organisms, not all cells retain the ability to divide
Mitosis produces genetically identical daughter cells, which is useful for gorwth, repair and asexual reproduction
The eukaryotic cell cycle has 3 main stages
- Interphase consists of two growth phases (G1 and G2), and a DNA synthesis stage (S).
  - The cell may exit at the G0
- Mitosis is the cuclear division
- Cytokinesis is when the cell splits in two forming two identical daughter cells
  - In animal cells, the plasma membrane folds inwards until the two dents meet to separate the two daughter cells
  - In plant cells, cellulose starts to build up at the equator (the end plate). Plasma membrane forms in the middle of the end plate resulting in two fully separated plant cells

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Meiosis produces 4 haploid daughter cells that are genetically different from each other
Variation results from independent assortment of chromosomes and crossing over during meiosis 1. Also, random fertilisation of the gametes.

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