Cell membrane (x 12000)

4.2.2 Cell membranes Functions: 1 A boundary between the cell and its environment 2 It is selectively permeable Structure The protein-phospholipid sandwich (Davson-Danielli model)

Structure The protein-phospholipid sandwich (Davson-Danielli model) - phospholipid molecules form a bimolecular layer: hydrophobic tails - attracts water hydrophilic heads - repels water - protein layer: on either side of the phospholipid layer

The fluid-mosaic model (Singer-Nicholson model) - phospholipid molecules unchanged but is capable of movement (fluid) - protein molecules varied in size and have less regular arrangement (mosaic): some proteins occur on the surface of the phospholipid laer (peripheral); some extend into the phospholipid layer (integral); some extend completely across the phospholipid layer (transmembrane) - proteins appear dotted throughout the phospholipid layer in a mosaic pattern - proteins have many functions, e.g. support, transport and their specificity enables them to perform many specific functions which vary from cell to cell

mosaic fluid transmembrane peripheral integral

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Liver cell nucleus (EM) Nucleus (EM) (x 12000 app.)

Membranous Organelles 4.2.3 The nucleus nuclear envelope: double-membraned, with pores to allow large molecules (RNA) to pass through  nucleoplasm: cytoplasm of the nucleus chromatin: coils of DNA bound to proteins, invisible (visible as chromosomes) heterochromatin: denser, more darkly staining areas of chromatin nucleolus: 1 or 2 round bodies manufacturing ribosomal RNA and assemble ribose It is not an organelle. Why? Because it is not membrane bounded

Functions of a nucleus: 1 To contain the genetic material (chromosomes) of a cell 2 To act as a control centre for activities of a cell 3 DNA carries the instructions for the synthesis of proteins 4 Produces ribosomes and RNA 5 Essential for cell division

Chloroplast

4.2.4 The chloroplast (4-10 um long & 30 nm thick) plastids: double membraned organelles containing chlorophyll & carotenoids and carry out photosynthesis chloroplast envelope: outer membrane similar to plasma membrane, inner membrane is folded into a series of lamellae stroma: a colourless, structureless matrix (background fluid) grana: stacks of coin-like structure containing thylakoids thylakoid: flattened sacs of granum holding chlorophyll for photosynthesis, intergranal lamellae (large thylakoid) join grana together starch grains: temporary stores for the products of photosynthesis DNA & oil droplets: DNA as genetic materials of chloroplast; oil droplets as energy reserve

Mitochondria Mitochondrion x15000

Mitochondria (EM) (x 15000 app.)

4.2.5 The mitochondrion (lengths vary from 1 to 10um) Double membrane: outer controls the entry & exit of chemicals; inner is folded to form cristae to increase the surface area for respiration to occur. The surface of the cristae have stalked granules along its length. Matrix: contains protein, lipids and traces of DNA & electron dense granules; functions as sites for stages of respiration; Highly active cells may possess up to 1000 mitochondria in a single cell, e.g. sperms & muscles

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