3.2.1.1 Structure of eukaryotic cells

Each type of cell has a special internal structure suited to its job. This is known as the cell ultrastructure. Eukaryotic cells have a distinct nucleus and membrane bound organelles. The most important stuffs in the cell are as follows:

• cell-surface membrane (covered in 3.2.3)
• nucleus (containing chromosomes, consisting of protein-bound, linear DNA, and one or more nucleoli) 
• this contains the organisms hereditary material, manufactures rRNA and ribosomes, and acts as the control centre of the cell through the production of mRNA and tRNA (hence protein synthesis)
• has a nuclear envelope (double membraned) whose outer surface is continuous with the endoplasmic reticulum. This controls the entry/exit of materials in and out of the nucleus
• Nuclear pores allow the passage of large molecules
• The nucleoplasm makes up the bulk of the nucleus (granular jelly-like material)
• Chromosomes consist of protein (histone) bound linear DNA
• mitochondria
• These are the sites of the aerobic stages of respiration so are responsible for the production of ATP.
• These are double membraned. This controls entry and exit of material. The inner membrane is folded to form cristae which provide a large surface area for attachment of enzymes involved in respiration (respiration occurs on the mitochondrial membrane)
• The matrix contains proteins, lipids, ribosomes, and DNA which allows the mitochondria to control the production of some of their own proteins.
• chloroplasts (in plants and algae)
• These are organelles that carry out photosynthesis
• the chloroplast envelope is a double plasma membrane that surrounds the organelle. Being highly selective, it controls what enters and exits the chloroplast
• the grana are stacks of up to 100 disc like structures (thylakoids). Within thylakoids there is chlorophyll (the photosynthetic pigment). The granal membranes provide a large surface area for the attachment of chlorophyll/electron carriers/enzymes that carry out the light-dependant reactions
• the stroma is a fluid-filled matrix where the second stage of photosynthesis occurs. The stroma contains other structures such as starch grains. The stroma possesses all the enzymes needed to make sugars in the second stage of photosynthesis.
• They contain DNA and ribosomes to quickly and easily manufacture some of the proteins needed for photosynthesis
• Golgi apparatus and Golgi vesicles
• This is similar to the SER but is more compact. It consists of a stack of membranes that make up flattened sacs (cisternae) will vesicles (small rounded hollow structures called vesicles).
• The proteins and lipids produced by the ER are passed through the Golgi apparatus which modifies the proteins (e.g adding prosthetic components) and labels them allowing them to be accurately sorted and sent to their correct destinations. Once sorted the proteins and lipids are transported in Golgi vesicles.
• The Golgi apparatus can:
• add carbohydrate to proteins to form glycoproteins
• produce secretory enzymes
• secrete carbohydrates 
• transport, modify, and store lipids
• form lysosomes
• lysosomes (a type of Golgi vesicle that releases lysozymes) 
• lysosomes are formed when the vesicles produced by the Golgi apparatus contain enzymes (e.g proteases and lipases).
• They hydrolyse material ingested by phagocytic cells
• they release enzymes to the outside of the cell (exocytosis) to destroy external material
• they digest worn out organelles
• they completely break down cells (autolysis)
• ribosomes 
• small cytoplasmic granules with two subunits - one small and one large. Each subunit contains rRNA and protein. They are the site of protein synthesis.
• may be associated with the RER
• 80S are larger and occur in eukaryotic cells
• 70S are smaller and occur in prokaryotic cells, mitochondria, and chloroplasts
• rough endoplasmic reticulum and smooth endoplasmic reticulum
• This is a system of membranes that spread through the cytoplasm of the cells. It is continuous with the outer membrane and encloses a network of tubules and flattened sacs called cisternae
• The rough endoplasmic reticulum has ribosomes present on the outer surface of the membranes. It provides a large surface area for the synthesis of proteins and glycoproteins and provides a pathway for the transport of materials through the cell (especially proteins)
• The smooth endoplasmic reticulum lacks ribosomes on its surface and might be more tubular in appearance. It synthesises, stores, and transports lipids and carbohydrates.
• cell wall (in plants, algae and fungi)
• consists of microfibrils (cellulose) embedded in a matrix. microfibrils have considerable strength and so contribute to the overall strength of the cell wall
• The cell wall consists of a number of polysaccharides (such as cellulose) and there is a thin layer (the middle lamella) which marks the boundary between adjacent cell walls and cements adjacent cells together
• The cell wall provides mechanical strength in order to prevent the cell bursting under changing osmotic pressure, gives mechanical strength as a whole to the plant, allows water through to contribute to the movement of water through the plant
• cell vacuole (in plants)
• a fluid-filled sac bounded by a single membrane (tonoplast)
• contains a solution of mineral salts, sugars, amino acids, wastes, and sometimes pigments such as anhthocyanins
• they support herbaceous plants by making cells turgid
• the sugars and amino acids can act as a temporary food store
• the pigments may colour petals which might attract pollinating insects

In complex multicellular organisms, eukaryotic cells become specialised for specific functions. Specialised cells are organised into:

• tissues
• for working efficiency cells are often aggregated together. A collection of similar cells that perform a specific function is known as a tissue. Examples include:
• Epithelial tissue
• xylem
• organs
• tissues are aggregated into organs. It therefore follows that an organ is a combination of tissues that are coordinated to perform a variety of functions (although they often have one predominant major function). Examples include:
• the stomach
• a leaf
• systems
• organs work together as a single unit known as an organ system. Examples include:
• the digestive system
• the respiratory system
• the circulatory system