Cell Structure - Basics

Cell Membrane

  • Phodpholipid bilayer with embedded proteins.
  • Selectively permeable - enables control of passage of substances in and out of cell.
  • Barrier between internal and external environment of cell.
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The Nucleus

Function:

  • Acts as the control centreof the cell making RNA (mRNA and tRNA) that is needed for protein synthesis.
  • Retains the genetic material of the cell in the form of DNA/Chromosomes.
  • Manufactures ribosomes and RNA

Main Features:

  • Nuclear Envelope - Double envelope to control what enters and leaves the nucleus and contain the reactions within.
  • Nuclear Pores - Small enough (40-100nm) to allow large molecules such as RNA to pass out of the nucleus.
  • Nucleoplasm - Jelly like material rich in chromatin (DNA & proteins).
  • Nucleolus - Dense sperical patch which is the site of RNA and ribosome synthesis.
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Rough Endoplasmic Reticulum

Functions:

  • The site of protein synthesis (membrane proides a large surface area this process)
  • Provides a pathway for transport of materials especially proteins through the cell.

Main Features:

  • Consists of folded sheets of membranes forming flattened sacs called cisternae.
  • The membranes are continuous with the outer membrane of the nuclear envelope and are covered in small granullar structures called ribosomes.
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Smooth Endoplasmic Reticulum

Functions:

Involved in the synthesis, transport and storage of lipids and carbohydrates (membranes increase surface area for these processes).

Main Features:

  • Consists of folded sheets of membranes forming flattened sacs called cisternae.
  • There are no ribosomes attatched to the membranes.
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Golgi Body/Apparatus

Main Features:

Consists of folded sheets of membranes forming flattened sacs called cisternae. The membranes are curved and are found in smaller patches than ER (Endoplasmic Reticulum)

Main Functions:

  • Modifies proteins often adding carbohydrates to form glycoproteins.
  • Produces secretory enzymes.
  • Forms Lysosomes.
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Mitochondria

Functions:

The site of the reactions of aerobic respiration which makes ATP for processes such as active transport.

Main Features:

  • Double Membrane - The outer membrane controls what enters and leaves the organelle. The inner membrane is folded to form extensions known as cisternae.
  • Cristae - Infoldings of the inner membrane provide increased surface area for the attatchment of enzymes involved in respiration.
  • Matrix - Fairly rigid cytoplasm like material containing traces of DNA and ribosomes (70s) allowing mitochondria to produce their own proteins. The matrix contains many enzymes involved in respiration.
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Chloroplasts

Main Features:

  • Double Membrane - The outer membrane controls what enters and leaves the organelle. The inner membrane is folded inwards to form flattened sacs called thylakoids which make up dark areas called the grana.
  • Chlorophyll - Attatched to the thylakoid membranes which are the site of the first stage of photosynthesis.
  • Stroma - The jelly like material filling the chloroplast is rich in photosynthetic enzymes and contains DNA and small 70s ribosomes allowing proteins to be made.
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Ribosomes

Main Features:

  • Tiny Granules - Around 25nm in diameter. Made of rRNA and protein. They have a large subunit and a small subunit.
  • Found free in the cytoplasm and attatched to the membranes of the rough ER and to the outer membrane of the nuclear envelope.

Function:

  • Protein Synthesis
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Lysosomes

Formed when the vesicles produced by the Golgi Apparatus/Body have enzymes within them such as proteases and lipases.

Main Features:

  • Small spherical sacs surrounded by a single membrane.
  • They contain digestive enzymes including protease and lipase.

Functions:

  • Autolysis - Digestion of old or damaged cells and organelles.
  • Phagocytosis and digestion of viruses and bacteria (in white blood cells)
  • Release enzymes outside cells (exocytosois) to destroy material around cells.
  • Abundent in secretory cells and in phagocytic cells.
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The Cell Wall

Functions:

  • To provide mechanical strength in order to prevent the cell bursting under pressure created by the osmotic entry of water.
  • To give mechanical strength to the plant as a whole.
  • To allow water to pass along it and so contribute to the movement of water through the plant.

Main Features:

  • In plants - consists of microfibrils of cellulose embedded in a matrix. Cellulose microfibrils have high mechanical strength. There is a thin layer, in the middle lamella, which marks the boundry between adjacent cell walls and cements adjacent walls together.
  • In Algae - Consists of either cellulose or glycoproteins, or a mixture of both.
  • In fungi - Consits of a mixture of chitin (a nitrogen-containing polysaccharide), glycan (polysaccharide) and glycoproteins.
  • Does not contain cellulose.
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Vacuoles

Main Features:

  • A fluid-filled sac bounded by a single membrane called tonoplast.
  • A plant vacuole contains a solution of mineral salts, sugars, amino acids, waste and sometimes pigments.

Functions:

  • Supports plants by making cells turgid
  • May act as a temporary food store (sugars and amino acids)
  • Pigments may colour petals to attract pollinating insects.
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