Plant Organelles

Plant Structure and Function

What is a vacuole?

A fluid-filled space inside the cytoplasm surrounded by a membrane.

How do vacuoles differ in plant and animal cells?

Animal cell vacuoles are usually small and temporary, whereas plant cells usually have a large permanent vacuole.

What is the permanent vacuole?

A large fluid-filled organelle found in plant cells that contains cell sap and is surrounded by the tonoplast.

How much of the volume of a plant cell can the permanent vacuole occupy?

Up to about 80%.

What is the tonoplast?

The specialised membrane surrounding the permanent vacuole of a plant cell.

What is the function of the tonoplast?

It controls the movement of substances into and out of the vacuole and helps control the water potential of the cell.

What does the tonoplast contain?

Protein channels and carrier proteins that control movement of substances across it.

What is cell sap?

The aqueous solution that fills the permanent vacuole of a plant cell.

What does cell sap contain?

Water containing a range of dissolved substances such as ions, sugars, pigments, waste products and other chemicals.

How does the permanent vacuole help water enter a plant cell?

Dissolved substances in the cell sap lower its water potential, causing water to enter the cell by osmosis.

What is osmosis?

The net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.

What happens when water enters the permanent vacuole by osmosis?

The vacuole expands and pushes the cytoplasm against the cell wall.

How does the permanent vacuole provide support to plants?

Water entering the vacuole creates turgor pressure, pressing the cytoplasm against the cell wall and keeping cells turgid.

What does turgid mean?

Swollen and firm because water inside the cell produces pressure against the cell wall.

Why is turgor pressure important in non-woody plants?

It provides structural support and helps keep the plant upright.

What happens to a plant when its cells lose turgor?

The cells become flaccid and the plant wilts.

What is one major function of plant vacuoles besides maintaining turgor?

They store a variety of substances.

How can vacuoles contribute to the colour of plants?

They can store pigments such as betacyanin.

Where is the pigment betacyanin stored in beetroot cells?

In the vacuoles.

Why does pigment leak from beetroot cells when the tissue is heated?

Heat damages the membranes surrounding the cells and vacuoles, allowing pigment to escape.

What substances can plant vacuoles store?

Pigments, proteins, enzymes, waste products and other chemicals.

What function can vacuolar enzymes perform?

They can perform a similar digestive role to lysosomal enzymes in animal cells.

Why can storing harmful substances in vacuoles be useful?

It isolates them from the cytoplasm so they do not interfere with cell processes.

What is digitalis?

A chemical found in foxgloves that can act as a heart drug but can also be poisonous.

Where is digitalis stored in foxglove cells?

In the vacuoles.

What are chloroplasts?

Plant organelles adapted to carry out photosynthesis.

Where are chloroplasts mainly found?

In green, photosynthetic parts of plants, especially leaves.

Are chloroplasts found in every plant cell?

No. They are mainly found in cells exposed to light where photosynthesis occurs.

Why are chloroplasts important to plants?

They allow plants to absorb light energy and use it to produce organic molecules by photosynthesis.

What pigment is found in chloroplasts?

Chlorophyll.

What is chlorophyll?

A green pigment that absorbs light energy for photosynthesis.

Why are chloroplasts green?

They contain the green pigment chlorophyll.

What is the approximate size of a chloroplast?

About 4–10 µm long and 2–3 µm thick.

What is the general shape of a chloroplast?

A biconvex, disc-like organelle.

What surrounds a chloroplast?

A double membrane consisting of an outer membrane and an inner membrane.

What is the stroma?

The fluid-filled matrix inside a chloroplast.

What occurs in the stroma?

Many enzyme-controlled reactions of photosynthesis, including the light-independent reactions.

What are thylakoids?

Flattened membrane sacs inside chloroplasts that contain chlorophyll and are involved in the light-dependent reactions of photosynthesis.

What is a granum?

A stack of thylakoid membranes inside a chloroplast.

What is the plural of granum?

Grana.

Why are thylakoids arranged into grana?

The stacked membranes provide a large surface area for the light-dependent reactions of photosynthesis.

Where is chlorophyll located in a chloroplast?

In the thylakoid membranes.

What are intergranal lamellae?

Thylakoid membranes that connect different grana within a chloroplast.

What other structures may be present inside chloroplasts?

Ribosomes, DNA, starch grains and lipid droplets.

Why are starch grains found in chloroplasts?

Glucose produced during photosynthesis can be converted into starch for temporary storage.

Why do chloroplasts contain their own DNA?

They can produce some of their own proteins and replicate partly independently of the cell.

What type of ribosomes are found in chloroplasts?

Small 70S ribosomes.

What similarities do chloroplasts and mitochondria have?

Both have a double membrane, their own DNA, ribosomes and extensively folded internal membranes that provide a large surface area for reactions.

How do chloroplasts and mitochondria differ in function?

Chloroplasts carry out photosynthesis, whereas mitochondria carry out aerobic respiration.

How do chloroplasts provide a large surface area for reactions?

Their internal thylakoid membranes are extensively arranged into stacks called grana.

What does the endosymbiotic theory suggest about chloroplasts?

They evolved from free-living prokaryotic organisms that were engulfed by other cells and developed a symbiotic relationship.

What evidence supports the endosymbiotic origin of chloroplasts?

They contain their own circular DNA and 70S ribosomes and are surrounded by a double membrane.

What is the main difference between chloroplasts and mitochondria regarding pigments?

Chloroplasts contain chlorophyll, whereas mitochondria do not.

What is the main difference between chloroplasts and mitochondria regarding photosynthesis?

Chloroplasts are the site of photosynthesis, whereas mitochondria are not.

Why do some plant cells contain no chloroplasts?

They are not exposed to light or do not carry out photosynthesis.

Why do root cells generally lack chloroplasts?

Roots are usually underground and receive little or no light, so photosynthesis cannot occur.

Can some non-green plant tissues develop chloroplasts?

Yes. In suitable conditions, particularly exposure to light, some cells can develop chloroplasts and become green.

What are amyloplasts?

Colourless plant organelles specialised for storing starch.

What is the main function of amyloplasts?

To store starch.

Where are large numbers of amyloplasts commonly found?

In plant tissues specialised for starch storage, such as potato tubers.

Why are amyloplasts colourless?

They do not contain chlorophyll.

What is stored inside amyloplasts?

Starch made from amylose and amylopectin.

What are the two main components of starch?

Amylose and amylopectin.

Why is starch stored in amyloplasts?

It acts as an energy store that can later be broken down into glucose when the plant needs energy.

What happens to stored starch when a plant cell needs energy?

The starch can be converted into glucose, which can then be used in respiration.

How do amyloplasts differ from chloroplasts?

Amyloplasts are colourless and store starch, whereas chloroplasts contain chlorophyll and carry out photosynthesis.

What structures are characteristic of plant cells but not animal cells?

A cellulose cell wall, chloroplasts and a large permanent vacuole.

What structures are found in both plant and animal cells?

A cell surface membrane, cytoplasm, nucleus, mitochondria, ribosomes, rough and smooth endoplasmic reticulum and Golgi apparatus.

How does the vacuole of a typical plant cell differ from vacuoles in animal cells?

A plant cell usually has one large permanent vacuole, while animal cell vacuoles are generally small and temporary.

How does the cell wall affect the shape of a plant cell?

It provides rigidity and gives plant cells a more regular shape.

Why do animal cells not need chloroplasts?

Animals do not carry out photosynthesis and obtain organic nutrients by feeding on other organisms.

Why do plant cells still require mitochondria even though they have chloroplasts?

Plants need mitochondria to carry out aerobic respiration and produce ATP for cellular processes.

How can you identify a chloroplast in an electron micrograph?

Look for a double-membraned organelle containing stacks of internal thylakoid membranes called grana.

How can you identify a permanent vacuole in an electron micrograph?

It appears as a large, relatively empty-looking region occupying much of the cell and surrounded by the tonoplast.

How can you identify an amyloplast in a micrograph?

It appears as a colourless plastid containing large starch grains, commonly in storage tissues.