CELL TRANSPORT & CELL DIVISION

Question 1

cell membranes

Answer 1

• Cell membranes act as barriers and can control what passes into
  and out of cells and organelles
• The cell membrane is composed
  of phospholipids, proteins,
  glycoproteins, glycolipids and
  cholesterol.
• Glycolipids and glycoproteins allow cell adherence, stability and
  act as recognition sites
• Cholesterol has a hydrophilic end and a hydrophobic end &
  regulates membrane fl uidity by intercalating between the
  phospholipids.
• At high temperatures the phospholipid bilayer increases its
  fl uidity and permeability, increasing the risk of the proteins
  becoming denatured. At low temperatures, the phospholipids
  are compressed, making the membrane stiff
• Organic solvents disrupt the bilayer and dissolve the membrane

Question 2

passive transport

Answer 2

• Passive transport involves exchange of substances without
  requiring metabolic energy from the cell
• Diff usion is the net movement of particles from an area of
  higher concentration to an area of lower concentration (down
  their concentration gradient).
• Facilitated diff usion is the net movement of particles down
  their concentration gradient across a partially permeable cell
  membrane via
  carrier or channel
  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 higher (less negative) 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 diff usion can be increased by increasing the
  number of channel & carrier proteins, the surface area of the
  cell membrane, reducing the diff usion distance and creating a
  steeper concentration gradient.

Question 3

active transport

Answer 3

• Active transport is the movement of particles from an area of
  low con centration 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 molecules into 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 a cell.
  A vesicle fuses with the cell membrane, releasing its material.

Question 4

mitosis

Answer 4

• Within multicellular organisms, not all cells retain the ability to
  divide
• Mitosis produces genetically identical daughter cells, which is
  useful for growth, repair and asexual reproduction
• The eukaryotic cell cycle has three
  main stages:
• Interphase consists of two growth
  phases (G1&G2) and a DNA synthesis
  stage (S). The cell may exit the cell
  cycle at G0
• Mitosis is the nuclear 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

Question 5

stages of mitosis

Answer 5

Prophase - DNA condenses & coils, nuclear envelope breaks down, centrioles move to opposite poles
Metaphase - spindle fibres attach to centromeres and chromosomes line at the equator
Anaphase - centromeres divide and chromatids move to opposite poles
Telophase - chromosomes uncoil, nuclear envelope reforms

Question 6

cell cycle

Answer 6

cell cycle checkpoints ensure the cell only divide at the appropriate time when ready and helps detect DNA damage .

G/S checkpoint - between g and s. Decides whether cell proceeds to S phase
G1 checkpoint - end of g1. Checks cell is ready for s-phase
G2/M checkpoint - between g2 and mitosis. Checks and repairs all of the dna before the cell enters mitosis
Mitosis checkpoint - in the middle of mitosis. Checks the cel, is ready to proceed in mitosis. Ensuring that the cell doesn’t proceed to the next stage before it’s ready

Question 7

meiosis

Answer 7

• Meiosis produces 4 haploid daughter cells (gametes) that are
• In meiosis 1, homologous chromosomes are separated from each other, with one chromosome from each pair going into
  one of the two daughter cells. In the second meiotic division, the sister chromatids from each chromosome are separated
  Prophase 1 - chromatic cindenses and coils, nuclear envelope breaks down, centrioles move to opposite poles. Crossing over between homologous chromosomes
  Metaphase 1 - spindle fibres attach to centromeres and chromosomes line at the equator Anaphase
  Anaphase 1 - homologous chromosomes move to opposite poles
  Telophase 1 and cytokineses - chromosomes uncoil, nuclear envelope reforms. Animal cells divide by cytokineses
  Prophase 2 - chromatin condenses and coils , nuclear envelope breaks down, centrioles move to opposite poles
  Metaphase 2 - spindle fibres attach to the centromeres and chromosomes line up at the equator
  Anaphase 2 # centromeres divide and the chromatids are pulled to opposite poles
  Telophase 2 and cytokineses - chromosomes uncoil, nuclear envelope reforms. Cells divide to form 4 haploid cells
  Variation occurs from independent assortment of chromosomes and crossing over during meiosis 1 . Also random fertilisation of gametes