Cell structure

Prokaryotic cell structure

Their ability to retain a dye called crystal violet classifies a group of bacteria as Gram-positive, they appear blue/violet after exposure to the dye
Slime capsule is a thick polysaccharide layer used for protection against desiccation (drying out) and phagocytosis
The cell wall acts as a protection and maintains the shape of the cell, it is made of a carbohydrate macromolecule called murein/peptidoglycan (a glycoprotein);
The plasma membrane acts as a selective membrane that lets sufficient amounts of oxygen and other nutrients enter and leave the cell as needed.
The cytoplasm holds and suspends the organelles of specialized function.
Prokaryotes do not have cytoplasmic organelles apart from ribosomes. Prokaryote ribosomes are smaller than those of eukaryotes: they are 70S whereas eukaryote ribosomes are 80S. Ribosomes are the main site for protein synthesis. This part of the cytoplasm appears darker in electron micrographs.
In many electron micrographs of prokaryotes, part of the cytoplasm appears lighter than the rest. This region is called nucleoid and contains naked DNA( DNA not associated with histone proteins).
Plasmids are accessory chromosomes which are small loops of DNA and have the drug-resistance gene.;
Plasmids are commonly used in genetic engineering to make copies of genes or large quantities of proteins or hormones. They contain genes for antibiotic resistance.
The short attachment pili (hollow protein structures) are for adhesion. Some bacteria can produce a special pilus called a conjugation or sex pilus that enables conjugation, the transfer of DNA from a donor bacterium to a recipient to enable genetic recombination.
Flagella are for locomotion, acting like a propeller.

A2.2.6 . Eukaryote cell structure

All eukaryotes have a compartmentalised cell structure

There are several advantages to being compartmentalized:

The efficiency of metabolism - enzymes and substrates for a particular process can be much more concentrated than if spread throughout the cytoplasm.
Localised conditions - pH and other such factors can be kept at optimal levels for a particular process, which may be different to the levels needed for other processes in a cell.
Toxic/damaging substances can be isolated, e.g. digestive enzymes (that could digest the cell itself) are stored in lysosomes
The numbers and locations of organelles can be changed depending on the cell’s requirements.

Features common to eukaryote cells include:

A plasma membrane enclosing a compartmentalized cytoplasm with 80S ribosomes;
A nucleus with chromosomes made of DNA bound to histones, contained in a double membrane with pores;
Membrane-bound cytoplasmic organelles including mitochondria, endoplasmic reticulum, Golgi apparatus
A variety of vesicles or vacuoles including lysosomes;
A cytoskeleton of microtubules and microfilaments.

Fig: Ultrastructure of a eukaryotic cell – a plant cell (palisade mesophyll cell of a leaf).

Organelles of Eukaryotic cells	Structure	Functions
Plasma membrane	The membrane is described as being ‘partially permeable’. The plasma membrane is formed from a bilayer of phospholipids.	All cells are surrounded by a plasma membrane which controls the exchange of materials between the internal cell environment and the external environment.
Mitochondria	A double membrane surrounds mitochondria. The inner nucleolus is invaginated to form structures called cristae. The fluid inside is called the matrix. The shape of mitochondria is variable but usually spherical or ovoid.	A site of cellular respiration in which ATP is generated.
80S Free Ribosomes	The free ribosomes appear as dark granules in the cytoplasm and are not surrounded by a membrane.	The sites of protein synthesis. Free ribosomes produce proteins used inside the cell itself. The nucleolus is involved in the production of ribosomes.
Nucleus	The nuclear membrane is double and has pores through it. The nucleus contains the chromosomes, consisting of DNA associated with histone proteins.	The nucleus is where DNA is replicated and transcribed to form mRNA, which is exported via the nuclear pores to the cytoplasm.
Smooth endoplasmic reticulum	The Smooth endoplasmic reticulum consists of a branched network of tubular membranes.	It is used to synthesize lipids, phospholipids and steroids. A special type of smooth ER stores calcium ions in muscle when it is relaxed.
Rough endoplasmic reticulum	The rER consists of flattened membrane sacs, called cisternae. Ribosomes are attached to the outside of these cisternae. They are larger than in prokaryotes and are classified as 80S	rER to synthesize protein for secretion from the cell. Protein synthesized by the ribosomes of the rER passes into its cisternae. It is then carried by vesicles, which bud of and are moved to the Golgi apparatus. These proteins are usually for use outside of the cell.
Golgi apparatus	This organelle consists of flattened membrane sacs called cisternae (as in rER). However, these cisternae are not as long, are often curved, do not have attached ribosomes and have many vesicles nearby.	The Golgi apparatus processes proteins brought in vesicles from the rER. Most of these proteins are then carried in vesicles to the plasma membrane for secretion.
Lysosomes ( absent from plant cells)	These are approximately spherical with a single membrane. They are formed from Golgi vesicles.	They contain digestive enzymes, which can be used to break down ingested food in vesicles. These enzymes can also break down organelles or even whole cells
Chloroplast ( absent from animal cells)	A double membrane surrounds the chloroplast. Inside are stacks of thylakoids, which are flattened sacs of membrane. The shape of chloroplasts is variable but usually spherical or ovoid.	They produce glucose and a wide variety of other organic compounds by photosynthesis. If chloroplasts have been photosynthesizing rapidly, they may contain starch grains.

Vacuoles & vesicles	These organelles consist of a single membrane with fluid inside. Many plant cells have large vacuoles that occupy more than half of the cell volume. Some animals absorb foods from the outside and digest them inside vacuoles.	Some unicellular organisms use vacuoles to expel excess water. Vesicles are very small vacuoles used to transport materials inside the cell.
Cytoskeleton	The cytoskeleton is constructed from several types of protein fibre. Tubulin is used to make microtubules and actin is used to make microfilaments. These structures can easily be constructed or deconstructed, so the cytoskeleton is dynamic.	Microtubules guide the movement of components within the cell. They help plant cells to construct cell walls. A layer of microfilaments just inside the plasma membrane helps animal cells to maintain their shape. Microtubules present in centrioles, cilia & flagella. Centrioles form an anchor point for microtubules during cell division and also for microtubules inside cilia and flagella.
Microtubules & centrioles		The cytoplasm of cells contains small cylindrical fibres called microtubules. They have a variety of roles, including moving chromosomes during cell division. Animal cells have structures called centrioles, which consist of two groups of nine triple microtubules. Centrioles form an anchor point for microtubules during cell division and also for microtubules inside cilia and flagella.
Cilia & flagella		These are whip-like structures projecting from the cell surface. They contain a ring of nine double microtubules plus two central ones. Flagella are larger and usually only one is present, as in a sperm. Cilia are smaller and many are present. Cilia and agella can be used for locomotion. Cilia can also be used to create a current in the fluid next to a cell.

A2.2.8 Differences in eukaryotic cell structure between animals, fungi & plants

Organelles	Plant cells	Animal cells	Fungi
Cell walls (A rigid layer outside the plasma membrane to strengthen and protect the cell)	Present made of cellulose	Absent	Made of chitin
Vacuoles (A flexible fluid compartment surrounded by a single membrane)	Large permanent used for storage of substances and pressurizing the cell	Small temporary vacuoles expel excess water or digest food or pathogens taken in by endocytosis	Large permanent used for storage of substances and pressurizing the cell
Plastids (A family of organelles with two outer membranes and internal membrane sacs)	Plastids of various types such as chloroplasts ( for photosynthesis) and amyloplasts ( to store starch)	absent	absent
Centrioles (Cylindrical organelles that organise the assembly of structures composed of microtubules)	Absent, except in plants with swimming male gametes, which have a centriole at the base of the flagellum	Present: used to construct the spindle that moves chromosomes in mitosis and the 9+2 microtubules in cilia & flagella	Absent, except in fungi with swimming male gametes, which have a centriole at the base of the flagellum
Undulipodia (Cilia and flagella used to generate movement of a cell or movement of fluid adjacent to a cell)	Absent except in plants with male gametes that swim using flagella (Tails)	Cilia and flagella are present in many animal cells, including the tail of male gametes	Absent except in fungi with male gametes that swim using flagella (Tails)

A2.2.7 Life processes in unicellular organisms

In a multicellular organism, different cell types are specialized to perform these functions, but the single cell of a unicellular organism must perform them all.

Unicellular (single-celled) and multicellular (many cells) organisms must carry out the following functions to stay alive:

Metabolism - all the enzyme-catalysed reactions occurring in a cell, including cell respiration
Reproduction - the production of offspring. It may be sexual or asexual
Homeostasis - the ability to maintain and regulate internal conditions within tolerable limits, including temperature
Growth - the permanent increase in size
Response - (or sensitivity), the ability to respond to external or internal changes (stimuli) in their environment. Thus improving their chance of survival
Excretion - the disposal of metabolic waste products, including carbon dioxide from respiration
Nutrition - the acquisition of energy and nutrients for growth and development, either by, absorbing organic matter or by synthesising organic molecules (e.g. photosynthesis)

A2.2.9 Atypical cell structure in eukaryotes:

Striated muscle fibres (fused muscle cells) are:
- Longer than typical cells
- Have multiple nuclei surrounded by a single membrane (sarcolemma)
- Striated muscle cells are formed from multiple cells which have fused that work together as a single unit
- These features challenge the concept that cells work independently of each other even in a multicellular organism
Aseptate fungi
- Fungi have many long, narrow branches called hyphae
- Hyphae have cell membranes, and cell walls and some have septa
- Aseptate fungal hyphae do not have septa, thus these cells are multinucleated with continuous cytoplasm
- The cells have no end walls making them appear as one cell
- challenges the idea that a cell is a single unit.

Red blood cells

Red blood cells, a type of animal cell, are unique in that they do not contain a nucleus
The reason for this is to enable the cell to carry a large volume of the oxygen-binding pigment haemoglobin
The biconcave shape of red blood cells means they have maximum surface area to improve their oxygen-carrying capacity

IV Phloem sieve tube elements

These serve a plant by transporting dissolved substances, such as sucrose, around the plant
These unique tissues have no end cell wall and lack many cell organelles such as nuclei, mitochondria and ribosomes
Because of the lack of their organelles, sieve tube elements can only survive due to the presence of companion cells which sit alongside next to the sieve tube elements and help to maintain the cytoplasm of the sieve tubes.