3.2.1.1 Eukaryotic Cell Structures

The Nucleus

Structure:

• Contains the nucleolus (for rRNA manufacture & ribosomes assembly)

• Nuclear envelope = double membrane that surrounds the nucleus + has ribosomes on the surface

• Nuclear pores allow the passage of large molecules (e.g. mRNA)

• Nucleoplasm = granular, jelly-like material → bulk of the nucleus

• Contains genetic information in the form of chromosomes (protein-bound, linear DNA)

Function:

• Controls cell activities → contains DNA with instructions to produce protein

• (Nucleolus) synthesises ribosomes & ribosomal RNA

Mitochondria

Structure:

• Surrounded by a double membrane in which the inner membrane is folded to form structures known as cristae

• Cristae → provides large SA for attachment of enzymes & other proteins involved in respiration

• Contain an enzyme-rich liquid called matrix — makes up the remainder of the cell

• Contain their own DNA + ribosomes

Function:

• Site of aerobic respiration — Produces ATP.

Chloroplast

Only difference between plant cell & algae cell is they have different shape chloroplasts.

Structure:

• The chloroplast envelope — a double plasma membrane that surrounds the organelle (highly selective = high entry / exit requirements)

• Contain fluid-filled sacs known as thylakoids which are stacked up to form grana → connected by lamella (intercellular “glue”)

• Fluid-filled matrix (contains enzymes needed to make sugars) within known as stroma → 2^nd stage of photosynthesis takes place here

• Contain their own DNA + ribosomes.

Function:

• Site of photosynthesis — These reactions take place in the grana and stroma

Rough Endoplasmic Reticulum (RER)

Structure:

• Contains a network of membranes enclosing a fluid-filled space, known as cisternae.

• The surface of the cisternae is covered with ribosomes.

Function:

• Provide large SA for the synthesis and transport of proteins - The proteins are made using the ribosomes.

• Provide a pathway for the transport of materials especially proteins, throughout the cell

Smooth Endoplasmic Reticulum (SER)

Structure:

• Same as the rough endoplasmic reticulum without the ribosomes.

• More tubular in appearance

Function:

• Synthesis, storage, and transport of lipids and carbohydrates - For example, cholesterol and steroid hormones.

Golgi Apparatus

Structure:

• Similar to SER except it’s more compact.

• Contain fluid-filled, membrane-bound sacs known as cisternae.

• Vesicles — small rounded hollow structures → may move to cell surface, where they fuse with the membrane + release their contents to the outside of cell.

Functions:

• Add carbohydrate to proteins to form glycoproteins

• Produce secretory enzymes (e.g. those secreted by the pancreas)

• Secrete carbohydrates, such as those used in making cell walls in plants

• Transport, modify & store lipids = “post office” → ‘labels’ glycoproteins to sort accurately and sent to their correct destination

• Form lysosomes (specialised vesicles)

Especially well developed in secretory cells → e.g. epithelial cells that line the intestines.

Lysosomes

Formed when vesicles produced by Golgi apparatus contain enzymes such as proteases & lipases.

Structure:

• Contain hydrolytic enzymes + lysozymes (as many as 50).

• Surrounded by a membrane to keep enzymes separate from the cytoplasm of the cell.

Functions:

• Digest pathogens — This process uses enzymes.

• Break down waste material — This material includes old organelles and cells → useful chemicals are made + can be reused

• Release enzymes to the outside of the cell (exocytosis) in order to destroy material around the cell

• Completely break down cells after they have died = autolysis

Especially abundant in secretory cells, such as epithelial cells & phagocytic cells.

Ribosomes

May be in the cytoplasm OR associated with the RER + two types:

• 80S — found in eukaryotic cells = around 25mm in diameter

• 70S — found in prokaryotic cells, mitochondria & chloroplast = slightly smaller.

Structure:

• Made up of proteins and rRNA (ribosomal RNA) — made in the nucleolus.

• Consist of a large and a small subunit.

• Not surrounded by a membrane.

• Occur in vast numbers → may account fo rup to 25% of the dry mass of a cell.

Function:

• Site of protein synthesis - Involved in the process of translation.

Cell Wall

Structure:

• Made up of microfibrils of the polysaccharide cellulose, embedded in a matrix (for plants)

• Contains channels (gaps) known as plasmodesmata (or plasmodesma if just one).

• Middle lamella — thin layer, which marks the boundary between adjacent cell walls and cement adjacent cells together.

Functions:

• Supports the cell - Contents of the cell press against the cell wall to make it rigid.

• Prevents the cell from bursting (mechanical strength) - The cell wall can withstand high osmotic pressure.

• Allows exchange of substances between cells - Plasmodesmata connects neighbouring cells.

Cell wall of algae: cellulose / glycoproteins or a mixture of both

Cell wall of fungi: nitrogen containg polysaccharide called chitin

Vacuoles

Structure:

• Contains cell sap (solution of mineral salts, sugars, amino acids, wastes & sometimes pigment such as anthocyanins).

• Surrounded by a single selectively permeable membrane known as a tonoplast.

Function:

• Helps to maintain pressure within the cell, which keeps the cell rigid and stops the plant from wilting.

• Support herbaceous plants & herbaceous parts of woody plants → by making cells turgid

• The sugars & amino acids may act as a temporary food store

• The pigments (anthocyanins) may colour petals to attract pollinating insects