Cell Structure

Cell theory is based on three concepts:

1. All organisms are composed of one or more cells.
   • Unicellular (one celled) organisms include bacteria and protists (small eukaryotes, such as amoebas).
   • Multicellular (many celled) organisms include plants and animals.
2. The cell is the fundamental unit of organisms.
   • A cell is the basic structural unit in organisms.
   • It is contained within a plasma membrane which maintains the internal environment.
   • The cell is capable of metabolic reactions.
3. New cells can only arise from pre-existing cells.
   • All cells must arise by division of pre-existing cells.

Modern interpretations of the cell theory include three additional concepts:

1. Energy flow occurs within cells.
   • Cells perform chemical reactions, which involves transfer of energy.
   • The total activity of an organism is dependent on the activity within individual cells.
2. Heritable information is passed to daughter cells when a cell divides, in the form of DNA.
   • Cells divide to produce daughter cells
   • Each daughter cell contains a copy of the original parental cell’s DNA, i.e. a heritable ‘blueprint’ for the cell.
3. Cells have a similar chemical composition.
   • Cells share features across species such as the basic structure, including the cell membrane and organelles, the genetic code, metabolic activity, and the ability to divide.

Membrane-bound organelles are exclusive to eukaryotic cells. They have a specialised function and an internal environment that differs from its surroundings.

Specialised macromolecular assemblies are not bound by a membrane and examples include ribosomes, cytosol, the cytoskeleton, the plasma membrane and the cell wall.

The nucleus stores the majority of DNA within chromosomes.

Transcription of DNA into mRNA takes place within the nucleus.

mRNA is processed to protect it from immediate degradation within the cytoplasm, where it is translated by ribosomes.

The mitochondria is the site of cellular respiration – a process that converts energy stored in chemical bonds of biological fuels into chemical bonds of a molecule called adenosine triphosphate (ATP).

Glucose undergoes glycolysis before entering the Krebs cycle and is then later converted into ATP.

Glycerol, fatty acids and amino acids may enter the Krebs cycle at different points and are also converted to ATP.

ATP can be directly used by most cellular enzymes.

The number of mitochondria is directly proportional to the energy demands of the cell.

The mitochondrial matrix contains:

• Proteins
• A small amount of mitochondrial DNA (mtDNA) chromosomes which are circular, small and encode only 13 proteins (involved in respiration)
• Ribosomes which translate the mtDNA.

Note: the majority of proteins in mitochondria are encoded by nuclear DNA, translated in the cytosol and then transported.

Chloroplasts are the main site of photosynthesis, a process that converts solar energy to chemical energy by synthesising sugars from carbon dioxide and water.

Chloroplasts are unique to plant cells and some algae.

Smooth ER has a variety of metabolic functions, which depend mostly on the cell type. These include:

• Synthesis of all lipid and steroid hormones (ovarian, testicular and adrenal cells)
• Carbohydrate metabolism (liver and kidney cells)
• Detoxification (liver cells)
• Calcium ion storage (neurons and muscle cells)

Rough ER has a variety of different functions and structural features.

The cytosol-facing side of its membrane contains attached ribosomes, making its surface appear rough when seen through a microscope. These ribosomes are mostly involved in translating proteins that are secreted by the cells.

The synthesised polypeptide chain moves from the ribosome through a pore into the ER lumen. Here, the polypeptide folds and becomes a functional protein.

The rough ER membrane continuously folds to form buds, which are released as membrane-bound transport vesicles. As they bud off, transport vesicles take some of the contents of the ER lumen with them. These vesicles can travel to the outside of the cell through the Golgi apparatus.

At the Golgi apparatus:

• Vesicles arrive at the cis face and fuse with the Golgi membrane, releasing their contents.
• Contents undergo processing and maturation: for example, proteins can pick up post-translational modifications, carbohydrates can have their sugar monomers exchanged, and lipid composition can be altered.
• The processed molecules reach the trans face and are sorted and packaged into new vesicles.
• Many of the vesicles contain proteins with specific signal sequences (molecular tags ~20 amino acids long) that target them to the plasma membrane. They are called secretory vesicles. The contents are released by exocytosis, and the cell membrane is regenerated.

Membrane-embedded proteins can trigger release of vesicles at organelles, or movement back to the cis face of the Golgi apparatus, depending on the molecular tag.

Features of the cell membrane, such as protein ion channels, are formed through the same process but they are only partly moved into the ER lumen, effectively embedding them in the ER membrane. When the vesicle becomes part of the cell membrane, they also become a fixed feature.

Lysosomes are a specialised type of vesicle containing liquid and hydrolytic enzymes that can digest proteins, lipids and carbohydrates. They function at acidic pH, while the rest of the cell maintains a neutral pH.

Lysosomes are necessary for intracellular digestion. They break down damaged or excess cell components, destroy viruses and bacteria and recycle useful building blocks, such as amino acids from redundant proteins. Lysosomes can release amino acids, carbohydrate monomers and simple fatty acids, into the cytosol for further processing.

Phagocytosis is the engulfing of smaller organisms or non-living organic material into small vesicles called food vacuoles. When these vacuoles fuse with lysosomes, food within is exposed to the lysosomal enzymes. These enzymes are highly resistant to degradation, which prevents lysosomal proteases from digesting themselves.

Vacuoles are very large vesicles. There are several different types, including:

• Food vacuoles, which result from phagocytosis
• Contractile vacuoles, which are mainly found in freshwater, unicellular eukaryotes
• Central vacuoles, which are found in plants and fungi.

In freshwater conditions, water is constantly entering the cell via osmosis. To maintain an optimal ion concentration within the cell, a contractile vacuole pulsates and expels the excess water out of the cell.

Central vacuoles are the largest of vacuoles, typically formed by fusion of many smaller vacuoles. In plants, the solution inside a central vacuole is what we recognise as the plant’s sap, juice or oil. It acts as a storage of sugars, proteins, lipids, inorganic compounds, pigments to help attract pollinators and toxins to deter herbivores. Central vacuoles are also important in growth: as the vacuole absorbs water, the cell becomes larger.

Ribosomes are the site of mRNA-to-protein translation.

They are found free-floating in the cytoplasm or bound to the cytoplasm-facing side of the rough ER. The number of ribosomes in a cell is correlated with its rate of protein synthesis.

The cytoplasm is all the contents within a cell, between the cell membrane and the nuclear envelope.

The cytoplasm includes all organelles (except the nucleus), and a jelly-like liquid in which they are all suspended. This liquid is the cytosol.

The cytosol is mainly water but also contains ions, soluble proteins and their complexes, and many other macromolecules.

The size and volume of macromolecules is significant enough to induce an effect called macromolecular crowding, which means that proteins within a cell are more likely to assemble larger structures (both functional protein complexes and aggregates) and polypeptide chains are more likely to fold into three-dimensional shapes. As a result, metabolic pathways are more efficient than they would be in a test tube within an ideal solution.

Microtubules radiate from a centrosome, a region typically located near the nucleus.

The centrosome consists of two centrioles, which are simply rings of microtubular tubes. Centrioles are the seeding point of microtubule growth and organise the mitotic spindle during cell division.

They are found in flagella and cilia – microtubule projections covered by the plasma membrane that extend from eukaryotic cells. Flagella are much longer than cilia and a cell typically has one flagellum.

Flagella are involved in cell motility (movement). A cell can have hundreds or thousands of cilia, which can be motile or non-motile. Motile cilia can move cells or can be fixed in a tissue layer, moving fluid over the tissue surface (e.g. in the trachea).

Non-motile or sensory cilia are rich in membrane receptors that detect and send signals about the cell’s environment.

Actin filaments reside near the plasma membrane.

The outer layer of the cytoplasm is denser and more gel-like than the inner layer. They support the cell’s shape by resisting tension forces and assist changes in cell shape in muscle cell contraction, pinching of the cell during division, and extension of the plasma membrane to engulf bacteria in phagocytosis.

Intermediate filaments are similar to actin filaments with respect to their ability to resist tension.

They are found just below the plasma membrane; but they are also found within and around organelles, which helps keep organelles in place and tether them to the rest of the cytoskeleton. For example, the nuclear lamina is made entirely of intermediate filaments.

The plasma membrane is a phospholipid bilayer at the boundary of each cell that separates its internal contents from the external environment. Plasma membranes have many components, including:

Transmembrane proteins that span the whole width of the plasma membrane and peripheral or integral proteins, which face only the extracellular or intracellular face of the membrane, respectively.

Membrane proteins have many functions, including, but not limited to transportation, metabolism, selectivity, signalling and adhesion.

The plasma membrane is a phospholipid bilayer at the boundary of each cell that separates its internal contents from the external environment. Plasma membranes have many components, including:

Glycolipids coat the external surface of the plasma membrane. These are lipids with a carbohydrate attachment and are crucial for cell recognition. Such interactions are important for immune cells to recognise invading bacteria or senescent cells, and can trigger processes like phagocytosis, cell division or apoptosis (cell death).

Cell walls are only found in plants, fungi and some unicellular eukaryotes.

The cell wall is a rigid extracellular structure that protects the cell, supports its shape and prevents excess water influx. It is ~1000 times thicker than the plasma membrane.

The cell wall is composed of cellulose (plants) or chitin (fungi) microfibres embedded in a matrix of other polysaccharides and proteins.

In plants, the cell wall enables growth despite the pull of gravity.