International A Level Biology / Plant structure and function

Plant cell ultrastructure

Edexcel International A Level Biology (YBI11), chapter 10: Plant structure and function, specification point 4.1, 4.2. Free lesson.

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Plant cells share most organelles with animal cells, but have extra structures including a cellulose wall, plastids and a large vacuole. Knowing what these structures do helps you identify plant cells and distinguish chloroplasts from amyloplasts.

What the eight plant structures look like

The cell wall lies outside the cell surface membrane. It is made of cellulose microfibrils in a matrix. Where two cells meet, the middle lamella is a thin layer between their walls. Plasmodesmata are channels through the wall, lined with membrane and filled with cytoplasm. A pit is a very thin region of the wall, found in pairs in neighbouring cells. A chloroplast has two membranes and stacks of flattened discs called thylakoids. Each stack is a granum. An amyloplast has two membranes but no thylakoids. It contains starch grains. The vacuole is a large space filled…

How the wall and its links work

The wall and its connections support the cell and help substances move between neighbouring cells. When water enters a plant cell, the wall pushes back against the contents. This makes the cell turgid, or firm, and stops it bursting. The cellulose wall gives strength and support, and resists the pressure inside a turgid cell. The middle lamella contains calcium pectate, which sticks neighbouring cells together like glue. Plasmodesmata connect the cytoplasm of neighbouring cells, so substances such as sucrose can pass directly between them. Pits let water and dissolved substances pass…

What the structures inside the cell do

The four structures inside the cell each have a different role. The vacuole's effect on firmness is especially important: when water enters by osmosis, it pushes the cytoplasm against the wall, making the cell turgid. This helps support the soft parts of a plant. A chloroplast is the site of photosynthesis. Chlorophyll in the thylakoids absorbs light. Chloroplasts also have their own circular DNA and ribosomes, like mitochondria. An amyloplast makes starch from glucose and stores it. Amyloplasts are common in storage organs such as potatoes and in roots. The vacuole stores…

How plant and animal cells differ

Plant and animal cells share a nucleus, mitochondria, rough and smooth ER, Golgi and ribosomes. Their main differences are the wall, plastids, vacuoles, centrioles and stored carbohydrate. Plant cells have a cellulose wall, with a middle lamella, plasmodesmata and pits. Animal cells have only a cell surface membrane. Plants have plastids, including chloroplasts and amyloplasts. Chloroplasts occur in green parts, so root cells do not have them. Plant cells have a large, permanent vacuole with a tonoplast. Animal cells have small, temporary vacuoles, if any. Most plant cells have no…

How to tell a chloroplast from an amyloplast

Both plastids have two membranes, so the membranes alone do not distinguish them. Look inside the organelle in a micrograph. The internal structures and starch grains give you the evidence. A chloroplast is found in green parts such as leaves. It has grana, which are stacks of thylakoids, and may contain a few small starch grains. An amyloplast is found in storage tissue and roots. It has no thylakoids, no green colour and is packed with large starch grains. Amylose and amylopectin are molecules. The organelle that stores starch is an amyloplast.

How to explain why roots have amyloplasts

Roots are underground, so they do not receive the light needed for photosynthesis. They can still receive sucrose made in the leaves, which can be stored as starch.

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