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Cell components

An organelle is a structure inside a cell that is surrounded by a membrane (or several membranes).

The membrane separates the inside of the organelle from the rest of the cell. Inside this compartment created by the organelle, organelles can carry out specialised functions.

Organelles are only found in more complex organisms (eukaryotes).

The mitochondria produce energy for the cell.

This drawing of a typical animal cell highlights some typical organelles.
This drawing of a typical animal cell highlights some typical organelles.

Organelles allow cells to separate different cellular processes. This is useful because these processes are often doing opposite things and need different chemical conditions.

Lysosomes break down old proteins and the process requires an acidic environment. The endoplasmic reticulum and ribosomes create new proteins and could not function under acidic conditions.

Organelles are analogous to organs of the human body in that each organelle has a dedicated function.

The nucleus contains the genetic information of a cell.

Only more complex organisms have a nucleus. Organisms with a nucleus in their cells are called eukaryotes, and their cells are eukaryotic cells.

The cells of animals and plants have a nucleus. Bacteria do not have a nucleus. They carry their genetic material in the main body of the cell.

The nucleus is the "brain" of the cell. It controls most of the cell's activities, such as growth, repair and cell division.

Having the cell's genetic material inside an organelle protects the genetic material against potentially harmful substances in the cytoplasm.

The nucleus can be identified by its large, spherical shape. In animal cells the nucleus accounts for roughly 10% of the cell's volume.

Some specialised cell types lose their nucleus during development. This is the case for red blood cells.

These onion cells have clearly visible nuclei (bright green-blue circles) because they have been stained.
These onion cells have clearly visible nuclei (bright green-blue circles) because they have been stained.

The nuclear envelope is the "skin" of the nucleus. It consists of two membranes on the outside of the nucleus. This is protects the DNA against damage from enzymes in the cytosol.

The nuclear envelope has small holes to allow chemical messengers in and out. These holes are called nuclear pores.

The nucleolus is a structure in the nucleus. It transcribes the RNA that is used to make ribosomes (small structures used to make proteins).

The nucleus. The nucleolus is the smaller orange sphere and occupies a significant amount of space in the nucleus.
The nucleus. The nucleolus is the smaller orange sphere and occupies a significant amount of space in the nucleus.

The mitochondrion (plural: mitochondria) is the energy-production centre of the cell.

Cells need energy to break down molecules and build new ones. Mitochondria produce energy using aerobic respiration.

Aerobic respiration produces adenosine triphosphate (ATP) molecules, which are used as an energy source by other parts of the cell.

A transmission electron microscope image and a schematic diagram of a mitochondrion.
A transmission electron microscope image and a schematic diagram of a mitochondrion.

Mitochondria are unusual in two important aspects:

  • Mitochondria contain their own genetic information.
  • Mitochondria multiply independently from the cell. This process is similar to the multiplication of bacteria.

It is commonly thought that mitochondria evolved from free living single-cell organisms with no nucleus (prokaryotes).

Mitochondria are rod-shaped cell organelles around $$5-10\mu \text{ m}$$ in length and $$0.5\mu \text{ m}$$ in diameter. A mitochondrion is enclosed by two highly specialised membranes.

The two membranes create two separate mitochondrial compartments: the inter membrane space, which lies between the inner and outer membrane, and the internal matrix in the centre.

The inner membrane has a bigger surface than the outer membrane and is folded many times. The spaces created by these folds are called cristae. More active cells have mitochondria with more cristae.

Oxidative phosphorylation (ATP production) occurs on the cristae.

The Krebs cycle (the series of reactions that generate energy) takes place in the matrix.

The chloroplast is the site of photosynthesis in photosynthesising organisms such as plants.

Chloroplasts contain a green pigment called chlorophyll that absorbs light.

Animal cells do not have chloroplasts!

Chloroplasts have a flattened disc shape (a bit like a round cheese). They are small and a cell may contain many of them.

To maximise light exposure, they orient themselves so that the flat side faces the light source. This maximises the surface area for light absorption.

Chloroplasts are around $$2-10 \, \mu \text{m}$$ in diameter.

Important! Plants still need mitochondria to convert food produced by chloroplasts into a usable energy source (ATP).

Plant cells containing chloroplasts (the small green discs).
Plant cells containing chloroplasts (the small green discs).

Like mitochondria, chloroplasts have a double membrane and reproduce independently of the cell. Similar to mitochondria, they are thought to derive from simple separate organisms.

The chloroplast is the site of photosynthesis in plants. It is surrounded by two membranes. The area between the inner and the outer membrane is the intermembrane space.

The fluid inside the inner membrane space is known as the stroma, and it contains 50% of the proteins of the chloroplast.

In the stroma there is a network of stacked discs known as thylakoids. Stacks of these discs are called grana. Thylakoids are the site of photosynthesis.

The membrane of the thylakoid contains chlorophyll, which converts sunlight into energy for photosynthesis.

The sugars produced by photosynthesis are stored as starch grains in the chloroplast.

The endoplasmic reticulum (ER) is a large network of membrane compartments. It is the largest organelle in most animal and plant cells.

Rough endoplasmic reticulum

The rough endoplasmic reticulum plays an important role in protein synthesis.

The roughness of the rough ER is due to ribosomes (responsible for assembly proteins) on the surface of the tubes making up the ER network.

The ribosomes secrete newly-made proteins into the rough ER which then modifies them and delivers them to the Golgi body. From there they are delivered to other organelles or outside the cell.

The smooth endoplasmic reticulum has no ribosomes.

The smooth ER produces lipids and is responsible for breaking down toxins.

Most cells have very little smooth ER. However, cells that are specialised on neutralising toxins, such as liver cells, may contain lots of smooth ER.

Smooth endoplasmic reticulum

The Golgi body (or Golgi apparatus) consists of a group of stacked membranes that sends proteins to their final destinations in other organelles or outside the cell.

The Golgi apparatus can be thought of as the postal centre of the cell. The ER tells the Golgi where to deliver proteins.
The Golgi apparatus can be thought of as the postal centre of the cell. The ER tells the Golgi where to deliver proteins.

The Golgi apparatus can be thought of as the postal centre of the cell:

  1. The Golgi body receives newly created proteins from the endoplasmic reticulum.
  2. The Golgi apparatus adds polysaccharides (sugars) that serve as transport markers.
  3. Proteins are packaged into vesicles; small, round organelles that store and transport substances.
  4. Once in a vesicle, the proteins can be stored or transported to their final target.

The Golgi apparatus delivers proteins to other organelles or secretes them from the cell.

Secreted proteins travel in vesicles until they reach the cell membrane.

Ribosomes produce and assemble all the proteins in the cell. All cells, even very simple ones like bacteria, have ribosomes.

Ribosomes are embedded in the endoplasmic reticulum.
Ribosomes are embedded in the endoplasmic reticulum.

Ribosomes are composed of very large proteins and ribosomal RNA. A ribosome does not have a membrane and therefore is not an organelle.

  • Membrane-bound ribosomes are attached to the rough endoplasmic reticulum. They produce proteins used in organelles or outside the cell.
  • Ribosomes give rough endoplasmic reticulum its rough appearance.

  • Free ribosomes float freely in the cell. They produce proteins used within the cell.

A lysosome is an organelle that can be thought of as a recycling and garbage disposal centre.

Lysosomes contain digestive enzymes that break down viruses, bacteria and spent molecules. Many of the small products of this process are harmless and can be reused.

The enzymes are stored in lysosomes to prevent them from breaking down the useful molecules in the cytoplasm.

The environment in the lysosome is more acidic than the cytoplasm. The pH is around 4.8, compared to 7.2 in the cytoplasm. The acidity is maintained by "pumping" hydrogen ions into the organelle.

The digestive enzymes can only function in an acidic environment. Escaping the lysosome would cause the enzymes to stop working.

Lysosomes are bubble shaped and around $${0.1-1.2\mu \text{ m}}$$ in size (around the same size as a bacterium). Lysosomes are exclusive to animal cells.

Lysosomes are membrane-bound bubbles containing enzymes.
Lysosomes are membrane-bound bubbles containing enzymes.

A vacuole is a fluid-filled sac that is present in all plant cells. Only some types of animal cells have vacuoles.

Plant cells use the vacuole

  • to store food or waste material and
  • to maintain turgidity (stiffness).

The vacuole is primarily filled with water with dissolved ions. In addition, it can contain food or waste for storage.

By filling the vacuole with water, the cell can have a bigger volume (without changing the concentration of chemicals in the cell's main body). As a result of this increase in volume, its membrane is pressed against the cell wall providing structural stability to the cell and the plant as a whole.

Animal cells cannot maintain turgidity as they have no cell wall to stop them from bursting!

This diagram shows the large vacuole of a plant cell.
This diagram shows the large vacuole of a plant cell.

Cytoplasm is the gel-like mixture that fills each cell.

The cytoplasm consists of

  • the organelles of the cells and
  • the gel-like liquid surrounding the organelles (the cytosol).

The main component of cytoplasm is water but the cytoplasm also contains a complex combination of ions, proteins, lipids and other substances.

Diagram of a plant cell. The fluid inside chloroplasts is a type of cytoplasm.
Diagram of a plant cell. The fluid inside chloroplasts is a type of cytoplasm.

Centrioles are cylindrical structures made of tube-shaped proteins called microtubules.

Microtubules are tubular structures made out of proteins.

Centrioles are present in most animal cells and are absent in plants.

Centrosomes are cell components that contain two centrioles. When animal cells divide, the centrosome helps to organise the movement of the chromosomes.

The centriole is made of microtubule bundles.
The centriole is made of microtubule bundles.

The cell wall is a protective barrier that surrounds the outside of the cell membrane in some cells.

All plant cells have a cell wall. In plants, the cell wall is made from cellulose. Cellulose is a strong polysaccharide made from glucose.

Bacteria also have a cell wall but it is not made from cellulose.

In plants, the cell wall provides support and protection.

The cell wall prevents plant cells from bursting when filled with water. It stops the membrane of plant cells from over-expanding.

Unlike the cell membrane, the cell wall does not regulate the flow of substances into and out of the cell.

Animal cells can burst because they do not have a cell wall.

Plant cells have both a cell wall and cell membrane.
Plant cells have both a cell wall and cell membrane.

The cytoskeleton is a network of protein fibres spanning the cytoplasm. It gives cells stability, shape and the ability to move.

In this magnified image of a cell, actin is stained red and microtubules are green.
In this magnified image of a cell, actin is stained red and microtubules are green.

The cytoskeleton anchors organelles in place and aids the multiplication of cells. It is made up of three types of fibres:

Microtubules play a role in cell division and serve as tracks along which organelles can travel.

A lysosome moves along a microtubule to reach a food vacuole.

Intermediate filaments help to anchor some organelles. They are permanent structures, while the other filament types are frequently reassembled.

Actin filaments within the cell give stability. This is particularly important for cells that lack a cell wall. They also play a role in cell movement.

Prokaryotic cells have cytoskeletons made of different types of fibre.

All cells have a cell membrane (also called the plasma membrane) that encloses the cytoplasm and the organelles in it.

The cell membrane is selectively permeable. This means the membrane controls what substances can move through it. This helps to maintain a stable environment within the cell.

Selective permeability is also called partial permeability or semi-permeability.

The cell membrane has two components:

  • Lipids: The cell membrane is formed from two layers of lipids (lipid bilayer). Other lipids control the flexibility of the membrane.
  • Proteins:Proteins embedded in the lipid bilayer control the permeability of the membrane.
Illustration of the cell membrane showing the lipids in red and yellow (A) and proteins in blue (B).
Illustration of the cell membrane showing the lipids in red and yellow (A) and proteins in blue (B).

Plant and animal cells do not have the same set of organelles.

Diagram showing the similarities and differences between plant and animal cells.
Diagram showing the similarities and differences between plant and animal cells.
Animal cell Plant cell
No cell wall Has a cell wall
No chloroplasts Has chloroplasts
Small/absent vacuole Cell-sap filled vacuole

Cell structures can be distinguished by their function and shape:

Component Function and shape
Nucleus Contains the cell's genetic material; large sphere
Mitochondria Produce energy for the cell; rounded rods with second folded membrane inside
Chloroplasts Produce sugars by photosynthesis; oval disk
Ribosomes Synthesise proteins; very small
Endoplasmic reticulum Folds and delivers new proteins to the Golgi body; network of membrane compartments
Golgi body Delivers proteins to their destinations; group of stacked flat membrane enclosures
Cell membrane Encloses the cytoplasm and all organelles
Cell wall Protects the cell and defines cell shape