# Structure of a cell

Structure and function of each type of organelle




Cell wall
All plant cells are surrounded by cell wall made of cellulose. Cellulose is a carbohydrate (polysaccharide). Animal cells never have cell walls. Cellulose form fibres, those make the cell wall very strong. This helps to protect and support the cell. It also prevents the cell from bursting when it absorbs too much water. Fibres have big spaces that make it easy for large molecules to go through the cellulose cell wall. Therefore the cell wall is fully permeable.

Cytoplasm
Cytoplasm is a complex solution Cytoplasm is a clear jelly. About 70% of it consists of water. It contains many substances dissolved in it, especially proteins. Many metabolic reactions take place in the cytoplasm.

Vacuole
Most cells contain vacuoles A vacuole is a space in a cell, surrounded by a membrane. Most plant cells have a big vacuole filled with a fluid called the cell sap. The fluid inside the vacuole contains a solution of sugar and other substances. The vacuole membrane in plant cells is called the tonoplast. Animal cells have much smaller vacuoles, which may contain nutrients of water.

Lysosome
• Spherical vesicles • Contain digestive enzymes which are produced in the Golgi apparatus or ER

• Breakdown of imported food vacuoles • Cause self-digestion of broken-down organelles

Chloroplasts'
Chloroplasts trap energy of sunlight Chloroplasts are never found in animal cells. All green parts of plants contain cells with chloroplasts that contain the green pigment chlorophyll. Chlorophyll absorbs sunlight, and the energy of sunlight is then used to produce sugars during the process of photosynthesis. Starch grains are formed during the process of photosynthesis and are used to store the sugars (in the form of starch) that are produced during photosynthesis.

Mitochondria
Mitochondria release energy from food Most cells contain mitochondria. The mitochondrion is the organelle in which aerobic respiration takes place. This process releases energy after the break down of glucose in the cells for the different cell activities. The more active a cell, the more mitochondria it has.

Nucleus
The nucleus stores inherited information The nucleus stores the chromosomes. Chromosomes are very long, but thin and cannot easily be see with a light microscope. However, when a cell divides, the chromosomes become short and thick and can be seen with a good light microscope. Chromosomes carry the inherited information and are responsible for the formation of proteins in organisms.

• Cavities filled with cell sap/fluid containing water and dissolved solutes e.g. glucose, salts, amino acids, hormones • Tonoplast membrane surrounds the cell sap • Small vacuoles in young plant cell untie to form a large vacuole, filling up to 80% of cell volume. Often formed by Golgi apparatus

Nucleus in plant and animal cells • Largest organelle in eukaryotic cells • Surrounded by double membrane called nuclear envelope, similar to cell membrane. • Membrane is perforated by many pores which allow the exchange of substances between nucleus and cytoplasm • Nucleoplasma with chromosomes • Carry genetic material in chromosomes • Controls hereditary • Controls the structure, functions and activities of the cell • Regulates the production of all proteins including enzymes and hormones • Essential during cell division Most cells contain nuclei Exceptions: Mature red blood cells do not contain a nucleus cells and sieve tube

Cell membrane in plant and animal cells
Made up of two groups • Phospholipids: double layer (bi-layer) with hydrophobic lipid tails (repel water) are directed inwards hydrophilic phosphate heads (attract water) are directed outwards • Proteins: irregular arrangement, some on the surface, others extended across • Surrounds cell content • Assists in active transport of material across the membrane • Regulates substances passing in and out of the cell because it is partially permeable

Whatever type of animal or plant they come from, all cells have a cell surface membrane surrounding the cell. Inside the cell membrane is a jelly-like substance called cytoplasm, in which many small structures are found, the organelles. The most obvious of these is usually the nucleus. The cell membrane sometimes is also called the plasma membrane. In plant cellist is difficult to see the membrane because it is right against the cell wall. The cell surface membrane is a very thin layer of protein and fat. It controls the transport over the membrane. The membrane is partially permeable, which means it will let some substances through and others not.

Mitochondria in plant and animal cells • Cylindrical, rod-shaped organelles • Large organelles • Found in all cells • Has double membrane: outer membrane is smooth and selective permeable, inner membrane is infolded to form cristae where ADP combines with P to form ATP • Inside contains a aqueous solution of metabolites and enzymes, called the matrix which controls the Krebs cycle • Releases energy while breaking down glucose during the process of aerobic respiration • NB: the amount of mitochondria in a cell is determined by the activities and energy requirements of the cell
Endoplasmic reticulum (ER) in plant and animal cells • Network of folded membranes forming interconnected sheets, tubes or sacs • Originates from outer membrane of nucleus to which it is attached to the surface membrane • Two types present:

     smooth ER = no ribosomes
rough ER = with ribosomes	•	Smooth ER is concerned with the production of oil/lipids and steroids


• Gives a large surface area for two reasons: exchange of substances ribosomes to attach to (for the process of protein synthesis) • Transport substances through the cell e.g. RNA to ribosomes for protein synthesis

Chloroplasts in plant cells • Outer smooth membrane is selective permeable for CO2 and H2O in and glucose and O2 out • Members of the plastids, other plastids are leucoplasts in which starch is stored and chromoplasts in which pigment like carotene is stored • Inner membrane: gives rise to strands called thylakoids. Thylakoids are arranged to form flattened circular piles, grana/granum. Thykaloid contain chlorophyll. Grana are connected by lamella. • Liquid inside the chloroplast is called matrix and contain starch gains which act as temporary food storage • Produces glucose/carbohydrates during photosynthesis