Organelles

Cell ultrastructure - the organelles within cells and their function for the OCR exam board.

Hope they help!

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  • Created by: Joanne
  • Created on: 15-09-12 15:19

Cell surface membrane

  • surrounds the cell forming a barrier between the contents of the cell and the extracellular evironment
  • thin layer made up of lipids (fats) and protein molecules
  • phospholipid bilayer
  • function: controls what enters/leaves the cell (selectively permeable)

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Nucleus

  • variable location (not always in the centre)
  • surrounded by a pair of membranes called the nuclear envelope that has small gaps throughout it (nuclear pores)
  • function of nucleus: contains most of the cell's genetic material which regulates activities of the cell
  • nucleolus contains DNA that makes ribosomes

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Ribosomes

  • free in the cytoplasm or bound to rough endoplasmic reticulum
  • made up of ribosomal RNA (ribonucleic acid) and proteins composed of two subunites - one large and one small
  • function: synthesis of proteins

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Mitochondria

  • located in the cytoplasm, surrounded by a double membrane
  • site of aerobic respiration, glucose and oxygen used to make ATP (energy)
  • found in large numbers in metabolically active cells e.g. muscle
  • the inner layer of the double membrane forms partitions called cristae
  • mitochondria contain some DNA

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Rough endoplasmic reticulum (RER)

  • continuous in the nuclear envelope, extending to the cytoplasm
  • complex system of membranous tubules studded with ribosomes
  • function: synthesis of proteins
  • as protein molecules are made, they are collected in the cisternae (enclosed spaces formed by the membranes) and get transported throughout the cell

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Smooth endoplasmic reticulum (SER)

  • located in the cytoplasm, structure is similar to RER but lacking ribosomes
  • function: synthesis of lipids/phospholipids/steroids, carbohydrate metabolism, transport of these materials within the cell, detoxification of drugs/poisons
  • different roles in different cells e.g. in liver cells, it breaks down toxins but in the ovary/testis, it is the site of production of steroid hormones

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Golgi apparatus

  • located in the cytoplasm, associated with the endoplasmic reticulum
  • structure: stack of flattened, membranous sacks called cisternae
  • not a permanent structure - constantly changes
  • function: modification of proteins/lipids, transports these using vesicles, these vesicles bud off from the ends of the golgi body, some of the vesicles form lysosomes that contain hydrolytic enzymes

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Centrioles

  • found in the cytoplasm of animal cells but not plant cells
  • part of the cytoskeleton usually next to the nucleus
  • found as a pair, each one composed of nine sets of triplet microtubules arranged in a ring
  • function: make and organise tiny structures called microtubules

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Vacuoles

  • located in the cytoplasm
  • membrane-bound organelle that contains fluid
  • found in both animal and plant cells, but they are not permanent in animals
  • contains the cell sap (which contains sugars, pigments and enzymes) and plants have a permanent vacuole
  • membrane that surrounds vacuole is called the tonoplast

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Vesicles

  • often produced by the golgi apparatus
  • involved in transport of proteins which can by modified
  • can contain hydrolytic enzymes which act as lysosomes (which digest and break down worn out organelles)

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Cytoskeleton

  • it is a network throughout the cytoplasm
  • a dynamic system of microtubules, microfilaments, intermediate filaments which are all made of proteins and provide structure for cells
  • function: shape and mechanical support for the cell
  • regulation of cellular activities e.g. guiding secretary vesicles
  • involved in cell movement - helps direct the movement of organelles by providing 'tracks'
  • microtubules act as 'motors' using energy from ATP to pull organelles along the tracks from one place to another
  • the cytoskeleton helps the whole cell to move

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Chloroplasts

  • found in the cytoplasm of plant cells and never animal cells
  • specialised cells containing the pigment chlorophyll
  • consists of a double membrane (envelope) surrounding it
  • site of photosynthesis
  • inside are stacks of flattened sacs called thylakoids which are stacked together to form grana
  • contains some DNA

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Cell wall

  • surrounds the plant cell and lies outside the plasma membrane but is not an organelle as it is not inside the cell (never in animal cells)
  • made up of cellulose fibres
  • function: protects the cell, maintains the cell shape, prevents excessive uptake of water - it stops the cell bursting

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Lysosomes

  • type of vesicle and only found in the cytoplasm of animal cells
  • tiny bags of digestive enzymes - single membrane-bound sacs of hydrolytic enzymes
  • they bud off the golgi apparatus
  • functio: destroy worn out or unwanted organelles within the cell (the hydrolytic enzymes break down the molecule), fuse with other vesicles that contain something that needs to be digested (process called endocytosis)

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Cilia and flagella

  • found in animal cells anchored to the cell membrane
  • core of microtubules that are long, thin extensions from the cell surface
  • microtubules arranged in a 9 + 2 pattern (9 doublets of microtubules arranged in a ring with two single microtubules in the middle of the ring)
  • function: cell mobility, move fluid across the cell surface
  • movement produced by microtubules sliding against each other

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Plant and animal cells

Plant cell

Animal cell

Nucleus

yes

yes

Cell surface membrane

yes

yes

Ribosomes

yes

yes

Mitochondria

yes

yes

Golgi apparatus

yes

yes

Rough and smooth endoplasmic reticulum

yes

yes

Cytoskeleton

yes

yes

Vacuoles and vesicles

yes

yes but not permanent

Chloroplast

yes

Cellulose cell wall

yes

Lysosomes

yes

Cilia and flagella

yes

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