ocr biology year 1: cell structure

describe the nucleus’ structure

• Contains DNA

• Contained within a double membrane (nuclear envelope) which has nuclear pores to allow molecules to move in and out.

describe the nucleus’ function

• DNA replication and transcription

• Controls metabolic activities of the cell

describe the structure of the nucleolus

• Area within the nucleus

• Densely staining

• Composed of proteins and RNA 

describe the function of the nucleolus

• Produces ribosomes

describe the structure of the mitochondrion

• Has a double membrane

• Inner membrane is folded into cristae

• The fluid interior is called the matrix

• Contain mitochondrial DNA (mtDNA) 

describe the function of the mitochondrion

• Site of final stages of aerobic cell respiration (produces ATP)

• The membrane forming the cristae contains the enzymes used in respiration

describe the structure of the vesicles

• Membranous sacs with storage and transport roles

• Have a single membrane with fluid inside

describe the function of the vesicles

• Transport of materials within the cell

describe the structure of the lysosome

• Specialised vesicle containing hydrolytic enzymes

• Densely stained in electron micrograph

describe the function of the lysosome

• Breaking down waste material in cells including old/damaged organelles and pathogens ingested by phagocytic cells

describe the structure of the rough endoplasmic reticulum

• Made up of flattened membranous sacs called cisternae

• Has ribosomes bound to the surface 

describe the function of the rough endoplasmic reticulum

• Synthesise proteins usually for secretion from the cells (in vesicles which travel via the Golgi apparatus)

describe the structure of the smooth endoplasmic reticulum

• Made up of flattened membranous sacs called cisternae

• NO ribosomes bound to the surface

describe the function of the smooth endoplasmic reticulum

• Lipid and carbohydrate synthesis and storage

describe the structure of the free ribosomes

• Not surrounded by a membrane

• Made of RNA

describe the function of the free ribosomes

• Site of protein synthesis

describe the structure of the Golgi apparatus

• Similar in structure to SER

• Made up of flattened membranous sacs (cisternae) from which vesicles can bud off

describe the function of the Golgi apparatus

• Role in modifying proteins made in the RER and packaging them into vesicles 

describe the structure of the peroxisomes

• small, membrane-bound organelles

describe the function of the peroxisomes

• break down waste products from metabolic reactions

describe the structure of the centrioles

• part of the cytoskeleton (made of microtubules)

• two centrioles make up a centrosome

describe the function of the centrioles

• makes up the spindle fibre for cell division

describe the structure of microfilaments

• fibres made of actin

• smallest part of the cytoskeleton

describe the function of microfilaments

• moves the cell

• cell contraction during cytokinesis (splitting cytoplasm for cell division)

• helping to produce pseudopodia

describe the structure of microtubules

• tubes made of globular tubulin proteins

• scaffold-like structure

• largest part of the cytoskeleton

describe the function of microtubules

• form the mitotic spindle (spindle fibres)

• make up the flagellae and cilia

• transport organelles inside the cell (they act like a track, guiding the organelles around the cell)

• moves the cell around

describe the structure of the intermediate fibres

• material depends (can be keratin)

• medium-sized part of the cytoskeleton

describe the function of the intermediate fibres

• give mechanical strength to cells, helping them to resist physical stress

what are the flagellae/undulipodia of a eukaryotic cell?

• one or two long extensions from a cell

• made up of 2 central microtubules surrounded by 9 pairs of microtubules arranged like a wheel (9+2 arrangement)

what are the ciliae of a eukaryotic cell?

• many short extensions from cells

• made up of 2 central microtubules surrounded by 9 pairs of microtubules arranged like a wheel (9+2 arrangement)

what is the structure of a chloroplast?

• double membrane (for higher surface area)

• stroma (gel-like fluid inside with enzymes)

• thylakoids stacked into grana and connected by lamellae (contain chlorophyll for photosynthesis (light-dependent reaction))

• starch grains (for energy storage)

• nucleoid (containing DNA)

• free ribosomes (to make proteins)

describe the structure and function of the cell wall

• made up of cellulose (a complex carbohydrate)

• provides support and structure

• protects cell from pathogens

describe the structure and function of the vacuole

• if in a plant cell, they are large and permanent

• made up of the tonoplast (outer membrane) and cell sap (liquid inside)

• provides turgor and is selectively permeable to let some substances in

outline the process of protein synthesis

1. in the nucleus, DNA is transcribed to make mRNAm which leaves via a nuclear pore

2. ribosomes on the rough ER (which has a membrane that is continuous with the nuclear envelope), translate the mRNA and the resulting protein is fed into the rough ER’s cisternae

3. a vesicle containing the protein buds off the cisterna and travels via microtubules to the cis face of the Golgi Apparatus

4. the Golgi processes and packages the protein

5. another vesicle buds off the trans face of the Golgi and travels via microtubules to the plasma membrane for exocytosis

what is endocytosis and exocytosis?

• endocytosis: process by which a cell engulfs a material, forming a phagocytic vacuole (sac) around it

• exocytosis: process by which a material is transported out of a cell in a secretory vesicle

how is genetic material kept in eukaryotic cells?

• in a nucleus

• linear

• associated with proteins (histones)

• extra-chromosomal DNA is in mitochondria and chloroplasts

how is genetic material kept in prokaryotic cells?

• in a nucleoid region (not membrane-bound)

• circular

• not associated with proteins

• extra-chromosomal DNA is free in the cytoplasm and is in the form of plasmids

what is the type of ribosomes in eukaryotic cells?

80S (bigger)

what is the type of ribosomes in eukaryotic cells?

70S (smaller)

what is the motile organelle in eukaryotic cells?

cytoskeleton (microfilaments, intermediate fibres, microtubules in a 9+2 arrangement that can form ciliae+flagellae)

what are the motile organelles in prokaryotic cells?

a less complex cytoskeleton (no 9+2 arrangement, thinner)

can have flagellae (NOT the ones made of microtubules)

what is the cell wall (if present) made of in eukaryotic cells?

• chitin (for fungi)

• cellulose (for plants)

what is the cell wall made of in prokaryotic cells?

peptidoglycan

how do eukaryotic cells reproduce?

mitosis and/or meiosis (asexual and/or sexual)

how do prokaryotic cells reproduce?

binary fission (asexual)

what are differences in cell division between plants and animals?

mature plant cells do not divide, but elongate, whereas animal cells continue to divide regardless of maturity

what is the endosymbiont theory?

the theory that mitochondria and chloroplasts began as free-living bacteria before being taken into a cell as an endosymbiont (an organism living in another organism)

evidence: own DNA, can make their own proteins as a result, double-membraned

describe the functions of cell membranes

• compartmentalisation (keeping cell contents separate from surroundings/organelles separate from cytoplasm, because metabolism often involves incompatible reactions which need different conditions to be maintained (e.g. chemical gradients))

• cell recognition and signalling (e.g. binding of neurotransmitters)

• providing sites for chemical reactions (e.g. inner membrane of mitochondria contain molecules needed for ATP production)

• holding components of metabolic pathways in place

• regulating transport

• being partially permeable to control what goes in or out (e.g. plasma membrane allows small uncharged particles to pass through and the proteins channels and transporters control passage of larger charged particles)

describe the fluid-mosaic model

• phospholipids and proteins are free to move within the membrane, making the membrane flexible

• the proteins embedded in the membrane are various shapes and sizes

what is the structure of the plasma membrane?

phospholipid bilayer with proteins and cholesterol embedded throughout it

what are the three types of proteins in the plasma membrane?

• channel proteins

• carrier proteins

• glycoproteins

what is the difference between intrinsic/integral and extrinsic/peripheral proteins?

• intrinsic/integral: embedded through both layers of the membrane

• extrinsic/peripheral: embedded on one side of the membrane

describe the structure and function of channel proteins

• hydrophilic channel

• allows passive movement (diffusion) of polar molecules and ions across the membrane

• held in place by hydrophobic interactions between core of membrane and side chains on the outside of the protein

describe the function of carrier proteins

• allows passive and active transport by changing shape

describe the structure and function of glycoproteins

• carbohydrate chains are attached

• helps with cell adhesion

• act as receptors for cell signalling (e.g. for neurotransmitters)

describe the structure and function of cholesterol

• steroid with a hydrophilic hydroxyl group and a hydrophobic tail (which is attracted to the hydrophobic tails of phospholipids in the centre of the membrane)

describe the structure and function of glycolipids

• lipids with carbohydrate chains attached

• act as cell markers (so the body can tell which cells are foreign and which cells are the body’s own cells)