Eukaryotic_Cell_Organisation.docx

Eukaryotic Cell Organisation

·         Animals and plants are Eukaryotes

·         Bacteria are Prokaryotes

Plasma Membrane

·         The plasma membrane of the cell encloses a viscous fluid called protoplasm

·         Protoplasm is divided into the nucleus (protoplasm enclosed within the nuclear membrane called nucleoplasm) and cytoplasm (protoplasm outside the nucleus).

Nucleus

Is a structure comprised of:

·         Nuclear membrane

·         Nucleoplasm

·         Nucleolus (variable in number)

·         Chromatin

Nuclear Membrane

·         Delimits the nuclear boundary.  It is a double membrane – similar to the plasma membrane.  Each half is around 10nm thick

·         Separated by a space of 10-15 nm wide

·         The membrane is perforated by nuclear pores – 40-50nm diameter.  These occupy up to 25% of the membrane

·         The pores allow selective exchange of materials between the nucleus and cytoplasm.  In particular they allow the exit from the nucleus of Ribonucleic Acid (RNA) molecules. 

Nucleolus

·         A darkly stained region seen only in the non-dividing nucleus

·         It is a round/oval deposit of RNA which is associated with a particular site (Nucleolar Organiser) on specific chromosomes

·         Not membrane-bound

·         Manufactures the chemical materials from which ribosomes are assembled.

Nucleoplasm

·         Communicates via the nuclear pores and bathes the other nuclear components

·         Less viscous than cytoplasm and contains fewer ribosomes

·         Concentration of enzymes for the synthesis of deoxyribonucleic Acid (DNA) and RNA is greater in the nucleoplasm than in the cytoplasm

Chromatin

·         Main chemical constituent of the nucleus

·         Is composed of DNA, RNA and protein

·         When the cell is not dividing, this material is unravelled and inter-twined to form a diffuse network

·         In the dividing cell the chromatin condenses and is concentrated into structures called chromosomes -  which carry the genetic information (genes)

Centrosome

Composed of two centrioles.  Is involved in spindle formation during cell division

Cytoplasm

Three sets of components:

A)     General ground substances

      Cytosol: is a watery fluid.  It bathes the other structures.

      Microtubules: these direct the movement of cytoplasm and act as a cell ‘skeleton’

      Microfilaments:  are contractile and act as ‘inttracellular musculature’

B)     Cell organelles

These divide the cell into different compartments where the different biochemical rections of the cell metabolism can occur.

Chloroplasts (plants only)

Lysosomes

Microbodies

Mitochondria

Ribosomes

C)    Membranes

      Within the cell is the:

-          Endoplasmic reticulum

-          Golgi Apparatus

Cell Organelles

Chloroplasts (plant cells only)

·         One of many groups know as plastids

·         All are bound by a double membrane – each around6nm thick

There are two types of plastid:

·         Leucoplasts which are colourless and concerned with storage and metabolism of oils and starches

·         Chromoplasts which contain pigments, lens-shaped (4-10µm long) e.g. chloroplasts containing the photosynthetic pigment chlorophyll giving leaves their green colour

Chromoplasts are the site of photosynthesis – the use of light energy to fix carbon dioxide from the air making it into organic compounds (by reduction) to sugars and other carbohydrates.

Photosynthesis involves two sets of reactions:

The ‘light reaction’ (photochemical)

The light energy is trapped by chlorophyll and ultimately is used to split water (photolysis) into oxygen (released to the atmosphere) and hydrogen (reduces the hydrogen carrier nicotinamide adenine dinucleotide phosphate NADP to NADPH)

The ‘dark reaction’ (enzymatic)

Uses the NADPH and ATP produced in the light reaction to reduce carbon dioxide to sugars.

Microbodies

·         Spherical structures (0.1-1.7µm diameter) bound by a single membrane

·         Sometimes called peroxisomes because they contain enzymes (peroxidises) e.g. catalase which breaks down hydrogen peroxide in the cell

·         Peroxisomes are found in animal cells and photosynthetic cells of higher plants

·         Higher plants (particularly germinating seeds which utilise fat reserves) have microbodies which contain enzymes of the glyoxylate cycle (part of the pathway by which fats and oils are converted to sugars) known as glyoxysomes

·         Smaller microbodies (microperoxisomes) are present in most cells and are involved in the breakdown of fatty acids

Mitochondria

·         Spherical.elliptical in shape (0.5µm, 1-10µm long)

·         Bounded by a double membrane

·         The outer membrane is smooth in outline.  Resembles the plasma membrane

The inner membrane is folded into finger-like projections (cristae)

Internal surface is covered by stalked granules

·         The centre of the mitochondrion is fluid-filled which bathes the inner surface of the cristae and contains mitochondrial ribosomes (attached to the inner membrane) and mitochondrial DNA.

·         Mitochondria are capable of dividing by binary fission

·         Certain stages of respiration occur within the mitochondria: pyruvic acid (from glycolysis which occurs in cytoplasm – where one glucose molecule is converted to two pyruvate molecules) enters the mitochondria and is broken down to simpler molecules to produce ‘energy’ in the form of ATP (Adenosine Triphosphate).

This involves:

·         Tricarboxylic acid cycle (also called Kreb’s cycle) – oxidises pyruvic acid (from glycolysis) and produces carbon dioxide and reducing equivalents in the form of Nicotinamide Adenine Dinucleotide NADH (occurs in mitochondrial matrix)

·         Electron transport chain (cytochrome chain) – removes hydrogen from reduced NADH or FADH2 (reduced Flavin Adenine Dinucleotide), splits it into H+ ions (protons) and electrons and passes the electrons through a series of carriers of decreasing redox potential – this is called Oxidative Phosphorylation.  The energy from passage of the electron is used to drive a proton ‘pump’ which generates ATP from ADP (Adenosine Diphosphate) and inorganic phosphate (Pi) by means of an enzyme called ATP synthase which is located on the stalked particles of the mitochondria.  The transported electron finally combines with ½ O2 which is then reduced by H+ to form water.

·         These processes of cellular respiration will not occur in the absence of oxygen.

·         This enables the cell to make much more ATP than from just anaerobic glycolysis

·         Some poisons (e.g. cyanide) work by stopping oxygen acting as the final electron acceptor and this stops mitochondrial oxidative phosphorylation.

Ribosomes

·         Not bound by a membrane

·         Composed of protein and RNA

·         Are involved in protein synthesis

·         Once the ribosomal unit is formed it moves along the mRNA stand (messenger RNA) and ‘reads’ the encoded message to synthesise protein

·         The ribosome synthesises the correct type of protein molecule (amino acids joined in correct order after being carried in by tRNA (transport RNA).  The protein remains attached to the ribosome until ribosome reaches the end of the mRNA after which the protein is released

·         The cell can speed up the process of protein synthesis by forming polyribosomes (polysomes) which are large numbers of ribosomes which read the same strand of mRNA at the same time – each ribosome moves along the same direction.

Endoplasmix Reticulum (ER)

·         An interconnected system of membranes distributed throughout the cytoplasm

·         May appear continuous with the nuclear membrane

·         On cellular disruption, the ER breaks and gives rise to the microsomal fraction

·         The ER creates enclosed or semi-enclosed spaces in the cell in the form or tubes or cisternae (cavities)

·         Compartments created by the ER allows reactions to occur in isolation from the rest of the cell

Two types of ER

Rough ER (RER) coated with ribosomes on its outer surface

·         The site of protein synthesis for secretion (export) from the cell or use within the cell (lysosomal proteins)

·         Proteins are produced on the outer surface of the RER and then pass to the ER for temporary storage before they are passed onto the Golgi Apparatus.  The ribosomes which are free in the cytoplasm synthesise proteins for use within the cell rather than export.

Smooth ER (SER) – lacks ribosomes

·         Concerned with synthesis of triglycerides and steroids and with biotransformation of harmful naturally occurring or foreign (xenobiotic) substances, e.g. drugs, alcohol, etc.

·         Smooth ER of the liver cells are particularly responsible for drug detoxification.

Lysosomes

·         Lysosomal enzyme proteins are made in the Rough ER and transported to the Golgi Body where they are packed into lysosomes which are budded off vesicles.

·         They contain mainly acid hydrolase enzymes (optimum activity at pH 5) which have ‘digested’ functions within the cell – unwanted intra-cellular material and ‘worn out’ organelles; endocytosed extra-cellular material such as large molecules, bacteria or may be secreted outside the cell to aid in extra cellular digestions of e.g. bone.

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