Subcellular Components & Organelles

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The Nucleus

The nucleus is a big organelle that contains the cell’s DNA and is surrounded by a nuclear envelope. The largest sub-structure within the nucleus is the nucleolus.

Structure of the nuclear envelope

The nuclear envelope is a double-membrane structure that has a number of pores.
Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers.

Function of the nuclear envelope

The pores in the nuclear envelope control the passage of ions, molecules and RNA between the nucleoplasm and cytoplasm.

Structure of the nucleus

The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus.
In eukaryotes, the nucleus contains linear chromosomes that are made up of DNA.
There is an area within the nucleus called the nucleolus (plural = nucleoli).

Function of the nucleus

The nucleus controls the actions of the cell.
DNA in the nucleus contains instructions for the synthesis of proteins.
Ribosomal RNA is joined together with associated proteins in the nucleolus to assemble the ribosomal subunits.

Structure of the nucleolus

The nucleolus is the largest sub-structure within the nucleus.
The nucleolus is made of proteins, RNA and DNA.

Function of the nucleolus

The main role of the nucleolus is to act as the site at which ribosomes are made.
The nucleolus also has secondary roles, such as immobilizing proteins and forming signal recognition particles.

Mitochondria and Chloroplasts

Mitochondria produce adenosine triphosphate (ATP) via aerobic respiration. Chloroplasts are plant cell organelles that carry out photosynthesis.

Structure of mitochondria

Mitochondria are oval-shaped, double-membrane organelles that have their own ribosomes and DNA.
Each membrane is a phospholipid bilayer embedded with proteins.
- The inner layer of the membrane has folds called cristae, this increases the surface area available for ATP synthesis.
The area surrounded by the folds is called the mitochondrial matrix.
- The mitochondrial matrix contains enzymes used for respiration, such as those required for the Krebs cycle.

Function of mitochondria

Mitochondria make ATP via aerobic respiraton.
- Muscle cells have a very high concentration of mitochondria because they are highly active and need a lot of energy to keep the body moving.

Structure of chloroplasts

Chloroplasts have their own DNA, ribosomes and have inner and outer membranes.
The space enclosed by the inner membrane contains a set of interconnected and stacked fluid-filled membrane sacs called thylakoids.
- Each stack of thylakoids is called a granum (plural = grana).
- Grana are linked by lamellae.
- Lamellae are flat, thin parts of thylakoid membrane.
The fluid enclosed by the inner membrane that surrounds the grana is called the stroma.

Reactions of photosynthesis

The light-dependent reaction of photosynthesis occurs in the grana. The thylakoid membrane contains chlorophyll to absorb light and the electron transport proteins required for photosynthesis.
The light-independent, or carbon-fixation, reaction of photosynthesis occurs in the stroma.
- These reactions are covered in greater detail in the "cellular energetics" unit coming up.

Function of chloroplasts

Photosynthesis is the series of reactions that use carbon dioxide, water and light energy to make glucose and oxygen.
This process happens in chloroplasts and allows plants to make their own food (e.g. sugars).
Chloroplasts are found in plant and algal cells but not in animal cells.