sábado, 31 de octubre de 2009

The Ultrastructure of Plant Cells

Cell wall: made of rigid cellulose. This makes the plant a more uniform and gives a regular shape. The ones of neighboring cells are cemented together by a thin layer of pectic substances which form the middle lamella.
Plasmodesmata: fine membrane-lined pores through which pass cytoplasmic threads. Each one has a central tubular core. It enables substances to be transported easily.
Chloroplast: large organelle where photosynthesis takes place. It has an envelope of two membranes and contains stroma (gel-like; contains ribosomes, DNA and photosynthetic enzymes; stores starch and lipids) and a system of membranes. The membranes contain chlorophyll which harvests light energy for photosynthesis.
Large central vacuole: sac bounded by a membrane (tonoplast). It contains sap (watery solution of various substances). When full of sap, the vacuole causes the cell surface membrane to press against the cell wall (pressure helps to support plant). Sometimes it functions as a lysosome.

Eukaryotic cells
These types of cells are double membraned, with a well defined nucleus containing genetic material. For example: plants, animals, fungi, protoctists.
Prokaryotes cells
These types of cells lack the double membrane organelles and do not have a membrane-bound nucleus.
Bacteria cell
Cell surface membrane: encloses the cytoplasm. Contains enzymes, ribosomes and food granules.
Cell wall: surrounds the cell surface membrane.
Capsule: encloses the cell wall.
Simple flagella
• Nucleoid: Single strand of DNA coiled into the centre of the cell.
Plasmids: additional circular pieces of genetic material.

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