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Peroxisomes that are responsible for collecting toxic chemicals

Fluid mosaic model of cell membranes

  1. Membranes are fluid and are rather viscous – like vegetable oil.
  2. The molecules of the cell membrane are always in motion, so the phospholipids are able to drift across the membrane, changing places with their neighbour.
  3. Proteins, both in and on the membrane, form a mosaic, floating in amongst the phospholipids.
  4. Because of this, scientists call the modern view of membrane structure the ‘Fluid Mosaic Model’.
  5. The mosaic of proteins in the cell membrane is constantly changing.


Membrane proteins

  1. A variety of protein molecules are embedded in the basic phospholipid bilayer
  2. Some proteins are attached to the surface of the cell membrane on both the internal and external surface. These may be hormone receptors, enzymes or cell recognition proteins (or antigens)
  3. Other proteins are embedded in the phospholipid bilayer itself. These are often associated with transporting molecules from one side of the membrane to the other and are referred to as carrier proteins.
  4. Some of these form channels or pores through which certain substances can pass (facilitated diffusion), whilst others bind to a substance on one side of the membrane and carry it to the other side of the membrane (active transport)
  5. Proteins exposed to the cell’s external environment often have carbohydrates attached to them which act as antigens (e.g. blood groups A & B – group AB has both; group O has neither).
  6. Some viruses may also bind here too.


The nucleus (pl. nuclei)

  1. The Nucleus is normally the largest organelle within a Eukaryotic cell. But it is NOT the ‘BRAIN’ of the cell!!
  2. Prokaryotes have no nucleus, having a nuclear body instead. This has no membrane and a loop of DNA (and no chromatin proteins)
  3. The nucleus contains the cell’s chromosomes (human, 46, fruit fly 6, fern 1260) which are normally uncoiled to form a chromatinic network, which contain both linear DNA and proteins, known as histones. These proteins coil up (dehydrate) at the start of nuclear division, when the chromosomes first become visible.
  4. Whilst most cells have a single nucleus some cells (macrophages, phloem companion cells) have more than one.
  5. Fungi have many nuclei in their cytoplasm – they are coenocytic (= common cytoplasm throughout)
  6. The nucleus is surrounded by a double membrane called the nuclear envelope.
  7. The nuclear envelope has many nuclear pores through which mRNA, and proteins can pass. These dimples make it look like a golf ball.
  8. Most nuclei contain at least one nucleolus (plural, nucleoli). The nucleoli are where ribosomes are synthesised. Ribosomes, you remember, translate mRNA into proteins.
  9. When a nucleus prepares to divide, the nucleolus disappears.


Comparison of structures between animal and plant cells

Typical animal cell Typical plant cell
OrganellesNucleusNucleolus (within nucleus)Rough ERSmooth ER80S RibosomesCytoskeletonGolgi apparatusCytoplasmMitochondriaVesiclesVacuolesLysosomesCentriolesNucleusNucleolus (within nucleus)Rough ERSmooth ER80S RibosomesCytoskeletonGolgi apparatusCytoplasmMitochondrionVesicleChloroplast and other plastidsCentral vacuoleTonoplast (central vacuole membrane)
Additional structuresCiliaFlagellaePlasma membranePlasma membraneCellulose cell wallPlasmodesmata


Mitochondria

  1. Mitochondria are found scattered throughout the cytosol, and are relatively large organelles (second only to the nucleus and chloroplasts)
  2. Mitochondria are the sites of aerobic respiration, in which energy from organic compounds is transferred to ATP. For this reason they are sometimes referred to as the ‘powerhouse’ of the cell
  3. ATP is the molecule that most cells use as their main energy ‘currency’
  4. Mitochondria are more numerous in cells that have a high energy requirement – our muscle cells contain a large number of mitochondria, as do liver, heart and sperm cells
  5. Mitochondria are surrounded by two membranes, indicating that they were once free-living organisms that have become mutualistic and then a part of almost every eukaryotic cell (not RBC’s and xylem vessels)
    1. The smooth outer membrane serves as a boundary between the mitochondria and the cytosol.
    2. The inner membrane has many long folds, known as cristae, which greatly increase the surface area of the inner membrane, providing more space for ATP synthesis to occur.
  6. Mitochondria have their own DNA, and new mitochondria arise only when existing ones grow and divide. They are thus semi-autonomous organelles.

Ribosomes

  1. Unlike most other organelles, ribosomes are not surrounded by a membrane
  2. Ribosomes are the site of protein synthesis in a cell
  3. They are the most common organelles in almost all cells
  4. Some are free in the cytoplasm (Prokaryotes)
    others line the membranes of rough endoplasmic reticulum (rough ER)
  5. They exist in two sizes: 70S are found in all Prokaryotes, chloroplasts and mitochondria, suggesting that they have evolved from ancestral Prokaryotic organisms. They are free-floating. 80S found in all eukaryotic cells – attached to the rough ER (they are rather larger)
  6. Groups of 80s ribosomes, working together, are known as a polysome.